Amino acid-sensing mTOR signaling is involved in modulation of lipolysis by chronic insulin treatment in adipocytes

miR-720 Regulates Insulin Secretion by Targeting Rab35

miRNAs pose a good prospect in the diagnosis and treatment of type 2 diabetes (T2D). This study is aimed at investigating whether miR-720 targets Rab35 to regulate insulin secretion in MIN6 cells and its molecular mechanism and the clinical value of miR-720 as a specific biomarker of T2D. Fifty-five samples of new diagnosis T2D patients and normal control were collected. Levels of miR-720, fasting blood glucose, insulin, and other indicators of glucose and lipid metabolism were determined. We increased and decreased the miR-720 expression using miR-720 mimic and inhibitor to identify the effect of miR-720 on insulin secretion in MIN6 cells, respectively. Then, we used miR-720 mimic, miR-720 inhibitor, and dual luciferase reporter gene assays to prove miR-720 which regulates insulin secretion by targeting Rab35 in MIN6 cells. In addition, we overexpressed and silenced the Rab35 gene to detect the expression of PI3K, Akt, and mTOR in MIN6 cells by RT-PCR and western blot. In this study, circulating miR-720 was significantly higher in the T2D group than the control group, and miR-270 was positive correlated with FBG, while negatively correlated with FINS. The overexpression of miR-720 inhibited insulin secretion, and miR-720 downregulation promoted insulin secretion. miR-720 regulated insulin secretion by targeting Rab35 in MIN6 cells. Compared with the control group, the expression of PI3K, Akt, and mTOR was significantly decreased by the overexpression of the Rab35 gene, while the silencing Rab35 gene could induce the expression of PI3K, Akt, and mTOR. Furthermore, miR-720 mimic could activate the PI3K pathway. We conclude that miR-720 may be a potential biomarker for the diagnosis of T2D. Increase of miR-720 reduced the Rab35 expression then activate the PI3K/Akt/mTOR signal pathway, thus inhibiting insulin secretion.

The Role of Mammalian Target of Rapamycin (mTOR) in Insulin Signaling

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that controls a wide spectrum of cellular processes, including cell growth, differentiation, and metabolism. mTOR forms two distinct multiprotein complexes known as mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), which are characterized by the presence of raptor and rictor, respectively. mTOR controls insulin signaling by regulating several downstream components such as growth factor receptor-bound protein 10 (Grb10), insulin receptor substrate (IRS-1), F-box/WD repeat-containing protein 8 (Fbw8), and insulin like growth factor 1 receptor/insulin receptor (IGF-IR/IR). In addition, mTORC1 and mTORC2 regulate each other through a feedback loop to control cell growth. This review outlines the current understanding of mTOR regulation in insulin signaling in the context of whole body metabolism.

mTORC1 signaling and the metabolic control of cell growth

mTOR [mechanistic target of rapamycin] is a serine/threonine protein kinase that, as part of mTORC1 (mTOR complex 1), acts as an important molecular connection between nutrient signals and the metabolic processes indispensable for cell growth. While there has been pronounced interest in the upstream mechanisms regulating mTORC1, the full range of downstream molecular targets through which mTORC1 signaling stimulates cell growth is only recently emerging. It is now evident that mTORC1 promotes cell growth primarily through the activation of key anabolic processes. Through a diverse set of downstream targets, mTORC1 promotes the biosynthesis of macromolecules, including proteins, lipids, and nucleotides to build the biomass underlying cell, tissue, and organismal growth. Here, we focus on the metabolic functions of mTORC1 as they relate to the control of cell growth. As dysregulated mTORC1 underlies a variety of human diseases, including cancer, diabetes, autoimmune diseases, and neurological disorders, understanding the metabolic program downstream of mTORC1 provides insights into its role in these pathological states.

Oncogenic PI3K and K-Ras stimulate de novo lipid synthesis through mTORC1 and SREBP

An enhanced capacity for de novo lipid synthesis is a metabolic feature of most cancer cells that distinguishes them from their cells of origin. However, the mechanisms through which oncogenes alter lipid metabolism are poorly understood. We find that expression of oncogenic PI3K (H1047R) or K-Ras (G12V) in breast epithelial cells is sufficient to induce de novo lipogenesis, and this occurs through the convergent activation of the mechanistic target of rapamycin complex 1 (mTORC1) downstream of these common oncogenes. Oncogenic stimulation of mTORC1 signaling in this isogenic setting or a panel of eight breast cancer cell lines leads to activation of the sterol regulatory element-binding proteins (SREBP1 and SREBP2) that are required for oncogene-induced lipid synthesis. The SREBPs are also required for the growth factor-independent growth and proliferation of oncogene-expressing cells. Finally, we find that elevated mTORC1 signaling is associated with increased mRNA and protein levels of canonical SREBP targets in primary human breast cancer samples. These data suggest that the mTORC1/SREBP pathway is a major mechanism through which common oncogenic signaling events induce de novo lipid synthesis to promote aberrant growth and proliferation of cancer cells.

mTOR and tau phosphorylated proteins in the hippocampal tissue of rats with type 2 diabetes and Alzheimer's disease

This study explored the effects of mammalian target of rapamycin (mTOR) on the increased risk of developing Alzheimer's disease (AD) in rats with type 2 diabetes mellitus (T2DM). Male Sprague-Dawley rats were randomly divided into four groups: control, T2DM, AD and T2DM+AD. Changes in the learning and memory abilities of the rats were observed using the Morris water maze. mTOR activity and tau protein hyperphosphorylation in the hippocampus were analyzed by immunohistochemical staining and RT-PCR. The learning and memory abilities of the experimental rats were weakened compared with those of the control group. The T2DM+AD group revealed significant changes over the T2DM and AD groups. Compared with the control, T2DM and AD groups, the mTOR protein and mRNA levels, hyperphosphorylation of tau protein and total tau protein mRNA levels were significantly increased in the T2DM+AD group. T2DM may excessively activate mTOR in the hippocampal tissue by impairing insulin signaling, thereby increasing the extent of tau hyperphosphorylation and promoting the occurrence of AD.

The multifaceted role of mTORC1 in the control of lipid metabolism

The mechanistic target of rapamycin is a protein kinase that, as part of the mechanistic target of rapamycin complex 1 (mTORC1), senses both local nutrients and, through insulin signalling, systemic nutrients to control a myriad of cellular processes. Although roles for mTORC1 in promoting protein synthesis and inhibiting autophagy in response to nutrients have been well established, it is emerging as a central regulator of lipid homeostasis. Here, we discuss the growing genetic and pharmacological evidence demonstrating the functional importance of its signalling in controlling mammalian lipid metabolism, including lipid synthesis, oxidation, transport, storage and lipolysis, as well as adipocyte differentiation and function. Defining the role of mTORC1 signalling in these metabolic processes is crucial to understanding the pathophysiology of obesity and its relationship to complex diseases, including diabetes and cancer.

Hepatitis C virus core protein decreases lipid droplet turnover: a mechanism for core-induced steatosis

Steatosis is a frequent complication of hepatitis C virus infection. In mice, this condition is recapitulated by the expression of a single viral protein, the nucleocapsid core. Core localizes to the surface of lipid droplets (LDs) in infected liver cells through a process dependent on host diacylglycerol acyltransferase 1 (DGAT1), an enzyme that synthesizes triglycerides in the endoplasmic reticulum. Whether DGAT1 also plays a role in core-induced steatosis is uncertain. Here, we show that mouse embryonic fibroblasts isolated from DGAT1(-/-) mice are protected from core-induced steatosis, as are livers of DGAT1(-/-) mice expressing core, demonstrating that the steatosis is DGAT1-dependent. Surprisingly, core expression did not increase DGAT1 activity or triglyceride synthesis, thus excluding the possibility that core activates DGAT1 to cause steatosis. Instead, we find that DGAT1-dependent localization of core to LDs is a prerequisite for the steatogenic properties of the core. Using biochemical and immunofluorescence microscopy techniques, we show that the turnover of lipids in core-coated droplets is decreased, providing a physiological mechanism for core-induced steatosis. Our results support a bipartite model in which core first requires DGAT1 to gain access to LDs, and then LD-localized core interferes with triglyceride turnover, thus stabilizing lipid droplets and leading to steatosis.

Chronic leucine supplementation improves glycemic control in etiologically distinct mouse models of obesity and diabetes mellitus

BACKGROUND

Leucine may function as a signaling molecule to regulate metabolism. We have previously shown that dietary leucine supplementation significantly improves glucose and energy metabolism in diet-induced obese mice, suggesting that leucine supplementation could potentially be a useful adjuvant therapy for obesity and type 2 diabetes. Since the underlying cause for obesity and type 2 diabetes is multifold, we further investigated metabolic effects of leucine supplementation in obese/diabetes mouse models with different etiologies, and explored the underlying molecular mechanisms.

METHODS

Leucine supplementation was carried out in NONcNZO10/LtJ (RCS10) - a polygenic model predisposed to beta cell failure and type 2 diabetes, and in B6.Cg-Ay/J (Ay) - a monogenic model for impaired central melanocortin receptor signaling, obesity, and severe insulin resistance. Mice in the treatment group received the drinking water containing 1.5% leucine for up to 8 months; control mice received the tap water. Body weight, body composition, blood HbA1c levels, and plasma glucose and insulin levels were monitored throughout and/or at the end of the study period. Indirect calorimetry, skeletal muscle gene expression, and adipose tissue inflammation were also assessed in Ay mice.

RESULTS

Leucine supplementation significantly reduced HbA1c levels throughout the study period in both RCS10 and Ay mice. However, the treatment had no long term effect on body weight or adiposity. The improvement in glycemic control was associated with an increased insulin response to food challenge in RCS10 mice and decreased plasma insulin levels in Ay mice. In leucine-treated Ay mice, energy expenditure was increased by ~10% (p < 0.05) in both dark and light cycles while the physical activity level was unchanged. The expression levels of UCP3, CrAT, PPAR-alpha, and NRF-1, which are known to regulate mitochondrial oxidative function, were significantly increased in the soleus muscle of leucine-treated Ay mice whereas the expression levels of MCP-1 and TNF-alpha and macrophage infiltration in adipose tissue were significantly reduced.

CONCLUSIONS

Chronic leucine supplementation significantly improves glycemic control in multiple mouse models of obesity and diabetes with distinct etiologies. The metabolic benefits of leucine supplementation are likely mediated via multiple mechanisms in different tissues, but are not necessarily dependent of weight reduction.

Mammalian target of rapamycin and diabetes: what does the current evidence tell us?

New-onset diabetes mellitus after transplantation (NODAT) is a serious complication in organ transplantation; not only does it enhance the risk of graft dysfunction, it also increases cardiovascular morbidity and mortality. The mammalian target of rapamycin (mTOR) is regulated independently by insulin, amino acids, and energy sufficiency. It integrates signal from growth factors, hormones, nutrients, and cellular energy levels to regulate protein translation and cell growth, proliferation, and survival. In addition, mTOR generates an inhibitory feedback loop on insulin receptor substrate (IRS) proteins. Therefore, it was suggested that mTOR might link nutrient excess with both obesity and insulin resistance. In this review, we summarize the role of mTOR and its inhibitor sirolimus (SRL) on chronic hyperglycemia and insulin resistance in beta cells, adipose tissue, liver, and muscle. We further hypothesize, based on data from the literature and generated in our laboratory, that SRL could counteract the development of NODAT in stable glucose homeostasis due to its positive effects on insulin-stimulated glucose uptake, whereas in conditions that require an adaptive beta cell proliferation (such as pregnancy and weight increase), the administration of SRL might have effects that would promote the development of NODAT. Therefore, it seems crucial for patient outcome to consider these potentially contrasting effects of SRL.