MicroRNAs (miRNAs) are small non-coding RNAs with the capability of modulating gene expression at the post-transcriptional level either by inhibiting messenger RNA (mRNA) translation or by promoting mRNA degradation. The outcome of a myriad of physiological processes and pathologies, including cancer, cardiovascular and metabolic diseases, relies highly on miRNAs. However, deciphering the precise roles of specific miRNAs in these pathophysiological contexts is challenging due to the high levels of complexity of their actions. Indeed, regulation of mRNA expression by miRNAs is frequently cell/organ specific; highly dependent on the stress and metabolic status of the organism; and often poorly correlated with miRNA expression levels. Such biological features of miRNAs suggest that various regulatory mechanisms control not only their expression, but also their activity and/or bioavailability. Several mechanisms have been described to modulate miRNA action, including genetic polymorphisms, methylation of miRNA promoters, asymmetric miRNA strand selection, interactions with RNA-binding proteins (RBPs) or other coding/non-coding RNAs. Moreover, nucleotide modifications (A-to-I or C-to-U) within the miRNA sequences at different stages of their maturation are also critical for their functionality. This regulatory mechanism called “RNA editing” involves specific enzymes of the adenosine/cytidine deaminase family, which trigger single nucleotide changes in primary miRNAs. These nucleotide modifications greatly influence a miRNA’s stability, maturation and activity by changing its specificity towards target mRNAs. Understanding how editing events impact miRNA’s ability to regulate stress responses in cells and organs, or the development of specific pathologies, e.g., metabolic diseases or cancer, should not only deepen our knowledge of molecular mechanisms underlying complex diseases, but can also facilitate the design of new therapeutic approaches based on miRNA targeting. Herein, we will discuss the current knowledge on miRNA editing and how this mechanism regulates miRNA biogenesis and activity.

Non-alcoholic fatty liver disease (NAFLD) is associated with a thorough reprogramming of hepatic metabolism. Epigenetic mechanisms, in particular those associated with deregulation of the expressions and activities of microRNAs (miRNAs), play a major role in metabolic disorders associated with NAFLD and their progression towards more severe stages of the disease. In this review, we discuss the recent progress addressing the role of the many facets of complex miRNA regulatory networks in the development and progression of NAFLD. The basic concepts and mechanisms of miRNA-mediated gene regulation as well as the various setbacks encountered in basic and translational research in this field are debated. miRNAs identified so far, whose expressions/activities are deregulated in NAFLD, and which contribute to the outcomes of this pathology are further reviewed. Finally, the potential therapeutic usages in a short to medium term of miRNA-based strategies in NAFLD, in particular to identify non-invasive biomarkers, or to design pharmacological analogues/inhibitors having a broad range of actions on hepatic metabolism, are highlighted.

Phosphatase and tensin homolog (PTEN) is a regulator of phosphoinositide 3-kinase signaling and an important tumor suppressor mutated/deleted in human cancers. PTEN deletion in the liver leads to insulin resistance, steatosis, inflammation, and cancer. We recently demonstrated that unsaturated fatty acids trigger steatosis by down-regulating PTEN expression in hepatocytes via activation of a mammalian target of rapamycin (mTOR)/nuclear factor kappa B (NF-κB) complex, but the molecular mechanisms implicated in this process are still unknown. Here, we investigated potential genetic and epigenetic mechanisms activated by fatty acids leading to PTEN down-regulation. Our results indicate that unsaturated fatty acids down-regulate PTEN messenger RNA expression in hepatocytes through mechanisms unrelated to methylation of the PTEN promoter, histone deacetylase activities, or repression of the PTEN promoter activity. In contrast, unsaturated fatty acids up-regulate the expression of microRNA-21, which binds to PTEN messenger RNA 3′-untranslated region and induces its degradation. The promoter activity of microRNA-21 was increased by mTOR/NF-κB activation. Consistent with these data, microRNA-21 expression was increased in the livers of rats fed high-fat diets and in human liver biopsies of obese patients having diminished PTEN expression and steatosis. Conclusion: Unsaturated fatty acids inhibit PTEN expression in hepatocytes by up-regulating microRNA-21 synthesis via an mTOR/NF-κB–dependent mechanism. Aberrant up-regulation of microRNA-21 expression by excessive circulating levels of fatty acids exemplify a novel regulatory mechanism by which fatty acids affect PTEN expression and trigger liver disorders. (Hepatology 2009.)

BACKGROUND AND PURPOSE mTOR inhibitors are currently used as immunosuppressants in transplanted patients and as promising anti‐cancer agents. However, new‐onset diabetes is a frequent complication occurring in patients treated with mTOR inhibitors such as rapamycin (Sirolimus). Here, we investigated the mechanisms associated with the diabetogenic effects of chronic Sirolimus administration in rats and in in vitro cell cultures.EXPERIMENTAL APPROACH Sirolimus was administered to rats fed either a standard or high‐fat diet for 21 days. Metabolic parameters were measured in vivo and in ex vivo tissues. Insulin sensitivity was assessed by glucose tolerance tests and euglycaemic hyperinsulinaemic clamps. Rapamycin effects on glucose metabolism and insulin signalling were further evaluated in cultured myotubes.KEY RESULTS Sirolimus induced a decrease in food intake and concomitant weight loss. It also induced specific fat mass loss that was independent of changes in food intake. Despite these beneficial effects, Sirolimus‐treated rats were glucose intolerant, hyperinsulinaemic and hyperglycaemic, but not hyperlipidaemic. The euglycaemic hyperinsulinaemic clamp measurements showed skeletal muscle is a major site of Sirolimus‐induced insulin resistance. At the molecular level, long‐term Sirolimus administration attenuated glucose uptake and metabolism in skeletal muscle by preventing full insulin‐induced Akt activation and altering the expression and translocation of glucose transporters to the plasma membrane. In rats fed a high‐fat diet, these metabolic defects were exacerbated, although Sirolimus‐treated animals were protected from diet‐induced obesity.CONCLUSIONS AND IMPLICATIONS Taken together, our data demonstrate that the diabetogenic effect of chronic rapamycin administration is due to an impaired insulin action on glucose metabolism in skeletal muscles.

Objective Hepatocellular carcinoma (HCC) development occurs with non-alcoholic fatty liver disease (NAFLD) in the absence of cirrhosis and with an increasing incidence due to the obesity pandemic. Mutations of tumour suppressor (TS) genes and oncogenes (ONC) have been widely characterised in HCC. However, mounting evidence indicates that non-genomic alterations of TS/ONC occur early with NAFLD, thereby potentially promoting hepatocarcinogenesis in an inflammatory/fibrotic context. The aim of this study was to identify and characterise these alterations. Design The proteome of steatotic liver tissues from mice spontaneously developing HCC was analysed. Alterations of TSs/ONCs were further investigated in various mouse models of NAFLD/HCC and in human samples. The inflammatory, fibrogenic and oncogenic functions of S100A11 were assessed through in vivo, in vitro and ex-vivo analyses. Results A whole set of TSs/ONCs, respectively, downregulated or upregulated was uncovered in mice and human with NAFLD. Alterations of these TSs/ONCs were preserved or even exacerbated in HCC. Among them, overexpression of S100A11 was associated with high-grade HCC and poor prognosis. S100A11 downregulation in vivo significantly restrains the development of inflammation and fibrosis in mice fed a choline/methionine-deficient diet. Finally, in vitro and ex-vivo analyses revealed that S100A11 is a marker of hepatocyte de-differentiation, secreted by cancer cells, and promoting cell proliferation and migration. Conclusion Cellular stress associated with NAFLD triggers non-genomic alterations of a whole network of TSs/ONCs fostering hepatocarcinogenesis. Among those, overexpression of the oncogenic factor S100A11 promotes inflammation/fibrosis in vivo and is significantly associated with high-grade HCC with poor prognosis.

The phosphoinositide 3-kinase (PI3K)/phosphatase and tensin homolog (PTEN)/Akt axis is a key signal transduction node that regulates crucial cellular functions, including insulin and other growth factors signaling, lipid and glucose metabolism, as well as cell survival and apoptosis. In this pathway, PTEN acts as a phosphoinositide phosphatase, which terminates PI3K-propagated signaling by dephosphorylating PtdIns(3,4)P₂ and PtdIns(3,4,5)P₃. However, the role of PTEN does not appear to be restricted only to PI3K signaling antagonism, and new functions have been recently discovered for this protein. In addition to the well-established role of PTEN as a tumor suppressor, increasing evidence now suggests that a dysregulated PTEN expression and/or activity is also linked to the development of several hepatic pathologies. Dysregulated PTEN expression/activity is observed with obesity, insulin resistance, diabetes, hepatitis B virus/hepatitis C virus infections, and abusive alcohol consumption, whereas mutations/deletions have also been associated with the occurrence of hepatocellular carcinoma. Thus, it appears that alterations of PTEN expression and activity in hepatocytes are common and recurrent molecular events associated with liver disorders of various etiologies. These recent findings suggest that PTEN might represent a potential common therapeutic target for a number of liver pathologies.