The role of renin in the adipose tissue renin-angiotensin system.

Abstract

The renin angiotensin system (RAS) has been implicated in a variety of adipose tissue functions including tissue growth, differentiation, metabolism, and inflammation. While expression of all components necessary for a locally derived adipose tissue RAS have been demonstrated within adipose tissue, independence of local adipose RAS component concentrations from corresponding plasma RAS fluctuations has not been addressed. To analyze this, we varied in vivo rat plasma concentrations of two RAS components, renin and angiotensinogen (AGT), to determine the influence of their plasma concentrations on adipose and cardiac tissue levels in both perfused (plasma removed) and nonperfused samples. Variation of plasma RAS components was accomplished by 4 treatment groups: Normal, DOCA-salt, Bilateral nephrectomy, and Losartan. Adipose and cardiac tissue AGT concentrations correlated positively with plasma values. Perfusion of adipose tissue decreased AGT concentrations by 11.1% indicating that adipose tissue AGT was in equilibrium with plasma. Cardiac tissue renin levels positively correlated with plasma renin concentration for all treatments. In contrast, adipose tissue renin levels did not correlate with plasma renin, with the exception of extremely high plasma renin concentrations achieved in the Losartan treated group. These results suggest that adipose tissue may control its own local renin concentration independently of plasma renin as a potential mechanism for maintaining a functional local adipose RAS. Whereas adipose tissue possesses a local renin-angiotensin system, the synthesis and regulated release of renin has not been addressed. To that end we utilized differentiating 3T3-L1 cells and analyzed renin expression and secretion. Renin mRNA expression and protein enzymatic activity were not detectable in preadipocytes. However, upon differentiation, renin mRNA and both intracellular and extracellular renin activity were up regulated. In differentiated adipocytes, forskolin treatment resulted in a 28-fold increase in renin mRNA while TNF alpha treatment decreased renin mRNA 4-fold. IL-6, insulin, and angiotensin II (Ang II) were without effect. In contrast, forskolin and TNF alpha each increased renin protein secretion by 12- and 7-fold, respectively. Although both forskolin and TNFalpha induce lipolysis in adipocytes, fatty acids, prostaglandin E2 or lipopolysaccharide had no effect on renin mRNA or secretion. To evaluate mechanism(s) by which forskolin and/or TNF alpha are able to regulate renin secretion, a general lipase inhibitor (E600) and PKA inhibitor (H89) were used. Both inhibitors attenuated forskolin induced renin release while having no effect on TNF alpha regulated secretion. In contrast, E600 potentiated forskolin-stimulated renin mRNA levels while H89 had no effect. Neither inhibitor had any influence on TNFalpha regulation of renin mRNA. Relative to lean controls, renin expression was reduced 78% in the epididymal adipose tissue of obese male C57Bl/6J mice, consistent with TNF alpha-mediated down regulation of renin mRNA in the culture system. In conclusion, the expression and secretion of renin are regulated under a complex series of hormonal and metabolic determinants in mature 3T3-L1 adipocytes.