Effect of ageing in the early biochemical signals elicited by parathyroid hormone in intestinal cells

GENTILI CLAUDIA

Caxambú, Brasil

Simposio; Congreso anual de la Sociedad Brasilera de Bioquímica e Biologia Molecular (SBBq); 2004

Sociedad Brasilera de Bioquímica e Biologia Molecular (SBBq)

Parathyroid hormone (PTH) is an 84-amino-acid polypeptide hormone functioning as a major mediator of bone remodeling and as an essential regulator of calcium homeostasis. Although the effects of PTH in bone cells is well characterized, the hormone action in rat intestinal cells is not completely defined. Previous studies demonstrated that in rat enterocytes, signaling through PTH includes stimulation of adenylyl cyclase, increases of intracellular calcium and activation of phospholipase C, mechanisms that suffer alterations with ageing. In the present study we first examined the localization, expression and binding properties of the PTH receptor type 1 (PTHR1) in enterocytes isolated from 3 and 24 month old rats by several analysis and the results showed that the affinity of the receptor for PTH was not affected by age but the protein expression of PTHR1 was reduced in the aged rat. Furthermore, we demonstrated that activation of PTHR1 in these cells rapidly and transiently leads to phosphorylation and activation of a number of intracellular proteins, the most prominent are the mitogen-activated protein kinases (MAPK) ERK1 and ERK2 (which leads to an increase in DNA synthesis), phosphoinositide 3-kinase (PI3K), which plays an important role in mitogenesis and cytosolic phospholipase A2 (cPLA2) which catalyzes the hydrolysis of membrane glycerophospholipids to liberate arachidonic acid (AA) and lysophosholipids (both represent a class of lipid mediators and could also play a role in proliferation). Our results revealed that PI3K contributes to MAPK phosphorylation by PTH. With ageing, we analysed alterations in PI3K activity and response to PTH. Moreover, although the relative levels of MAPK did not change with age, the magnitude of PTH-dependent MAPK phosphorylation was significantly lower in enterocytes of aged rats compared with those of young animals. We found that PTH stimulate AA release in rat duodenal cells, effect that is greatly potentiated by ageing. We also found that hormone-induced AA release in young animals is Ca2+-dependent via cPLA2, while AA released by PTH in cells from aged rats is due to the activation of cPLA2 and the Ca2+-independent PLA2 (iPLA2). The increased cPLA2 activity in these stimulated cells is attributed in part to phosphorylation by MAPK. The results of this study suggest that age-related declines in PTH regulation of signal transduction pathways in rat enterocytes may be due, in part, to the loss of hormone receptors. Impairment of PTH activation of ERK 1/ 2, PI3K and cPLA2 activity upon ageing may result in abnormal proliferation in the duodenum and the source of several diseases, such as osteoporosis. <!--[if !supportEmptyParas]--> <!--[endif]-->