Angiotensin II-induced down-regulation of inositol trisphosphate receptors in WB rat liver epithelial cells - Evidence for involvement of the proteasome pathway

Chronic stimulation of WB rat liver epithelial cells by angiotensin II (Ang II) resulted in the down-regulation of both type I and type III myo-inositol 1,4,5-trisphosphate receptors (IP(3)Rs), Stimulation with vasopressin, bradykinin, epidermal growth factor, or 12-O-tetradecanoylphorbol-13-acetate was without effect. Ang II-induced down-regulation of IP(3)Rs could be detected within 2 h and resulted in an inhibition of IP3-induced Ca2+ release from permeabilized cells, IP3R down-regulation was reversible, and both home- and heterooligomers of IP(3)Rs were equally susceptible to Ang II-induced degradation. Chloroquine and NH4Cl increased the basal levels of IP(3)Rs by a-fold, suggesting that the basal turnover of IP(3)Rs occurs via a lysosomal pathway, However, Ang II-induced degradation of IP3R was not affected by these inhibitors, suggesting that stimulated degradation of IP(3)Rs occurs via a non-lysosomal pathway, The cysteine protease and proteasomal inhibitor N-acetyl-Leu-Leu-norleucinal completely prevented Ang II-mediated down-regulation of IP(3)Rs, whereas the structural analog N-acetyl-Leu-Leu-methioninal was without effect, Lactacystin, a highly specific proteasome inhibitor, also blocked Ang II-mediated IP3R degradation, Stimulation with Ang II increased the amount of LP3R immunoprecipitated by anti-ubiquitin antibodies, We conclude that Ang II-stimulated IP3R degradation involves enhanced ubiquitination of the protein and degradation by the proteasome pathway.