Abstract
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-AT1 receptor (N111G cells) showed elevated levels of inositol phosphates and frequent spontaneous intracellular Ca2+ oscillations. Interestingly, Ca2+ 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 Ca2+ and were also obtained when endogenous muscarinic and purinergic receptors were activated, revealing a heterologous desensitization process. The desensitized component of the Ca2+ signaling cascade was neither the G protein Gq nor phospholipase C. The intracellular Ca2+ store of N111G cells and their mechanism of Ca2+ 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 (IP3R) as revealed by reduced IP3-induced Ca2+ release, lowered [3H]IP3 binding capacity, diminished IP3R immunoreactivity, and accelerated IP3R degradation involving the lysosomal pathway. Treatment with the inverse agonist EXP3174 reversed the desensitized phenotype of N111G cells. Down-regulation of IP3R represents a reversible adaptive response to protect cells against the adverse effects of constitutively active Ca2+-mobilizing receptors.
Down-regulation of the Inositol 1,4,5-Trisphosphate Receptor in Mouse Eggs Following Fertilization or Parthenogenetic Activation
