Isolation from a human MDR lung cell line of multiple clonal subpopulations which exhibit significantly different drug resistance.

The heterogeneous nature of an adriamycin-selected human MDR squamous lung cell line, DLKP-A, was investigated by isolating and characterising 9 of its clonal subpopulations. The DLKP-A cell line exhibits resistance to the classical MDR drugs, overexpresses P-glycoprotein and displays reduced topoisomerase II amounts. The clonal cell lines exhibit a wide range of resistance extents, with the most resistant clone displaying 9 times the extent of adriamycin resistance observed in the least resistant clone. A number of clones exhibit sensitivity to the concentration of adriamycin in which the parental cell line was selected, possibly indicating cooperation between the more and less resistant cells. Detailed analysis of 4 of the clonal subpopulations revealed broadly similar drug resistance mechanisms. Alterations in expression of the MDR-associated genes MDR1 and Topo IIalpha were observed, with no detectable changes in the expression of MDR3, MRP, GSTpi, Topo IIbeta, Topo I and CYP1A1 noted. However, each clonal cell line displayed a distinct extent of expression of MDR1 and Topo IIalpha and further characterisation of the clones indicated that other modes of drug resistance may exist in at least one of the cell lines. In particular, 2 of the clones (DLKPA6B and DLKPA11B) which have almost identical drug resistance profiles appear to have quite different mechanisms of resistance. The clonal subpopulations possess individual growth rates, amounts of adriamycin accumulation and susceptibility to toxicity-enhancement by MDR-modulating agents. It was possible to generate a cell line with a drug toxicity profile similar to DLKP-A by mixing some of the clonal subpopulations. Our results provide evidence of heterogeneity within an MDR human cell population with respect to resistance and expression of MDR-associated genes.