Abstract
BackgroundCelastrol is a natural proteasome inhibitor that exhibits promising anti-tumor effects in human malignancies, especially the androgen-independent prostate cancer (AIPC) with constitutive NF-κB activation. Celastrol induces apoptosis by means of proteasome inhibition and suppresses prostate tumor growth. However, the detailed mechanism of action remains elusive. In the current study, we aim to test the hypothesis that celastrol suppresses AIPC progression via inhibiting the constitutive NF-κB activity as well as modulating the Bcl-2 family proteins.Methodology/Principal FindingsWe examined the efficacy of celastrol both in vitro and in vivo, and evaluated the role of NF-κB in celastrol-mediated AIPC regression. We found that celastrol inhibited cell proliferation in all three AIPC cell lines (PC-3, DU145 and CL1), with IC50 in the range of 1–2 µM. Celastrol also suppressed cell migration and invasion. Celastrol significantly induced apoptosis as evidenced by increased sub-G1 population, caspase activation and PARP cleavage. Moreover, celastrol promoted cleavage of the anti-apoptotic protein Mcl-1 and activated the pro-apoptotic protein Noxa. In addition, celastrol rapidly blocked cytosolic IκBα degradation and nuclear translocation of RelA. Likewise, celastrol inhibited the expression of multiple NF-κB target genes that are involved in proliferation, invasion and anti-apoptosis. Celastrol suppressed AIPC tumor progression by inhibiting proliferation, increasing apoptosis and decreasing angiogenesis, in PC-3 xenograft model in nude mouse. Furthermore, increased cellular IκBα and inhibited expression of various NF-κB target genes were observed in tumor tissues.Conclusions/SignificanceOur data suggest that, via targeting the proteasome, celastrol suppresses proliferation, invasion and angiogenesis by inducing the apoptotic machinery and attenuating constitutive NF-κB activity in AIPC both in vitro and in vivo. Celastrol as an active ingredient of traditional herbal medicine could thus be developed as a new therapeutic agent for hormone-refractory prostate cancer.
Highlights
The proteasome is a multicatalytic protease complex responsible for the degradation of multiple intracellular proteins that are involved in various cellular events, including DNA repair, cell cycle, survival and apoptosis
We have found that celastrol, a natural proteasome inhibitor, suppresses androgen-independent prostate cancer (AIPC) progression by modulating two pathways
We report that celastrol potently inhibits PC-3 growth, migration, invasion, and angiogenesis, and induces apoptosis that is associated with NF-kB attenuation both in vitro and in vivo
Summary
The proteasome is a multicatalytic protease complex responsible for the degradation of multiple intracellular proteins that are involved in various cellular events, including DNA repair, cell cycle, survival and apoptosis. The elevated NF-kB surviving signaling pathway in the androgen-independent prostate cancer (AIPC) seems to be correlated with high proteasome activity, as reflected by a faster turnover of IkBa [8,9]. Based on this evidence, targeting proteasomes might decrease NF-kB activity and be a useful strategy in AIPC intervention. Celastrol is a natural proteasome inhibitor that exhibits promising anti-tumor effects in human malignancies, especially the androgen-independent prostate cancer (AIPC) with constitutive NF-kB activation. We aim to test the hypothesis that celastrol suppresses AIPC progression via inhibiting the constitutive NF-kB activity as well as modulating the Bcl-2 family proteins
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