THE AMPHIBIAN MELANIZATION INHIBITING FACTOR (MIF) BLOCKS THE ALPHA-MSH EFFECT ON MOUSE MALIGNANT MELANOCYTES

We have found that a melanization inhibitory factor (MIF) extracted fr om the ventral skin of Rana forreri has a slight inhibitory effect on the activity levels of tyrosinase and dopachrome tatuomerase in B16/F1 0 and Cloudman S-91 murine melanoma cell lines. Furthermore, this fact or appears to block the effects of alpha-MSH on these enzymatic activi ties. However, MIF treatment does not affect the melanogenic action of theophylline on the same cells, suggesting that MIF acts proximal to MSH-mediated cAMP formation, possibly by interaction with the MSH rece ptor. In this way, we show that this amphibian factor has biological a ctivity on mammalian melanocytes. This suggests the existence of mamma lian counterparts of amphibian MIF in the mouse integument that might regulate epidermal melanocytes. These peptides might be related to the agouti protein, as they share similar mechanisms of action. The inter action of different peptides with the MSH receptor would be a complex but general mechanism responsible for many mammalian coat color varian ts.