Dopamine D1 receptor-mediated inhibition of NADPH oxidase activity in human kidney cells occurs via protein kinase A-protein kinase C cross talk

Dopamine cellular signaling via the D1 receptor (D1R) involves both protein kinase A (PKA) and protein kinase C (PKC), but the PKC isoform involved has not been determined. Therefore, we tested the hypothesis that the D1R-mediated inhibition of NADPH oxidase activity involves cross talk between PKA and a specific PKC isoform(s). In HEK-293 cells heterologously expressing human D1R (HEK-hD1), fenoldopam, a D1R agonist, and phorbol 12-myristate 13-acetate (PMA), a PKC activator, inhibited oxidase activity in a time- and concentration-dependent manner. The D1R-mediated inhibition of oxidase activity (68.1 ± 3.6%) was attenuated by two PKA inhibitors, H89 (10 μmol/L; 88 ± 8.1%) and Rp-cAMP (10 μmol/L; 97.7 ± 6.7%), and two PKC inhibitors, bisindolylmaleimide I (1 μmol/L; 94 ± 6%) and staurosporine (10 nmol/L; 93 ± 8%), which by themselves had no effect (n = 4-8/group). The inhibitory effect of PMA (1 μmol/L) on oxidase activity (73 ± 3.2%) was blocked by H89 (100 ± 7.8%; n = 5 or 6/group). The PMA-mediated inhibition of NADPH oxidase activity was accompanied by an increase in PKCθS676, an effect that was also blocked by H89. Fenoldopam (1 μmol/L) also increased PKCθS676 in HEK-hD1 and human renal proximal tubule (RPT) cells. Knockdown of PKCθ with siRNA in RPT cells prevented the inhibitory effect of fenoldopam on NADPH oxidase activity. Our studies demonstrate for the first time that cross talk between PKA and PKCθ plays an important role in the D1R-mediated negative regulation of NADPH oxidase activity in human kidney cells.