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Regulation of P-glycoprotein expression in cyclic AMP-dependent protein kinase mutants
1Department of Medicine, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, New Brunswick 08901, USA.
Abstract:
Multidrug resistance (MDR) in cancer poses a major obstacle to the success of chemotherapy. We previously reported that cyclic AMP (cAMP)-resistant mutants of the Chinese hamster ovary and the mouse adrenal cortical carcinoma cells harboring defective regulatory (RI alpha) subunits of the cAMP-dependent protein kinase (PKA) are more sensitive than wild-type cells to chemotherapeutic agents that are substrates for P-glycoprotein. In addition, a transfectant overexpressing a mutant RI alpha cDNA showed similar increased sensitivity to these drugs. The altered drug sensitivity in the RI alpha mutants results from reduced expression of the mdr gene, suggesting that PKA may regulate its expression. In this study, we evaluated the sensitivity of several Chinese hamster ovary catalytic (C) subunit mutants to various anticancer drugs. Like the RI alpha subunit mutant, the C subunit mutants also exhibit decreased kinase activity and unresponsiveness to growth inhibition by cAMP. However, in contrast to the RI alpha subunit mutant, the C subunit mutants are not multidrug sensitive and maintain P-glycoprotein expression levels comparable to those of wild-type cells. Furthermore, the C subunit mutants display the same resistance patterns as wild-type cells to P-glycoprotein substrates, including Adriamycin, Taxol, and colchicine. No significant difference was observed in their sensitivity to non-MDR drugs, such as 5-fluorodeoxyuridine, between wild-type, RI alpha, and C subunit mutant cells. These results suggest that the increased multidrug sensitivity in the PKA mutant cells results from alteration of the RI alpha subunit and not the kinase activity, thus implying novel functions for the RI alpha subunit. Therefore, genetic alteration of the RI alpha subunit of PKA may modulate drug resistance in cancer.
Insights
Altered regulatory (RI alpha) subunits of protein kinase A (PKA) increase cancer cells' sensitivity to chemotherapy drugs. This effect stems from the RI alpha subunit itself, not PKA
Area of Science:
- Molecular Biology
- Cancer Research
- Biochemistry
Background:
- Multidrug resistance (MDR) is a significant challenge in cancer chemotherapy.
- Previous work showed cAMP-resistant mutants with defective regulatory (RI alpha) subunits of protein kinase A (PKA) are more sensitive to P-glycoprotein substrates.
Purpose of the Study:
- To investigate the role of catalytic (C) subunit mutants of PKA in anticancer drug sensitivity.
- To determine if altered PKA kinase activity or specific subunits mediate changes in multidrug resistance.
Main Methods:
- Evaluation of drug sensitivity in Chinese hamster ovary (CHO) cells with mutations in RI alpha and C subunits of PKA.
- Comparison of P-glycoprotein expression levels and resistance patterns to various anticancer drugs (e.g., Adriamycin, Taxol, colchicine, 5-fluorodeoxyuridine) between wild-type and mutant cell lines.
Main Results:
- C subunit mutants, unlike RI alpha mutants, did not exhibit increased multidrug sensitivity and maintained wild-type P-glycoprotein expression levels.
- Both RI alpha and C subunit mutants showed decreased kinase activity and cAMP unresponsiveness.
- No significant difference in sensitivity to non-MDR drugs was observed across cell lines.
Conclusions:
- Increased multidrug sensitivity in PKA mutant cells is specifically linked to alterations in the RI alpha subunit, not PKA kinase activity.
- The RI alpha subunit of PKA plays a novel role in modulating P-glycoprotein expression and cancer drug resistance.
- Genetic modification of the RI alpha subunit presents a potential strategy for overcoming MDR in cancer therapy.