The increase in ACE2 after MI suggests that it plays an important role in the negative modulation of the renin angiotensin system in the generation and degradation of angiotensin peptides after cardiac injury.

Background: Angiotensin converting enzyme (ACE) 2 is a recently identified homologue of ACE that may counterregulate the actions of angiotensin (Ang) II by facilitating its breakdown to Ang 1-7. The reninangiotensin system (RAS) has been implicated in the pathogenesis of cirrhosis but the role of ACE2 in liver disease is not known. Aims: This study examined the effects of liver injury on ACE2 expression and activity in experimental hepatic fibrosis and human cirrhosis, and the effects of Ang 1-7 on vascular tone in cirrhotic rat aorta. Methods: In sham operated and bile duct ligated (BDL) rats, quantitative reverse transcriptase-polymerase chain reaction was used to assess hepatic ACE2 mRNA, and western blotting and immunohistochemistry to quantify and localise ACE2 protein. ACE2 activity was quantified by quenched fluorescent substrate assay. Similar studies were performed in normal human liver and in hepatitis C cirrhosis. Results: ACE2 mRNA was detectable at low levels in rat liver and increased following BDL (363-fold; p,0.01). ACE2 protein increased after BDL (23.5-fold; p,0.05) as did ACE2 activity (fourfold; p,0.05). In human cirrhotic liver, gene (.30-fold), protein expression (97-fold), and activity of ACE2 (2.4 fold) were increased compared with controls (all p,0.01). In healthy livers, ACE2 was confined to endothelial cells, occasional bile ducts, and perivenular hepatocytes but in both BDL and human cirrhosis there was widespread parenchymal expression of ACE2 protein. Exposure of cultured human hepatocytes to hypoxia led to increased ACE2 expression. In preconstricted rat aorta, Ang 1-7 alone did not affect vascular tone but it significantly enhanced acetylcholine mediated vasodilatation in cirrhotic vessels. Conclusions: ACE2 expression is significantly increased in liver injury in both humans and rat, possibly in response to increasing hepatocellular hypoxia, and may modulate RAS activity in cirrhosis.

The metallopeptidase angiotensin-converting enzyme (ACE) plays a pivotal role in the cardiovascular system by generating the vasoconstrictor peptide angiotensin II. A homolog of ACE with different substrate specificity, ACE2, has recently been cloned that shows an expression pattern restricted to endothelial cells of the heart and kidney, epithelial cells of the distal tubule of the kidney, and the testis. Although the importance of ACE2 to cardiac function is already evident, its role in the testis remains unknown. In this study, we report the cloning and expression of human testicular ACE2 and confirm that it is identical to the somatic form of the enzyme. ACE2 catalytic activity was present in membrane preparations of whole testes and Leydig cells from adult rats; expression of the protein in Leydig cells was confirmed by Western immunoblot analysis. Using immunohistochemistry, ACE2 expression was confined to the Leydig cells in the rat testis and to Leydig and Sertoli cells in the human testis. Ablation of the Leydig cells in the rat by the specific toxin, ethane dimethane sulfonate, eliminated ACE2-positive cells from the interstitium. Expression of ACE2 in rat Leydig cells was up-regulated during the development of adult-type Leydig cells at puberty and after ethane dimethane sulfonate treatment. Expression of ACE2 activity in the testis was not significantly altered by manipulation of the pituitary-testicular hormonal axis with sc testosterone implants. These data suggest that ACE2 is a constitutive product of adult-type Leydig cells and may participate in the control of testicular function by as yet unknown mechanisms.

RAS activation in chronic liver injury is associated with upregulation of ACE2, Mas and hepatic conversion of angiotensin II to angiotensin-(1-7) leading to increased circulating angiotensin-(1-7). These results support the presence of an ACE2-angiotensin-(1-7)-Mas axis in liver injury which may counteract the effects of angiotensin II.

Angiotensin IV (Ang IV) exerts profound effects on memory and learning, a phenomenon ascribed to its binding to a specific AT 4 receptor. However the AT 4 receptor has recently been identified as the insulin-regulated aminopeptidase (IRAP). In this study, we demonstrate that AT 4 receptor ligands, including Ang IV, branes with high affinity, which was up to 200-fold greater than in the catalytic assay; this difference was not consistent among the peptides, and could not be ascribed to ligand degradation. Although some AT 4 ligands were subject to minor cleavage by HEK293T membranes, none were substrates for IRAP. Of a range of peptides tested, only vasopressin, oxytocin, and met-enkephalin were rapidly cleaved by IRAP. We propose that the physiological effects of AT 4 ligands result, in part, from inhibition of IRAP cleavage of neuropeptides involved in memory processing.