Angiotensin II receptors: protein and gene structures, expression and potential pathological involvements

Two distinct types of cell-surface angiotensin II receptors (AT1 and AT2) have been defined pharmacologically and cDNAs encoding each type have been identified by expression cloning. These pharmacological studies showed the AT1 receptors to mediate all the known functions of angiotensin II in regulating salt and fluid homeostasis. Further complexity in the angiotensin II receptor system was revealed when homology cloning showed the existence of two AT1 subtypes in rodents and in situ hybridization and reverse transcription-polymerase chain reaction analyses showed their level of expression to be regulated differently in different tissues: AT1A is the principal receptor in the vessels, brain, kidney, lung, liver, adrenal gland and fetal pituitary, while AT1B predominates in the adult pituitary and is only expressed in specific regions of the adrenal gland (zona glomerulosa) and kidney (glomeruli). Expression of AT1A appears to be induced by angiotensin II in vascular smooth-muscle cells but is inhibited in the adrenal gland. Preliminary analysis of the AT1 promoters is also suggestive of a high degree of complexity in their regulation. Investigation of a potential role for altered AT1 receptor function has commenced at a genetic level in several diseases of the cardiovascular system. No mutations affecting the coding sequence have been identified in Conn adenoma and no linkage has been demonstrated with human hypertension by sib-pair analysis. None the less, certain polymorphisms that do not alter the protein structure have been found to be associated with hypertension and to occur at an increased frequency in conjunction with specific polymorphisms in the ACE gene in individuals at increased risk for myocardial infarction. Further characterization of the regions of the AT1 gene that regulate its expression are therefore needed. The physiological importance of the AT2 gene product still remains a matter of debate.