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  • Recent studies offer novel insight into the relationship bet


    Recent studies offer novel insight into the relationship between SIRT1, insulin resistance, and metabolic syndrome. SIRT1 activity is decreased in many cell types in diabetes, including adipocytes, hepatocytes, and myoblasts [7]. SIRT1 has been shown to play an important role in oxidative stress, mitochondrial dysfunction, and inflammation, which are hallmarks of insulin resistance and type 2 diabetes [8]. Evidence shows that decreased SIRT1 expression may contribute to muscle insulin resistance, as indicated in muscle biopsies obtained from type 2 diabetes patients [9]. Inhibition of SIRT1 induces insulin resistance in cultured insulin-sensitive Fosfomycin calcium and tissues, whereas SIRT1 activation leads to metabolic improvements, such as enhanced glucose utilization and insulin sensitivity [10]. SIRT1 can activate AMP-activated protein kinase (AMPK) by deacetylating its upstream kinase LKB1, which promotes LKB1 translocation from the nucleus to the cytosol, where it is activated and phosphorylates and activates AMPK [11], [12]. Similarly, AMPK can activate SIRT1 by increasing the NAD+/NADH ratio or the expression/activity of nicotinamide phosphoribosyltransferase (Nampt), the rate-limiting enzyme in NAD+ biosynthesis [13]. Metformin is a biguanide drug commonly used to treat type 2 diabetes, decreasing hyperglycemia mostly by suppressing glucose production and release by the liver and increasing insulin-stimulated glucose uptake by peripheral tissues, such as muscles. Metformin decreases the rate of urine albumin excretion in patients with type 2 diabetes [14] and exerts beneficial effects in patients with impaired renal function [15]. Metformin's mechanism of action has been linked to the activation of AMPK [16]. We have shown that, in podocytes, metformin decreases the production of reactive oxygen species (ROS) through a reduction in NAD(P)H oxidase activity [17]. We have also demonstrated that metformin affects purinergic signaling through inhibition of ecto-ATPase, causing an increase in the extracellular ATP concentration and activation of P2 receptors with subsequent activation of AMPK and reduction in NAD(P)H oxidase activity [18]. Thus, metformin may act via multiple cellular targets to protect cell function. Recently, metformin was reported to indirectly induce hepatic SIRT1 through AMPK-mediated induction of nicotinamide phosphoribosyltransferase (Nampt) [19]. We previously reported that the exposure of podocytes to high glucose (HG) concentrations leads to decreased SIRT1 protein expression and activity, with concomitant reduction in AMPK phosphorylation levels and abolition of the stimulating effect of insulin on glucose uptake, suggesting that this mechanism may be involved in the development of insulin resistance in podocytes cultivated in the presence of HG concentrations, mimicking diabetic conditions [20]. In the current study, we examined the hypothesis that metformin modulates HG-induced insulin resistance in primary rat podocytes via activation of SIRT1 and AMPK.
    Materials and methods
    Discussion Several studies have linked the insulin-signaling pathways with podocyte function. At first, glomerular podocytes were shown to possess a heterogeneous glucose transport system consisting of facilitative glucose transporters (GLUTs), and that the insulin response of podocytes occurs with an increase in PI3K and MAPK signaling, resulting in increased glucose uptake via the facilitative glucose transporters GLUT1 and GLUT4 [22], [25]. Growing evidence indicates that the insulin resistance of podocytes may be an important initiator for many of the pathological processes observed in DN [26], [27]. A previous study revealed that exposure of primary rat podocytes to HG concentrations attenuates the insulin responsiveness of podocytes in association with decreased AMPK phosphorylation levels, resulting in the abolition of insulin-stimulated glucose uptake into these cells [21]. We also demonstrated previously that exposure of podocytes to hyperglycemic conditions increases the albumin permeability of a podocyte monolayer [24] and that HG-mediated induction of insulin resistance in podocytes is associated with downregulation of SIRT1 protein, decreased enzymatic activity, and a subsequent decrease in AMPK phosphorylation [20]. The SIRT1 level positively correlates with insulin sensitivity [28] and is directly or indirectly involved in insulin signaling [29]. SIRT1 downregulation was demonstrated to be involved in the pathogenesis and development of DN, and was correlated with the level of proteinuria [30]. As a major regulator of cellular metabolism, AMPK was also involved in improved insulin sensitivity and glucose homeostasis [31]. Downregulation of AMPK in response to HG exposure was first shown to occur in skeletal muscle cells [32]. The same effect, with a parallel decrease in SIRT1 activity, was observed in cultured HepG2 cells; SIRT1 inhibitor NAM decreased both SIRT1 and AMPK activity, whereas incubation with quercetin, a SIRT1 activator, increased both activities [33]. These studies led to examining a possible link between SIRT1 and AMPK, finding that AMPK and SIRT1 positively regulate each other and share many common target molecules [34]. Several studies suggest that SIRT1 functions as a regulator of AMPK activity through AMPK upstream LKB1 kinase [35], [36], though AMPK suppression was also shown to be caused by a decrease in SIRT1 activity due to decreased AMPK-mediated Nampt expression [37], [38]. The precise interactions between SIRT1 and AMPK in podocytes remain unclear, but our previous results indicated that the decreased AMPK phosphorylation in HG-cultured cells occurs in a SIRT1-dependent manner. AMPK phosphorylation was reduced in SIRT1-depleted cells and the stimulating effect of insulin on AMPK phosphorylation was suppressed, suggesting that SIRT1 is involved in the regulation of AMPK activity in podocytes [20].