Fructose-1,6-Bisphosphatase Overexpression in Pancreatic β-Cells Results in Reduced Insulin Secretion : A New Mechanism for Fat-Induced Impairment of β-Cell Function

OBJECTIVE—Fructose-1,6-bisphosphatase (FBPase) is a gluconeogenic enzyme that is upregulated in islets or pancreatic β-cell lines exposed to high fat. However, whether specific β-cell upregulation of FBPase can impair insulin secretory function is not known. The objective of this study therefore is to determine whether a specific increase in islet β-cell FBPase can result in reduced glucose-mediated insulin secretion

Neprilysin Is Required for Angiotensin-(1-7)'s Ability to Enhance Insulin Secretion via Its Proteolytic Activity to Generate Angiotensin-(1-2)

Recent work has renewed interest in therapies targeting the renin-angiotensin system (RAS) to improve β-cell function in type 2 diabetes. Studies show that generation of angiotensin-(1–7) by ACE2 and its binding to the Mas receptor (MasR) improves glucose homeostasis, partly by enhancing glucose-stimulated insulin secretion (GSIS). Thus, islet ACE2 upregulation is viewed as a desirable therapeutic goal. Here, we show that, although endogenous islet ACE2 expression is sparse, its inhibition abrogates angiotensin-(1–7)–mediated GSIS. However, a more widely expressed islet peptidase, neprilysin, degrades angiotensin-(1–7) into several peptides. In neprilysin-deficient mouse islets, angiotensin-(1–7) and neprilysin-derived degradation products angiotensin-(1–4), angiotensin-(5–7), and angiotensin-(3–4) failed to enhance GSIS. Conversely, angiotensin-(1–2) enhanced GSIS in both neprilysin-deficient and wild-type islets. Rather than mediating this effect via activation of the G-protein–coupled receptor (GPCR) MasR, angiotensin-(1–2) was found to signal via another GPCR, namely GPCR family C group 6 member A (GPRC6A). In conclusion, in islets, intact angiotensin-(1–7) is not the primary mediator of beneficial effects ascribed to the ACE2/angiotensin-(1–7)/MasR axis. Our findings warrant caution for the concurrent use of angiotensin-(1–7) compounds and neprilysin inhibitors as therapies for diabetes

Glucose Metabolism In Vivo in Four Commonly Used Inbred Mouse Strains

OBJECTIVE—To characterize differences in whole-body glucose metabolism between commonly used inbred mouse strains

Neprilysin Deficiency Protects Against Fat-Induced Insulin Secretory Dysfunction by Maintaining Calcium Influx

Neprilysin contributes to free fatty acid (FFA)-induced cellular dysfunction in nonislet tissues in type 2 diabetes. Here, we show for the first time that with prolonged FFA exposure, islet neprilysin is upregulated and this is associated with reduced insulin pre-mRNA and ATP levels, oxidative/nitrative stress, impaired potassium and calcium channel activities, and decreased glucose-stimulated insulin secretion (GSIS). Genetic ablation of neprilysin specifically protects against FFA-induced impairment of calcium influx and GSIS in vitro and in vivo but does not ameliorate other FFA-induced defects. Importantly, adenoviral overexpression of neprilysin in islets cultured without FFA reproduces the defects in both calcium influx and GSIS, suggesting that upregulation of neprilysin per se mediates insulin secretory dysfunction and that the mechanism for protection conferred by neprilysin deletion involves prevention of reduced calcium influx. Our findings highlight the critical nature of calcium signaling for normal insulin secretion and suggest that interventions to inhibit neprilysin may improve β-cell function in obese humans with type 2 diabetes

cJUN N-terminal kinase (JNK) activation mediates islet amyloid-induced beta cell apoptosis in cultured human islet amyloid polypeptide transgenic mouse islets

Aims/hypothesisAggregation of human islet amyloid polypeptide (hIAPP) as islet amyloid is associated with increased beta cell apoptosis and reduced beta cell mass in type 2 diabetes. Islet amyloid formation induces oxidative stress, which contributes to beta cell apoptosis. The cJUN N-terminal kinase (JNK) pathway is a critical mediator of beta cell apoptosis in response to stress stimuli including oxidative stress and exogenous application of hIAPP. We determined whether amyloid formation by endogenous hIAPP mediates beta cell apoptosis through JNK activation and downstream signalling pathways.MethodshIAPP transgenic and non-transgenic mouse islets were cultured for up to 144 h in 16.7 mmol/l glucose to induce islet amyloid in the presence or absence of the amyloid inhibitor Congo Red or a cell-permeable JNK inhibitor. Amyloid, beta cell apoptosis, JNK signalling and activation of downstream targets in the intrinsic and extrinsic apoptotic pathways were measured.ResultsJNK activation occurred with islet amyloid formation in hIAPP transgenic islets after 48 and 144 h in culture. Neither high glucose nor the hIAPP transgene alone was sufficient to activate JNK independent of islet amyloid. Inhibition of islet amyloid formation with Congo Red reduced beta cell apoptosis and partially decreased JNK activation. JNK inhibitor treatment reduced beta cell apoptosis without affecting islet amyloid. Islet amyloid increased mRNA levels of markers of the extrinsic (Fas, Fadd) and intrinsic (Bim [also known as Bcl2l11]) apoptotic pathways, caspase 3 and the anti-apoptotic molecule Bclxl (also known as Bcl2l1) in a JNK-dependent manner.Conclusions/interpretationIslet amyloid formation induces JNK activation, which upregulates predominantly pro-apoptotic signals in both extrinsic and intrinsic pathways, resulting in beta cell apoptosis.<br /

Oxidative stress is induced by islet amyloid formation and time-dependently mediates amyloid-induced beta cell apoptosis

Aims/hypothesis Islet amyloid in type 2 diabetes contributes to loss of beta cell mass and function. Since islets are susceptible to oxidative stress-induced toxicity, we sought to determine whether islet amyloid formation is associated with induction of oxidative stress.Methods Human islet amyloid polypeptide transgenic and non-transgenic mouse islets were cultured for 48 or 144 h with or without the antioxidant N-acetyl-l-cysteine (NAC) or the amyloid inhibitor Congo Red. Amyloid deposition, reactive oxygen species (ROS) production, beta cell apoptosis, and insulin secretion, content and mRNA were measured.Results After 48 h, amyloid deposition was associated with increased ROS levels and increased beta cell apoptosis, but no change in insulin secretion, content or mRNA levels. Antioxidant treatment prevented the rise in ROS, but did not prevent amyloid formation or beta cell apoptosis. In contrast, inhibition of amyloid formation prevented the induction of oxidative stress and beta cell apoptosis. After 144 h, amyloid deposition was further increased and was associated with increased ROS levels, increased beta cell apoptosis and decreased insulin content. At this time-point, antioxidant treatment and inhibition of amyloid formation were effective in reducing ROS levels, amyloid formation and beta cell apoptosis. Inhibition of amyloid formation also increased insulin content.Conclusions/interpretation Islet amyloid formation induces oxidative stress, which in the short term does not mediate beta cell apoptosis, but in the longer term may feed back to further exacerbate amyloid formation and contribute to beta cell apoptosis.<br /

Increased StAR (steroidogenic acute regulatory protein) is detrimental to ß cells and promotes mitochondrial dysfunction.

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