Alteration of Type II Regulatory Subunit of cAMP‐dependent Protein Kinase in Human Cisplatin‐resistant Cells as a Basis of Collateral Sensitivity to 8‐Chloro‐cAMP

A cyclic adenosine 3′,5′‐monophosphate (cAMP) analogue, 8‐chloro‐cAMP (8‐Cl‐cAMP), had a collateral growth‐inhibitory effect on a cis‐diamminedichloroplatinum(II) (CDDP)‐resistant human cancer cell lines (PC‐14/CDDP). The non‐selective analogues dibutyryl‐cAMP, 8‐bromo‐cAMP and forskolin, which are cAMP agonists, showed far less cytotoxicity than 8‐Cl‐cAMP in both cell lines. There was no significant difference in cAMP content between PC‐14 and PC‐14/CDDP. Because 8‐Cl‐cAMP has been shown to bind selectively to the site I receptor of the type II regulatory subunit (RII) of cAMP‐dependent protein kinase, we determined the level of expression of regulatory subunits in PC‐14 and PC‐14/CDDP cells by photoaffinity labeling. PC‐14/CDDP cells had a higher RII level, low site I receptor of type I regulatory subunit (RI) level, and a lower RI/RII ratio than the parental PC‐14 cells. Exposure to 8‐Cl‐cAMP increased the RI and RII level in PC‐14/CDDP cells in dose‐ and time‐dependent manners. On the other hand, in parental PC‐14 cells, RII was not detected and the levels of RI and RII were not increased by exposure to 8‐Cl‐cAMP. These results suggested that the change in RI and/or RII levels caused by 8‐Cl‐cAMP was correlated with 8‐Cl‐cAMP‐induced growth inhibition and that the collateral sensitivity to 8‐Cl‐cAMP in CDDP‐resistant cells was due to the increased RII level. Our results suggest that 8‐Cl‐cAMP can be used in combination with CDDP and that measurement of RI and RII levels and/or the RI/RII ratio is a useful tool to predict CDDP sensitivity.