These findings demonstrate that AT2-R is re-expressed by cardiac fibroblasts present in fibrous regions in failing CM hearts and that the increased AT2-R exerts an anti-AT1-R action on the progression of interstitial fibrosis during cardiac remodeling by inhibiting both fibrillar collagen metabolism and growth of cardiac fibroblasts.

Abstract-The expression pattern of angiotensin (Ang) II type 2 receptor (AT 2 -R) in the remodeling process of human left ventricles (LVs) remains poorly defined. We analyzed its expression at protein, mRNA, and cellular levels using autopsy, biopsy, or operation LV samples from patients with failing hearts caused by acute (AMI) or old (OMI) myocardial infarction and idiopathic dilated cardiomyopathy (DCM) and also examined functional biochemical responses of failing hearts to Ang II. In autopsy samples from the nonfailing heart group, the ratio of AT 1 -R and AT 2 -R was 59% and 41%, respectively. The expression of AT 2 -R was markedly increased in DCM hearts at protein (3.5-fold) and mRNA (3.1-fold) levels compared with AMI or OMI. AT 1 -R protein and mRNA levels in AMI hearts showed 1.5-and 2.1-fold increases, respectively, whereas in OMI and DCM hearts, AT 1 -R expression was significantly downregulated. AT 1 -R-mediated response in inositol phosphate production was significantly attenuated in LV homogenate from failing hearts compared with nonfailing hearts. AT 2 -R sites were highly localized in the interstitial region in either nonfailing or failing heart, whereas AT 1 -R was evenly distributed over myocardium at lower densities. Mitogen-activated protein kinase (MAPK) activation by Ang II was significantly decreased in fibroblast compartment from the failing hearts, and pretreatment with AT 2 -R antagonist caused an additional significant increase in Ang II-induced MAPK activity (36%). Cardiac hypertrophy suggested by atrial and brain natriuretic peptide levels was comparably increased in OMI and DCM, whereas accumulation of matrix proteins such as collagen type 1 and fibronectin was much more prominent in DCM than in OMI. These findings demonstrate that (1) AT 2 -R expression is upregulated in failing hearts, and fibroblasts present in the interstitial regions are the major cell type responsible for its expression, (2) AT 2 -R present in the fibroblasts exerts an inhibitory effect on Ang II-induced mitogen signals, and (3) AT 1 -R in atrial and LV tissues was downregulated during chronic heart failure, and AT 1 -R-mediated functional biochemical responsiveness was decreased in the failing hearts. Thus, the expression level of AT 2 -R is likely determined by the extent of interstitial fibrosis associated with heart failure, and the expression and function of AT 1 -R and AT 2 -R are differentially regulated in failing human hearts. (Circ Res. 1998;83:1035-1046.)Key Words: angiotensin II type 2 receptor Ⅲ AT 2 receptor Ⅲ angiotensin II type 1 receptor Ⅲ AT 1 receptor, angiotensin II T he presence of 2 isoforms of angiotensin (Ang) II receptor was originally proposed on the basis of differences in sensitivity of receptor-ligand binding to dithiothreitol. Ang type 2 receptor (AT 2 -R), which is insensitive to dithiothreitol and has a high affinity for PD123319 and CGP42112A, was isolated, and this receptor was shown to have the same seventransmembrane domain of AT 1 -R but only minimal homology (see Review i...

Mechanical stress plays a pivotal role in the development of cardiac hypertrophy during hemodynamic overload, and angiotensin (Ang) II secreted from stretched myocytes plays an important role in mechanical stretch-induced hypertrophy. In the present study, we examined stretch-induced expression of Ang II receptors in an in vitro stretch model using 1-day-old rat myocytes. Both Ang II type 1 receptor (AT1-R) and type 2 receptor (AT2-R) mRNA levels were upregulated by myocyte stretching with similar time courses: significant increases were evident 6 hours after stretching, maximal levels (2.8- and 3.3-fold, respectively) were observed at 12 hours, and these were sustained for up to 18 hours. Ang II receptor expression in fibroblast-rich cultures was not affected by stretching. Conditioned medium in which myocytes were stretched for 12 hours significantly downregulated AT1-R and AT2-R mRNA levels in recipient myocytes, and this effect was almost completely blocked by AT1-R antagonists but not AT2-R antagonists. Stretch-induced expression of AT1-R and AT2-R mRNAs was further increased by 27% and 31%, respectively, after pretreatment with AT1-R antagonists, suggesting that Ang II secreted from stretched myocytes downregulates both AT1-R and AT2-R. Western blot and binding assays showed that the number of AT1-Rs and AT2-Rs increased by 2.4- and 2.6-fold, respectively, without affecting receptor affinities. Inositol phosphate response to 0.5 mumol/L Ang II was enhanced 2.1-fold in stretched myocytes. Nuclear runoff assays and treatment with actinomycin D revealed that stretch-induced upregulation of AT1-R was mainly due to increased transcription, whereas that of AT2-R resulted from a stabilizing effect on AT2-R mRNA metabolism. Stretch-induced changes in levels of Ang II receptors were inhibited by genistein but not by H-7, staurosporin, and protein kinase C depletion or by BAPTA-AM. Exposure to cycloheximide did not affect stretch-induced changes. These findings indicate that nonsecretory pathways activated by myocyte stretching upregulate the expression of Ang II receptor subtypes transcriptionally and posttranscriptionally through mechanisms involving stretch-activated tyrosine kinases independently of de novo protein synthesis and that the AT1-R-mediated action of Ang II is functionally enhanced in stretched cardiac myocytes.