Mechanisms of paracrine and endocrine actions of Angiotensin II

Adipose tissue expresses high levels of angiotensinogen (agt), the only known precursor of Angiotensin II (Ang II) and we have previously shown that this hormone plays a paracrine role in regulation of adipocyte metabolism by increasing activity and gene expression of two key liogenic enzymes, fatty acid synthase (FAS) and glycerol-3-phosephate dehydrogenase (GPDH). However, molecular mechanisms by which Ang II regulates an adipocyte lipogenic gene, namely FAS and transcription factors mediating this regulation are largely unknown. Furthermore, signaling mechanisms whereby Ang II induces adiposity need to be further explored. Accordingly, our studies were designed to investigate transcriptional regulation of FAS gene by Ang II and determine signaling mechanisms of Ang II leading to activate FAS gene transcription. We demonstrated that Ang II responsive element is an E box within adipocyte FAS gene and the adipocyte determination and differentiation factor1/sterol-regulatory element binding protein 1c (ADD1/SERBP1c) functions as a transcription factor mediating these actions. Ang II induced FAS transcription as well as ADD1 gene expression in a glucose-dependent manner. Using immunoprecipitation and western blot analyses, we found that Ang II regulates adipocyte metabolism via activation of insulin signaling molecules including IRβ, IRS, PI-3K and Akt. This signaling mechanism was mainly mediated by angiotensin receptor type 1 receptors (AT1) and in part by angiotensin receptor type 2 receptors (AT2). Next, with agt knockout (agt -/-, KO) and transgenic (Tg-KO mice expressing agt exclusively in adipose tissue and Tg-WT mice overexpressing agt in adipose tissue) mouse models, we examined changes in adipose tissue metabolism in vivo and renal gene regulation in these mice and further dissected endocrine effects of adipocyte agt. We found that compared with those of WT mice, body weight gain and fat pad weight were lower in response to high fat diet in KO mice exhibiting hypotension and renal abnormalities. Targeted expression of the agt gene only in adipose tissue (Tg-KO) partly rescued these phenotypes whereas agt overexpression in adipose tissue of transgenic mice (Tg-WT) was associated with increased epididymal fat pad mass and blood pressure. Subsequent western blot and renal gene expression analyses indicated that adipocyte agt participates in systemic blood pressure regulation at least partly by increasing renal agt and AT1 receptor production in Tg-WT mice. Additionally, microarray data revealed that adipose tissue-specific agt restoration was able to correct altered expression of genes associated with blood pressure homeostasis and renal function in KO mice. In conclusion, ANG II plays a hypertrophic role in adipocytes by a paracrine/autocrine mechanism. Furthermore, adipocyte Ang II exerts endocrine effects on renal function and gene expression, thereby contributing to systemic blood pressure regulation and kidney homeostasis. This study may provide valuable approaches in treatments of obesity and obesity-associated metabolic alternations such as insulin resistance and hypertension.