Metallothionein Content Correlates with the Sensitivity of Human Small Cell Lung Cancer Cell Lines to Cisplatin1

We have established m-diamminedichloroplatinum(II) (cisplatin) re sistant human small cell lung cancer cell lines, H69/CDDP0.2 and H69/ CDDP, to investigate the mechanism of acquired resistance to cisplatin. H69/CDDP02 and H69/CDDP »ere6and 11-fold resistant to cisplatin compared with the H69 parental cell line. H69/CDDP was also resistant to cadmium chloride (2-fold), m-diammine(glycolato)platinum (4-fold), 4-hydroperoxycyclophosphamide (3-fold) and 3-|(4-amino-2-methyl-5pyrimidinyl)methyl|-l-(2-chloroethyl)-l-nitrosourea (4-fold) if the drug concentrations that inhibit cell growth by 50% from growth inhibition assay were compared. There was no significant difference in the cisplatin accumulation among these cell lines. Although DNA interstrand cross link formations, determined by filter elution assay in H69/CDDP(U and H69/CDDP, was decreased to 20 to 30% of that in H69 parental cells, the repair capacity of DNA interstrand cross-links was equivalent in all three cell lines. Intracellular glutathione content was equal in all cell lines. H69/CDDP had the highest glutathione S-transferase activity (H69, 11 nmol/min/mg protein, H69/CDDP(u, 12 nmol/min/mg protein; H69/CDDP, 74 nmol/min/mg protein, respectively) and an overexpression of glutathione 5-transferase TTmRNA. The drug concentrations that inhibit cell growth by 50% for cisplatin in all cell lines were decreased by treatment with ethacrynic acid, an inhibitor of glutathione S-transferase ir. but this did not alter the relative degree of resistance. Intracellular metallothionein content (H69, 14 pmol/mg protein, H69/CDDP|).2, 22 pmol/mg protein; H69/CDDP, 33 pmol/mg protein, respectively) and expression of metallothionein mRNA were correlated with the drug concentrations that inhibit cell growth by 50% of the three cell lines for cisplatin and cadmium chloride. The present study suggested the impor tance of metallothionein in the mechanisms of cisplatin resistance.