Enhancement of Adipocyte Browning by Angiotensin II Type 1 Receptor Blockade

Abstract

Browning of white adipose tissue (WAT) has been highlighted as a new possible therapeutic target for obesity, diabetes and lipid metabolic disorders, because WAT browning could increase energy expenditure and reduce adiposity. The new clusters of adipocytes that emerge with WAT browning have been named 'beige' or 'brite' adipocytes. Recent reports have indicated that the renin-angiotensin system (RAS) plays a role in various aspects of adipose tissue physiology and dysfunction. The biological effects of angiotensin II, a major component of RAS, are mediated by two receptor subtypes, angiotensin II type 1 receptor (AT1R) and type 2 receptor (AT2R). However, the functional roles of angiotensin II receptor subtypes in WAT browning have not been defined. Therefore, we examined whether deletion of angiotensin II receptor subtypes (AT1aR and AT2R) may affect white-to-beige fat conversion in vivo. AT1a receptor knockout (AT1aKO) mice exhibited increased appearance of multilocular lipid droplets and upregulation of thermogenic gene expression in inguinal white adipose tissue (iWAT) compared to wild-type (WT) mice. AT2 receptor-deleted mice did not show miniaturization of lipid droplets or alteration of thermogenic gene expression levels in iWAT. An in vitro experiment using adipose tissue-derived stem cells showed that deletion of the AT1a receptor resulted in suppression of adipocyte differentiation, with reduction in expression of thermogenic genes. These results indicate that deletion of the AT1a receptor might have some effects on the process of browning of WAT and that blockade of the AT1 receptor could be a therapeutic target for the treatment of metabolic disorders.

Fig 1. Effect of angiotensin subtype receptor deletion on body weight and adipose tissue weight.

(A) Body weight at 10 weeks old in WT, AT_1aKO and AT₂KO mice. (B) Ratio of iWAT weight to body weight at 10 weeks old in WT, AT_1aKO and AT₂KO mice. Data are expressed as mean±SEM. n = 15 for each group. **P<0.01 vs. WT.

Fig 2. Effect of angiotensin subtype receptor deletion on inguinal white adipose tissue morphology.

(A) Representative images of aldehyde-fuchsin staining and UCP1 immunohistochemical staining of iWAT. (B) Adipocyte number in each cell size group (0–200, 200–500, >500 μm²). Data are expressed as mean±SEM. n = 5 for each group. (C) Representative images (×400) of UCP1 immunohistochemical staining of brown (BAT) and inguinal white adipose tissue.

Fig 3. mRNA expression in iWAT of WT, AT_1aKO, and AT₂KO mice.

(A) Expression of adipocyte differentiation factors in iWAT. (B) Expression of thermogenic genes in iWAT. (C) Expression of beige-selective genes in iWAT. Data are expressed as mean±SEM. n = 15 for each group. *P<0.05, **P<0.01 vs. WT.

Fig 4. Effect of angiotensin II receptor subtype deletion on adipocyte differentiation from ASC and thermogenic gene expression in differentiated ASC.

(A) Representative images of differentiated ASC. (B) Lipid accumulation determined by Oil Red O staining in differentiated ASC. (C) mRNA expression of thermogenic genes in differentiated ASC. Data are expressed as mean±SEM. n = 6 for each group.

Fig 5. mRNA expression of angiotensin receptors in iWAT (A) and differentiated ASC (B) of WT, AT1aKO, AT2KO mice.

Data are expressed as mean±SEM. n = 15 for each group in iWAT. n = 6 for each group in differentiated ASC.