Insulin is a hormone produced by the beta cells of the pancreas to regulate circulating blood glucose for energy. Glucose enters the bloodstream after a meal leading to an increase in insulin concentrations in the body which directs glucose uptake by muscle and fat cells. While the precise cause of insulin resistance is not known, elevated levels of the hormone (hyperglycemia) remain in circulation, do to a loss of insulin action in its target tissues. As a result, blood glucose levels remain elevated exacerbating the metabolic consequences of hyperglycemia.
As insulin resistance progresses over time, islet beta-cells are very dynamic in nature and can adapt to this metabolic stress by proliferating in order to accommodate the increased demand for insulin. Since all of the body’s cells need glucose for energy, the beta-cells of the pancreas continually produce more insulin to ensure proper glucose homeostasis. This process can go on for years without notice up to the point where type-2 diabetes finally develops.
Dr. Poy and his colleagues carefully addressed how beta cells make use of the microRNA (miRNA) pathway to proliferate and secrete insulin during changes in insulin sensitivity. “Recent evidence has shown that the miRNA pathway is an important regulator of gene expression in response to metabolic stress”, Dr. Poy pointed out. “It also has been known that the miRNA pathway has an essential role in the proliferation of beta-cells, however the extent of its contribution is unclear”, he said.
MicroRNAs are small ribonucleic acidsmolecules with about 22 nucleotides that are known transcriptional products of DNA. Only within the last 10 years, researchers have discovered that microRNAs play an important role in virtually all mammalian cells. While their precise function remains unclear, miRNAs have been established as significant regulators of gene expression. That is, miRNAs control which proteins and how much of a protein cells secrete.
Sudhir G. Tattikota, Thomas Rathjen and Dr. Poy now have identified two different miRNAs with independent roles in the insulin metabolism including miRNA-184 (miR-184). The researchers could show that this microRNA is silenced as insulin resistance develops – in mice as wells as in humans who have acquired type-2 diabetes.
As a result of the silencing of this miRNA, expression of targeted genes including Argonaute2 (Ago2), are upregulated in the beta cells. Ago2 belongs to the Argonaute family of proteins that are known to guide the miRNAs to their target genes. According to the researchers this protein plays a key role during insulin resistance by facilitating beta-cell proliferation and the production of more insulin to compensate for insulin resistance.
The researchers worked with obese mice that had already developed insulin resistance. Obesity is one among others a risk factor for the developing of diabetes type-2. When the researchers deleted Ago2 specifically in the beta cells of the pancreas in these mice, the compensatory proliferation of the beta-cells was reduced. “This underlines the integral role of Ago2 and the miRNA pathway in this process”, Dr. Poy and his colleagues stressed.
How a Low-carbohydrate diet restores function of beta-cells in obese mice
Previous studies in mice and humans have shown that a high fat, low carbohydrate or “ketogenic” diet can improve insulin sensitivity. The researchers showed that obese mice fed this diet restored the function of miR-184, thereby controlling both beta cell proliferation as well as the release of insulin by targeting Ago2 as well as another gene (Slc25a22) respectively.
With this study, the researchers have shown that targeting of Ago2 by miRNA-184 is an essential component of the compensatory response to regulate proliferation of beta cells according to insulin sensitivity. Dr. Poy: “Our observations on the effects of the ketogenic diet on microRNA function in the beta-cell unify several poorly understood mechanisms and reinforce the potential in studying the role of small RNAs in physiologic stresses”. He and his colleagues hope that research on the miRNA pathway should bring clarity in understanding how the network of small RNAs contribute to maintaining essential metabolic processes and how their failure ultimately leads to disease.
*Cell Metabolism, http://dx.doi.org/10.1016/j.cmet.2013.11.015
Argonaute2 mediates compensatory expansion of the pancreatic beta-cell
Sudhir G. Tattikota1,12, Thomas Rathjen1,12, Sarah J. McAnulty1, Hans-Hermann Wessels1, Ildem Akerman4, Martijn van de Bunt5, Jean Hausser6,Jonathan L.S. Esguerra7, Anne Musahl1, Amit K. Pandey1, Xintian You1, Wei Chen1, Pedro L. Herrera8, Paul R. Johnson5,9,10, Donal O’Carroll11, Lena Eliasson7, Mihaela Zavolan6, Anna L. Gloyn5,9, Jorge Ferrer4, Ruby Shalom-Feuerstein3, Daniel Aberdam2, and Matthew N. Poy1#
1 Max Delbrueck Center for Molecular Medicine, 13125 Berlin, Germany
2 INSERM U976, University of Paris Diderot, 75475 France
3 Department of Anatomy and Cell Biology, The Rappaport Family Institute for Research in the Medical Sciences, Faculty of Medicine, Technion-Israel Institute of Technology, 31096 Haifa, Israel
4Genomic Programming of Beta-cells Laboratory, Institut d’Investigacions Biomèdiques August Pi I Sunyer (IDIBAPS), 08036 Barcelona, Spain
5 Oxford Centre for Diabetes, Endocrinology & Metabolism, University of Oxford, OX3 7LJ Oxford, UK
6 Computational and Systems Biology, Biozentrum, University of Basel, 4056 Basel, Switzerland
7 Lund University Diabetes Center, Department of Clinical Sciences, Lund University, Malmö University Hospital, SE-205 02 Malmö, Sweden
8 Department of Genetic Medicine and Development, Faculty of Medicine, University of Geneva, 1211 Geneva, Switzerland
9NIHR Oxford Biomedical Research Centre, ORH Trust, OCDEM, Churchill Hospital, OX3 7LJ Oxford, UK
10Nuffield Department of Surgery, University of Oxford, OX3 9DU Oxford, UK
11 European Molecular Biology Laboratory, 00015 Monterotondo Scalo Italy
12 these authors contributed equally
Max Delbrück Center for Molecular Medicine (MDC) Berlin-Buch
in the Helmholtz Association
Robert-Rössle-Straße 10; 13125 Berlin; Germany
Phone: +49 (0) 30 94 06 – 38 96
Fax: +49 (0) 30 94 06 – 38 33