Thiazole Antibiotic Thiostrepton Synergize with Bortezomib to Induce Apoptosis in Cancer Cells

Thiazole antibiotic, thiostrepton was recently identified as proteasome inhibitor. We investigated the therapeutic potential of the combination of thiostrepton and proteasome inhibitor bortezomib (Velcade) on various human tumor cell lines. Combination of sub-lethal concentrations of thiostrepton and bortezomib induced potent apoptosis and inhibition of long-term colony formation in a wide variety of human cancer cell lines. The synergistic relationship between thiostrepton and bortezomib combination was also quantitatively demonstrated by calculating their combination index values that were much lower than 1 in all studied cell lines. The synergy between these drugs was based on their proteasome inhibitory activities, because thiostrepton modification, thiostrepton methyl ester, which did not have intact quinaldic acid ring and did not inhibit proteasome activity failed to demonstrate any synergy in combination with bortezomib

Proteasome Inhibitors Induce p53-Independent Apoptosis in Human Cancer Cells

Proteasome inhibitors are used against human cancer, but their mechanisms of action are not entirely understood. For example, the role of the tumor suppressor p53 is controversial. We reevaluated the role of p53 in proteasome inhibitor-induced apoptosis by using isogenic human cancer cell lines with different p53 status. We found that well-known proteasome inhibitors such as MG132 and bortezomib, as well as the recently discovered proteasome inhibitor thiostrepton, induced p53-independent apoptosis in human cancer cell lines that correlated with p53-independent induction of proapoptotic Noxa but not Puma protein. In addition, these drugs inhibited growth of several cancer cell lines independently of p53 status. Notably, thiostrepton induced more potent apoptosis in HepG2 cells with p53 knockdown than in parental cells with wild-type p53. Our data confirm that proteasome inhibitors generally induce p53-independent apoptosis in human cancer cells

FoxM1 knockdown sensitizes human cancer cells to proteasome inhibitor-induced apoptosis but not to autophagy

Apoptosis has been widely accepted as the primary mechanism of drug-induced cell death. Recently, a second type of cell death pathway has been demonstrated: autophagy, also called programmed type II cell death. Autophagy is a highly regulated process, by which selected components of a cell are degraded. It primarily functions as a cell survival mechanism under stress. However, persistent stress can also promote extensive autophagy leading to cell death. Forkhead Box M1 (FoxM1), an oncogenic transcription factor that is abundantly expressed in a wide range of human cancers. Here, we evaluated the role of FoxM1 in sensitivity of human cancer cells to proteasome inhibitor-induced apoptosis and autophagy. We found that FoxM1 knockdown sensitized the human cancer cells to apoptotic cell death induced by proteasome inhibitors, such as, MG132, bortezomib and thiostrepton, while it did not affect the levels of autophagy following treatment with these drugs

Proteasome inhibitors suppress expression of NPM and ARF proteins

Proteasome inhibitors stabilize numerous proteins by inhibiting their degradation. Previously, we have demonstrated that proteasome inhibitors thiostrepton, MG132 and bortezomib paradoxically inhibit transcriptional activity and mRNA/protein expression of FOXM1. Here, we demonstrate that, in addition to FOXM1, the same proteasome inhibitors also decrease mRNA and protein expression of NPM and ARF genes. These data suggest that proteasome inhibitors may suppress gene expression by stabilizing their transcriptional inhibitors

Abstract 1036: ARC synergizes with ABT-737 to induce apoptosis in human cancer cells

Abstract Previously, we reported that the nucleoside analog/transcriptional inhibitor ARC was able to induce p53-independent apoptosis in multiple cancer cell lines of different origin. This occurred at least in part by the suppression of Mcl-1 expression, which a short-lived, pro-survival member of the Bcl-2 family. Interestingly, pan-caspase inibitor Z-VAD-FMK protected leukemia cells from ARC-induced mitochondrial damage and apoptosis, and Mcl-1 mRNA and protein from down-regulation by ARC. At the same time treatment of human cancer cells with the pan-Bcl-2 inhibitor ABT-737 alone led to up-regulation of Mcl-1 protein expression. Combination of sub-apoptotic concentrations of ABT-737 and ARC suppressed Mcl-1 expression, induced mitochondrial injury and potent apoptosis in wide variety of human cancer cell lines. In prostate cancer cells co-treatment with ARC and ABT-737 resulted in synergistic induction of cell death with combination index 0.7-0.8. These data suggest that ABT-737/ARC combination that simultaneously targets Bcl-2 and Mcl-1 may be efficient against human cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1036.</jats:p

Proteasome inhibitory activity of thiazole antibiotics

Thiopeptides are sulfur containing highly modified macrocyclic antibiotics with a central pyridine/tetrapyridine/dehydropiperidine ring with up to three thiazole substituents on positions 2, 3 and 6. Thiazole antibiotics with central pyridine nucleus have a macrocyclic loop connecting thiazole rings at position 2 and 3 described as ring A. In addition antibiotics with central tetrahydropyridine nucleus have a quinaldic acid macrocycle also connected to thiazole on position 2 described as ring B. We have demonstrated before that thiazole antibiotics thiostrepton and Siomycin A act as proteasome inhibitors in mammalian tumor cells. Here we decided to test whether other known thiazole antibiotics such as berninamycin, micrococcin P1 and P2, thiocillin and YM-266183 (lacking the quinaldic acid ring B) demonstrate this activity. We found that none of them act as proteasome inhibitors. Moreover, structural modification of thiostrepton to thiostrepton methyl ester (with open B ring) also did not demonstrate this activity. These data suggest that B ring of thiostrepton and Siomycin A that is absent in other thiazole antibiotics determines the proteasome inhibitory activity of these drugs

Mutual Regulation of FOXM1, NPM and ARF Proteins

Abstract ARF, NPM and FOXM1 proteins interact with each other in mammalian cells. We showed previously that proteasome inhibitors suppress not only FOXM1 expression, but also the expression of ARF and NPM proteins. Using RNA interference we found that the depletion of each of these proteins by RNAi in human cancer HeLa cells leads to down-regulation of the two other partners, suggesting that these proteins stabilize each other in human cancer cells. Since the suppression of FOXM1 is one of hallmarks of proteasome inhibition, suppression of ARF and NPM by proteasome inhibitors may be explained in part as a secondary effect of downregulation of FOXM1 that modulate stability of ARF and NPM1 proteins

Abstract 2626: Thiazole antibiotic/proteasome inhibitor thiostrepton against cancer

Abstract Thiazole antibiotic, thiostrepton was identified as an inhibitor of oncogenic transcription factor FoxM1, later demonstrated to exhibit proteasome inhibitory activity. We found that polymeric micelle-encapsulated thiostrepton reduced tumor growth rate in MDA-MB-231-luc breast cancer xenografts. Encapsulation of thiostrepton into polymeric micelles can aid its solubilization and increase its accumulation into tumor sites. Furthermore, its anti-cancer effects on breast cancer xenografts were found to be through reducing cell proliferation and inducing cell death. Thiostrepton is sulfur containing highly modified macrocyclic antibiotic with a central pyridine/tetrapyridine/dehydropiperidine ring with up to three thiazole substituents on positions 2, 3 and 6, which has macrocyclic loop connecting thiazole rings at position 2 and 3 described as ring A. In addition, it has a quinaldic acid macrocycle also connected to thiazole on position 2 described as ring B. Structural modification of thiostrepton to thiostrepton methyl ester (with open B ring) did not demonstrate proteasome inhibitory activity. These data suggest that B ring of thiostrepton and similar thiazole antibiotic Siomycin A that is absent in other thiazole antibiotics determines the proteasome inhibitory activity of these drugs. We investigated the therapeutic potential of the combination of thiostrepton and proteasome inhibitor bortezomib (Velcade) on various human tumor cell lines. Combination of sublethal concentrations of thiostrepton and bortezomib induced potent apoptosis and inhibition of long-term colony formation in a wide variety of human cancer cell lines. The synergistic realtionship between thiostrepton and bortezomib combination was also quantitatively demonstrated by low combination index between 0.1 and 0.8. The synergy between these drugs was based on their proteasome inhibitory activities of both drugs, because structurally similar thiostrepton modification, thiostrepton methyl ester that did not have proteasome inhibitory activity failed to increase apoptosis in combination with bortezomib Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2626. doi:10.1158/1538-7445.AM2011-2626</jats:p