Inhibition of retinoblastoma protein translation by UVB in human melanocytic cells and reduced cell cycle arrest following repeated irradiation.

In human melanocytes and a human melanoma cell line (MM96L), the level of the retinoblastoma gene product (pRB) detected by Western blotting transiently decreased to 55% and 70% of controls respectively 9-12 h after a noncytostatic exposure (75 Jm-2) to UVB (280-315 nm) and to 2% and 14% 48 h after a cytostatic exposure (300 Jm-2). The pRB levels in fibroblasts and HeLa showed minimal loss, and under some conditions increased compared with unirradiated cells. Equitoxic doses of gamma radiation, cisplatin or the antimetabolite deoxyinosine had little effect on pRB levels. UVC (254 nm) was less inhibitory compared with equitoxic UVB. No loss of pRB mRNA was found in MM96L after UVB, nor was pRB protein stability significantly affected. Synthesis of new pRB in MM96L 24 h after UVB was 16% of controls, suggesting that loss of pRB results from a UVB-specific inhibition of translation. Compared with HeLa cells and fibroblasts, MM96L cells exhibited reduced cycle arrest if irradiated when pRB was depleted by a previous UVB exposure. These results suggest a mechanism whereby down-regulation of pRB translation by UVB may play a role in genesis of melanoma.

Tumor selectivity and transcriptional activation by azelaic bishydroxamic acid in human melanocytic cells.

Azelaic bishydroxamic acid (ABHA), a potent differentiating agent for lymphoid cells, was selectively toxic for 5 human tumor cell lines and transformed human melanocytes and keratinocytes (dose for 37% survival, D37, 30-100 microg/mL) compared with normal cells (melanocytes, fibroblasts; D37 > 300 microg/mL). Dendritic morphology was the only indicator found for increased differentiation, markers for the pigmentation pathway being unchanged or inhibited by ABHA. In contrast to hexamethylene bisacetamide and azelaic acid, ABHA significantly increased the HIV LTR, SV40 and c-fos promoter activities during a 24 hr treatment. Metallothionein promoter activity was enhanced by 5 hr treatment with ABHA in a sensitive melanoma cell line (MM96L) but was inhibited in a more resistant line (HeLa); c-fos promoter activity was inhibited in HeLa during this time. Transcription from a p53 binding response element was inhibited in MM96L by a 24 hr ABHA treatment but enhanced in HeLa. ABHA may represent a structural prototype for designing more potent and selective anti-melanoma agents.