Serum Metabolite Profiling of Human Colorectal Cancer Using GC

Aug 13, 2009 - Oncology, Shanghai Medical College, Fudan University, Shanghai 200032, China, David H. Murdock Research. Institute, Kannapolis, North ...
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Serum Metabolite Profiling of Human Colorectal Cancer Using GC-TOFMS and UPLC-QTOFMS Yunping Qiu,§,‡ Guoxiang Cai,†,‡ Mingming Su,|,‡ Tianlu Chen,⊥,‡ Xiaojiao Zheng,⊥ Ye Xu,† Yan Ni,⊥ Aihua Zhao,⊥ Lisa X. Xu,⊥ Sanjun Cai,*,† and Wei Jia*,§,| Department of Nutrition, University of North Carolina at Greensboro, North Carolina Research Campus, Kannapolis North Carolina 28081, Department of Colorectal Surgery, Cancer Hospital, and Department of Oncology, Shanghai Medical College, Fudan University, Shanghai 200032, China, David H. Murdock Research Institute, Kannapolis, North Carolina 28081, and School of Pharmacy, and Shanghai Center for Systems Biomedicine, Shanghai Jiao Tong University, Shanghai, P. R. China, 200240 Received December 18, 2008

Abstract: Colorectal carcinogenesis involves the overexpression of many immediate-early response genes associated with growth and inflammation, which significantly alters downstream protein synthesis and small-molecule metabolite production. We have performed a serum metabolic analysis to test the hypothesis that the distinct metabolite profiles of malignant tumors are reflected in biofluids. In this study, we have analyzed the serum metabolites from 64 colorectal cancer (CRC) patients and 65 healthy controls using gas chromatography time-of-flight mass spectrometry (GCTOFMS) and Acquity ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry (Acquity UPLC-QTOFMS). Orthogonal partial leastsquares discriminate analysis (OPLS-DA) models generated from GC-TOFMS and UPLC-QTOFMS metabolic profile data showed robust discrimination from CRC patients and healthy controls. A total of 33 differential metabolites were identified using these two analytical platforms, five of which were detected in both instruments. These metabolites potentially reveal perturbation of glycolysis, arginine and proline metabolism, fatty acid metabolism and oleamide metabolism, associated with CRC morbidity. These results suggest that serum metabolic profiling has great potential in detecting CRC and helping to understand its underlying mechanisms. Keywords: Colorectal cancer • metabonomics • gas chromatography time-of-flight mass spectrometry • ultraper* To whom correspondence should be addressed. Sanjun Cai, M.D., Department of Colorectal Surgery, Cancer Hospital, Fudan University; Department of Oncology, Shanghai Medical College, Fudan University, 270 Dong an Road, Shanghai 200032, P.R.China. Phone, (86)21-64175590 ext 1208; fax, (86)21-64035387, e-mail, [email protected]. Wei Jia, Department of Nutrition, University of North Carolina at Greensboro, North Carolina Research Campus, Kannapolis NC 28081, USA. Phone, 704-250-5803; fax, 704-250-5809; e-mail, [email protected]. § University of North Carolina at Greensboro. ‡ These authors contributed equally to this work. † Fudan University. | David H. Murdock Research Institute. ⊥ Shanghai Jiao Tong University.

4844 Journal of Proteome Research 2009, 8, 4844–4850 Published on Web 08/13/2009

formance liquid chromatography quadrupole time-offlight mass spectrometry • serum

Introduction Colorectal cancer (CRC) is the third most common type of cancer and the fourth most frequent cause of cancer mortality in the world.1 The American Cancer Society estimated that a total of 1 437 180 new cancer cases and 565 650 deaths from cancer, including 148 810 new CRC cases and 49 960 deaths from CRC, would occur in the United States in 2008.2 Recent decades have witnessed a rapid increase in CRC morbidity in fast developing countries like China, especially in major cities where significant lifestyle alterations have occurred.3 Early and accurate diagnosis of CRC is of central importance for 5-year survival and for less complicated surgery.4 Although CRC is a highly treatable and often curable disease when localized to the bowel, the prognosis for late stage CRC (e.g., recurrent metastatic disease) remains poor and is often fatal.2 A fundamental reason for the relative lack of progress worldwide in treating CRC is that the biology of this malignant disease is not sufficiently understood. Experimental studies have focused largely on understanding the transcriptional regulation of cancer-associated gene expression,5 whereas less research has been devoted to determining how this perturbed post-transcriptional regulation leads to the abnormal expression of downstream proteins and metabolites in this complex disease. Therefore, more effort is needed to improve our understanding of CRC biology in order to identify new molecular targets and improve current cancer treatment and prevention strategies. Overexpression of many immediate-early response genes associated with growth and inflammation (e.g., proto-oncogenes, inflammatory mediators, and angiogenic growth factors) is commonly observed in CRC cells. These genetic modifications, associated with colorectal carcinogenesis, allow the transformed cell to escape apoptosis while promoting proliferation, angiogenesis, and metastasis.6 These changes lead to significant alterations in downstream biochemical substances such as proteins and small-molecule metabolites. Smallmolecule metabolites are the products of systemic biochemical regulations, and their expression levels can be regarded as the response of biological systems to genetic and environmental changes.7 10.1021/pr9004162 CCC: $40.75

 2009 American Chemical Society

technical notes

Serum Metabolite Profiling of Human Colorectal Cancer Currently, a lack of detailed information about the diseaseassociated metabonome limits the ability of cancer biologists to understand the roles of metabolic pathways associated with CRC and its treatment. To address this gap in knowledge, emerging metabonomics/metabolomics technology uses multivariate statistical techniques to analyze the highly complex data sets generated by high-throughput spectroscopy, such as nuclear magnetic resonance (NMR) and mass spectrometry (MS).7-9 Identifying metabolites that account for the differences between the metabolic profiles of people with CRC and healthy counterparts can reveal important underlying molecular mechanisms of the disease. To date, global metabolic profiling of human biofluids (e.g., urine and sera) has been used to visualize the distinctive metabolic profiles of patients with prostate cancer,10 inflammatory bowel disease,11 type 2 diabetes12 and so on. Recently, metabolic profiling of tissue specimens of CRC patients has revealed significant variations in the metabolites of tumor tissue and normal mucosa.13-15 However, the metabolite profile of serum associated with CRC morbidity still remains poorly understood and would be of particular interest since the study is noninvasive and provides global pathophysiological information. We have previously used a combined gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) method to study the aristolochic acid induced nephrotoxicity in a rat model.16 The combined use of different types of spectroscopic platforms, such as GC-MS and LC-MS, takes advantage of complementary analytical outcomes and, therefore, provides a broadened metabolic “window” for explaining the biological variations associated with pathophysiological conditions. In the current study, we attempted to combine two profiling instruments, gas chromatography-time-of-flight mass spectrometry (GC-TOFMS) and Acquity ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry (Acquity UPLC-QTOFMS) to maximize the spectrum of the detectable serum metabolites. In this study, we conducted a comprehensive analysis of the serum metabolites from 129 participants (65 healthy individuals and 64 CRC patients diagnosed as stage I, II, III, or IV, TNM Classification) using GC-TOFMS and UPLC-QTOFMS in conjunction with multivariate statistical techniques. The purpose of this comprehensive metabolic analysis was to determine whether variations in the CRC metabonome are reflected in serum, thereby making an alternative, noninvasive means for cancer detection possible. The study was also intended to gain knowledge of important metabolic variations associated with CRC morbidity, which can be utilized for improved CRC detection, diagnosis, and therapeutic strategies.

Materials and Methods Chemicals. HPLC grade methanol, acetonitrile, and formic acid were purchased from Merck Chemicals (Darmstadt, Germany). Chloroform, pyridine, and anhydrous sodium sulfate were analytical grade and purchased from China National Pharmaceutical Group Corporation (Shanghai, China). L-2chlorophenylalanine was purchased from Intechem Tech. Co. Ltd. (Shanghai, China). BSTFA (1% TMCS), heptadecanoic acid, methoxyamine, leucine-enkephalin were purchased from SigmaAldrich (St. Louis, MO). Clinical Samples. The patients, ages 42-74 years and diagnosed with CRC (32 colon cancers and 32 rectal cancers), were categorized according to histopathological features and stages according to TNM classification of malignant tumors:

Table 1. Demographic and Clinical Chemistry Characteristics of Human Subjects

n Age (median, range) Male/female ratio BMI (median, range) CEA (>5.0 ng/mL) Stage I Stage II Stage III Stage IV Colon cancer Rectal cancer

CRC patients

healthy controls

64 59, 42-74 35/29 22.2, 16.4-28.9 26 9 27 20 8 32 32

65 55, 42-69 34/31 23.5, 17.8-27.4 N/A / / / / / /

stage I, 9 patients; stage II, 27 patients; stage III, 20 patients; stage IV, 8 patients. Patients enrolled in this research were not on any medication before sample collection. The clinical diagnosis and pathological reports of all the patients were obtained from the hospital. The healthy volunteers, ages 42-69 years, were selected by a routine physical examination and any subjects with inflammatory conditions or gastrointestinal tract disorders were excluded. Body mass index (BMI) and carcinoembryonic antigen (CEA) levels for each CRC patient were also assessed. Clinical information on participants is provided in Table 1. Venous blood was collected in the morning before breakfast from a total of 64 CRC patients and 65 healthy volunteers at Cancer Hospital, Shanghai Medical College, Fudan University (Shanghai, China). The protocol was approved by the Cancer Hospital Institutional Review Board and all participants gave informed consent before they were involved in the study. GC-TOFMS Spectral Acquisition of Serum Samples and Data Pretreatment. Serum metabolites were analyzed with chemical derivatization following our previously published procedure with minor modifications.17,18 A 100-µL aliquot of serum sample was spiked with two internal standard solutions (10 µLof L-2-chlorophenylalanine in water, 0.3 mg/mL; 10 µL of heptadecanoic acid in methanol, 1 mg/mL) and vortexed for 10 s. The mixed solution was extracted with 300 µL of methanol/chloroform (3:1) and vortexed for 30 s. After storing for 10 min at -20 °C, the samples were centrifuged at 12 000g for 10 min. An aliquot of the 300-µL supernatant was transferred to a glass sampling vial to vacuum-dry at room temperature. The residue was derivatized using a two-step procedure. First, 80 µL of methoxyamine (15 mg/mL in pyridine) was added to the vial and kept at 30 °C for 90 min, followed by 80 µL of BSTFA (1% TMCS) at 70 °C for 60 min. Each 1-µL aliquot of the derivatized solution was injected in splitless mode into an Agilent 6890N gas chromatograph coupled with a Pegasus HT time-of-flight mass spectrometer (Leco Corporation, St. Joseph, MI). The CRC and control samples were run in the order of “control-CRC-control”, alternately, to minimize systematic analytical deviations. Separation was achieved on a DB-5ms capillary column (30 m × 250 µm i.d., 0.25-µm film thickness; (5%-phenyl)-methylpolysiloxane bonded and cross-linked; Agilent J&W Scientific, Folsom, CA), with helium as the carrier gas at a constant flow rate of 1.0 mL/min. The temperature of injection, transfer interface, and ion source was set to 270, 260, and 200 °C, respectively. The GC temperature programming was set to 2 min isothermal heating at 80 °C, followed by 10 °C/min oven temperature ramps to 180 °C, 5 °C/min to 240 °C, and 25 °C/ Journal of Proteome Research • Vol. 8, No. 10, 2009 4845

technical notes min to 290 °C, and a final 9 min maintenance at 290 °C. Electron impact ionization (70 eV) at full scan mode (m/z 30-600) was used, with an acquisition rate of 20 spectra/s in the TOFMS setting. GC-TOFMS Data Analysis. The acquired MS files from GC-TOFMS analysis were exported in NetCDF format by ChromaTOF software (v3.30, Leco Co., CA). CDF files were extracted using custom scripts (revised Matlab toolbox hierarchical multivariate curve resolution (H-MCR), developed by Par Jonsson et al.19,20) in the MATLAB 7.0 (The MathWorks, Inc.) for data pretreatment procedures such as baseline correction, denoising, smoothing, alignment, time-window splitting, and multivariate curve resolution (based on multivariate curve resolution algorithm).20 The resulting three-dimensional data set includes sample information, peak retention time and peak intensities. Internal standards and any known artificial peaks, such as peaks caused by noise, column bleed and BSTFA derivatization procedure, were removed from the data set. The resulting data was mean centered and unit variance scaled during chemometric data analysis in the SIMCA-p 12.0 Software package (Umetrics, Umeå, Sweden). Principal component analysis (PCA) and orthogonal projection to latent structuresdiscriminant analysis (OPLS-DA) were carried out to discriminate between CRC patients and healthy controls. In this study, the default 7-round cross-validation in SIMCA-p software package was applied with one-seventh of the samples being excluded from the mathematical model in each round, in order to guard against overfitting. On the basis of a variable importance in the projection (VIP) threshold of 1 from the 7-fold cross-validated OPLS-DA model, a number of metabolites responsible for the difference in the metabolic profiles of diseased individuals and healthy controls could be obtained. In parallel, the metabolites identified by the OPLS-DA model were validated at a univariate level using the nonparametric Wilcoxon-Mann-Whitney test from the Matlab statistical toolbox with the critical p-value set to 0.05. The corresponding fold change shows how these selected differential metabolites varied between the CRC and healthy control groups. Additionally, compound identification was performed by comparing the mass fragments with NIST 05 Standard mass spectral databases in NIST MS search 2.0 (NIST, Gaithersburg, MD) software with a similarity of more than 70% and finally verified by available reference compounds. UPLC-QTOFMS Spectral Acquisition of Serum Samples and Data Pretreatment. Each 100 µL serum was used for metabolite extraction prior to UPLC-QTOFMS analysis. The metabolite extraction procedure was carried out after adding 100 µL of water (containing 0.1 mg/mL L-2-chlorophenylalanine as the internal standard) and 400 µL of a mixture of methanol and acetonitrile (5:3) in the serum. After vortexing for 2 min, the mixture was kept at room temperature for 10 min, then centrifuged at 14 500g for 20 min. The supernatant was filtered through a syringe filter (0.22 µm) and placed into the sampling vial pending UPLC-QTOFMS analysis. A 5 µL aliquot of the filtrate was injected into a 10 cm × 2.1 mm, 1.7 µm BEH C18 column (Waters) held at 40 °C using an Acquity ultraperformance liquid chromatography system (Waters). Samples were run Control-CRC alternately. The column was eluted with a linear gradient of 1-20% B over 0-1 min, 20-70% B over 1-3 min, 70-85% B over 3-8 min, 85-100% B over 8-9 min; the composition was held at 100% B for 0.5 min. For positive ion mode (ESI+) where A ) water with 0.1% formic acid and B ) acetonitrile with 0.1% formic acid, while 4846

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Qiu et al. A ) water and B ) acetonitrile for negative ion mode (ESI-). The flow rate was 0.4 mL/min. All the samples were kept at 4 °C during the analysis. The mass spectrometric data was collected using a Waters Q-TOF micro MS (Manchester, U.K.) equipped with an electrospray ionization source operating in either positive or negative ion mode. The source temperature was set at 120 °C with a cone gas flow of 50 L/h, a desolvation gas temperature of 300 °C with a desolvation gas flow of 600 L/h. In the case of positive and negative ion modes, the capillary voltage was set to 3.2 and 3 kV, and the cone voltage of 35 and 50 V, respectively. Centroid data was collected from 50 to 1000 m/z with a scan time of 0.3 s and interscan delay of 0.02 s over a 9.5 min analysis time. MassLynx software (Waters) was used for system controlling and data acquisition. Leucine enkephalin was used as the lock mass (m/z 556.2771 in ESI+ and 554.2615 in ESI-) at a concentration of 100 ng/mL and flow rate of 0.2 mL/min, with a lockspray frequency of 20 s. UPLC-QTOFMS Data Analysis. The UPLC-QTOFMS ES+ and ES- raw data were analyzed by the MarkerLynx Applications Manager version 4.1 (Waters, Manchester, U.K.) using parameters reported in our previous work.21 A list of the ion intensities of each peak detected was generated, using retention time (RT) and the m/z data pairs as the identifier for each ion. The resulting three-dimensional matrix contained arbitrarily assigned peak indexes (retention time-m/z pairs), sample names (observations), and ion intensity information (variables). To obtain consistent differential variables, the resulting matrix was further reduced by removing any peaks with missing value (ion intensity ) 0) in more than 40% of the samples from both CRC and healthy groups. The ion peaks generated by the internal standard were also removed. The data was then normalized by dividing the sum of all peak intensities within the sample. Similar to the GC-TOFMS data analysis, the resulting data set was imported to SIMCA-p software for PCA and OPLS-DA analysis and Matlab toolbox for WilcoxonMann-Whitney test.

Results GC-TOFMS Analysis. Typical GC-TOFMS total ion current (TIC) chromatograms of serum samples from a cancer patient and a healthy control are shown in Figure 1A. The resulting spectrum was analyzed with H-MCR toolbox in Matlab and SIMCA-p software. After removing two internal standards, a total of 223 variables were used in the following analysis. In the PCA scores plot, we can see the separation trends between CRC patients and controls as shown in Figure 1A of Supporting Information (SI). The OPLS-DA model demonstrated satisfactory modeling and predictive abilities using one predictive component and two orthogonal components (R2Ycum ) 0.901, Q2cum ) 0.758), achieving a distinct separation between the metabolite profiles of the two groups (Figure 1B). Notably, nine early stage CRC patients (stage I), in addition to the patients belonging to stages II-IV, were correctly discriminated from the healthy controls by the OPLS-DA model. The loading plots were provided in the Supporting Information (SI Figure 2A). However, a separate OPLS-DA model without the healthy group failed to discriminate different pathological stages (I-IV) of CRC patients. Twenty-two metabolites were identified using MS spectral databases and 14 were confirmed using reference standards among the differential variables using VIP values (VIP > 1) in the OPLS-DA model and the Wilcoxon-Mann-Whitney test

technical notes

Serum Metabolite Profiling of Human Colorectal Cancer

Figure 1. Results from GC-TOFMS analysis. (A) Typical total ion current (TIC) chromatograms of biological samples obtained from a CRC patient and a healthy control (the keys can be found in Table 2). The TIC chromatogram for the control group is shown below with the CRC samples. (B) OPLS-DA scores plot discriminating the serum from CRC patients and healthy control using GC-TOFMS analysis.

Table 2. Differential Metabolites Derived from OPLS-DA Mode of GC-TOFMS Analysis with Wilcoxon-Mann-Whitney Test No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

metabolite

a

Pyruvateb Lactate 2-hydroxybutanoic acid 3-hydroxybutanoic acidb Urea Valineb Leucineb Prolineb Threonineb Threonic acid Malic acid 4-hydroxyprolineb Citrullineb 2-Piperidinecarboxylic acid Ornithineb Hippurate Lysineb Tyrosineb Tryptophanb Oleic acid Oleamideb Uridine

Retention time (Min)

VIPc

Pd

FCe R/C

Chemical class

5.343 5.497 6.452 6.956 7.231 7.762 8.563 8.948 10.11 10.727 11.442 11.928 13.059 15.261 16.028 16.32 17.551 17.807 22.225 22.495 25.135 25.392

2.9 1.3 1.4 1.1 1.5 1.6 1.6 1.1 1.4 1.5 1.4 1.6 1.8 1.9 1.6 1.3 1.3 1.6 2 1.1 4.5 1.1

9.57 × 10-13 8.99 × 10-3 2.84 × 10-4 0.017 1.15 × 10-3 3.09 × 10-5 1.20 × 10-4 0.0101 0.0053 5.51 × 10-5 3.82 × 10-3 1.71 × 10-4 3.93 × 10-4 0.0079 3.93 × 10-4 0.0377 0.00291 9.89 × 10-5 2.04 × 10-5 0.0423 2.04 × 10-15 1.14 × 10-6

2.1 1.3 1.4 1.4 -1.4 -1.5 -1.5 -1.3 -1.4 -1.6 1.3 -1.5 -1.4 -1.3 -1.4 -1.5 -1.4 -1.5 -1.6 1.1 -3 -1.7

Organic acid Organic acid Organic acid Organic acid Amine Amino acid Amino acid Amino acid Amino acid Organic acid Organic acid Amino acid Amino acid Organic acid Amino acid Aromatic compound Amino acid Amino acid Amino acid Fatty acid Fatty acid amine Pyridine

a Metabolites are identified using available library databases. b Verified by reference compounds. c Variable importance in the projection (VIP) was obtained from OPLS-DA with a threshold of 1.0. d P-value and fold change (FC) are calculated from nonparametric Wilcoxon-Mann-Whitney test. e FC with a positive value indicates a relatively higher concentration present in CRC patients, while a negative value means a relatively lower concentration as compared to the healthy controls.

(p < 0.05) (Table 2). Among the identified metabolites, oleamide was the serum metabolite found to be most depleted in the CRC patients, compared to controls, showing the greatest fold change (FC ) -3). Pyruvate was the metabolite most increased (FC ) 2.1) in CRC patients. The most significantly altered serum metabolites included decreased lysine, tryptophan, citrulline and tyrosine, and elevated lactate, 2-hydroxybutrate, and 3-hydroxybutrate in the serum of CRC patients compared with healthy controls. UPLC-QTOFMS Analysis. Because of an insufficient volume of serum samples, one CRC patient and two healthy controls were not analyzed in the UPLC-QTOFMS platform. After removing those peaks with missing value in more than 40% of individuals in both groups, a total of 1570 peaks of ESI+ and 450 peaks of ESI- were obtained from the acquired data using MarkerLynx software. These two sets of data were normalized and induced to SIMCA-P 12.0, respectively. PCA scores plots showed the separation trend between CRC patients and controls in negative ion mode (SI Figure 1B) and positive ion

mode (SI Figure 1C). OPLS-DA model score plots from both negative (Figure 2A) and positive (Figure 2B) ion mode showed clear separations between CRC patients and healthy controls using one predictive component and three orthogonal components, with satisfactory modeling and predictive abilities (R2Ycum ) 0.881, Q2cum ) 0.633 for negative and R2Ycum ) 0.918, Q2cum ) 0.812 for positive mode, respectively). The loading plots were provided in the Supporting Information (SI Figure 2B,C). Similar to GC-TOFMS analysis, the model failed to distinguish the stages of CRC. A total of 16 metabolites identified from UPLC-QTOFMS positive ion mode and negative ion mode are demonstrated in Table 3 using a method similar to GC-TOFMS analysis. Among these, five metabolites were also detected in the GC-TOFMS analysis with the same direction. Those metabolites include increased levels of pyruvate and lactate, and decreased levels of tryptophan, tyrosine, and uridine in the CRC patients compared to healthy controls. Journal of Proteome Research • Vol. 8, No. 10, 2009 4847

technical notes

Qiu et al.

Figure 2. Results from UPLC-QTOFMS analysis. (A) OPLS-DA scores plot discriminating the serum from CRC patients and healthy control using UPLC-QTOFMS negative ion mode analysis. (B) OPLS-DA scores plot discriminating the serum from CRC patients and healthy control using UPLC-QTOFMS positive ion mode analysis. Table 3. Differential Metabolites Derived from OPLS-DA Mode of UPLC-QTOFMS Analysis with Wilcoxon-Mann-Whitney Test no.

metabolite

a

VIPb

retention time (min)

Pc

1 2 3 4 5 6 7 8 9 10

Glycerol phosphate Pyruvic acid Lactate Tyrosine Uridine Phenylalanine Tryptophan Myristic acid Palmitic acid Nervonic acid

0.683 0.698 0.718 1.119 1.287 1.457 1.653 3.983 4.655 6.082

Negative Ion Mode 1.7 3.89 × 10-5 1.3 0.0001 1.3 0.0013 1.1 0.0098 1.1 0.0114 1.0 0.0334 1.0 0.0381 1.2 0.0034 1.1 0.0009 1.3 0.0032

12 13 14 15 16

Arginine Carnitine Glutamic acid Nicotinamide Dopamine

0.7031 0.7249 0.7303 1.2926 1.3779

Positive Ion Mode 1.2 1.2 2.0 1.0 1.2

0.0129 0.0073 1.21 × 10-6 0.0032 0.0098

FCd R/C

chemical class

1.5 1.5 1.4 -1.3 -1.3 -1.2 -1.2 -1.3 -1.4 -1.3

Organic acid Organic acid Organic acid Amino acid Pyridine Amino acid Amino acid Fatty acid Fatty acid Fatty acid

-1.3 1.3 -1.7 -1.2 -1.3

Amino acid Carnitines Amino acid Amino acid Aromatic compound

a Metabolites are identified by comparing the exact mass and retention time of our reference metabolites in our laboratory. b Variable importance in the projection (VIP) was obtained from OPLS-DA with a threshold of 1.0. c P-value and fold change (FC) are calculated from nonparametric Wilcoxon-Mann-Whitney test. d FC with a positive value indicates a relatively higher concentration present in CRC patients, while a negative value means a relatively lower concentration as compared to the healthy controls.

Discussion Since the metabonomic data typically contains a large number of variables that are interrelated, multivariate statistical methods such as PCA and OPLS-DA coupled with a univariate statistical method such as Wilcoxon-Mann-Whitney test were used in this study. Therefore, feature selection from variables was performed using two parameters, a threshold of 1 by VIP and a p-value set to 0.05, to identify variables with biological significance as endpoints of altered interdependent biochemical pathways. GC-TOFMS based metabonomic study identified significant variations between CRC patients and healthy controls in 22 metabolites including oleamide, proline, citrulline, ornithine, and 3-hydroxybutrate. UPLC-QTOFMS was able to identify 16 differential metabolites including arginine, glutamate, palmitate, and carnitine. Among them, 5 differential metabolites, pyruvate, lactate, tryptophan, tyrosine, and uridine, were identified in both analytical platforms with the same alteration direction (up- or down-regulation). Thus, the combination of the two analytical platforms significantly broadened the spectrum of detected metabonomes and cross-validated each other. The OPLS-DA models derived from our current GC-TOFMS and UPLC-QTOFMS (both positive and negative ion mode) 4848

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metabolic analysis showed good and similar separations between patients with CRC and healthy controls, highlighting the diagnostic potential of this noninvasive analytical approach. Similar to recent metabonomic reports of CRC tissue samples,13 our attempt to stratify CRC patients based on pathological classifications (stage I-IV) was not successful. We suspect that the variability for patient metabonomes at each stage was too great to allow further stratification. We also conceive that metabolic phenotypic variations may not be in direct association with the different CRC pathological stages. The histopathological diagnosis of CRC reflects a chronological development of impairment in the intestinal tract, as a relatively “static” readout of the disease, while variations in metabolites can be regarded as “real time” readout, reflecting the dynamic states of the CRC pathophysiology. There are a number of serum metabolites which showed a consistent trend of alteration (up- or down-regulation) from stage I to IV of CRC patients (SI Table 1, Figure 3). To facilitate this metabolic approach into clinical use and to understand the underlying biological alterations associated with CRC morbidity, metabolite identification is critically important. Detection using both instruments allows greater confidence to be placed on the measurement made for this

technical notes

Serum Metabolite Profiling of Human Colorectal Cancer

Figure 3. A simplified metabolism pathway linking urea cycle to proline metabolism, while glutamate is an important bridge. Down arrow (V) means down-regulation in the CRC group compared with control group.

subset of differential metabolites, including higher levels of pyruvate and lactate, and lower levels of tryptophan, tyrosine, and uridine in the CRC patients compared to the healthy controls. Pyruvate and lactate are the intermediate and the endpoint product of glycolysis. Increased glycolysis is associated with many tumors or cancer cells, even in the presence of oxygen, which is known as the Warburg effect.22 Recent proteomic analysis of colonic tissues from CRC patients has identified increased glycolysis as an important metabolic variation associated with CRC morbidity.23 The elevated level of lactate was also noted in recently published metabolic profilesofCRCtissuescomparedwiththeirnormalcounterparts.13,15 Therefore, we summarize that the abnormal accumulation of pyruvate and lactate in the CRC patients may be the result of a higher energy demand in the solid colorectal tumor tissues. Many serum amino acids were found to be down-regulated in CRC patients compared with healthy controls (Tables 2 and 3). This observation may indicate an overutilization of amino acids in the tumor tissue, as evidenced by recent tissue based metabolic profile research that up-regulation of most amino acids was detected in CRC carcinoma tissue compared to normal tissue.14 The complementary results between serum and CRC tissue are similar to previous reports of decreased histamine levels in the serum of colon cancer patients compared with healthy controls,24 and an up-regulation in colorectal cancer tissue compared to normal tissue.25 The results of our UPLC-QTOFMS positive ion mode analysis showed a decrease in the level of arginine in CRC patients. L-Arginine has been reported to inhibit chemicalinduced colorectal cancer and can reduce cell proliferation in patients with colorectal adenoma.26,27 It has been suggested that the underlying correlation between arginine and colorectal cancer is its regulation of the immune system via nitric oxide.27,28 Arginine is an intermediate of the urea cycle as illustrated in Figure 3. The disturbed arginine metabolism associated with CRC morbidity can be confirmed by our observations in GC-TOFMS analysis, where ornithine, citrulline and urea were found to be down-regulated in CRC patients compared to healthy controls. Moreover, arginine metabolism is connected with glutamate metabolism and proline metabolism via glutamic semialdehyde.29 Our observations of depleted glutamate, proline, and hydroxyproline in the serum of CRC patients may further support the down-regulation of arginine metabolism, and reflect disturbed proline metabolism in CRC patients as well. The abnormal proline level in CRC patient serum may also indicate disturbed expression of proline

oxidase (catalyzes the first step of proline degradation to pyrroline-5-carboxylate), which was reported to highly correlate with p53-dependent inhibition of apoptosis in colorectal cancer.30 Thus, our finding of depletion of arginine and proline metabolism may be an indication of an immune system impairment related to nitric oxide metabolism and may correlate with abnormal apoptosis in CRC patients. Integrating the information obtained from the two metabolic profile analyses, we noticed alternation of fatty acid metabolism in our serum based CRC metabonomic study. GC-TOFMS analysis detected higher levels of 3-hydroxybutanoic acid, the endpoint product of fatty acids β-oxidation, in the CRC patients, while UPLC-QTOFMS analysis revealed decreased levels of palmitic acid, myristic acid and carnitine, the carrier of fatty acid, in the CRC patients compared to healthy controls. These findings are an indication of the dysfunction of fatty acid β-oxidation metabolism, which can be confirmed by previous proteomic research.31 Oleamide (or cis-9, 10-octadecenoamide) was the most down-regulated metabolite observed in all of the differential metabolites obtained from GC-TOFMS analysis. It is a primary fatty acid amide reported to mediate conjugated linoleic acid inhibition of Caco-2 in colon cancer cell growth.32 Oleamide has also been reported to enhance the activity of certain types of serotonin receptors (e.g., 5-HT1A, 5-HT2A, and 5-HT2C).33,34 Our finding of tryptophan down-regulation, the precursor of serotonin, in CRC patients may correlate with the altered serotonin function. Oleamide can further influence CRC morbidity through competitively inhibiting the degradation of endocannabinoids via fatty acid amides hydrolase.35 As evidence shows that endocannabinoids have the ability to influence tumor cell proliferation and cell apoptosis in vivo and in vitro through activating cannabinoid receptors (CB1 and CB2) and vanilloid receptors,36,37 endocannbinoids have been suggested as a potential target for CRC therapy.38 We detected differently expressed metabolites in CRC patients with multivariate and univariate statistical significance. These distinctions collectively constitute a metabolic window into CRC morbidity, providing metabolic endpoints that complement the interpretation of genomic, proteomic, and epidemiological data. The results of this study also highlight the potential of this sufficiently robust and noninvasive profiling approach for detection of CRC. A limitation of the current study is that, to our current knowledge, many differential variables between CRC patients and healthy controls are yet to be identified. The next step of our study will be to continue to identify these key differential variables.

Acknowledgment. This work was funded by the National Basic Research Program of China (Project numbers are 2007CB914700 and 2004CB518605). Supporting Information Available: PCA scores plots; loading plots of OPLS-DA models; the metabolites with a consistent trend of alteration from stage I-IV of CRC patients. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Weitz, J.; Koch, M.; Debus, J.; Hohler, T.; Galle, P. R.; Buchler, M. W. Colorectal cancer. Lancet 2005, 365 (9454), 153–165. (2) Jemal, A.; Siegel, R.; Ward, E.; Hao, Y.; Xu, J.; Murray, T.; Thun, M. J. Cancer statistics, 2008. CA Cancer J. Clin. 2008, 58 (2), 71– 96.

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