Inhibition of type I and IIIIP(3)Rs by TGF-beta is associated with impaired calcium release in mesangial cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) mediate cytosolic free cal cium concentration ([Ca2+](c)) signals in response to a variety of agonists that stimulate mesangial cell contraction and proliferation. In the presen t study, we demonstrate that mesangial cells express both type I and III IP (3)Rs and that these receptors occupy different cellular locations. Chronic treatment with transforming growth factor-beta 1 (TGF-beta 1; 10 ng/ml, 24 h) leads to downregulation of both type I and III IP(3)Rs as measured by i mmunoblot and confocal analysis. TGF-beta 1 treatment does not affect IP3 l evels, and downregulation of type I IP3R is not due to enhanced degradation of the protein, as the half-life of type I IP3R is unchanged in the presen ce or absence of TGF-beta 1. Functional effects of TGF-beta 1-induced downr egulation of the IP(3)Rs were evaluated by measuring [Ca2+](c) changes in r esponse to epidermal growth factor (EGF) in intact cells and sensitivity of [Ca2+](c) release to IP3 in permeabilized cells. TGF-beta 1 pretreatment l ed to a significant decrease of [Ca2+](c) release induced by EGF in intact cells and by submaximal IP3 (400 nm) in permeabilized cells. Total IP3-sens itive [Ca2+](c) stores were not changed, as assessed by stimulation with ma ximal doses of IP3 (10.5 mu m) and thapsigargin-mediated calcium release in permeabilized cells. We conclude that prolonged exposure to TGF-beta 1 lea ds to downregulation of both type I and III IP(3)Rs in mesangial cells and this is associated with impaired sensitivity to IP3.