Influence of antioxidant therapy on islet graft survival

Nrf2 and Antioxidant Response in Animal Models of Type 2 Diabetes

This perspective examines the proposition that chronically elevated blood glucose levels caused by type 2 diabetes (T2D) harm body tissues by locally generating reactive oxygen species (ROS). A feed-forward scenario is described in which the initial onset of defective beta cell function T2D becomes sustained and causes chronic elevations in blood glucose, which flood metabolic pathways throughout the body, giving rise to abnormally high local levels of ROS. Most cells can defend themselves via a full complement of antioxidant enzymes that are activated by ROS. However, the beta cell itself does not contain catalase or glutathione peroxidases and thereby runs a greater risk of ROS-induced damage. In this review, previously published experiments are revisited to examine the concept that chronic hyperglycemia can lead to oxidative stress in the beta cell, how this relates to the absence of beta cell glutathione peroxidase (GPx) activity, and whether this deficiency might be ameliorated by genetic enrichment of beta cell GPx and by oral antioxidants, including ebselen, a GPx mimetic.

Redox-Dependent Inflammation in Islet Transplantation Rejection

Type 1 diabetes is an autoimmune disease that results in the progressive destruction of insulin-producing pancreatic β-cells inside the islets of Langerhans. The loss of this vital population leaves patients with a lifelong dependency on exogenous insulin and puts them at risk for life-threatening complications. One method being investigated to help restore insulin independence in these patients is islet cell transplantation. However, challenges associated with transplant rejection and islet viability have prevented long-term β-cell function. Redox signaling and the production of reactive oxygen species (ROS) by recipient immune cells and transplanted islets themselves are key players in graft rejection. Therefore, dissipation of ROS generation is a viable intervention that can protect transplanted islets from immune-mediated destruction. Here, we will discuss the newly appreciated role of redox signaling and ROS synthesis during graft rejection as well as new strategies being tested for their efficacy in redox modulation during islet cell transplantation.

Tat-enhanced delivery of metallothionein can partially prevent the development of diabetes

Metallothioneins (MTs) are intracellular low-molecular-weight, cysteine-rich proteins with potent metal-binding and redox functions, but with limited membrane permeativity. The aim of this study was to investigate whether we could enhance delivery of MT-1 to pancreatic islets or β cells in vitro and in vivo. The second goal was to determine whether increased MT-1 could prevent cellular toxicity induced by high glucose and free fatty acids in vitro (glucolipotoxicity) and ameliorate the development of diabetes induced by streptozotocin in mice or delay the development of diabetes by improving insulin secretion and resistance in the OLETF rat model of type 2 diabetes. Expression of HIV-1 Tat-MT-1 enabled efficient delivery of MT into both INS-1 cells and rat islets. Intracellular MT activity increased in parallel with the amount of protein delivered to cells. The formation of reactive oxygen species, glucolipotoxicity, and DNA fragmentation due to streptozotocin decreased after treating pancreatic β cells with Tat-MT in vitro. Importantly, in vivo, intraperitoneal injection resulted in delivery of the Tat-MT protein to the pancreas as well as liver, muscle, and white adipose tissues. Multiple injections increased radical-scavenging activity, decreased apoptosis, and reduced endoplasmic reticulum stress in the pancreas. Treatment with Tat-MT fusion protein delayed the development of diabetes in streptozotocin-induced mice and improved insulin secretion and resistance in OLETF rats. These results suggest that in vivo transduction of Tat-MT may offer a new strategy to protect pancreatic β cells from glucolipotoxicity, may improve insulin resistance in type 2 diabetes, and may have a protective effect in preventing islet destruction in type 1 diabetes.

Pancreatic Islet Culture and Preservation Strategies: Advances, Challenges, and Future Outlook

Postisolation islet survival is a critical step for achieving successful and efficient islet transplantation. This involves the optimization of islet culture in order to prolong survival and functionality in vitro. Many studies have focused on different strategies to culture pancreatic islets in vitro through manipulation of culture media, surface modified substrates, and the use of various techniques such as encapsulation, embedding, scaffold, and bioreactor culture strategies. This review aims to present and discuss the different methodologies employed to optimize pancreatic islet culture in vitro as well as address their respective advantages and drawbacks.

Islet transplantation and antioxidant management: A comprehensive review

Islet transplantation as a promising treatment for type 1 diabetes has received widespread attention. Oxidative stress plays an essential role in cell injury during islet isolation and transplantation procedures. Antioxidants have been used in various studies to improve islet transplantation procedures. The present study reviews the role of oxidative stress and the benefits of antioxidants in islet transplantation procedures. The bibliographical databases Pubmed and Scopus were searched up to November 2008. All relevant human and animal in-vivo and in-vitro studies, which investigated antioxidants on islets, were included. Almost all the tested antioxidants used in the in-vitro studies enhanced islet viability and insulin secretion. Better control of blood glucose after transplantation was the major outcome of antioxidant therapy in all in-vivo studies. The data also indicated that antioxidants improved islet transplantation procedures. Although there is still insufficient evidence to draw definitive conclusions about the efficacy of individual supplements, the benefits of antioxidants in islet isolation procedures cannot be ignored.

Glutathione peroxidase protein expression and activity in human islets isolated for transplantation

BACKGROUND

Overexpression of antioxidant enzymes has been reported to protect rodent beta cells from oxidative stress. However, very little is known about protein expression and activity of antioxidant enzymes in human islets.

METHOD/RESULTS

Human islet protein levels by Western analysis and enzymatic activity for the key antioxidant enzymes superoxide dismutases (SODs), catalase, and glutathione peroxidase-1 (GPx) were examined. Enzyme protein expression and activity were in the order SODs > catalase > GPx. Human islet GPx protein expression was significantly less than that found for catalase (p < 0.0001) and levels of GPx activity were virtually undetectable. As glucose and estrogens have been proposed to alter antioxidant enzyme levels, we examined islet data from male and female donors separately and under varying glucose concentrations. We found significantly less (p < 0.001) GPx protein expression in islets from females compared to males, but no significant regulation by glucose in either gender.

CONCLUSIONS

Human islets have very low protein and activity levels for GPx, the essential enzyme for protection against excessive levels of intracellular lipid peroxides. GPx mimetics may be especially valuable in providing human islets with the broadest spectrum of protection against oxidative stress during isolation and transplantation.

Methods of human islet culture for transplantation

The ability to maintain isolated human islet preparations in tissue culture has recently been adopted by most islet transplant centers, and improves the safety as well as the practicality of islet transplantation. Maintaining islet viability and recovery, however, remains challenging in a clinical setting, due to stringent conditions required for culture. Islet culture is further complicated by the fact that islets do not form a monolayer. This review aims to clarify media, supplementation, and conditions that have been shown to be relevant to human islets, as well as to offer avenues of future research. Factors examined that may influence islet survival include base medium, glucose concentration, vitamin, inorganic ion, lipid, hormone, growth factor, amino acid, and binding protein composition and concentration, as well as culture temperature and seeding density. In addition, this article reviews novel techniques, such as coculture and matrices, that have been employed in an attempt to improve islet survival and functional viability.

Inflammation-mediated dysfunction and apoptosis in pancreatic islet transplantation: implications for intrahepatic grafts

Recent advances in clinical protocols have improved the outcomes of pancreatic islet transplantation (PIT), yet PIT recipients typically require pancreatic islet grafts derived from multiple donors to achieve insulin independence. This along with experimental models of syngeneic PIT, showing that up to 60% of pancreatic islet tissue undergoes apoptosis within the first several days post-transplantation, strongly suggest the involvement of nonalloantigen-specific, inflammatory events in partial destruction of the graft following PIT. Interleukin-1beta appears to be among the most important inflammatory mediators, causing pancreatic islet dysfunction and apoptosis through the up-regulation of inducible nitric oxide (NO) synthase and cyclooxygenase-2. Kupffer cells secrete many molecules, including cytokines, NO, and free radicals, which are known to be directly toxic to the pancreatic islets, and depletion or inhibition of Kupffer cells improves outcomes following experimental PIT. Immediately after transplantation, the pancreatic islets are perfused only by portal vein blood until the process of angiogenesis restores arterial blood flow some 7-10 days later. This delayed vascularization may have implications for the expression of leukocyte adhesion molecules, the effects of free radicals, and the role of ischemia-reperfusion injury. Finally, in the immediate post-transplant period, hepatocytes may contribute to pancreatic islet injury through the production of NO. This paper reviews literature regarding the inflammatory events that follow PIT as well as the pathogenesis of diabetes and the pathophysiology of hepatic ischemia-reperfusion and their relation to the survival and function of intrahepatic pancreatic islet grafts.

Metallothionein Protects Islets from Hypoxia and Extends Islet Graft Survival by Scavenging Most Kinds of Reactive Oxygen Species

Islet transplantation is a promising therapy for Type 1 diabetes, but many attempts have failed due to early graft hypoxia or immune rejection, which generate reactive oxygen species (ROS). In the current study, we determined that transgenic overexpression of the antioxidant metallothionein (MT) in pancreatic beta cells provided broad resistance to oxidative stress by scavenging most kinds of ROS including H2O2, peroxynitrite radical released from streptozotocin, 3-morpholinosydnonimine (SIN-1), and superoxide radical produced by xanthine/xanthine oxidase. MT also reduced nitric oxide-induced beta cell death. A direct test of hypoxia/reperfusion sensitivity was made by exposing FVB and MT islets to hypoxia (1% O2). MT markedly reduced ROS production and improved islet cell survival. Because MT protected beta cells from a broad spectrum of ROS and from hypoxia, we considered it to be an ideal candidate for improving islet transplantation. We first tested syngeneic transplantation by implanting islets under the kidney capsule of the same strain, FVB mice, thereby eliminating the immune rejection component. Under these conditions, MT islets maintained much greater insulin content than control islets. Allotransplantation was then tested. MT transgenic and normal FVB islets were implanted under the kidney capsule of BALB/c mice that were previously treated with streptozotocin to induce diabetes. We found that MT islets extended the duration of euglycemia 2-fold longer than nontransgenic islets. The benefit of MT was due to protection from ROS since nitrotyrosine staining, an indicator of free radical damage, was much lower in MT grafts than in FVB grafts. The time course of protection suggested that the major mode of MT action may have been protection from hypoxia or hypoxia/reperfusion. These data demonstrate that treatment with a broad spectrum antioxidant protects islets from ROS damage such as that produced during the early phase of islet transplantation.