- Research article
- Open Access
- Open Peer Review
Bicyclic triterpenoid Iripallidal induces apoptosis and inhibits Akt/mTOR pathway in glioma cells
© Koul et al; licensee BioMed Central Ltd. 2010
- Received: 5 January 2010
- Accepted: 24 June 2010
- Published: 24 June 2010
The highly resistant nature of glioblastoma multiforme (GBM) to chemotherapy prompted us to evaluate the efficacy of bicyclic triterpenoid Iripallidal against GBM in vitro.
The effect of Iripallidal on proliferation and apoptosis in glioma cell lines was evaluated by MTS, colony formation and caspase-3 activity. The effect of iripallidal to regulate (i) Akt/mTOR and STAT3 signaling (ii) molecules associated with cell cycle and DNA damage was evaluated by Western blot analysis. The effect of Iripallidal on telomerase activity was also determined.
Iripallidal (i) induced apoptosis, (ii) inhibited Akt/mTOR and STAT3 signaling, (iii) altered molecules associated with cell cycle and DNA damage, (iv) inhibited telomerase activity and colony forming efficiency of glioma cells. In addition, Iripallidal displayed anti-proliferative activity against non-glioma cancer cell lines of diverse origin.
The ability of Iripallidal to serve as a dual-inhibitor of Akt/mTOR and STAT3 signaling warrants further investigation into its role as a therapeutic strategy against GBM.
- Glioma Cell
- Glioma Cell Proliferation
- Acute Myeloid Leukemic Cell
- Colon Cancer Cell Line HT29
- Cervical Cancer Cell Line HeLa
Iripallidal [(-) (6R,10S,11S,18R,22S)-26-Hydroxy-22-α-methylcycloirid-16-enal NSC 631939]- a bicyclic triterpenoid isolated from Iris pallida belongs to the terpenoid family as Paclitaxel. Paclitaxel is an effective chemotherapy for several types of neoplasms . Iripallidal inhibited cell growth in a NCI 60 cell line screen  and induced cytotoxicity in human tumor cell lines . Besides the fact that Iridals are ligands for phorbol ester receptors with modest selectivity for RasGRP3 , not much is known regarding its mechanism of action.
Despite recent advances in understanding molecular mechanisms involved in GBM progression, the prognosis of the most malignant brain tumor continues to be dismal. Ras activation occurs in GBMs  and this high level of active Ras has been a target for glioma therapy. RasGRP3- is an exchange factor that catalyzes the formation of the active GTP-bound form of Ras-like small GTPases . Importantly, Ras activation stimulates its downstream effector Akt that plays a major role in glioblastoma development as ~80% of GBM cases express high Akt levels . Akt activates mammalian target of rapamycin (mTOR), which is deregulated in glioblastoma . mTOR phosphorylates p70 ribosomal S6 kinase (p70S6 kinase) that regulates translation of proteins involved in cellular proliferation and formation. Moreover, blocking mTOR signaling reduces glioma cell proliferation . Given the importance of Akt/mTOR signaling in glioma cell survival, significant efforts are being invested in identifying inhibitors that target this pathway [8–10]. In addition to aberrant PI3K/Akt signaling; heightened STAT3 activation plays a critical role in glioblastoma and STAT3 inhibitors have shown promise as therapeutics for GBM [11–13]. In addition to RasGRP3 Iripallidal also binds to PKCα  which is known to induce cells ectopically expressing hyperactive Ras to undergo apoptosis . Not only is STAT3 essential for Ras transformation  but constitutively activated STAT3 is negatively regulated by PKC-activated tyrosine phosphatase(s) . As Iridals interacts with PKCα and RasGRP3-molecules that regulate Akt and STAT3 signaling, and since inhibition of Akt/mTOR and STAT3 signaling are being targeted for GBM treatment we evaluated the effect of Iripallidal on glioma cell proliferation and these signaling pathways.
Cell culture and treatment
Glioblastoma cell lines A172, LN229, T98G and U87MG were obtained from American Type Culture Collection and cultured in DMEM supplemented with 10% fetal bovine serum. Peripheral blood mononuclear cells (PBMC) were isolated by Ficoll/Histopaque density gradient centrifugation. Adherent monocytes were purified from PBMC following adherence on glass petri-dish for three hours after flushing the non-adherent cells by extensive washing with PBS. All experiments with human PBMC were conducted under an approved institutional Human Ethics Committee protocol.
On attaining semi-confluence, cells were switched to serum free media and after 6 hours, cells were treated with different concentration of Iripallidal (in Dimethyl sulphoxide, DMSO) in serum free media for 24 hours. DMSO treated cells were used as controls. Iripallidal was purchased from Calbiochem, USA. All reagents were purchased from Sigma unless otherwise stated. Colon cancer cell line HT29, breast cancer line MCF-7, cervical cancer cell line HeLa, hepatocellular carcinoma cell line HepG2, acute myeloid leukemic cell line THP1 and human monocytes were similarly treated with Iripallidal.
Determination of cell viability
Viability of Iripallidal treated monocytes and cancer cell lines was assessed using the [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxy-phenyl)-2-(4-sulfophenyl)- 2H-tetrazolium, inner salt] (MTS) (Promega) as described earlier .
Assay of Caspase 3 activity
The Colorimetric Assay kits for caspase 3 (Sigma) were used to determine its enzymatic activity in Iripallidal treated glioma cells as described previously .
Western Blot Analysis
Protein from whole cell lysates were isolated as described previously . Protein (20-50 μg) isolated from control and Iripallidal treated cells was electrophoresed on 6% to 10% polyacrylamide gel and Western blotting performed as described . Antibodies were purchased from Cell Signaling Technology (Danvers, MA) unless otherwise mentioned. The following antibodies were used: p21 (BD Biosciences), p27 (Abcam), pSTAT3 (Tyr705), pmTOR (Ser2448), mTOR, Akt, pAkt (Ser473), Cyclin D1 (Abcam), phospho-p70S6K (Thr389), cMyc (Santa Cruz), phospho-S6K (Ser235/236), pH2AX Ser139 (Upstate), cleaved-PARP and β actin. Secondary antibodies were purchased from Vector Laboratories. After addition of chemiluminescence reagent (Amersham), blots were exposed to Chemigenius, Bioimaging System (Syngene, UK) for developing and images were captured using Genesnap software (Syngene). The blots were stripped and reprobed with anti-β-actin to determine equivalent loading as described .
TeloTAGGGTelomerase PCR ELISA PLUS
Telomerase activity was determined using the TeloTAGGG Telomerase PCR ELISA PLUS kit (Roche, Germany) as described previously .
Colony formation in soft agar
The soft agar colony formation assay was performed using CytoSelect™ 96-Well Cell Transformation Assay kit (Cell Biolabs, Inc), as described previously .
All comparisons between groups were performed using two-tailed Paired student's t-Test. All values of p less than 0.05 were taken as significant.
Iripallidal decreases viability and induces apoptosis in glioma cells
Iripallidal inhibits Akt/mTOR signaling in glioblastoma cells
mTOR activation results in phosphorylation of effector molecule p70S6K and S6 ribosomal protein, which subsequently leads to mTOR-dependent gene transcription that regulates cell growth, protein synthesis, and metabolism. We therefore determined the effect of Iripallidal on the status of p70S6K and pS6 kinase. Iripallidal inhibited phosphorylation of mTOR targets 70S6K and ribosomal protein S6 (Fig. 2). These results indicate that iripallidal acts as a dual inhibitor of Akt/mTOR pathway.
Iripallidal downregulates STAT3 phosphorylation in glioma cells
Iripallidal affects expression of molecules involved in cell cycle regulation and DNA damage response
As maintained DNA breaks induce apoptosis  and since H2AX is phosphorylated at sites of DNA double-strand breaks , we determined the expression of γ-H2AX in Iripallidal treated cells. While an increased γ-H2AX expression was observed in Iripallidal treated cells (Fig. 4b), the levels of total H2AX was unaffected (Fig. 4b).
Iripallidal suppresses telomerase activity in glioma cells
Iripallidal inhibits proliferation of non-glioma cancer cells of diverse origin in vitro
In vitro screening of compounds with anticancer properties by NCI identified Iridals for their anti-proliferative activity. Besides its ability to bind PKCα and RasGRP3 , nothing is known regarding the mechanism of action or bioavailability of Iripallidal. Our studies suggest that Iripallidal induce apoptosis in glioma cells and inhibits the Akt/mTOR pathway. The efficacy of mTOR inhibitors in glioblastoma cell lines [8, 10] has prompted their clinical trials for GBM [9, 34]. As rapamycin activates Akt pathway by a negative feedback loop involving phosphorylation of insulin receptor substrate (IRS) by mTOR effector molecule S6 kinase [35, 36], it was therefore not surprising that Rapamycin treatment induced Akt activation in some GBM patients in a Phase I clinical trial . Moreover, dual inhibition of Akt and mTOR has proven effective in pre-clinical model of GBM , suggesting that dual Akt/mTOR inhibitor can effectively overcome the effects of feeback loop efficiently than a single inhibitor selectively targeting mTOR. As mTOR blockade is a biomarker of therapeutic efficacy in glioma , the unique ability of Iripallidal to inhibit both Akt and mTOR can be exploited as novel anti-glioma therapy. In addition to inhibiting Akt/mTOR axis, Iripallidal also inhibited STAT3 signaling. PKC inhibitor attenuates Ras activation and this attenuation correlates with an inhibition of RasGRP3 phosphorylation . Interestingly, PKCα regulates mTOR  as well as STAT3 activation . It is possible that Iripallidal effects Akt/mTOR and STAT3 signaling pathways through its ability to bind PKC.
Iripallidal mediated decrease in STAT3 activation was concurrent with decreased cyclin D1 and increased p21 expression. While cyclin D1 overexpression and STAT3 activation are mutually exclusive events , p21 inhibits STAT3 signaling . Besides, inhibition of mTOR signaling induces cell cycle arrest through regulation of Cyclin D and p27 . As telomerase inhibition is known to cause apoptosis in human cancers , the ability of Iripallidal to down-regulate telomerase activity may also represent a mechanism for its anti-proliferative effect on glioma cells. Besides glioma cell lines, Iripallidal also decreased the viability of several other cancer cell types although to different extents. It is known that cytotoxic responses is a reflection of an integrated readout of all targets and/or biochemical pathways affected upon drug exposure . As strong co-relation exists between chemo-responsiveness and gene expression , it is likely that differential expression of cellular pathways in cancer cell types of diverse origin could have resulted in differences in sensitivity to Iripallidal.
Taken together our studies suggest that (i) Iripallidal induces glioma cell apoptosis and (ii) inhibits Akt/mTOR and STAT3 pathway. This ability of Iripallidal to act as a multi-inhibitor that blocks Akt/mTOR and STAT3 pathways suggest that its potential as a chemotherapeutic agent against GBM should be further evaluated. Importantly, Iripallidal is not only a promising candidate for the treatment of GBM but a wide variety of malignancies, since it elicits cell death in many tumor cell types.
"Bicyclic triterpenoid Iripallidal as a novel anti-glioma and anti-neoplastic therapy in vitro" has been filed for Indian patent (#2915/DEL/2008) and International Patent (PCT/IN09/000336) through Department of Biotechnology, Govt. of India.
This work was supported by core grant from DBT to the National Brain Research Centre, Manesar. DD, VS and NK are supported by a research fellowship from Council of Scientific and Industrial Research (CSIR, Government of India). The authors thank Mr Uttam Kumar Saini for technical assistance.
- Holmes FA, Walters RS, Theriault RL, Forman AD, Newton LK, Raber MN, Buzdar AU, Frye DK, Hortobagyi GN: Phase II trial of taxol, an active drug in the treatment of metastatic breast cancer. J Natl Cancer Inst. 1991, 83 (24): 1797-1805.View ArticlePubMedGoogle Scholar
- Shao L, Lewin NE, Lorenzo PS, Hu Z, Enyedy IJ, Garfield SH, Stone JC, Marner FJ, Blumberg PM, Wang S: Iridals are a novel class of ligands for phorbol ester receptors with modest selectivity for the RasGRP receptor subfamily. Journal of medicinal chemistry. 2001, 44 (23): 3872-3880. 10.1021/jm010258f.View ArticlePubMedGoogle Scholar
- Bonfils JP, Pinguet F, Culine S, Sauvaire Y: Cytotoxicity of iridals, triterpenoids from Iris, on human tumor cell lines A2780 and K562. Planta medica. 2001, 67 (1): 79-81. 10.1055/s-2001-10625.View ArticlePubMedGoogle Scholar
- Guha A, Lau N, Huvar I, Gutmann D, Provias J, Pawson T, Boss G: Ras-GTP levels are elevated in human NF1 peripheral nerve tumors. Oncogene. 1996, 12 (3): 507-513.PubMedGoogle Scholar
- Ebinu JO, Bottorff DA, Chan EY, Stang SL, Dunn RJ, Stone JC: RasGRP, a Ras guanyl nucleotide- releasing protein with calcium- and diacylglycerol-binding motifs. Science (New York, NY). 1998, 280 (5366): 1082-1086.View ArticleGoogle Scholar
- Sonoda Y, Ozawa T, Aldape KD, Deen DF, Berger MS, Pieper RO: Akt pathway activation converts anaplastic astrocytoma to glioblastoma multiforme in a human astrocyte model of glioma. Cancer research. 2001, 61 (18): 6674-6678.PubMedGoogle Scholar
- Guertin DA, Sabatini DM: Defining the role of mTOR in cancer. Cancer cell. 2007, 12 (1): 9-22. 10.1016/j.ccr.2007.05.008.View ArticlePubMedGoogle Scholar
- Paternot S, Roger PP: Combined inhibition of MEK and mammalian target of rapamycin abolishes phosphorylation of cyclin-dependent kinase 4 in glioblastoma cell lines and prevents their proliferation. Cancer research. 2009, 69 (11): 4577-4581. 10.1158/0008-5472.CAN-08-3260.View ArticlePubMedGoogle Scholar
- Cloughesy TF, Yoshimoto K, Nghiemphu P, Brown K, Dang J, Zhu S, Hsueh T, Chen Y, Wang W, Youngkin D, et al: Antitumor activity of rapamycin in a Phase I trial for patients with recurrent PTEN-deficient glioblastoma. PLoS medicine. 2008, 5 (1): e8-10.1371/journal.pmed.0050008.View ArticlePubMedPubMed CentralGoogle Scholar
- Wei LH, Su H, Hildebrandt IJ, Phelps ME, Czernin J, Weber WA: Changes in tumor metabolism as readout for Mammalian target of rapamycin kinase inhibition by rapamycin in glioblastoma. Clin Cancer Res. 2008, 14 (11): 3416-3426. 10.1158/1078-0432.CCR-07-1824.View ArticlePubMedGoogle Scholar
- Hussain SF, Kong LY, Jordan J, Conrad C, Madden T, Fokt I, Priebe W, Heimberger AB: A novel small molecule inhibitor of signal transducers and activators of transcription 3 reverses immune tolerance in malignant glioma patients. Cancer research. 2007, 67 (20): 9630-9636. 10.1158/0008-5472.CAN-07-1243.View ArticlePubMedGoogle Scholar
- Iwamaru A, Szymanski S, Iwado E, Aoki H, Yokoyama T, Fokt I, Hess K, Conrad C, Madden T, Sawaya R, et al: A novel inhibitor of the STAT3 pathway induces apoptosis in malignant glioma cells both in vitro and in vivo. Oncogene. 2007, 26 (17): 2435-2444. 10.1038/sj.onc.1210031.View ArticlePubMedGoogle Scholar
- Lo HW, Cao X, Zhu H, Ali-Osman F: Constitutively Activated STAT3 Frequently Coexpresses with Epidermal Growth Factor Receptor in High-Grade Gliomas and Targeting STAT3 Sensitizes Them to Iressa and Alkylators. Clin Cancer Res. 2008, 14 (19): 6042-6054. 10.1158/1078-0432.CCR-07-4923.View ArticlePubMedPubMed CentralGoogle Scholar
- Zhu T, Tsuji T, Chen C: Roles of PKC isoforms in the induction of apoptosis elicited by aberrant Ras. Oncogene. 29 (7): 1050-1061. 10.1038/onc.2009.344.Google Scholar
- Gough DJ, Corlett A, Schlessinger K, Wegrzyn J, Larner AC, Levy DE: Mitochondrial STAT3 supports Ras-dependent oncogenic transformation. Science (New York, NY). 2009, 324 (5935): 1713-1716.View ArticleGoogle Scholar
- Oka M, Sumita N, Sakaguchi M, Iwasaki T, Bito T, Kageshita T, Sato K, Fukami Y, Nishigori C: 12-O-tetradecanoylphorbol-13-acetate inhibits melanoma growth by inactivation of STAT3 through protein kinase C-activated tyrosine phosphatase(s). The Journal of biological chemistry. 2009, 284 (44): 30416-30423. 10.1074/jbc.M109.001073.View ArticlePubMedPubMed CentralGoogle Scholar
- Sharma V, Tewari R, Sk UH, Joseph C, Sen E: Ebselen sensitizes glioblastoma cells to Tumor Necrosis Factor (TNFalpha)-induced apoptosis through two distinct pathways involving NF-kappaB downregulation and Fas-mediated formation of death inducing signaling complex. International journal of cancer. 2008, 123 (9): 2204-2212. 10.1002/ijc.23771.View ArticlePubMedGoogle Scholar
- Sharma V, Koul N, Joseph C, Dixit D, Ghosh S, Sen E: HDAC inhibitor Scriptaid induces glioma cell apoptosis through JNK activation and inhibits telomerase activity. Journal of cellular and molecular medicine. 2009Google Scholar
- Sharma V, Joseph C, Ghosh S, Agarwal A, Mishra MK, Sen E: Kaempferol induces apoptosis in glioblastoma cells through oxidative stress. Molecular cancer therapeutics. 2007, 6 (9): 2544-2553. 10.1158/1535-7163.MCT-06-0788.View ArticlePubMedGoogle Scholar
- Dixit D, Sharma V, Ghosh S, Koul N, Mishra PK, Sen E: Manumycin inhibits STAT3, telomerase activity and growth of glioma cells by elevating intracellular reactive oxygen species generation. Free radical biology & medicine. 2009Google Scholar
- Kumar S, Lavin MF: The ICE family of cysteine proteases as effectors of cell death. Cell death and differentiation. 1996, 3 (3): 255-267.PubMedGoogle Scholar
- Choe G, Horvath S, Cloughesy TF, Crosby K, Seligson D, Palotie A, Inge L, Smith BL, Sawyers CL, Mischel PS: Analysis of the phosphatidylinositol 3'-kinase signaling pathway in glioblastoma patients in vivo. Cancer research. 2003, 63 (11): 2742-2746.PubMedGoogle Scholar
- Fan QW, Knight ZA, Goldenberg DD, Yu W, Mostov KE, Stokoe D, Shokat KM, Weiss WA: A dual PI3 kinase/mTOR inhibitor reveals emergent efficacy in glioma. Cancer cell. 2006, 9 (5): 341-349. 10.1016/j.ccr.2006.03.029.View ArticlePubMedPubMed CentralGoogle Scholar
- Rajan P, Panchision DM, Newell LF, McKay RD: BMPs signal alternately through a SMAD or FRAP-STAT pathway to regulate fate choice in CNS stem cells. The Journal of cell biology. 2003, 161 (5): 911-921. 10.1083/jcb.200211021.View ArticlePubMedPubMed CentralGoogle Scholar
- Zhang C, Yang N, Yang CH, Ding HS, Luo C, Zhang Y, Wu MJ, Zhang XW, Shen X, Jiang HL, et al: S9, a novel anticancer agent, exerts its anti-proliferative activity by interfering with both PI3K-Akt-mTOR signaling and microtubule cytoskeleton. PloS one. 2009, 4 (3): e4881-10.1371/journal.pone.0004881.View ArticlePubMedPubMed CentralGoogle Scholar
- Zhou BB, Elledge SJ: The DNA damage response: putting checkpoints in perspective. Nature. 2000, 408 (6811): 433-439. 10.1038/35044005.View ArticlePubMedGoogle Scholar
- Furuta T, Takemura H, Liao ZY, Aune GJ, Redon C, Sedelnikova OA, Pilch DR, Rogakou EP, Celeste A, Chen HT, et al: Phosphorylation of histone H2AX and activation of Mre11, Rad50, and Nbs1 in response to replication-dependent DNA double-strand breaks induced by mammalian DNA topoisomerase I cleavage complexes. The Journal of biological chemistry. 2003, 278 (22): 20303-20312. 10.1074/jbc.M300198200.View ArticlePubMedGoogle Scholar
- Zhang X, Mar V, Zhou W, Harrington L, Robinson MO: Telomere shortening and apoptosis in telomerase-inhibited human tumor cells. Genes & development. 1999, 13 (18): 2388-2399.View ArticleGoogle Scholar
- Ram R, Uziel O, Eldan O, Fenig E, Beery E, Lichtenberg S, Nordenberg Y, Lahav M: Ionizing radiation up-regulates telomerase activity in cancer cell lines by post-translational mechanism via ras/phosphatidylinositol 3-kinase/Akt pathway. Clin Cancer Res. 2009, 15 (3): 914-923. 10.1158/1078-0432.CCR-08-0792.View ArticlePubMedGoogle Scholar
- Zhou C, Gehrig PA, Whang YE, Boggess JF: Rapamycin inhibits telomerase activity by decreasing the hTERT mRNA level in endometrial cancer cells. Molecular cancer therapeutics. 2003, 2 (8): 789-795.PubMedGoogle Scholar
- Konnikova L, Simeone MC, Kruger MM, Kotecki M, Cochran BH: Signal transducer and activator of transcription 3 (STAT3) regulates human telomerase reverse transcriptase (hTERT) expression in human cancer and primary cells. Cancer research. 2005, 65 (15): 6516-6520. 10.1158/0008-5472.CAN-05-0924.View ArticlePubMedGoogle Scholar
- Dikmen ZG, Gellert GC, Jackson S, Gryaznov S, Tressler R, Dogan P, Wright WE, Shay JW: In vivo inhibition of lung cancer by GRN163L: a novel human telomerase inhibitor. Cancer research. 2005, 65 (17): 7866-7873.PubMedGoogle Scholar
- Schlessinger K, Levy DE: Malignant transformation but not normal cell growth depends on signal transducer and activator of transcription 3. Cancer research. 2005, 65 (13): 5828-5834. 10.1158/0008-5472.CAN-05-0317.View ArticlePubMedPubMed CentralGoogle Scholar
- Doherty L, Gigas DC, Kesari S, Drappatz J, Kim R, Zimmerman J, Ostrowsky L, Wen PY: Pilot study of the combination of EGFR and mTOR inhibitors in recurrent malignant gliomas. Neurology. 2006, 67 (1): 156-158. 10.1212/01.wnl.0000223844.77636.29.View ArticlePubMedGoogle Scholar
- O'Reilly KE, Rojo F, She QB, Solit D, Mills GB, Smith D, Lane H, Hofmann F, Hicklin DJ, Ludwig DL, et al: mTOR inhibition induces upstream receptor tyrosine kinase signaling and activates Akt. Cancer research. 2006, 66 (3): 1500-1508. 10.1158/0008-5472.CAN-05-2925.View ArticlePubMedPubMed CentralGoogle Scholar
- Tremblay F, Marette A: Amino acid and insulin signaling via the mTOR/p70 S6 kinase pathway. A negative feedback mechanism leading to insulin resistance in skeletal muscle cells. The Journal of biological chemistry. 2001, 276 (41): 38052-38060.PubMedGoogle Scholar
- Fan QW, Cheng CK, Nicolaides TP, Hackett CS, Knight ZA, Shokat KM, Weiss WA: A dual phosphoinositide-3-kinase alpha/mTOR inhibitor cooperates with blockade of epidermal growth factor receptor in PTEN-mutant glioma. Cancer research. 2007, 67 (17): 7960-7965. 10.1158/0008-5472.CAN-07-2154.View ArticlePubMedPubMed CentralGoogle Scholar
- Teixeira C, Stang SL, Zheng Y, Beswick NS, Stone JC: Integration of DAG signaling systems mediated by PKC-dependent phosphorylation of RasGRP3. Blood. 2003, 102 (4): 1414-1420. 10.1182/blood-2002-11-3621.View ArticlePubMedGoogle Scholar
- Quintanilla-Martinez L, Kremer M, Specht K, Calzada-Wack J, Nathrath M, Schaich R, Hofler H, Fend F: Analysis of signal transducer and activator of transcription 3 (Stat 3) pathway in multiple myeloma: Stat 3 activation and cyclin D1 dysregulation are mutually exclusive events. The American journal of pathology. 2003, 162 (5): 1449-1461.View ArticlePubMedPubMed CentralGoogle Scholar
- Coqueret O, Gascan H: Functional interaction of STAT3 transcription factor with the cell cycle inhibitor p21WAF1/CIP1/SDI1. The Journal of biological chemistry. 2000, 275 (25): 18794-18800. 10.1074/jbc.M001601200.View ArticlePubMedGoogle Scholar
- Covell DG, Wallqvist A, Huang R, Thanki N, Rabow AA, Lu XJ: Linking tumor cell cytotoxicity to mechanism of drug action: an integrated analysis of gene expression, small-molecule screening and structural databases. Proteins. 2005, 59 (3): 403-433. 10.1002/prot.20392.View ArticlePubMedGoogle Scholar
- The pre-publication history for this paper can be accessed here:http://www.biomedcentral.com/1471-2407/10/328/prepub
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.