Protein kinase C antagonizes pertussis-toxin-sensitive coupling of the calcitonin receptor to adenylyl cyclase

The calcitonin receptor is known to couple to G(s) and G(q), activating adenylyl cyclase and phospholipase C, respectively. The observation of pertussis-toxin-sensitive responses to calcitonin suggests that the receptor is capable of coupling to G(i/o) as well. However, the calcitonin-dependent activation of adenylyl cyclase in HEK-293 cells that stably express the cloned rabbit calcitonin receptor, as in many other cells that express calcitonin receptors, shows little pertussis toxin sensitivity. Calcitonin treatment of these cells stimulates protein kinase C, which is reported to antagonize the receptor-dependent activation of G(i). The possibility that protein kinase C could be antagonizing G(alpha i)-adenylyl cyclase coupling was tested by examining the effects of protein kinase C inhibitors (chelerythrine chloride and sphingosine) or of chronic treatment with phorbol ester to deplete protein kinase C. All three treatments led to a reduction of calcitonin-induced adenylyl cyclase activity that was reversed by pertussis toxin. Inhibiting or depleting protein kinase C had no effect on the activation of adenylyl cyclase by cholera toxin, indicating that G(s) and adenylyl cyclase were not affected by these treatments. Calcitonin treatment of HEK-293 cells, that stably express a myc-tagged rabbit calcitonin receptor, induced the formation of complexes of the receptor and G(alpha i) subunits, confirming that the calcitonin receptor interacts with G(i). Thus, the calcitonin receptor can couple to G(i), but the inhibition of adenylyl cyclase by G(alpha i) is negatively regulated by protein kinase C.