Age-related decline in mitogen-activated protein kinase phosphorylation in PTH-stimulated rat enterocytes

In the present study we analyzed whether parathyroid hormone (rPTH[1-34]; PTH) stimulates the tyrosine phosphorylation of the growth-related protein mitogen-activated protein (MAP) kinases (p42/44-MAPK), also known as extracellular signal-regulated kinases (ERK1/2), in duodenal enterocytes isolated from young (3 months) and aged (24 months) rats. Western blot analysis revealed that PTH rapidly stimulates MAPK phosphorylation. The hormone effects on MAPK were evident within 30 s, peaking at 1 min (4-fold). PTH response was dose-dependent (10(-11)-10(-7) M) with maximal stimulation achieved at 10(-9)-10(-8) M. PTH-induced MAPK phosphorylation was effectively suppressed by the tyrosine-kinase inhibitors, genistein (100 muM) and herbimycin (2 muM). Moreover, the tyrosine phosphorylation and activation of MAPK was dependent on Src kinase, since PP1 (10 and 20 muM), a specific Src family tyrosine-kinase inhibitor, blacked PTH-induced MAPK activation. With aging, the response to PTH was significantly reduced. However, The amount of basal protein expression determined by Western blot analysis for MAPK was not different in the enterocytes from young and aged rats. In conclusion, the results obtained in this work expand our knowledge on the mechanism of action of PTH in duodenal cells, revealing that protein tyrosine phosphorylation is linked to the PTH regulation of enterocyte MAPK activation, and that this mechanism is impaired with aging. Understanding the molecular mechanisms for the age-related differences in PTH signaling will require more information about the subtle mechanisms that modulate the PTH receptor-MAPK signaling pathway. (C) 2000 Elsevier Science Inc. All rights reserved.