Angiotensin II receptor subtypes AT1 and AT2 are down-regulated by angiotensin II through AT1 receptor by different mechanisms

The regulatory effects of angiotensin II (AngII) on its receptor subtypes, AT(1) and AT(2), were studied using cultured bovine adrenal cells (BAG), which express both receptor subtypes, and PC12W and R3T3 cells, which express only AT(2) receptors. In BAG, AngII caused a decrease in AT(1)- and AT(2)-binding sites and their corresponding messenger RNAs (mRNAs), but with different kinetics. AT(1)-binding sites decreased by more than 50% within the first 3 h, whereas AT(1) mRNA started to decline after a lag period of 3 h. Both AT(2)-binding sites and mRNA remained stable within the first 6 h of AngII treatment. Then, AT(2) mRNA decreased rapidly with an apparent half-life of 2-3 h, whereas AT(2)-binding sites declined with an apparent half-life of about 16 h. Measurement of transcription rate and mRNA half-life by the [H-3]uridine-thiouridine method revealed that AngII reduced by 90% the rate of AT(1) transcription, but had no effect on AT(1) mRNA half-life, whereas it slightly reduced AT(2) transcription, but markedly reduced AT(2) mRNA stability. All of the effects of AngII on both AT(1) and AT(2) receptors were blocked by losartan, indicating that they were mediated exclusively through the AT(1) receptor. In PC1PW cells, AngII was unable to modify AT(2)-binding sites or mRNA. Moreover, in BAG, [I-125]AngII was internalized through the AT(1) receptor, whereas occupancy of AT(2) receptors in either BAC or PC12W did not produce internalization of the hormone. These results indicate that AngII, through the AT1(r)eceptor, down-regulates both AT(1) and AT(2), but by different mechanisms; AT1 receptor is regulated through internalization-degradation of the occupied receptor and inhibition of transcription, whereas AT2 receptor is regulated mainly by decreasing the stability of its mRNA. Moreover, the phorbol ester phorbol 12-myristate 13-acetate mimicked most of the effects of AngII in BAC and decreased both AT(2)-binding sites and mRNA on PC12W cells, indicating that the hormonal regulation of both AT(1) and AT(2) receptors is mediated through protein kinase C activation.