SGK1 is regulated by metabolic-related factors in 3T3-L1 adipocytes and overexpressed in the adipose tissue of subjects with obesity and diabetes
Introduction
Metabolic syndrome is a combination of interrelated risk factors of metabolic origin, including arterial hypertension, dyslipidemias, glucose intolerance, abdominal obesity, and a proinflammatory and prothrombotic state [1]. Importantly, these risk factors are associated with increased cardiovascular morbidity and mortality [2]. However, the mechanisms linking these abnormalities are not fully known. No single pathogenesis has been identified [3].
Serum and glucocorticoid-inducible protein kinase 1 (SGK1) is the newest member of the AGC kinase family [4]. In mammals, SGK1 is ubiquitously expressed and participates in the regulation of transport, hormone release, neuroexcitability, cell proliferation, and apoptosis [5]. The primary physiological function of this kinase has been studied in the kidneys, in which it controls sodium reabsorption by regulating the epithelial sodium channel (ENaC) [6]. Recently, increasing studies have been conducted and addressed its diverse pathologic role in adverse ventricular remodeling [7], metabolic syndrome [5], [8], tumor growth [9], [10], infertility [11], and platelet hyper-responsiveness [12]. Of note, SGK1 is hypothesized to be a common signaling molecule that fosters the development of metabolic syndrome [8].
One piece of convincing evidence linking SGK1 to metabolic syndrome is that a certain variant of the SGK1 gene [the combined presence of distinct polymorphisms in intron 6 (I6CC) and exon 8 (E8CC/CT)] has been shown to be associated with increased blood pressure [13], [14], obesity and type 2 diabetes mellitus [15], [16]. It is hypothesized that this SGK1 gene variant is associated with increased SGK1 expression and activity. Additional possible mechanisms include the following: (1) SGK1 stimulates ENaC, thus increasing renal salt retention and predisposing individuals to hypertension [13]; (2) SGK1 upregulates sodium-glucose cotransporter 1 (SGLT1) expression and enhances intestinal glucose absorption, leading to excessive insulin release and triggering repeated glucose uptake and thus obesity [15], [17]; and (3) SGK1 contributes to a prothrombotic state and participates in the stimulation of fibrosis, leading to the development of diabetic nephropathy [8].
In our previous study of the regulatory effect of aldosterone on adipocytokines [18], we observed that SGK1 mRNA was expressed in adipose tissue and was regulated by aldosterone in adipocytes. A recent study by Di Pietro et al. confirmed SGK1 expression in adipose tissue, and they also reported that SGK1 influences adipocyte differentiation [19]. Given the essential role of adipose tissue in the development of metabolic syndrome, it is crucial to further assess the functional significance of SGK1 in differentiated adipocytes and adipose tissue. The present study was performed to explore SGK1 expression in adipose tissue, to investigate SGK1 regulation in differentiated 3T3-L1 adipocytes and to determine if SGK1 expression was altered in the adipose tissue of subjects with obesity and diabetes.
Section snippets
Materials
Insulin, dexamethasone, 3-isobutyl-1-methylxanthine, aldosterone and tumor necrosis factor (TNF)-α were purchased from Sigma (Saint Louis, MO, USA). Rosiglitazone was generously provided by Shanghai Sunve Pharmaceutical Co., Ltd. The rabbit monoclonal antibody against SGK1 was obtained from Cell Signaling Technology (Beverly, MA, USA). The DIG Northern Starter Kit was purchased from Roche.
Human subjects
Human visceral adipose tissue was obtained through surgical resection at the Nanjing Drum Towel Hospital.
SGK1 mRNA and protein expression in adipose tissue
SGK1 mRNA was detected with RT-PCR in different mouse tissues, including the liver, kidney, muscle, small intestine, brain, lung, heart, and adipose tissue (Fig. 1A). SGK1 protein expression in adipose tissue was confirmed with immunohistochemistry and was detected as brown staining (Fig. 1B and C).
SGK1 mRNA expression is regulated by metabolic-related factors in differentiated 3T3-L1 adipocytes
Aldosterone [18], [21], glucocorticoids [22], peroxisome proliferator-activated receptor-γ (PPAR-γ) agonists [23] and TNF-α have all been reported to regulate adipocyte function. SGK1 is also a
Discussion
The role of SGK1 in the development of metabolic syndrome is an interesting and relevant topic but has not been fully elucidated. As previously shown, the kidneys and intestines are the most commonly analyzed tissues because of previous assumptions made about the mechanism through which SGK1 contributes to metabolic syndrome [5]. Obesity is considered a principal causative factor in the development of metabolic syndrome. The dysregulated production of adipocytokines participates in the
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant number 81000350) and Jiangsu Province's Key Discipline of Medicine (XK201105).
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These authors contributed equally to this work.