Malignant progression is greatly affected by dynamic cross-talk between stromal and cancer cells. Exosomes are secreted nanovesicles that have key roles in cell–cell communication by transferring nucleic acids and proteins to target cells and tissues. Recently, MicroRNAs (miRs) and their delivery in exosomes have been implicated in physiological and pathological processes. Tumor-delivered miRs, interacting with stromal cells in the tumor microenvironment, modulate tumor progression, angiogenesis, metastasis and immune escape. Altered cell metabolism is one of the hallmarks of cancer. A number of different types of tumor rely on mitochondrial metabolism by triggering adaptive mechanisms to optimize their oxidative phosphorylation in relation to their substrate supply and energy demands. Exogenous exosomes can induce metabolic reprogramming by restoring the respiration of cancer cells and supress tumor growth. The exosomal miRs involved in the modulation of cancer metabolism may be potentially utilized for better diagnostics and therapy.

MiR126 affects mitochondrial energy metabolism, resulting in MM tumor suppression. Since MM is a fatal neoplastic disease with a few therapeutic options, this finding is of potential translational importance.

BackgroundMitochondrial dysfunction induces insulin resistance in myocytes via a reduction of insulin receptor substrate-1 (IRS-1) expression. However, the effect of mitochondrial dysfunction on insulin sensitivity is not understood well in hepatocytes. Although research has implicated the translational repression of target genes by endogenous non-coding microRNAs (miRNA) in the pathogenesis of various diseases, the identity and role of the miRNAs that are involved in the development of insulin resistance also remain largely unknown.MethodologyTo determine whether mitochondrial dysfunction induced by genetic or metabolic inhibition causes insulin resistance in hepatocytes, we analyzed the expression and insulin-stimulated phosphorylation of insulin signaling intermediates in SK-Hep1 hepatocytes. We used qRT-PCR to measure cellular levels of selected miRNAs that are thought to target IRS-1 3′ untranslated regions (3′UTR). Using overexpression of miR-126, we determined whether IRS-1-targeting miRNA causes insulin resistance in hepatocytes.Principal FindingsMitochondrial dysfunction resulting from genetic (mitochondrial DNA depletion) or metabolic inhibition (Rotenone or Antimycin A) induced insulin resistance in hepatocytes via a reduction in the expression of IRS-1 protein. In addition, we observed a significant up-regulation of several miRNAs presumed to target IRS-1 3′UTR in hepatocytes with mitochondrial dysfunction. Using reporter gene assay we confirmed that miR-126 directly targeted to IRS-1 3′UTR. Furthermore, the overexpression of miR-126 in hepatocytes caused a substantial reduction in IRS-1 protein expression, and a consequent impairment in insulin signaling.Conclusions/SignificanceWe demonstrated that miR-126 was actively involved in the development of insulin resistance induced by mitochondrial dysfunction. These data provide novel insights into the molecular basis of insulin resistance, and implicate miRNA in the development of metabolic disease.

Abbreviations: GLUT, glucose transporter; HBP, hexosamine biosynthetic pathway; IRS, insulin receptor substrate; PDK-1, phosphatidylinositol dependent protein kinase-1; PKC-λ/ζ, protein kinase C-λ/ζ; PI3K, phospatidylinositol 3-kinase; PM, plasma membrane IntroductionThe pathophysiology of non-insulin-dependent diabetes mellitus (NIDDM) results from impaired peripheral tissue sensitivity to insulin and reduction of insulin secretion. Glucose uptake in mammalian cells is mediated by a family of intrinsic membrane proteins known as facilitative glucose transporters, GLUTs (Mueckler, 1994). GLUT4, the insulin-responsive glucose transporter, is selectively expressed in adipocytes and muscle (Bryant et al., 2002). GLUT4 in these cells constantly recycles between the plasma membrane (PM) and intracellular storage pools (Cushm an and W ardzala, 1980). About 90 percent of GLUT4 is sequestered intracellularly in the absence of insulin or other stimuli such as exercise (Cushman and Wardzala, 1980). Insulin stimulates glucose uptake in these cells primarily by inducing net translocation of GLUT4 from the intracellular storage sites to the PM (Lee et al., 1999, Lee et al., 2000b. The translocation of GLUT4 by insulin results from a cascade of signal transduction, which is composed of a series of molecules (Pessin et al., 1999). Insulin-stimulated GLUT4 translocation and glucose uptake are mediated largely through the activation/phosphorylation of IRS-1 and its downstream effectors, such as PI3K, PDK-1, PKC-λ/ζ, and Akt2 (Saltiel and Pessin, 2002;Kanzaki and Pessin, 2003). An impaired GLUT4 translocation and/or insulin signaling pathway would result in insulin resistance and hyperglycemia, the primary characteristics and hallmarks of NIDDM (Watson and Pessin, 2001). O-GlcNAc m odification on IRS-1 and Akt2 by PUGNAc inhibits their phosphorylation and induces insulin resistance in rat prim ary adipocytes O-GlcNAc modification and insulin resistance 221It has been known that O-GlcNAc modification of protein plays an important role in transcription, translation, nuclear transport and cytoskeletal assembly (Comer and Hart, 2000;Wells et al., 2001). Recently, O-GlcNAc modification as well as phosphorylation/ dephosphorylation has been focused on as a main regulation mechanism of cellular signal transduction pathways. Post-translational O-GlcNAc modification on proteins is defined as the O-linked attachment of single GlcNAc moiety to specific hydroxyl groups of Ser or Thr residue (Buse et al., 2002). Since Oglycosylation of cellular proteins via Ser/Thr takes place dynamically in various ways and the modification site is usually located near the O-phosphorylation site or sometimes at the same site, O-GlcNAc modification plays as a dynamic negative regulator against protein phosphorylation (Snow and Hart, 1998, Zachara and. Therefore, O-glycosylation and O-phosphorylation are known to participate in the modification of protein interaction and regulation of signal transduction by changing the protein reaction site through the mutua...

a b s t r a c tMicroRNAs (miRNAs) play an important role in insulin signaling and insulin secretion, but the role of miRNAs in the association between obesity and hepatic insulin resistance is largely unknown. This study reports that saturated fatty acid (SFA) and high fat diet (HFD) significantly induce miR-195 expression in hepatocytes, and that the insulin receptor (INSR), not insulin receptor substrate-1 (IRS-1), is a direct target of miR-195. Furthermore, the ectopic expression of miR-195 suppresses the expression of INSR, thereby impairing the insulin signaling cascade and glycogen synthesis in HepG2 cells. These findings suggest that the dysregulation of miR-195 by SFA is a detrimental factor for hepatic insulin sensitivity.