DOWN-REGULATION OF MDR-1 EXPRESSION BY 8-CL-CAMP IN MULTIDRUG-RESISTANT MCF-7 HUMAN BREAST-CANCER CELLS

8-Cl-cAMP, a site-selective analogue of cAMP, decreased mdr-1 expressi on in multidrug-resistant human breast cancer cells, A sixfold reducti on of mdr-1 mRNA expression by 8-Cl-cAMP began within 8 h of treatment and was associated with a decrease in the synthesis of P-glycoprotein and with an increase in vinblastine accumulation A reduction in mdr-1 expression after. 8-Cl-cAMP treatment was also observed in multidrug- resistant human ovarian cancer cell hues, 8-Cl-cAMP is known to change the ratio between the two regulatory subunits, RI and RII, of protein kinase A (pKA), We observed that RI alpha decreased within 24 h of 8- Cl-cAMP treatment, that RII beta increased after as few as 3 h of trea tment, and that PKA catalytic activity remained unchanged during 48 h of 8-Cl-cAMP treatment, The results are consistent with the hypothesis that mdr-1 expression is regulated in part by changes in PKA isoenzym e levels, Although 8-Cl-cAMP has been used to differentiate cells in o ther model systems, the only differentiating effect that could be dete cted after 8-Cl-cAMP treatment in the MCF-7TH cells was an increase in cytokeratin expression, Evil dence that the reduction of mdr-1 mRNA o ccurred at the level of gene transcription was obtained by measuring c hloramphenicol acetyltransfer;ase (CAT) mRNA in MCF-7TH cells transfec ted with an mdr-1 promoter-CAT construct prior to 8-Cl-cAMP treatment, Thus, 8-Cl-cAMP is able to downregulate mdr-1 expression and suggests a new approach to reversal of drug resistance in human breast cancer.