Glutamate is an essential mediator in glutamine‐amplified insulin secretion

Abstract

Aims/Introduction Glutamine is the most abundant amino acid in the circulation. In this study, we investigated cell signaling in the amplification of insulin secretion by glutamine. Materials and Methods Clonal pancreatic β‐cells MIN6‐K8, wild‐type B6 mouse islets, glutamate dehydrogenase (GDH) knockout clonal β‐cells (Glud1KOβCL), and glutamate‐oxaloacetate transaminase 1 (GOT1) knockout clonal β‐cells (Got1KOβCL) were studied. Insulin secretion from these cells and islets was examined under various conditions, and intracellular glutamine metabolism was assessed by metabolic flux analysis. Intracellular Ca2+ concentration ([Ca2+]i) was also measured. Results Glutamine dose‐dependently amplified insulin secretion in the presence of high glucose in both MIN6‐K8 cells and Glud1KOβCL. Inhibition of glutaminases, the enzymes that convert glutamine to glutamate, dramatically reduced the glutamine‐amplifying effect on insulin secretion. A substantial amount of glutamate was produced from glutamine through direct conversion by glutaminases. Glutamine also increased [Ca2+]i at high glucose, which was abolished by inhibition of glutaminases. Glutamic acid dimethylester (dm‐Glu), a membrane permeable glutamate precursor that is converted to glutamate in cells, increased [Ca2+]i as well as induced insulin secretion at high glucose. These effects of glutamine and dm‐Glu were dependent on calcium influx. Glutamine also induced insulin secretion in clonal β‐cells MIN6‐m14, which otherwise exhibit no insulin secretory response to glucose. Conclusions Glutamate converted from glutamine is an essential mediator that enhances calcium signaling in the glutamine‐amplifying effect on insulin secretion. Our data also suggest that glutamine exerts a permissive effect on glucose‐induced insulin secretion.

Branched‐chain α‐keto acids and glutamate/glutamine: Biomarkers of insulin resistance in childhood obesity

“…Glutamate is produced via multiple transamination reactions including the BCAT‐dependent step of BCAA catabolism 42 . An increase in plasma glutamate may enhance insulin secretion since glutamate can serve as an insulin secretagogue via its conversion to α‐ketoglutarate by glutamate dehydrogenase 58,59 ; it is also a substrate for hepatic gluconeogenesis 60 . Stefan et al recently found that elevated plasma glutamate levels are associated with higher liver fat content, lower insulin sensitivity and increased carotid intima media thickness independent of total body fat mass and visceral fat mass 14 .…”

Ferroptosis Signaling in Pancreatic β-Cells: Novel Insights & Therapeutic Targeting

“…Additionally, glutamine provides precursors for GSH production, resulting in a decrease in the steady-state level of lipid oxidation products, a crucial component of cell viability [ 104 , 105 ]. In this context, glutamine availability is sensed by Glutaminase 1 (GLS1), which converts glutamine into glutamate for GSH synthesis and plays an important role in insulin secretion [ 106 , 107 , 108 , 109 ]. Notably, we discovered that endogenous GLS1 mRNA and protein expression were suppressed upon exposure to diabetic milieu conditions (hyperglycemia, streptozotocin, and H 2 O 2 ), leading to a reduction in GSH synthesis.…”

Glucose-dependent miR-125b is a negative regulator of β-cell function

“…Interestingly, MIR125B-Tg islets displayed a 40% reduction on Imphd1 mRNA (padj 0.016) as well as lower levels of glutamine synthetase (GS, GLUL), a limiting enzyme for glutamine synthesis the inhibition of which reduced GSIS 46 . Of note, glutamine has been shown to strongly increase Ca 2+ influx in the presence of glucose, without changes in ATP content 47 .…”

“…Interestingly, MIR125B-Tg islets displayed a 40% reduction on Imphd1 mRNA (padj 0.016) as well as lower levels of glutamine synthetase (GS, GLUL ), a limiting enzyme for glutamine synthesis the inhibition of which reduced GSIS 46 . Of note, glutamine has been shown to strongly increase Ca 2+ influx in the presence of glucose, without changes in ATP content 47 . Secondly, decreased insulin secretion following KCl-mediated membrane depolarization in the absence of reduced intracellular Ca 2+ suggests the presence of functional voltage-dependent Ca 2+ channels in transgenic islets and points towards defects in the final steps of exocytosis.…”

“…This points towards defects at different levels on the secretory pathway, as supported by the striking changes in gene expression. First, K ATP -independent amplification pathways(32) might be impaired in MIR125B-Tg islets, which display lower levels of genes encoding enzymes limiting for the production of guanine nucleotides and glutamine such as Imphd1 and GLUL that can promote insulin exocytosis in an ATP/ADP independent manner(33; 34). Secondly, voltage-clamp electrophysiology experiments confirmed the presence of functional voltage-dependent Ca 2+ -channels in MIR125B-Tg β-cells.…”