HUMAN-MELANOMA CELL-LINES SHOW LITTLE RELATIONSHIP BETWEEN EXPRESSIONOF PIGMENTATION GENES AND PIGMENTARY BEHAVIOR IN-VITRO

Several laboratories are pursuing the question of whether the expressi on of pigment genes can be used as a useful marker for tumour progress ion. However, many melanoma rumours are amelanotic in vivo. The purpos e of this study was to examine the relationship between the expression of tyrosinase-related genes [tyrosinase, tyrosinase-related protein-1 (TRP-1) and tyrosinase-related protein-2 (TRP-2)] and pigmentation of melanoma cells. Fourteen cutaneous melanoma cell lines were examined for visible pigment, melanin content, and dopa oxidase activity and fi ndings were related to the previously determined expression of the thr ee tyrosinase-related genes in these cells in culture. Four of the cel l lines were also stimulated with alpha-MSH, isobutylmethylxanthine, a nd forskolin to examine the relationship between induced pigmentation and upregulation of pigmentation genes. There was no simple correlatio n between pigmentation gene expression and dopa oxidase activity or to tal melanin content of the 14 melanoma cell lines in culture, In the m ajority of cells, there was no appreciable pigment, whereas, in contra st, half of the cells showed significant dopa oxidase activity. Upregu lation of dopa oxidase activity was achieved by alpha-MSH in two out o f four cell lines examined in detail and with IBMX in three out of fou r of these cell lines. IBMX increased tyrosinase gene expression in al l four cell lines; Q-MSH was without effect; and TRP-1 and TRP-2 expre ssion were largely unaffected by IBMX or a-MSH. Modest changes in morp hology were noted in response to IBMX, Overall, however human melanoma cell lines were, with two exceptions, amelanotic in culture despite t he fact that 10 out of the 14 lines expressed tyrosinase-related genes . We conclude that measurable pigmentation is not a necessary conseque nce of the expression of pigmentation genes. An implication of this wo rk is that amelanotic tumours in vivo may nevertheless be positive for tyrosinase-related genes.