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proteins and c-SRC tyrosine kinase in parathyroid hormone-induced signal transduction in rat enterocytes
Departamento de Biología, Bioquímica y Farmacia, Universidad Nacional del Sur, San Juan 670, 8000 Bahia Blanca, Argentina
(Requests for offprints should be addressed to A R de Boland; Email: aboland{at}criba.edu.ar)
Parathyroid hormone (PTH) interacts in target tissues with a G protein-coupled receptor (GPCR) localized in the plasma membrane. Although activation of GPCR can elicit rapid stimulation of cellular protein tyrosine phosphorylation, the mechanism by which G proteins activate protein-tyrosine kinases is not completely understood. In the present work, we demonstrate that PTH rapidly increases the activity of non-receptor tyrosine kinase c-Src in rat intestinal cells (enterocytes). The response is biphasic, the early phase is fast and transient, peaking at 30 s (+120%), while the second phase progressively increases up to 5 min (+220%). The hormone activates c-Src in intestinal cells through fast changes in tyrosine phosphorylation of the enzyme. The first event in the activation of c-Src is the dephosphorylation of Tyr527 (which happens after a few seconds of PTH treatment), followed by a second event of activation with phosphorylation at Tyr416 (+twofold, 5 min). Removal of external Ca2+ (EGTA, 0.5 mM) and chelation of intracellular Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetracetic acid acetoxymethyl ester (BAPTA) (5 µM) suppressed Tyr527 dephosphorylation and Tyr416 phosphorylation, indicating that Ca2+ is an upstream activator of c-Src in enterocytes stimulated with PTH. The G protein subunits, G
s and Gß, are associated with c-Src in basal conditions and this association increases two- to threefold in cells treated with PTH. Blocking of Gß subunits by preincubation of cells with a Gß antibody abolished hormone-dependent c-Src Tyr416 phosphorylation and ERK1/ERK2 activation. The results of this work indicate that PTH activates c-Src in intestinal cells through conformational changes via G proteins and calcium-dependent modulation of tyrosine phosphorylation of the enzyme, and that PTH receptor activation leads via Gß
–c-Src to the phosphorylation of the MAP kinases, ERK1 and ERK2.
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