Cellular sialic acid level and phenotypic expression in B16 melanoma cells: comparison of spontaneous variations and bromodeoxyuridine- and theophylline-induced changes

The relation of the total cellular content of sialic acid to phenotypic expression of B16 mouse melanoma cells was examined by using phenotype-modifying reagents and more than 10 cloned cell lines with spontaneous phenotypic variations. The sialic acid content changed in a growth phase-dependent manner with a peak in the early log phase of growth. This peak completely disappeared when cells were treated with 5-bromodeoxyuridine (BrdU), suggesting its relation to quasi-normal phenotypes of the treated cells. BrdU treatment also reduced the cellular sialic acid content itself and resulted in the suppression of the activity of tyrosinase, the key enzyme for melanogenesis, and a considerable increase in cell-to-substratum adhesiveness. Treatment with theophylline, in contrast, markedly elevated the sialic acid content, which was accompanied by dramatic increments in tyrosinase activity and pigmentation as well as a slight increase in adhesiveness. The results show a correlation of sialic acid level with tyrosinase expression but not with cell adhesion. From comparison of spontaneous phenotypic variations, the correlation of sialic acid level with tyrosinase activity was confirmed, while there was only a slight correlation with adhesiveness. It is thus suggested that sialylation/desialylation, being reflected as variations in cellular sialic acid content, is implicated in melanoma cell differentiation in terms of tyrosinase expression.