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-tetradecano ylphorbol-13-acetate was without effect. Ang II-induced down-regulatio n of IP(3)Rs could be detected within 2 h and resulted in an inhibitio n of IP3-induced Ca2+ release from permeabilized cells, IP3R down-regu lation was reversible, and both home- and heterooligomers of IP(3)Rs w ere equally susceptible to Ang II-induced degradation. Chloroquine and NH4Cl increased the basal levels of IP(3)Rs by a-fold, suggesting tha t the basal turnover of IP(3)Rs occurs via a lysosomal pathway, Howeve r, Ang II-induced degradation of IP3R was not affected by these inhibi tors, suggesting that stimulated degradation of IP(3)Rs occurs via a n on-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 protea some inhibitor, also blocked Ang II-mediated IP3R degradation, Stimula tion with Ang II increased the amount of LP3R immunoprecipitated by an ti-ubiquitin antibodies, We conclude that Ang II-stimulated IP3R degra dation involves enhanced ubiquitination of the protein and degradation by the proteasome pathway.