Distinct β-Arrestin- and G Protein-dependent Pathways for Parathyroid Hormone Receptor-stimulated ERK1/2 Activation

Abstract

The type I PTH/PTH-related peptide receptor (PTH1R), 2 a seventransmembrane receptor (7TMR) highly expressed in the kidney and bone, plays a fundamental role in the regulation of calcium homeostasis, as well as in bone formation and resorption. Ligands for PTH1R including PTHrp and PTH are involved in the etiology and treatment of disease processes such as hypercalcemia of malignancy and osteoporosis. The actions of PTH, however, are complex. PTH is known for both anabolic and catabolic effects on bone, which are dependent upon intermittent or persistent exposure, respectively (1-3). The mechanistic basis of these effects on bone remodeling are not well understood.The intracellular signaling pathways activated by PTH and PTHrP via the PTH1R receptor include G s -mediated activation of adenylate cyclase, resulting in cAMP production and PKA activation, and G q/11 -mediated PLC␤ stimulation, leading to inositol 1,4,5-trisphosphate (IP 3 ) production, calcium mobilization, and PKC activation (4 -7). It has also been demonstrated that PTH activates the Raf-MEK-ERK MAP kinase (MAPK) cascade through both PKA and PKC in a cell-specific and G protein-dependent manner (8 -10). MAPKs activated in response to stimulation by many different classes of cell surface receptors, including growth factor receptor tyrosine kinases and 7TMRs, regulate cell growth, division, differentiation, and apoptosis (11). PTHstimulated activation of MAPK is known to have proliferative effects in kidney and bone (12, 13).There is growing evidence for novel 7TMR signaling mechanisms that are distinct from the classical G protein second messenger-dependent pathways. One such mechanism involves ␤-arrestins, a small family of cytosolic proteins initially identified for their central role in 7TMR desensitization. ␤-Arrestins are recruited to agonist-occupied 7TMRs that have been phosphorylated by specialized G protein-coupled receptor kinases (GRKs) and sterically inhibit receptor-G protein coupling resulting in homologous receptor desensitization. Additionally, ␤-arrestins act as adaptors in clathrin-mediated receptor endocytosis (14, 15). The role of ␤-arrestins acting as signal transducers through the formation of scaffolding complexes with accessory effector molecules such as Src, Ras, ERK1/2, JNK3, and MAPK kinase 4 (MKK4) is becoming increasingly recognized (16 -20).The potential signaling diversity of 7TMRs suggests the possible existence of multiple discrete "active" receptor conformations. This implies that specific ligands might direct distinct signaling responses by preferentially stabilizing one or more of these active conformations. In the simple two-state model of receptor activation, agonists are defined as drugs that stabilize the active receptor conformation, which in turn promotes G protein activation. Conversely, an inverse agonist preferentially binds to the inactive receptor conformational state thereby reduc-

Physical Interaction of Calmodulin with the 5-Hydroxytryptamine2C Receptor C-Terminus Is Essential for G Protein-independent, Arrestin-dependent Receptor Signaling

“…Although ␤-arrestin 2 was identified as the predominant ␤-arrestin isoform interacting with 5-HT 2C receptors (Marion et al, 2004), depletion of either ␤-arrestin 1 or ␤-arrestin 2 by RNA interference strongly reduced 5-HT 2C receptor-mediated ERK1,2 phosphorylation. These results are coherent with several reports indicating that both ␤-arrestin isoforms are required for G protein-independent ERK1,2 signaling mediated by various GPCRs such as ␤ 2 -adrenergic and parathyroid hormone type 1 receptors (Gesty-Palmer et al, 2006;Shenoy et al, 2006). Nonetheless, activation of ERK1,2 by other GPCRs, including angiotensin II type 1a and V2 receptors, is critically dependent on ␤-arrestin 2, whereas ␤-arrestin 1 has a minor influence or an antagonistic action on ERK1,2 signaling (Ahn et al, 2004;Ren et al, 2005), indicating that specificity of ␤-arrestin isoforms for ERK signaling is dependent on the GPCR.…”

Distinct conformational changes in β-arrestin report biased agonism at seven-transmembrane receptors

“…We have previously reported that stimulation of PTH1R with PTH-(1-34) leads to both G protein-dependent and ␤-arrestin-dependent signaling, whereas stimulation with the PTH analogue [D-Trp 12 ,Tyr 34 ]PTH-(7-34), which is an inverse agonist (PTH-IA), results in only ␤-arrestin signaling (19). As shown in Fig.…”

Molecular and cellular mechanisms of the anabolic effect of intermittent PTH

“…PTHR1 also stimulates Gα q -mediated activation of protein kinase C (PKC). In addition, PTH activates β-arrestin-mediated activation of extracellular regulated kinase (ERK) signaling [39,40]. β-arrestin is also involved in desensitization of cAMP signaling by PTHR1.…”