Down-regulation of inositol 1,4,5-trisphosphate receptor in cells stably expressing the constitutively active angiotensin II N111G-AT(1) receptor

The diverse cellular changes brought about by the expression of a constitutively active receptor are poorly understood. QBI-human embryonic kidney 293A cells stably expressing the constitutively active N111G-AT(1) receptor (N111G cells) showed elevated levels of inositol phosphates and frequent spontaneous intracellular Ca(2+) oscillations. Interestingly, Ca(2+) transients triggered with maximal doses of angiotensin II were much weaker in N111G cells than in wild-type cells. These blunted responses were observed independently of the presence or absence of extracellular Ca(2+) and were also obtained when endogenous muscarinic and purinergic receptors were activated, revealing a heterologous desensitization process. The desensitized component of the Ca(2+) signaling cascade was neither the G protein G(q) nor phospholipase C. The intracellular Ca(2+) store of N111G cells and their mechanism of Ca(2+) entry also appeared to be intact. The most striking adaptive response of N111G cells was a down-regulation of their inositol 1,4,5-trisphosphate receptor (IP(3)R) as revealed by reduced IP(3)-induced Ca(2+) release, lowered (3)H]IP(3) binding capacity, diminished IP(3)R immunoreactivity, and accelerated IP(3)R degradation involving the lysosomal pathway. Treatment with the inverse agonist EXP3174 reversed the desensitized phenotype of N111G cells. Down-regulation of IP(3)R represents a reversible adaptive response to protect cells against the adverse effects of constitutively active Ca(2+)-mobilizing receptors.