The possible mechanisms for the reduced
melanin content and poor melanogenic response to
MSH was investigated in B16-F10DD differentiation deficient
melanoma cells. In particular, the
MSH receptor status and associated signal transduction pathway linking to
tyrosinase activity in these cells was studied for evidence of any defects. F10DD cells contained high-affinity binding sites for
alpha-MSH, with KD values similar to those previously reported for other variants of the
B16 melanoma. SDS-PAGE analysis after radioactive
ligand cross-linking showed no evidence of gross structural alterations of the receptor. The F10DD cells expressed approximately twice as many receptors as the F10 parent cell line, suggesting a possible feedback response attempting to compensate for the amelanotic condition. The functional integrity of the
MSH 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 of
tyrosinase mRNA and
protein, point to a structural defect in
tyrosinase or in the post-translational control mechanisms by which the activity of this
enzyme is regulated.