Fluorescent Nanoparticles Consisting of Lipopeptides and Fluorescein

Jul 11, 2011 - Fluorescent Nanoparticles Consisting of Lipopeptides and Fluorescein-Modified Polyanions for Monitoring of Protein Kinase Activity ... ...
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Fluorescent Nanoparticles Consisting of Lipopeptides and Fluorescein-Modified Polyanions for Monitoring of Protein Kinase Activity Haruka Koga,†,‡ Riki Toita,†,‡,§ Takeshi Mori,†,|| Tetsuro Tomiyama,† Jeong-Hun Kang,||,^ Takuro Niidome,†,||,# and Yoshiki Katayama*,†,||,#,r )

†

Graduate School of Systems Life Sciences, Department of Applied Chemistry, Faculty of Engineering, and #Center for Future Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan r Center for Advanced Medical Innovation, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan

bS Supporting Information ABSTRACT: Protein kinase (PK)-responsive nanoparticles (NPs) comprising a hydrophobically modified peptide substrate for PKs and a fluorescein-labeled polyanion (pA-F) were reported for monitoring PK activity via fluorescence intensity measurements. In this system, the formation of NPs by mixing lipopeptides and pA-Fs results in fluorescence quenching, while the quenched fluorescence recovered following dissociation of the NPs owing to the phosphorylation reaction of PKs. Eleven lipopeptides with different hydrophobic moieties (hydrocarbon and lithocholic acid) and four pA-Fs having main chains with differing flexibilities and fluorescein contents were synthesized and used to fabricate a series of twenty-four PK-responsive NP probes. The responses of the PK-responsive NP probes to PKs were evaluated to screen the most suitable NP probes. The assay system was then used to determine the IC50 values for five inhibitors, the results of which were very similar to those previously reported. Thus, PK-responsive NPs are useful tools for high-throughput screening (HTS) of PK inhibitors.

’ INTRODUCTION Protein kinases (PKs) have important roles in regulating various cellular functions (e.g., growth, differentiation, motility, apoptosis, and survival) through the signal transduction system. It has been estimated that the human genome involves over 500 different genes encoding PKs, which corresponds to around 20,000 distinct phosphorylation reactions occurring in human cells.1,2 Because the dysregulation of PK activities is associated with devastating diseases such as cancers, diabetes, autoimmune disorders, and neurological diseases, PK inhibitors have been regarded as potential drugs for their treatment.35 In addition, PKs are also related to stem cell modulation (e.g., differentiation, self-renewal, reprogramming), and as such, they are of particular interest in stemcell biology and regenerative medicine.6 Thus, the development of chemical modulators for PK activity (e.g., inhibitors, activators) is of great significance. As such, powerful high-throughput screening (HTS) systems are necessary for the discovery of novel chemical compounds to regulate the activity of particular PKs. Pharmaceutical companies have large libraries of chemical compounds that need to be evaluated for the discovery of PK inhibitors. As such, the development of homogeneous assays (mix-and-measure), which do not require substrates to be immobilized onto a solid phase or a washing step, are considered to be suitable for HTS formats as compared with heterogeneous assays (e.g., enzyme-linked immunosorbent assays).7 Of the many homogeneous PK activity detection systems known, r 2011 American Chemical Society

fluorescence-based assays have certain advantages as practical HTS formats due to their simple and sensitive readout and their high potential for miniaturization and automation.727 Fluorescence PK assays can be classified into two large groups: antibodybased assays which employ antibodies of phospho-amino acid and non-antibody-based assays. In the case of antibody-based assays, direct detection with fluorophore-labeled antibodies,811 fluorescence polarization12,13 and fluorescence resonance energy transfer (FRET)14,15 have been reported to date. However, most of the antibody-based assays have only been applied to end-point assays. In addition, unlike the well-known anti-phospho-tyrosine, antibodies against phospho-serine and phospho-threonine having satisfactory affinity and specificity have yet to be developed. This is the biggest obstacle to the wide application of antibodybased assays for many well-known PKs.13 In the case of non-antibody-based assays, a number of peptide-1623 and polymer-based probes2426 have been reported. Of these assays, some are available not only for HTS but also for PK imaging in living cells. Although polymer-based probes show good performance in cell-free and living-cell assays, their synthesis is difficult when compared with that of peptide-based probes, which are not only easy to synthesize but also have high reproducibility Received: February 3, 2011 Revised: June 27, 2011 Published: July 11, 2011 1526

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Bioconjugate Chemistry when used in automated systems. Imperiali and co-workers reported fluorophore-modified peptide probes that enhanced the original fluorescence intensity after the phosphorylation reaction via chelation of Mg2+ between the phosphate groups and the internal chelator of the probes.1922 Lee’s group reported the synthesis of self-assembled micelle-like probes comprising fluorophores and long alkyl chain modified peptide substrates with a neutral net charge that elevate fluorescence intensity through a phosphorylation reaction owing to disassembly of the micelle.23 However this probe should have cationic or anionic amino acids added to the peptide substrate to neutralize the net charges, although this modification may reduce the reactivity of the peptide toward target Ser/Thr and Tyr PKs.2833 Thus, we aimed to develop a novel probe according to the following criteria: (1) homogeneous, (2) non-antibody use, (3) simple synthesis, (4) real time measurement, and (5) without any change of original peptide sequence. Especially, criterion (5) is important to avoid reducing both the reactivity and specificity of the peptide for target PKs. In this report, we described the novel homogeneous fluorescent-PK assay using nanoparticles (NPs), each consisting of a hydrophobically modified peptide substrate (i.e., a lipopeptide) and a fluorescein-labeled polyanion (pA-F). The well-known Ser/The PKs, cAMP-dependent protein kinase (PKA)3436 and protein kinase C (PKC)R,3638 were selected as target PKs. The hyperactivations of these PKs have often been observed in many cancers, and thus, these PKs show high potential as drug targets. As specific and sensitive peptide substrates, Kemptide28 and Alphatomega3942 were used for the NP probes of PKA and PKCR, respectively. When the NPs are formed between pA-Fs and lipopeptides via the electrostatic interaction, fluorescence is effectively self-quenched owing to the concentration of the fluorescein moieties. In contrast, after the phosphorylation reaction of the lipopeptides in the NPs, the fluorescence intensity is amplified owing to dissociation of the NPs because phosphorylation reaction with PKs reduces the net cationic charges of the lipopeptides. The system was available to evaluate the IC50 values of some well-known inhibitors of PKA and PKCR. Our system offers a simple, rapid, sensitive and accurate monitoring of PK activity which is suitable to the high-throughput screening (HTS) of PK inhibitors.

’ EXPERIMENTAL SECTION Synthesis of Lipopeptides. Various lipopeptides modified with methacrylic acid or a fatty acid (hexanoic acid, decanoic acid, tetradecanoic acid, octadecanoic acid, and lithocholic acid; all from Sigma-Aldrich, Tokyo, Japan) at the N-terminus were synthesized according to standard Fmoc-chemistry procedures, as described previously.3942 The peptide sequences used for the PKA and PKCR substrates were X-LRRASLG-NH2 and X-FKKQGSFAKKK-NH2, respectively: X indicates a hydrophobic moiety. Crude lipopeptides were purified by reverse phase HPLC by using a linear AB gradient at a flow rate of 3 mL/min, where eluent A was 0.1% TFA in water and eluent B was 0.1% TFA in acetonitrile, and analyzed by MALDI-ToF MS (Bruker, Kanagawa, Japan). Reverse phase HPLC and MALDI-ToF-MS analysis showed that the lipopeptides were of >95% purity. Synthesis of Poly(L-aspartic acid) Modified with Fluorescein (pAsp-F-X). Poly aspartic acid in the sodium salt form (100 mg, 10 μmol) (Mw 10,500, COOH 720 μmol; Sigma-Aldrich), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride

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(EDC) (138 mg, 720 μmol) (DOJINDO, Kumamoto, Japan), and HOBt monohydrate (110 mg, 720 μmol) (Watanabe Chem., Hiroshima, Japan) were dissolved in NaHCO3 buffer (6.8 mL, 0.1 M, pH 8.5), followed by addition of various amounts of a DMF solution of Boc-ethylenediamine (Fluka, Tokyo, Japan) at 0 C. The reaction mixture was stirred at room temperature for one day and dialyzed against distilled water by using a dialysis membrane (molecular weight cutoff, 3500) for four days, followed by lyophilization to obtain a white powder. After deprotection of the Boc groups using TFA/DCM (1/1, v/v), the reactant was precipitated in an excess volume of diethyl ether. This suspension was centrifuged at 5000g for 5 min at 4 C, and the precipitate was dried in vacuo to obtain a white powder. The primary amine contents in the polymers were determined by 2,4,6-trinitrobenezenesulfonic acid (TNBS) assay. Fluorescein modification of pAsp-ethylenediamine was carried out briefly as follows: pAsp-ethylenediamine (100 mg) with different amine contents and a two-times excess equivalent of 5- (and 6-) carboxyfluorescein succinimidyl ester (Pierce, Tokyo, Japan) were dissolved in 350 μL of DMSO/pyridine (5/2 = v/v) containing a five-times equivalent of diisopropylethylamine, and the resulting mixture was stirred overnight. After adjusting the pH of the solution to11 with 2 M NaOH, the solution was dialyzed against water for five days by using a dialysis membrane with a molecular weight cutoff of 3,500, followed by lyophilization to obtain a yellow powder. The fluorescein content was determined by measuring the absorbance at 494 nm in PBS (pH 7.4) (ε = 68,000) using UVvis spectroscopy (Shimadzu, Kyoto, Japan) and pAsp-F-X, where X means the content of fluorescein as mol %. Synthesis of poly(acrylic acid)-Modified with Fluorescein (pAA-F). A mixture of poly(acrylic acid) in the free salt form (26.1 mg, Wako, Osaka, Japan) and EDC (138 mg, 720 μmol) were dissolved in NaHCO3 buffer (6.0 mL, 0.1 M, pH 8.5), followed by the addition of 5-(aminoacetamido)fluorescein (4 mg, 10 μmol, Invitrogen, Tokyo, Japan) at 0 C. The mixture was stirred overnight and then dialyzed against water for five days by using a dialysis membrane with a molecular weight cutoff of 3,500, followed by lyophilization to obtain a yellow powder. The fluorescein content was calculated using the method described above. Quenching Assay for Optimization of NP Preparation. Lipopeptide/pAsp-F-X NPs at various cation/anion (C/A) ratios were prepared by simply mixing lipopeptide and pAsp-FX in water. The C/A ratio is defined as the ratio of residual molar concentration of amino groups in lipopeptides to that of carboxyl groups in pAsp-F-X. The NPs were incubated on ice for 15 min, after which the final volume was adjusted to 100 μL with PKA reaction buffer [0.2 mM ATP, 1 mM MgCl2, and 10 mM HEPES, pH 7.3; all as final concentrations] or PKCR reaction buffer [0.2 mM ATP, 1 mM MgCl2, 0.5 mM CaCl2, 2.0 μg/mL diacylglycerol (DAG), 2.5 μg/mL phosphatidylserine (PS), and 10 mM HEPES, pH 7.3; all as final concentrations]. The fluorescence intensity was measured at 37 C using the multilabel counter ARVO Sx (Wallac Inc., Turku, Finland). Excitation and emission wavelengths were 485 and 535 nm, respectively. The relative fluorescence intensity (RFI) was determined using the following equation: relativefluorescenceintensityðRFIÞ ¼ ðFobs  Fb Þ=ðFpAsp-F-X  Fb Þ

In this formula, Fobs, Fb, and FpAsp-F are the fluorescence intensities of the lipopeptide/pAsp-F-X NPs at each C/A ratio, 1527

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that of reaction buffer, and that of pAsp-F-X without lipopeptides, respectively. Fluorescent Amplification Assay. Lipopeptide/pAsp-F-X NPs were prepared at optimal C/A ratios, as shown in Table 1. The NPs were incubated on ice for 15 min, after which the final volume was adjusted to 100 μL with PKA or PKCR reaction buffer containing 0.1 U/μL PKA (Promega, Tokyo, Japan) or 1.1 ng/μL PKCR (Sigma-Aldrich), respectively. Before the addition of PKs, the fluorescence intensity of the sample solution was measured and was plotted at t = 0 in all figures. Fluorescence measurements of the sample solutions were taken at each time interval using the multilabel counter ARVO Sx following the addition of PK or heat-inactivated PK. The inactivation of the PKs was performed by heating the sample solution at 97 C for 5 min. The phosphorylation ratio at each time interval was also determined by MALDI-ToF-MS as described previously.3941 Measurement of Light-Scattering Intensity (LSI). Dissociation of the lipopeptide/pAsp-F-X NPs by PKs was confirmed by monitoring the LSI using Zetasizer Nano Series particle analyzer (Malvern Instruments, U.K.), as described previously.40 The final Table 1. Efficiency of Quenching and Fluorescence Amplification in Various Lipopeptides and pAsp-F-Xs lipopeptides Ma-PKAe 6-PKAe 10-PKA

14-PKA

18-PKA

Lith-PKA

a

pAsp-F-X all types all types

C/Aa

QEb/%

RFI changesc

timed/min

f 

 

 

 

pAsp-F-1.1

3.0

85

3.9

5

pAsp-F-3.7

3.0

90

6.3

5

pAsp-F-10.8

2.0

90

2.3

5

pAsp-F-1.1

3.0

62





pAsp-F-3.7

1.5

87

4.1

5

pAsp-F-10.8

3.0

90





pAsp-F-1.1 pAsp-F-3.7

0.5 1.0

39 75

 3.1

 30

pAsp-F-10.8

1.5

84





pAsp-F-1.1

3.0

21





pAsp-F-3.7

3.0

64

2.7

20

pAsp-F-10.8

4.0

75





C/A ratio showing the highest quenching efficiency. b QE means quenching efficiency. QE is calculated from the following equation: QE/% = (1  RFINP)  100. RFINP is relative fluorescent intensity of nanoparticle. c Maximum RFI changes. d The time of maximum RFI changes. e Quenching assays and PKA amplification assays were not performed because NP formation was not observed in the DLS experiment. f “” means “not determined”.

volume of the NP dispersion was adjusted to 100 μL with PKA or PKCR reaction buffer as described above. The reaction was initiated by adding PKs to each buffer at 37 C. Determination of IC50. The time course of fluorescence amplification in the PK reaction in the presence of various concentrations of known PK inhibitors was measured as described above. IC50 factors were calculated by using Graph Prism sigmoidal dose response software. EC50 factors were also calculated according to the same procedures.

’ RESULTS AND DISCUSSION Design of PK-Responsive Fluorescent NPs. The concept of PK-responsive fluorescent NPs is shown in Figure 1. For this strategy, various cationic lipopeptides (the net charge for the PKA and PKCR peptide substrates is +2 and +5, respectively) modified with different lengths of fatty acids or lithocholic acid at the N-terminus of the peptide substrates were synthesized to increase their interaction (Chart 1). The electrostatic formation of NPs between these cationic lipopeptides and fluoresceinmodified polyanions (pA-Fs) results in fluorescence quenching because the fluorescein moieties associated with the pA-Fs are in close proximity to each other so as to allow intermolecular energy transfer owing to the compaction of the pA-F chains (i.e., the F€orster radius of fluoresceinfluorescein = 4.4 nm).43 On the other hand, when the lipopeptide was phosphorylated by the target PKs, the NPs dissociated because the electrostatic interactions between the pA-Fs and the lipopeptides were weakened owing to a decrease of the net cationic charges of the lipopeptides as a result of the introduction of negatively charged phosphate groups into the lipopeptides (i.e., +2 to 0 in the PKA peptide substrate and +5 to +3 in the PKCR peptide substrate), leading to amplification of the fluorescence intensity. Effect of Main Chain Structures of pA-F. Two types of polyanions, poly(aspartic acid) (pAsp) and poly(acrylic acid) (pAA) (Chart 1), were compared as pA-Fs: pAsp is more rigid than pAA. Each pA-F, which contains fluorescein from 1 to 11 mol %, was dissolved in HEPES buffer (10 mM, pH 7.3) or PK reaction buffer (10 mM HEPES buffer (pH 7.3) with 100 μM ATP and 1 mM MgCl2), and then their fluorescence intensity was measured using a microplate reader (Figure S1 in the Supporting Information). No obvious difference in fluorescence intensity was detected between the two buffered pAsp-F-X solutions (X means the content of fluorescein as mol %). In contrast, the fluorescence intensity in the two buffered pAA-F-1.3 solutions was four times lower in the PK reaction buffer containing Mg2+ than in the HEPES buffer indicating

Figure 1. Schematic illustration of the PK-responsive system. Fluorescence is quenched via the formation of nanoparticles (NPs) between cationic lipopeptides and fluorescein-labeled polyanions (pA-Fs). Phosphorylation reactions by PKs decrease the net cationic charges of the lipopeptides, causing the NPs to dissociate, and then the fluorescence intensity is amplified. 1528

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Bioconjugate Chemistry Chart 1. Chemical Structures of Lipopeptide and pA-F

self-quenching of pAA-F-1.3 in the PK reaction buffer. This difference in quenching behavior between pAsp-F and pAA-F is considered to be due to the difference in the rigidity of the polymer chain. The pAA chain will be flexible enough to form a compaction state by hydrophobic interaction of the fluorescein moieties. Another possibility to explain the difference observed in the two polyanions is the hydrophobicity of the pA-F main chain. In the PK reaction buffer, Mg2+ ions will be chelated by the carboxylic acid moiety of the main chain, which will enhance the compaction of pA-F due to the decrease in the electrostatic repulsion. More hydrophilic nature of the main chain of pAsp-FX may contribute to avoid the self-aggregation. These results suggest that a rigid and hydrophilic polymer main chain is more suitable to avoid self-quenching without lipopeptides. Thus, pAsp-F-X was selected as the optimal chemical structure for the polyanions for this strategy.

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Fluorescence Quenching by NP Formation. We examined the optimum C/A ratio for mixing of lipopeptides and pAsp-F-X to find a suitable quenching condition in NP formation; the results of the study are summarized in Table 1. First, we investigated the effects of the N-terminus hydrophobic moiety of the lipopeptides on the formation of NPs using dynamic light scattering (DLS) measurement. The less hydrophobic Ma-PKA and 6-PKA did not form detectable NPs with the pAsp-F-X used in this study at any C/A ratio due to their low hydrophobicity. On the other hand, those longer fatty acid- or lithocholic acidmodified lipopeptides (10-PKA, 14-PKA, 18-PKA and Lith-PKA) were able to form NPs with pAsp-F-X with a diameter of 200 1000 nm depending on the content of fluorescein in pAsp-F-X and the C/A ratio. Previous reports demonstrated that NPs comprising polycations and polyanions modified with hydrophobic groups are generally more stable than NPs consisting of unmodified polycations and polyanions even in high salt concentration owing to hydrophobic interactions.4446 Thus we concluded that hydrophobic interactions among the N-terminus hydrophobic moieties of the lipopeptides play a critical role in increasing the interaction with polyanions. To our knowledge, this is the first report demonstrating the importance of hydrophobic groups on NP formation between polyanions and cationic peptides with low net cationic charges. Next, we examined the quenching efficiency (QE) of pAsp-FX/lipopeptide NPs in PKA reaction buffer without PKA, and the result is shown in Figure 2 and Table 1. The relative fluorescence intensity (RFI) of NPs with 10-PKA, 14-PKA and Lith-PKA decreased with increasing C/A ratio, and became saturated above particular C/A ratios depending on the combination of pAsp-F-X and lipopeptides. In contrast, NPs with 18-PKA showed a different behavior. For example, the RFI of the 18-PKA/pAspF-3.7 NPs decreased with increasing C/A ratio, and reached a minimum at a C/A ratio of 1.0, while a further increase of the C/ A ratio led to an elevation of the RFI. Similar behaviors were also observed in the systems of pAsp-F-1.1 and pAsp-F-10.8. This observation is not fully understood as yet, but it is considered that, at higher C/A ratios, 18-PKA may assemble with itself to form micelles. In comparison with the pAsp-F-X/Lith-PKA system, the RFI reached about 0.4 at a C/A ratio of 1.5 in pAsp-F-3.7 and pAsp-F10.8 NPs (Figure 2B,C), while that in the pAsp-F-1.1 NPs was reduced only to 0.8 even at a C/A ratio of 3 (Figure 2A). In addition, weak quenching caused by the formation of NPs in pAsp-F-1.1 was also observed with 14-PKA and 18-PKA, as compared with those in pAsp-F-3.7 and pAsp-F-10.8 (Table 1). These results indicated a lower QE for the pAsp-F-1.1 NPs compared with others. This lower QE was due to the low content of fluorescein in pAsp-F-X. Thus, we concluded that pAsp-F-1.1 was not suitable as a pAsp-F-X to achieve effective quenching. In the case of pAsp-F-3.7 NPs, 18-PKA (75%) and Lith-PKA (64%) showed insufficient fluorescent quenching at C/A ratios of 1 and 3, respectively. In contrast, both 10-PKA and 14-PKA showed more effective fluorescent quenching at about 90% at C/ A ratios of 3 and 1.5, respectively (Figure 2B and Table 1). Similarly in the system of pAsp-F-10.8 NPs, slightly higher quenching efficiencies from both 10-PKA and 14-PKA (90%) were detected compared with 18-PKA (84%) and Lith-PKA (75%) (Figure 2C). Clearly, as the hydrophobicity of the N-terminus moiety in the lipopeptides increased, the quenching efficiency tended to reduce probably due to steric hindrance of the N-terminus moieties. Thus, the less hydrophobic 1529

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Figure 2. Evaluation of fluorescence intensity of NPs at each C/A ratio. After the formation of NPs between lipopeptides and (A) pAsp-F-1.1, (B) pAspF-3.7 and (C) pAsp-F-10.8 in PKA reaction buffer without PKA, the fluorescence intensity was measured using a microplate reader. Data are represented as relative fluorescence intensity (RFI), as described in the Experimental Section.

Figure 4. Change of fluorescence intensity of various 10-PKA NPs by PKA (closed marks) and heat-inactivated PKA (open marks). Diamonds, triangles and circles indicate 10-PKA/pAsp-F-1.1, 10-PKA/pAsp-F-3.7 and 10-PKA/pAsp-F-10.8 NPs, respectively. Data are represented as fold fluorescence changes after the phosphorylation reaction.

Figure 3. Fluorescence amplification of 10-PKA/pAsp-F-3.7 NPs (C/A ratio 3.0) by the phosphorylation reaction with PKA. (A) Change of fluorescence intensity after addition of 100 U/mL PKA (closed circles) and heat-inactive PKA (open squares) to a dispersion containing 10-PKA/pAsp-F-3.7 NPs. (B) Fluorescence images from microplate wells 20 min after PKA addition. (C) Change of light scattering intensity (LSI) of NP dispersion after addition of PKA (closed circles) and heatinactivated PKA (open squares). Data are represented as relative LSI, where the initial LSI was defined as 1. (D) Time course of phosphorylation ratio. Phosphorylation ratios of NPs were determined by MALDIToF-MS analysis.

lipopeptides such as 10-PKA and 14-PKA were superior in quenching efficiency. Fluorescent Amplification by Phosphorylation Reaction. Six NPs (summarized in Table 1) were investigated to see whether their fluorescence intensities were amplified by the phosphorylation reaction with PKA. After addition of active or heat-inactivated PKA (negative control) to the NP dispersions in 96-well-microplates, the time course of fluorescence intensity was monitored using a microplate reader. As shown in Figure 3A,B, the RFI of the 10-PKA/pAsp-F-3.7 NPs did not change for 20 min after addition of heat-inactivated PKA. On the other hand, a drastic increase of RFI was detected after addition of PKA, in which the RFI reached a maximum after 5 min (6.3-fold increase).

To clarify whether fluorescence amplification is caused by dissociation of the NPs during the phosphorylation reaction with PKA, the time course of both the light scattering intensity (LSI) and the phosphorylation ratio of 10-PKA/pAsp-F-3.7 was monitored with DLS and MALDI-ToF MS analysis, respectively. As shown in Figure 3C, when heat-inactivated PKA was added to the 10-PKA/pAsp-F-3.7 NP dispersion, no change in LSI was detected. In contrast, an immediate and drastic decrease of LSI was observed after addition of PKA. Moreover, the time course of the phosphorylation ratio of 10-PKA/pAsp-F-3.7 NPs was almost concerted with that of the RFI and LSI changes (Figure 3D). These results strongly suggest that the 10-PKA/ pAsp-F-3.7 NPs dissociated as a result of the phosphorylation reaction with PKA, leading to amplification of the fluorescence intensity. Effect of Fluorescein Content in pAsp-F-X and Hydrophobicity of Lipopeptide. A fluorescence amplification assay with PKA was conducted against NPs consisting of 10-PKA and three kinds of pAsp-F-X with different fluorescein contents (Figure 4). In all of the 10-PKA/pAsp-F-X systems, a gradual increase of the fluorescence intensity was observed by the addition of active PKA, but such amplification was not observed with inactivated PKA. However, there was a significant difference in fluorescence amplification; 3.9-, 6.3-, and 2.3-fold for 10-PKA/pAsp-F-1.1, 10-PKA/pAsp-F-3.7 and 10-PKA/pAsp-F-10.8, respectively, at 5 min after PKA addition in which the changes in RFI were saturated. From a comparison of the 10-PKA/pAsp-F-1.1 and 10-PKA/pAsp-F-3.7 NPs (Table 1), it was speculated that a higher quenching efficiency is better for larger RFI changes. 1530

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Figure 5. Effect of the hydrophobic moiety of the lipopeptide on fluorescence amplification by PKA. (A) Time course of fluorescence change of NPs involving various lipopeptides and pAsp-F3.7 by PKA. Data are represented as fold changes with initial RFI. (B) Change of phosphorylation ratio of various pAsp-F-3.7 NPs.

However, the RFI change in 10-PKA/pAsp-F-10.8 was the lowest in spite of the high quenching efficiency. This poor fluorescence recovery of the 10-PKA/pAsp-F-10.8 NPs is considered to be due to the incomplete dissociation of the NPs to recover their original fluorescence intensity. These results indicated that effective dissociation as well as effective quenching is important to achieve large fluorescent amplification. We found that pAsp-F-3.7 was the best polyanion in this study because it achieved not only effective quenching but effective dissociation. Figure 5A shows a comparison of pAsp-F-3.7 NPs with each lipopeptide. The fluorescence intensities of the NPs increased with PKA addition in all cases. However, those NPs with more hydrophobic lipopeptides (18-PKA and Lith-PKA) showed less RFI changes than those with less hydrophobic lipopeptides (10PKA and 14-PKA). This was due to less quenching of the more hydrophobic lipopeptides as discussed above (Table. 1). In addition, when the hydrophobicity of the lipopeptides was increased, longer reaction times were needed to reach the maximum RFI. Thus the phosphorylation time course of each NP system was determined by MALDI-ToF-MS (Figure 5B). The NPs of 10PKA/pAsp-F-3.7 and 14-PKA/pAsp-F-3.7 were phosphorylated with a comparable time course, and they were almost concerted with that of the RFI changes. In contrast, the phosphorylation ratio of Lith-PKA/pAsp-F-3.7 and 18-PKA/pAsp-F-3.7 proceeded more slowly compared with that of 10-PKA/pAsp-F-3.7 and 14PKA/pAsp-F-3.7. Thus, the slow RFI development in NPs involving highly hydrophobic lipopeptides reflected a slow phosphorylation reaction. We concluded that the 10-PKA/pAsp-F-3.7 and 14-PKA/pAsp-F-3.7 NPs were suitable to achieve drastic and rapid fluorescence amplification on reaction with PKA. Optimal NPs for High-Throughput Assay of PK Inhibitors. To evaluate the sensitivities of this system, fluorescence amplification during phosphorylation reactions was monitored against 14-PKA/pAsp-F-3.7 and 10-PKA/pAsp-F-3.7 NPs in the presence of several concentrations of PKA (0.1100 U/mL) (Figure S2A,B in the Supporting Information). The 10-PKA/ pAsp-F-3.7 NPs showed fluorescence amplification with phosphorylation above 50 U/mL of PKA, whereas the 14-PKA/pAspF-3.7 NPs increased the RFI above 10 U/mL. One of the reasons for the higher sensitivity of the 14-PKA/pAsp-F-3.7 NPs is the much smaller size of the 14-PKA/pAsp-F-3.7 NPs (627 and 194 nm for 10-PKA/pAsp-F-3.7 and 14-PKA/pAsp-F-3.7 NPs, respectively), which corresponds to a larger specific surface area, enabling the efficient PKA reaction. The EC50 value, which is the

concentration of enzyme required for 50% of maximum phosphorylation of the peptide, was determined to be 11 U/mL for 14-PKA/pAsp-F-3.7 NPs (Figure S2C in the Supporting Information). Thus, the 14-PKA/pAsp-F-3.7 NPs were used for determination of the IC50 of the PKA inhibitor described below. Application to Another Kinase, PKCr. To demonstrate the generality of our assay, we applied the system to PKCR, another Ser/Thr kinase. Because PKCR plays key roles in abnormal proliferation of cancer cells, it is hyperactivated in many cancer cells or tissues (e.g., melanoma, hepatoma, glioma, and breast cancer).3641 Thus, monitoring of PKCR activity is considered to be useful for cancer diagnosis. A PKCR-specific peptide (HFKKQGSFAKKK-NH2)3840 was used as a motif of lipopeptides (Table 1). However, there were two issues for applying the peptide to our system. First, PKCR needs a lipid cofactor for enzyme activation. It was reported that cationic detergent-like molecules with longer alkyl chains (i.e., 1,12-diaminododecane and octadodecylamine) act as competitive inhibitors against phospholipid-sensitive PKC families including PKCR, whereas octylamine, which has a shorter alkyl chain, showed no inhibitory effect.47,48 Because the chemical structures of lipopeptides resemble cationic detergents, they may have some effect on PKCR activity. Thus, we examined the phosphorylation ratios of lipopeptides involved in NPs with PKCR by MALDI-ToF-MS to select the optimal hydrophobic moieties of the PKC lipopeptides (Figure S3 in the Supporting Information). The phosphorylation ratio of NPs with 10-PKC or Lith-PKC was similar to that of the free peptide substrate (Ma-PKC), but NPs with 14-PKC or 18PKC showed lower phosphorylation ratios, suggesting that lipopeptides with longer fatty acids are not suitable as lipopeptides for PKCR. Thus, we selected 10-PKC for the fluorescence amplification assay of PKCR. Another issue is the high cationic charge (+5) of the peptide. The peptide still has +3 net charges after phosphorylation in contrast to the PKA substrate that changes its net charge to neutral. This may be unfavorable for fluorescence development with phosphorylation of the NPs, because the responsiveness of the system is based on the decrease of positive charge on the peptide. Thus, we investigated the fluorescence characteristics during NP formation and its phosphorylation. When 10-PKC formed NPs with pAsp-F-3.7 or pAsp-F-10.8, the fluorescence intensities were effectively quenched (70%) above a C/A ratio of 0.5 or 1.5, respectively (Figure 6A). After forming 10-PKC/ pAsp-F-10.8 NPs at a C/A ratio of 2.0, the fluorescence intensity 1531

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Figure 6. Fluorescence amplification of NPs by PKCR. (A) Fluorescence quenching of 10-PKC/pAsp-F-3.7 (open squares) or 10-PKC/ pAsp-F-10.8 (closed triangles) NPs at each C/A ratio in PKCR reaction buffer without PKCR. (B) Change of RFI after addition of 1.1 pg/μL PKCR (closed circles) and heat-inactive PKCR (open squares) to dispersions containing 10-PKC/pAsp-F-10.8 NP (C/A ratio = 3.0). (C) Fluorescence images from microplate wells obtained 20 min after PKCR addition. (D) Change of LSI of 10-PKC/pAsp-F-10.8 (closed circles) and 10-PKC-A/pAsp-F-10.8 (open circles) NP dispersions by PKCR. 10-PKC-A was used as the negative control of 10-PKC, where a phosphorylation site serine was substituted with alanine, meaning no phosphorylation occurred with PKCR.

was monitored in the presence of active or inactivated PKCR. No change of RFI was detected after addition of heat-inactivated PKCR, whereas the RFI increased after adding PKCR (Figure 6B). These changes were also clearly detected visually under a UV illuminator (Figure 6C). In addition, dissociation of the 10-PKC/pAsp-F-10.8 NPs by phosphorylation was confirmed by a decrease in LSI after addition of PKCR (Figure 6D). These results demonstrated the wide applicability of our system for PKs that prefer cationic substrate peptides having net charges ranging from +2 to +5. Such PKs include many kinds of Ser/Thr kinases such as Ca2+/calmodulin-dependent PK II (CaMK-II), mitogen-activated PK-2 (MAPK-2), MAP-activated PK-2 (MAPKAPK-2), and PKB/Akt.2933,49 However, it may be difficult to apply our methodology to PKs that phosphorylates acidic peptides (e.g., casein kinase50). Determination of IC50 Values of PK Inhibitors. Finally, we applied the PK-responsive fluorescent NPs for evaluation of the IC50 values of certain PK inhibitors. 14-PKA/pAsp-F-3.7 and 10PKC/pAsp-F-10.8 NPs were used for the IC50 determination of inhibitors against PKA and PKCR, respectively. When the fluorescence intensity was monitored during the PKA reaction in the presence of various concentrations of staurosporine and H-89, a concentration-dependent suppression of the fluorescence amplification was observed (Figure 7A for staurosporine), which could also be observed visually (Figure 7B). By using the fluorescence intensity value obtained 20 min after the PKA reaction (Figure 7C), we determined the IC50 values of staurosporine and H-89 as 18 nM and 241 nM, respectively. These values were consistent with those previously reported (15 nM and 135 nM) (Table 2). The use of the 10-PKC/pAsp-F-10.8

Figure 7. Determination of IC50 values of PKA inhibitors using 14PKA/pAsp-F-3.7 NPs. (A) Change of fluorescence intensity by PKA in the presence of various concentrations of staurosporine (0.11000 nM). (B) Fluorescence optical images of microplate wells obtained 20 min after PKA addition. (C) IC50 data for H-89 (closed circles) and staurosporine (open circles).

Table 2. IC50 Values for Each Inhibitor Determined by Using Optimized NPs IC50 value/nM a

target PKs PKA

inhibitors staurosporine H-89

PKCR

Ro-31-7549 Ro-31-8425 rottlerin

found

reported

18

1551

241

13552

64 17

5353 853

>5,000

30,00054

a

14-PKA/pAsp-F-3.7 and 10-PKC/pAsp-F-10.8 NPs were used for PKA and PKCR assay, respectively.

NPs enabled us to calculate the IC50 values of certain PKCR inhibitors (Table 2). These results show that our NP systems are useful in high-throughput screening (HTS) to search for novel PK inhibitors. 1532

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’ ASSOCIATED CONTENT

bS

Supporting Information. Characterizations of NPs. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +81-92-802-2849. Fax: +81-92-802-2849. E-mail: [email protected]. Present Addresses §

Innovation Center for Medical Redox Navigation, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan. ^ Laboratory for Advanced Diagnostic Devices and Department of Biomedical Engineering, Advanced Medical Engineering Center, National Cardiovascular Center Research Institute, 5-7-1 Fujishirodai, Suita, Osaka 565-8565, Japan. Author Contributions ‡

These authors contributed equally to this work.

’ ACKNOWLEDGMENT This work was financially supported by a grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. R.T. is grateful to the Japan Society for the Promotion of Science (JSPS) for the doctor course scholarship. ’ REFERENCES (1) Hunter, T. (2000) Signaling—2000 and beyond. Cell 100, 113–127. (2) Cohen, P. (2001) The role of protein phosphorylation in human health and disease. Eur. J. Biochem. 268, 5001–5010. (3) Zhang, J., Yang, P. L., and Gray, N. S. (2009) Targeting cancer with small molecule kinase inhibitors. Nat. Rev. Cancer 9, 28–39. (4) Knight, Z. A., Lin, H., and Shokat, K. M. (2010) Targeting the cancer kinome through polypharmacology. Nat. Rev. Cancer 10, 130– 137. (5) Lapenna, S., and Giordano, A. (2009) Cell cycle kinases as therapeutic targets for cancer. Nat. Rev. Drug Discovery 8, 547–566. (6) Xu, Y., Shi, Y., and Ding, S. (2008) A chemical approach to stemcell biology and regenerative medicine. Nature 453, 338–344. (7) Ahsen, O. V., and B€omer, U. (2005) High-throughput screening for kinase inhibitors. ChemBioChem 6, 481–490. (8) Kupcho, K., Somberg, R., Bulleit, B., and Goueli, S. A. (2003) A homogeneous, nonradioactive high-throughput fluorogenic protein kinase assay. Anal. Biochem. 317, 210–217. (9) Rodems, M., Hamman, B. D., Lin, C., Zhao, J., Shah, S., Heidary, D., Makings, L., Stack, J. H., and Pollok, B. A. (2002) A FRET-based assay platform for ultra-high density drug screening of protein kinases and phosphatases. Assay Drug Dev. Technol. 1, 9–19. (10) Park, Y. W., Cummings, R. T., Wu, L., Zheng, S., Cameron, P. M., Woods, A., Zaller, D. M., Marcy, A. I., and Hermes, J. D. (1999) Homogeneous proximity tyrosine kinase assays: scintillation proximity assay versus homogeneous time-resolved fluorescence. Anal. Biochem. 269, 94–104. (11) Turek-Etienne, T. C., Lei, M., Terracciano, J. S., Langsdorf, E. F., Bryant, R. W., Hart, R. F., and Horan, A. C. (2004) Use of red-shifted dyes in a fluorescence polarization AKT kinase assay for detection of biological activity in natural product extracts. J. Biomol. Screening 9, 52–61. (12) Fowler, A., Swift, D., Longman, E., Acornley, A., Hemsley, P., Murray, D., Unitt, J., Dale, I., Sullivan, E., and Coldwell, M. (2002) An evaluation of fluorescence polarization and lifetime discriminated

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