Pj. Odwyer et al., INCREASED DT-DIAPHORASE EXPRESSION AND CROSS-RESISTANCE TO MITOMYCIN-C IN A SERIES OF CISPLATIN-RESISTANT HUMAN OVARIAN-CANCER CELL-LINES, Biochemical pharmacology, 52(1), 1996, pp. 21-27
In a series of ovarian carcinoma cell lines selected in vitro for resi
stance to cisplatin by continuous exposure to increasing drug concentr
ations, the lever of resistance is proportional to the expression of g
amma- glutamylcysteine synthetase (gamma-GCS). To determine if other d
etoxicating genes are coordinately expressed, we measured the activity
of DT-diaphorase and cytochrome P450 reductase. The specific activity
of DT-diaphorase, but not that of cytochrome P450 reductase, increase
d with increasing resistance to cisplatin. Steady-state mRNA levels fo
r DT diaphorase correlated with enzyme activity and hence with cisplat
in resistance. Since the activity of DT diaphorase has been associated
with sensitivity to quinones, we studied the cytotoxicity of mitomyci
n C under oxic conditions. Unexpectedly, resistance to mitomycin C inc
reased proportionally with that to cisplatin (r = 0.997). Pretreatment
with buthionine sulfoximine, which inhibits glutathione (GSH) synthes
is, failed to sensitize either the sensitive or the resistant lines to
mitomycin C. Thus, the basis for collateral resistance to mitomycin C
in the cisplatin resistant lines under oxic conditions is unrelated t
o overproduction of GSH. Under hypoxia, the toxicity of mitomycin C to
the most sensitive (A2780) cell line was unchanged. However, the most
resistant (C200) line was 2-fold more resistant to mitomycin C under
hypoxic conditions. The coordinate overexpression of DT diaphorase and
gamma-GCS in the resistant cell lines is thus associated with hypoxic
cell resistance, and supports the involvement of shared mechanisms of
gene regulation in the observed resistant phenotype.