G alpha q family members couple parathyroid hormone (PTH)/PTH-related peptide and calcitonin receptors to phospholipase C in COS-7 cells

The PTH/PTH-related peptide (PTHrP) receptor and the calcitonin receptor mediate the action of their physiological ligands by activating two different effecters, adenylyl cyclase and phospholipase C. Whereas regulation of adenylyl cyclase via both receptors is thought to involve the G protein G(s), it is not known whether activation of phospholipase C results from coupling of the receptors to G(q) family members or whether beta gamma-subunits released from receptor-activated G(s) lead to phospholipase C activation. To elucidate the mechanism of this type of dual signaling, we reconstituted the signal transduction of the PTH/PTHrP and the calcitonin receptor in COS-7 and HEK293 cells. In COS-7 cells expressing the receptor alone, addition of the respective ligands resulted in the accumulation of cAMP and inositol phosphates. When cells were cotransfected with the cDNAs of receptor and different alpha-subunits of the G(q) family (G alpha(q), G alpha(11), G alpha(14), G alpha(15), and G alpha(16)), a severalfold increase in the ligand-dependent inositol phosphate production could be observed, indicating that the receptors functionally interacted with all alpha-subunits of the G alpha(q) family. Additionally, whereas PTH treatment of HEK293 cells coexpressing both the PTH/PTHrP receptor and G alpha(q), increased both second messengers, the same treatment in cells expressing the PTH/PTHrP receptor alone increased only cAMP. Under all conditions tested, activation of phospholipase C via the PTH/PTHrP and calcitonin receptor required higher ligand concentrations than receptor-mediated adenylyl cyclase activation. Our data strongly support the idea that dual signaling of the PTH/PTHrP and calcitonin receptors is due to the activation of different G proteins belonging to the G(s) and G(q) families.