Acquisition of neuroendocrine characteristics by prostate tumor cells is reversible: Implications for prostate cancer progression

Neuroendocrine (NE) cells occur as scattered foci within prostatic adenocar cinoma, similar to their distribution within ductal epithelial cells of the normal prostate. However, the density of NE cells is often greater in pros tate carcinomas than in normal tissue, and the frequency of NE cells correl ates with tumor grade, loss of androgen sensitivity, autocrine/paracrine ac tivity, and poor prognosis. Although NE cells are nonmitotic, proliferating cells are found in direct proximity to them, suggesting that NE cells prov ide paracrine stimuli for surrounding carcinoma cells. In vitro, differenti ation of the LN-CaP and PC3M prostatic tumor cell lines to a ME phenotype c an be induced by dibutyryl cyclic AMP (cAMP), suggesting that physiological agents that increase intracellular concentrations of cAMP might regulate N E differentiation in vivo. Indeed, we demonstrate in this report that LNCaP cells acquire NE characteristics in response to treatment with physiologic al and pharmacological agents that elevate intracellular cAMP, agents such as epinephrine, isoproterenol, forskolin, and dibutyryl cAMP. The androgen- independent LNCaP-derived cell line C4-2 also responded to these agents, in dicating that cells representing later stages of tumor progression are also capable of differentiation. The NE phenotype in this study was monitored b y the appearance of dense core granules in the cytoplasm, the extension of neuron-like processes, loss of mitogenic activity, and expression of the NE markers neuron-specific enolase, parathyroid hormone-related peptide, neur otensin, serotonin, and chromogranin A. However, contrary to previous repor ts, we observed rapid loss of the NE phenotype in both LNCaP and C4-2 cells upon withdrawal of inducing agents. Withdrawal also resulted in a rapid, d ramatic increase in tyrosine kinase and mitogen-activated protein kinase ac tivities, suggesting that activation of these intracellular signaling enzym es may be important for reentry into the cell cycle. Together, these result s indicate that chronic cAMP-mediated signaling is required to block prolif eration of prostate tumor cells and to induce NE differentiation.