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Blockade of the Ras/Raf/ERK and Ras/PI3K/Akt pathways by monacolin K reduces the expression of GLO1 and induces apoptosis in U937 cells Chun-Chia Chen, Mei-Li Wu, Chi-Tang Ho, and Tzou-Chi Huang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/jf505275s • Publication Date (Web): 08 Jan 2015 Downloaded from http://pubs.acs.org on January 13, 2015

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Journal of Agricultural and Food Chemistry

Blockade of the Ras/Raf/ERK and Ras/PI3K/Akt pathways by monacolin K reduces the expression of GLO1 and induces apoptosis in U937 cells

Chun-Chia Chen†, Mei-Li Wu†, Chi-Tang Ho‡, Tzou-Chi Huang*,†



Department of Food Science, National Pingtung University of Science and

Technology, Pingtung 91201, Taiwan ‡

Department of Food Science, Rutgers University, New Brunswick, NJ 08901, USA

*

Correspondence: Dr. Tzou-Chi Huang, Department of Food Science, National

Pingtung University of Science and Technology, 1, Shuehfu Road, Neipu, Pingtung 91201, Taiwan E-mail: [email protected] Tel: +886-8-7703202 ext: 5196 Fax: +886-8-7740582

Coauthors,

Chun-Chia

Chen

([email protected]),

Mei-Li

([email protected]), and Chi-Tang Ho ([email protected]).

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Wu

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Abstract

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Monacolin K, a hydrolytic product of icaritin, is the major active component in the

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traditional fermented Monascus purpureus. Monacolin K inhibits the proliferation of

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acute myeloid leukemia (AML) and underlying mechanisms remain to be identified.

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In the present study, we demonstrated that Monacolin K inhibits the proliferation of

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human AML cell line U937, in a dose-dependent manner. Importantly, morphological,

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DNA fragmentation and image cytometry analyses indicated that monacolin K

8

induced U937 cells apoptosis. Monacolin K could inactivate Ras translocation from

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cell membranes to cytosol. Monacolin K could also reduce the Ras-dependent

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phosphorylation of ERK and Akt, and the subsequent translocation of nuclear factor

11

kappa B (NF-kB) from cytosol to nucleus in U937 cells. The underlying mechanisms

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of apoptotic activity of monacolin K were associated with inhibition of the

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Ras/Raf/ERK and Ras/PI3K/Akt signals and downregulation of HMG-CoA reductase

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and Glyoxalase 1. Based on results obtained using specific inhibitors, U0126,

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LY294002 and JSH-23, the Ras/Raf/ERK/NF-κB/GLO1 and Ras/Akt/NF-κB/GLO1

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pathways were proposed for the apoptotic effect of monacolin K in U937 cells.

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Keywords: monacolin K / acute myeloid leukemia / glyoxalase 1 / HMG-CoA

19

reductase / inactivate Ras translocation / apoptosis 2

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Introduction

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Red yeast rice (RYR) is a traditional fermented product of Monascus purpureus and

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white rice.1 It is popular in some Asian countries and is known to have properties of

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lowering total cholesterol (TC) and low-density lipoprotein-cholesterol (LDL-C)

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levels.2 Accumulating evidence also showed that RYR inhibited proliferation of

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various cancer cells in vitro.3,4 The major bioactive compound in RYR, monacolin K,

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also showed tumor-suppressive activity in animal models.5 RYR is a potentially

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useful over-the-counter cholesterol-lowering agent. Typical RYR dose used in clinical

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trials is 1200-2400 mg/day which contains approximately 5 mg monacolin K.2

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The structure of monacolin K is identical to the statin drug, lovastatin. Case

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reports and early clinical studies suggest a therapeutic potential for statins in acute

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myeloid leukemia (AML). Statins are 3-hydroxy-3-methylglutaryl-coenzyme A

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reductase (HMGCR) inhibitors that act on the mevalonate pathway and inhibit

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synthesis of cholesterol, geranylgeranyl pyrophosphate (GGPP) and farnesyl

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pyrophosphate (FPP).6 It is well established that mevalonate is essential for

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post-translational modification of Ras, which can lead to increased levels of

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farnesylation of Ras by farnesyl-pyrophosphate transferase. Inhibition of mevalonate

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synthesis leads to a depletion of its downstream intermediates, such as FPP and GGPP,

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precursors for sterols, dolichols, Coenzyme Q, isoprenoids, and carotenoids.7 FPP and 3

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GGPP are substrates for the post-translational modification of proteins including the

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Ras and Rho family GTPases, well-known proto-oncogenes for control cell growth,

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migration, survival and differentiation in mammalian cells. Statins inactivate Ras

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translocation and downstream MEK/ERK and PI3K/Akt signalings leading to cancer

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cell apoptosis by interrupting the biosynthesis of mevalonate.8 Genetic mutations of

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Ras protein in cancer cells trigger dysregulation of Ras/Raf/MEK/ERK and

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Ras/PI3K/PTEN/Akt/mTOR pathway signalings, leading to uncontrolled cellular

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proliferation and decreased sensitivity to agents that ordinarily can trigger apoptosis.

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It is well established that glyoxalase 1 (GLO1) plays a pivotal role in the growth

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and survival of cancer cells. GLO1 is a key enzyme for protecting cancer cells from

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apoptosis, and it works through the elimination of excess methylglyoxal (MG), which

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is a toxic metabolite of the major energy source, glucose. The increased survival

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activity of tumors, including higher glycolytic activity and a consequent rise in MG

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production, requires an increased detoxifying action of GLO1.9 The increase of GLO1

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expression has been shown to be associated with increased proliferative activity of

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tumors. In antitumor agent-resistant leukemia cells, GLO1 was frequently

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overexpressed.10 Immunohistochemical analysis confirmed the increase of GLO1

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among patients with colon carcinoma,11 or prostate cancer.12 Significantly high

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glyoxalase activity has been detected during differentiation of leukemia cells.13 4

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Endogenously produced MG has been shown to cause apoptosis in cultured leukemia

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cells.14

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In contrast with that in normal cells, the cholesterol content of malignant tumors

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in the body is particularly high. The primary source of cholesterol synthesis in cancer

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cells is glucose. Cholesterol is currently known to be synthesized from a series of

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reactions involving the conversion of citrate in the tricarboxylic acid (TCA) cycle to

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generate acetyl coenzyme A, which then reacts with several enzymes in the

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mevalonate pathway to yield cholesterol.15 It is well established that HMGCR activity

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was three-fold higher in mononuclear blood cells from patients with leukemia,

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compared with cells from healthy subjects.16

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We here provide a detailed mechanistic investigation of the apoptotic effect of

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monacolin K and how it is related to the inhibition of GLO1. In the present study, we

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hypothesize that monacolin K showed its apoptotic activity through the inhibition of

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HMGCR, inactivation of Ras translocation, down regulation of PI3K/Akt and

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Raf/ERK1/2 signal pathways leading to suppression of GLO1 and HMGCR in U937

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cells.

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Materials and methods

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Chemicals and Antibodies 5

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Monacolin K, perchloric acid, 2-methylquinoxaline (2-MQ), o-phenylenediamine,

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3-(4,5-dimethylthiazol-2-y-l)-2,5-diphenyltetrazolium

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sulphoxide (DMSO) and 4,6-diamidino-2-phenylindole (DAPI) were purchased from

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Sigma-Aldrich (St. Louis, MO). Cell culture media were purchased from Invitrogen

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(Carlsbad, CA). Antibodies against phospho-p44/42 MAPK (Thr202/Tyr204),

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phospho-Akt (Ser473), ERK1/2, Akt1/PKBα and actin, were obtained from Millipore

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(Billerica, MA). Antibodies against GLO-1, NF-κB (p65), H-Ras, Raf-1 and lamin A

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were obtained from Santa Cruz Biotechnology (Santa Cruz, CA). Antibodies against

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K-Ras and N-Ras were obtained from GeneTex (San Antonio, TX). Alexa Fluor® 594

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Goat Anti-Mouse IgG (H+L) was purchased from Invitrogen (Carlsbad, CA). U0126

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(MEK1/2 inhibitor), LY294002 (PI3K inhibitor), JSH-23 (NF-κB inhibitor), IgG

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HRP-conjugated goat anti-mouse and IgG HRP-conjugated goat anti-rabbit were

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obtained from Chemicon (Temecula, CA).

bromide

(MTT),

dimethyl

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Cell Culture

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U937 cells were purchased from the American Type Culture Collection (Rockville,

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MD). The cells were cultured in RPMI 1640 medium with 10% fetal bovine serum

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(GibcoBRL; Grand Island, NY), 5 mM L-glutamine (GibcoBRL) and 1%

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penicillin/streptomycin (10000 units of penicillin/mL and 10 mg/mL streptomycin) at 6

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37 °C in a humidified 5% CO2 incubator.

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Cell Viability Assay

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Cell viability was assessed by MTT assay.17 Briefly, cells at 1×105/mL were treated

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with various concentrations of monacolin K in 96-well plates for 48 h, followed by

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treatment with MTT for 4 h at 37 °C. The medium was removed and DMSO was

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added to dissolve the blue formazan residue and color intensity was measured using

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an ELISA reader at 570 nm.

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Morphological Assay

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After treatment with various concentrations of monacolin K for 48 h, cells were

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harvested and fixed with 4% paraformaldehyde in PBS for 20 min at room

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temperature, and stained with 2.5 mg/mL DAPI solution for 30 min at room

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temperature. The cells were washed two more times with PBS and analyzed via a

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confocal microscope (CARV II Confocal Imager; BD Biosciences).

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DNA Fragmentation Assay

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After treatment with various concentrations of monacolin K for 48 h, washed with

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ice-cold PBS, the cells were resuspended in 180 µL DNA lysis buffer (0.5% 7

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lauroylsarcosine, 10 mM EDTA, 0.5 mg/mL proteinase K and 50 mM Tris-HCl at pH

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8.0) overnight at 56 °C, and treated with RNaseA (0.5 µg/mL) for 1 h at 37 °C. Total

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DNA

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phenol/chloroform/isoamyl alcohol (Sigma-Aldrich, St. Louis, MO). 10 µL of DNA

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were analyzed on 1.8% agarose gel.

was

extracted

from

the

cells

with

an

equal

volume

of

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Image Cytometry Analysis

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Cell sub-G1 phase was analyzed by the DNA content using DAPI staining method.

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After treatment with various concentrations of monacolin K for 48 h, U937 cells were

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centrifuged at 1,500 rpm for 5 min, and the cell pellets were fixed in 70% ice-cold

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ethanol overnight at 4 °C. Fixed cells were washed with PBS, incubated with 0.5 mL

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hypotonic buffer containing 0.5% Triton X-100 and 0.5 mg/mL RNase A for 30 min

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at 37 °C, and then stained with 50 µg/mL DAPI, and the sub-G1 phase content was

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determined on a NucleoCounter NC-3000 cytometer (ChemoMetec, Allerod,

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Denmark).

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RNA Isolation and Reverse Transcription-PCR Amplification (RT-PCR)

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Total RNA was extracted from U937 cells using a commercially available RNA

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extraction kit (Favorgen, Pingtung, Taiwan) and following the manufacturer’s 8

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instruction. First-stranded cDNA was synthesized using 1 µg total RNA by RT-PCR

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kit (Roche, Indianapolis). The relative expression of GLO1 and HMGCR was

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analyzed using RT-PCR with GAPDH as an internal control. The primers used were

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GLO1,

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5’-CTACATTAAGGTTGCCATTTTGTTAG-3’, yielding a product of length 564 bp;

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HMGCR,

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5’-CAAGCCTAGAGACATAATCATC-3’, yielding a product of length 252 bp;

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GAPDH,

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5’-CAAGCCTAGAGACATAATCATC-3’, yielding a product of length 248 bp. A 10

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µL volume reaction for PCR included 2 µL of cDNA, 1 µL each of sense and

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antisense primers and 5 µL of PCR master mix. The conditions were: 94 °C for 5 min,

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94 °C for 40 sec, 60 °C for 35 sec (GLO1), 58 °C for 30 sec (HMGCR) and 64 °C for

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30 sec (GAPDH), 72 °C for 40 sec, and 72 °C for 7 min, for 26 cycles. The PCR

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products were separated by electrophoresis on 1.5% agarose gels and the results were

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analyzed using a Kodak Gel logic imaging system (Hyland Scientific, Stanwood,

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WA).

sense

5’-ATGGCAGAACCGCAGCCCCCGT-3’

sense

sense

5’-TACCATGTCAGGGGTACGTC-3’

5’-TACCATGTCAGGGGTACGTC-3’

and

and

and

antisense

antisense

antisense

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Western Blot Analysis

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The whole cells protein were extracted via the addition of 100 µL of gold lysis buffer 9

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(50 mM Tris-HCl, 150 mM NaCl, 1 mM EDTA, 1% Triton X-100, and 1% protease

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inhibitor cocktail at pH 7.4), followed by incubation at 4 °C for 1 h, centrifugation,

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and collection of cell lysate. Subcellular Protein Fractionation Kit was used to

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determine the levels of protein expression in cytosol, nucleus, and membrane. Extract

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preparation followed instruction of the manufacturer’s published protocol (Thermo,

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Rockford, IL). Protein concentrations were measured with a Bio-Rad protein assay

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reagent (Bio-Rad Laboratories, Munich, Germany). Equal amounts of lysate protein

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(30 µg/lane) were subjected to sodium dodecyl sulfate polyacrylamide gel

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electrophoresis

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electrotransferred to a polyvinylidene difluoride (PVDF) membranes (Millipore,

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Billerica, MA), blocked at 37 °C for 1 h with 1% BSA in TBST (20 mM/L Tris-base;

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125 mM/L NaCl; 0.2% Tween 20; 0.1% sodium azide), and then immunoblotted with

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primary antibodies overnight at 4 °C, and then washed with TBST solution (0.05%

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Tween 20 in TBS). After washing six times with TBST buffer, the blot was washed,

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exposed to horseradish peroxidase conjugated secondary antibodies for 1 h,

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immunoreactive proteins were visualized with an enhanced chemiluminescence (ECL)

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detection system (GE Healthcare, Little Chalfont, UK).

(SDS-PAGE).

After

electrophoresis,

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Total Cholesterol Quantitation 10

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proteins

were

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The total cholesterol concentration was measured using Cholesterol/Cholesterol Ester

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Quantitation Kit (Calbiochem, Novabiochem, Boston, MA). After treatment with

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monacolin K for 48 h, cells were washed with cold PBS and centrifuged. The pellet

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was re-suspended in 0.2 mL of chloroform:isopropanol:NP-40, homogenize for 3 min

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and cleared by centrifugation at 14,000 rpm for 10 min. Supernatants were decanted

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and air-dry at 50 °C, and vacuum-dry for 30 min to remove residual solvents. The

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dried lipids were dissolved in 0.2 mL of the reaction buffer and the absorbance was

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measured at 570 nm for total cholesterol detection.

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Methylglyoxal (MG) Measurement

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MG was measured by an o-Phenylenediamine method.18 MG was derivatized with

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o-phenylenediamine to form the quinoxaline product 2-MQ, which is very specific for

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MG. Briefly, the supernatant of the cell was incubated with 100 mM

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o-phenylenediamine for 3 h at room temperature. The quantity of the quinoxaline

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derivative of MG (2-MQ) was measured using a Hitachi L-6200 HPLC system

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(Hitachi, Ltd., Mississauga, Ontario, Canada). The mobile phase was generally 68

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vol% of 10 mM KH2PO4 (pH 2.5) and 32 vol% of HPLC grade acetonitrile. The

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analysis conditions were as follows: detector wavelength, 315 nm; mobile phase flow

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rate, 1.0 mL/min; and column temperature, 20 °C. The whole series of samples in a 11

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single experiment was run in duplicate.

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Immunofluorescence Microscopy

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To detect the localization of NF-κB in monacolin K treated U937 cells, the U937 cells

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were first seeded in a 12 well dish. The cells were fixed with 4% paraformaldehyde

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for 30 min and permeabilized with PBS containing 0.1% Triton X-100 for 5 min at

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room temperature. After specimens were blocked with 5% BSA for 1 h at room

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temperature, and further incubated with NF-κB (p65) antibody for 2 h. After three

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washes in TPBS, specimens were then stained with Alexa Fluor® 594 Goat

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Anti-Mouse IgG for 1 h at room temperature. After being washed three times with

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TPBS, cells were stained with 0.5 mg/mL of DAPI for 10 min to counter stain the

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nucleus, and then washed three times for 5 min with TPBS. Images were collected by

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confocal microscope.

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Statistical Analysis

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The data were analyzed using the SAS program (SAS Institute, Cary, NC). The

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significance of the difference among mean values was determined by one-way

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analysis of variance followed by the t-test; (*) p < 0.05 and (**) p < 0.01 were

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considered statistically significant. Values are the mean ± SD of three independent

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experiments. 12

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Results

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Monacolin K Induced Apoptosis in U937 Cells

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MTT assays were performed to evaluate the potential cytotoxic effects of monacolin

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K. As shown in Figure 1A, U937 cells were incubated with increased concentrations

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of monacolin K for 48 h. Exposure of monacolin K to U937 cells resulted in a

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significant decrease in viable cells in a dose-dependent fashion. Treatment with 20

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µM monacolin K achieved 57% inhibition (p < 0.01). To determine whether an

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increase in apoptosis was associated with the observed decrease in cell number after

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monacolin K treatment, the cells were stained with DAPI and analyzed by confocal

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microscope. As shown in Figure 1B, the nuclei of U937 cells exposed to monacolin K

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exhibited nuclear condensation and fragmented chromatin of typical apoptosis in a

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dose-dependent manner. Apoptosis was also detected by DNA fragmentation assay.

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DNA fragmentation was significantly increased after treatment with monacolin K in a

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dose-dependent manner (Figure 1C). Furthermore, the degree of apoptosis was

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quantified by analyzing the amount of sub-G1 DNA content in the U937 cells treated

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with monacolin K using an image cytometer. As shown in Figure 1D, in untreated

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control, the percentage of untreated control cells with fragmented DNA (sub-G1) was

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only 3%. Treatment with 20 µM monacolin K increased the percentage of cells with

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fragmented DNA (sub-G1) to 68%. Taken together, these findings indicated that 13

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exposure to monacolin K resulted in the inhibition of cell proliferation and a striking

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induction of apoptosis in U937 cells.

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Monacolin K Downregulated HMGCR Expression and Reduced Cholesterol

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Generation

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In order to elucidate the effects of monacolin K on HMGCR production on the

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transcriptional level, cells were treated with various concentrations of monacolin K

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for 24 h. The cDNA was amplified by PCR with primers specific for HMGCR or

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GAPDH (as a control gene), and the results indicated that HMGCR mRNA

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expression was suppressed in the presence of monacolin K as shown in Figure 2A.

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The suppression of HMGCR mRNA expression correlated to Western Blot analysis.

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Treatment of U937 cells with monacolin K inhibited HMGCR protein expression also

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in a dose-dependent manner (Figure 2B). After incubation with 20 µM monacolin K,

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we measured the intracellular total cholesterol content. As shown in Figure 2C,

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monacolin K showed significant inhibitory effect on cholesterol level at

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approximately 36% (p < 0.01), as compared with control cells.

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Monacolin K Downregulated GLO1 Expression and Increase MG Levels

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To understand the critical roles of GLO1 in apoptosis, expression of GLO1 in U937 14

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cells was monitored following treatment with monacolin K. U937 cells were treated

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with various concentrations of monacolin K for 48 h or 20 µM of monacolin K for

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various times. The cDNA was amplified by PCR with primers specific for GLO1 or

253

GAPDH (as a control gene). As shown in Figure 3A, monacolin K induced time- and

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dose-dependent downregulation of GLO1 mRNA levels. Additionally, monacolin K

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administration resulted in a marked decrease in the GLO1 protein level as compared

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with control (Figure 3B). To investigate the effect of GLO1 expression blockage on

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MG level, U937 cells were treated with monacolin K at a concentration of 20 µM for

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48 h. A significant increase in MG level was observed in comparison to levels

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produced by untreated cells (Figure 3B). Monacolin K caused an increase in MG

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accumulation by 2.7-fold as compared with that of control (p < 0.05).

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Effects of Monacolin K on Ras Protein Expression and Downstream Signaling

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Pathways Regulation

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Ras is considered a key enzyme for proliferative, survival, and oncogenic signals. We

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examined the regulatory effect of monacolin K on membrane protein Ras and its

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downstream signaling proteins, i.e., Raf-1, Akt and ERK. As shown in Figure 4A,

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after incubation with 20 µM monacolin K for 24 h, monacolin K significantly

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decreased the levels of H-Ras, K-Ras, N-Ras and Raf-1 in cell membrane. In contrast, 15

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the H-Ras, K-Ras, N-Ras and Raf-1 levels in cytosolic fraction were significantly

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higher in the monacolin K treatments. To assess whether monacolin K mediated

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and/or inhibited the phosphorylation of Akt and ERK. U937 cells were treated with

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various concentrations of monacolin K for 24 h. Figure 4B showed that monacolin K

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significantly inhibited the activation of Akt and ERK as shown by the decrease in the

274

phosphorylation of Akt and ERK.

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Monacolin K Abrogated NF-κB Signaling by Blocking the Nuclear Translocation

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of NF-κB (p65)

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To explore the underlying mechanism of NF-κB inactivation by monacolin K, we

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analyzed the protein expression of p65 by immunoblotting. As shown in Figure 5A,

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treatment of the U937 cells with 20 µM monacolin K for 24 h resulted in a significant

281

decreased level of p65 in cell nucleus. In contrast, the p65 levels in cytosolic fraction

282

were significantly increased. Confocal microscope analysis revealed that monacolin K

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inhibited p65 translocation from cytosol to nucleus (Figure 5B). Therefore, it was

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possible that the inhibitory effect of monacolin K on the NF-κB translocation was due

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to inactivation of Akt and ERK pathways, leading to a reduction in GLO1 expression.

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Inhibitory Effects of Several Specific Inhibitors on Expression of Akt, ERK, and 16

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GLO1 and on Cell Viability in U937 Cells

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Several specific inhibitors were used to assess whether a specific signaling pathway

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was involved in inhibition of monacolin K induced GLO1 and other protein

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expressions. LY294002, U0126 and JSH-23 were used in the treatment of U937 cells.

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After treatment, cell extracts were prepared to determine the levels of GLO1. As

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shown in Figure 6A, three LY294002, U0126 and JSH-23 inhibitors could effectively

294

suppress the GLO1 protein expressions. In addition, Figure 6B showed that Akt and

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ERK phosphorylation decreased after LY294002 and U0126 treatment for 24 h, and

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JSH-23 had little effects on the phosphorylation of Akt and ERK (Figure 6B). Next,

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we administered these inhibitors, to U937 cells to determine whether suppression of

298

the Ras/PI3K/Akt/NF-κB and Ras/Raf/ERK/NF-κB signaling pathways would inhibit

299

cell viability. As shown in Figure 6C, these inhibitors had a strong inhibitory effect on

300

U937 cells growth (p < 0.01). These results confirmed that the apoptotic effect of

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monacolin K followed the Ras/Akt/NF-κB/GLO-1 and Ras/Raf/ERK/NF-κB/GLO-1

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signaling pathways.

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Mevalonate Attenuated Monacolin K Induced Apoptosis and Protein Expression

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in U937 Cells

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Overexpression of mevalonate, a precursor of cholesterol, has been reported to be 17

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associated with cell survival and proliferation of cancer cells. In the study, the

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treatment of mevalonate with monacolin K fully reversed its induced apoptosis in

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U937 cells as evidenced by DNA fragmentation and cell viability analysis (p < 0.01)

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(Figure 7A and Figure 7B). Next, co-incubation of mevalonate with monacolin K for

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24 h reversed monacolin K triggered loss of Akt and ERK phosphorylation (Figure

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7C). We further investigated whether mevalonate blocked monacolin K suppresses

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GLO1 expression. After cells pre-incubated with mevalonate for 1 h and then treated

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with monacolin K for 48 h, as shown in Figure 7D and Figure 7E, mevalonate

315

prevented the decrease in GLO1 mRNA and protein by monacolin K. These results

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indicated that the down-regulation of mevalonate pathway is highly associated with

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monacolin K inhibition on GLO1 as well as the induction of apoptosis in U937 cells.

318 319

Discussion

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The anti-cancer effect of monacolin K was demonstrated by its growth inhibitory

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activity on various cancer cell lines including human acute myeloid leukemia,19 breast

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cancer,20 and pancreatic cancer.21 A structure activity relationship analysis showed the

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alkyl function group increased lipophilicity of monacolin K leading to a direct

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diffusion through the cell membrane and affected cell proliferation, survival, and

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motility.4 We hypothesize that monacolin K may induce apoptosis in U937 cells by 18

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inhibiting the mevalonate pathway and impairing Ras localization to the plasma

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membrane. It interrupted downstream Akt and ERK pathways leading to the

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decreased HMGCR and GLO1 proteins expression.

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Tumors, including leukemia, transformed from normal cells alter metabolism.

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Mutations of oncogenes and tumor-suppressor genes are believed to switch the

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glucose metabolism from oxidative phosphorylation to glycolysis in cancer cells.22

332

Previous reports showed that cancer cells tend to use glycolysis pathway to

333

metabolize glucose due to lacking or mutated p53 gene.23 Positron emission

334

tomography (PET) data demonstrated that tumor cells accumulated extraordinary

335

amounts of glucose. Elevated glucose metabolism is seen in activated lymphocytes in

336

the draining lymph nodes of Moloney Murine Sarcoma and Leukemia Virus

337

Complex-challenged mice using a 18F-FDG probe.24

338

MG can be derived non-enzymatically from phosphorylated glycolytic

339

intermediates dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. MG

340

causes DNA fragmentation at physiological concentrations on U937 cells25 and HL-60

341

cell lines.26 Recent studies have shown that MG specifically inhibits mitochondrial

342

respiration in human leukaemic leucocytes leading to a tumoricidal effect.27 We

343

postulate that U937 cancer cell may use glycolysis to get the required energy and

344

increase the GLO1 expression to eliminate excess MG for surviving and proliferation. 19

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345

Detoxification of MG is mainly activated via the glyoxalase pathway present in

346

the cytosol of all mammalian cells that use GSH as a cofactor.28 The apoptotic effects

347

of MG were found in high glucose-treated VL-17A cells29 and brain microvascular

348

endothelial cells.30 It has been shown that GLO1 expression were significantly

349

up-regulated in prostate cancer LNCaP,31 hepatocellular carcinoma,32 and human

350

gastric cancer,33 when compared with adjacent non-tumorous tissue. Recently, a novel

351

mechanism of MG cytotoxicity in prostate cancer cells was proposed.31 The silenced

352

GLO1 in prostate LNCaP and PC3 cancer cells was found to be responsible for the

353

accumulation of excess MG that led to the apoptosis via the cell survival NF-κB

354

signaling pathway.

355

Ras has been reported to be frequently mutated in patients with acute myeloid

356

leukemia (AML). Ras gene mutation leads to the expression of activated Ras proteins,

357

and triggers constitutively overactive signaling inside the cell.34 A previous study

358

found N-Ras mutations (11%) and K-Ras mutations (5%), in AML patients.35 Among

359

the Ras isoforms, only H-Ras is palmitoylated and integrated into lipid rafts.36

360

Activated Ras recruits Raf family proteins to the lipid raft and activate the subsequent

361

ERK pathway. Constitutively activated Raf/MEK/ERK pathway has been

362

characterized in both acute myeloid leukemia (AML) and acute lymphocytic leukemia

363

(ALL).37 Overexpression of activated Raf proteins is closely related to cell cycle 20

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364

arrest in human promyelocytic leukemia and some hematopoietic cells.38 It is well

365

established that the cross-talk between Ras/Raf/MEK/ERK and Ras/PI3K/PTEN/Akt

366

pathways regulates cell growth and tumorigenesis in many leukemia samples.37 Both

367

pathways may trigger the phosphorylation of downstream targets and regulate cell

368

survival and proliferation.

369

PI3K activity is another predominant growth factor activated pathway in

370

varieties of human tumors including leukemia.39 Accumulated evidence shows that

371

PI3K is heavily involved in the malignant transformation of cells through the

372

activation of the downstream Akt.40 Blalock et al.41 reported that MEK-mediated

373

transformation of certain hematopoietic cells requires the activation of the PI3K

374

pathway. Combined treatment with the Chk1 inhibitor 7-hydroxystaurosporine

375

(UCN-01) and lovastatin was found to synergistically disrupt Ras prenylation and its

376

translocation to membrane leading to the inhibition of the phosphorylation of both

377

ERK 1/2 and Akt and enhanced apoptosis in U937 cells.42 Using immunoprecipitation

378

analysis, Chiu et al.43 showed that Raf-1, Akt and Ras can accumulate in the

379

membrane to increase cell migration/invasion through PI3K/Akt and Raf-1/ERK

380

activation. A redox modulated up-regulation of survival signaling, involving the ERK

381

and PI3K/Akt phosphorylation pathways has been reported in U937 cells.44 Recent

382

studies demonstrated that monacolin K can inhibit the Akt and ERK pathways leading 21

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383

to the decreased Bcl-2, while increased Bax expression in human colon carcinoma

384

HCT-116 cell line.45

385

The expression and activity of the transcription factor, NF-κB, have been shown

386

to be regulated by the Ras/PI3K/AKT and RAS/MAPK pathways.46 The transcription

387

factor NF-κB promotes cell proliferation and prevents apoptosis through its regulation

388

of various gene products. Monacolin K and other natural statins were found to reduce

389

NF-κB DNA-binding activity in TNF-α (tumor necrosis Factor-α)-stimulated human

390

myeloid leukemia KBM-5 cells.47

391

She et al.48 reported that farnesyltransferase inhibitor manumycin A (manumycin)

392

induced nitric oxide production and apoptosis in two myeloid leukemia cell lines

393

(U937 and HL-60). Manumycin reduced the amount of functional Ras localized at the

394

cytoplasmic membrane, thus blocking Raf-1 association with Ras. Manumycin

395

inhibits the phosphorylation of both ERK1/2 and Akt and interferes with the

396

association of PI3K and Ras in HepG2 cells.49 Monacolin K and other statins

397

stimulate cell death, which was accompanied by the collapse of the mitochondrial

398

membrane potential (MMP) in isolated Sprague-Dawley rats hepatocytes.18 This

399

observation indicates that cholesterol levels play a major role in the integrity of cell

400

membranes, and the inactivation of HMGCR activity by monacolin K may be

401

responsible for the apoptotic effect. 22

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402

Our results reveal that the translocation of both Ras and Raf-1 to membrane and

403

the levels of p-Akt in the PI3K/AKT/mTOR signaling pathway as well as p-ERK in

404

the MAPK signal pathway were down-regulated by monacolin K in the U937 cell line.

405

We postulate that monacolin K, a HMG CoA reductase inhibitor, blocks the

406

biosynthesis of mevalonate and the subsequent farnesyl pyrophosphate through the

407

inactivation of HMG CoA reductase. Lack of farnesyl pyrophosphate results in the

408

inhibition of the isoprenylation of Ras protein, and leading to the repression of

409

accumulation

410

phosphorylation of both Akt and ERK pathways will be thus abrogated. Overall, these

411

results provided evidence of the specific molecular pathways by which monacolin K

412

induces apoptosis: it decreases the activation of GLO1 and lowers cholesterol levels

413

through the inhibition of Ras/Raf/ERK/NF-κB and Ras/Akt/NF-κB pathways (Figure

414

8). These findings provide evidence that it will be worthwhile to investigate the

415

potential of monacolin K for the treatment of leukemia and potentially other types of

416

human cancers.

of

putative

lipid

raft-associated

proteins.

The

subsequent

417 418

Abbreviations Used

419

GLO1,

420

extracellular-signal-regulated kinases; Akt, protein Kinase B; NF-κB, nuclear factor

421

kappa-B ; MG, methylglyoxal; RYR, red yeast rice

glyoxalase

1;

HMGCR,

HMG-CoA

23

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reductase;

ERK,

Journal of Agricultural and Food Chemistry

422 423

Acknowledgments

424

This work was supported by the Ministry of Science and Technology (Taiwan) (NSC

425

103-2313-B-020-008-).

426 427

Reference

428

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429

Constituents of red yeast rice, a traditional Chinese food and medicine. J. Agric. Food

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Chem. 2000, 48, 5220-5225.

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enhancement of apoptosis by statins through down-regulation of the NF-kappaB

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Figure captions

594

Figure 1. Inhibition of cell viability and apoptosis-inducing effects of monacolin K on

595

U937 cells. (A) U937 cells were seeded (1 × 105 cells/well) in a 96-well plate, the cells

596

were treated with variable concentrations of monacolin K for 48 h and cell viability

597

was measured by MTT assay. (B) Nuclear condensation in apoptotic cells detected by

598

DAPI staining. U937 cells were treated with various concentrations of monaclin K for

599

48 h. (C) DNA fragmentation was detected by gel electrophoresis following

600

stimulation with 2.5-20 µM monacolin K for 48 h. (D) Image cytometry analysis

601

shows the percentage of sub-G1 phase in U937 cells treated with monacolin K for 48 h.

602

All data represent means ± SD of triplicates (n = 3). In panel A, **p < 0.01, compared

603

with untreated cells (control groups).

604 605

Figure 2. Monacolin K specifically suppresses HMGCR expression and cholesterol

606

level in U937 cells. (A) Cells were incubated with various concentrations of

607

monacolin K for 24 h, HMGCR and GAPDH mRNA levels were analyzed by RT-PCR.

608

(B) Immunoblots of whole cell lysates were probed with antibodies specific to

609

HMGCR and Actin. (C) U937 cells were treated with 20 µM monacolin K for 48 h,

610

and measured the intracellular total cholesterol content. All data represent means ± SD

611

of triplicates (n = 3). In panel C, **p < 0.01, compared with untreated cells (control 33

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612

groups).

613 614

Figure 3. Down-regulation of GLO1 and increase MG levels by monacolin K in U937

615

cells. U937 cells were incubated with various concentrations of monacolin K for 48 h,

616

or with monacolin K (20 µM) for the indicated periods. (A) GLO1 and GAPDH

617

mRNA levels were analyzed by RT-PCR. (B) Immunoblots of whole cell lysates were

618

probed with antibodies specific to GLO1 and Actin. (C) MG levels were estimated by

619

HPLC as described in Materials and methods. Monacolin K caused an increase in MG

620

accumulation by 2.7-fold as compared with that of control (Figure 3C). All data

621

represent means ± SD of triplicates (n = 3). In panel C, **p < 0.05, compared with

622

untreated cells (control groups).

623 624

Figure 4. Down-regulation of Ras and its downstream signaling molecules by

625

monacolin K in U937 cells. (A) After treatment with monacolin K for 24 h, membrane

626

and cytosol proteins were extracted and used for immunoblotting of Ras (H, K and N)

627

and Raf-1. (B) Cells were lysed and the total protein (30 mg) was submitted to

628

SDS-PAGE and immunodetected with anti-phospho- or anti-(ERK or Akt) antibodies.

629

Tests were performed in triplicate and all experiments repeated three times (n = 3).

630 34

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631

Figure 5. Monacolin K inhibits the translocation of NF-κB (p65) to the nucleus. (A)

632

U937 cells were incubated for 24 h with indicated concentrations of monacolin K and

633

nuclear extracts were immunoblotted for p65 and the loading control Lamin A. (B)

634

After treated with 24 h, cells were fixed, stained with p65 antibody and Alexa Fluor®

635

594 Goat Anti-Mouse (red), and counterstained with DAPI (blue). Tests were

636

performed in triplicate and all experiments repeated three times (n = 3).

637 638

Figure 6. Down-regulation of Ras downstream signaling molecules and cell viability

639

by various inhibitors in U937 cells. (A) U937 cells were treated with 20 µM various

640

inhibitors for 48 h. Total cell extracts were generated and immunoblotted with

641

antibodies against GLO1. (B) U937 cells were treated with 20 µM of various

642

inhibitors and incubated for 24 h, immunoblots of whole cell lysates were probed with

643

antibodies specific to p-Akt and p-ERK. (C) The cell viabilities of U937 cells treated

644

with 20 µM various inhibitors for 48 h were measured by MTT assay. All data

645

represent means ± SD of triplicates (n = 3). In panel C, **p < 0.01, compared with

646

untreated cells (control groups).

647 648

Figure 7. Recover of monacolin K-induced apoptosis and suppress Ras downstream

649

signaling molecules in U937 cells by adding mevalonate. Cells were preincubated 35

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650

with 1 mM mevalonate for 1 h and then treated with 20 µM monacolin K for 48 h (A

651

and B). (C) Cells were pretreated with 1 mM mevalonate for 1 h and then treated with

652

20 µM monacolin K for 24 h, immunoblots of whole cell lysates were probed with

653

antibodies specific to anti-phospho- or anti-(ERK or Akt). (D) GLO1 and GAPDH

654

mRNA levels were analyzed by RT-PCR. (E) Immunoblots of whole cell lysates were

655

probed with antibodies specific to GLO1 and Actin. All data represent means ± SD of

656

triplicates (n = 3). In panel C, **p < 0.01, compared with untreated cells (control

657

groups).

658 659

Figure 8. Diagram depicting the hypothesis of signaling regulation on GLO1

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expression through both Ras/Raf/ERKNF-κB and Ras/PI3K/Akt/NF-κB pathways

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following monacolin K exposure in U937 cells. Farnesyl-PP, farnesyl pyrophosphate;

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FPTase, farnesyl prenyl transferase; GEF, guanine nucleotide exchange factor; GAP,

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GTPase-activating protein.

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