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OBJECTIVE-Phogrin and IA-2, autoantigens in insulin-dependent diabetes, have been shown to be involved in insulin secretion in pancreatic β-cells; however, implications at a molecular level are confusing from experiment to experiment We analyzed biological functions of phogrin in β-cells by an RNA interference technique.
RESEARCH DESIGN AND METHODS-Adenovirus-mediated expression of short hairpin RNA specific for phogrin (shPhogrin) was conducted using cultured β-cell lines and mouse islets. Both glucose-stimulated insulin secretion and cell proliferation rate were determined in the phogrin-knockdown cells. Furthermore, protein expression was profiled in these cells. To see the binding partner of phogrin in β-cells, coimmunoprecipitation analysis was carried out.
RESULTS-Adenoviral expression of shPhogrin efficiently decreased its endogenous expression in pancreatic β-cells. Silencing of phogrin in β-cells abrogated the glucose-mediated mitogenic effect, which was accompanied by a reduction in the level of insulin receptor substrate 2 (IRS2) protein, without any changes in insulin secretion. Phogrin formed a complex with insulin receptor at the plasma membrane, and their interaction was promoted by high-glucose stimulation that in turn led to stabilization of IRS2 protein. Corroboratively, phogrin knockdown had no additional effect on the proliferation of β-cell line derived from the insulin receptor-knockout mouse.
CONCLUSIONS-Phogrin is involved in ß-cell growth via regulating stability of IRS2 protein by the molecular interaction with insulin receptor. We propose that phogrin and IA-2 function as an essential regulator of autocrine insulin action in pancreatic β-cells. Diabetes 58:682-692, 2009
Glucose is a principle regulator of pancreatic β-cell survival and growth as well as insulin secretion (1). It is a potent mitogen on pancreatic β-cells and regulates islet β-cell mass through their replication (2). Recent studies have suggested that insulin secreted in response to elevated glucose exerts autocrine/paracrine effects, including promotion of insulin biosynthesis and proliferation of β-cells (3,4). The importance of insulin signaling in maintaining β-cell mass was demonstrated by targeted knockouts of the insulin receptor and insulin receptor substrate 2 (IRS2) (5-8). Although insulin receptor knockout had a restricted effect on β-cell mass (J), its mitogenic function on β-cells was clearly shown by short interfering RNA (siRNA)- based silencing of insulin receptor in β-cell- derived MLN6 cells (9,10). More recently, another pathway was demonstrated showing that glucose metabolism leads to increased β-cell mass through the transcriptional activation of IRS2...