Biochemical and Biophysical Research Communications
AMPK regulates KATP channel trafficking via PTEN inhibition in leptin-treated pancreatic β-cells
Introduction
Leptin is an adipocyte-derived hormone that controls food intake, glucose homeostasis, and energy expenditure in response to the amount of body fat [1], [2]. In addition to its central action, leptin directly affects pancreatic β-cell functions at different levels involving insulin secretion and cell viability [3], [4], [5], [6], [7]. The defect in leptin signaling in pancreatic β-cells leads to uncontrolled insulin secretion and a failure of glucose homeostasis [1], [8], [9]. Therefore, unraveling the leptin signaling pathways in pancreatic β-cells may hold key to explaining the epidemiological link between obesity and diabetes.
ATP-sensitive potassium (KATP) channel, which comprises pore-forming Kir6.2 and regulatory SUR1 subunits, plays a crucial role in regulating pancreatic β-cell excitability and insulin secretion [10]. Recently, trafficking of KATP channels to the plasma membrane has emerged as an important mechanism controlling pancreatic β-cell excitability [11]. We also found that leptin causes KATP channel trafficking in pancreatic β-cells via calmodulin-dependent protein kinase kinase β (CaMKKβ)-dependent activation of AMP-activated protein kinase (AMPK) [7], [12], providing a molecular mechanism for previous observations that leptin activates KATP channels [5], [7], [13]. Therefore, a mechanistic understanding of leptin-induced KATP channel trafficking will provide a new perspective on the development of obesity-associated diabetes.
Phosphatase and tensin homologue (PTEN) controls not only cell growth and survival, but also metabolic signaling [14]. Comparable to leptin, several lines of evidence indicate that PTEN is involved in the regulation of insulin secretion and glucose homeostasis: suppression of PTEN expression reduces serum insulin levels in ob/ob mice [15]; and PTEN loss enhances pancreatic β-cell growth and survival [16], [17]. In addition, a previous report argued a functional relationship between leptin and PTEN in pancreatic β-cells: leptin leads to PTEN inhibition and KATP channel activation [18]. These results raise the possibility that leptin-induced PTEN inhibition is mediated by AMPK, but the link between PTEN and AMPK has never been identified. In this study, we discovered that leptin inhibits PTEN through AMPK-mediated activation of glycogen synthase kinase 3β (GSK3β) and this pathway is crucial for KATP channel trafficking. Our results will provide insight into the molecular mechanisms that underlie leptin action on pancreatic β-cell physiology and glucose homeostasis.
Section snippets
Cell culture and reagents
Insulin-secreting INS-1 cells (passage 20–50) and primary pancreatic β-cells were grown as described in our previous paper [19]. All animal experimental procedures for isolating pancreatic β-cells were conducted in accordance with the guidelines of the University Committee on Animal Resources at Seoul National University (Approval No.: SNU-120216-02). All cell culture reagents were purchased from Invitrogen. The cells were treated with leptin (Calbiochem), AICAR (Calbiochem), compound C (CC)
AMPK mediates leptin-induced PTEN inactivation
Based on two key observations that leptin inhibits PTEN [18] and leptin activates AMPK [12], we questioned whether AMPK mediates leptin-induced PTEN inhibition. Because PTEN is inactivated by phosphorylation at Ser-380/Thr-382/Thr-383 residues [18], PTEN inhibition was assessed by measuring phospho-PTEN (pPTEN) levels. Western blot analysis showed that PTEN phosphorylation was obviously observed for 5 min mark after treatment with leptin (approximately a three-fold increase) and maintained
Discussion
In this study, we identified the signaling components connecting leptin to KATP channel trafficking in pancreatic β-cells: leptin inhibits PTEN via AMPK-mediated GSK3β activation and this leads to KATP channel trafficking. Pharmacologic or genetic interventions of AMPK, GSK3β, or PTEN abrogated leptin-induced KATP channel trafficking. Our results indicate that the improper function of leptin signaling constituents leads to aberrant insulin secretion in pancreatic β-cells. Our findings
Acknowledgment
This research was supported by the National Research Foundation of Korea Grant (2010-0029394) funded by the Ministry of Science and Future Planning.
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