Role of miR-199a-5p in the post-transcriptional regulation of ABCA1 in response to hypoxia in peritoneal macrophages.

Hypoxia is a crucial factor contributing to maintenance of atherosclerotic lesions. The ability of ABCA1 to stimulate the efflux of cholesterol from cells in the periphery, particularly foam cells in atherosclerotic plaques, is an important anti-atherosclerotic mechanism. The posttranscriptional regulation by miRNAs represents a key regulatory mechanism of a number of signaling pathways involved in atherosclerosis. Previously, miR-199a-5p has been shown to be implicated in the endocytic and retrograde intracellular transport. Although the regulation of miR-199a-5p and ABCA1 by hypoxia has been already reported independently, the role of miR-199a-5p in macrophages and its possible role in atherogenic processes such us regulation of lipid homeostasis through ABCA1 has not been yet investigated. Here, we demonstrate that both ABCA1 and miR-199a-5p show an inverse regulation by hypoxia and Ac-LDL in primary macrophages. Moreover, we demonstrated that miR-199a-5p regulates ABCA1 mRNA and protein levels by directly binding to its 3'UTR. As a result, manipulation of cellular miR-199a-5p levels alters ABCA1 expression and cholesterol efflux in primary mouse macrophages. Taken together, these results indicate that the correlation between ABCA1-miR-199a-5p could be exploited to control macrophage cholesterol efflux during the onset of atherosclerosis, where cholesterol alterations and hypoxia play a pathogenic role.

New Insights on the Regulation of the Insulin-Degrading Enzyme: Role of microRNAs and RBPs.

The evident implication of the insulin-degrading enzyme (IDE) in Alzheimer's disease (AD) and type 2 diabetes mellitus (T2DM), among its capacity to degrade insulin and amyloid-ß peptide (Aß), suggests that IDE could be an essential link in the relation between hyperinsulinemia, insulin resistance and AD. However, little is known about the cellular and molecular regulation of IDE expression, and even less has been explored regarding the post-transcriptional regulation of IDE, although it represents a great molecular target of interest for therapeutic treatments. We recently described that miR-7, a novel candidate for linking AD and T2DM at the molecular level, regulates IDE and other key genes in both pathologies, including some key genes involved in the insulin signaling pathway. Here, we explored whether other miRNAs as well as other post-transcriptional regulators, such as RNA binding proteins (RBP), could potentially participate in the regulation of IDE expression in vitro. Our data showed that in addition to miR-7, miR-125, miR-490 and miR-199 regulate IDE expression at the post-transcriptional level. Moreover, we also found that IDE contains multiple potential binding sites for several RBPs, and a narrow-down prediction analysis led us to speculate on a novel regulation of IDE by RALY and HuD. Taken together, these results demonstrate the novel players controlling IDE expression that could represent potential therapeutical targets to treat several metabolic diseases with a high impact on human health, including AD and T2DM.

Posttranscriptional Regulation of Insulin Resistance: Implications for Metabolic Diseases.

Insulin resistance defines an impairment in the biologic response to insulin action in target tissues, primarily the liver, muscle, adipose tissue, and brain. Insulin resistance affects physiology in many ways, causing hyperglycemia, hypertension, dyslipidemia, visceral adiposity, hyperinsulinemia, elevated inflammatory markers, and endothelial dysfunction, and its persistence leads to the development metabolic disease, including diabetes, obesity, cardiovascular disease, or nonalcoholic fatty liver disease (NAFLD), as well as neurological disorders such as Alzheimer's disease. In addition to classical transcriptional factors, posttranscriptional control of gene expression exerted by microRNAs and RNA-binding proteins constitutes a new level of regulation with important implications in metabolic homeostasis. In this review, we describe miRNAs and RBPs that control key genes involved in the insulin signaling pathway and related regulatory networks, and their impact on human metabolic diseases at the molecular level, as well as their potential use for diagnosis and future therapeutics.

MicroRNA 7 Impairs Insulin Signaling and Regulates Aß Levels through Posttranscriptional Regulation of the Insulin Receptor Substrate 2, Insulin Receptor, Insulin-Degrading Enzyme, and Liver X Receptor Pathway.

Brain insulin resistance is a key pathological feature contributing to obesity, diabetes, and neurodegenerative disorders, including Alzheimer's disease (AD). Besides the classic transcriptional mechanism mediated by hormones, posttranscriptional regulation has recently been shown to regulate a number of signaling pathways that could lead to metabolic diseases. Here, we show that microRNA 7 (miR-7), an abundant microRNA in the brain, targets insulin receptor (INSR), insulin receptor substrate 2 (IRS-2), and insulin-degrading enzyme (IDE), key regulators of insulin homeostatic functions in the central nervous system (CNS) and the pathology of AD. In this study, we found that insulin and liver X receptor (LXR) activators promote the expression of the intronic miR-7-1 in vitro and in vivo, along with its host heterogeneous nuclear ribonucleoprotein K (HNRNPK) gene, encoding an RNA binding protein (RBP) that is involved in insulin action at the posttranscriptional level. Our data show that miR-7 expression is altered in the brains of diet-induced obese mice. Moreover, we found that the levels of miR-7 are also elevated in brains of AD patients; this inversely correlates with the expression of its target genes IRS-2 and IDE. Furthermore, overexpression of miR-7 increased the levels of extracellular Aß in neuronal cells and impaired the clearance of extracellular Aß by microglial cells. Taken together, these results represent a novel branch of insulin action through the HNRNPK-miR-7 axis and highlight the possible implication of these posttranscriptional regulators in a range of diseases underlying metabolic dysregulation in the brain, from diabetes to Alzheimer's disease.

Adult Mice Lacking Mct8 and Dio2 Proteins Present Alterations in Peripheral Thyroid Hormone Levels and Severe Brain and Motor Skill Impairments.

Background: Mutations in the thyroid hormone (TH) transporter monocarboxylate transporter 8 (MCT8) lead to peripheral hyperthyroidism and profound psychomotor alterations in humans. Mice lacking Mct8 present peripheral hyperthyroidism but no gross neurological abnormalities due to brain compensatory mechanisms involving the enzyme deiodinase type 2 (Dio2). Methods: Here we have analyzed the endocrine and neurologic phenotype of mice lacking both Mct8 and Dio2 at three and six months of age. Thyroxine (T4) and 3,5,3' triiodothyronine (T3) levels/content were measured by specific radioimmunoassays; motor skill performance was evaluated by the footprint, rotarod, four limb hanging wire, and balance beam tests; and brain histological analysis was performed by immunostaining for neurofilament and parvalbumin. Results: We have found that this mouse model presents peripheral hyperthyroidism and brain hypothyroidism. Interestingly, the severity of the brain hypothyroidism seems permanent and varies across regions, with the striatum being a particularly affected area. We have also found brain alterations at the histological level compatible with TH deficiency and impaired motor skills. Conclusions: These findings indicate the potential of Mct8/Dio2-deficient mice to represent a model for human MCT8 deficiency, to understand the mechanisms underlying its pathophysiology, and ultimately design therapeutic interventions for human patients.