Prostate cancer and neuroendocrine differentiation. Molecular aspects in prostate cancer development

Abstract: Hormone refractory prostate cancer occurs when androgen-deprivation therapy (ADT) fails to stop the growth of prostate cancer for any longer. Recent studies point towards a role for neuroendocrine (NE) differentiation in the development of hormone refractory disease. It has been hypothesized that NE-differentiated malignant tumor cells survive during ADT and promote development of androgen-independent disease through autocrine and paracrine signaling pathways. In vitro studies have shown that androgen depletion, stimulation with growth factors such as IL-6 and cAMP inducing agents cause NE-differentiation in prostate cancer cell lines. In this thesis, we have analyzed the tumorogenic characteristics of NE tumor cells with regard to expression of key transcription factors, stem cell markers and clinical prognostic markers. Hypoxia has been shown to induce increased tumor growth by promoting angiogenic and glycolytic pathways. Tumors overexpressing hypoxia inducible factors (HIF1a and HIF2a), important transcriptional activators of oxygen-regulated genes, are resistant to chemo- and radiotherapy. We found overexpression and cytoplasmic stabilization of HIF1a and HIF2a in androgen receptor negative NE cells in benign and malignant prostate tissue. Furthermore, we showed that stabilization of HIF1a in NE cells in prostate cancer is due to the presence of a HIF1a isoform, HIF1a1.2. This isoform is co-localized with ARNT, a subunit important for the function of HIF1 transcription factor. This finding indicates that the HIF1a1.2 isoform might sequester ARNT in the cytoplasm. We also studied presence of stem cell marker Oct 3/4 in prostate cancer and benign prostate hyperplasia. Transcription of type 1 of Oct 3/4 as well as protein expression with nuclear localization of Oct 3/4 were not detected in any of prostate tumors or benign prostate hyperplasia but we found only the cytoplasmic isoform 2 of Oct3/4 in prostate tumors with NE-differentiation and also in benign prostate hyperplasia. We studied NE-differentiation in hormone-naive high-grade prostate adenocarcinoma with four immunohistochemial NE markers, Chromogranin A (CgA), synaptophysin, serotonin and neuron-specific enolase (NSE). We showed that synaptophysin immunoreactivity detects the highest number of NE-differentiated tumors. In addition, NE-differentiation measured with synaptophysin, serotonin and NSE immunoreactivity, was correlated to Gleason grade 4 rather than Gleason grade 5 cancer. Univariate statistical analysis revealed that the immunoexpression of CgA, serotonin and NSE is correlated to longer patient survival time. In conclusion, NE-differentiation in hormone-naive high-grade prostate adenocarcinoma, measured with three of four NE markers, is correlated to Gleason grade 4 cancer, a better prognosis and longer patient survival time.