Somatostatin effects on the proteome of human prostate cancer cells

Abstract: The transition of prostate cancer from androgen dependent to the therapy resistant androgen independent state, is a large and persistent problem in the clinical management of prostate cancer. The aim of this thesis was to study somatostatin effects on the proteome of prostate cancer cells, to identify differences in protein expression in androgen dependent and androgen independent prostate cancer cells, and to explore possibilities to enhance the somatostatin receptor expression through pre-treatment with sms, 5-aza and TSA (DNA methylation/HDAC inhibitors). The results may give insight into the feasibility of using somatostatin as a part of the therapy of advanced prostate cancer. Somatostatin (sms) is important in the regulation and physiological control of many organs including the prostate. Sms can inhibit tumor growth and angiogenesis through binding to its specific receptors (SSTRs) transducing growth inhibitory, anti secretory and apoptotic signals. Because of these properties sms is of interest for the management of prostate cancer and also of other malignancies. Sms is an unstable peptide and therefore more stable sms derivatives have been developed that are suitable for clinical use. Sms and its derivative smsdx (smsdx is a stable long acting glycosylated derivative of natural sms) were incubated with LNCaP and DU145 cells. Proteomic analysis using two-dimensional electrophoresis (2-DE) was performed to determine and compare the effect of the sms and smsdx incubation. Protein expression patterns were analysed with 2-DE and mass spectrometry (MS). Using cDNA obtained from these cell lines, the difference in expression level of SSTR mRNA transcripts before and after sms/smsdx, 5-aza and TSA treatments was analyzed by RT-PCR. Marked quantitative differences were observed in the protein expression profiles in sms/smsdx treated LNCaP and DU145 cells compared to the control cells. One third of the detected proteins were differentially expressed (PRDXs, hnRNPs). Concordance in protein expression patterns were observed between smsdx and sms treated cells with strong agreement between the up/down regulation of proteins. Catalytic mitochondrial and mitochondrial-associated proteins were significantly affected (fold change ~2 or higher). The incubation triggered up-regulation of the catalytic mitochondrial proteins. Apoptotic related proteins were also affected. An increased induction of mRNA expression of all five SSTR subtypes was observed in the LNCaP cells when incubated with sms/smsdx (dose dependent). The results indicate a positive feedback loop between sms and its receptors. Inhibition of DNA methylation and histone acetylation resulted in up regulation of SSTR5 mRNA expression. The results show that smsdx affects important proteins of the proteome in prostate cancer cells both in androgen dependent and in androgen independent prostate cancer cells. Different treatment protocols in terms of derivatives, doses, dose frequency and treatment duration may trigger/enhance effects on proteins involved in the survival/death of the tumor cells. Epigenetic manipulation in combination with sms, might be a possibility to enhance treatment sensibility for somatostatin therapy. Further clinical research on sms is needed to reveal its full potential in the management of prostate cancer.