RESUMO
Some malignant melanoma cells regress spontaneously by terminal differentiation, and understanding the mechanisms of this spontaneous regression can contribute to the development of a new therapy not only for melanoma but also for other cancers. The signal transducing G protein is one component of the signaling pathways for the differentiation-inducing molecules such as alpha-melanocyte-stimulating hormone (alpha-MSH) and cAMP. To investigate the role of G proteins in the differentiation process, we analyzed the expression of various G proteins by quantitative Western blot and cAMP response in human malignant melanoma cell lines. SK-MEL-3 cells expressed the largest amount of stimulatory G protein alpha subunit (G(s) alpha) and the largest amount of inhibitory G protein alpha subunit (G(i) alpha) was expressed in Malme-3M cells among the 4 melanoma cell lines analyzed in this experiment. The SK-MEL-28 cells exhibited largest amount of alpha subunit of G(q) and the beta subunits. The cAMP formation by forskolin stimulation was largest in the Malme-3M. The amount of cAMP formation did not show any correlation with the expression of G(s) alpha nor that of G(i) alpha. The population doubling time was longest in Malme-3M cells. In this experiment, we found that the melanoma cells vary widely both in the expression of various G proteins and in cAMP production depending on the cell lines.