| Neuroendocrine (NE) differentiation in prostatic adenocarcinomas has been reported to be an early marker for development of androgen independence. However, the cellular origin, molecular mechanisms controlling differentiation, and effects on tumor growth of prostatic NE cells are unresolved. To address questions regarding NE differentiation, LNCaP cells were co-treated with agents that signal through cAMP-dependent protein kinase (PKA) and with the cytokine, interleukin-6 (IL-6). Co-treatment with these agents reversibly enhanced the acquisition of NE characteristics including neuritic morphology, mitotic arrest, neuron-specific enolase (NSE) and βIII-tubulin expression, and mitogenic neuropeptide secretion. Convergent IL-6 and PKA signaling resulted in potentiated mitogen-activated protein kinase (MAPK) activation without effecting the level of signal transducer and activator of transcription (STAT) or PKA activation observed with these agents alone. Co-treatment with Epi and IL-6 resulted in MAPK-dependent potentiation of transcription of neuronal specific, growth regulatory and other genes important for NE differentiation. Most importantly, convergent signaling from PKA and IL-6 pathways was found to synergistically increase secretion of mitogenic neuropeptides. These observations suggest that the ability of PKA and IL-6 signaling to potentiate acquisition of a NE phenotype by LNCaP cells may contribute to the prevalence of NE cells in androgen-independent prostate cancer.; To test the ability of NE cells to affect tumor growth, LNCaP cells were engineered to inducibly express a constitutively activated form of the PKA catalytic subunit, which was found to be sufficient for acquisition of NE characteristics. Forskolin-, epinephrine- and isoproterenol-dependent NE differentiation of LNCaP cells was significantly inhibited by expression of a dominant negative mutant of the PKA regulatory subunit, demonstrating that prostatic NE differentiation in response to these agents depends on PKA. Cell lines that express the active form of PKA enhanced the growth of surrounding prostate tumor cells under a variety of conditions, including co-culture, anchorage independence and in vivo androgen-independent tumorigenesis. These results suggest that NE cells of prostatic tumors have the potential to enhance androgen-independent tumor growth in a paracrine manner, thereby contributing to progression of the disease. |
