Epidemiological, clinical and experimental evidence suggests a link between type 2 diabetes and Alzheimer's disease (AD). Insulin modulates metabolism of ␤-amyloid precursor protein (APP) in neurons, decreasing the intracellular accumulation of ␤-amyloid (A␤) peptides, which are pivotal in AD pathogenesis. The present study investigates whether the widely prescribed insulin-sensitizing drug, metformin (Glucophage R ), affects APP metabolism and A␤ generation in various cell models. We demonstrate that metformin, at doses that lead to activation of the AMP-activated protein kinase (AMPK), significantly increases the generation of both intracellular and extracellular A␤ species. Furthermore, the effect of metformin on A␤ generation is mediated by transcriptional up-regulation of ␤-secretase (BACE1), which results in an elevated protein level and increased enzymatic activity. Unlike insulin, metformin exerts no effect on A␤ degradation. In addition, we found that glucose deprivation and various tyrphostins, known inhibitors of insulin-like growth factors/insulin receptor tyrosine kinases, do not modulate the effect of metformin on A␤. Finally, inhibition of AMP-activated protein kinase (AMPK) by the pharmacological inhibitor Compound C largely suppresses metformin's effect on A␤ generation and BACE1 transcription, suggesting an AMPK-dependent mechanism. Although insulin and metformin display opposing effects on A␤ generation, in combined use, metformin enhances insulin's effect in reducing A␤ levels. Our findings suggest a potentially harmful consequence of this widely prescribed antidiabetic drug when used as a monotherapy in elderly diabetic patients. A lzheimer's disease (AD) is a devastating neurodegenerative disorder, with aging, genetic, and environmental factors contributing to its development and progression. AD is not only characterized by pathological deposition of A␤ peptides and neurofibrillary tangles but is also associated with microgliamediated inflammation and dysregulated lipid homeostasis and glucose metabolism. Amyloid peptides are derived from sequential proteolytic cleavages of full-length amyloid precursor protein (APP) by ␤-secretase (BACE1) and ␥-secretase. Full-length APP can undergo alternative processing by ␣-secretase, releasing a soluble fragment (sAPP␣) extracellularly, which precludes A␤ formation. Compelling evidence indicates that A␤, especially the oligomers, are toxic to neurons; excessive generation and accumulation of A␤ peptides in neurons is believed to initiate the pathological cascade in AD (1-3).Epidemiological studies strongly suggest that metabolic defects correlate with the functional alterations associated with aging of the brain and with AD pathogenesis (4-11). The vast majority of AD cases are late onset and sporadic in origin with aging being the most profound risk factor. Insulin signaling is known to be involved in the process of brain aging (12)(13)(14)(15)(16)(17)(18)(19)(20). Insulin dysfunction/resistance in diabetes mellitus (DM) is not only a common syndrome ...

Vision loss from ischemic retinopathies commonly results from the accumulation of fluid in the inner retina [macular edema (ME)]. Although the precise events that lead to the development of ME remain under debate, growing evidence supports a role for an ischemia-induced hyperpermeability state regulated, in part, by VEGF. Monthly treatment with anti-VEGF therapies is effective for the treatment of ME but results in a major improvement in vision in a minority of patients, underscoring the need to identify additional therapeutic targets. Using the oxygen-induced retinopathy mouse model for ischemic retinopathy, we provide evidence showing that hypoxic Müller cells promote vascular permeability by stabilizing hypoxia-inducible factor-1α (HIF-1α) and secreting angiogenic cytokines. Blocking HIF-1α translation with digoxin inhibits the promotion of endothelial cell permeability in vitro and retinal edema in vivo. Interestingly, Müller cells require HIF-but not VEGF-to promote vascular permeability, suggesting that other HIF-dependent factors may contribute to the development of ME. Using gene expression analysis, we identify angiopoietinlike 4 (ANGPTL4) as a cytokine up-regulated by HIF-1 in hypoxic Müller cells in vitro and the ischemic inner retina in vivo. ANGPTL4 is critical and sufficient to promote vessel permeability by hypoxic Müller cells. Immunohistochemical analysis of retinal tissue from patients with diabetic eye disease shows that HIF-1α and ANGPTL4 localize to ischemic Müller cells. Our results suggest that ANGPTL4 may play an important role in promoting vessel permeability in ischemic retinopathies and could be an important target for the treatment of ME.diabetes | retinal vein occlusion | angiogenesis | transcription factor I schemic retinopathies include a diverse group of retinal diseases, in which immature retinal vasculature (e.g., retinopathy of prematurity or incontinentia pigmenti) or damage to mature retinal vessels (e.g., diabetic retinopathy, retinal vein occlusion, or sickle cell retinopathy) leads to retinal ischemia (1). Although diverse (and poorly understood) etiologies may lead to insufficient perfusion of the retina, all lead to a common sequelae: the formation of abnormal leaky blood vessels that can manifest clinically with the accumulation of fluid in the inner retina [i.e., macular edema (ME)] and often, a profound loss of vision (2). Indeed, ME in patients with ischemia-induced retinopathies remains the leading cause of vision loss in the working age population in the developed world (3).The concept that ischemic retinopathies are driven by ischemia-induced angiogenic factors was proposed over half a century ago (4). A single transcriptional activator, hypoxia-inducible factor-1 (HIF-1), has recently emerged as the master regulator of these angiogenic mediators. HIF-1 is a heterodimeric protein composed of an exquisitely oxygen-sensitive α-subunit and a ubiquitous β-subunit. Under hypoxic conditions, degradation of the oxygensensitive HIF-1α subunit is reduced, whereas its trans...

SignificanceIn proliferative diabetic retinopathy (PDR), the most vision-threatening sequela of diabetic eye disease, retinal ischemia leads to increased expression of angiogenic factors that promote neovascularization. Although therapies targeting the potent angiogenic mediator vascular endothelial growth factor have been remarkably successful for the treatment of diabetic macular edema, this approach has not proven sufficient to prevent the development of retinal neovascularization, implicating additional angiogenic factor(s) in PDR pathogenesis. We demonstrate here that angiopoietin-like 4 is a potent angiogenic mediator with markedly increased expression in the eyes of PDR patients. Our studies identify a novel therapeutic target for the treatment of ocular neovascular disease and may have broad implications for the treatment of other diseases dependent on pathologic angiogenesis.