Insulin rescues retinal neurons from apoptosis by a phosphatidylinositol 3-kinase/Akt-mediated mechanism that reduces the activation of caspase-3

Mitochondrial dysfunction and reactive oxygen species in excitotoxicity and apoptosis: implications for the pathogenesis of neurodegenerative diseases

In recent years we have witnessed a major interest in the study of the role of mitochondria, not only as ATP producers through oxidative phosphorylation but also as regulators of intracellular Ca2+ homeostasis and endogenous producers of reactive oxygen species (ROS). Interestingly, the mitochondria have been also implicated as central executioners of cell death. Increased mitochondrial Ca2+ overload as a result of excitotoxicity has been associated with the generation of superoxide and may induce the release of proapoptotic mitochondrial proteins, proceeding through DNA fragmentation/condensation and culminating in cell demise by apoptosis and/or necrosis. In addition, these processes have been implicated in the pathogenesis of many neurodegenerative diseases, which share several features of cell death: selective brain areas undergo neurodegeneration, involving mitochondrial dysfunction (mitochondrial complexes are affected), loss of intracellular Ca2+ homeostasis, excitotoxicity, and the extracellular or intracellular accumulation of insoluble protein aggregates in the brain.

Insulin induces cobalt uptake in a subpopulation of rat cultured primary sensory neurons

Previous findings show that both the vanilloid receptor 1 and the insulin receptor are expressed on small primary sensory neurons. As insulin evokes activity in second messengers which could induce opening of the vanilloid receptor 1, we examined, by using the cobalt-uptake technique, whether or not insulin can activate cultured rat primary sensory neurons through activating the vanilloid receptor 1. Capsaicin (50, 100 and 500 nm) induced concentration-dependent labelling in primary sensory neurons. Preincubation of cells in insulin (10 micromoles) for 10 min followed by a 2-min wash did not produce significant change in the capsaicin-induced labelling. Coapplication of insulin (10 micromoles) with capsaicin, however, potentiated the 50 and 100 nm capsaicin-evoked staining. Insulin itself also produced cobalt labelling in a concentration-dependent manner. The size-frequency distributions of neurons showing capsaicin- or insulin-induced cobalt accumulation were similar. The insulin-induced cobalt labelling was significantly reduced by the tyrosine kinase inhibitor, tyrphostin AG1024, the vanilloid receptor 1 antagonists, ruthenium red and capsazepine, the protein kinase inhibitor, staurosporine and the phospholipase C inhibitor neomycin. Double immunostaining of cultured primary sensory neurons and sections from dorsal root ganglia revealed that about one-third of the cells coexpress the insulin receptor and vanilloid receptor 1. These findings suggest that insulin activates a subpopulation of primary sensory neurons, probably through phosphorylation- and/or phosphatidylinositol(4,5)biphosphate hydrolysis-evoked activation of the vanilloid receptor 1. Although the insulin-induced activation of vanilloid receptor 1 seems to be a short-lived effect in vitro, in vivo it might play a role in the development of burning pain sensation in hyperinsulinism.

Neuronal necrosis inhibition by insulin through protein kinase C activation

In the serum-free culture of rat embryonic neurons, most neurons rapidly died by necrosis, which was revealed by propidium iodide (PI)-positive staining as early as 3 h after the start of culture and by marked membrane disruption and mitochondrial swelling in transmission electron microscopic (TEM) analysis. However, neither nuclear condensation/fragmentation stained with Hoechst 33342 nor activated caspase-3-like immunoreactivity was observed. In the serum-deprived culture, on the other hand, neurons showed apoptotic features, such as caspase-3 activation and nuclear damages in TEM analysis. Insulin at relatively higher concentrations, up to 100 microg/ml, ameliorated the rapid decrease in survival activity measured with 2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt WST-8 assay and PI staining in the serum-free culture, despite the fact that brain-derived neurotrophic factor and insulin-like growth factor-I had no survival effect even at concentrations up to 100 microg/ml. Insulin-induced survival effects were abolished by the protein kinase C (PKC) inhibitor calphostin C but not by the phosphatidyl inositol-3-OH-kinase inhibitor wortmannin or the mitogen-activated protein kinase inhibitors PD98059 or U0126. Insulin significantly stimulated the PKC activity in cell lysates and suppressed the mitochondrial swelling and membrane disruption in TEM analysis in a calphostin C-reversible manner. All of these findings suggest that insulin inhibited the neuronal necrosis resistant to known neurotrophic factors under the serum-free culture through PKC mechanisms.

Characterization of insulin signaling in rat retina in vivo and ex vivo

Insulin receptor (IR) signaling cascades have been studied in many tissues, but retinal insulin action has received little attention. Retinal IR signaling and activity were investigated in vivo in rats that were freely fed, fasted, or injected with insulin by phosphotyrosine immunoblotting and by measuring kinase activity. A retina explant system was utilized to investigate the IR signaling cascade, and immunohistochemistry was used to determine which retinal cell layers respond to insulin. Basal IR activity in the retina was equivalent to that in brain and significantly greater than that of liver, and it remained constant between freely fed and fasted rats. Furthermore, IR signaling increased in the retina after portal vein administration of supraphysiological doses of insulin. Ex vivo retinas responded to 10 nM insulin with IR beta-subunit (IRbeta) and IR substrate-2 (IRS-2) tyrosine phosphorylation and AktSer473 phosphorylation. The retina expresses mRNA for all three Akt isoforms as determined by in situ hybridization, and insulin specifically increases Akt-1 kinase activity. Phospho-AktSer473 immunoreactivity increases in retinal nuclear cell layers with insulin treatment. These results demonstrate that the retinal IR signaling cascade to Akt-1 possesses constitutive activity, and that exogenous insulin further stimulates this prosurvival pathway. These findings may have implications in understanding normal and dysfunctional retinal physiology.

Alterations in the expression of MHC class I glycoproteins by B16BL6 melanoma cells modulate insulin receptor-regulated signal transduction and augment [correction of augments] resistance to apoptosis

In a variety of malignancies, the immune-escape phenotype is associated, in part, with the inability of tumor cells to properly present their Ags to CTLs due to a deranged expression of MHC class I glycoproteins. However, these molecules were found to possess broader nonimmune functions, including participation in signal transduction and regulation of proliferation, differentiation, and sensitivity to apoptosis-inducing factors; processes, which are characteristically impaired during malignant transformation. We investigated whether the deranged expression of MHC class I expression by tumor cells could affect proper receptor-mediated signal transduction and accentuate their malignant phenotype. The malignant and H-2K murine MHC class I-deficient B16BL6 melanoma cells were characterized by an attenuated capacity to bind insulin due to the retention of corresponding receptor in intracellular stores. The restoration of H-2K expression in these cells, which abrogated their capacity to form tumors in mice, enhanced membrane translocation of the receptor, presumably, by modulating its glycosylation. The addition of insulin to H-2K-expressing melanoma cells cultured in serum-free conditions precluded apoptotic death by up-regulating the activity of protein kinase B (PKB)/Akt. In contrast, the deficiency for H-2K characteristic to the malignant clones was associated with a constitutive high activity of PKB/Akt, which rendered them resistant to apoptosis, induced by deprivation of serum-derived growth factors. The possibility to correct the regulation of PKB/Akt activity by restoration of H-2K expression in B16BL6 melanoma cells may be considered as an attractive approach for cancer therapy, since an aberrant activation of this enzyme is characteristic to resistant malignancies.

A new view of diabetic retinopathy: a neurodegenerative disease of the eye

Diabetic retinopathy (DR) is a common complication of diabetes and a leading cause of legal blindness in working-age adults. The clinical hallmarks of DR include increased vascular permeability, leading to edema, and endothelial cell proliferation. Much of the research effort has been focused on vascular changes, but it is becoming apparent that other degenerative changes occur beyond the vascular cells of the retina. These include increased apoptosis, glial cell reactivity, microglial activation, and altered glutamate metabolism. When occurring together, these changes may be considered as neurodegenerative and could explain some of the functional deficits in vision that begin soon after the onset of diabetes. This review will present the current evidence that neurodegeneration of the retina is a critical component of DR. There are two basic hypotheses that account for loss of cells in the neural retina. First, the loss of blood-retinal barrier integrity, which initially manifests as an increase in vascular permeability, causes a failure to control the composition of the extracellular fluid in the retina, which in turn leads to edema and neuronal cell loss. Alternatively, diabetes has a direct effect on metabolism within the neural retina, leading to an increase in apoptosis, which in turn causes breakdown of the blood-retinal barrier. It is not clear which hypothesis will be found to be correct, and, in fact, it is likely that vascular permeability and neuronal apoptosis are closely linked components of DR. However, the gradual loss of neurons suggests that progress of the disease is ultimately irreversible, since these cells cannot usually be replaced. In light of this possibility, new treatments for DR should be preventive in nature, being implemented before overt clinical symptoms develop. While vascular permeability is the target that is primarily considered for new treatments of DR, evidence presented here suggests that apoptosis of neurons is also an essential target for pharmacological studies. The vision of people with diabetes will be protected only when we have discovered a means to prevent the gradual but constant loss of neurons within the inner retina.

Diabetic retinopathy: more than meets the eye

Retinal microvascular dysfunction in diabetes is a major component of diabetic retinopathy. This review highlights recent observations regarding the cellular anatomy that contributes to the blood-retinal barrier and its breakdown, the alterations of macroglial, neuronal, and microglial cells in diabetes, and how these changes lead to loss of vision. In addition, the effects of systemic pathophysiologic influences, including metabolic control, blood pressure, and fluid volume on the formation of diabetic macular edema are discussed. Finally, an overview of inflammatory mechanisms and responses in the retina in diabetes is provided. Together, these new observations provide a broader clinical and research perspective on diabetic retinal vascular dysfunction than previously considered, and provide new avenues for improved treatments to prevent loss of vision.

In IGF-I receptor-deficient leiomyosarcoma cells autocrine IGF-II induces cell invasion and protection from apoptosis via the insulin receptor isoform A

One of the two isoforms of the human insulin receptor (isoform A or IR-A) binds IGF-II with high affinity and is predominantly expressed in fetal tissues and malignant cells. We evaluated the biological relevance of IR-A in human myosarcoma cells. Six myosarcoma cell lines were studied. All produced high amounts of IGF-II and five of them predominantly expressed IR-A. SKUT-1 leiomyosarcoma cells, that do not express the IGF-IR, were identified as a suitable model to study the effects of IR-A in the absence of the interference of IGF-IR. In these cells, which express high levels of IR with an IR-A relative abundance of approximately 95%, IGF-II elicits biological effects exclusively via IR-A activation and IGF-I is almost ineffective. Blockade of autocrine IGF-II reduced unstimulated cell viability and migration. Although both insulin and IGF-II activate IR-A, these two ligands showed a different ability to activate different intracellular signaling pathways and to elicit different biological effects. Insulin was more potent than IGF-II in activating the PI3-K/Akt pathway and in protecting cells from apoptosis. In contrast, IGF-II was more potent than insulin in activating the Shc/ERK pathway and in stimulating cell migration. These data indicate that IGF-II sensitive IR-A is the predominant IR isoform in a variety of myosarcoma cells. In SKUT-1 leiomyoma cells this fetal IR isoform may vicariate the IGF-IR for cell response to both insulin and IGF-II. Acting on the same IR-A receptor IGF-II is more potent than insulin in stimulating cancer cell migration.

Insulin-like growth factor-I blocks Bcl-2 interacting mediator of cell death (Bim) induction and intrinsic death signaling in cerebellar granule neurons

Cerebellar granule neurons depend on insulin-like growth factor-I (IGF-I) for their survival. However, the mechanism underlying the neuroprotective effects of IGF-I is presently unclear. Here we show that IGF-I protects granule neurons by suppressing key elements of the intrinsic (mitochondrial) death pathway. IGF-I blocked activation of the executioner caspase-3 and the intrinsic initiator caspase-9 in primary cerebellar granule neurons deprived of serum and depolarizing potassium. IGF-I inhibited cytochrome c release from mitochondria and prevented its redistribution to neuronal processes. The effects of IGF-I on cytochrome c release were not mediated by blockade of the mitochondrial permeability transition pore, because IGF-I failed to inhibit mitochondrial swelling or depolarization. In contrast, IGF-I blocked induction of the BH3-only Bcl-2 family member, Bim (Bcl-2 interacting mediator of cell death), a mediator of Bax-dependent cytochrome c release. The suppression of Bim expression by IGF-I did not involve inhibition of the c-Jun transcription factor. Instead, IGF-I prevented activation of the forkhead family member, FKHRL1, another transcriptional regulator of Bim. Finally, adenoviral-mediated expression of dominant-negative AKT activated FKHRL1 and induced expression of Bim. These data suggest that IGF-I signaling via AKT promotes survival of cerebellar granule neurons by blocking the FKHRL1-dependent transcription of Bim, a principal effector of the intrinsic death-signaling cascade.

In vivo regulation of phosphoinositide 3-kinase in retina through light-induced tyrosine phosphorylation of the insulin receptor beta-subunit

Recently, we have shown that phosphoinositide 3-kinase (PI3K) in bovine rod outer segment (ROS) is activated in vitro by tyrosine phosphorylation of the C-terminal tail of the insulin receptor (Rajala, R. V. S., and Anderson, R. E. (2001) Invest. Ophthal. Vis. Sci. 42, 3110-3117). In this study, we have investigated the in vivo mechanism of PI3K activation in the rodent retina and report the novel finding that light stimulates tyrosine phosphorylation of the beta-subunit of the insulin receptor (IRbeta) in ROS membranes, which leads to the association of PI3K enzyme activity with IRbeta. Retinas from light- or dark-adapted mice and rats were homogenized and immunoprecipitated with antibodies against phosphotyrosine, IRbeta, or the p85 regulatory subunit of PI3K, and PI3K activity was measured using PI-4,5-P(2) as substrate. We observed a light-dependent increase in tyrosine phosphorylation of IRbeta and an increase in PI3K enzyme activity in isolated ROS and in anti-phosphotyrosine and anti-IRbeta immunoprecipitates of retinal homogenates. The light effect was localized to photoreceptor neurons and is independent of insulin secretion. Our results suggest that light induces tyrosine phosphorylation of IRbeta in outer segment membranes, which leads to the binding of p85 through its N-terminal Src homology 2 domain and the generation of PI-3,4,5-P(3). We suggest that the physiological role of this process may be to provide neuroprotection of the retina against light damage by activating proteins that protect against stress-induced apoptosis.

Glucose promotes pancreatic islet beta-cell survival through a PI 3-kinase/Akt-signaling pathway

The concentration of glucose in plasma is an important determinant of pancreatic beta-cell mass, whereas the relative contributions of hypertrophy, proliferation, and cell survival to this process are unclear. Glucose results in depolarization and subsequent calcium influx into islet beta-cells. Because depolarization and calcium (Ca(2+)) influx promote survival of neuronal cells, we hypothesized that glucose might alter survival of islet beta-cells through a similar mechanism. In the present studies, cultured mouse islet beta-cells showed a threefold decrease in apoptosis under conditions of 15 mM glucose compared with 2 mM glucose (P < 0.05). MIN6 insulinoma cells incubated in 25 mM glucose for 24 h showed a threefold decrease in apoptosis compared with cells in 5 mM glucose (1.7 +/- 0.2 vs. 6.3 +/- 1%, respectively, P < 0.001). High glucose (25 mM) enhanced survival-required depolarization and Ca(2+) influx and was blocked by phosphatidylinositol (PI) 3-kinase inhibitors. Glucose activation of the protein kinase Akt was demonstrated in both insulinoma cells and cultured mouse islets by means of an antibody specific for Ser(473) phospho-Akt and by an in vitro Akt kinase assay. Akt phosphorylation was dependent on PI 3-kinase but not on MAPK. Transfection of insulinoma cells with an Akt kinase-dead plasmid (Akt-K179M) resulted in loss of glucose-mediated protection, whereas transfection with a constitutively active Akt enhanced survival in glucose-deprived insulinoma cells. The results of these studies defined a novel pathway for glucose-mediated activation of a PI 3-kinase/Akt survival-signaling pathway in islet beta-cells. This pathway may provide important targets for therapeutic intervention.

Tumor necrosis factor-alpha-induced accumulation of tumor suppressor protein p53 and cyclin-dependent protein kinase inhibitory protein p21 is inhibited by insulin in ME-180S cells

The tumor suppressor protein p53 plays an important role in the protection against the development of cancer and is inactivated in many human malignancies. Since p53 is an important inhibitor of cell growth, keeping p53 function under control is critical for survival of cell. One of the principal mechanisms by which cells achieve this is by regulating the p53 protein level, although its phosphorylation and cellular localization also contribute to the regulation of its function. Since many tumors secrete growth factor(s) that inhibit apoptosis and support the growth of cancer cells, we wanted to know whether insulin would have an effect on antitumor and p53-inducing activities of human tumor necrosis factor-alpha (TNF-alpha). Here we show that treatment of human cervical carcinoma cell line, ME-180S, with TNF-alpha results in time-dependent accumulation of p53 and its transcriptional target, p21. However, pretreatment of these cells with insulin inhibits TNF-alpha-dependent cell killing, induction of p53, p21 and apoptosis.

A Role for phosphoinositide 3-kinase in the control of cell division and survival during retinal development

Neurogenesis in the retina requires the concerted action of three different cellular processes: proliferation, differentiation, and apoptosis. Class IA phosphoinositide 3-kinase (PI3K) is a heterodimer composed of a p85 regulatory and a p110 catalytic subunit. p110alpha has been shown to regulate cell division and survival. Little is known of its function in development, however, as p110alpha knockout mice exhibit CNS defects, but death at early embryonic stages impairs further study. Here, we examine the role of PI3K in mouse retina development by expressing an activating form of PI3K regulatory subunit, p65(PI3K), as a transgene in the retina. Mice expressing p65(PI3K) showed severely disrupted retina morphogenesis, with ectopic cell masses in the neuroepithelium that evolved into infoldings of adult retinal cell layers. These changes correlated with an altered cell proliferation/cell death balance at early developmental stages. Nonetheless, the most affected cell layer in adult retina was that of photoreceptors, which correlated with selectively increased survival of these cells at developmental stages at which cell division has ceased. These results demonstrate the relevance of accurate PI3K regulation for normal retinal development, supporting class IA PI3K involvement in induction of cell division at early stages of neurogenesis. These data also show that, even after cell division decline, PI3K activation mediates survival of differentiated neurons in vivo.

Functions of PI 3-kinase in development of the nervous system

In the nervous system, receptor regulated phosphoinositide (PI) 3-kinases (PI 3-kinases) participate in fundamental cellular activities that underlie development. Activated by trophic factors, growth factors, neuregulins, cytokines, or neurotransmitters, PI 3-kinases have been implicated in neuronal and glial survival and differentiation. PI 3-kinases produce inositol lipid second messengers that bind to pleckstrin homology (PH) domains in diverse groups of signal transduction proteins, and control their enzymatic activities, subcellular membrane localization, or both. Downstream targets of the inositol lipid messengers include protein kinases and regulators of small GTPases. The kinase Akt/PKB functions as a key component of the PI 3-kinase dependent survival pathway through its phosphorylation and regulation of apoptotic proteins and transcription factors. Furthermore, since members of the Rho GTPase and Arf GTPase families have been implicated in regulation of the actin cytoskeleton, vesicular trafficking, and transcription, the downstream targets of PI 3-kinase that control these GTPases are excellent candidates to mediate aspects of PI 3-kinase dependent neuronal and glial differentiation.

Glycogen synthase kinase 3: an emerging therapeutic target

Glycogen synthase kinase 3 (GSK-3) is a serine/threonine protein kinase that has recently emerged as a key target in drug discovery. It has been implicated in multiple cellular processes and linked with the pathogenesis of several diseases. GSK-3 inhibitors might prove useful as therapeutic compounds in the treatment of conditions associated with elevated levels of enzyme activity, such as type 2 diabetes and Alzheimer's disease. The pro-apoptotic feature of GSK-3 activity suggests a potential role for its inhibitors in protection against neuronal cell death, and in the treatment of traumatic head injury and stroke. Finally, selective inhibitors of GSK-3 could mimic the action of mood stabilizers such as lithium and valproic acid and be used in the treatment of bipolar mood disorders.

Excessive hexosamines block the neuroprotective effect of insulin and induce apoptosis in retinal neurons

In addition to microvascular abnormalities, neuronal apoptosis occurs early in diabetic retinopathy, but the mechanism is unknown. Insulin may act as a neurotrophic factor in the retina via the phosphoinositide 3-kinase/Akt pathway. Excessive glucose flux through the hexosamine biosynthetic pathway (HBP) is implicated in the development of insulin resistance in peripheral tissues and diabetic complications such as nephropathy. We tested whether increased glucose flux through the HBP perturbs insulin action and induces apoptosis in retinal neuronal cells. Exposure of R28 cells, a model of retinal neurons, to 20 mm glucose for 24 h attenuated the ability of 10 nm insulin to rescue them from serum deprivation-induced apoptosis and to phosphorylate Akt compared with 5 mm glucose. Glucosamine not only impaired the neuroprotective effect of insulin but also induced apoptosis in R28 cells in a dose-dependent fashion. UDP-N-acetylhexosamines (UDP-HexNAc), end products of the HBP, were increased approximately 2- and 15-fold after a 24-h incubation in 20 mm glucose and 1.5 mm glucosamine, respectively. Azaserine, a glutamine:fructose-6-phosphate amidotransferase inhibitor, reversed the effect of 20 mm glucose, but not that of 1.5 mm glucosamine, on attenuation of the ability of insulin to promote cell survival and phosphorylate Akt as well as accumulation of UDP-HexNAc. Glucosamine also impaired insulin receptor processing in a dose-dependent manner but did not decrease ATP content. By contrast, in L6 muscle cells, glucosamine impaired insulin receptor processing but did not induce apoptosis. These results suggest that the excessive glucose flux through the HBP may direct retinal neurons to undergo apoptosis in a bimodal fashion; i.e. via perturbation of the neuroprotective effect of insulin mediated by Akt and via induction of apoptosis possibly by altered glycosylation of proteins. The HBP may be involved in retinal neurodegeneration in diabetes.