A SELECTIVE PROTEIN-KINASE-C INHIBITOR (CGP-41251) POSITIVELY AND NEGATIVELY MODULATES MELANOMA CELL MSH RECEPTORS

Melanoma cells express receptors for melanocyte-stimulating hormone (M SH) in variable abundance. CGP 41251, a derivative of staurosporine wi th an increased selectivity for protein kinase C (PKC) inhibition, was found to modulate MSH receptors in human D10 and HBL cells and in the mouse B16 cell line. Up-regulation was observed in D10 and B16 cells at a concentration of 290 nM and 190 nM, respectively. In HBL cells, h owever, the PKC inhibitor induced a pronounced MSH receptor down-regul ation with an EC(50) of only 32 nM. In D10 and HBL cells, alpha-MSH an d CGP 41251 synergistically regulated MSH receptors whereas these agen ts had an antagonistic effect in B16 cells. PKC stimulation by short-t erm treatment with phorbol ester had an opposite effect on MSH recepto rs as compared to CGP 41251. In B16 cells, CGP 41251 at a concentratio n of 100 nM increased the sensitivity to MSH-induced melanogenesis. Th e staurosporine derivative inhibited proliferation of HBL, B16, and D1 0 cells at EC(50)s of 180 nM, 190 nM, and 520 nM, respectively. Furthe rmore, CGP 41251 increased the dendricity of the cells. In a concentra tion range between 300 nM and 1 mu M, CGP 41251 induced a sharp increa se of the mean cell diameter from 16 mu m to 19 mu m. Thus, the effect s of the selective PKC inhibitor on MSH receptors are induced at lower concentrations than needed for the inhibition of proliferation or for the change in cell morphology. These results suggest that the number of MSH receptors expressed on the surface of cultured melanoma cells c orrelates with the level of constitutive PKC activity in individual ce ll Lines.