Inhibitors of histone deacetylases have been approved for clinical application in cancer treatment. On the other hand, histone acetyltransferase (HAT) inhibitors have been less extensively investigated for their potential use in cancer therapy. In prostate cancer, the HATs and coactivators p300 and CBP are upregulated and may induce transcription of androgen receptor (AR)-responsive genes, even in the absence or presence of low levels of AR. To discover a potential anticancer effect of p300/CBP inhibition, we used two different approaches: (i) downregulation of p300 and CBP by specific short interfering RNA (siRNA) and (ii) chemical inhibition of the acetyltransferase activity by a newly developed small molecule, C646. Knockdown of p300 by specific siRNA, but surprisingly not of CBP, led to an increase of caspase-dependent apoptosis involving both extrinsic and intrinsic cell death pathways in androgen-dependent and castration-resistant prostate cancer cells. Induction of apoptosis was mediated by several pathways including inhibition of AR function and decrease of the nuclear factor kappa B (NF-κB) subunit p65. Furthermore, cell invasion was decreased upon p300, but not CBP, depletion and was accompanied by lower matrix metalloproteinase (MMP)-2 and MMP-9 transcriptions. Thus, p300 and CBP have differential roles in the processes of survival and invasion of prostate cancer cells. Induction of apoptosis in prostate cancer cells was confirmed by the use of C646. This was substantiated by a decrease of AR function and downregulation of p65 impairing several NF-κB target genes. Taken together, these results suggest that p300 inhibition may be a promising approach for the development of new anticancer therapies. Mol Cancer Ther; 10(9); 1644–55. ©2011 AACR.

Interleukin-6 (IL-6) is suggested to have a pathogenic role in the progression of prostate cancer (PC), therefore representing an attractive target for new therapies. However, due to the pleiotropy of this cytokine, targeting IL-6 results in different and unpredictable responses. In order to better understand the mechanisms underlying the different responses to the cytokine, we focused our attention on IL-6 receptors (IL-6Rs) that represent the first element in the cascade of cytokine-activated signalling pathways. IL-6 signal transduction may indeed occur through the membrane IL-6R (classical signalling) and/or through the less studied soluble IL-6R (sIL-6R; IL-6 trans-signalling (IL-6TS)). We provide the first evidence how responses to IL-6 may depend on the different content of IL-6Rs in PC. In particular, the studies of ³H-thymidine incorporation and exploitation of different approaches (i.e. activation or inhibition of IL-6TS in sIL-6R-negative and -positive cell lines and transfection of IL-6R siRNA) allowed us to demonstrate that IL-6TS specifically accounts for an anti-proliferative effect of the cytokine in three PC cell lines that are known to respond differently to IL-6. Additionally, by applying migration-, scratch- and adhesion assays, we show that IL-6TS increases motility and migration and decreases adhesion of prostate cells facilitating thereby processes that determine metastasis initiation and spread. Finally, by western analyses, we uncovered an IL-6- and sIL-6R-dependent downregulation of the tumour suppressor maspin. Collectively, these data suggest that selective targeting of IL-6TS might allow to refine the currently available experimental anti-IL-6 therapies against PC.

Suppressor of cytokine signaling-3 (SOCS-3) acts as a negative feedback regulator of the Janus-activated kinase/signal transducers and activators of transcription factors signaling pathway and plays an important role in the development and progression of various cancers. To better understand the role of SOCS-3 in prostate cancer, SOCS-3 expression was down-regulated in DU-145, LNCaP-IL-6+, and PC3 cells by consecutive SOCS-3 small interfering RNA transfections. SOCS-3 mRNA and protein expression as measured by quantitative reverse transcription-PCR and Western blot, respectively, were decreased by ∼70% to 80% compared with controls. We observed a significant decrease in cell proliferation and viability in all SOCS-3–positive cell lines but not in the parental LNCaP cell line, which is SOCS-3 negative. In this study, we show that down-regulation of SOCS-3 leads to an increased cell death in prostate cancer cell lines. We found a considerable increase in the activation of the proapoptotic caspase-3/caspase-7, caspase-8, and caspase-9. A significant up-regulation of cleaved poly(ADP-ribose) polymerase and inhibition of Bcl-2 expression was observed in all SOCS-3–positive cell lines. Overexpression of Bcl-2 could rescue cells with decreased SOCS-3 levels from going into apoptosis. Tissue microarray data prove that SOCS-3 is highly expressed in castration-refractory tumor samples. In conclusion, we show that SOCS-3 is an important protein in the survival machinery in prostate cancer and is overexpressed in castration-resistant tumors. SOCS-3 knockdown results in an increase of cell death via activation of the extrinsic and intrinsic apoptosis pathways. [Cancer Res 2009;69(18):7375–84]

Antihormonal and chemotherapy are standard treatments for nonorgan-confined prostate cancer. The effectivity of these therapies is limited and the development of alternative approaches is necessary. In the present study, we report on the use of the multikinase inhibitor sorafenib in a panel of prostate cancer cell lines and their derivatives which mimic endocrine and chemotherapy resistance.³H-thymidine incorporation assays revealed that sorafenib causes a dose-dependent inhibition of proliferation of all cell lines associated with downregulation of cyclin-dependent kinase 2 and cyclin D1 expression. Apoptosis was induced at 2 μM of sorafenib in androgen-sensitive cells, whereas a higher dose of the drug was needed in castration-resistant cell lines. Sorafenib stimulated apoptosis in prostate cancer cell lines through downregulation of myeloid cell leukemia-1 (MCL-1) expression and Akt phosphorylation. Although concentrations of sorafenib required for the antitumor effect in therapy-resistant sublines were higher than those needed in parental cells, the drug showed efficacy in cells which became resistant to bicalutamide and docetaxel respectively. Most interestingly, we show that sorafenib has an inhibitory effect on androgen receptor (AR) and prostate-specific antigen expression. In cells in which AR expression was downregulated by short interfering RNA, the treatment with sorafenib increased apoptosis in an additive manner. In summary, the results of the present study indicate that there is a potential to use sorafenib in prostate cancers as an adjuvant therapy option to current androgen ablation treatments, but also in progressed prostate cancers that become unresponsive to standard therapies.