Functional alterations of G-proteins in diabetic rat retina: a possible explanation for the early visual abnormalities in diabetes mellitus

Towards a New Biomarker for Diabetic Retinopathy: Exploring RBP3 Structure and Retinoids Binding for Functional Imaging of Eyes In Vivo

Diabetic retinopathy (DR) is a severe disease with a growing number of afflicted patients, which places a heavy burden on society, both socially and financially. While there are treatments available, they are not always effective and are usually administered when the disease is already at a developed stage with visible clinical manifestation. However, homeostasis at a molecular level is disrupted before visible signs of the disease are evident. Thus, there has been a constant search for effective biomarkers that could signal the onset of DR. There is evidence that early detection and prompt disease control are effective in preventing or slowing DR progression. Here, we review some of the molecular changes that occur before clinical manifestations are observable. As a possible new biomarker, we focus on retinol binding protein 3 (RBP3). We argue that it displays unique features that make it a very good biomarker for non-invasive, early-stage DR detection. Linking chemistry to biological function and focusing on new developments in eye imaging and two-photon technology, we describe a new potential diagnostic tool that would allow rapid and effective quantification of RBP3 in the retina. Moreover, this tool would also be useful in the future to monitor therapeutic effectiveness if levels of RBP3 are elevated by DR treatments.

Preventing diabetic retinopathy by mitigating subretinal space oxidative stress in vivo

Patients with diabetes continue to suffer from impaired visual performance before the appearance of overt damage to the retinal microvasculature and later sight-threatening complications. This diabetic retinopathy (DR) has long been thought to start with endothelial cell oxidative stress. Yet newer data surprisingly finds that the avascular outer retina is the primary site of oxidative stress before microvascular histopathology in experimental DR. Importantly, correcting this early oxidative stress is sufficient to restore vision and mitigate the histopathology in diabetic models. However, translating these promising results into the clinic has been stymied by an absence of methods that can measure and optimize anti-oxidant treatment efficacy in vivo. Here, we review imaging approaches that address this problem. In particular, diabetes-induced oxidative stress impairs dark-light regulation of subretinal space hydration, which regulates the distribution of interphotoreceptor binding protein (IRBP). IRBP is a vision-critical, anti-oxidant, lipid transporter, and pro-survival factor. We show how optical coherence tomography can measure subretinal space oxidative stress thus setting the stage for personalizing anti-oxidant treatment and prevention of impactful declines and loss of vision in patients with diabetes.

Transducin1, Phototransduction and the Development of Early Diabetic Retinopathy

Purpose

Recent evidence suggests that retinal photoreceptor cells have an important role in the pathogenesis of retinal microvascular lesions in diabetes. We investigated the role of rod cell phototransduction on the pathogenesis of early diabetic retinopathy (DR) using Gnat1-/- mice (which causes permanent inhibition of phototransduction in rod cells without degeneration).

Methods

Retinal thickness, oxidative stress, expression of inflammatory proteins, electroretinograms (ERG) and optokinetic responses, and capillary permeability and degeneration were evaluated at up to 8 months of diabetes.

Results

The diabetes-induced degeneration of retinal capillaries was significantly inhibited in the Gnat1-/- diabetics. The effect of the Gnat1 deletion on the diabetes-induced increase in permeability showed a nonuniform accumulation of albumin in the neural retina; the defect was inhibited in diabetic Gnat1-/- mice in the inner plexiform layer (IPL), but neither in the outer plexiform (OPL) nor inner nuclear (INL) layers. In Gnat1-deficient animals, the diabetes-induced increase in expression of inflammatory associated proteins (iNOS and ICAM-1, and phosphorylation of IĸB) in the retina, and the leukocyte mediated killing of retinal endothelial cells were inhibited, however the diabetes-mediated induction of oxidative stress was not inhibited.

Conclusions

In conclusion, deletion of transducin1 (and the resulting inhibition of phototransduction in rod cells) inhibits the development of retinal vascular pathology in early DR.

Differential effect of a carotenoid-rich diet on retina function in non-diabetic and diabetic rats

Objective: This study was designed to examine the supplementation of a carotenoid-rich carrot powder, on retina function and carotenoid metabolism in non-diabetic control and type 1 diabetic animals. Methods: Male Wistar rats (n = 30) were randomly assigned to diets supplemented with (n = 15) or without (n = 15) carrot powder enriched diets (150 g/kg diet). After 3 weeks of diet adaptation, 8 rats in each group were treated with streptozotocin (iv) to induce type 1 diabetes and fed for a further 9 wk. Retinal function was assessed with the electroretinogram (ERG). Hepatic and plasma retinoids and carotenoids were measured by ultra-performance liquid chromatography. Results: Non-diabetic control rats fed the carrot diet had significantly (p < 0.02) higher rod- and cone- driven post-synaptic b-wave amplitudes, respectively, compared to those fed the control diet. These functional changes correlated with higher (p < 0.05) liver levels of carotenoids (α- and β- carotene) and retinoids. In diabetic rats, carrot diet exacerbated retina dysfunction; the amplitudes for most of rod- and cone-driven ERG components were the lowest amplitudes among all groups (p < 0.02). Diabetic rats fed the carrot diet had lower hepatic retinol and retinyl palmitate, while having higher α- and β-carotene levels, indicating diminished hepatic conversion of carotenoids into retinoids. Discussion: Dietary supplementation of high dose dietary carotenoids plays a beneficial role on healthy rat retina function, but exerts a detrimental effect in diabetes, which warrants undertaking detailed mechanistic studies.

Molecular Mechanisms Mediating Diabetic Retinal Neurodegeneration: Potential Research Avenues and Therapeutic Targets

Diabetic retinopathy (DR) is a devastating complication of diabetes with a prevalence rate of 35%, and no effective treatment options. Since the most visible clinical features of DR are microvascular irregularities, therapeutic interventions often attempt to reduce microvascular injury, but only after permanent retinal damage has ensued. However, recent data suggests that diabetes initially affects retinal neurons, leading to neurodegeneration as an early occurrence in DR, before onset of the more noticeable vascular abnormalities. In this review, we delineate the sequence of initiating events leading to retinal degeneration in DR, considering neuronal dysfunction as a primary event. Key molecular mechanisms and potential biomarkers associated with retinal neuronal degeneration in diabetes are discussed. In addition to glial reactivity and inflammation in the diabetic retina, the contribution of neurotrophic factors, cell adhesion molecules, apoptosis markers, and G protein signaling to neurodegenerative pathways warrants further investigation. These studies could complement recent developments in innovative treatment strategies for diabetic retinopathy, such as targeting retinal neuroprotection, promoting neuronal regeneration, and attempts to re-program other retinal cell types into functional neurons. Indeed, several ongoing clinical trials are currently attempting treatment of retinal neurodegeneration by means of such novel therapeutic avenues. The aim of this article is to highlight the crucial role of neurodegeneration in early retinopathy progression, and to review the molecular basis of neuronal dysfunction as a first step toward developing early therapeutic interventions that can prevent permanent retinal damage in diabetes. ClinicalTrials.gov: NCT02471651, NCT01492400.

Nucleoside diphosphate kinase B deficiency causes a diabetes-like vascular pathology via up-regulation of endothelial angiopoietin-2 in the retina

AIMS

Nucleoside diphosphate kinase B (NDPKB) is capable of maintaining the cellular nucleotide triphosphate pools. It might therefore supply UTP for the formation of UDP-GlcNAc from glucose. As NDPKB contributes to vascular dysfunction, we speculate that NDPKB might play a role in microangiopathies, such as diabetic retinopathy (DR). Therefore, we investigated the impact of NDPKB on retinal vascular damage using NDPKB(-/-) mice during development of DR and its possible mechanisms.

METHODS

Pericyte loss and acellular capillary (AC) formation were assessed in streptozotocin-induced diabetic NDPKB(-/-) and wild-type (WT) mice. Expression of angiopoietin-2 (Ang2) and protein N-acetylglucosamine modification (GlcNAcylation) were assessed by western blot and/or immunofluorescence in the diabetic retinas as well as in endothelial cells depleted of NDPKB by siRNA and stimulated with high glucose.

RESULTS

Similar to diabetic WT retinas, non-diabetic NDPKB(-/-) retinas showed a significant decrease in pericyte coverage in comparison with non-diabetic WT retinas. Hyperglycemia further aggravates pericyte loss in diabetic NDPKB(-/-) retinas. AC formation was detected in the diabetic NDPKB(-/-) retinas. Similar to hyperglycemia, NDPKB deficiency induced Ang2 expression and protein GlcNAcylation that were not further altered in the diabetic retinas. In cultured endothelial cells, stimulation with high glucose and NDPKB depletion comparably increased Ang2 expression and protein GlcNAcylation.

CONCLUSIONS

Our data identify NDPKB as a protective factor in the retina, which controls Ang2 expression and the hexosamine pathway. NDPKB-deficient mice are a suitable model for studying mechanisms underlying diabetic retinal vascular damage.

Photoreceptors in diabetic retinopathy

Although photoreceptors account for most of the mass and metabolic activity of the retina, their role in the pathogenesis of diabetic retinopathy has been largely overlooked. Recent studies suggest that photoreceptors might play a critical role in the diabetes-induced degeneration of retinal capillaries, and thus can no longer be ignored. The present review summarizes diabetes-induced alterations in photoreceptor structure and function, and provides a rationale for further study of a role of photoreceptors in the pathogenesis of the retinopathy.

Glial and neuronal dysfunction in streptozotocin-induced diabetic rats

Neuronal dysfunction has been noted very soon after the induction of diabetes by streptozotocin injection in rats. It is not clear from anatomical evidence whether glial cell dysfunction accompanies the well-documented neuronal deficit. Here, we isolate the Müller cell driven slow-P3 component of the full-field electroretinogram and show that it is attenuated at 4 weeks following the onset of streptozotocin-hyperglycaemia. We also found a concurrent reduction in the sensitivity of the phototransduction cascade, as well as in the components of the electroretinogram known to indicate retinal ganglion cell and amacrine cell integrity. Our data support the idea that neuronal and Müller cell dysfunction occurs at the same time in streptozotocin-induced hyperglycaemia.

Investigating structural and biochemical correlates of ganglion cell dysfunction in streptozotocin-induced diabetic rats

The aim of this study was to determine whether inner retinal dysfunction in diabetic rats is correlated with structural and/or biochemical changes in the retina and optic nerve. Using the electroretinogram (ERG; -5.83 to 1.28 log cd.s.m(-2)) retinal function (photoreceptor, bipolar, amacrine and ganglion cell components) was measured in control (n=13; citrate buffer) and diabetic (n=13; streptozotocin, STZ, 50 mg kg(-1)) rats, 12 weeks following treatment. Retinae and optic nerves were analyzed for structural changes and retinae were assessed for alterations in growth factor/cytokine expression using quantitative real-time PCR. We found that phototransduction efficiency was reduced 12 weeks after STZ-induced diabetes (-30%), leading to reduced amplitude of ON-bipolar (-18%) and amacrine cell (-29%) dominated responses; ganglion cell dysfunction (-84%) was more profound. In the optic nerve, nerve fascicle area and myelin sheath thickness were reduced (p<0.05), whereas the ratio of blood vessels and connective tissue to total nerve cross-sectional area was increased (p<0.05) in diabetic compared to control rats. In the retina, connective tissue growth factor (CTGF), transforming growth factor beta, type 2 receptor (TGFbeta-r2) mRNA and platelet-derived growth factor B (PDGF-B) mRNA were increased (p<0.035). Reduced ganglion cell function was correlated with increased CTGF and TGFbeta-r2, but not PDGF-B mRNA. In summary, the ganglion cell component exhibited the greatest level of dysfunction within the ERG components examined after 12 weeks of STZ-induced diabetes; the level correlated with increased CTGF and TGFbeta-r2 mRNA, but not with gross morphological changes in the retina or optic nerve.

Time-Dependent Course of Electroretinograms in the Spontaneous Diabetic Goto-Kakizaki Rat

PURPOSE

This study evaluates the relevance to human retinopathy of electroretinograms (ERGs) from the spontaneously diabetic Goto-Kakizaki (GK) rat.

METHODS

Starting from 4 weeks of age, we obtained ERGs every 4 weeks from six GK rats and seven Wistar (control) rats, and from two GK and two Wistar rats at 14 days of age. The a-wave, b-wave, and oscillatory potentials (OPs) were recorded after stimulation with a single bright flash. We compared the amplitudes and implicit times and measured a-wave latencies to evaluate photoreceptor function.

RESULTS

The amplitudes of the a-wave, b-wave, and OPs (O1 and O2) of the GK rats were reduced between 4 and 48 weeks of age. The time-dependent courses of change in a-wave, b-wave, and O2 amplitude did not differ between the two groups. The a-wave latencies in GK rats were significantly prolonged, but not the implicit times of OPs. At 14 days of age, the a-wave amplitudes were significantly smaller in GK than in Wistar rats.

CONCLUSION

Functional abnormalities of photoreceptors might be induced by inheritable degeneration at an early age in the GK rat. Although hyperglycemia would cause retinal hypoxia, it would not be severe enough to disturb the generation of OPs.

Changes in rhodopsin kinase and transducin in the rat retina in early-stage diabetes

To establish changes in phototransduction in diabetes, the effects of high glucose on rhodopsin kinase (RK) and transducin (G(t)), as well as recoverin, were examined in the retina of STZ-induced diabetic rats. Diabetes was induced by single intraperitoneal injection of STZ (50mg/kg) to Sprague-Dawley (SD) rats and the animals were sacrificed after 6 weeks. Immunohistochemistry (IHC) and Western blot analysis were carried out using antibodies against RK and G(talpha) (alpha subunit of G(t)) in the STZ-induced diabetic retina and the control retina. The expression level of recoverin protein was also analysed. In the diabetic retina, while the expression of RK protein increased, that of G(talpha) and recoverin proteins decreased. RK immunoreactivity (IR) appeared generally in the retina, and its signal increased in the outer limiting membrane (OLM), some rod cells in the outer segment layer (OSL) and at the tip of the outer plexiform layer (OPL) in the diabetic retina. G(talpha)-IR also appeared in the OPL and in photoreceptor layer. In the diabetic retina, G(talpha)-IR significantly decreased in the OPL, indicating RK-IR increase. This study illustrates the alterations in RK, G(talpha) and recoverin in the diabetic retina that may induce dysfunctions in phototransduction even in early-stage diabetes.

Streptozotocin-induced diabetes impairs G-protein linked signal transduction in vascular smooth muscle

The present studies were undertaken to examine if the impaired vascular function observed in diabetes is attributed to the altered levels of G-protein. Diabetes was induced in Sprague Dawley rats by a single intraperitoneal injection of streptozotocin (STZ) (60 mg/kg body wt) and after a period of 5 days, the aorta were used for adenylyl cyclase activity determination and protein quantification. A temporal relationship between the expression of Gialpha proteins and development of diabetes was also examined on day 1, 2, 3, 4 and 5 of injection of STZ. Blood glucose levels were significantly increased from day 1 in STZ-rats as compared to their counterpart control rats and reached to about 20 mM on 3rd day and 30 mM on 5th day. The expression of Gialpha-2 and Gialpha-3 proteins as determined by immunoblotting techniques was decreased by about 70 and 50% respectively in aorta from STZ rats compared to the control rats after 5 days of treatment, whereas 40% decrease in Gialpha-2 and Gialpha-3 was observed after 3rd day of STZ injection. On the other hand, the expression of Gsalpha was unaltered in STZ rats. In addition, the stimulatory effect of cholera toxin (CT) on GTP-mediated stimulation of adenylyl cyclase was not different in STZ as compared to the control group. However, the stimulatory effects of isoproterenol, glucagon, NaF and FSK on adenylyl cyclase activity were significantly enhanced in STZ rats as compared to control rats, whereas basal adenylyl cyclase activity was significantly lower in STZ-rats as compared to control rats. In addition, GTPgammaS inhibited FSK-stimulated adenylyl cyclase activity in concentration-dependent manner (receptor-independent functions of Gialpha) in control rats which was completely attenuated in STZ-rats. In addition, receptor-mediated inhibitions of adenylyl cyclase by angiotensin II, oxotremorine, atrial natriuretic peptide (ANP99-126) and C-ANP4-23 were also attenuated (receptor-dependent functions of Gialpha) in STZ-rats. These results indicate that aorta from diabetic rats exhibit decreased levels of cAMP and decreased expression of Gialpha. The decreased expression of Gialpha may be responsible for the altered responsiveness of adenylyl cyclase to hormonal stimulation and inhibition in STZ-rats. It may thus be suggested that the impaired adenylyl cyclase-Gialpha protein signaling may be one of the possible mechanisms responsible for the impaired vascular functions in diabetes.

alpha2D/A-adrenergic receptor gene induction in the retina by phorbol ester: involvement of an AP-2 element

BACKGROUND

The alpha2-adrenergic receptor (alpha2-AR) expressed in the bovine retina has been demonstrated to be of the alpha2D subtype. The bovine alpha2D-adrenergic receptor (alpha2D/A-AR) gene has been cloned and characterized. This report describes the induction of this gene by phorbol- 12, 13-myristate acetate (PMA), an activator of protein kinase C (PKC).

RESULTS

Treatment of the bovine retina for 60 min with PMA (1 micrometer) resulted in significant and similar increases in alpha2D/A-AR mRNA level and gene transcription. This indicates that PMA causes alpha2D/A-AR gene induction and that this induction takes place directly at the transcriptional level. In C6 cells, treatment with PMA at a concentration which was as low as 0.1 micrometer induced endogenous alpha2D/A-AR mRNA after 60 min. Luciferase reporter assays in C6 cells mapped the PMA-responsive element to a region between -247 bp and -163 bp on the alpha2D/A-AR promoter. Electrophoretic mobility shift assays showed an increased binding of nuclear factor(s) from PMA-treated bovine retina to this promoter region. Competition assays indicate that an AP-2 element may be involved in the PMA-dependent induction.

CONCLUSION

These findings demonstrate for the first time, the direct induction of the alpha2D/A-AR gene by PMA and support a role for an AP-2 element in the induction mechanism.

Glucose activates the carboxyl methylation of gamma subunits of trimeric GTP-binding proteins in pancreatic beta cells. Modulation in vivo by calcium, GTP, and pertussis toxin

The gamma subunits of trimeric G-proteins (gamma1, gamma2, gamma5, and gamma7 isoforms) were found to be methylated at their carboxyl termini in normal rat islets, human islets and pure beta [HIT-T15] cells. Of these, GTPgammaS significantly stimulated the carboxyl methylation selectively of gamma2 and gamma5 isoforms. Exposure of intact HIT cells to either of two receptor-independent agonists--a stimulatory concentration of glucose or a depolarizing concentration of K+--resulted in a rapid (within 30 s) and sustained (at least up to 60 min) stimulation of gamma subunit carboxyl methylation. Mastoparan, which directly activates G-proteins (and insulin secretion from beta cells), also stimulated the carboxyl methylation of gamma subunits in intact HIT cells. Stimulatory effects of glucose or K+ were not demonstrable after removal of extracellular Ca2+ or depletion of intracellular GTP, implying regulatory roles for calcium fluxes and GTP; however, the methyl transferase itself was not directly activated by either. The stimulatory effects of mastoparan were resistant to removal of extracellular Ca2+, implying a mechanism of action that is different from glucose or K+ but also suggesting that dissociation of the alphabetagamma trimer is conducive to gamma subunit carboxyl methylation. Indeed, pertussis toxin also markedly attenuated the stimulatory effects of glucose, K+ or mastoparan without altering the rise in intracellular calcium induced by glucose or K+. Glucose-induced carboxyl methylation of gamma2 and gamma5 isoforms was vitiated by coprovision of any of three structurally different cyclooxygenase inhibitors. Conversely, exogenous PGE2, which activates Gi and Go in HIT cells and which thereby would dissociate alpha from beta(gamma), stimulated the carboxyl methylation of gamma2 and gamma5 isoforms and reversed the inhibition of glucose-stimulated carboxyl methylation of gamma subunits elicited by cyclooxygenase inhibitors. These data indicate that gamma subunits of trimeric G-proteins undergo a glucose- and calcium-regulated methylation-demethylation cycle in insulin-secreting cells, findings that may imply an important role in beta cell function. Furthermore, this is the first example of the regulation of the posttranslational modification of G-protein gamma subunits via nonreceptor-mediated activation mechanisms, which are apparently dependent on calcium influx and the consequent activation of phospholipases releasing arachidonic acid.

Streptozotocin-induced diabetes elicits the phosphorylation of hepatocyte Gi2 alpha at the protein kinase C site but not at the protein kinase A-controlled site

Streptozotocin-induced diabetes caused a profound increase in the steady-state level of phosphorylation of the alpha-subunit of the adenylate cyclase inhibitory protein Gi2 in hepatocytes. Unlike hepatocytes from control animals, those from streptozotocin-diabetic animals showed no increase in the phosphorylation of Gi2 alpha in response to a challenge with the protein kinase C activator phorbol myristate acetate. However, a stimulatory effect of 8-bromo-cAMP on Gi2 alpha phosphorylation was evident in hepatocytes from diabetic animals but this was severely reduced compared with that observed in hepatocytes from normal animals. Two-dimensional tryptic phosphopeptide mapping showed that Gi2 alpha in resting hepatocytes from diabetic animals was phosphorylated exclusively at the protein kinase C site (C-site) but no labelling was evident at the protein kinase A-regulated site (AN-site). Treatment of hepatocytes from diabetic animals with phorbol myristate acetate did not change this pattern of labelling. In contrast, challenge of hepatocytes from diabetic animals with 8-bromo-cAMP led to the appearance of a new labelled phosphopeptide that was consistent with labelling at the AN-site. Analysis of the C-site and AN-site phosphopeptides from hepatocytes of diabetic animals treated with 8-bromo-cAMP showed that the increase in labelling of Gi2 alpha caused by this ligand could be attributed almost entirely to labelling at the AN-site. Thus streptozotocin diabetes appears to cause enhanced labelling of hepatocyte Gi2 alpha by exclusively increasing phosphorylation at the C-site. It is suggested that the increased labelling at the C-site reflects an augmentation of the protein kinase C signalling system in hepatocytes from streptozotocin-induced diabetic animals. This may have wide-spread functional consequences for these cells and may result either from an increased protein kinase C activity and/or a reduction in protein phosphatase 1 and/or 2A activity.

High-glucose incubation of human umbilical-vein endothelial cells does not alter expression and function either of G-protein alpha-subunits or of endothelial NO synthase

Alterations in G-protein-controlled signalling pathways (primarily pathways controlled by Gs and Gi) have been reported to occur in animal models of diabetes mellitus. We have therefore studied the effect of a long-term exposure of human umbilical vein endothelial cells to elevated concentrations of glucose on expression and function of G-protein subunits and endothelial NO synthase. Long-term incubation in high glucose (30 mM for 15 days) did not affect the levels of Gialpha-2, Gqalpha, the splice variants (long and short form) of Gsalpha, and the G-protein beta-subunits or adenylate cyclase activity; basal, as well as isoprenaline-, forskolin- and guanosine 5'-[gamma-thio]triphosphate-stimulated enzyme activities were comparable in high- and low-glucose-treated cells, thus ruling out any functional changes in the stimulatory pathway. Pretreatment of endothelial cells with pertussis toxin blocked a substantial fraction (50%) of the mitogenic response to serum factor(s) which depend(s) of functional Gi2. The sensitivity of cells cultured in high glucose was comparable with that of the paired controls maintained in normal glucose (EC50 = 3.1 +/- 0.5 and 3.3 +/- 0.4 ng/ml respectively). Similarly, we failed to detect any differences in endothelial NO synthase expression, or intracellular distribution and basal activity of the enzyme in endothelial cells cultured in high glucose. Stimulation of NO synthase in intact cells revealed a comparable response to the calcium ionophore (A23187). In contrast, stimulation with histamine (which acts via H1-receptors predominantly coupled to Gq) resulted in a significantly increased response in the cells maintained in high glucose. These data are suggestive of an altered H1-histamine receptor-Gq-phospholipase C pathway in endothelial cells cultured in high glucose concentrations, but rule out any glucose-induced functional changes in Gs- and Gi-controlled signalling pathways.

Signal transduction alterations in peripheral nerves from streptozotocin-induced diabetic rats

We have previously determined the presence of muscarinic receptors and the expression of several G proteins in homogenates and myelin fractions from rat sciatic nerves. In the present study we investigated whether changes in several signal transduction pathways in peripheral nerves might be responsible for some of the biochemical abnormalities (e.g., phosphoinositide metabolism) present in sciatic nerves from streptozotocin-induced diabetic rats. Sciatic nerves from 5 week diabetic rats that were prelabelled with [3H]-myo-inositol displayed a significant increase in the basal release of inositol mono- and bis-phosphate, while carbamylcholine-stimulated release was significantly smaller. Basal- and forskolin-stimulated adenylyl cyclase activity was significantly decreased in sciatic nerve homogenates from diabetic animals. However, we were unable to detect any significant differences in the levels of cAMP in intact nerves or in nerve segments that were incubated in the presence or absence of forskolin. ADP-ribosylation experiments showed that in sciatic nerves from experimentally diabetic rats there was a significant increase in the ADP-ribosylation catalyzed by cholera and pertussis toxins. Measurements of the levels of alpha-subunits of G proteins revealed that the expression of Gq/11 alpha, Gs alpha, and Gi-3 alpha was increased by 30 to 50%. These results indicate that during the course of experimental diabetes, peripheral nerves exhibit an abnormal production of inositol phosphates and cAMP, together with an abnormal expression and/or function of G proteins. One of the consequences of such alterations is the diminished release of inositol phosphates triggered by muscarinic agonists in diabetic sciatic nerves.

Subcellular localization and kinetic characterization of guanine nucleotide binding proteins in normal rat and human pancreatic islets and transformed beta cells

The subcellular localization and the kinetics of the GTPase activities of monomeric and heterotrimeric GTP-binding proteins were investigated in normal rat and human pancreatic islets and were compared to those obtained using a transformed hamster beta cell line (HIT cells). The [alpha-32P]GTP overlay technique revealed the presence of at least four low-molecular-mass proteins (approx. 20-27 kDa) in normal rat islets, which were enriched in the secretory granule fraction compared to the membrane fraction (with little abundance of these proteins in the cytosolic fraction). In contrast, in HIT cells, these proteins (at least six) were predominantly cytosolic. Three of these proteins were immunologically identified as rab3A, rac2, and CDC42Hs in islets as well as in HIT cells. In addition, pertussis toxin augmented the ribosylation of at least one heterotrimeric G-protein of about 39 kDa (probably G(i) and/or G(o)) in the membrane and secretory granule fractions of normal rat and human islets, whereas at least three such Ptx substrates (36-39 kDa) were found in HIT cell membranes. Kinetic activities revealed the presence of at least three such activities (Km for GTP of 372 nM, 2.2 microM, and 724 microM) in islet homogenates which were differentially distributed in various subcellular fractions; similar activities were also demonstrable in HIT cell homogenates. Thus, these studies demonstrate the presence of both monomeric G-proteins intrinsic to the secretory granules of normal rat islets which can be ascribed to beta cells; since these G-proteins are regulated by insulinotropic lipids (as described in the accompanying article), such proteins may couple the activation of phospholipases (endogenous to islets) to the exocytotic secretion of insulin. These findings also suggest that caution is necessary in extrapolating data concerning G-proteins from cultured, transformed beta cell lines to the physiology of normal islets, in view of both qualitative and quantitative differences between the two preparations.

Regulation of guanine-nucleotide binding proteins in islet subcellular fractions by phospholipase-derived lipid mediators of insulin secretion

In the accompanying article (Kowluru, A., Rabaglia, M.E., Mose, K.E. and Metz, S.A. (1994) Biochim. Biophys. Acta 1222, 348-359) we identified three specific GTPase activities in islet subcellular fractions; most notably, two of these were enriched in the secretory granules. In the present study, we describe the regulation of GTPase activity in subcellular fractions of normal rat and human islets by insulinotropic lipids with a similar rank order as their insulin-releasing capacity. Arachidonic acid (AA), lysophosphatidylcholine (LPC), or phosphatidic acid (PA) inhibited the GTPase activities significantly (by 60-80%) in islet homogenates; each also selectively inhibited certain GTPases in specific individual fractions. Less insulinotropic fatty acids, such as linoleic acid and oleic acid, inhibited GTPase to a lesser degree, whereas lysophosphatidic acid (LPA), phosphatidylcholine (PC) or palmitic acid, which do not acutely promote secretion, were ineffective. Similar inhibitory effects of these lipids were also demonstrable in fractions of human islets as well as those of transformed beta-cells (HIT cells). The effects of lipids were not attributable to their detergent properties (since several detergents failed to mimic lipid effects) or to inhibition of GTP binding (since they actually increased GTP gamma S binding modestly, and moreover, in reconstituted fractions, they potentiated GDP/GTP exchange activity up to 2-fold). These data indicate that the insulinotropic nature of the lipids might be due, in part, to their ability to maintain G-proteins in their GTP-bound (active) configuration by increasing GTP binding and decreasing its hydrolysis. These studies comprise the first evidence for the regulation by biologically active lipids of endocrine cell G-proteins at a locus distal to plasma membrane events (i.e., on endocrine secretory granules), and provide thereby a possible novel mechanism whereby the activation of islet endogenous phospholipases might culminate in insulin exocytosis.

Analysis of the adenylate cyclase signalling system, and alterations induced by culture with insulin, in a novel SV40-DNA-immortalized hepatocyte cell line (P9 cells)

An immortalized cell line, called P9, was derived from hepatocytes by transfection with SV40 DNA. These cells expressed enzyme activities characteristic of hepatocytes, namely glucose-6-phosphatase, glycogen phosphorylase, bilirubin glucuronyltransferase and both glucagon- and prostaglandin E1 (PGE1)-stimulated adenylate cyclase activities, albeit at decreased levels compared with native hepatocytes. Levels of the G-protein subunits alpha-Gi-2, alpha-Gi-3, G beta and the 'long' form of alpha-G2 (45 kDa) were approximately 4-fold higher relative to native hepatocytes, whereas those of the 'short' form of alpha-G2 (42 kDa) were lower by approximately 40%. Associated with this were marked alterations in the guanine nucleotide regulation of adenylate cyclase. Receptor-mediated stimulation, achieved by either PGE1 or glucagon, was apparent in P9 cells, although the latter was only evident upon amplification with forskolin. Glucagon-stimulated cyclic AMP accumulation in P9 cells did not exhibit desensitization, as in hepatocytes, nor was the phosphorylation of alpha-Gi-2 evident. Culture of P9 cells with insulin led to a dose-dependent decrease (EC50 0.2 +/- 0.1 nM) in the ability of PGE1 to stimulate adenylate cyclase activity, with the maximum effect attained after approximately 6 h. A comparable attenuation of stimulation was seen for glucagon- and guanine-nucleotide-stimulated adenylate cyclase activities. In cells cultured with insulin, lower levels of GTP were required to stimulate adenylate cyclase, ADP-ribosylation of the 45 kDa form of alpha-Gs with cholera toxin was attenuated, and the expression of both alpha Gi-2 and alpha-Gi-3 was increased. It is suggested that the expression of alpha-Gi-2 and alpha-Gi-3 may be directly regulated by the action of insulin in hepatocytes and P9 cells.

Stimulation by prostaglandin E2 of a high-affinity GTPase in the secretory granules of normal rat and human pancreatic islets

Recent reports of a pertussis-toxin (Ptx)-sensitive inhibition of glucose-induced insulin release by prostaglandin E2 (PGE2) in transformed beta-cells prompted us to look for the presence of prostaglandin-regulatable GTP-binding proteins (G-proteins) on the secretory granules of normal pancreatic islets. PGE2 (but not PGF2 alpha, PGA2, PGB2 or PGD2) stimulated in a concentration-dependent manner a high-affinity GTPase activity in the secretory-granule-enriched fractions of both normal rat and human islets. Similar results were found after sucrose-density-gradient-centrifugation-based isolation of secretory granules to those after a differential-centrifugation procedure. Half-maximal stimulation occurred at 800 nM PGE2, a concentration known to inhibit both phases of glucose-induced insulin secretion from pure beta-cell lines. The GTPase stimulatory effect of PGE2 was blocked virtually totally by Ptx pretreatment; it was not due to an effect on substrate binding since no measurable effect of PGE2 on binding of guanosine 5'-[gamma-[35S]thio]triphosphate was observed in cognate fractions. Other Ptx-sensitive inhibitors of insulin secretion (such as adrenaline or clonidine) also stimulated GTPase activity, suggesting that one (or more) inhibitory exocytotic G-proteins (i.e. a putative GEi) is located on the secretory granules. These studies demonstrate, for the first time in an endocrine gland, the presence of a regulatable G-protein, strategically located on the secretory granules where it might regulate the exocytotic cascade distal to both plasma-membrane events and the generation of soluble mediators of insulin secretion.

Guanosine-5'-(3-O-thio)triphosphate-mediated stimulation of phosphoinositidase C in solubilized rat peripheral nerve myelin and its alteration in streptozotocin-induced diabetes

The regulation of phosphoinositidase C (PIC) activity by guanosine-5'-(3-O-thio)triphosphate (GTP gamma S) was characterized in a cholate-solubilized peripheral myelin-enriched fraction from rat sciatic nerve. The GTP analog maximally enhanced PIC-catalyzed hydrolysis of exogenous phosphatidylinositol-4,5-bisphosphate (PIP2) in a dose-dependent manner only within a narrow range of cholate concentrations. Maximal stimulation was attained at 0.6 microM GTP gamma S and could be completely prevented by 1 microM guanosine-5'-(2-O-thio)diphosphate. Neither adenylyl-imidodiphosphate nor adenosine triphosphate (ATP) enhanced PIC activity. Carbamoylcholine (1 mM) added together with GTP gamma S increased the extent of PIP2 hydrolysis over that elicited by GTP gamma S alone and this stimulation was blocked by the muscarinic receptor antagonist, atropine (50 microM). In detergent-solubilized myelin preparations from streptozotocin-induced diabetic rats, a higher concentration of the guanine nucleotide analog was required to achieve stimulation comparable to that obtained with corresponding preparations from normal animals. These results suggest that sciatic nerve myelin possesses muscarinic receptors coupled via a GTP-binding protein to PIC and that this system can be reconstituted in detergent-solubilized extracts. It is possible that the function of G proteins in cell signaling is impaired in experimental diabetic neuropathy.

Alterations in retinal Na+, K(+)-ATPase in diabetes: streptozotocin-induced and Zucker diabetic fatty rats

The temporal pattern of changes in the specific activities of retinal Na+, K(+)-ATPase (Na, K-ATPase) and Mg(2+)-ATPase (Mg-ATPase) were determined at several time intervals following the onset of diabetes in streptozotocin-induced diabetic (STZ: at 1, 2, 4 and 6 months) Long-Evans hooded rats, spontaneously diabetic Zucker diabetic fatty (ZDF: at 1, 2 and 4 months) rats and their age-matched controls. These animals were utilized as models for insulin-dependent diabetes mellitus (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM), respectively. Na, K-ATPase specific activity, using 10(3) M ouabain, was decreased (-6% to -14%) at all time points after the appearance of hyperglycemia in the ZDF rat, but was reduced only after 4 and 6 months in the STZ rat (-8% and -14%, respectively). In contrast, Mg-ATPase activity was significantly increased (13%) after 4 months in the ZDF rat and after 6 months in the STZ rat (8%). The concentration-dependent inhibitory effects of ouabain (10(-9) to 10(-3) M) on the activity of Na, K-ATPase in diabetic rats and age-matched controls was used to assess the time-dependent effects of diabetes on the alpha 3-high ouabain affinity or the alpha 1-low ouabain affinity retinal Na, K-ATPase isozymes. The retinal Na, K-ATPase activity for the alpha 3 isozyme was significantly lower at all times examined for the ZDF (-5% to -26%) and STZ-induced diabetic rats (-8% to -14%). This was reflected in the markedly decreased half-maximal inhibitory concentrations (IC50) of ouabain for the alpha 3 isozyme. For example, after four months of diabetes, the mean +/- SEM IC50 values were 12 +/- 3 nM in the STZ rats and 48 +/- 6 nM in the age-matched controls and 19 +/- 3 nM in the ZDF rats and 30 +/- 4 nM in the age-matched controls. In contrast, the activity of the alpha 1 isozyme was slightly, but significantly, decreased at 2 and 4 months in the ZDF rats (-4% to -7%) and after 4 and 6 months in the STZ-induced diabetic rats (-3% to -9%) while the IC50 values were unchanged. Moreover, the Hill coefficient for the alpha 3 isozyme was decreased in both diabetic groups while it was unchanged for the alpha 1 isozyme.(ABSTRACT TRUNCATED AT 400 WORDS)