Localization and regulation of pancreatic selenoprotein P

Abstract

Progressive loss of pancreatic b-cell mass is a crucial feature of type 2 diabetes mellitus. As b-cells express very low amounts of the antioxidant enzymes catalase and glutathione peroxidase (GPx), they appear to be particularly vulnerable to oxidative damage in the pathogenesis of diabetes. Here, we investigated the pancreatic expression pattern and regulation of selenoprotein P (Sepp1), which may serve as an additional antioxidant enzyme inside and outside of cells. Sepp1 was detected in rodent pancreas by immunofluorescence and real-time RT-PCR. Regulation of Sepp1 biosynthesis in INS-1 rat insulinoma cells was investigated by real-time RT-PCR, luciferase gene reporter assay, and immunoblotting. Sepp1 and Gpx1 gene expressions in rat pancreas were 58 and 22% respectively of the liver values. Pancreatic Sepp1 expression was restricted to the endocrine tissue, with Sepp1 being present in the a-and b-cells of mouse islets. In INS-1 insulinoma cells, Sepp1 expression was stimulated by the selenium compound sodium selenate and diminished in the presence of high glucose (16 . 7 vs 5 mM) concentrations. Sepp1 mRNA stability was also lowered at 16 . 7 mM glucose.Moreover, Sepp1 mRNA levels were decreased in isolated murine islets cultured in highglucose (22 mM) medium compared with normal glucose (5 . 5 mM) medium. Pancreatic Sepp1 expression was elevated upon treatment of mice with the b-cell toxin streptozotocin. This study shows that pancreatic islets express relatively high levels of Sepp1 that may fulfill a function in antioxidant protection of b-cells. Downregulation of Sepp1 expression by high glucose might thus contribute to glucotoxicity in b-cells. Key Words" pancreatic islet

The PACAP-Regulated Gene Selenoprotein T Is Abundantly Expressed in Mouse and Human β-Cells and Its Targeted Inactivation Impairs Glucose Tolerance

“…It localized in the endoplasmic reticulum of insulinoma MIN6 cells as also observed previously in neuronal cells (6,7). Although tissue-distribution studies of other selenoproteins in the pancreas are still scarce, similar or overlapping distributions to that of SelT have been reported for other selenoproteins such as thioredoxin, thioredoxin reductase, or SeP in the endocrine pancreas (26,27). Interestingly, SelT contains a conserved motif called the thioredoxin-like fold (5), which is involved in the regulation of intracellular redox equilibrium (28), as described for other thioredoxins or thioredoxin reductases (29).…”

“…Indeed, inactivation of the corresponding genes impairs glucose tolerance and decreases pancreatic ␤-cell mass and insulin concentration. In addition, SeP seems to play a major role in glucose homeostasis by acting as a hepatokine that modulates insulin sensitivity (10) and by protecting against glucotoxicity in pancreatic ␤-cells (27). Our present data show that SelT is also involved in the regulation of blood glucose because SelT-deficient mice exhibited higher blood glucose compared with controls, indicating impaired glucose tolerance in these animals.…”

“…The alteration of islet morphology as revealed by their smaller size in SelT-knockout mice could be responsible for this deficiency. In support of this hypothesis, several studies have shown the important role of selenoproteins for ␤-cell integrity and function (27,30). Selenoproteins are known to protect different cell types from the harmful effects of oxidative stress as mentioned above, by exerting protective antioxidant actions that promote cell survival.…”

“…Several studies have previously shown that reactive oxygen species elevation due to glucotoxicity impairs insulin secretion by ␤-cells (27,34,35). Insulin secretion is actually linked to SelT expression because reduction of SelT through RNA silencing inhibited the insulinotropic effect of the neuropeptide PACAP in MIN6 cells.…”

Glutathione Peroxidase Mimic Ebselen Improves Glucose-Stimulated Insulin Secretion in Murine Islets

“…The SOD mimic copper diisopropylsalicylate (CuDIPs) attenuated the streptozotocin (STZ)-induced diabetes in rats (20) and restored the suppressed foxa2 expression in the SKO islets (68). As an essential micronutrient, Se is a component of 25 human selenoproteins (36) that are involved in antioxidant defense (10,11) and regulation of b-cells (61). Either overexpression or deficiency of selenoproteins disturbed glucose homeostasis in mice (37).…”