HORMONAL-REGULATION OF ANGIOTENSIN-II TYPE-1 RECEPTOR EXPRESSION AND AT(1)-R MESSENGER-RNA LEVELS IN HUMAN ADRENOCORTICAL-CELLS

Human adrenocortical H295R cells express AII receptors which are predominantly of the AT(1) but not AT(2) 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 AT(1)-R mRNA levels which in turn preceded a time-dependent (maximal by 12 h) and dose-dependent loss of [I-125]AII binding and phosphoinositidase G activation on subsequent AII challenge. Thus, both decreased AT(1)-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 G pathways by treatment with AII also caused a rapid (maximal by 3 h) and dose-dependent loss in AT(1)-R mRNA, but mRNA levels subsequently rose again, approaching control levels by 36 h. Treatment with All for 48 h had little effect on either [I-125]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 AT(1)-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 K+, also resulted in a decrease in AT(1)-R mRNA levels. Thus in the H295R cell, control of AT(1)-R expression appears to be complex, being achieved at least in part through control of the level of AT(1)-R mRNA by multiple independent signaling pathways including protein kinase A, protein kinase C and Ca2+.