Opioid-somatostatin interactions in regulating cancer cell growth

Opioids and somatostatin mediate their cellular effects through specific membrane receptors. Three major receptor classes (delta, mu and kappa) were identified for opioids, while for somatostatin, five different receptor classes (SSTR1-5) have been cloned. Through the interaction with their receptors, opioids and somatostatin exert their effects on cell growth, proliferation, differentiation and secretion. Specific actions of each receptor type have been reported, to be implicated in one or more of the cell functions referred above but have been mainly correlated with cell growth control. In several systems the effect of either neuropeptide is the reverse, inducing cell growth rather than antiproliferative and proapoptotic signals. In recent years, a growing number of reports indicate a possible interaction between opioid and somatostatin system. This could occur at the receptor level, through a cross-interaction of either neuropeptide with either receptor type, or receptor hetero-dimerization, and at a post-receptor level, via interaction with specific signaling molecules. These interactions provide new directions for the identification of specific molecules acting at the receptor and post-receptor level, mimicking the effects of both categories of agents.