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Parathyroid hormone (PTH), a major physiologic regulator  of proximal renal tubule cell sodium-phosphate cotransport, stimulates several  signal transduction pathways including extracellular signal-regulated kinases  (ERK). The physiologic role of PTH-stimulated ERK is unknown. The purpose of  the present study was to identify signaling components involved in  PTH-stimulated ERK activity and to determine the role of PTH-stimulated ERK  activity in regulation of phosphate transport. PTH-stimulated ERK activity was  measured in opossum kidney (OK) cell lysates as phosphorylation of myelin  basic protein by an  in vitro  kinase assay. PTH stimulated a  dose-dependent increase in ERK activity with a peak at 10 -7  M. The  time course was biphasic with an early peak at 10 min and a later peak at 20  min. Pretreatment of OK cells with the nonreceptor tyrosine kinase inhibitors  genistein and herbimycin A or with the phosphatidylinositol 3-kinase (PI-3K)  inhibitors wortmannin and LY294002 blocked the early and late peaks of  PTH-stimulated ERK activity. Pretreatment with the protein kinase C inhibitor  calphostin C blocked only the later phase of PTH-stimulated ERK. To determine  the role of ERK in regulation of phosphate transport, PTH inhibition of  phosphate uptake and PTH regulation of sodium-phosphate cotransporter (NaPi-4)  expression were measured in OK cells pretreated with the MEK inhibitor  PD098059. PD098059 significantly attenuated PTH inhibition of phosphate uptake  but did not prevent PTH downregulation of NaPi-4. It is concluded that PTH  stimulates ERK through two signal transduction pathways: an early pathway  dependent on tyrosine kinase and PI-3K and a late pathway dependent on protein  kinase C. PTH-stimulated ERK regulates phosphate transport by a mechanism  other than downregulation of NaPi-4 expression.

Parathyroid Hormone Stimulates Extracellular Signal-Regulated Kinase (ERK) Activity Through Two Independent Signal Transduction Pathways