Silencing inducible nitric oxide synthase protects rat pancreatic islet

Amniotic Membrane Extract Protects Islets From Serum-Deprivation Induced Impairments and Improves Islet Transplantation Outcome

Islet culture prior to transplantation is a standard practice in many transplantation centers. Nevertheless, the abundant islet mass loss and function impairment during this serum-deprivation culture period restrain the success of islet transplantation. In the present study, we used a natural biomaterial derived product, amniotic membrane extract (AME), as medium supplementation of islet pretransplant cultivation to investigate its protective effect on islet survival and function and its underlying mechanisms, as well as the engraftment outcome of islets following AME treatment. Results showed that AME supplementation improved islet viability and function, and decreased islet apoptosis and islet loss during serum-deprived culture. This was associated with the increased phosphorylation of PI3K/Akt and MAPK/ERK signaling pathway. Moreover, transplantation of serum-deprivation stressed islets that were pre-treated with AME into diabetic mice revealed better blood glucose control and improved islet graft survival. In conclusion, AME could improve islet survival and function in vivo and in vitro, and was at least partially through increasing phosphorylation of PI3K/Akt and MAPK/ERK signaling pathway.

Insulin secretion: The nitric oxide controversy

Nitric oxide (NO) is a gas that serves as a ubiquitous signaling molecule participating in physiological activities of various organ systems. Nitric oxide is produced in the endocrine pancreas and contributes to synthesis and secretion of insulin. The potential role of NO in insulin secretion is disputable - both stimulatory and inhibitory effects have been reported. Available data indicate that effects of NO critically depend on its concentration. Different isoforms of NO synthase (NOS) control this and have the potential to decrease or increase insulin secretion. In this review, the role of NO in insulin secretion as well as the possible reasons for discrepant findings are discussed. A better understanding of the role of NO system in the regulation of insulin secretion may facilitate the development of new therapeutic strategies in the management of diabetes.

A novel inhibitor of inducible NOS dimerization protects against cytokine-induced rat beta cell dysfunction

BACKGROUND AND PURPOSE

Beta cell apoptosis is a major feature of type 1 diabetes, and pro-inflammatory cytokines are key drivers of the deterioration of beta cell mass through induction of apoptosis. Mitochondrial stress plays a critical role in mediating apoptosis by releasing cytochrome C into the cytoplasm, directly activating caspase-9 and its downstream signalling cascade. We aimed to identify new compounds that protect beta cells from cytokine-induced activation of the intrinsic (mitochondrial) pathway of apoptosis.

EXPERIMENTAL APPROACH

Diabetogenic media, composed of IL-1β, IFN-γ and high glucose, were used to induce mitochondrial stress in rat insulin-producing INS1E cells, and a high-content image-based screen of small molecule modulators of Casp9 pathway was performed.

KEY RESULTS

A novel small molecule, ATV399, was identified from a high-content image-based screen for compounds that inhibit cleaved caspase-9 activation and subsequent beta cell apoptosis induced by a combination of IL-1β, IFN-γ and high glucose, which together mimic the pathogenic diabetic milieu. Through medicinal chemistry optimization, potency was markedly improved (6-30 fold), with reduced inhibitory effects on CYP3A4. Improved analogues, such as CAT639, improved beta cell viability and insulin secretion in cytokine-treated rat insulin-producing INS1E cells and primary dispersed islet cells. Mechanistically, CAT639 reduced the production of NO by allosterically inhibiting dimerization of inducible NOS (iNOS) without affecting its mRNA levels.

CONCLUSION AND IMPLICATIONS

Taken together, these studies demonstrate a successful phenotypic screening campaign resulting in identification of an inhibitor of iNOS dimerization that protects beta cell viability and function through modulation of mitochondrial stress induced by cytokines.

The good and bad effects of cysteine S-nitrosylation and tyrosine nitration upon insulin exocytosis: a balancing act

As understanding of the mechanisms driving and regulating insulin secretion from pancreatic beta cells grows, there is increasing and compelling evidence that nitric oxide (•NO) and other closely-related reactive nitrogen species (RNS) play important roles in this exocytic process. •NO and associated RNS, in particular peroxynitrite, possess the capability to effect signals across both intracellular and extracellular compartments in rapid fashion, affording extraordinary signaling potential. It is well established that nitric oxide signals through activation of guanylate cyclase-mediated production of cyclic GMP. The intricate intracellular redox environment, however, lends credence to the possibility that •NO and peroxynitrite could interact with a wider variety of biological targets, with two leading mechanisms involving 1) Snitrosylation of cysteine, and 2) nitration of tyrosine residues comprised within a variety of proteins. Efforts aimed at delineating the specific roles of •NO and peroxynitrite in regulated insulin secretion indicate that a highly-complex and nuanced system exists, with evidence that •NO and peroxynitrite can contribute in both positive and negative regulatory ways in beta cells. Furthermore, the ultimate biochemical outcome within beta cells, whether to compensate and recover from a given stress, or not, is likely a summation of contributory signals and redox status. Such seeming regulatory dichotomy provides ample opportunity for these mechanisms to serve both physiological and pathophysiologic roles in onset and progression of diabetes. This review focuses attention upon recent accumulating evidence pointing to roles for nitric oxide induced post-translational modifications in the normal regulation as well as the dysfunction of beta cell insulin exocytosis.

Regulation of pancreatic β-cell survival by nitric oxide: clinical relevance

The reduction of pancreatic β-cell mass is an important factor in the development of type 1 and type 2 diabetes. Understanding the mechanisms that regulate the maintenance of pancreatic β-cell mass as well as β-cell death is necessary for the establishment of therapeutic strategies. In this context, nitric oxide (NO) is a diatomic, gaseous, highly reactive molecule with biological activity that participates in the regulation of pancreatic β-cell mass. Two types of cellular responses can be distinguished depending on the level of NO production. First, pancreatic β-cells exposed to inflammatory cytokines, lipid stress or hyperglycaemia produce high concentrations of NO, mainly due to the activation of inducible NO synthase (iNOS), thus promoting cell death. Meanwhile, under homeostatic conditions, low concentrations of NO, constitutively produced by endothelial NO synthase (eNOS), promote cell survival. Here, we will discuss the current knowledge of the NO-dependent mechanisms activated during cellular responses, emphasizing those related to the regulation of cell survival.

Akt2 knockout mitigates chronic iNOS inhibition-induced cardiomyocyte atrophy and contractile dysfunction despite persistent insulin resistance

Increased levels of inducible nitric oxide synthase (iNOS) during cardiac stress such as ischemia-reperfusion, sepsis and hypertension may display both beneficial and detrimental roles in cardiac contractile performance. However, the precise role of iNOS in the maintenance of cardiac contractile function remains elusive. This study was designed to determine the impact of chronic iNOS inhibition on cardiac contractile function and the underlying mechanism involved with a special focus on the NO downstream signaling molecule Akt. Male C57 or Akt2 knockout [Akt2(-/-)] mice were injected with the specific iNOS inhibitor 1400W (2 mg/kg/d) or saline for 7 days. Both 1400W and Akt2 knockout dampened glucose and insulin tolerance without additive effects. Treatment of 1400W decreased heart and liver weights as well as cardiomyocyte cross-sectional area in C57 but not Akt2 knockout mice. 1400W but not Akt2 knockout compromised cardiomyocyte mechanical properties including decreased peak shortening and maximal velocity of shortening/relengthening, prolonged relengthening duration, reduced intracellular Ca(2+) release and decay rate, the effects of which were ablated or attenuated by Akt2 knockout. Akt2 knockout but not 1400W increased the levels of intracellular Ca(2+) regulatory proteins including SERCA2a and phospholamban phosphorylation. 1400W reduced the level of anti-apoptotic protein Bcl-2, the effect of which was unaffected by Akt2 knockout. Neither 1400W nor Akt2 knockout significantly affected ER stress, autophagy, the post-insulin receptor signaling Akt, GSK3β and AMPK, as well as the stress signaling IκB, JNK, ERK and p38 with the exception of elevated IκB phosphorylation with jointed effect of 1400W and Akt2 knockout. Taken together, these data indicated that an essential role of iNOS in the maintenance of cardiac morphology and function possibly through an Akt2-dependent mechanism.