The first druglike selective angiotensin II AT(2) receptor agonist (21) with a K(i) value of 0.4 nM for the AT(2) receptor and a K(i) > 10 microM for the AT(1) receptor is reported. Compound 21, with a bioavailability of 20-30% after oral administration and a half-life estimated to 4 h in rat, induces outgrowth of neurite cells, stimulates p42/p44(mapk), enhances in vivo duodenal alkaline secretion in Sprague-Dawley rats, and lowers the mean arterial blood pressure in anesthetized, spontaneously hypertensive rats. Thus, the peptidomimetic 21 exerts a similar biological response as the endogenous peptide angiotensin II after selective activation of the AT(2) receptor. Compound 21, derived from the prototype nonselective AT(1)/AT(2) receptor agonist L-162,313 will serve as a valuable research tool, enabling studies of the function of the AT(2) receptor in more detail.

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This study was aimed at establishing whether specific activation of angiotensin II (ANG II) type 2 receptor (AT2R) modulates adipocyte differentiation and function. In primary cultures of subcutaneous (SC) and retroperitoneal (RET) preadipocytes, both AT2R and AT1R were expressed at the mRNA and protein level. Cells were stimulated with ANG II or the AT2R agonist C21/M24, alone or in the presence of the AT1R antagonist losartan or the AT2R antagonist PD123,319. During differentiation, C21/M24 increased PPAR␥ expression in both RET and SC preadipocytes while the number of small lipid droplets and lipid accumulation solely increased in SC preadipocytes. In mature adipocytes, C21/M24 decreased the mean size of large lipid droplets. Upon abolishment of AT2R expression using AT2R-targeted shRNAs, expressions of AT2R, aP2, and PPAR␥ remained very low, and cells were unable to differentiate. In Wistar rats fed a 6-wk high-fat/highfructose (HFHF) diet, a significant shift toward larger adipocytes was observed in RET and SC adipose tissue depots. C21/M24 treatments for 6 wk restored normal adipocyte size distribution in both these tissue depots. Moreover, C21/M24 and losartan decreased hyperinsulinemia and improved insulin sensitivity impaired by HFHF diet. A strong correlation between adipocyte size area and glucose infusion rate during euglycemic-hyperinsulinemic clamp was observed. These results indicate that AT2R is involved in early adipocyte differentiation, while in mature adipocytes and in a model of insulin resistance AT2R activation restores normal adipocyte morphology and improves insulin sensitivity.angiotensin type 2 receptor; adipocyte; differentiation; PPAR␥; highfat/high-fructose diet SEVERAL RECENT REVIEWS clearly implicate the renin-angiotensin system (RAS) in various aspects of adipose tissue physiology and dysfunction, linking RAS abnormalities with the development of insulin resistance and type 2 diabetes (7,25,36,50). Classically, the hormone angiotensin II (ANG II) mediates its action via two major receptors, namely the ANG II type 1 receptor (AT1R) and the type 2 receptor (AT2R). AT1R is closely associated with the regulation of blood pressure and hydro-mineral balance, with long-term ANG II-induced AT1R activation playing a well-documented role in hypertension, type 2 diabetes, and cardiovascular diseases (7,25,36,50). In contrast, AT2R is highly expressed in fetal life, while in the adult its expression is much more variable, according to tissues and pathophysiological situations (for recent reviews see Refs. 25,45,46,51,56). In conditions of insulin resistance, several studies, although not all (38), suggest that AT1R blockade provides end-organ protection independently of its antihypertensive effect by decreasing the size of adipocytes in visceral adipose tissue and by reversing the profile of adipokine secretion associated with obesity and type 2 diabetes (13,17,34,48,61) (for review see Refs. 20,25). Therapeutic responses to angiotensin receptor blockers (ARBs) may result from enhanced AT2R...

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A para substitution pattern of the phenyl ring is a characteristic feature of the first reported selective AT(2) receptor agonist M024/C21 (1) and all the nonpeptidic AT(2) receptor agonists described so far. Two series of compounds structurally related to 1 but with a meta substitution pattern have now been synthesized and biologically evaluated for their affinity to the AT(1) and AT(2) receptors. A high AT(2)/AT(1) receptor selectivity was obtained with all 41 compounds synthesized, and the majority exhibited K(i) ranging from 2 to 100 nM. Five compounds were evaluated for their functional activity at the AT(2) receptor, applying a neurite outgrowth assay in NG108-15 cells. Notably, four of the five compounds, with representatives from both series, acted as potent AT(2) receptor antagonists. These compounds were found to be considerably more effective than PD 123,319, the standard AT(2) receptor antagonist used in most laboratories. No AT(2) receptor antagonists were previously reported among the derivatives with a para substitution pattern. Hence, by a minor modification of the agonist 1 it could be transformed into the antagonist, compound 38. These compounds should serve as valuable tools in the assessment of the role of the AT(2) receptor in more complex physiological models.

In this investigation, it is demonstrated that the first nonpeptide AT(1) receptor agonist L-162,313 (1), disclosed in 1994, also acts as an agonist at the AT(2) receptor. In anesthetized rats, administration of compound 1 intravenously or locally in the duodenum increased duodenal mucosal alkaline secretion, effects that were sensitive to the selective AT(2) receptor antagonist PD-123,319. The data strongly suggest that 1 is an AT(2) receptor agonist in vivo. To the best of our knowledge, this substance is the first nonpeptidic low-molecular weight compound with an agonistic effect mediated through the AT(2) receptor.

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