Contribution of glutathione and glutathione‐dependent enzymes in the reversal of adriamycin resistance in colon carcinoma cell lines

Four human colon cancer cell lines (SW620, LS 180, DLD‐I, and HCT‐15) and sub‐lines isolated in vitro by selection with Adriamycin were studied for reversal of intrinsic and acquired Adriamycin resistance, using buthionine sulfoximine (BSO) to deplete cellular glutathione alone and in combination with the P‐glycoprotein antagonist verapamil. GSH levels varied among the parental cell lines but did not increase with resistance. In the parental SW620, DLD‐1 and HCT‐15 and their drugresistant derivatives, there was no relation between the effect of the glutathione‐depleting agent BSO, the mRNA expression of both selenium‐dependent glutathione peroxidase (GPx) and glutathione S‐transferase pi (GSTπ), bulk glutathione S‐trans‐ferase (GST) activity, and the degree of resistance. However, in LS 180 and its derivative sub‐lines, which do not principally rely on P‐glycoprotein (Pgp) for Adriamycin resistance, treatment with BSO demonstrated a relatively diminished GSH depletion and enhanced recovery. In comparison with the other acquired cell lines, BSO specifically reversed acquired resistance in the LS 180 Adriamycin‐resistant subline (LS 180 Ad 150) after short‐term drug exposure. Furthermore, the LS 180 Ad 150 cells demonstrated an increase in both GPx and GSTπ mRNA expression. These observations suggest that glutathione‐mediated detoxification of Adriamycin may play a role in the resistance of this sub‐line. Verapamil enhanced Adriamycin cytotoxicity 1.2‐ to 12‐fold in the intrinsically resistant cells and as much as 15‐fold in cell lines with acquired resistance. Combination of BSO with verapamil resulted in additive, but not synergistic, reversal of resistance. The results underscore the complex nature of Adriamycin resistance, and suggest a role for drug‐resistance‐modulating agents in the treatment of colon carcinoma.