br The lipoxygenase pathway in the pancreatic

The 12/15-lipoxygenase pathway in the pancreatic islet and in diabetes mellitus Homeostasis of blood glucose is maintained by hormone secretion from the pancreatic islets of Langerhans. More specifically, insulin produced by the β hyPerFUsion™ high-fidelity PCR Kit of the islet plays a major role in proper maintenance of blood glucose. Normal levels of insulin are required to promote glucose uptake in muscle and in adipose tissue, and to suppress gluconeogenesis and promote glycogenesis in the liver. Therefore, either a loss of production of insulin or a defect in the release of insulin from the β cell creates dysregulation of overall glucose homeostasis in the body. Recent evidence indicates that both forms of diabetes (types 1 and 2) are associated with a significant loss of β cells. A primary underlying cause of β cell loss arises as a consequence of inflammatory mechanisms. Expression and/or activity of 12-LOX in human islets is upregulated by hyperglycemia and by inflammatory cytokines. These observations indicate a role for 12-LOX in mediating the loss of functional insulin secretion and in the insulin resistance commonly associated with inflammation [30], [31], [32], [33], [34]. Early studies of the role of 12-LOX in islet function were guided by experiments using enzymatic inhibition. Initial studies led to incorrect conclusions due to the lack of specificity in the chemical inhibitors used. This has underscored the need to generate specific isotype-selective inhibitors. Gene-based knockout studies and targeted protein knockdown approaches have provided clarity in more recent studies of the important role of 12-LOX in islet function. Insulin resistance and impairment in islet function that develops on a high-fat diet were prevented in leukocyte-12-LOX (12/15-LOX) knockout mice, suggesting that 12/15-LOX activity is relevant to type 2 diabetes, and to β cell dysfunction in obese states [35], [36]. Additionally, diabetic Zucker fatty rats that have a defect in insulin secretion have elevated 12-LOX, further supporting a role for 12-LOX in the pathogenesis of type 2 diabetes [34]. Bleich et al. reported on a mouse model deficient in the leukocyte-derived 12/15-LOX. In contrast to control C57BL/6J mice, the 12/15-LOX knockout mice (on the same genetic background) were resistant to the induction of diabetes by low-dose streptozotocin [37]. This streptozotocin protocol induced immune-mediated islet destruction similar to type 1 diabetes. The 12/15-LOX knockout mice lacked the cytokine-induced conversion of AA to 12HETE, implying that 12HETE generation was cytotoxic to β cells [37]. The role of 12-LOX as a key mediator in the development of autoimmune diabetes is further supported by the work of McDuffie et al., who developed a congenic 12/15-LOX knockout in non-obese diabetic (NOD) mice. The phenotype of the female NOD mouse includes the spontaneous development of autoimmune type 1 diabetes. 12/15-LOX knockout mice resulted in a significant reduction (2.5% vs. >60% in control animals) in the development of diabetes [38]. The mechanisms for how the deletion of 12-LOX protects against type 1 diabetes development are an active area of current investigation. Interestingly, 12-LOX activity mediates the expression of interleukin-12 (IL-12) [39], [40], [41], [42]. IL-12 is a key cytokine driving the Th1 autoimmune response via STAT4 second-messenger signaling and the induction of interferon gamma (IFNγ). A direct role of pro-inflammatory cytokines in stimulating 12-LOX activity is further supported by observations of cytokine-induced production of 12HETE in both islets and β cell lines [31], [43]. Moreover, the addition of 12-LOX products (12HETE and 12HPETE) to human islets resulted in a decrease in glucose-stimulated insulin secretion associated with a decrease in islet viability [30]. These studies also reported a partial restoration in glucose-stimulated insulin secretion if 12HETE was combined with lisofylline, an inhibitor of IL-12 signaling. Collectively these data support a predicted role of IL-12 in mediating the immune damage caused by the 12-LOX pathway.