ACTIVATED RAS ONCOGENE AND SPECIFICALLY ACQUIRED-RESISTANCE TO CISPLATIN IN HUMAN MAMMARY EPITHELIAL-CELLS - INDUCTION OF DNA CROSS-LINKS AND THEIR REPAIR

A human non-malignant mammary epithelial cell line, HBL100, and the ra s-transformed HBL100/ras1 cell line were examined for their sensitivit y to cis-diamminedichloroplatinum(II) (cisplatin). The clonogenic cell survival assay showed that HBL100/ras1 exhibited a 2.7-fold increased resistance compared to the parental HBL100 cell line. The responses t o other agents interacting with DNA, such as mitomycin C, 8-methoxypso ralen plus WA or doxorubicin, were very similar in both cell lines. Th e same is true for ionizing radiation (Alapetite et al., Int J. Radiat . Biol., 59, 385-396, 1991). In other words, the mechanism of acquired resistance in HBL100 appears to be limited to cisplatin. No differenc e was observed between the two cell lines in cisplatin uptake as deter mined by atomic absorption spectrometry. Alkaline elution showed that less interstrand cross-links were formed by this drug in the resistant HBL100/ras1 cells compared to HBL100 and, moreover, the removal of th ese adducts was clearly more efficient in the former cell line. This w as confirmed by an in vitro excision repair assay which revealed a 2.2 -fold increase in DNA repair activity in the extracts from HBL100/ras1 versus HBL100 cells. It is concluded that the transformation of human epithelial HBL100 cells by the ras gene resulted in an acquired resis tance apparently limited to cisplatin, a feature associated with a red uced proportion of induced interstrand cross-links and a higher effici ency in their removal. The mechanism of involvement of the ras gene pr oduct in this process is still a matter of speculation.