Parathyroid Hormone Activates Mitogen-activated Protein Kinase via a cAMP-mediated Pathway Independent of Ras

Abstract

In a previous study, we demonstrated that parathyroid hormone (PTH) inhibits mitogen-activated protein (MAP) kinase activation in osteosarcoma cells via a protein kinase A-dependent pathway. Here, we show that PTH can induce a transient activation of MAP kinase as well. This was observed in both Chinese hamster ovary R15 cells stably expressing high levels of rat PTH/PTHrelated peptide receptor and parietal yolk sac carcinoma cells expressing the receptor endogenously. PTH was a strong activator of adenylate cyclase and phospholipase C in Chinese hamster ovary R15 cells. PTHinduced MAP kinase activation did not depend on activation of G i , phorbol ester-sensitive protein kinase C, elevated intracellular calcium levels, or release of G␤␥ subunits. It could, however, be mimicked by addition of forskolin or 8-bromo-cAMP to these cells. Prolonged treatment with forskolin caused sustained protein kinase A activity, whereas MAP kinase activity returned to basal levels. Subsequent treatment with PTH or 8-bromo-cAMP did not result in MAP kinase activation, whereas phorbol ester-or insulin-induced MAP kinase activation was unaffected. Finally, expression of a dominant negative form of Ras (Ras Asn-17 ), which completely blocked insulin-induced MAP kinase activation, did not affect activation by PTH or cAMP. In conclusion, PTH regulates MAP kinase activity in a cell type-specific fashion. The activation of MAP kinase by PTH is mediated by cAMP and independent of Ras.Mitogen activated protein (MAP) 1 kinases are protein serine and threonine kinases that play an important role in the regulation of cell growth and differentiation (1-3). The activity of MAP kinase is under the control of external stimuli that mediate their effects by binding to cell surface receptors. Protein tyrosine kinase receptors transduce the signal by autophosphorylation of tyrosine residues, allowing the receptor to interact with Src homology 2 domain-containing proteins, such as Grb2, which will recruit son of sevenless, resulting in the activation of Ras. This will cause the successive activation of Raf-1, MAP kinase kinase (MEK), and MAP kinase (4).G protein-coupled receptors (GPCRs) regulate MAP kinase activity, depending on the identity of the G protein, the receptor, and the cell type involved. G i -coupled receptors, such as the M2 muscarine acetylcholine receptor, the ␣ 2 -adrenergic receptor, or the receptors for lysophosphatidic acid (LPA) or thrombin, stimulate MAP kinase in a Ras-dependent manner (5-7). Recent reports have shown that MAP kinase activation through G i involves the release of G␤␥ subunits (8 -11), which via an as yet unidentified tyrosine kinase induce the phosphorylation of Shc, leading to the formation of a Shc-Grb2-son of sevenless complex (12, 13) and activation of Ras.For receptors coupled to G q , such as the M1 acetylcholine receptor, the ␣ 1 -adrenergic receptor, or the bombesin receptor, both ␤␥ subunit-dependent (10) and ␤␥ subunit-independent (8, 11) activation of MAP kinase has been reported. The ␤␥ subunit-indu...

The cytoplasmic domain close to the transmembrane region of the glucagon-like peptide-1 receptor contains sequence elements that regulate agonist-dependent internalisation

“…Similar results have been reported previously in both RIN and CHO-GLPR cells (Montrose-Rafizadeh et al 1999). Other members of this family of G-protein-coupled receptors, such as PTH receptors, also undergo increases in the phospho-ERK-1 form following agonist stimulation (Verheijen & Defize 1997). The effect observed in cells expressing GLPR 435R and GLPR 418R mutant receptors was similar, but the increase in phosphorylated forms of ERK-1/ERK-2 persisted longer.…”

Parathyroid Hormone–Related Protein Induces Insulin Expression Through Activation of MAP Kinase–Specific Phosphatase-1 That Dephosphorylates c-Jun NH2-Terminal Kinase in Pancreatic β-Cells

“…2). Because PTHrP is known to interact with the MAP kinase pathway (16,17) and the cAMP pathway has been reported to interact with this pathway in an enhancing or suppressing manner depending on the cell types (12-15), we examined the involvement of the MAP kinases in insulin expression by PTHrP using MAP kinase pathway inhibitors. We initially used two inhibitors: PD98059 as a MEK inhibitor and SB203580 as a SAPK (p38 and JNK) inhibitor.…”

“…In contrast, cAMP elevation by EGF or nerve growth factor further enhances ERK1/2 MAP kinase activation in PC12 cells and LNCaP prostate carcinoma cells (14,15). The effect of PTHrP on MAP kinase activation has been little studied, except for studies on PTHrP-inhibited EGF-dependent ERK2 MAP kinase activation in the osteoblastoid cells UMR 106 and ROS17/2.8 (16) and on activated MAP kinase p42 in Chinese hamster ovary R15 cells (17).…”

Glucose-dependent Insulinotropic Polypeptide Activates the Raf-Mek1/2-ERK1/2 Module via a Cyclic AMP/cAMP-dependent Protein Kinase/Rap1-mediated Pathway

“…Therefore, it may be expected that cAMP can positively influence ERK1/2 activity and cellular fate in both cell types. cAMP regulation of ERK1/2 activity in CHO cells has been controversial, with studies showing both activating and inhibitory affects (51,54,55). A previous report (13) investigating GIP signaling demonstrated an inhibitory effect of cAMP on ERK1/2 activity in CHO cells.…”