br Materials and methods br Results br Discussion

Materials and methods
Results
Discussion Diabetic kidney disease (DKD) is the most common cause of end-stage renal disease [15], [16]. The pathogenesis of DKD is not fully understood, and there is no effective treatment. Although previous studies suggested that early lesions of DKD are found in glomeruli [17], [18], recent works have shown that tubulointerstitial lesions play a major role in the pathogenesis of DKD after the initial kidney insults [19]. Adiponectin is an anti-inflammatory factor and has been shown to reduce insulin resistance [20], [21], [22]. A cross-sectional study of Pima Indians with diabetes showed that higher serum adiponectin levels were associated with increased albuminuria and worse renal function [23]. Another study showed that patients with serum adiponectin levels above 4 μg/ml had significantly faster renal disease progression [24]. Interestingly, ARBs also alleviate DKD. These studies suggest that the increases in serum adiponectin levels and activation of AT1 may play a role in the development of kidney disease. These findings reveal the existence of adiponectin resistance. AT1 receptor may contribute to adiponectin resistance by inhibiting AdipoR activation and signaling through heterodimerization. AMPK is an essential player in adiponectin signaling pathway that regulates energy metabolism. The fact that suppression of AMPK activity by AI-10-49 C largely diminished candesartan-mediated inhibition of NFκB via blocking AT1 also suggests that the AT1-mediated effect is at least partly resulted from direct inhibition of AdipoR1 and AdipoR2 via heterodimerization (Fig. 4). Thus, direct stimulation of AdipoR1 and AdipoR2 with adiponectin or their agonists may confer similar beneficial actions. In agreement with our findings and speculations, extensive evidence from the literature shows that adiponectin regulates glucose and lipid metabolism, improves insulin sensitivity, and exerts anti-atherosclerotic and anti-inflammatory effects [25], [26]. These effects depend on the interplay of its two receptor subtypes: AdipoR1 and AdipoR2 [27]. AdipoR1 is mainly expressed in skeletal muscle and AdipoR2 in the liver. As reported in one study [28], two types of islet β cells, macrophages, and endothelial cells contain cell surface AdipoR1 and R2 receptors. AdipoR1 and AdipoR2 have very similar structures containing seven transmembrane domains, both of which can activate peroxisome proliferator-activated receptor-α (PPARα), PPARγ, AMPK, p38 mitogen-activated protein kinase (p38MAPK) and other signaling pathways [11], [29], [30], [31], [32]. AdipoRs-induced activation of AMPK pathway was mainly involved in the fatty acid oxidation improved by adiponectin, independent of glucose uptake. PPARα, p38MAPK signal pathway and adiponectin oxidation of fatty acids and glucose uptake were correlated. Adiponectin inhibits the high expression of MCP-1 mRNA in renal cortical tissues of diabetic mice, thus exerting a protective effect [33]. We have shown in this study that AT1 and AT2 also form heterodimer. Under HG condition, this heterodimerization showed a downward trend. Since AT2 antagonizes the function of AT1 in almost all AT1-mediated actions, such heterozimerization may be beneficial. At last, the counteractions mediated by candesartan and compound C in serum-free medium without AngII and adiponectin stimulation (Fig. 4) suggest that the heterodimerized AT1 is constitutively active and the heterodimerized adiponectin receptors are inactive. Since chronic kidney disease (CKD) including DKD is also an independent risk factor for atherogenic vascular diseases, alleviation of DKD with ARBs in diabetic patients even without hypertension can be largely beneficial [34]. Our result provides mechanistic support for this clinical practice.
Disclosure
Acknowledgments This work was supported by grants from the National Science Foundation of China (81170679 to X.W and 81400694 to D.Z).