Identification of Chelerythrine as an Inhibitor of BclXL Function

Abstract

The identification of small molecule inhibitors of antiapoptotic Bcl-2 family members has opened up new therapeutic opportunities, while the vast diversity of chemical structures and biological activities of natural products are yet to be systematically exploited. Here we report the identification of chelerythrine as an inhibitor of BclXL-Bak Bcl-2 homology 3 (BH3) domain binding through a high throughput screening of 107,423 extracts derived from natural products. Chelerythrine inhibited the BclXL-Bak BH3 peptide binding with IC50 of 1.5 μm and displaced Bax, a BH3-containing protein, from BclXL. Mammalian cells treated with chelerythrine underwent apoptosis with characteristic features that suggest involvement of the mitochondrial pathway. While staurosporine, H7, etoposide, and chelerythrine released cytochrome c from mitochondria in intact cells, only chelerythrine released cytochrome c from isolated mitochondria. Furthermore BclXL-overexpressing cells that were completely resistant to apoptotic stimuli used in this study remained sensitive to chelerythrine. Although chelerythrine is widely known as a protein kinase C inhibitor, the mechanism by which it mediates apoptosis remain controversial. Our data suggest that chelerythrine triggers apoptosis through a mechanism that involves direct targeting of Bcl-2 family proteins. The identification of small molecule inhibitors of antiapoptotic Bcl-2 family members has opened up new therapeutic opportunities, while the vast diversity of chemical structures and biological activities of natural products are yet to be systematically exploited. Here we report the identification of chelerythrine as an inhibitor of BclXL-Bak Bcl-2 homology 3 (BH3) domain binding through a high throughput screening of 107,423 extracts derived from natural products. Chelerythrine inhibited the BclXL-Bak BH3 peptide binding with IC50 of 1.5 μm and displaced Bax, a BH3-containing protein, from BclXL. Mammalian cells treated with chelerythrine underwent apoptosis with characteristic features that suggest involvement of the mitochondrial pathway. While staurosporine, H7, etoposide, and chelerythrine released cytochrome c from mitochondria in intact cells, only chelerythrine released cytochrome c from isolated mitochondria. Furthermore BclXL-overexpressing cells that were completely resistant to apoptotic stimuli used in this study remained sensitive to chelerythrine. Although chelerythrine is widely known as a protein kinase C inhibitor, the mechanism by which it mediates apoptosis remain controversial. Our data suggest that chelerythrine triggers apoptosis through a mechanism that involves direct targeting of Bcl-2 family proteins. Proteins of the Bcl-2 family are central regulators of apoptosis. While the precise molecular mechanisms by which these proteins confer their biological activities remain to be determined, they are thought to act directly on the mitochondria (1Gross A. McDonnell J.M. Korsmeyer S.J. Genes Dev. 1999; 13: 1899-1911Crossref PubMed Scopus (3224) Google Scholar). Members of the Bcl-2 family can be divided into three subfamilies based on several conserved sequence motifs known as Bcl-2 homology (BH) 1The abbreviations used are: BH, Bcl-2 homology; CytC, cytochrome c; H7, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine; GST, glutathione S-transferase; Z, benzyloxycarbonyl; fmk, fluoromethyl ketone; FP, fluorescence polarization; HPLC, high pressure liquid chromatography. 1The abbreviations used are: BH, Bcl-2 homology; CytC, cytochrome c; H7, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine; GST, glutathione S-transferase; Z, benzyloxycarbonyl; fmk, fluoromethyl ketone; FP, fluorescence polarization; HPLC, high pressure liquid chromatography. domains. The antiapoptotic members (Bcl-2, BclXL, Mcl-1, Bcl-1-w, and Ced-9) share all four BH domains, designated as BH1–4; the proapoptotic members Bax, Bak, Bok, and BclXs contain BH1–3 domains, but other proapoptotic members (Bid, Bad, Bik, and Egl1) only have a BH3 domain. Antiapoptotic Bcl-2 family members appear to function, at least in part, by interacting with and antagonizing proapoptotic family members (2Cory S. Adams J.M. Nat. Rev. Cancer. 2002; 2: 647-656Crossref PubMed Scopus (3258) Google Scholar). The BH1–3 domains of BclXL form an elongated hydrophobic groove, which is the docking site for the BH3 domains of proapoptotic binding partners (3Sattler M. Liang H. Nettesheim D. Meadows R.P. Harlan J.E. Eberstadt M. Yoon H.S. Shuker S.B. Chang B.S. Minn A.J. Thompson C.B. Fesik S.W. Science. 1997; 275: 983-986Crossref PubMed Scopus (1278) Google Scholar). It has been demonstrated that BH3 domains from Bak and Bad proteins are required for binding to BclXL and for mediating their proapoptotic effect (4Chittenden T. Flemington C. Houghton A.B. Ebb R.G. Gallo G.J. Elangovan B. Chinnadurai G. Lutz R.J. EMBO J. 1995; 14: 5589-5596Crossref PubMed Scopus (437) Google Scholar). A synthetic peptide derived from the Bak BH3 domain binds recombinant BclXL protein in vitro with high affinity (3Sattler M. Liang H. Nettesheim D. Meadows R.P. Harlan J.E. Eberstadt M. Yoon H.S. Shuker S.B. Chang B.S. Minn A.J. Thompson C.B. Fesik S.W. Science. 1997; 275: 983-986Crossref PubMed Scopus (1278) Google Scholar). Furthermore the Bak BH3 peptide alone is able to induce apoptosis when introduced into various cell lines (5Holinger E.P. Chittenden T. Lutz R.J. J. Biol. Chem. 1999; 274: 13298-13304Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar), suggesting that the interaction between the BH3 domains from proapoptotic proteins and BclXL is important in mediating the proapoptotic signal. Therefore small molecular weight compounds that inhibit the BclXL-BH3 domain interaction could potentially act as apoptotic modulators. Indeed significant progress has been made in isolating compounds of this nature (6Baell J.B. Huang D.C. Biochem. Pharmacol. 2002; 64: 851-863Crossref PubMed Scopus (130) Google Scholar). BH3I-1 and BH3I-2 were discovered by screening a library of 16,320 compounds for ones that disrupted the interaction between BclXL and a Bak BH3 peptide (7Degterev A. Lugovskoy A. Cardone M. Mulley B. Wagner G. Mitchison T. Yuan J. Nat. Cell Biol. 2001; 3: 173-182Crossref PubMed Scopus (536) Google Scholar), whereas antimycin A was discovered serendipitously (8Tzung S.P. Kim K.M. Basanez G. Giedt C.D. Simon J. Zimmerberg J. Zhang K.Y. Hockenbery D.M. Nat. Cell Biol. 2001; 3: 183-191Crossref PubMed Scopus (435) Google Scholar). A few other compounds have also been discovered in silico, and they are of diverse structures (9Wang J.L. Liu D. Zhang Z.J. Shan S. Han X. Srinivasula S.M. Croce C.M. Alnemri E.S. Huang Z. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 7124-7129Crossref PubMed Scopus (1144) Google Scholar, 10Enyedy I.J. Ling Y. Nacro K. Tomita Y. Wu X. Cao Y. Guo R. Li B. Zhu X. Huang Y. Long Y.Q. Roller P.P. Yang D. Wang S. J Med. Chem. 2001; 44: 4313-4324Crossref PubMed Scopus (309) Google Scholar, 11Lugovskoy A.A. Degterev A.I. Fahmy A.F. Zhou P. Gross J.D. Yuan J. Wagner G. J. Am. Chem. Soc. 2002; 124: 1234-1240Crossref PubMed Scopus (118) Google Scholar). Natural products cover a molecular diversity not available from synthetic libraries with an unrivaled success rate as drug leads (12Nielsen J. Curr. Opin. Chem. Biol. 2002; 6: 297-305Crossref PubMed Scopus (94) Google Scholar). We have, therefore, carried out a large scale high throughput screen of natural product extracts to uncover compounds that would disrupt the interaction between BclXL and the Bak BH3 peptide. Here we report the identification of chelerythrine (1,2-dimethoxy-12-methyl[1,3]benzodioxolo[5,6-c]phenanthridinium), which is a natural benzophenanthridine alkaloid and a known protein kinase C inhibitor (13Herbert J.M. Augereau J.M. Gleye J. Maffrand J.P. Biochem. Biophys. Res. Commun. 1990; 172: 993-999Crossref PubMed Scopus (1182) Google Scholar), as an inhibitor of BclXL-Bak BH3 peptide binding. Chelerythrine released cytochrome c (CytC) from isolated mitochondria and induced apoptosis in BclXL-overexpressing cells that were completely resistant to staurosporine or etoposide. Chelerythrine thus represents the first BH3 mimetic identified through high throughput screening of natural products. Reagents and Cell Lines—Human SH-SY5Y and MCF7 cells were maintained as described previously (14Chan S.L. Tan K.O. Zhang L. Yee K.S. Ronca F. Chan M.Y. Yu V.C. J. Biol. Chem. 1999; 274: 32461-32468Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar). HCT116 cells and FDC-P1 cells were gifts from Bert Vogelstein (Dana Farber Cancer Institute) and David C. Huang (The Walter and Eliza Hall Institute), respectively. LipofectAMINE (Invitrogen) was used for transfections according to the user's manual, and BclXL stably transfected SH-SY5Y cells were selected with 400 μg/ml hygromycin B (Calbiochem) and maintained in medium containing 100 μg/ml hygromycin B after 2–3 weeks of selection. The peptide protease inhibitor z-VAD-fmk was from Enzyme System Products, Livermore, CA. Staurosporine, H7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine), and etoposide were from Sigma, and BH3I-1 was from Calbiochem. Fluorescence Polarization (FP) Assay—The Bak BH3 peptide labeled with fluorescein at the N terminus was synthesized by Mimotopes (Clayton, Victoria, Australia) and purified by HPLC. The peptide was dissolved in Me2SO at 1 mm. The reaction was carried out in a total volume of 50 μl/well containing 50 μg/ml glutathione S-transferase (GST)-BclXLΔC19 and a 60 nm concentration of the labeled peptide in assay buffer (50 mm Tris, pH 8, 150 mm NaCl, and 0.1% bovine serum albumin). 5 μl of natural product extracts were added to the wells, the reaction was incubated at room temperature for 1 h, and FP values were determined using a Tecan Ultra plate reader. Isolation of Chelerythrine—A MeOH/CH2Cl2 (1:1) extract from the stems of Bocconia vulcanica (Papaveraceae) was separated using a modified Kupchan solvent partition method (15Pettit G.R. Kamano Y. Aoyagi R. Herald C.L. Doubek D.L. Schmidt J.M. Rudloe J.J. Tetrahedron. 1985; 41: 985-994Crossref Scopus (89) Google Scholar) to give an active CH2Cl2 fraction. The active fraction was fractionated using gradient C18 HPLC (20–30% ACN, H2O buffered with 0.1% formic acid) and preparative TLC on silica gel (1% MeOH, CHCl3) to yield chelerythrine (0.9 mg), identical in all respects to that reported previously (16Hanaoka M. Motonishi T. Mukai C. J. Chem. Soc. Perkin Trans. 1986; 1: 2253-2256Crossref Scopus (41) Google Scholar). Experiments subsequent to the identification of the compound were performed with chelerythrine chloride purchased from Sigma. Antibodies—Anti-BCL-2 (100), anti-Bcl-xS/L (S-19), anti-Bax (N-20), and anti-Bid (C-20) antibodies were purchased from Santa Cruz Biotechnology. Anti-Bak (Ab2) was from Oncogene Research Products. Anti-CytC (7H8.2C12) antibody was from BD Biosciences Pharmingen, and antibodies against actin and cytochrome oxidase IV were from Sigma. Production of GST-BclXL— The DNA sequence encoding BclXLΔC19 was inserted into the GST fusion protein vector pGEX-TK4E (14Chan S.L. Tan K.O. Zhang L. Yee K.S. Ronca F. Chan M.Y. Yu V.C. J. Biol. Chem. 1999; 274: 32461-32468Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar). The plasmid was transformed into the Escherichia coli strain BL 21, and the fusion protein was isolated as described previously (14Chan S.L. Tan K.O. Zhang L. Yee K.S. Ronca F. Chan M.Y. Yu V.C. J. Biol. Chem. 1999; 274: 32461-32468Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar). GSTBclXLΔ19 was eluted with 100 mm glutathione, 50 mm Tris-HCl (pH 8.0). Eluate was dialyzed against phosphate-buffered saline containing 15% glycerol and concentrated to 1 μg/ml using Amicon centrifugal concentrating devices. In Vitro Binding Analysis—Labeled Bax was prepared by in vitro transcription/translation of pXJHA-Bax (17Tan K.O. Tan K.M. Chan S.L. Yee K.S. Bevort M. Ang K.C. Yu V.C. J. Biol. Chem. 2001; 276: 2802-2807Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar) using the TnT T7-coupled reticulocyte lysate system from Promega. The GST binding assay was performed as described previously (14Chan S.L. Tan K.O. Zhang L. Yee K.S. Ronca F. Chan M.Y. Yu V.C. J. Biol. Chem. 1999; 274: 32461-32468Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar) except that increasing concentrations of chelerythrine were incubated with GSTBclXLΔ19 30 min prior to the addition [35S]Bax. CytC Release from Isolated Mitochondria—Mitochondria were isolated from SH-SY5Y cells. Cells were suspended in isolation buffer (320 mm sucrose, 1 mm EDTA, 50 mm HEPES (pH 7.5), 1 μm dithiothreitol) and disrupted by 10 expulsions through a 27-gauge needle. Disrupted cells were centrifuged two times at 1000 × g for 5 min to remove cell debris and nucleus. The supernatants were centrifuged at 3000 × g to pellet the mitochondria. The mitochondria pellets were resuspended in assay buffer (250 mm sucrose, 2 mm KH2PO4, 5 mm sodium succinate, 2 mm EGTA, and 10 mm HEPES (pH 7.5)) at 0.5 mg/ml and treated at room temperature with the indicated compounds for 15 min followed by centrifugation. CytC released into the supernatant was subjected to fractionation by 10% SDS-PAGE followed by Western blotting analysis. Flow Cytometry—For detection of sub-G1 DNA, cells were washed once, resuspended in 200 μl of phosphate-buffered saline, and fixed in a 50-fold excess of ice-cold 70% ethanol. Cells were recovered by centrifugation at 1000 × g for 5 min at 4 °C, washed, stained with 50 mg/ml propidium iodide for 30 min at room temperature, and analyzed in a FACScan flow cytometer (BD Biosciences). Mitochondrial potential change as measured by JC-1 staining was performed in accordance with the manufacturer's instructions (Molecular Probes). A minimum of 10,000 cells/sample were analyzed. Identification of Chelerythrine as an Inhibitor of BclXL and BH3 Peptide Interaction—A high throughput screen based on FP (7Degterev A. Lugovskoy A. Cardone M. Mulley B. Wagner G. Mitchison T. Yuan J. Nat. Cell Biol. 2001; 3: 173-182Crossref PubMed Scopus (536) Google Scholar) was devised to identify compounds that disrupt the interaction between BclXL and the BH3 domain of Bak. A total of 107,423 extracts prepared from plants, actinomycetes, fungi, marine invertebrates, and marine bacteria were screened. Twelve extracts were chosen for isolation of active compounds, and the active principle of four extracts from plants was found to be chelerythrine (Fig. 1A). Chelerythrine displaced the fluorescently labeled BH3 domain peptide from a recombinant GST-BcLXL fusion protein with IC50 of 1.5 μm (Fig. 1B). Similar concentrations of chelerythrine with GST protein and the labeled peptide produced no significant change in polarization (Fig. 1B). Chelerythrine Disrupts the Interaction between BclXL and Bax—The ability of chelerythrine to displace the Bak BH3 peptide in the FP assay suggests that it may be able to displace BH3-containing proteins from BclXL. In vitro translated [35S]Bax bound specifically to GST-BclXL immobilized on glutathione beads, and the addition of chelerythrine resulted in a dose-dependent decrease in Bax binding (Fig. 1, C and D). Chelerythrine was, however, unable to disrupt interaction between the Caenorhabditis elegans sex determination proteins [35S]FEM3 and GST-FEM2 (18Tan K.M. Chan S.L. Tan K.O. Yu V.C. J. Biol. Chem. 2001; 276: 44193-44202Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar) immobilized on glutathione beads (data not shown), suggesting that the action of chelerythrine on Bax and BclXL was specific. The solution structure of Bcl-2 has been solved (19Petros A.M. Medek A. Nettesheim D.G. Kim D.H. Yoon H.S. Swift K. Matayoshi E.D. Oltersdorf T. Fesik S.W. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 3012-3017Crossref PubMed Scopus (357) Google Scholar), and the data suggest that Bcl-2 and BclXL have highly similar three-dimensional structures, including the hydrophobic groove. Interestingly we found that the binding of Bax to Bcl-2 was disrupted by chelerythrine in a dose-dependent manner (data not shown). Chelerythrine-mediated Apoptosis Exhibits Characteristic Features Similar to Cell Death Induced by Proapoptotic Members of the Bcl-2 Family—Since the mitochondria play a key role in the control of apoptosis and it is the main site where BclXL and Bcl-2 exert their function, we evaluated mitochondrial function in response to chelerythrine with the fluorescent dye JC-1 that allows the analysis of mitochondrial potential (ΔΨm). Treatment of human neuroblastoma SH-SY5Y cells (20Ronca F. Chan S.L. Yu V.C. J. Biol. Chem. 1997; 272: 4252-4260Abstract Full Text Full Text PDF PubMed Scopus (58) Google Scholar) with chelerythrine at 2.5 and 5 μm for induced a decrease in mitochondrial potential as indicated by an in JC-1 fluorescence (Fig. A and mitochondrial potential were inhibited by the inhibitor (Fig. A and similar to that mitochondrial potential change was sensitive to D.M. J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus Google Scholar). Treatment of SH-SY5Y cells with chelerythrine also induced the of sub-G1 DNA that is of apoptosis (Fig. C and D). The of sub-G1 DNA is by the addition of (Fig. C and which is with the that DNA is on M. H. H. K. A. S. PubMed Scopus Google Scholar). The cells sub-G1 DNA, however, were not they were unable to on (data not shown). Similar to apoptosis by proapoptotic members of the Bcl-2 of only but not the cell J. Korsmeyer S.J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, S. T. Korsmeyer S.J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). The change in mitochondrial potential and the sub-G1 DNA chelerythrine were in two other cell a cell and a cell (data not shown), suggesting that the effect is not to SH-SY5Y cells. Chelerythrine CytC Release from Isolated stimuli apoptosis through the of CytC from the mitochondrial to thus this to Treatment of SH-SY5Y cells with etoposide, staurosporine, and H7 (20Ronca F. Chan S.L. Yu V.C. J. Biol. Chem. 1997; 272: 4252-4260Abstract Full Text Full Text PDF PubMed Scopus (58) Google Scholar) as as chelerythrine induced mitochondrial potential CytC from the mitochondria (Fig. A and and the of sub-G1 DNA (data not shown), which are of apoptosis. the action of chelerythrine is on BclXL or Bcl-2 on the it be able to CytC directly from isolated mitochondria as with proapoptotic Bcl-2 family members S. Curr. Biol. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar, M. S. T. Chittenden T. Lutz R.J. H. Y. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). mitochondria were isolated from SH-SY5Y cells and subjected to with various Chelerythrine released CytC from isolated mitochondria in a dose-dependent manner (Fig. C and D). and other protein kinase C inhibitors as H7 and staurosporine were unable to (Fig. C and at concentrations the required to induce apoptosis in intact cells (Fig. and data not shown). The of Bax or Bak with BclXL in the mitochondrial to be (data not shown). It was to was a in between the proteins chelerythrine It is that the CytC represents a direct effect of chelerythrine on Chelerythrine Apoptosis in BclXL-overexpressing SH-SY5Y of Bcl-2 or BclXL is able to cell induced by of and (2Cory S. Adams J.M. Nat. Rev. Cancer. 2002; 2: 647-656Crossref PubMed Scopus (3258) Google Scholar, A.A. S.W. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). The concentrations of the apoptotic the mitochondria BclXL to these the other a compound directly on BclXL, it be able to the effect of of the protein protein in a of is in to concentrations with small molecular weight we SH-SY5Y cells that BclXL. In these cells the of BclXL is while other members of the Bcl-2 family as Bax, Bak, and with a of Bcl-2 (Fig. Treatment of BclXL-overexpressing cells with staurosporine up to 1 μm not induce cell as indicated by the of mitochondrial potential change (Fig. data not as as the of sub-G1 DNA (Fig. In the vector was sensitive to apoptosis. of the cells mitochondrial potential and of the cells sub-G1 DNA when only a 100 nm concentration of the drug was added (Fig. B and the apoptotic of etoposide (Fig. B and and H7 (data not were by BclXL the and BclXL-overexpressing cells not they were at the and of the cell the are with that cell induced by are not by BclXL A.J. C.M. Thompson C.B. 1995; PubMed Google Scholar). of BclXL was able to confer to the effect of chelerythrine at concentration of up to 2 chelerythrine the effect of BclXL and induced apoptosis in these cells (Fig. Similar were with a cell in which chelerythrine at concentrations μm was able to the effect of Bcl-2 (data not shown). The data suggest that staurosporine, H7, and etoposide, apoptosis by of antiapoptotic proteins in cells has been in to available (2Cory S. Adams J.M. Nat. Rev. Cancer. 2002; 2: 647-656Crossref PubMed Scopus (3258) Google Scholar, A.A. S.W. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). Chelerythrine has been to against and cells and cells S.J. A. Cancer Res. 2000; 6: Google Scholar). It in an with to the S.J. A. Cancer Res. 2000; 6: Google Scholar). Our that chelerythrine may act as a BH3 mimetic that is able to the antiapoptotic in transformed cells and thus can be as a potential The inhibitors of BclXL-BH3 interaction identified are all proapoptotic in the diverse these compounds suggest that they may act through mechanisms in the Bcl-2 family proteins. Interestingly BclXL against and while it the cells antimycin (8Tzung S.P. Kim K.M. Basanez G. Giedt C.D. Simon J. Zimmerberg J. Zhang K.Y. Hockenbery D.M. Nat. Cell Biol. 2001; 3: 183-191Crossref PubMed Scopus (435) Google Scholar). The identification of chelerythrine as a inhibitor of BclXL-BH3 interaction to the of that are in the molecular mechanisms by which proteins of the Bcl-2 family their We are to Bert David C. and B. for the of We Cao for of of for the of plants used in this with