Malignant melanoma is the skin cancer with the most significant impact on man, carrying the highest risk of death from metastasis. Both incidence and mortality rates continue to rise each year, with no effective long-term treatment on the horizon. In part, this reflects lack of identification of critical genes involved and specific therapies targeted to correct these defects. We report that selective activation of the Akt3

Diabetic retinopathy remains a frightening prospect to patients and frustrates physicians. Destruction of damaged retina by photocoagulation remains the primary treatment nearly 50 years after its introduction. The diabetes pandemic requires new approaches to understand the pathophysiology and improve the detection, prevention, and treatment of retinopathy. This perspective considers how the unique anatomy and physiology of the retina may predispose it to the metabolic stresses of diabetes. The roles of neural retinal alterations and impaired retinal insulin action in the pathogenesis of early retinopathy and the mechanisms of vision loss are emphasized. Potential means to overcome limitations of current animal models and diagnostic testing are also presented with the goal of accelerating therapies to manage retinopathy in the face of ongoing diabetes. Diabetes 55:2401-2411, 2006 D espite years of clinical and laboratory investigation, diabetic retinopathy remains the leading cause of vision impairment and blindness among working-age adults, yet the fundamental cause(s) remains uncertain. Retinal photocoagulation to reduce neovascularization and macular edema was developed in the 1950s and is still the standard of care (1). The number of people worldwide at risk of developing vision loss from diabetes is predicted to double over the next 30 years (2), so it is imperative to develop better means to identify, prevent, and treat retinopathy in its earliest stages rather than wait for the onset of vision-threatening lesions. Progress in these areas requires a new perspective on the problem that includes the roles of the neural retina, impaired insulin action, and inflammation. In this way, established neurobiological principles can inform us how diabetes impairs vision, and knowledge of metabolism, inflammation, and regenerative medicine may lead to new treatments.This perspective will discuss how the unique anatomy and physiology of the retina may render it vulnerable to the metabolic derangements of diabetes and lead to impaired vision. The intent of this unconventional approach is to encourage consideration of new opportunities for investigations that will advance the field. NORMAL RETINAL STRUCTURE AND PHYSIOLOGY Topographic and cellular organization of the retina.It is instructive to consider the functional organization of the retina (literally a network) to better understand the impact of diabetes (http://webvision.med.utah.edu). The retina is a transparent layer of neural tissue between the retinal pigmented epithelium and the vitreous body. Normal vision depends on intact cell-cell communication among the neuronal, glial, microglial, vascular, and pigmented epithelial cells of the retina. The fundamental functions of the retina are to capture photons, convert the photochemical energy into electrical energy, integrate the resulting action potentials, and transmit them to the occipital lobe of the brain, where they are deciphered and interpreted into recognizable images. The retina is partitioned from the syst...

Diabetic retinopathy is characterized by early onset of neuronal cell death. We previously showed that insulin mediates a prosurvival pathway in retinal neurons and that normal retina expresses a highly active basal insulin receptor/Akt signaling pathway that is stable throughout feeding and fasting. Using the streptozotocin-induced diabetic rat model, we tested the hypothesis that diabetes diminishes basal retinal insulin receptor signaling concomitantly with increased diabetes-induced retinal apoptosis. The expression, phosphorylation status, and/or kinase activity of the insulin receptor and downstream signaling proteins were investigated in retinas of age-matched control, diabetic, and insulin-treated diabetic rats. Four weeks of diabetes reduced basal insulin receptor kinase, insulin receptor substrate (IRS)-1/2-associated phosphatidylinositol 3-kinase, and Akt kinase activity without altering insulin receptor or IRS-1/2 expression or tyrosine phosphorylation. After 12 weeks of diabetes, constitutive insulin receptor autophosphorylation and IRS-2 expression were reduced, without changes in p42/p44 mitogen-activated protein kinase or IRS-1. Sustained systemic insulin treatment of diabetic rats prevented loss of insulin receptor and Akt kinase activity, and acute intravitreal insulin administration restored insulin receptor kinase activity. Insulin treatment restored insulin receptor-␤ autophosphorylation in rat retinas maintained ex vivo, demonstrating functional receptors and suggesting loss of ligand as a cause for reduced retinal insulin receptor/Akt pathway activity. These results demonstrate that diabetes progressively impairs the constitutive retinal insulin receptor signaling pathway through Akt and suggests that loss of this survival pathway may contribute to the initial stages of diabetic retinopathy.

We recently demonstrated that ceramide-coated balloon catheters limit vascular smooth muscle cell (VSMC) growth after stretch injury in vivo. In that study, inhibition of VSMC growth was correlated with a decrease in phosphorylation of the cell survival kinase Akt (protein kinase B). Utilizing cultured A7r5 VSMCs, we have now examined the mechanism by which ceramide inhibits Akt phosphorylation/activation. Our initial studies showed that ceramide-induced inhibition of Akt phosphorylation was not mediated through diminution in phosphoinositide 3-kinase activity. As we have previously demonstrated that protein kinase C (PKC) is a target of ceramide, we proposed an alternative signaling mechanism by which ceramide induces inhibition of Akt through activation of PKC. We demonstrate that C 6 -ceramide (but not the inactive analog dihydro-C 6 -ceramide) induced PKC activity and also caused a selective increase in the association between Akt and PKC, without affecting PKC⑀, in A7r5 cells. In addition, the ability of ceramide to significantly decrease plateletderived growth factor-induced Akt phosphorylation or cell proliferation was abrogated in A7r5 cells overexpressing a dominant-negative mutant of PKC. Taken together, these data suggest that ceramide-mediated activation of PKC leads to diminished Akt activation and consequent growth arrest in VSMCs. The therapeutic potential for ceramide to limit dysregulated VSMC growth has direct applicability to vascular diseases such as restenosis and atherosclerosis.

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 β-subunit (IRβ) 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.

Abstract-Neointimal hyperplasia at the site of surgical intervention is a common and deleterious complication of surgery for cardiovascular diseases. We hypothesized that direct delivery of a cell-permeable growth-arresting lipid via the balloon tip of an embolectomy catheter would limit neointimal hyperplasia after stretch injury. We have previously demonstrated that sphingolipid-derived ceramide arrested the growth of smooth muscle cell pericytes in vitro. Here, we show that ceramide-coated balloon catheters significantly reduced neointimal hyperplasia induced by balloon angioplasty in rabbit carotid arteries in vivo. This ceramide treatment decreased the number of vascular smooth muscle cells entering the cell cycle without inducing apoptosis. In situ autoradiographic studies demonstrated that inflating the balloon catheter forced cell-permeable ceramide into the intimal and medial layers of the artery. Intercalation of ceramide into vascular smooth muscle cells correlated with rapid inhibition of trauma-associated phosphorylation of extracellular signal-regulated kinase and protein kinase B. These studies demonstrate the utility of cell-permeable ceramide as a novel therapy for reducing neointimal hyperplasia after balloon angioplasty. (Circ Res. 2000;87:282-288.)

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In addition to the well-documented role of nitric oxide (NO) as a vasodilator, NO has also been implicated in vascular smooth muscle cell (VSMC) growth arrest. Signaling mechanisms responsible for growth factor receptor-mediated VSMC proliferation include the extracellular signal-regulated kinase (ERK) and possibly the protein kinase B (PKB) cascade. Thus the present study was designed to test the hypothesis that, in A7r5 vascular smooth muscle-derived cells, platelet-derived growth factor (PDGF)-induced activation of either ERK or PKB is regulated by NO, which then modulates cellular proliferation and/or apoptosis. PKB-alpha was the predominant isoform of PKB expressed in A7r5 cells and was also expressed in rabbit carotid arteries and aortae. Phosphorylation of PKB-alpha and ERK induced by PDGF-BB was maximal within 5-15 min in A7r5 cells. Preincubation of A7r5 cells with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) resulted in a biphasic regulation of PDGF-stimulated PKB-alpha phosphorylation and bioactivity. Acute exposure to SNAP significantly augmented PDGF-induced activation of PKB-alpha, whereas prolonged incubation led to a marked diminution in PDGF-induced activation of PKB-alpha. In contrast, SNAP did not affect PDGF-induced activation of ERK at any time point. The cGMP-independent effects of SNAP on PDGF-induced activation of PKB-alpha were established with the use of an inhibitor of soluble guanylyl cyclase, ODQ, as well as a cell-permeable analog of cGMP, 8-bromo-cGMP. Prolonged treatment of A7r5 cells with SNAP led to a significant decrease in DNA synthesis without an appreciable increase in apoptosis. These data suggest that, after prolonged exposure to SNAP, NO selectively attenuates PDGF-induced increase in PKB-alpha activation, which in turn may contribute to diminished VSMC proliferation by mechanisms involving growth arrest but not apoptosis.