XPR1 mediates the pancreatic B-cell phosphate flush

Glucose-stimulated insulin secretion is the hallmark of the pancreatic β-cell, a critical player in the regulation of blood glucose concentration. In 1974, the remarkable observation was made that an efflux of intracellular inorganic phosphate (P ) accompanied the events of stimulated insulin secretion. The mechanism behind this "phosphate flush," its association with insulin secretion, and its regulation have since then remained a mystery. We recapitulated the phosphate flush in the MIN6m9 β-cell line and pseudoislets. We demonstrated that knockdown of XPR1, a phosphate transporter present in MIN6m9 cells and pancreatic islets, prevented this flush. Concomitantly, XPR1 silencing led to intracellular P accumulation and a potential impact on Ca signaling. XPR1 knockdown slightly blunted first-phase glucose-stimulated insulin secretion in MIN6m9 cells, but had no significant impact on pseudoislet secretion. In keeping with other cell types, basal P efflux was stimulated by inositol pyrophosphates, and basal intracellular P accumulated following knockdown of inositol hexakisphosphate kinases. However, the glucose-driven phosphate flush occurred despite inositol pyrophosphate depletion. Finally, while it is unlikely that XPR1 directly affects exocytosis, it may protect Ca signaling. Thus, we have revealed XPR1 as the missing mediator of the phosphate flush, shedding light on a 45-year-old mystery

Lipid Mediators Are Critical in Resolving Inflammation: A Review of the Emerging Roles of Eicosanoids in Diabetes Mellitus

The biosynthesis pathway of eicosanoids derived from arachidonic acid, such as prostaglandins and leukotrienes, relates to the pathophysiology of diabetes mellitus (DM). A better understanding of how lipid mediators modulate the inflammatory process may help recognize key factors underlying the progression of diabetes complications. Our review presents recent knowledge about eicosanoid synthesis and signaling in DM-related complications, and discusses eicosanoid-related target therapeutics

Insulin Influences LPS-Induced TNF-α and IL-6 Release Through Distinct Pathways in Mouse Macrophages from Different Compartments

Background/Aims: Diabetic subjects are more susceptible to infections, which is partially due to insulin deficiency and hyperglycemia. We hypothesized that insulin influences cytokine release by macrophages from diabetic C57BL/6 mice stimulated with lipopolysaccharides (LPS). Methods: Bone marrow-derived macrophages (BMDM) and tissue-specific macrophages from diabetic (alloxan 60 mg/kg, i.v.) male C57BL/6 mice were stimulated by LPS (100 ng/mL) and/or treated by insulin (1 mU/mL). Results: Using BMDM from diabetic mice, we showed that LPS induced an increase in TNF-α and IL-6 release and p38, SAPK/JNK, ERK 1/2, and Akt (308-Thr and 473-Ser) phosphorylation but not in PKCα/β II and delta. Insulin increased TNF-α and IL-6 secretion in LPS-stimulated macrophages as well as p-p38, p-SAPK/JNK, p-ERK 1/2, p-PI3K (p55) and p-Akt (473-Ser) expression. Furthermore, PI3-kinase inhibition by wortmannin decreased TNF-α release, and inhibition by LY294002 decreased both TNF-α and IL-6 levels after LPS-insulin treatment. PD98059, which inhibits the ERK upstream activators MAPK kinase (MKK) 1 and MKK2, reduced the effect promoted by insulin in BMDM stimulated by LPS In tissue-specific macrophages, insulin reduced LPS-induced TNF-α, IL-6 and IL-1β secretion in alveolar and peritoneal macrophages. Conclusion: These data suggest that insulin through the modulation of PI3-kinase and ERK 1/2 pathways drive different responses in macrophages, thereby enhancing our understanding of the plasticity of these cells

Protein kinase- and lipase inhibitors of inositide metabolism deplete IP7 indirectly in pancreatic β-cells: Off-target effects on cellular bioenergetics and direct effects on IP6K activity

Inositol pyrophosphates have emerged as important regulators of many critical cellular processes from vesicle trafficking and cytoskeletal rearrangement to telomere length regulation and apoptosis. We have previously demonstrated that 5-di-phosphoinositol pentakisphosphate, IP7, is at a high level in pancreatic β-cells and is important for insulin exocytosis. To better understand IP7 regulation in β-cells, we used an insulin secreting cell line, HIT-T15, to screen a number of different pharmacological inhibitors of inositide metabolism for their impact on cellular IP7. Although the inhibitors have diverse targets, they all perturbed IP7 levels. This made us suspicious that indirect, off-target effects of the inhibitors could be involved. It is known that IP7 levels are decreased by metabolic poisons. The fact that the inositol hexakisphosphate kinases (IP6Ks) have a high Km for ATP makes IP7 synthesis potentially vulnerable to ATP depletion. Furthermore, many kinase inhibitors are targeted to the ATP binding site of kinases, but given the similarity of such sites, high specificity is difficult to achieve. Here, we show that IP7 concentrations in HIT-T15 cells were reduced by inhibitors of PI3K (wortmannin, LY294002), PI4K (Phenylarsine Oxide, PAO), PLC (U73122) and the insulin receptor (HNMPA). Each of these inhibitors also decreased the ATP/ADP ratio. Thus reagents that compromise energy metabolism reduce IP7 indirectly. Additionally, PAO, U73122 and LY294002 also directly inhibited the activity of purified IP6K. These data are of particular concern for those studying signal transduction in pancreatic β-cells, but also highlight the fact that employment of these inhibitors could have erroneously suggested the involvement of key signal transduction pathways in various cellular processes. Conversely, IP7’s role in cellular signal transduction is likely to have been underestimated. [Display omitted] •In pancreatic β-cells several inhibitors of signal transduction reduce IP7 levels.•There is a positive correlation between IP7 reduction and decrease in ATP/ADP.•Inhibitors deplete IP7 levels indirectly by decreasing ATP/ADP levels.•Some purportedly specific cell-signaling inhibitors directly target IP6K activity.•Caution is required in interpreting data obtained using inhibitors of inositide metabolism