Inhibition of type I and III IP₃Rs by TGF-β is associated with impaired calcium release in mesangial cells

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) mediate cytosolic free calcium concentration ([Ca²⁺](c)) signals in response to a variety of agonists that stimulate mesangial cell contraction and proliferation. In the present study, we demonstrate that mesangial cells express both type I and III IP₃Rs and that these receptors occupy different cellular locations. Chronic treatment with transforming growth factor-β1 (TGF-β1; 10 ng/ml, 24 h) leads to downregulation of both type I and III IP₃Rs as measured by immunoblot and confocal analysis. TGF-β1 treatment does not affect IP₃ levels, 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 presence or absence of TGF-β1. Functional effects of TGF-β1-induced downregulation of the IP₃Rs were evaluated by measuring [Ca²⁺](c) changes in response to epidermal growth factor (EGF) in intact cells and sensitivity of [Ca²⁺](c) release to IP₃ in permeabilized cells. TGF-β1 pretreatment led 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-sensitive [Ca²⁺](c) stores were not changed, as assessed by stimulation with maximal doses of IP₃ (10.5 μm) and thapsigargin-mediated calcium release in permeabilized cells. We conclude that prolonged exposure to TGF-β1 leads to downregulation of both type I and III IP₃Rs in mesangial cells and this is associated with impaired sensitivity to IP₃.