Molecular and Biological Control of Melanogenesis Through Tyrosinase Genes and Intrinsic and Extrinsic Regulatory Factors

Non-premelanosomal melanogenic compartments and their melanogenesis-controlling functions have been further elucidated. In addition to enzymatic and nonenzymatic controlling factors, we have also been exploring the role of melanogenesis-related genes. Naturally occurring intrinsic melanogenic inhibitors, MW <6,000(α), 6,000-30,000(β), and >30,000(γ), having different modes of action, have been identified within melanoma cells. One of the α-type melanogenic inhibitors of isolated tyrosinase(Ty) nonsuppressive types, later identified as lactic acid, induces depigmentation of cultured B-16 cells by the reduction in Ty activity level due to the inhibition of its mRNA expression. The transfection of Ty cDNA, rather than nuclear DNA-binding master regulatory gene, can induce, within both Ty-deficient amelanotic melanoma cells and also within fibroblasts, melanin polymer formation. This multisequential step occurs not only by the induction of Ty synthesis but also by the induction of other regulatory proteins and factors such as dopachrome tautomerase, DHICA-oxidase, catalase, Ty-glycosylation in GERL, and Ty-transfer by coated vesicles to newly assigned melanogenic vacuoles in which not only eumelanin but also rather pronounced concomitant pheomelanin formation is seen. Investigation of melanin-producing vacuoles in transfected fibroblasts and reexamination of premelanosomes in pigment cells has revealed the following: 1) Melanosomes possess phagocytic ability; 2) melanosomes receive tyrosinase and hydrolases via coated vesicles from GERL; 3) melanosomes possess lysosome-associated membrane protein 1 (LAMP-1); 4) amelanotic melanoma contains lysosome-like vacuoles with myelin figures that acquire typical premelanosome structure after Ty-cDNA transfection. Thus it is proposed that melanosomes are specialized lysosomes in pigment cells. Coated vesicles synthesize melanin monomers such as DHICA and some DHI, and have a monomer-stabilizing system. Thus they can transport them in intact form with Ty to premelanosomes, which subsequently polymerize these monomers by the action of DHICA-oxidase and T₃-Ty. Selective eradication and diagnosis of malignant melanoma using our ¹⁰B-dopa analogue has been successfully performed in human melanoma patients using combined thermal neutron irradiation for the former and positron emission CT for the latter.