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.
All Science Journal Classification (ASJC) codes
- Calcium mobilization
- Inositol 1,4,5-trisphosphate receptors
- Mesangial cells
- Transforming growth factor-β1