Experimental anti-Mcl-1 Therapy in Prostate Cancer

Mcl-1 oncogene is highly expressed in prostate cancer and is up-regulated by interleukin-6. In our previous study, we showed that treatment of cells with an anti-IL-6 antibody leads to an increased apoptotic rate in prostate cancer cells. We have previously demonstrated specific inhibition of Mcl-1 by siRNA in prostate cancer cells. To investigate improvement of prostate cancer therapy by targeting Mcl-1, we plan to test develop in vitro and in vivo approaches. Well investigate the expression of Mcl-1 in collection of prostate cancer samples obtained before and after androgen ablation therapy. Regulation of expression of Mcl-1 protein and mRNA after treatment with androgen and growth factors will be investigated in prostate cancer cell lines. These findings may serve as a basis for improvement of prostate cancer therapy. Docetaxel is a microtubule inhibitor that is approved for chemotherapy and we plan to investigate whether its combination with Mcl-1 siRNA leads to a synergistic effect on prostate cancer cell lines. It is also known that Akt is increasingly phosphorylated in prostate cancer patients and is a bad prognostic factor. In the present project, well address the question whether the Mcl-1 siRNA treatment may lead to inhibition of Akt phosphorylation. For in vivo studies, androgen receptor-positive and androgen receptor- negative prostate cancer cells will be stably transfected with either specific or control siRNA and inoculated into nude mice. Tumor volume will be measured and at the end of the experiment material analyzed by immunohistochemistry. Well also investigate whether Mcl-1 expression increases in relapsed xenografts. If this hypothesis is confirmed, well ask whether Mcl-1 siRNA approach prior to castration may lead to a delayed tumour progression and whether this approach is effective after regrowth of a tumour that was initially androgen- dependent. Taken together, the experiments proposed may establish a basis for development of small molecule inhibitors for Mcl-1 that could be used in combination with other drugs.

Current therapies for advanced prostate cancer are based on androgen ablation and treatment with nonsteroidal anti-androgens. After a certain period of time, most patients with tumors show a progression which is reflected in an increase in levels of prostate-specific antigen and metastatic spread. In our previous work, we showed that the treatment of prostate cancer cells with a pro-inflammatory cytokine interleukin-6 leads to an increase in expression of the oncogene Mcl-1. An experimental therapy approach with the anti-IL-6 antibody siltuximab led to inhibition of expression of Mcl-1. We demonstrated that androgen ablation leads to an increase in the expression of Mcl-1. Thus, during endocrine therapy there is a compensatory effect of oncogene up-regulation which may facilitate the development of therapy resistance. In this project, we focused on inhibition of Mcl-1 expression by the tyrosine kinase inhibitor sorafenib. Treatment with sorafenib leads to inhibition of proliferation of prostate cancer cell lines and sublines which are resistant to endocrine or chemotherapy. Sorafenib inhibited Mcl-1 expression also in those sublines. In summary, the results imply that an inhibition of Mcl-1 is an option for prostate cancer therapy also after endocrine or docetaxel treatment.