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Negative Regulation of Parathyroid Hormone (PTH)-Activated Phospholipase C by PTH/PTH-Related Peptide Receptor Phosphorylation and Protein Kinase A

Endocrine Unit (H.A.W.T.), Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114; and Endocrine Division (A.B.A.-S.), Wayne State University School of Medicine, Detroit, Michigan 48201

Address all correspondence and requests for reprints to: Hesham Tawfeek, M.D., Endocrine Unit, Massachusetts General Hospital, 55 Fruit Street, Thier 1051, Boston, Massachusetts 02114. E-mail: tawfeek{at}helix.mgh.harvard.edu.

PTH binding to the PTH/PTHrP receptor activates adenylate cyclase/protein kinase A (PKA) and phospholipase C (PLC) pathways and increases receptor phosphorylation. The mechanisms regulating PTH activation of PLC signaling are poorly understood. In the current study, we explored the role of PTH/PTHrP receptor phosphorylation and PKA in PTH activation of PLC. When treated with PTH, LLCPK-1 cells stably expressing a green fluorescent protein (GFP)-tagged wild-type (WT) PTH/PTHrP receptor show a small dose-dependent increase in PLC signaling as measured by inositol trisphosphate accumulation assay. In contrast, PTH treatment of LLCPK-1 cells stably expressing a GFP-tagged receptor mutated in its carboxyl-terminal tail so that it cannot be phosphorylated (PD-GFP) results in significantly higher PLC activation (P < 0.001). The effects of PTH on PLC activation are dose dependent and reach maximum at the 100 nM PTH dose. When WT receptor-expressing cells are pretreated with H89, a specific inhibitor of PKA, PTH activation of PLC signaling is enhanced in a dose-dependent manner. H89 pretreatment in PD-GFP cells causes a further increase in PLC activation in response to PTH treatment. Interestingly, PTH and forskolin (adenylate cyclase/PKA pathway activator) treatment causes an increase in PLCβ3 phosphorylation at the Ser1105 inhibitory site and that increase is blocked by the PKA inhibitor, H89. Expression of a mutant PLCβ3 in which Ser1105 was mutated to alanine (PLCβ3-SA), in WT or PD cells increases PTH stimulation of inositol 1,4,5-trisphosphate formation. Altogether, these data suggest that PTH signaling to PLC is negatively regulated by PTH/PTHrP receptor phosphorylation and PKA. Furthermore, phosphorylation at Ser1105 is demonstrated as a regulatory mechanism of PLCβ3 by PKA.