ACE2, the first known human homologue of angiotensin-converting enzyme (ACE), was identified from 5' sequencing of a human heart failure ventricle cDNA library. ACE2 has an apparent signal peptide, a single metalloprotease active site, and a transmembrane domain. The metalloprotease catalytic domains of ACE2 and ACE are 42% identical, and comparison of the genomic structures indicates that the two genes arose through duplication. In contrast to the more ubiquitous ACE, ACE2 transcripts are found only in heart, kidney, and testis of 23 human tissues examined. Immunohistochemistry shows ACE2 protein predominantly in the endothelium of coronary and intrarenal vessels and in renal tubular epithelium. Active ACE2 enzyme is secreted from transfected cells by cleavage N-terminal to the transmembrane domain. Recombinant ACE2 hydrolyzes the carboxy terminal leucine from angiotensin I to generate angiotensin 1-9, which is converted to smaller angiotensin peptides by ACE in vitro and by cardiomyocytes in culture. ACE2 can also cleave des-Arg bradykinin and neurotensin but not bradykinin or 15 other vasoactive and hormonal peptides tested. ACE2 is not inhibited by lisinopril or captopril. The organ- and cell-specific expression of ACE2 and its unique cleavage of key vasoactive peptides suggest an essential role for ACE2 in the local renin-angiotensin system of the heart and kidney. The full text of this article is available at http://www. circresaha.org.

RNA interference (RNAi) holds considerable promise as a therapeutic approach to silence disease-causing genes, particularly those that encode so-called 'non-druggable' targets that are not amenable to conventional therapeutics such as small molecules, proteins, or monoclonal antibodies. The main obstacle to achieving in vivo gene silencing by RNAi technologies is delivery. Here we show that chemically modified short interfering RNAs (siRNAs) can silence an endogenous gene encoding apolipoprotein B (apoB) after intravenous injection in mice. Administration of chemically modified siRNAs resulted in silencing of the apoB messenger RNA in liver and jejunum, decreased plasma levels of apoB protein, and reduced total cholesterol. We also show that these siRNAs can silence human apoB in a transgenic mouse model. In our in vivo study, the mechanism of action for the siRNAs was proven to occur through RNAi-mediated mRNA degradation, and we determined that cleavage of the apoB mRNA occurred specifically at the predicted site. These findings demonstrate the therapeutic potential of siRNAs for the treatment of disease.

The carboxypeptidase ACE2 is a homologue of angiotensin-converting enzyme (ACE). To clarify the physiological roles of ACE2, we generated mice with targeted disruption of the Ace2 gene. ACE2-deficient mice were viable, fertile, and lacked any gross structural abnormalities. We found normal cardiac dimensions and function in ACE2-deficient animals with mixed or inbred genetic backgrounds. On the C57BL/6 background, ACE2 deficiency was associated with a modest increase in blood pressure, whereas the absence of ACE2 had no effect on baseline blood pressures in 129/SvEv mice. After acute Ang II infusion, plasma concentrations of Ang II increased almost 3-fold higher in ACE2-deficient mice than in controls. In a model of Ang II-dependent hypertension, blood pressures were substantially higher in the ACE2-deficient mice than in WT. Severe hypertension in ACE2-deficient mice was associated with exaggerated accumulation of Ang II in the kidney, as determined by MALDI-TOF mass spectrometry. Although the absence of functional ACE2 causes enhanced susceptibility to Ang II-induced hypertension, we found no evidence for a role of ACE2 in the regulation of cardiac structure or function. Our data suggest that ACE2 is a functional component of the renin-angiotensin system, metabolizing Ang II and thereby contributing to regulation of blood pressure.

No abstract

Abstract-The scavenger receptor class B type I (SR-BI) is a lipoprotein receptor that has been shown to be important in high density lipoprotein cholesterol (HDL-C) metabolism in mice. To determine its role in humans, we have characterized the human SR-BI gene and investigated its genetic variation in 489 white men and women. Five variants were demonstrated: 2 in introns (3 and 5) and 3 in exons (1, 8, and 11). Three variants at exons 1 and 8 and intron 5 with allele frequencies Ͼ0.1 were used to examine associations with lipid or anthropometric variables. The exon 1 variant was significantly (PϽ0.05) associated with increased HDL-C and lower low density lipoprotein cholesterol (LDL-C) values in men, but no associations were observed in women. The exon 8 variant was associated in women with lower LDL-C concentrations (3.05Ϯ0.98 mmol/L and 3.00Ϯ0.93 mmol/L for heterozygotes and homozygotes, respectively) compared with women homozygous for the common allele T he scavenger receptor class B type I (SR-BI) is a multilipoprotein receptor found in the liver and steroidogenic glands of both mice 1 and humans 2,3 (for a review, see Reference 4). The cDNA for human SR-BI (also known as CLA-1) was originally cloned by homology to human CD36 and rat LIMPII, which are members of a family of transmembrane proteins. 5 An independent expression cloning study identified the hamster homologue by its ability to mediate the binding of modified LDL, and it was also shown to bind native LDL. 6 Subsequently, murine SR-BI was shown to mediate the uptake of lipid, but not apoprotein, from HDL into cells, 1 a process described as selective uptake. [7][8][9] This finding established SR-BI as the first HDL transmembrane receptor to be identified and cloned. Further studies of the human homologue demonstrated that it also is a multilipoprotein receptor that binds HDL, LDL, and VLDL. 2,10 Further analysis in vivo in mice and rats has supported a role for SR-BI in cholesterol metabolism. Targeted disruption of apoAI, the major protein component of HDL, leads to an increase in SR-BI expression in the adrenal glands of mice, 11 where HDL-C is used for steroid hormone synthesis. In addition, SR-BI expression levels in the adrenal glands are increased in response to adrenocorticotropic hormone and decreased in response to dexamethasone. 12 Estrogen treatment at high doses in rats greatly reduces SR-BI expression in the liver while it increases SR-BI expression in the adrenal gland and ovarian corpus luteal cells. 13 Transient overexpression of SR-BI in the livers of mice by adenoviral infection leads to a marked reduction in plasma HDL levels and a concomitant increase in plasma LDL/IDL cholesterol levels. 14 Finally, targeted disruption of the SR-BI gene in mice leads to a significant increase in plasma HDL 15,16 and reduced selective uptake of cholesterol from HDL into the liver. 16 Thus, SR-BI has clearly been shown to be a very important player in HDL metabolism in mice. However, although mice have HDL as the major cholesterol-carrying lipopr...