Mice lacking the poly(ADP-ribose) polymerase gene are resistant to pancreatic beta-cell destruction and diabetes development induced by streptozocin

PARP1 gene knock-out increases resistance to retinal degeneration without affecting retinal function

Retinitis pigmentosa (RP) is a group of inherited neurodegenerative diseases affecting photoreceptors and causing blindness in humans. Previously, excessive activation of enzymes belonging to the poly-ADP-ribose polymerase (PARP) group was shown to be involved in photoreceptor degeneration in the human homologous rd1 mouse model for RP. Since there are at least 16 different PARP isoforms, we investigated the exact relevance of the predominant isoform - PARP1 - for photoreceptor cell death using PARP1 knock-out (KO) mice. In vivo and ex vivo morphological analysis using optic coherence tomography (OCT) and conventional histology revealed no major alterations of retinal phenotype when compared to wild-type (wt). Likewise, retinal function as assessed by electroretinography (ERG) was normal in PARP1 KO animals. We then used retinal explant cultures derived from wt, rd1, and PARP1 KO animals to test their susceptibility to chemically induced photoreceptor degeneration. Since photoreceptor degeneration in the rd1 retina is triggered by a loss-of-function in phosphodiesterase-6 (PDE6), we used selective PDE6 inhibition to emulate the rd1 situation on non-rd1 genotypes. While wt retina subjected to PDE6 inhibition showed massive photoreceptor degeneration comparable to rd1 retina, in the PARP1 KO situation, cell death was robustly reduced. Together, these findings demonstrate that PARP1 activity is in principle dispensable for normal retinal function, but is of major importance for photoreceptor degeneration under pathological conditions. Moreover, our results suggest that PARP dependent cell death or PARthanatos may play a major role in retinal degeneration and highlight the possibility to use specific PARP inhibitors for the treatment of RP.

Reg2 protects mouse insulinoma cells from streptozotocin-induced mitochondrial disruption and apoptosis

We reported previously that pancreas-specific ablation of IGF-I in mice induced an increased expression of regenerating family proteins Reg2 and Reg3β in the pancreas and protected them from streptozotocin (Stz)-induced β-cell damage. We, therefore, assessed the effect of ectopically introduced Reg2 on Stz-induced apoptosis in MIN6 mouse insulinoma cells and report here that Reg2 protects MIN6 cells from Stz-induced apoptosis by attenuating its ability to disrupt mitochondrial membrane integrity, activate caspase-3 and promote poly-ADP ribose polymerase cleavage, and induce apoptosis. These changes correlated with suppression of c-jun N-terminal kinase (JNK) phosphorylation by Stz. Reg2 inhibited Stz-induced proapoptotic events as well as the inactivation of JNK. Inclusion of chemical inhibitor of JNK to Reg2 expressing cells rendered them sensitive to Stz. These data demonstrate that Reg2 protects insulin-producing cells against Stz-induced apoptosis by interfering with its cytotoxic signaling upstream of the intrinsic proapoptotic events by preventing its ability to inactivate JNK.

Comparative study of the binding characteristics to and inhibitory potencies towards PARP and in vivo antidiabetogenic potencies of taurine, 3-aminobenzamide and nicotinamide

BACKGROUND

Poly(ADP-ribose) is a NAD+-requiring, DNA-repairing, enzyme playing a central role in pancreatic beta-cell death and in the development of endothelial dysfunction in humans and experimental animals. PARP activation is also relevant to the development of complications of diabetes. Hence, agents capable of inhibiting PARP may be useful in preventing the development of diabetes and in slowing down complications of diabetes.

METHODS

PARP inhibition was assessed with a colorimetric assay kit. Molecular docking studies on the active site of PARP were conducted using the crystalline structure of the enzyme available as Protein Data Bank Identification No. 1UK1. Type 2 diabetes was induced in male Sprague-Dawley rats with streptozotocin (STZ, 60 mg/kg, i.p.). The test compounds (3-aminobenzamide = 3-AB, nicotinamide = NIC, taurine = TAU) were given by the i.p. route 45 min before STZ at 2.4 mM/kg (all three compounds) or 1.2 and 3.6 mM/kg (only NIC and TAU). Blood samples were collected at 24 hr after STZ and processed for their plasma. The plasma samples were used to measure glucose, insulin, cholesterol, triglycerides, malondialdehyde, nitric oxide, and glutathione levels using reported methods.

RESULTS

3-AB, NIC and TAU were able to inhibit PARP, with the inhibitory potency order being 3-AB>NIC> or =TAU. Molecular docking studies at the active site of PARP showed 3-AB and NIC to interact with the binding site for the nicotinamide moiety of NAD+ and TAU to interact with the binding site for the adenine moiety of NAD+. While STZ-induced diabetes elevated all the experimental parameters examined and lowered the insulin output, a pretreatment with 3-AB, NIC or TAU reversed these trends to a significant extent. At a dose of 2.4 mm/kg, the protective effect decreased in the approximate order 3-AB>NIC> or =TAU. The attenuating actions of both NIC and TAU were dose-related except for the plasma lipids since NIC was without a significant effect at all doses tested.

CONCLUSIONS

At equal molar doses, 3-AB was generally more potent than either TAU or NIC as an antidiabetogenic agent, but the differences were not as dramatic as would have been predicted from their differences in PARP inhibitory potencies. NIC and TAU demonstrated dose-related effects, which in the case of TAU were only evident at doses > or =2.4 mM/kg. The present results also suggest that in the case of NIC and TAU an increase in dose will enhance the magnitude of their attenuating actions on diabetes-related biochemical alterations to that achieved with a stronger PARP inhibitor such as 3-AB. Hence, dosing will play a critical role in clinical studies assessing the merits of NIC and TAU as diabetes-preventing agents.

Pancreatic acinar-specific overexpression of Reg2 gene offered no protection against either experimental diabetes or pancreatitis in mice

Reg proteins are normally expressed in pancreatic acinar cells, and the level of several of these proteins was significantly induced upon damage to the endocrine or exocrine pancreas. It has been established that Reg1 and pancreatic islet neogenesis-associated protein [INGAP, Reg3delta] promote the growth or regeneration of the endocrine islet cells. Recent reports suggest that Reg2 is an autoantigen normally expressed in islet beta-cells. Reg2 overexpression in vitro offered protection to insulinoma cells. Overexpressed Reg3alpha increased cyclin D1 and CDK4 levels and the rate of proliferation in insulinoma cells. Acinar-specific overexpression of INGAP increased beta-cell mass and protected the animals from streptozotocin-induced diabetes. Moreover, Reg2 gene expression was induced during pancreatitis. We hypothesized that Reg2 is a secreted protein that promotes the growth, survival, and/or regeneration of pancreatic endocrine and exocrine cells. To test its effectiveness, we used elastase-1 promoter (Ela-Reg2) to develop an acinar cell-specific overexpression of the Reg2 gene. Western blot analysis, real-time PCR, and immunohistochemistry revealed barely detectable levels of endogenous Reg2 in the pancreas of normal wild-type mice and increased Reg2 levels in the pancreas of Ela-Reg2 mice that were similar to or higher than Reg2 levels induced in experimental diabetes or pancreatitis. Compared with wild-type littermates, growth, blood glucose and insulin levels, and glucose tolerance were normal in Ela-Reg2 mice; pancreatic histology revealed no change in endocrine or exocrine tissues. Acinar-specific overexpression of the Reg2 gene offered no protection against streptozotocin-induced beta-cell damage and diabetes, in hyperglycemia and weight loss, and no advantage in restoring glucose homeostasis and islet function within 3 mo. Furthermore, serum amylase level and pancreatic histochemistry showed that Reg2 overexpression did not protect acinar cells against caerulein-induced acute pancreatitis. In contrast to INGAP or Reg3beta, exocrine overexpression of Reg2 offered no protection to the endocrine or exocrine pancreas, indicating clear subtype specificities of the Reg family of proteins.

Role of nicotinamide in DNA damage, mutagenesis, and DNA repair

Nicotinamide is a water-soluble amide form of niacin (nicotinic acid or vitamin B3). Both niacin and nicotinamide are widely available in plant and animal foods, and niacin can also be endogenously synthesized in the liver from dietary tryptophan. Nicotinamide is also commercially available in vitamin supplements and in a range of cosmetic, hair, and skin preparations. Nicotinamide is the primary precursor of nicotinamide adenine dinucleotide (NAD⁺), an essential coenzyme in ATP production and the sole substrate of the nuclear enzyme poly-ADP-ribose polymerase-1 (PARP-1). Numerous in vitro and in vivo studies have clearly shown that PARP-1 and NAD⁺ status influence cellular responses to genotoxicity which can lead to mutagenesis and cancer formation. This paper will examine the role of nicotinamide in the protection from carcinogenesis, DNA repair, and maintenance of genomic stability.

Poly(ADP-ribose) polymerase-1 (PARP-1) gene deficiency alleviates diabetic kidney disease

Poly(ADP-ribose)polymerase (PARP) inhibitors prevent or alleviate diabetic nephropathy. This study evaluated the role for PARP-1 in diabetic kidney disease using the PARP-1-deficient mouse. PARP-1-/- and the wild-type (129S1/SvImJ) mice were made diabetic with streptozotocin, and were maintained for 12 weeks. Final blood glucose concentrations were increased ∼ 3.7-fold in both diabetic groups. PARP-1 protein expression (Western blot analysis) in the renal cortex was similar in non-diabetic and diabetic wild-type mice (100% and 107%) whereas all knockouts were PARP-1-negative. PARP-1 gene deficiency reduced urinary albumin (ELISA) and protein excretion prevented diabetes-induced kidney hypertrophy, and decreased mesangial expansion and collagen deposition (both assessed by histochemistry) as well as fibronectin expression. Renal podocyte loss (immunohistochemistry) and nitrotyrosine and transforming growth factor-β₁ accumulations (both by ELISA) were slightly lower in diabetic PARP-1-/- mice, but the differences with diabetic wild-type group did not achieve statistical significance. In conclusion, PARP-1-/- gene deficiency alleviates although does not completely prevent diabetic kidney disease.

Role of JNK activation in pancreatic beta-cell death by streptozotocin

c-Jun N-terminal kinase (JNK) is activated by cellular stress and plays critical roles in diverse types of cell death. However, role of JNK in beta-cell injury is obscure. We investigated the role for JNK in streptozotocin (STZ)-induced beta-cell death. STZ induced JNK activation in insulinoma or islet cells. JNK inhibitors attenuated insulinoma or islet cell death by STZ. STZ-induced JNK activation was decreased by PARP inhibitors, suggesting that JNK activation is downstream of PARP-1. Phosphatase inhibitors induced activation of JNK and abrogated the suppression of STZ-induced JNK activation by PARP inhibitors, suggesting that the inhibition of phosphatases is involved in the activation of JNK by STZ. STZ induced production of reactive oxygen species (ROS) as potential inhibitors of phosphatases, which was suppressed by PARP inhibitors. PARP-1 siRNA attenuated insulinoma cell death and JNK activation after STZ treatment, which was reversed by MKP (MAP kinase phosphatase)-1 siRNA. These results suggest that JNK is activated by STZ downstream of PARP-1 through inactivation of phosphatases such as MKP, which plays important roles in STZ-induced beta-cell death.

The secret life of NAD+: an old metabolite controlling new metabolic signaling pathways

A century after the identification of a coenzymatic activity for NAD(+), NAD(+) metabolism has come into the spotlight again due to the potential therapeutic relevance of a set of enzymes whose activity is tightly regulated by the balance between the oxidized and reduced forms of this metabolite. In fact, the actions of NAD(+) have been extended from being an oxidoreductase cofactor for single enzymatic activities to acting as substrate for a wide range of proteins. These include NAD(+)-dependent protein deacetylases, poly(ADP-ribose) polymerases, and transcription factors that affect a large array of cellular functions. Through these effects, NAD(+) provides a direct link between the cellular redox status and the control of signaling and transcriptional events. Of particular interest within the metabolic/endocrine arena are the recent results, which indicate that the regulation of these NAD(+)-dependent pathways may have a major contribution to oxidative metabolism and life span extension. In this review, we will provide an integrated view on: 1) the pathways that control NAD(+) production and cycling, as well as its cellular compartmentalization; 2) the signaling and transcriptional pathways controlled by NAD(+); and 3) novel data that show how modulation of NAD(+)-producing and -consuming pathways have a major physiological impact and hold promise for the prevention and treatment of metabolic disease.

Protective effects of the volatile oil of Nigella sativa seeds on beta-cell damage in streptozotocin-induced diabetic rats: a light and electron microscopic study

The aim of this study was to evaluate the possible protective effects of the volatile oil of Nigella sativa (NS) seeds on insulin immunoreactivity and ultrastructural changes of pancreatic beta-cells in STZ-induced diabetic rats. STZ was injected intraperitoneally at a single dose of 50 mg/kg to induce diabetes. The rats in NS treated groups were given NS (0.2 ml/kg) once a day orally for 4 weeks starting 3 days prior to STZ injection. To date, no ultrastructural changes of pancreatic beta-cells in STZ induced diabetic rats by NS treatment have been reported. Islet cell degeneration and weak insulin immunohistochemical staining was observed in rats with STZ-induced diabetes. Increased intensity of staining for insulin, and preservation of beta-cell numbers were apparent in the NS-treated diabetic rats. The protective effect of NS on STZ-diabetic rats was evident by a moderate increase in the lowered secretory vesicles with granules and also slight destruction with loss of cristae within the mitochondria of beta-cell when compared to control rats. These findings suggest that NS treatment exerts a therapeutic protective effect in diabetes by decreasing morphological changes and preserving pancreatic beta-cell integrity. Consequently, NS may be clinically useful for protecting beta-cells against oxidative stress.

Nucleic acid-associated autoantigens: pathogenic involvement and therapeutic potential

Autoimmunity to ubiquitously expressed macromolecular nucleic acid-protein complexes such as the nucleosome or the spliceosome is a characteristic feature of systemic autoimmune diseases. Disease-specificity and/or association with clinical features of some of these autoimmune responses suggest pathogenic involvement which, however, has been proven in only a few cases so far. Although the mechanisms leading to autoimmunity against nucleic acid-containing complexes are still far from being fully understood, there is increasing experimental evidence that the nucleic acid component may act as a co-stimulator or adjuvans via activation of nucleic acid-binding receptor systems such as Toll-like receptors in antigen-presenting cells. Dysregulated apoptosis and inappropriate stimulation of nucleic acid-sensing receptors may lead to loss of tolerance against the protein components of such complexes, activation of autoreactive T cells and formation of autoantibodies. This has been demonstrated to occur in systemic lupus erythematosus and seems to represent a general mechanism that may be crucial for the development of systemic autoimmune diseases. This review provides a comprehensive overview of the most thoroughly-characterized nucleic acid-associated autoantigens, describing their structure and biological function, as well as the nature and pathogenic importance of the reactivities directed against them. Furthermore, recent advances in immunotherapy such as antigen-specific approaches targeted at nucleic acid-binding antigens are discussed.

Recent advances in physiological and pathological significance of NAD+ metabolites: roles of poly(ADP-ribose) and cyclic ADP-ribose in insulin secretion and diabetogenesis

Poly(ADP-ribose) synthetase/polymerase (PARP) activation causes NAD+ depletion in pancreatic beta-cells, which results in necrotic cell death. On the other hand, ADP-ribosyl cyclase/cyclic ADP-ribose hydrolase (CD38) synthesizes cyclic ADP-ribose from NAD+, which acts as a second messenger, mobilizing intracellular Ca2+ for insulin secretion in response to glucose in beta-cells. PARP also acts as a regenerating gene (Reg) transcription factor to induce beta-cell regeneration. This provides the new concept that NAD+ metabolism can control the cellular function through gene expression. Clinically, PARP could be one of the most important therapeutic targets; PARP inhibitors prevent cell death, maintain the formation of a second messenger, cyclic ADP-ribose, to achieve cell function, and keep PARP functional as a transcription factor for cell regeneration.

Nitrative stress and poly(ADP-ribose) polymerase activation in healthy and gestational diabetic pregnancies

AIMS/HYPOTHESIS

Increased oxidative-nitrosative stress, poly(ADP-ribose) polymerase (PARP) activation and subsequent cellular damage play important roles in the complications of both diabetes mellitus and pregnancy. Our aim was to investigate nitrative stress and PARP activity levels during normal and gestational diabetic (GDM) pregnancy in both maternal and fetal tissues.

METHODS

Blood samples were collected during pregnancy (weeks 16-29 and 36-40), and placental and umbilical cord tissues were harvested after delivery from healthy volunteers and GDM patients subjected to a carbohydrate-restricted diet or insulin treatment. Immunohistochemical staining was performed on leucocytes and tissue sections using anti-nitrotyrosine (NT), anti-poly(ADP-ribose) (PAR) and anti-apoptosis inducing factor antibodies.

RESULTS

In healthy pregnancies the intensity of NT and PAR staining of leucocytes correlated positively with gestational week (R (2) = 0.43, p < 0.01 and R (2) = 0.49, p < 0.001, respectively). In patients on a carbohydrate-restricted diet PAR staining was already strong in weeks 16-29 (p < 0.001 vs control) and did not increase further. In weeks 16-29 there was a correlation between PAR staining and the 2 h value of the oral glucose tolerance test (R (2) = 0.49, p < 0.001). Patients with the highest level of leucocyte PARP activity later required insulin therapy, which decreased the intensity of NT and PAR staining. Placental and umbilical cord tissues also had a higher level of nitrative stress markers in GDM pregnancies, but the highest level of PARP activity was observed after insulin therapy.

CONCLUSIONS/INTERPRETATION

Continuous elevation of tyrosine nitration and PARP activation may be considered physiological during pregnancy. However, the high level of PARP activity in early pregnancy may signal the subsequent development of severe GDM.

PARP inhibitors: new partners in the therapy of cancer and inflammatory diseases

Poly(ADP-ribose) polymerases (PARPs) are defined as cell signaling enzymes that catalyze the transfer of ADP-ribose units from NAD(+) to a number of acceptor proteins. PARP-1, the best characterized member of the PARP family, which currently comprises 18 members, is an abundant nuclear enzyme implicated in cellular responses to DNA injury provoked by genotoxic stress. PARP is involved in DNA repair and transcriptional regulation and is now recognized as a key regulator of cell survival and cell death as well as a master component of a number of transcription factors involved in tumor development and inflammation. PARP-1 is essential to the repair of DNA single-strand breaks via the base excision repair pathway. Inhibitors of PARP-1 have been shown to enhance the cytotoxic effects of ionizing radiation and DNA-damaging chemotherapy agents, such as the methylating agents and topoisomerase I inhibitors. There are currently at least five PARP inhibitors in clinical trial development. Recent in vitro and in vivo evidence suggests that PARP inhibitors could be used not only as chemo/radiotherapy sensitizers, but also as single agents to selectively kill cancers defective in DNA repair, specifically cancers with mutations in the breast cancer-associated genes (BRCA1 and BRCA2). PARP becomes activated in response to oxidative DNA damage and depletes cellular energy pools, thus leading to cellular dysfunction in various tissues. The activation of PARP may also induce various cell death processes and promotes an inflammatory response associated with multiple organ failure. Inhibition of PARP activity is protective in a wide range of inflammatory and ischemia-reperfusion-associated diseases, including cardiovascular diseases, diabetes, rheumatoid arthritis, endotoxic shock, and stroke. The aim of this review is to overview the emerging data in the literature showing the role of PARP in the pathogenesis of cancer and inflammatory diseases and unravel the solid body of literature that supports the view that PARP is an important target for therapeutic intervention in critical illness.

Poly(ADP-ribose) polymerase 1 at the crossroad of metabolic stress and inflammation in aging

Poly(ADP-ribose) polymerase 1 (PARP1) is a chromatin-associated nuclear protein, which functions as molecular stress sensor. Reactive oxygen species, responsible for the most plausible and currently acceptable global mechanism to explain the aging process, strongly activate the enzymatic activity of PARP1 and the formation of poly(ADP-ribose) (PAR) from NAD(+). Consumption of NAD(+) links PARP1 to energy metabolism and to a large number of NAD(+)-dependent enzymes, such as the sirtuins. As transcriptional cofactor for NF-kappaB-dependent gene expression, PARP1 is also connected to the immune response, which is implicated in almost all age-related or associated diseases. Accordingly, numerous experimental studies have demonstrated the beneficial effects of PARP inhibition for several age-related diseases. This review summarizes recent findings on PARP1 and puts them in the context of metabolic stress and inflammation in aging.

Pig islet xenotransplantation into non-human primate model

Allogeneic islet transplantation faces difficulties because (1) organ shortage is recurrent; (2) several pancreas donors are often needed to treat one diabetic recipient; and (3) the intrahepatic site of islet implantation may not be the most appropriate site. Another source of insulin-producing cells, therefore, would be of major interest, and pigs represent a possible and serious source for obtaining such cells. Pig islet grafts may seem difficult because of the species barrier, but recent reports demonstrate that pig islets may function in primates for at least 6 months. Pig islet xenotransplantation, however, must still overcome several hurdles before becoming clinically applicable. The actual consensus is to produce more preclinical data in the pig-to-primate model as a necessary requirement to envisage any pig-to-human transplantation of islets; therefore, a summary of the actual acquired knowledge of pig islet transplantation in primates seemed useful and is summarized in this overview.

New insights into metabolic signaling and cell survival: the role of beta-O-linkage of N-acetylglucosamine

The involvement of glucose in fundamental metabolic pathways represents a core element of biology. Late in the 20th century, a unique glucose-derived signal was discovered, which appeared to be involved in a variety of cellular processes, including mitosis, transcription, insulin signaling, stress responses, and potentially, Alzheimer's disease, and diabetes. By definition, this glucose-fed signaling system was a post-translational modification to proteins. However, unlike classical cotranslational N-glycosylation occurring in the endoplasmic reticulum and Golgi apparatus, this process occurs elsewhere throughout the cell in a highly dynamic fashion, similar to the quintessential post-translational modification, phosphorylation. This more recently described post-translational modification, the beta-O-linkage of N-acetylglucosamine (i.e., O-GlcNAc) to nucleocytoplasmic proteins, represents an under-investigated area of biology. This signaling system operates in all of the tissues examined and seems to have persisted throughout all multicellular eukaryotes. Thus, it comes with little surprise that O-GlcNAc signaling is an integral system and viable target for biomedical investigation. This system may be a boundless source for insight into a variety of diseases and yield numerous opportunities for drug design. This Perspective will address recent insights into O-GlcNAc signaling in the cardiovascular system as a paradigm for its involvement in other biological systems.

PARP activation and the alteration of vasoactive factors and extracellular matrix protein in retina and kidney in diabetes

AIMS

The development of diabetic complications is associated with increased oxidative stress which may damage DNA leading to the activation of nuclear enzyme poly (ADP-ribose) polymerase (PARP). PARP overactivation may further exacerbate the oxidative state of the cell through its consumption of nicotinamide adenine dinucleotide. In diabetic retinopathy and nephropathy, early characteristic features include increased production of vasoactive factors such as endothelin 1 (ET-1) and increased synthesis of extracellular matrix (ECM) proteins such as fibronectin (FN) and its splice variant extra domain B containing (EDB(+)) FN. We investigated the role of PARP in the development of diabetic retinopathy and nephropathy.

METHODS

Two models of diabetic complications were used. PARP-1 knockout mice and their respective wild type controls were fed a 30% galactose diet for 2 months. The rats were given injections of PARP inhibitor 3-aminobenzamide (30 mg/kg/day).

RESULTS

Analysis of the retinal and kidney tissues showed hyperhexosemia-induced oxidative stress and increased expression of ET-1, FN and EDB(+) FN in association with increased transcriptional co-activator p300 along with p300-dependent transcription factors, myocyte enhancing factors 2A and 2C. Furthermore, we showed increased PARP expression in the kidneys and retina of the diabetic rats. PARP blockade in both animal models prevented these hyperhexosemia-induced effects.

CONCLUSIONS

These findings suggests that hyperhexosemia and diabetes causes upregulation of ET-1, FN and EDB(+) FN at the transcriptional level in the retina and kidney via a signaling pathway mediated by PARP and an epigenetic mechanism involving p300 and MEF2 transcription factors. Understanding these mechanisms is important in identifying novel treatment targets.

Aire deficiency causes increased susceptibility to streptozotocin-induced murine type 1 diabetes

Aire-deficient mice are a model of the human monogenic disorder autoimmune polyendocrine syndrome type I (APS I) characterized by a progressive autoimmune destruction of multiple endocrine glands such as the adrenal cortex, the parathyroids and the beta-cells of the pancreas. The disease is caused by mutations in the autoimmune regulator (AIRE) gene, a putative transcription factor expressed in thymic medullary epithelial cells and in antigen-presenting cells of the myeloid lineage in peripheral lymphoid organs. As Aire(-/-) mice do not spontaneously develop endocrinopathies, we wanted to evaluate the autoimmune multiple low-dose streptozotocin (MLDSTZ) diabetes model in Aire(-/-) mice. Surprisingly, Aire heterozygote mice were most susceptible to MLDSTZ-induced diabetes, whereas Aire(-/-) mice displayed an intermediate sensitivity to diabetes. Furthermore, Aire(-/-) macrophages produced higher levels of TNF-alpha and lower levels of IL-10 following streptozotocin stimulation, and Aire(-/-) mice developed a higher frequency of islet cells autoantibodies as a sign of increased activation. However, the number of islet infiltrating F4/80(+) Aire(-/-) macrophages was significantly decreased which was attributed to an increased susceptibility to streptozotocin cytotoxicity of Aire(-/-) macrophages. In conclusion, Aire(-/-) macrophages display an increased activation after STZ stimuli, but suffer from increased susceptibility to STZ cytotoxicity. These results support an important function of Aire in the control of peripheral tolerance through myeloid antigen-presenting cells.

Autophagy and VMP1 expression are early cellular events in experimental diabetes

BACKGROUND/AIMS

We have described VMP1 as a new protein which expression triggers autophagy in mammalian cells. Here we show that experimental diabetes activates VMP1 expression and autophagy in pancreas beta cells as a direct response to streptozotocin (STZ).

METHODS

Male Wistar rats were treated with 65 mg/kg STZ and pancreas islets from untreated rats were incubated with 1 mM STZ.

RESULTS

RT-PCR analysis shows early VMP1 induction after STZ treatment. In situ hybridization reveals VMP1 mRNA in islet beta cells. Electron microscopy shows chromatin aggregation and autophagy morphology that was confirmed by LC3 expression and LC3-VMP1 co-localization. Apoptotic cell death and the reduction of beta cell pool are evident after 24 h treatment, while VMP1 is still expressed in the remaining cells. VMP1-Beclin1 colocalization in pancreas tissue from STZ-treated rats suggests that VMP1-Beclin1 interaction is involved in the autophagic process activation during experimental diabetes. Results were confirmed using pancreas islets, showing VMP1 expression and autophagy in beta cells as a direct effect of STZ treatment.

CONCLUSION

Pancreas beta cells trigger VMP1 expression and autophagy during the early cellular events in response to experimental diabetes.

Poly (ADP-ribose) polymerases (PARPs) 1-3 regulate astrocyte activation

Besides their traditional role in maintaining CNS homeostasis, astrocytes also participate in innate immune responses. Indeed, we have previously demonstrated that astrocytes are capable of recognizing bacterial pathogens such as Staphylococcus aureus, a common etiologic agent of CNS infections, and respond with the robust production of numerous proinflammatory mediators. Suppression of Poly (ADP-ribose) polymerase-1 (PARP-1), a DNA repair enzyme, has been shown to attenuate inflammatory responses in several cell types including mixed glial cultures. However, a role for PARP-1 in regulating innate immune responses in purified astrocytes and the potential for multiple PARP family members to cooperatively regulate astrocyte activation has not yet been examined. The synthetic PARP-1 inhibitor PJ-34 attenuated the production of several proinflammatory mediators by astrocytes in response to S. aureus stimulation including nitric oxide, interleukin-1 beta, tumor necrosis factor-alpha, and CCL2. The release of all four mediators was partially reduced in PARP-1 knockout (KO) astrocytes compared to wild-type cells. The residual inflammatory mediator expression detected in PARP-1 KO astrocytes was further blocked with PJ-34, suggesting either non-specific effects of the drug or actions on alternative PARP isoforms. Reduction in PARP-2 or PARP-3 expression by siRNA knock down revealed that these isoforms also contributed to inflammatory mediator regulation in response to S. aureus. Interestingly, the combined targeting of either PARP-1/PARP-2 or PARP-2/PARP-3 attenuated astrocyte inflammatory responses more effectively compared to knock down of either PARP alone, suggesting cooperativity between PARP isoforms. Collectively, these findings suggest that PARPs influence the extent of S. aureus-induced astrocyte activation.

Poly(ADP-ribose) polymerase-1 enhances transcription of the profibrotic CCN2 gene

In the fibrotic kidney, tubular epithelial cells express CCN2, formerly known as connective tissue growth factor. Because little is known about the transcriptional regulation of this profibrotic protein, this study investigated the mechanism underlying epithelial cell-selective upregulation of CCN2 in fibrosis. It was found that a previously unidentified cis-regulatory element located in the promoter of the murine CCN2 gene plays an essential role in basal and TGF-beta1-induced gene transcription in tubular epithelial cells; this element acts in conjunction with the Smad-binding element and the basal control element-1. By protein mass fingerprint analysis and de novo sequencing, poly(ADP-ribose) polymerase-1 (PARP-1) was identified as a trans-acting protein factor that binds to this promoter region, which we termed the PARP-1-binding element. In vivo, knockdown of PARP-1 in proximal tubular epithelial cells significantly reduced CCN2 mRNA levels and attenuated interstitial fibrosis in the obstructed kidney. Thus, the PARP-1/PARP-1 binding element complex functions as a nonspecific, fundamental enhancer of both basal and induced CCN2 gene transcription in tubular epithelial cells. This regulatory complex may be a promising target for antifibrotic therapy.

The mechanisms of alloxan- and streptozotocin-induced diabetes

Alloxan and streptozotocin are toxic glucose analogues that preferentially accumulate in pancreatic beta cells via the GLUT2 glucose transporter. In the presence of intracellular thiols, especially glutathione, alloxan generates reactive oxygen species (ROS) in a cyclic redox reaction with its reduction product, dialuric acid. Autoxidation of dialuric acid generates superoxide radicals, hydrogen peroxide and, in a final iron-catalysed reaction step, hydroxyl radicals. These hydroxyl radicals are ultimately responsible for the death of the beta cells, which have a particularly low antioxidative defence capacity, and the ensuing state of insulin-dependent 'alloxan diabetes'. As a thiol reagent, alloxan also selectively inhibits glucose-induced insulin secretion through its ability to inhibit the beta cell glucose sensor glucokinase. Following its uptake into the beta cells, streptozotocin is split into its glucose and methylnitrosourea moiety. Owing to its alkylating properties, the latter modifies biological macromolecules, fragments DNA and destroys the beta cells, causing a state of insulin-dependent diabetes. The targeting of mitochondrial DNA, thereby impairing the signalling function of beta cell mitochondrial metabolism, also explains how streptozotocin is able to inhibit glucose-induced insulin secretion.

KATP channel-deficient pancreatic beta-cells are streptozotocin resistant because of lower GLUT2 activity

In wild-type mice, a single injection of streptozotocin (STZ, 200 mg/kg body wt) caused within 4 days severe hyperglycemia, hypoinsulinemia, significant glucose intolerance, loss of body weight, and the disappearance of pancreatic beta-cells. However, in ATP-sensitive K(+) channel (K(ATP) channel)-deficient mice (Kir6.2(-/-) mice), STZ had none of these effects. Exposing isolated pancreatic islets to STZ caused severe damage in wild-type but not in Kir6.2(-/-) islets. Following a single injection, plasma STZ levels were slightly less in Kir6.2(-/-) mice than in wild-type mice. Despite the difference in plasma STZ, wild-type and Kir6.2(-/-) liver accumulated the same amount of STZ, whereas Kir6.2(-/-) pancreas accumulated 4.1-fold less STZ than wild-type pancreas. Kir6.2(-/-) isolated pancreatic islets also transported less glucose than wild-type ones. Quantification of glucose transporter 2 (GLUT2) protein content by Western blot using an antibody with an epitope in the extracellular loop showed no significant difference in GLUT2 content between wild-type and Kir6.2(-/-) pancreatic islets. However, visualization by immunofluorescence with the same antibody gave rise to 32% less fluorescence in Kir6.2(-/-) pancreatic islets. The fluorescence intensity using another antibody, with an epitope in the COOH terminus, was 5.6 times less in Kir6.2(-/-) than in wild-type pancreatic islets. We conclude that 1) Kir6.2(-/-) mice are STZ resistant because of a decrease in STZ transport by GLUT2 in pancreatic beta-cells and 2) the decreased transport is due to a downregulation of GLUT2 activity involving an effect at the COOH terminus.

PARP1 is required for adhesion molecule expression in atherogenesis

AIMS

Atherosclerosis is the leading cause of death in Western societies and a chronic inflammatory disease. However, the key mediators linking recruitment of inflammatory cells to atherogenesis remain poorly defined. Poly(ADP-ribose) polymerase 1 (PARP1) is a nuclear enzyme, which plays a role in acute inflammatory diseases.

METHODS AND RESULTS

In order to test the role of PARP in atherogenesis, we applied chronic pharmacological PARP inhibition or genetic PARP1 deletion in atherosclerosis-prone apolipoprotein E-deficient mice and measured plaque formation, adhesion molecules, and features of plaque vulnerability. After 12 weeks of high-cholesterol diet, plaque formation in male apolipoprotein E-deficient mice was decreased by chronic inhibition of enzymatic PARP activity or genetic deletion of PARP1 by 46 or 51%, respectively (P < 0.05, n >or= 9). PARP inhibition or PARP1 deletion reduced PARP activity and diminished expression of inducible nitric oxide synthase, vascular cell adhesion molecule-1, and P- and E-selectin. Furthermore, chronic PARP inhibition reduced plaque macrophage (CD68) and T-cell infiltration (CD3), increased fibrous cap thickness, and decreased necrotic core size and cell death (P < 0.05, n >or= 6).

CONCLUSION

Our data provide pharmacological and genetic evidence that endogenous PARP1 is required for atherogenesis in vivo by increasing adhesion molecules with endothelial activation, enhancing inflammation, and inducing features of plaque vulnerability. Thus, inhibition of PARP1 may represent a promising therapeutic target in atherosclerosis.

Deletion of aldose reductase leads to protection against cerebral ischemic injury

Previously, we reported that transgenic mice overexpressing endothelin-1 in astrocytes showed more severe neurological deficits and increased infarct after transient focal ischemia. In those studies, we also observed increased level of aldose reductase (AR), the first and rate-limiting enzyme of the polyol pathway, which has been implicated in osmotic and oxidative stress. To further understand the involvement of the polyol pathway, the mice with deletion of enzymes in the polyol pathway, AR, and sorbitol dehydrogenase (SD), which is the second enzyme in this pathway, were challenged with similar cerebral ischemic injury. Deletion of AR-protected animals from severe neurological deficits and large infarct, whereas similar protection was not observed in mice with SD deficiency. Most interestingly, AR(-/-) brains showed lowered expression of transferrin and transferrin receptor with less iron deposition and nitrotyrosine accumulation. The protection against oxidative stress in AR(-/-) brain was also associated with less poly(adenosine diphosphate-ribose) polymerase (PARP) and caspase-3 activation. Pharmacological inhibition of AR by Fidarestat also protected animals against cerebral ischemic injury. These findings are the first to show that AR contributes to iron- and transferrin-related oxidative stress associated with cerebral ischemic injury, suggesting that inhibition of AR but not SD may have therapeutic potential against cerebral ischemic injury.

Poly(ADP-ribose) polymerase-1 (PARP-1) controls lung cell proliferation and repair after hyperoxia-induced lung damage

Oxygen-based therapies expose lung to elevated levels of ROS and induce lung cell damage and inflammation. Injured cells are replaced through increased proliferation and differentiation of epithelial cells and fibroblasts. Failure to modulate these processes leads to excessive cell proliferation, collagen deposition, fibrosis, and chronic lung disease. Poly(ADP-ribose) polymerase-1 (PARP-1) is activated in response to DNA damage and participates in DNA repair, genomic integrity, and cell death. In this study, we evaluated the role of PARP-1 in lung repair during recovery after acute hyperoxia exposure. We exposed PARP-1 -/- and wild-type mice for 64 h to 100% hyperoxia and let them recover in air for 5-21 days. PARP-1-deficient mice exhibited significantly higher lung cell hyperplasia and proliferation than PARP-1 +/+ animals after 5 and 10 days of recovery. This was accompanied by an increased inflammatory response in PARP-1 -/- compared with wild-type animals, characterized by neutrophil infiltration and increased IL-6 levels in bronchoalveolar lavages. These lesions were reversible, since the extent of the hyperplastic regions was reduced after 21 days of recovery and did not result in fibrosis. In vitro, lung primary fibroblasts derived from PARP-1 -/- mice showed a higher proliferative response than PARP-1 +/+ cells during air recovery after hyperoxia-induced growth arrest. Altogether, these results reveal an essential role of PARP-1 in the control of cell repair and tissue remodeling after hyperoxia-induced lung injury.

A novel mechanism of action for statins against diabetes-induced oxidative stress

AIMS/HYPOTHESIS

Atorvastatin exerts beneficial vascular effects in diabetes, but the underlying mechanisms are yet to be elucidated. The aim of the present study was to determine whether Rac-1 is involved in the effect of atorvastatin on oxidative stress and vascular dysfunction.

MATERIALS AND METHODS

Using human aortic endothelial cells (HAECs) we evaluated the effect of high glucose levels on peroxide production by dihydrodichlorofluorescein and on Rac-1 activity using immunocytochemistry to detect Rac-1 translocation to the membrane. We evaluated vascular function, peroxide production by dihydroethidium and NADPH oxidase activity in vessels from atorvastatin-treated mice. Rac-1 activity was also assessed, both by immunoprecipitation of the Rac-p21-activated kinase complex and by analysis of Rac-1 translocation to the membrane. These experiments were also conducted in vessels infected with an adenoviral vector carrying a constitutively active mutant of Rac-1.

RESULTS

In HAECs exposed to high glucose levels, atorvastatin prevented oxidative stress, and this protection was associated with impaired Rac-1 activation. This effect was also observed in a murine model of diabetes mellitus. More importantly, the addition of geranylgeranyl pyrophosphate (GGPP) blocked the effects of atorvastatin in both glucose-exposed HAECs and diabetic vessels. Atorvastatin failed to afford protection against vascular abnormalities in the presence of a constitutively active mutant of Rac-1.

CONCLUSIONS/INTERPRETATION

The results of this study demonstrate that the vascular antioxidant effect of atorvastatin in diabetes is mediated through inhibition of Rac-1 via a reduction in GGPP. Thus, selective Rac-1 inhibition should be considered in the design of novel pharmacological strategies to reduce the impact of diabetes mellitus on vascular function.

Methylating agents and DNA repair responses: Methylated bases and sources of strand breaks

The chemical methylating agents methylmethane sulfonate (MMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) have been used for decades as classical DNA damaging agents. These agents have been utilized to uncover and explore pathways of DNA repair, DNA damage response, and mutagenesis. MMS and MNNG modify DNA by adding methyl groups to a number of nucleophilic sites on the DNA bases, although MNNG produces a greater percentage of O-methyl adducts. There has been substantial progress elucidating direct reversal proteins that remove methyl groups and base excision repair (BER), which removes and replaces methylated bases. Direct reversal proteins and BER, thus, counteract the toxic, mutagenic, and clastogenic effects of methylating agents. Despite recent progress, the complexity of DNA damage responses to methylating agents is still being discovered. In particular, there is growing understanding of pathways such as homologous recombination, lesion bypass, and mismatch repair that react when the response of direct reversal proteins and BER is insufficient. Furthermore, the importance of proper balance within the steps in BER has been uncovered with the knowledge that DNA structural intermediates during BER are deleterious. A number of issues complicate the elucidation of the downstream responses when direct reversal is insufficient or BER is imbalanced. These include inter-species differences, cell-type-specific differences within mammals and between cancer cell lines, and the type of methyl damage or BER intermediate encountered. MMS also carries a misleading reputation of being a radiomimetic, that is, capable of directly producing strand breaks. This review focuses on the DNA methyl damage caused by MMS and MNNG for each site of potential methylation to summarize what is known about the repair of such damage and the downstream responses and consequences if the damage is not repaired.

Partial protection against collagen antibody-induced arthritis in PARP-1 deficient mice

Poly(ADP-ribose) polymerase-1 (PARP-1) is a nuclear DNA-binding protein that participates in the regulation of DNA repair and maintenance of genomic integrity. In addition, PARP-1 has a role in several models of inflammation disease, where its absence or inactivation confers protection. The aim of this study was to analyze the impact of selective PARP-1 suppression in collagen antibody-induced arthritis. We show that PARP-1 deficiency partially reduces the severity of arthritis, although the incidence of disease was similar in control and deficient mice. Decreased clinical scores were accompanied by partial reduction of histopathological findings. Interestingly, quantitative real-time PCR and ELISA analysis revealed that the absence of PARP-1 down-regulated IL-1β and monocyte chemotactic protein 1 expression in arthritic joints whereas tumor necrosis factor-α transcription was not impaired. Our results provide evidence of the contribution of PARP-1 to the progression of arthritis and identify this protein as a potential therapeutic target for the treatment of rheumatoid arthritis.

Ros-induced histone modifications and their role in cell survival and cell death

Much is known about the distal DNA damage repair response. In particular, many of the enzymes and auxiliary proteins that participate in DNA repair have been characterized. In addition, knowledge of signaling pathways activated in response to DNA damage is increasing. In contrast, comparatively less is known of DNA damage-sensing molecules or of the specific alterations to chromatin structure recognized by such DNA damage sensors. Thus, precisely how chromatin structure is altered in response to DNA damage and how such alterations regulate DNA repair processes remain important unanswered questions. In vertebrates, phosphorylation of the histone variant H2A.X occurs rapidly after double-strand break formation, extends over megabase chromatin domains, and is required for stable accumulation of repair proteins at damage foci. We have shown that reactive oxygen species (ROS)-induced DNA single-strand breaks induce the incorporation of 32P specifically into histone H3. ADP-Ribosylation of histones may stimulate local chromatin relaxation to facilitate the repair process, and, indeed, histone ribosylation preceded DNA damage-induced histone H3 phosphorylation. However, H3 phosphorylation occurred concomitant with overall chromatin condensation, as revealed by decreased sensitivity of chromatin to digestion by micrococcal nuclease and by DAPI staining of nuclei. Inhibitors of the ERK and p38MAPK pathways and inhibition of poly(ADP-ribose) polymerase all reduced ROS-induced H3 phosphorylation, chromatin condensation, and cell death. Precisely how changes in the post-translational modification of histone H3 regulate the survival response remains unclear. Attempts to determine the precise site of histone H3 phosphorylation, putative histone H3 kinases, and histone H3 interacting proteins are underway.

Streptozotocin induces G2 arrest in skeletal muscle myoblasts and impairs muscle growth in vivo

Streptozotocin (STZ) is used extensively to induce pancreatic beta-cell death and ultimately diabetes mellitus in animal models. However, the direct effects of STZ on muscle are largely unknown. To delineate the effects of STZ from the effects of hypoinsulinemia/hyperglycemia, we injected young rats with 1) saline (control), 2) STZ (120 mg/kg) or 3) STZ and insulin (STZ-INS; to maintain euglycemia). STZ rats demonstrated significantly elevated blood glucose throughout the 48-h protocol, while control and STZ-INS rats were euglycemic. Body mass increased in control (13 +/- 4 g), decreased by 19 +/- 2 g in STZ and remained unchanged in STZ-INS rats (-0.3 +/- 2 g). Cross-sectional areas of gastrocnemius muscle fibers were smaller in STZ vs. control (1,480 +/- 149 vs. 1,870 +/- 40 microm(2), respectively; P < 0.05) and insulin treatment did not rescue this defect (STZ-INS: 1,476 +/- 143 microm(2)). Western blot analysis revealed a detectable increase in ubiquitinated proteins in the STZ skeletal muscles compared with control and STZ-INS. To further define the effects of STZ on skeletal muscle, independent of hyperglycemia, myoblasts were exposed to varying doses of STZ (0.25-3.0 mg/ml) in vitro. Both acute and chronic exposures of STZ significantly impaired proliferative capacity in a dose-dependent manner. Within STZ-treated myoblasts, increased reactive oxygen species was associated with significant G(2)/M phase cell-cycle arrest. Taken together, our findings show that the effects of STZ are not beta-cell specific and reveal that STZ should not be used for studies examining diabetic myopathy.

Caspase inhibitors promote alternative cell death pathways

The use of caspase inhibitors has revealed the existence of alternative backup cell death programs for apoptosis. The broad-spectrum caspase inhibitor zVAD-fmk modulates the three major types of cell death. Addition of zVAD-fmk blocks apoptotic cell death, sensitizes cells to necrotic cell death, and induces autophagic cell death. Several studies have shown a crucial role for the kinase RIP1 and the adenosine nucleotide translocator (ANT)-cyclophilin D (CypD) complex in necrotic cell death. The underlying mechanism of zVAD-fmk-mediated sensitization to necrotic cell death involves the inhibition of caspase-8-mediated proteolysis of RIP1 and disturbance of the ANT-CypD interaction. RIP1 is also involved in autophagic cell death. Caspase inhibitors and knockdown studies have revealed negative roles for catalase and caspase-8 in autophagic cell death. The positive role of RIP1 and the negative role of caspase-8 in both necrotic and autophagic cell death suggest that the pathways of these two types of cell death are interconnected. Necrotic cell death represents a rapid cellular response involving mitochondrial reactive oxygen species (ROS) production, decreased adenosine triphosphate concentration, and other cellular insults, whereas autophagic cell death first starts as a survival attempt by cleaning up ROS-damaged mitochondria. However, when this process occurs in excess, autophagy itself becomes cytotoxic and eventually leads to autophagic cell death. A better understanding of the molecular mechanisms of these alternative cell death pathways may provide therapeutic tools to combat cell death associated with neurodegenerative diseases, ischemia-reperfusion pathologies, and infectious diseases, and may also facilitate the development of alternative cytotoxic strategies in cancer treatment.

Poly(ADP-ribose): novel functions for an old molecule

The addition to proteins of the negatively charged polymer of ADP-ribose (PAR), which is synthesized by PAR polymerases (PARPs) from NAD(+), is a unique post-translational modification. It regulates not only cell survival and cell-death programmes, but also an increasing number of other biological functions with which novel members of the PARP family have been associated. These functions include transcriptional regulation, telomere cohesion and mitotic spindle formation during cell division, intracellular trafficking and energy metabolism.

Caffeine metabolites are inhibitors of the nuclear enzyme poly(ADP-ribose)polymerase-1 at physiological concentrations

The activity of the nuclear enzyme poly(ADP-ribose)polymerase-1 (E.C.2.4.2.30), which is highly activated by DNA strand breaks, is associated with the pathophysiology of both acute as well as chronic inflammatory diseases. PARP-1 overactivation and the subsequent extensive turnover of its substrate NAD+ put a large demand on mitochondrial ATP-production. Furthermore, due to its reported role in NF-kappaB and AP-1 mediated production of pro-inflammatory cytokines, PARP-1 is considered an interesting target in the treatment of these diseases. In this study the PARP-1 inhibiting capacity of caffeine and several metabolites as well as other (methyl)xanthines was tested using an ELISA-assay with purified human PARP-1. Caffeine itself showed only weak PARP-1 inhibiting activity, whereas the caffeine metabolites 1,7-dimethylxanthine, 3-methylxanthine and 1-methylxanthine, as well as theobromine and theophylline showed significant PARP-1 inhibiting activity. Further evaluation of these compounds in H2O2-treated A549 lung epithelial and RF24 vascular endothelial cells revealed that the decrease in NAD+-levels as well as the formation of the poly(ADP-ribose)polymer was significantly prevented by the major caffeine metabolite 1,7-dimethylxanthine. Furthermore, H2O2-induced necrosis could be prevented by a high dose of 1,7-dimethylxanthine. Finally, antioxidant effects of the methylxanthines could be ruled out with ESR and measurement of the TEAC. Concluding, caffeine metabolites are inhibitors of PARP-1 and the major caffeine metabolite 1,7-dimethylxanthine has significant PARP-1 inhibiting activity in cultured epithelial and endothelial cells at physiological concentrations. This inhibition could have important implications for nutritional treatment of acute and chronic inflammatory pathologies, like prevention of ischemia-reperfusion injury or vascular complications in diabetes.

Necrosis, a well-orchestrated form of cell demise: signalling cascades, important mediators and concomitant immune response

Necrosis has long been described as a consequence of physico-chemical stress and thus accidental and uncontrolled. Recently, it is becoming clear that necrotic cell death is as well controlled and programmed as caspase-dependent apoptosis, and that it may be an important cell death mode that is both pathologically and physiologically relevant. Necrotic cell death is not the result of one well-described signalling cascade but is the consequence of extensive crosstalk between several biochemical and molecular events at different cellular levels. Recent data indicate that serine/threonine kinase RIP1, which contains a death domain, may act as a central initiator. Calcium and reactive oxygen species (ROS) are main players during the propagation and execution phases of necrotic cell death, directly or indirectly provoking damage to proteins, lipids and DNA, which culminates in disruption of organelle and cell integrity. Necrotically dying cells initiate pro-inflammatory signalling cascades by actively releasing inflammatory cytokines and by spilling their contents when they lyse. Unravelling the signalling cascades contributing to necrotic cell death will permit us to develop tools to specifically interfere with necrosis at certain levels of signalling. Necrosis occurs in both physiological and pathophysiological processes, and is capable of killing tumour cells that have developed strategies to evade apoptosis. Thus detailed knowledge of necrosis may be exploited in therapeutic strategies.

Diabetic neuropathy and oxidative stress

This review will focus on the impact of hyperglycemia-induced oxidative stress in the development of diabetes-related neural dysfunction. Oxidative stress occurs when the balance between the production of reactive oxygen species (ROS) and the ability of cells or tissues to detoxify the free radicals produced during metabolic activity is tilted in the favor of the former. Although hyperglycemia plays a key role in inducing oxidative stress in the diabetic nerve, the contribution of other factors, such as endoneurial hypoxia, transition metal imbalances, and hyperlipidemia have been also suggested. The possible sources for the overproduction of ROS in diabetes are widespread and include enzymatic pathways, auto-oxidation of glucose, and mitochondrial superoxide production. Increase in oxidative stress has clearly been shown to contribute to the pathology of neural and vascular dysfunction in diabetes. Potential therapies for preventing increased oxidative stress in diabetic nerve dysfunction will be discussed.

The human apurinic/apyrimidinic endonuclease-1 suppresses activation of poly(adp-ribose) polymerase-1 induced by DNA single strand breaks

DNA single-strand breaks (SSB) activate poly (ADP-ribose) polymerase 1 (PARP1), which then polymerizes ADP-ribosyl groups on various nuclear proteins, consuming cellular energy. Although PARP1 has a role in repairing SSB, activation of PARP1 also causes necrosis and inflammation due to depletion of cellular energy. Here we show that the major mammalian apurinic/apyrimidinic (AP) endonuclease-1 (APE1), an essential DNA repair protein, binds to SSB and suppresses the activation of PARP1. APE1's high affinity for SSB requires Arg177, which is unique in mammalian APEs. PARP1's binding to the cleaved DNA was inhibited, and PARP1 activation was suppressed by the wild-type APE1, but not by the R177A mutant APE1 protein. Cells transiently transfected with the wild-type APE1 decreased the PARP1 activation after H2O2 treatment, while such suppression did not occur with the expression of the R177A APE1 mutant. These results suggest that APE1 suppresses the activation of PARP1 during the repair process of the DNA damage generated by oxidative stress, which may have an important implication for cells to avoid necrosis due to energy depletion.

MafA expression and insulin promoter activity are induced by nicotinamide and related compounds in INS-1 pancreatic beta-cells

Nicotinamide has been reported to induce differentiation of precursor/stem cells toward a beta-cell phenotype, increase islet regeneration, and enhance insulin biosynthesis. Exposure of INS-1 beta-cells to elevated glucose leads to reduced insulin gene transcription, and this is associated with diminished binding of pancreatic duodenal homeobox factor 1 (PDX-1) and mammalian homologue of avian MafA/l-Maf (MafA). Nicotinamide and other low-potency poly(ADP-ribose) polymerase (PARP) inhibitors were thus tested for their ability to restore insulin promoter activity. The low-potency PARP inhibitors nicotinamide, 3-aminobenzamide, or PD128763 increased expression of a human insulin reporter gene suppressed by elevated glucose. In contrast, the potent PARP-1 inhibitors PJ34 or INO-1001 had no effect on promoter activity. Antioxidants, including N-acetylcysteine, lipoic acid, or quercetin, only minimally induced the insulin promoter. Site-directed mutations of the human insulin promoter mapped the low-potency PARP inhibitor response to the C1 element, which serves as a MafA binding site. INS-1 cells exposed to elevated glucose had markedly reduced MafA protein and mRNA levels. Low-potency PARP inhibitors restored MafA mRNA and protein levels, but they had no affect on PDX-1 protein levels or binding activity. Increased MafA expression by low-potency PARP inhibitors was independent of increased MafA protein or mRNA stability. These data suggest that low-potency PARP inhibitors increase insulin biosynthesis, in part, through a mechanism involving increased MafA gene transcription.

Poly(ADP-ribose) Polymerase-1 Signaling to Mitochondria in Necrotic Cell Death Requires RIP1/TRAF2-mediated JNK1 Activation

Poly(ADP-ribose) polymerase-1 (PARP-1) hyperactivation-induced necrosis has been implicated in several pathophysiological conditions. Although mitochondrial dysfunction and apoptosis-inducing factor translocation from the mitochondria to the nucleus have been suggested to play very important roles in PARP-1-mediated cell death, the signaling events downstream of PARP-1 activation in initiating mitochondria dysfunction are not clear. Here we used the DNA alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine, a potent PARP-1 activator, to study PARP-1 activation-mediated cell death. We found, based on genetic knockouts and pharmacological inhibition, that c-Jun N-terminal kinase (JNK), especially JNK1, but not the other groups of mitogen-activated protein kinase, is required for PARP-1-induced mitochondrial dysfunction, apoptosis-inducing factor translocation, and subsequent cell death. We reveal that receptor-interacting protein 1 (RIP1) and tumor necrosis factor receptor-associated factor 2 (TRAF2), are upstream of JNK in PARP-1 hyperactivated cells, because PARP-1-induced JNK activation was attenuated in RIP1-/- and TRAF2-/- mouse embryonic fibroblast cells. Consistently, knockouts of RIP1 and TRAF2 caused a resistance to PARP-1-induced cell death. Therefore, our study uncovers that RIP1, TRAF2, and JNK comprise a pathway to mediate the signaling from PARP-1 overactivation to mitochondrial dysfunction.

Differential effect of saturated, monounsaturated, and polyunsaturated fatty acids on alloxan-induced diabetes mellitus

Earlier, we reported that oils rich in omega-3 eicosapentaenoic acid and docosahexaenoic acid and omega-6 gamma-linolenic acid and arachidonic acid prevented the development of alloxan-induced diabetes mellitus in experimental animals. Here we report the results of our studies with pure saturated stearic acid (SA), monounsaturated oleic acid (OA) and omega-6 arachidonic acid (AA) on alloxan-induced diabetes mellitus in Wistar male rats. Prior oral supplementation with AA prevented alloxan-induced diabetes mellitus, whereas both SA and OA were ineffective. Cyclo-oxygenase (COX) and lipoxygenase (LO) inhibitors did not block this protective action of AA against alloxan-induced diabetes, suggesting that both prostaglandins and leukotrienes are not involved, and that AA by itself is effective. Furthermore, AA restored the anti-oxidant status to normal range in various tissues. These results suggest that AA protects pancreatic beta cells against alloxan-induced diabetes in experimental animals by attenuating oxidant stress.

Streptozotocin-induced diabetes in large animals (pigs/primates): role of GLUT2 transporter and beta-cell plasticity

BACKGROUND

To induce irreversible diabetes in large animals, the efficiency of streptozotocin (STZ) was evaluated in pigs, primates and compared to the gold standard model in rats.

METHODS

Low (50 mg/kg) and high (150 mg/kg) doses of STZ were tested. Hepatic/renal function, glucose metabolism (intravenous glucose tolerance tests, fasting blood glucose) and histomorphometry were evaluated prior to, 1, and 4 weeks after STZ treatment.

RESULTS

In rats and primates, expressing a high level of GLUT2 expression on beta cells, a dose of 50 mg/kg STZ induced irreversible diabetes (due to the 97% destruction of beta cell mass) without provoking liver or renal failure. In pigs, despite the use of high STZ dose, partial correction of hyperglycaemia was observed four weeks after STZ injection (decreased fasting blood glucose and intravenous glucose tolerance tests; increased insulin production). The correction of hyperglycaemia was associated with significant hypertrophy of immature pig beta-cell clusters (+30%, P<0.05), whereas no hypertrophy was observed in rats/primates.

CONCLUSION

These results demonstrated that STZ might be used to induce irreversible diabetes in rats and primates. In contrast, the low STZ sensitivity in pigs related to a low expression of GLUT2, higher number of immature beta cells and compensatory beta-cell hypertrophy, renders STZ-induced diabetes inappropriate for studying islet allografts in swine.

Selective Rac-1 Inhibition Protects From Diabetes-Induced Vascular Injury

Diabetes mellitus is a main risk factor for vascular diseases. Vascular injury induced by diabetes mellitus is characterized by endothelial dysfunction attributable to an increased oxidative stress. So far, the molecular mechanisms involved in the vasculotoxic effects of diabetes are only partially known. We examined the effect of diabetes mellitus on oxidative stress and Rac-1 activation, a small G-protein involved in the activation of NADPH oxidase. Our results show that oxidative stress in vessels of different murine models of diabetes mellitus and in endothelial cells treated with high glucose is associated with an increased Rac-1/PAK binding and Rac-1 translocation from cytosol to plasma membrane, thus demonstrating an enhanced Rac-1 activity. More important, selective Rac-1 inhibition by an adenoviral vector carrying a dominant negative mutant of Rac-1 protected from oxidative stress and vascular dysfunction induced by diabetes mellitus. Our study demonstrates that Rac-1 plays a crucial role in diabetes-induced vascular injury, and it could be a target of novel therapeutic approaches to reduce vascular risk in diabetes mellitus.

Autoimmune destruction of pancreatic beta cells

Type 1 diabetes results from the destruction of insulin-producing pancreatic beta cells by a beta cell-specific autoimmune process. Beta cell autoantigens, macrophages, dendritic cells, B lymphocytes, and T lymphocytes have been shown to be involved in the pathogenesis of autoimmune diabetes. Beta cell autoantigens are thought to be released from beta cells by cellular turnover or damage and are processed and presented to T helper cells by antigen-presenting cells. Macrophages and dendritic cells are the first cell types to infiltrate the pancreatic islets. Naive CD4+ T cells that circulate in the blood and lymphoid organs, including the pancreatic lymph nodes, may recognize major histocompatibility complex and beta cell peptides presented by dendritic cells and macrophages in the islets. These CD4+ T cells can be activated by interleukin (IL)-12 released from macrophages and dendritic cells. While this process takes place, beta cell antigen-specific CD8+ T cells are activated by IL-2 produced by the activated TH1 CD4+ T cells, differentiate into cytotoxic T cells and are recruited into the pancreatic islets. These activated TH1 CD4+ T cells and CD8+ cytotoxic T cells are involved in the destruction of beta cells. In addition, beta cells can also be damaged by granzymes and perforin released from CD8+ cytotoxic T cells and by soluble mediators such as cytokines and reactive oxygen molecules released from activated macrophages in the islets. Thus, activated macrophages, TH1 CD4+ T cells, and beta cell-cytotoxic CD8+ T cells act synergistically to destroy beta cells, resulting in autoimmune type 1 diabetes.

Depression of glucose levels and partial restoration of pancreatic beta-cell damage by melatonin in streptozotocin-induced diabetic rats

Diabetes mellitus is a common but serious metabolic disorder associated with many functional and structural complications. Glucose metabolism is disturbed due to an absolute or relative insulin deficiency. The experiment was carried out to determine the effect of melatonin on blood glucose and insulin concentrations, and histopathology of pancreatic beta-cells in streptozotocin (STZ)-induced diabetic rats. The rats were randomly allocated into one of the four experimental groups: group A (control), group B (diabetic untreated), group C (diabetic treated with melatonin for 6 weeks) and group D (diabetic treated with melatonin for 8 weeks); each group contained ten animals. Diabetes was induced in B, C and D groups by a single intraperitoneal (i.p.) injection of STZ (50 mg/kg, freshly dissolved in 5 mmol/l citrate buffer, pH 4.5). The rats in melatonin-treated groups were subjected to the daily i.p injection of 10 mg kg(-1) of melatonin for 6 or 8 weeks starting the day after STZ injection. Control and diabetic untreated rats were injected with the same volume of isotonic NaCl as the melatonin treated groups. Almost all insulin-positive beta-cells were degranulated, degenerated or necrotic in the STZ-treated rats leading to decrease in insulin secretion and an increase in blood glucose concentration. Melatonin treatment caused a sharp decrease in the elevated serum glucose, a slight increase in the lowered serum insulin concentrations and small partial regeneration/proliferation of beta-cells of islets. It is concluded that the hypoglycemic action of melatonin could be partly due to small amelioration in the beta-cells of pancreatic islets causing a slight increase in insulin secretion, it is mostly due to the extrapancreatic actions of the melatonin.

Role of poly(ADP-ribose) polymerase-1 activation in the pathogenesis of diabetic complications: endothelial dysfunction, as a common underlying theme

Hyperglycemia-induced overproduction of superoxide by mitochondrial electron-transport chain triggers several pathways of injury involved in the pathogenesis of diabetic complications [protein kinase C (PKC), hexosamine and polyol pathway fluxes, advanced glycation end product (AGE) formation] by inhibiting glyceraldehyde- 3-phosphate dehydrogenase (GAPDH) activity. Increased oxidative and nitrosative stress activates the nuclear enzyme, poly(ADP-ribose) polymerase-1 (PARP). PARP activation, on the one hand, depletes its substrate, NAD+, slowing the rate of glycolysis, electron transport, and ATP formation. On the other hand, it inhibits GAPDH by poly(ADP-ribosy)lation. These processes result in acute endothelial dysfunction in diabetic blood vessels, which importantly contributes to the development of various diabetic complications. Accordingly, hyperglycemia-induced activation of PKC isoforms, hexosaminase pathway flux, and AGE formation is prevented by blocking PARP activity. Furthermore, inhibition of PARP protects against diabetic cardiovascular dysfunction in preclinical models. PARP activation is present in microvasculature of human diabetic subjects. The oxidative/nitrosative stress-PARP pathway leads to diabetes-induced endothelial dysfunction, which may be an important underlying mechanism for the pathogenesis of other diabetic complications (cardiomyopathy, nephropathy, neuropathy, and retinopathy). This review focuses on the role of PARP in diabetic complications and the unique therapeutic potential of PARP inhibition in the prevention or reversal of diabetic complications.