Stimulation of cyclic GMP efflux in human melanocytes by hypergravity generated by centrifugal acceleration

Gravity alteration (micro- and hypergravity) is known to influence cell functions. As guanosine 3',5'-cyclic monophosphate (cGMP) plays an important role in human melanocyte functions and different guanylyl cyclase isoforms are responsible for cGMP synthesis in human non-metastatic and metastatic melanoma cells, we investigated the effects of hypergravity on the regulation of cGMP levels in cultured human melanocytes and in melanoma cell lines with different metastatic potentials. Hypergravity was produced by horizontal centrifugal acceleration. Here we report that long-term application of hypergravity (up to 5 g for 24 h) stimulated cGMP efflux in cultured melanocytes and in non-metastatic melanoma cells in the presence of 0.1 mM 3-isobutyl-1-methylxanthine (IBMX), a non-selective phosphodiesterase (PDE) inhibitor. Under these conditions, cAMP synthesis and melanin production were up-regulated in pigmented melanocytes and non-metastatic melanoma cells. Hypergravity also stimulated cGMP transport in the presence of 1 microM trequinsin, an inhibitor of cGMP-binding PDE (PDE5) and of transport by multidrug resistance proteins MRP4/5, whereas 50 microM trequinsin partially inhibited cGMP transport. Transport was further inhibited by probenecid, an inhibitor of endogenous non-selective transporters as well as of MRP4/5 and by cycloheximide as an inhibitor of de novo protein synthesis. In contrast, hypergravity did not affect cGMP efflux in metastatic melanoma cells, which might be related to an up-regulated cGMP efflux at 1 g. The results of the present study indicate that hypergravity may stimulate cGMP efflux in melanocytes and in non-metastatic melanoma cells most probably by an enhanced expression of endogenous transporters and/or MRP4/5. Thus, an altered acceleration vector may induce signaling events in melanocytic cells.