Hormonal regulation of angiotensin II type 1 receptor expression and AT₁-R mRNA levels in human adrenocortical cells

Abstract

Human adrenocortical H295R cells express AII receptors which are predominantly of the AT₁ but not AT₂ subclass. These receptors are functionally coupled to phosphoinositidase C in a manner similar to that seen in fetal human, sheep and bovine adrenocortical cells. Treatment of H295R cells with forskolin or dbcAMP to activate the protein kinase A pathway caused a rapid (maximal by 3 h) and sustained decrease in AT₁-R mRNA levels which in turn preceded a time-dependent (maximal by 12 h) and dose-dependent loss of [¹²⁵I]AII binding and phosphoinositidase C activation on subsequent AII challenge. Thus, both decreased AT₁-R mRNA levels and functional receptor expression appear to parallel each other in response to activation of protein kinase A. Activation of the Ca²⁺/protein kinase C pathways by treatment with AII also caused a rapid (maximal by 3 h) and dose-dependent loss in AT₁-R mRNA, but mRNA levels subsequently rose again, approaching control levels by 36 h. Treatment with AII for 48 h had little effect on either [¹²⁵I]AII binding or the subsequent phosphoinositidase C response. The effect of AII, but not forskolin, was blocked by the presence of cycloheximide. The action of AII on AT₁-R mRNA was probably mediated through both protein kinase C and Ca²⁺-sensitive protein kinases as the effect at 4 h was not completely reproduced by phorbol ester alone, but was fully reproduced by a combination of phorbol ester and Ca²⁺ ionophore. However, increased Ca²⁺ influx alone, due to treatment with BAYK8644 or elevated extracellular KI, also resulted in a decrease in AT₁-R mRNA levels. Thus in the H295R cell, control of AT₁-R expression appears to be complex, being achieved at least in part through control of the level of AT₁-R mRNA by multiple independent signaling pathways including protein kinase A, protein kinase C and Ca²⁺.