MSH RECEPTORS AND FUNCTION IN AMELANOTIC B16 MELANOMA-CELLS

The possible mechanisms for the reduced melanin content and poor melan ogenic response to MSH was investigated in B16-F10DD differentiation d eficient melanoma cells. In particular, the MSH receptor status and as sociated signal transduction pathway linking to tyrosinase activity in these cells was studied for evidence of any defects. F10DD cells cont ained high-affinity binding sites for alpha-MSH, with K(D) values simi lar to those previously reported for other variants of the B16 melanom a. SDS-PAGE analysis after radioactive ligand cross-linking showed no evidence of gross structural alterations of the receptor. The F10DD ce lls expressed approximately twice as many receptors as the F10 parent cell line, suggesting a possible feedback response attempting to compe nsate for the amelanotic condition. The functional integrity of the MS H receptors in F10DD cells was confirmed by the presence of increased levels of cAMP in response to MSH stimulation. These results, coupled with the observation that F10 and F10DD cells express similar levels o f tyrosinase mRNA and protein, point to a structural defect in tyrosin ase or in the post-translational control mechanisms by which the activ ity of this enzyme is regulated.