AbstractWe have previously shown that in Chinese hamster ovary (CHO) cells, a mutant cell line with a defective regulatory subunit (RI) for the cAMP‐dependent protein kinase (Abraham et al: Mol. Cell. Biol., 7:3098–3106, 1987), and a transfectant cell line expressing the same mutant kinase, showed increased sensitivity to a number of drugs that are known to be substrates for the multidrug transporter (P‐glycoprotein). In the current study we have investigated the mechanism by which cAMP‐dependent protein kinase controls drug resistance. We report here that the sensitivity of the kinase defective CHO cell lines to multiple drugs results from decreased RNA levels for the multidrug‐resistance gene. Similar results were obtained with mouse Y1 adrenal cells. Wild‐type Y1 cells had high levels of P‐glycoprotein due to expression of both themdr1b andmdr2 genes, whereas the cAMP‐dependent protein kinase mutant Kin 8 cells had decreased RNA levels for these genes. A Kin 8 transfectant with restored cAMP‐dependent protein kinase activity recoveredmdrexpression, indicating a cause and effect relationship between the protein kinase mutations andmdrexpression. No changes in nuclear run‐off assays could be detected, suggesting a non‐transcriptional mechanism of regulation. Wild‐type Y1 cells are more drug sensitive despite having higher levels of P‐glycoprotein than the mutant cells. This paradoxical result may be explained by the higher rate of synthesis of steroids by the wild‐type Y1 cells, which appear to be inhibitors of P‐glycoprotein transport act
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