Opioid agonists modify breast cancer cell proliferation by blocking cells to the G2/M phase of the cycle: involvement of cytoskeletal elements.

Opioids decrease cell proliferation in different systems including breast, prostate, lung, kidney, and intestine, through an interaction with opioid as well as other membrane-receptor systems (somatostatin, cholinergic), through an unidentified mechanism. Recently, we have reported an interaction of taxol with opioid membrane sites (BBRC 235, 201-204, 1997), and an involvement of opioids to the modification of actin cytoskeleton in renal OK cells (J Cell Biochem. [19981 70:60-69), indicating a possible action of the opioid effect. In the present work, we have examined the effect of two general opioid agonists (ethylketocyclazocine and etorphine) on the cell cycle, in human breast cancer T47D cells, as well as a possible modification of the cellular cytoskeleton under their action, in order to explain the antiproliferative effect of these agents. These two opioids produce a dose-dependent and reversible decrease of the proliferation of T47D cells, with a maximum attained at 10(-8) M. The addition of 10(-8) M of either opioid produced a significant increase of the number of cells arrested in the G2/M phase. Confocal laser microscopy revealed a modification of the actin and tubulin microfilaments, with a clear redistribution at the periphery of the cell, reversed by the addition of the general opioid antagonist diprenorphine. Furthermore, differences between the two opioids were obvious, attributed to the different receptor affinity of each agent. The observed redistribution of actin and tubulin cytoskeletal elements gives therefore a possible answer of the antiproliferative action of opioids. The modification of the cytoskeleton, directly involved to cell division, might provoke a "mechanical" obstacle, which could be the reason of the antiproliferative effect of these agonists. Furthermore, the observed tubulin-opioid interaction by opioids provides a possible explanation of the arrest at the G2/M phase of T47D cells under opioid treatment. Nevertheless, although the observed interaction of opioids with cytoskeletal elements gives a plausible answer of the antiproliferative effects of the agents, this might not be the only action of these agents in cell proliferation. Other, direct or indirect, genomic actions, which which remains to be elucidated, might be taken into consideration.