Biochemical characterization of a new melanoma model--the minipig MeLiM strain.

Hereditary minipig melanomas, which have many histopathological and other features in common with their human counterparts, have recently become preferred melanoma models. The MeLiM (Melanoma-bearing Libechov Minipig) strain was selected by partial inbreeding. A high tumour incidence and malignant behaviour on the one hand together with the occurrence of spontaneous regression and high susceptibility to the devitalization technique as a new strategy in melanoma therapy, make the MeLiM strain a superior melanoma model to other hereditary swine melanomas. Biochemical analyses of the tumours revealed: (i) a high concentration of eumelanin and low concentration of phaeomelanin in melanoma cells, which makes the probability of photochemical regression of MeLiM melanomas negligible; (ii) an extremely high level of melanosomes (almost half of the melanoma dry weight), which suggests a high differentiation of the MeLiM melanoma and is consistent with its mechanical rigidity; (iii) the presence of typical melanoma enzymes--tyrosinase, alpha-mannosidase and gamma-glutamyltransferase; and (iv) in the tumours regressing as a consequence of devitalization treatment, alpha-mannosidase activity was reduced and tyrosinase activity approached the detection threshold, which is in accordance with the substitution of melanoma tissue for connective tissue in devitalized tumours observed histologically. (Immuno)histochemical comparisons (based on dopaoxidase reaction, S100 and HMB-45 reactivities) of the skin from white and pigmented minipigs revealed the absence of melanocytes in white skin. This is the first direct evidence supporting a possible explanation for the absence of melanoma in white minipigs. The similarities and dissimilarities of the MeLiM model compared with human melanoma are highlighted.

Keratinocytes control the pheo/eumelanin ratio in cultured normal human melanocytes.

The pheo/eumelanin ratio of cultured normal human melanocytes is distinct from the ratio observed for the same cells in vivo where they are in close contact with keratinocytes. To study the possible involvement of keratinocytes in the control of melanogenesis, we compared quantitatively and qualitatively the melanin production in melanocyte mono-cultures, in melanocyte-keratinocyte co-cultures and in pigmented reconstructed epidermis. Pheomelanin and eumelanin contents were assessed by high-performance liquid chromatography with electrochemical and fluorometric detection of their specific degradation products and revealed striking differences in the presence of keratinocytes. In the absence of keratinocytes (melanocyte mono-cultures), we observed a very limited eumelanin production and a very high pheomelanin synthesis. The pheo/eumelanin ratio in mono-cultures could be slightly influenced by changing the composition of the culture medium, however, the very strong imbalance in favor of pheomelanin remained unchanged. An induction of eumelanin synthesis accompanied by an important reduction of pheomelanin formation was only observed in the presence of keratinocytes. The pheo/eumelanin ratio in melanocyte mono-culture dropped from 1043 down to about 25 in the presence of keratinocytes (co-cultures). The same observations were made when the melanocytes were integrated into a reconstructed human epidermis. Interestingly, under co-culture conditions resulting in only a partial contact between melanocytes and keratinocytes, the reduction of the pheo/eumelanin ratio were less pronounced. From these results we conclude that keratinocytes play an important role in the melanin production, affecting the melanogenic pathways.