Multiple resistance mechanisms in Chinese hamster cells resistant to 9-hydroxyellipticine.

We have previously shown that Chinese hamster lung cells resistant to 9-hydroxyellipticine, DC-3F/9-OH-E, display multiple phenotypical alterations including cross-resistance to a variety of drugs as well as loss of tumorigenicity. We now analyze a DC-3F/9-OH-E subline that has been maintained for a prolonged period of time in drug-free medium in order to clarify the relationships between the various phenotypic traits. The absence of selection resulted in a partial recovery of the ability to form colonies in soft agar as well as of the tumorigenicity in nude mice. In contrast, no change was observed with respect to population-doubling time. Our results also show that the resistance to 9-hydroxyellipticine, which is associated with an altered topoisomerase II activity, is stable in the absence of drug for more than 1 year. In contrast, the cross-resistance to doxorubicin is partially reversible and the cross-resistance to vincristine is totally reversible in the absence of selection. The cross-resistance to vincristine and doxorubicin is accompanied by a decreased drug uptake. Northern blot analysis shows that the multidrug resistance-associated Mr 170,000-180,000 glycoprotein is overexpressed in the DC-3F/9-OH-E cells and that the overexpression is lost in the absence of selection. We conclude that (a) the DC-3F/9-OH-E cells exhibit multiple mechanisms of resistance which can be dissociated, (b) the tumorigenicity and the altered topoisomerase activity are independent biochemical events whereas the oncogenic potential appears to follow the expression of the multidrug resistance, and (c) the multidrug resistance phenotype may be induced by a drug which is not itself recognized by the multidrug resistance mechanism such as 9-hydroxyellipticine.