Pancreatic beta cell death is a hallmark of type 1 (T1D) and type 2 (T2D) diabetes, but the molecular mechanisms underlying this aspect of diabetic pathology are poorly understood. Here we report that expression of the microRNA (miR)-200 family is strongly induced in islets of diabetic mice and that beta cell-specific overexpression of miR-200 in mice is sufficient to induce beta cell apoptosis and lethal T2D. Conversely, mir-200 ablation in mice reduces beta cell apoptosis and ameliorates T2D. We show that miR-200 negatively regulates a conserved anti-apoptotic and stress-resistance network that includes the essential beta cell chaperone Dnajc3 (also known as p58IPK) and the caspase inhibitor Xiap. We also observed that mir-200 dosage positively controls activation of the tumor suppressor Trp53 and thereby creates a pro-apoptotic gene-expression signature found in islets of diabetic mice. Consequently, miR-200-induced T2D is suppressed by interfering with the signaling of Trp53 and Bax, a proapoptotic member of the B cell lymphoma 2 protein family. Our results reveal a crucial role for the miR-200 family in beta cell survival and the pathophysiology of diabetes.npg

Over recent years, metabolic disorders such as type 2 diabetes have finally become recognized as a major challenge to global health. The attention of scientists therefore has to focus on improving our understanding of the molecular mechanisms behind these diseases and towards the design of new drug therapy strategies. The pathophysiology of diabetes is undoubtedly complex, oftentimes characterized by varying states of insulin resistance and impaired b-cell function; however, the identification of new pathways is constantly improving our understanding of the disease. We and others have recently shown that microRNAs (miRNAs) can play a role in insulin secretion and glucose homostasis. Thus, in this review, we will discuss the potential role of miRNAs in type 2 diabetes and related metabolic diseases.

Systemic administration of synthetic small interfering RNAs (siRNAs) effectively silences hepatocyte gene expression in rodents and primates [1][2][3] . Whether or not in vivo gene silencing by synthetic siRNA can disrupt the endogenous microRNA (miRNA) pathway remains to be addressed. Here we show that effective target-gene silencing in the mouse and hamster liver can be achieved by systemic administration of synthetic siRNA without any demonstrable effect on miRNA levels or activity. Indeed, siRNA targeting two hepatocyte-specific genes (apolipo-protein B and factor VII) that achieved efficient (~80%) silencing of messenger RNA transcripts and a third irrelevant siRNA control were administered to mice without significant changes in the levels of three hepatocyte-expressed miRNAs (miR-122, miR-16 and let-7a) or an effect on miRNA activity. Moreover, multiple administrations of an siRNA targeting the hepatocyte-expressed gene Scap in hamsters achieved long-term mRNA silencing without significant changes in miR-122 levels. This study advances the use of siRNAs as safe and effective tools to silence gene transcripts in animal studies, and supports the continued advancement of RNA interference therapeutics using synthetic siRNA.Recently, it was reported that adeno-associated viral (AAV)-expressed short hairpin RNA (shRNA), when administered to mice, can result in profound toxicity, presumably by saturation of the cellular miRNA pathway 4 . AAV expression of shRNA seemed to elicit toxicological effects by interference of cellular pathways for miRNA biogenesis, including NIH Public Access Author ManuscriptNature. Author manuscript; available in PMC 2011 January 11. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript transport of nuclear miRNA precursors to the cytoplasm by the nuclear karyphorin exportin-5. shRNA-mediated toxicological effects were evident by marked reductions in cytoplasmic levels of mature miRNA, and occurred in a manner independent of both 'ontarget' silencing and shRNA sequence. The findings of ref. 4 can be explained by viral vector expression of shRNA at high levels, because lower-level shRNA expression seems to be tolerated 4,5 . However, the implications for cellular delivery of synthetic siRNA that acts downstream of miRNA biogenesis have remained undetermined.We interrogated the biochemical and functional status of the miRNA pathway after in vivo administration of synthetic siRNA formulated in liposomal nanoparticles such as those recently described 2,6 . Formulated, synthetic siRNA targeting two hepatocyte-expressed gene transcripts (si-Apob targeting apolipoprotein B (Apob) and si-FVII targeting factor VII (F7)) and an irrelevant gene (si-Luc targeting luciferase) were administered by a single intravenous bolus injection at different dose levels in groups of three mice. At both 2 and 30 days, silencing of on-target gene transcripts in liver homogenates was measured by the branched DNA assay 2 , and levels of endogenous miRNA (miR-122, miR-16 and let-7a) we...

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