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Geldanamycin (GA, NSC 122750), a benzoquinone ansamycin antibiotic, exhibits antitumor activity by binding to heat shock protein 90 (Hsp90). Recently, multiple synthetic analogs of GA have been developed. Some of them are in clinical trials, such as 17-AAG (NSC 330507). There are differences in sensitivity to GA between various cancer cell lines and individual patients. Identifying the genetic determinants of chemosensitivity and -resistance to GA and its analogs will be important for optimizing cancer therapy. Previously, we demonstrated that in NCI-60 cell panel the expression of the cystine-glutamate transporterSLC7A11 showed negative correlation with the growth inhibitory potency of GA, but not with that of 17-AAG. This suggested that by transporting cystine, which is critical for glutathione (GSH) production, SLC7A11 confers resistance selectively to GA. Non-small cell lung cancer cell line A549 is resistant to GA and expresses high levels of SLC7A11. Inhibition with SLC7A11 inhibitors glutamate or (s)-4-carboxyphenylglycine and down-regulation of SLC7A11 by RNAi in A549 cells lowered intracellular glutathione levels and resulted in increased sensitivity to GA, but with no effects on 17-AAG. In contrast, hepatocarcinoma cell line HepG2 is highly sensitive to GA and expresses significantly lower SLC7A11 at both mRNA and protein levels than A549 cells. Ectopic expression of SLC7A11 cDNA in HepG2 cells confers increased resistance to GA, but not to 17-AAG. The resistance mediated by SLC7A11 in the HepG2 cells is reversible by (s)-4-carboxyphenylglycine. To confirm the role of GSH in SLC7A11-mediated chemoresistance, we next evaluated the effects of N-acetylcysteine (NAC), a precursor of cysteine and GSH synthesis, in modulating the sensitivity to GA analogues. The presence of NAC completely blocked the cytotoxic effects of GA in A549 and HepG2 cells, but with no protective effect on 17-AAG. Thus, 17-AAG may bypass the resistance mechanism mediated by SLC7A11 and GSH. There is surprising coherence between SLC7A11-correlations and the degree of hepatotoxicity of GA analogs: GA showed significantly stronger hepatotoxicity than 17-AAG. In addition, the basal expression level of SLC7A11 is lower in human normal hepatocytes than in HepG2 cells. Thus, SLC7A11 and GSH may play a role in modulating hepatotoxicity, and therefore be factors determining the differential toxicity of the analogues observed in preclinical studies. In conclusion, the cystine-glutamate transporter SLC7A11 mediates chemoresistance by upregulating intracellular glutathione levels. This resistance mechanism is associated with structural properties of GA analogs, strongly affecting those with the methoxy group at C-17 (e.g., GA), but not C-17-amino congeners (e.g., 17-AAG).

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA