Here, we have demonstrated that obesity increased mortality in a mouse model of renal IRI. In addition, we found that there were no differences among the groups in terms of the glomerular filtration rate (creatinine clearance) at 48 h after IRI or sham surgery. However, obese + IRI group mice presented the lowest urinary osmolality, the highest urinary NGAL expression, the greatest renal tubular damage, and the highest caspase 3 expression. It is possible that obesity exerts those effects by upregulating oxidative stress molecules, as evidenced by the higher urinary TBARS levels and higher renal expression of HO-1 in our obese mice. It has long been known that renal IRI creates a state of decreased renal Klotho protein expression (20, 21). Accordingly, we observed markedly low levels of renal Klotho protein expression in our obese mice.
Obesity is recognized as a global epidemic and as a public health crisis in various countries (22), having also been shown to be an independent risk factor for the development of chronic kidney disease (11, 23). Although we found no differences among our groups of mice in terms of creatinine clearance, it is known that the determination of serum creatinine levels or even creatinine clearance is not the best means of estimating the glomerular filtration rate. There have been studies showing that tubular secretion of creatinine is higher among obese patients than among non-obese patients (24). However, to our knowledge, there have been no animal studies showing that obesity aggravates IRI-induced AKI. In the present study, we demonstrated that obesity leads to kidney damage, resulting in tubular necrosis and apoptosis, as well as greater urinary expression of NGAL. In a previous study, involving Zucker rats, the obese rats were found to develop AKI within the first 24 h after orthopedic trauma, although the same did not occur among the lean rats (25). In critically ill patients, obesity has also been shown to be an independent risk factor for the development of AKI, the risk of which was found to correlate directly with the body mass index (26). Obese patients often also have metabolic syndrome, diabetes, dyslipidemia, hypertension, or cardiovascular disease (27, 28). In our mouse model of IRI-induced AKI, the animals did not develop diabetes or dyslipidemia. Therefore, we can speculate that the obesity per se might be an aggravating factor in IRI-induced AKI. Despite not having been submitted to IRI, the obese group mice showed higher Mac-2 inflammatory cell counts in renal tissue. Adipocytes store excess energy by undergoing hypertrophy. Visceral adipocytes undergo hypertrophy when storing additional lipids, becoming poorly vascularized and hypoxic, resulting in increased inflammatory cytokine production, immune cell infiltration, as well as cell stress and apoptosis (29). When that occurs, fat is stored in the kidneys, liver, pancreas, heart, skeletal muscle, and other tissues, leading to a condition known as lipotoxicity. Tissue-resident and infiltrating macrophages regulate the innate immune system, which plays a crucial role in inflammation in adipose tissue. It has been shown that macrophage activation and macrophage infiltration into adipose tissue are both more pronounced among patients who are obese (29). Lipids are a significant component in the normal kidney, accounting for approximately 3% of its wet weight (30). In a model of high-fat diet-induced obesity, Laurentius et al. (31) demonstrated that the number of infiltrating monocytes/macrophages was significantly higher in the kidneys of rats fed a high-fat diet than in those of rats fed a control diet. We found it interesting that, in our study, the Mac-2 + cell counts were higher in the obese group than in the normal group, despite the fact that the mice in both of those groups underwent sham surgery (i.e., were not induced to AKI), and that the obese + IRI group counts were comparable to those observed for the obese and normal + IRI groups. In the present study, an anti-Mac-2 antibody was used in order to identify macrophages in renal tissue. In the kidney, the antibody labeled a fraction of the CSF1R + and CX3CR1 + macrophages but also stained tubular epithelial cells (32), which might have interfered with the identification of macrophage infiltration.
Serum levels of the protein adiponectin are lower in obese individuals, especially in those with visceral fat accumulation and altered lipid metabolism, which is paradoxical because adiponectin is derived from adipocytes (33). In another mouse model of acute IRI, Tsugawa-Shimizu et al. (34) detected adiponectin in the endothelium of the renal arterioles and in the peritubular spaces of the renal cortex, as well as in the inner and outer renal medulla. The authors found that there was more renal tubular damage and greater vascular permeability in adiponectin-knockout mice than in wild-type mice. They suggested that adiponectin, by binding to T-cadherin, plays a major role in preserving the capillary network and mitigating renal tubular injury (35). In the present study, renal expression of adiponectin was lower in the obese and obese + IRI groups than in the other groups, although the difference was not statistically significant. We believe that the lack of statistical difference was attributable to the great variability within the groups.
In a very elegant study, Cui et al. (4) analyzed the association between the visceral adiposity index and serum levels of the anti-aging protein Klotho protein, using (United States) National Health and Nutrition Examination Survey data. The authors found that, among adults in the United States, there was a non-linear association, as well as a dose–response relationship, between serum Klotho levels and the visceral adiposity index. They showed that a there was a negative correlation between the two factors, albeit only when the visceral adiposity index below 3.21. Among the individuals with a visceral adiposity index between 0.29 and 3.21, serum Klotho levels decreased as visceral adiposity increased, and those individuals were more prone to aging-related syndromes. In the present study, the mice that were fed a high-fat diet showed lower renal Klotho expression than did those that were fed a standard diet, although the difference was not statistically significant. Hu et al. (20) showed that IRI reduces Klotho in the kidneys, urine, and blood of rodents. The mice in our normal + IRI group also presented lower renal Klotho protein expression than did those in our normal group. Nevertheless, that expression was much lower in our obese + IRI group. Therefore, we can surmise that obesity is a state of Klotho deficiency, which becomes much more pronounced when obese animals are submitted to IRI. Klotho deficiency has been shown to increase endogenous ROS generation and accentuate oxidative stress (36, 37). Conversely, overexpression of Klotho induces resistance to oxidative stress (21). In addition, Klotho administration has been shown to effectively reduce oxidative stress and preserve mitochondrial function in mice (37).