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      Rictor/mTORC2 Is Essential for Maintaining a Balance Between β-Cell Proliferation and Cell Size

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          Abstract

          OBJECTIVE

          We examined the role of Rictor/mammalian target of rapamycin complex 2 (mTORC2), a key component of the phosphotidylinositol-3-kinase (PI3K)/mTORC2/AKT signaling pathway, in regulating both β-cell mass and function.

          RESEARCH DESIGN AND METHODS

          Mice with β-cell–specific deletions of Rictor or Pten were studied to determine the effects of deleting either or both genes on β-cell mass and glucose homeostasis.

          RESULTS

          Rictor null mice exhibited mild hyperglycemia and glucose intolerance caused by a reduction in β-cell mass, β-cell proliferation, pancreatic insulin content, and glucose-stimulated insulin secretion. Islets from these mice exhibited decreased AKT-S473 phosphorylation and increased abundance of FoxO1 and p27 proteins. Conversely, Pten null (β PtenKO) mice exhibited an increase in β-cell mass caused by increased cellular proliferation and size. Although β-cell mass was normal in mice lacking both Rictor and Pten (βDKO), their β-cells were larger than those in the β PtenKO mice. Even though the β-cell proliferation rate in the βDKO mice was lower than in the β PtenKO mice, there was a 12-fold increase the phosphorylation of AKT-T308.

          CONCLUSIONS

          PI3K/AKT signaling through mTORC2/pAKT-S473 plays a key role in maintaining normal β-cell mass. The phosphorylation of AKT-S473, by negatively regulating that of AKT-T308, is essential for maintaining a balance between β-cell proliferation and cell size in response to proliferative stimuli.

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          Most cited references 33

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          Type 2 diabetes-a matter of beta-cell life and death?

          In type 2 diabetes, the beta cells of the pancreas fail to produce enough insulin to meet the body's demand, in part because of an acquired decrease in beta-cell mass. In adults, pancreatic beta-cell mass is controlled by several mechanisms, including beta-cell replication, neogenesis, hypertrophy, and survival. Here, I discuss evidence supporting the notion that increased beta-cell apoptosis is an important factor contributing to beta-cell loss and the onset of type 2 diabetes. Interestingly, a key signaling molecule that promotes beta-cell growth and survival, insulin receptor substrate 2 (IRS-2), is a member of a family of proteins whose inhibition contributes to the development of insulin resistance in the liver and other insulin-responsive tissues. Thus, the IRS-2 pathway appears to be a crucial participant in the tenuous balance between effective pancreatic beta-cell mass and insulin resistance.
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            The two TORCs and Akt.

            The regulatory circuits that control the activities of the two distinct target of rapamycin (TOR) complexes, TORC1 and TORC2, and of Akt have been a focus of intense research in recent years. It has become increasingly evident that these regulatory circuits control some of the most fundamental aspects of metabolism, cell growth, proliferation, survival, and differentiation at both the cellular and organismal levels. As such, they also play a pivotal role in the genesis of diseases including cancer, diabetes, aging, and degenerative diseases. This review highlights recent developments aimed at deciphering the interplay between Akt and mTORCs as well as their role in embryonic development and in cancer.
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              PI3K/Akt: getting it right matters.

               T Franke (2008)
              The Akt serine/threonine kinase (also called protein kinase B) has emerged as a critical signaling molecule within eukaryotic cells. Significant progress has been made in clarifying its regulation by upstream kinases and identifying downstream mechanisms that mediate its effects in cells and contribute to signaling specificity. Here, we provide an overview of present advances in the field regarding the function of Akt in physiological and pathological cell function within a more generalized framework of Akt signal transduction. An emphasis is placed on the involvement of Akt in human diseases ranging from cancer to metabolic dysfunction and mental disease.
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                Author and article information

                Journal
                Diabetes
                diabetes
                diabetes
                Diabetes
                Diabetes
                American Diabetes Association
                0012-1797
                1939-327X
                March 2011
                21 February 2011
                : 60
                : 3
                : 827-837
                Affiliations
                1Center for Stem Cell Biology, Vanderbilt University Medical Center, Nashville, Tennessee
                2Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, Tennessee
                Author notes
                Corresponding author: Mark A. Magnuson, mark.magnuson@ 123456vanderbilt.edu .
                Article
                1194
                10.2337/db10-1194
                3046843
                21266327
                © 2011 by the American Diabetes Association.

                Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by-nc-nd/3.0/ for details.

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                Categories
                Islet Studies

                Endocrinology & Diabetes

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