In pancreatic ␤-cells, glucose metabolism signals insulin secretion by altering the cellular array of messenger molecules. ATP is particularly important, given its role in regulating cation channel activity, exocytosis, and events dependent upon its hydrolysis. Uncoupling protein (UCP)-2 is proposed to catalyze a mitochondrial inner-membrane H ؉ leak that bypasses ATP synthase, thereby reducing cellular ATP content. Previously, we showed that overexpression of UCP-2 suppressed glucose-stimulated insulin secretion (GSIS) in isolated islets (1). The aim of this study was to identify downstream consequences of UCP-2 overexpression and to determine whether insufficient insulin secretion in a diabetic model was correlated with increased endogenous UCP-2 expression. In isolated islets from normal rats, the degree to which GSIS was suppressed was inversely correlated with the amount of UCP-2 expression induced. Depolarizing the islets with KCl or inhibiting ATP-dependent K ؉ (K ATP ) channels with glybenclamide elicited similar insulin secretion in control and UCP-2-overexpressing islets. The glucose-stimulated mitochondrial membrane (⌿ m ) hyperpolarization was reduced in ␤-cells overexpressing UCP-2. ATP content of UCP-2-induced islets was reduced by 50%, and there was no change in the efflux of Rb ؉ at high versus low glucose concentrations, suggesting that low ATP led to reduced glucose-induced depolarization, thereby causing reduced insulin secretion. Sprague-Dawley rats fed a diet with 40% fat for 3 weeks were glucose intolerant, and in vitro insulin secretion at high glucose was only increased 8.5-fold over basal, compared with 28-fold in control rats. Islet UCP-2 mRNA expression was increased twofold. These studies provide further strong evidence that UCP-2 is an important negative regulator of ␤-cell insulin secretion and demonstrate that reduced ⌬⌿ m and increased activity of K ATP channels are mechanisms by which UCP-2-mediated effects are mediated. These studies also raise the possibility that a pathological upregulation of UCP-2 expression in the prediabetic state could contribute to the loss of glucose responsiveness observed in obesity-related type 2 diabetes in humans.

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These authors contributed equally to this work.Keywords: autophagy, autophagosome, BECN1, LC3, PtdIns3KAbbreviations: ATG, autophagy related; BafA1, bafilomycin A 1 ; BCL2L1/Bcl -xL, BCL2-like 1; BECN1, Beclin 1, autophagy related; BECN1P1/BECN2, Beclin 1, autophagy related, pseudogene 1; EM, electron microscopy; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GFP, green fluorescent protein; KO, knockout; MAP1LC3/LC3, microtubule-associated protein 1 light chain 3; MAP1LC3-I/LC3-I, soluble, proteolytically processed microtubule-associated protein 1 light chain 3; MAP1LC3-II/LC3-II, proteolytically processed and lipid-modified microtubule-associated protein 1 light chain 3; MAP1LC3B/LC3B, microtubule-associated protein 1 light chain 3 b; PtdIns3K, phosphatidylinositol 3-kinase; PIK3C3/VPS34, phosphatidylinositol 3-kinase, catalytic subunit type 3; PIK3R4/VPS15, phosphoinositide-3-kinase, regulatory subunit 4; SQSTM1/p62, sequestosome 1; TUBB, tubulin, b class I; TALEN, transcription activator-like effector nuclease; UVRAG, UV radiation resistance associated; ZFYVE1/DFCP1, zinc finger, FYVE domain containing 1.BECN1/Beclin 1 is regarded as a critical component in the class III phosphatidylinositol 3-kinase (PtdIns3K) complex to trigger autophagy in mammalian cells. Despite its significant role in a number of cellular and physiological processes, the exact function of BECN1 in autophagy remains controversial. Here we created a BECN1 knockout human cell line using the TALEN technique. Surprisingly, the complete loss of BECN1 had little effect on LC3 (MAP1LC3B/LC3B) lipidation, and LC3B puncta resembling autophagosomes by fluorescence microscopy were still evident albeit significantly smaller than those in the wild-type cells. Electron microscopy (EM) analysis revealed that BECN1 deficiency led to malformed autophagosome-like structures containing multiple layers of membranes under amino acid starvation. We further confirmed that the PtdIns3K complex activity and autophagy flux were disrupted in BECN1 ¡/¡ cells. Our results demonstrate the essential role of BECN1 in the functional formation of autophagosomes, but not in LC3B lipidation.

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