Combination of Proteasomal Inhibitors Lactacystin and MG132 Induced Synergistic Apoptosis in Prostate Cancer Cells

The proteasome inhibitor Velcade (bortezomib/PS-341) has been shown to block the targeted proteolytic degradation of short-lived proteins that are involved in cell maintenance, growth, division, and death, advocating the use of proteasomal inhibitors as therapeutic agents. Although many studies focused on the use of one proteasomal inhibitor for therapy, we hypothesized that the combination of proteasome inhibitors Lactacystin (AG Scientific, Inc., San Diego, CA) and MG132 (Biomol International, Plymouth Meeting, PA) may be more effective in inducing apoptosis. Additionally, this regimen would enable the use of sublethal doses of individual drugs, thus reducing adverse effects. Results indicate a significant increase in apoptosis when LNCaP prostate cancer cells were treated with increasing levels of Lactacystin, MG132, or a combination of sublethal doses of these two inhibitors. Furthermore, induction in apoptosis coincided with a significant loss of IKKα, IKKβ, and IKKγ proteins and NFκB activity. In addition to describing effective therapeutic agents, we provide a model system to facilitate the investigation of the mechanism of action of these drugs and their effects on the IKK-NFκB axis

Novel role of androgens in mitochondrial fission and apoptosis

Androgen and androgen receptors (AR) play critical roles in the proliferation of prostate cancer through transcriptional regulation of target genes. Here, we found that androgens upregulated the expression of dynamin-related protein 1 (Drp1), which is involved in the induction of mitochondrial fission (MF), a common event in mitosis and apoptosis. Clinical tissue samples and various prostate cancer cell lines revealed a positive correlation between Drp1 and AR levels. Treatment of androgen-sensitive cells with an AR agonist, R1881, and antagonist, bicalutamide, showed that Drp1 is transcriptionally regulated by androgens, as confirmed by an AR ChIP-seq assay. Live imaging experiments using pAcGFP1-Mito stably transfected LNCaP (mito-green) cells revealed that androgen did not induce significant MF by itself, although Drp1 was upregulated. However, when treated with CGP37157 (CGP), an inhibitor of mitochondrial Ca(2+) efflux, these cells exhibited MF, which was further enhanced by pre-treatment with R1881, suggesting that androgen-induced Drp1 facilitated CGP-induced MF. This enhanced MF was correlated with increased apoptosis. Transfection with DN-Drp1 (K38A) rescued cells from increased apoptosis, confirming the role of androgen-induced Drp1 in the observed apoptosis with combination treatment. Further, we found that CGP reduced the expression of Mfn1, a protein that promotes mitochondrial fusion, a process which opposes fission. We suggest that androgen-increased Drp1 enhanced MF leading to apoptosis. The present study demonstrates a novel role for androgens in the regulation of mitochondrial morphology that could potentially be utilized in prostate cancer therapy