Rapid recovery of a functional MDR phenotype caused by MRP after a transient exposure to MDR drugs in a revertant human lung cancer cell line.

Prior studies have shown that, in some human tumour cells, increased expression of the multidrug resistance gene MDR1 can be induced in response to certain stress conditions such as a transient exposure to cytotoxic agents. Little is known about the possibility of increasing the expression of the recently cloned multidrug resistance-associated protein (MRP) in response to a transient exposure to cytotoxic drugs. In order to examine this possibility, we have used sensitive assays (RT-PCR, flow cytometry) and the sensitive large cell lung cancer cell line, COR-L23/P, and the revertant line (COR-L23/Rev), generated by growing the doxorubicin-selected, MRP-overexpressing resistant variant COR-L23/R without drug exposure for 24-28 weeks. COR-L23/Rev overexpresses MRP, but to a lesser extent than COR-L23/R. COR-L23/Rev rapidly recovered similar levels of MRP mRNA, protein expression, resistance and drug accumulation deficit as COR-L23/R after a 48-72 h exposure to cytotoxic concentrations of doxorubicin or vincristine but not cisplatin. The increase in MRP mRNA could only be detected 3 to 4 days after the transient exposure to drugs. However, when the parental line, COR-L23/P, was exposed to equitoxic doses of doxorubicin, vincristine or cisplatin, no increase in the levels of MRP mRNA could be observed at higher doses (5- to 10-fold the IC50) of doxorubicin or vincristine (but not of cisplatin), we detected a transient increase in the levels of MDR1 mRNA immediately after short-term exposure. In conclusion, we have shown that a human revertant lung cancer cell line (COR-L23/Rev) has the ability to recover quickly, similar levels of MRP expression and resistance as COR-L23/R after a transient exposure to the MDR-drugs doxorubicin and vincristine.