Diabetes-induced alterations in the expression, functioning and phosphorylation state of the inhibitory guanine nucleotide regulatory protein Gi-2 in hepatocytes

The functional state of hormone-sensitive adenylyl cyclase signaling system in diabetes mellitus

Diabetes mellitus (DM) induces a large number of diseases of the nervous, cardiovascular, and some other systems of the organism. One of the main causes of the diseases is the changes in the functional activity of hormonal signaling systems which lead to the alterations and abnormalities of the cellular processes and contribute to triggering and developing many DM complications. The key role in the control of physiological and biochemical processes belongs to the adenylyl cyclase (AC) signaling system, sensitive to biogenic amines and polypeptide hormones. The review is devoted to the changes in the GPCR-G protein-AC system in the brain, heart, skeletal muscles, liver, and the adipose tissue in experimental and human DM of the types 1 and 2 and also to the role of the changes in AC signaling in the pathogenesis and etiology of DM and its complications. It is shown that the changes of the functional state of hormone-sensitive AC system are dependent to a large extent on the type and duration of DM and in experimental DM on the model of the disease. The degree of alterations and abnormalities of AC signaling pathways correlates very well with the severity of DM and its complications.

Diabetic neuropathy: inhibitory G protein dysfunction involves PKC-dependent phosphorylation of Goalpha

We examined the hypothesis that decreased inhibitory G protein function in diabetic neuropathy is associated with increased protein kinase C (PKC)-dependent phosphorylation of the Goalpha subunit. Streptozotocin-induced diabetic rats were studied between 4 and 8 weeks after onset of diabetes and compared with aged-matched healthy animals as controls. Opioid-mediated inhibition of forskolin-stimulated cyclic AMP was significantly less in dorsal root ganglia (DRGs) from diabetic rats compared with controls. Activation of PKC in DRGs from control rats was associated with a significant decrease in opioid-mediated inhibition of forskolin-stimulated cyclic AMP that was similar to the decrease in inhibition observed in DRGs from diabetic rats. Both basal and PKC-mediated labeling of Goalpha with 32Pi was significantly less in DRGs from diabetic rats, supporting increased endogenous PKC-dependent phosphorylation of Goalpha. Probing of immunoprecipitated Goalpha with an anti-phospho-serine/threonine specific antibody revealed a significant increase in baseline phosphorylation in diabetic DRGs. Activation of PKC produced a significant increase in phosphorylation in control DRGs but no significant increase in Goalpha in diabetic DRGs. Phosphorylation of PKC-alpha was increased, PKC-betaII was unchanged and PKC-delta decreased in diabetic DRGs. These results suggest that diminished inhibitory G protein function observed in DRGs neurons from diabetic rats involves an isoform-specific PKC-dependent pathway.

G protein and adenylate cyclase complex-mediated signal transduction in the rat heart during sepsis

Changes in the protein level of various subunits of GTP-binding protein and the activity of adenylate cyclase in the rat heart during different phases of sepsis were studied. Sepsis was induced by cecal ligation and puncture (CLP). Experiments were divided into three groups: control, early sepsis, and late sepsis. Early and late sepsis refers to those animals sacrificed at 9 and 18 h, respectively, after CLP. The protein levels of various subunits of GTP-binding protein were determined by Western blot analysis. The activity of adenylate cyclase was measured based on the rate of formation of cAMP from [alpha-32P]ATP. The results show that protein levels of G alphas and G beta remained stable during the early and the late phases of sepsis. The protein levels of G alpha i-2 and G alpha i-3 remained relatively unaltered during the early phase of sepsis, but they were increased by 46.5% (P < 0.05) and 61.3% (P < 0.01), respectively, during the late phase of sepsis. The basal adenylate cyclase activity remained unchanged during the early phase while it was decreased by 25.7% (P < 0.05) during the late phase of sepsis. The isoproterenol-stimulated adenylate cyclase activity was unchanged during early sepsis while it was decreased by 44.6% (P < 0.01) during late sepsis. These data demonstrate that during the late hypodynamic phase of sepsis, myocardial G alpha i-2 and G alpha i-3 protein levels were increased and the increases were coupled with a reduction in adenylate cyclase activity. Because GTP-binding proteins mediate sympathetic control of cardiac function, the present findings may have a pathophysiological significance in contributing to the understanding of the pathogenesis of cardiac dysfunction during the late stage of sepsis.

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.

Alterations in g-protein-linked signal transduction in vascular smooth muscle in diabetes

The present studies were undertaken to determine the levels of stimulatory and inhibitory guanine nucleotide regulatory proteins (Gs and Gi respectively) and their relationship with adenylyl cyclase activity in aorta from 5-day streptozotocin-induced diabetic (STZ) rats. The levels of Gi alpha-2 as determined by immunoblotting techniques using AS/7 antibody were significantly decreased by about 60% in STZ as compared to control rats, whereas the levels of Gs alpha were not altered. In addition, the stimulatory effect of cholera toxin (CT) on GTP-sensitive adenylyl cyclase was not different in STZ as compared to control rats. On the other hand, the stimulatory effects of GTPgammaS, isoproterenol, glucagon, forskolin (FSK) and sodium fluoride on adenylyl cyclase were enhanced in STZ-rats. Furthermore, GTPgammaS inhibited FSK-stimulated adenylyl cyclase activity in a concentration-dependent manner (receptor independent functions of Gi) in control rats which was almost completely abolished in STZ rats. In addition, receptor-mediated inhibition of adenylyl cyclase by angiotensin II (AII), oxotremorine and atrial natriuretic peptide (ANP) was attenuated in STZ rats. These results suggest that the decreased expression of Gi alpha, but not of Gs alpha, may be responsible for the observed altered responsiveness of adenylyl cyclase to hormonal stimulation and inhibition in STZ-rats. It may thus be suggested that the decreased Gi activity may be one of the possible mechanisms responsible for the impaired vascular functions in diabetes.

Evidence for the involvement of Gi2 in activation of extracellular signal-regulated kinases in hepatocytes

BACKGROUND

Activation of the extracellular signal-regulated kinases ERK1 and ERK2 in hepatocytes by prostaglandin (PG)F2alpha was recently found to be inhibited by pertussis toxin (PTX) suggesting a role for Gi proteins.

RESULTS

Targeting the Gi2alpha expression by a specific ribozyme inhibited the PGF2alpha -induced ERK1/2 activation in hepatocytes. On the other hand a non-cleaving form of the Gi2alpha ribozyme did not significantly decrease the ERK1/2 activation. In ribozyme-treated cells the Gi2alpha protein level was reduced, while the Gqalpha level was not affected thus confirming the specificity of the ribozyme.

CONCLUSION

The present data suggest an important role of Gi2 in PGF2alpha -induced ERK1/2 signaling in hepatocytes.

Effects of the luteinising hormone-releasing hormone (LH-RH) agonist leuprolide on adenylyl cyclase regulation through G-protein coupled receptors in rat ventral prostate

Luteinising hormone-releasing hormone (LH-RH) agonists are widely used for the therapy of advanced prostate cancer through the suppression of testosterone secretion. Furthermore, recent studies indicate the existence of prostate LH-RH receptors coupled to signalling pathways resulting in direct antiproliferative effects. In order to shed light on the mechanisms through which these compounds inhibit prostate cell growth, we investigated the effects of leuprolide (a LH-RH agonist) treatment of rats compared with the effects of surgical castration on the behaviour of G-protein coupled receptors acting through adenylyl cyclase in the ventral prostate. Important decreases of both plasma testosterone levels and ventral prostate weight were observed 5 weeks after subcutaneous (s.c.) injection of a leuprolide-depot preparation (1.5 mg/kg body weight (b.w.)) or 5 days after bilateral gonadectomy. However, leuprolide treatment increased the number of vasoactive intestinal peptide (VIP) receptors and the ability of this neuropeptide to stimulate adenylyl cyclase activity in prostate membranes, whereas surgical castration decreased both parameters. Moreover, leuprolide resulted in significant increases of prostate alpha(s) and alpha(i1-3) (but not alpha(i1) and beta) G-protein levels, while the four G-protein subunits were overexpressed after gonadectomy. The estimation of alpha(s) and alpha(i) activity by experiments with Gpp[NH]p and forskolin indicated a potentiation of the two arms of adenylyl cyclase regulation in leuprolide-treated rats. Present observations suggest that leuprolide treatment leads to an antimitogenic response by acting mainly through the activation of Gi proteins negatively coupled to adenylyl cyclase.

Effects of Pygeum africanum extract (Tadenan) on vasoactive intestinal peptide receptors, G proteins, and adenylyl cyclase in rat ventral prostate

BACKGROUND

Tadenan (a Pygeum africanum extract) is a drug used in the treatment of benign prostatic hyperplasia. Its effects on prostate fibroblast proliferation and bladder function after partial outlet obstruction have been demonstrated in various pharmacological studies. However, its effects at the molecular level are poorly documented.

METHODS

Tadenan was dissolved in peanut oil. Rats were orally given two daily doses of the drug (1 or 10 mg/kg b.w.) for 4 days. Vasoactive intestinal peptide (VIP) binding, adenylyl cyclase stimulation, and expression of G-protein subunits were studied in rat prostatic membranes by established procedures.

RESULTS

Tadenan treatment of castrated/testosterone-replaced rats was performed in order to interfere with prostatic cell proliferation. This experimental approach resulted in increases of: 1) VIP effect on adenylyl cyclase stimulation through alpha(s) G-subunit; 2) alpha(i) activation by low Gpp[NH]p doses (in the presence of forskolin); and 3) alpha(s), alpha(i1/2), and alpha(i3/0) levels. However, there were no modifications in membranes from quiescent, nonproliferating prostates (untreated rats).

CONCLUSIONS

The observed regulatory role of Tadenan on various prostatic components of the adenylyl cyclase system, together with previous findings on protein kinase C-mediated signal transduction, open a complex array of possibilities of direct actions of this phytotherapeutic agent in the prostate.

Decreased opioid-induced antinociception but unaltered G-protein activation in the genetic-diabetic NOD mouse

Previous evaluation of antinociceptive action in experimental diabetes has been conducted almost exclusively in chemically induced diabetes mellitus. The purpose of the present study was to evaluate antinociceptive response and G-protein activation by mu-opioid receptor and delta-opioid receptor agonists in the genetic non-obese diabetic (NOD) mouse, a model of type I insulin-dependent diabetes mellitus (IDDM). Tail-flick latency before and after hyperglycemia was unaltered. Hyperglycemic NOD mice were hyporesponsive to intracerebroventricular (i.c.v.) injections of [D-Ala(2)]deltorphin II but not to [D-Ala(2), N-MePhe(4), Gly-ol(5)]enkephalin (DAMGO); however, G-protein activation in pons/medulla assessed by [35S]GTPgammaS binding was not diminished. This suggests that a G-protein defect in signaling cannot account for the hyporesponsiveness of antinociception in this genetic model of IDDM.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Purification of A1 adenosine receptor-G-protein complexes: effects of receptor down-regulation and phosphorylation on coupling

We examined the effects of exposing A1 adenosine receptors (A1ARs) to an agonist on the stability and phosphorylation state of receptor-guanine nucleotide-binding regulatory protein (R-G-protein) complexes. Non-denatured recombinant human A1ARs extended on the N-terminus with hexahistidine (His6) and the FLAG (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys) epitope (H/F) were purified to near homogeneity from stably transfected Chinese-hamster ovary (CHO)-K1 cells. Purified receptors have pharmacological properties similar to receptors in membranes. G-proteins were co-purified with 15+/-2% of H/F-A1AR unless receptor-G-protein (R-G) complexes were uncoupled by pre-treating cell membranes with GTP. By silver staining, purified A1AR-G-protein complexes contain receptors, G-protein alpha and beta subunits and an unidentified 97 kDa protein. Pretreating intact cells with N6-cyclopentyladenosine (CPA) for 24 h decreased both the total number of receptors measured in membranes and the number of purified A1ARs by about 50%. In contrast, pretreating cells with CPA decreased the number of R-G complexes measured in membranes (54+/-6%) significantly less than it decreased the number of purified R-G complexes (78+/-3%) as detected by 125I-N6-(4-aminobenzyl)adenosine binding or by Western blotting Gialpha2. The effect of CPA to decrease the fraction of receptors purified as R-G complexes was not associated with any change in low-level A1AR phosphorylation (found on serine), or low-level phosphorylation of G-protein alpha or beta subunits or the 97 kDa protein. These experiments reveal a novel aspect of agonist-induced down-regulation, namely a diminished stability of receptor-G-protein complexes that is manifested as uncoupling during receptor purification.

N-Methyl-D-aspartate attenuates opioid receptor-mediated G protein activation and this process involves protein kinase C

The effects of N-methyl-D-aspartate (NMDA) on opioid receptor-mediated G protein activation were explored in neuroblastoma X glioma hybrid (NG108-15) cells. Treatment of the cells with NMDA resulted in a remarkable attenuation of [35S]guanosine-5'-O-(3-thio)triphosphate binding stimulated by [D-Pen2,D-Pen5]-enkephalin (DPDPE), a delta-opioid receptor agonist. The effects of NMDA were dose and time dependent with an IC50 value of 5 nM and could be blocked by NMDA receptor antagonists. After NMDA treatment, the DPDPE dose-response curve shifted to the right (EC50 value increased approximately 7-fold, from 6 to 40 nM), and the maximal response induced by DPDPE was reduced by approximately 60%. The effects of NMDA were reversible, and the DPDPE response could recover within 60 min. The functional responses of delta-, mu-, and kappa-opioid receptors in primarily cultured neurons also were attenuated significantly by NMDA treatment. The inhibitory effects of NMDA on opioid receptor-mediated G protein activation could be blocked by coadministration of the protein kinase C (PKC) inhibitors or by elimination of the extracellular Ca2+. Correspondingly, NMDA treatment of NG108 cells significantly elevated cellular PKC activity and stimulated Gialpha2 phosphorylation. Transient transfection into NG108-15 cells of the wild-type Gialpha2 and a mutated Gialpha2 (Ser144Ala) resulted in a 2-fold increase in DPDPE-stimulated G protein activation. The DPDPE responses were greatly inhibited by NMDA treatment in the wild-type Gialpha2-transfected cells but much less affected in the mutant Gialpha2-transfected cells. In summary, NMDA attenuates opioid receptor/G protein coupling, and this process requires activation of PKC.

G-proteins and beta-adrenergic stimulation of adenylate cyclase activity in the diabetic rat prostate

BACKGROUND

The consequences of experimental diabetes on membrane lipids, beta-adrenergic stimulation of adenylate cyclase activity, and G-protein levels in the prostate gland are not defined.

METHODS

Prostatic membranes from control and streptozotocin (STZ)-diabetic rats were used to study adenylate cyclase stimulation as well as for immunodetection of stimulatory (alpha s) and inhibitory (alpha i) G-protein subunits. Changes in membrane lipid composition were estimated by [1-14C] acetate incorporation into lipid subclasses.

RESULTS

The efficacy of isoproterenol on stimulation of adenylate cyclase activity and the levels of alpha s, alpha i1/2, and alpha i3/0 G-protein subunits were drastically reduced in prostatic membranes from STZ-diabetic rats. Insulin treatment of diabetic rats tended to normalize G-protein levels, but it was ineffective on the poor adenylate cyclase response to isoproterenol or forskolin. However, it prevented enzyme desensitization to vasoactive intestinal peptide. The pattern of [1-14C] acetate incorporation into lipid subclasses did not vary with diabetes or insulin treatment.

CONCLUSIONS

STZ-induced diabetes results in desensitization for the beta-adrenergic response of adenylate cyclase, as supported by previous data on the low density of beta-adrenergic receptors and the present results on the general decrease of Gs and Gi proteins levels and even of the enzyme itself in the diabetic rat prostate.

Corticosterone alters G protein alpha-subunit levels in the rat hippocampus

The hypothalamic-pituitary-adrenal axis regulates the synthesis and secretion of corticosteroid hormones. The hippocampus, a component of the limbic system, contains the highest concentration of corticosteroid receptors in the brain and may play an important role in regulating hypothalamic-pituitary-adrenal axis activity and mediating physiological responses to stress. The corticosteroid hormone corticosterone alters the response elicited by activation of several different G protein-linked neurotransmitter receptors in the hippocampus. In the present study we used Western blot and immunohistochemical techniques to determine the effects of chronic adrenalectomy (ADX), low basal (CT) and high (HCT) corticosterone treatments on Gs, Gi1 and 2 and Go alpha-subunit levels and intracellular location in the rat hippocampus. CT treatment increased Gs alpha-subunit levels and HCT treatment increased the levels of Gs, Gi1 and 2 and Go alpha-subunits when compared to sham as detected on Western blots. No change in the intracellular location of the G protein alpha-subunits was detected using immunohistochemistry. Based on our results, we conclude that corticosterone alters G protein alpha-subunit levels in the rat hippocampus without altering their intracellular location. These results provide an important piece of information towards understanding how corticosteroids alter G protein-linked neurotransmitter receptor-mediated responses.

Adenosine inhibitory effect on enhanced growth of aortic smooth muscle cells from streptozotocin-induced diabetic rats

1. There is evidence to suggest that adenosine may regulate arterial smooth muscle cell (SMC) growth and proliferation, which is a key event in atherogenesis. This regulation may be mediated via adenylate cyclase. As diabetes is a known risk factor for atherosclerosis, we investigated the growth of aortic SMC from diabetic rats in primary culture and their sensitivity to adenosine and to adenylate cyclase activity. 2. Diabetes was induced with streptozotocin (STZ, 66 mg kg-1, i.p.) Aortic SMC primary cultures were prepared from STZ-diabetic and age-matched rats 5 weeks after the STZ injection. 3. SMC from STZ-diabetic rats grew faster and reached greater densities at confluence than those from non-diabetic animals. 4. Adenosine inhibited growth in both control and diabetic SMC. However, cells from STZ-diabetic rats were apparently more sensitive to adenosine. 5. Direct activation of adenylate cyclase by forskolin induced a dose-dependent growth inhibition, similar in both groups of cells. 6. Cholera toxin, an activator of stimulatory GTP-binding protein (Gs), induced a similar growth inhibitory response in non-diabetic and diabetic SMC. Pertussis toxin (PTX), an inactivator of inhibitory GTP-binding protein (Gi), did not itself affect SMC growth. However, PTX increased dose-dependently the growth inhibition induced by adenosine in SMC from non-diabetic rats but not in SMC from diabetic rats. 7. These findings suggest a functional abnormality in Gi activity in SMC from diabetic rats, that would explain the increased sensitivity to the nucleoside. This impaired inhibitory pathway may reflect changes in the growth regulation of SMC in experimental diabetic states.

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.

Altered intracellular Ca2+ regulation in pancreatic acinar cells from acute streptozotocin-induced diabetic rats

We investigated intracellular Ca2+ regulation in pancreatic acinar cells from rats with diabetes induced by a single injection of streptozotocin (80 mg/kg). Experiments were performed 2 days and 7 days after the injection of streptozotocin. The density of muscarinic receptors, measured by [3H]N-methyl scopolamine binding, was unchanged in 2-day-diabetic rats, but was significantly increased in 7-day-diabetic rats. The percentage of high affinity receptors (RH) and low affinity receptors (RL) determined from the competitive curves with [3H]N-methyl scopolamine and carbachol was not change in 2-day-diabetic rats compared to controls, whereas 7-day-diabetic rats showed a decrease in %RH and an increase in %RL. The carbachol-evoked initial peak of intracellular Ca2+ concentration ([Ca2+]i) was increased in 2-day-diabetic rats and decreased in 7-day-diabetic rats, compared to controls. In the carbachol-induced sustained phase in [Ca2+]i, the response in 7-day-diabetic rats was significantly decreased; however, there was no difference between controls and 2-day-diabetic rats. Carbachol (100 microM)-induced [3H]inositol 1,3,4-trisphosphate generation was significantly lower in diabetic rats than in the controls. The addition of inositol 1,4,5-trisphosphate (1,4,5-IP3) significantly increased 45Ca2+ release from saponin-permeabilized cells in 2-day-diabetic rats, but did not do so in 7-day-diabetic rats. Ca2+ refilling into the intracellular stores, determined by second cholecystokinin-8 (10 nM) stimulation after 10 microM carbachol stimulation, was increased in 2-day-diabetic rats and decreased in 7-day-diabetic rats. These observations indicate that the alterations in intracellular Ca2+ regulation accompanied by changes in transmembrane signaling occur in the earlier stage of the diabetic state. The findings also suggest that the increase in the carbachol-evoked [Ca2+]i peak in 2-day-diabetic rats is related predominantly to the higher sensitivity of 1,4,5-IP3-sensitive Ca2+ stores and the increase in the capacity of Ca2+ refilling in these animals, whereas the reduction in the [Ca2+]i peak in 7-day-diabetic rats appears to be related to the essential decrease in receptor-mediated 1,4,5-IP3 generation and the decrease in Ca2+ refilling capacity.

Opiate-mediated inhibition of calcium signaling is decreased in dorsal root ganglion neurons from the diabetic BB/W rat

The effect of diabetes mellitus on opiate-mediated inhibition of calcium current density (I(D Ca) [pA pF-1]) and cytosolic calcium response ([Ca2+]i nM) to depolarization with elevated KCl and capsaicin was assessed. Experiments were performed on isolated, acutely dissociated dorsal root ganglion (DRG) neurons from diabetic, BioBreeding/Worcester (BB/W) rats and age-matched control animals. Sciatic nerve conduction velocity was significantly decreased in diabetic animals compared to controls. Mean I(DCa) and [Ca2+]i responses to capsaicin and elevated KCl recorded in DRGs from diabetic animals were significantly larger than those recorded in DRG neurons from controls. In neurons from diabetic animals, the opiate agonist dynorphin A (Dyn A; 1, 3, and 5 microM) had significantly less inhibitory effect on I(D Ca) and KCl-induced [Ca2+]i responses compared to controls. Omega-conotoxin GVIA (omega-CgTX; 10 microM) and pertussis toxin (PTX; 250 ng ml-1) abolished Dyn A-mediated inhibition of I(DCa) and [Ca2+]i in control and diabetic neurons, suggesting that Dyn A modulated predominantly N-type calcium channels coupled to opiate receptors via PTX-sensitive (Gi/o) inhibitory G proteins. These results suggest that opiate-mediated regulation of PTX-sensitive, G protein-coupled calcium channels is diminished in diabetes and that this correlates with impaired regulation of cytosolic calcium.

Insulin action impaired by deficiency of the G-protein subunit G ialpha2

Integration of information between tyrosine kinase and G-protein-mediated pathways is necessary, but remains poorly understood. Here we use cells from transgenic mice harbouring inducible expression of RNA antisense to the gene encoding G ialpha2 to show that G ialpha2 is critical for insulin action. G ialpha2 deficiency in adipose tissue and liver produces hyperinsulinaemia, impaired glucose tolerance and resistance to insulin in vivo. Insulin resistance affects glucose-transporter activity and recruitment, counterregulation of lipolysis, and activation of glycogen synthase, all of which are cardinal responses to insulin. G ialpha2 deficiency increases protein-tyrosine phosphatase activity and attenuates insulin-stimulated tyrosine phosphorylation of IRS (insulin-receptor substrate 1) in vivo. G ialpha2 deficiency creates a model for insulin resistance characteristic of noninsulin-dependent diabetes mellitus (NIDDM), implicating G ialpha2 as a positive regulator of insulin action.

Use of elastase as an internal standard in immunoblotting techniques

A non-radioactive method for relative, semi-quantitative analysis of immunoblots, based on the use of elastase as internal standard and conventional peroxidase staining was devised and applied to the immunoassay of Gs-proteins in crude membrane preparations of rat kidneys. We found that the coefficients of variation of samples, run within the same experiment or run within different experiments, are reduced to half or a quarter of their original value respectively when corrected for elastase as an internal standard, allowing meaningful comparison of these samples.

Reversal of defective G-proteins and adenylyl cyclase/cAMP signal transduction in diabetic rats by vanadyl sulphate therapy

Vanadium salts exhibit a wide variety of insulinomimetic effects. In the present studies, we have examined the modulation of G-protein levels and adenylyl cyclase activity in the liver of streptozotocin-induced chronic diabetic rats (STZD) by vanadyl sulfate treatment and compared it with that of insulin. The basal enzyme activity, as well as the stimulatory effects of guanine nucleotides, glucagon, N-Ethylcarboxamideadenosine (NECA), isoproterenol, forskolin and sodium fluoride (NaF) on adenylyl cyclase were significantly increased in STZ-D rat liver as compared to control. In addition, the levels of stimulatory (Gs alpha) as well as inhibitory (Gi alpha-2 and Gi alpha-3) as determined by immunoblotting techniques were also significantly higher in the STZ-D rat liver, however, the inhibitory effects of oxotremorine and low concentrations of GTP gamma S on adenylyl cyclase were not different in the two groups. Vanadyl sulfate and insulin treatments restored the augmented basal enzyme activity, the stimulations exerted by stimulatory inputs on adenylyl cyclase and the G-protein levels to various degrees, however, vanadyl sulfate was more effective than insulin. In addition, unlike vanadyl sulfate, insulin was unable to improve the stimulation exerted by glucagon and isoproterenol on adenylyl cyclase activity in STZD rats. These results suggest that vanadyl sulfate mimics the effects of insulin to restore the defective levels of G-proteins and adenylyl cyclase activity. From these results it may be suggested that one of the mechanisms by which vanadyl sulfate improves the glucose homeostasis in STZ-D rats may be through its ability to modulate the levels of G-proteins and adenylyl cyclase signal transduction system.

Evidence that the pertussis toxin-sensitive trimeric GTP-binding protein Gi2 is required for agonist- and store-activated Ca2+ inflow in hepatocytes

The role of a trimeric GTP-binding protein (G-protein) in the mechanism of vasopressin-dependent Ca2+ inflow in hepatocytes was investigated using both antibodies against the carboxyl termini of trimeric G-protein alpha subunits, and carboxyl-terminal alpha-subunit synthetic peptides. An anti-Gi1-2 alpha antibody and a Gi2 alpha peptide (Gi2 alpha) Ile345-Phe355), but not a Gi3 alpha peptide (Gi3 alpha Ile344-Phe354), inhibited vasopressin- and thapsigargin-stimulated Ca2+ inflow, had no effect on vasopressin-stimulated release of Ca2+ from intracellular stores, and caused partial inhibition of thapsigargin-stimulated release of Ca2+. An anti-Gq alpha antibody also inhibited vasopressin-stimulated Ca2+ inflow and partially inhibited vasopressin-induced release of Ca2+ from intracellular stores. Immunofluorescence measurements showed that Gi2 alpha is distributed throughout much of the interior of the hepatocyte as well as at the periphery of the cell. By contrast, Gq/11 alpha was found principally at the cell periphery. It is concluded that the trimeric G-protein, Gi2, is required for store-activated Ca2+ inflow in hepatocytes and acts between the release of Ca2+ from the endoplasmic reticulum (presumably adjacent to the plasma membrane) and the receptor-activated Ca2+ channel protein(s) in the plasma membrane.

Adrenergic desensitization in left ventricle from streptozotocin diabetic swine

Patients with diabetes mellitus that exhibit cardiac pump failure display compromised stroke volume, ejection fraction, and slower rates of rise and fall of left ventricular (LV) dP/dt in the absence of ischemic injury. We hypothesized that diabetic cardiomyopathy may involve decrements in adrenergic sensitivity, with specific molecular alterations in the beta-adrenergic receptor (beta AR)- G protein- adenylyl cyclase (AC) signal transduction system. We assessed the effects of 3 months of streptozotocin-induced diabetes (125 mg/kg i.v.; DIAB, n = 10) on myocardial signal transduction in mini-pigs. DIAB were hyperglycemic compared to controls (CON, n = 10; 20.92 +/- 2.64 v 5.24 +/- 0.35 mM glucose), and had lower fasting insulin levels (6.46 +/- 0.97 v 13.68 +/- 3.91 microU/ml). Transmural LV free wall homogenates from DIAB exhibited similar beta AR density as CON, but decreased cAMP production (pmol cAMP/mg prot.min) using these pharmacological stimulators: 10 microM Isoproterenol plus 100 microM GTP (74 +/- 5 v 97 +/- 11); 100 microM Gpp(NH)p (116 +/- 7 v 161 +/- 17); 10 mM fluoride ion (266 +/- 16 v 324 +/- 25). No differences between DIAB and CON were observed when stimulated by 100 microM forskolin (440 +/- 20 v 429 +/- 33), suggesting no alterations in the catalytic subunit of AC. In DIAB, quantitative immunoblotting indicated slightly depressed levels of Gs (552 +/- 44 v 630 +/- 59 pmol/g ww; NS), but a significant redistribution of alpha s from the sarcolemma to the cytosol (32.7 +/- 0.82% v 25.9 +/- 1.7%). Significantly elevated levels of cardiac Gi were seen in DIAB homogenates compared to CON ventricles (2326 +/- 145 v 1522 +/- 181 pmol/g ww), with no alpha i subunit redistribution. We conclude that despite maintained beta AR density, receptor-dependent and G protein-dependent stimulation of AC is depressed so that streptozotocin-induced diabetic LV is affected by increased cardiac Gi, redistribution of Gs alpha to the cytosol, and an increase in the Gi/Gs ratio. These results help explain depressed catecholamine responsiveness and cardiac performance exhibited by diabetic patients.

Voltage-dependent calcium currents are enhanced in dorsal root ganglion neurones from the Bio Bred/Worchester diabetic rat

1. Whole-cell, high-threshold, voltage-dependent calcium currents (ICa) were enhanced in acutely dissociated, capsaicin-sensitive dorsal root ganglion neurones from diabetic Bio Bred/Worchester (BB/W) rats, compared with those from age-matched, non-diabetic controls. The magnitude of the enhancement increased with the duration of diabetes, and reached significance at diabetic durations of 6 months (diabetic: 6.3 +/- 0.4 nA; current density (CD), 157 +/- 12 pA pF-1; means +/- S.E.M., n = 9, P < 0.01; control: 3.9 +/- 0.6 nA; CD, 116 +/- 11 pA pF-1; n = 18) and 8 months (diabetic: 7.6 +/- 0.4 nA; CD, 177 +/- 25 pA pF-1; n = 11, P < 0.005; control: 5.1 +/- 0.5 nA; CD, 111 +/- 26 pA pF-1; n = 15). Low-threshold, voltage-dependent ICa were also enhanced in neurones from animals diabetic for 8 months (diabetic: 2.5 +/- 0.7 nA, n = 4, P < 0.05; control: 0.7 +/- 0.5 nA, n = 6). 2. The ICa enhancement was prevented by long-term treatment of diabetic animals with an aldose reductase inhibitor (ARI; peak ICa at 6 months: 4.41 +/- 0.48 nA, n = 2; at 8 months: 4.32 +/- 0.60 nA, n = 9). 3. The ICa enhancement was not due to a shift in the voltage dependence of either the current-voltage relationship or steady-state inactivation. 4. The L channel antagonist nifedipine and preferential N channel antagonist omega-conotoxin GVIA (omega-CgTX) caused a greater inhibition of high-threshold ICa in diabetic neurones compared with controls (nifedipine: control: 25 +/- 3%, n = 26; diabetic: 36 +/- 7%, n = 11; omega-CgTX: control: 40 +/- 4%, n = 21; diabetic: 50 +/- 7%, n = 7). Diabetic neurones also demonstrated a significantly greater residual current (2.44 +/- 0.34 nA, n = 7) in the presence of both antagonists vs. controls (1.28 +/- 0.30 nA, n = 8, P < 0.05), suggesting that N-, L- and additional non-N-, non-L-type high-threshold ICa were enhanced.

Tissue-specific alterations in G protein expression in genetic versus diet-induced models of non-insulin-dependent diabetes mellitus in the mouse

Various tissues were obtained from the well-characterized genetic model (C57BL/6J-ob/ob) of non-insulin-dependent diabetes mellitus (NIDDM) and from a diet-induced model of NIDDM produced in the same genetic background (C57BL/6J). The objectives were to determine whether the previously observed changes in guanine nucleotide-binding regulatory protein (G protein) expression in adipose tissue from ob/ob mice were mirrored by concomitant changes in other tissues, and whether NIDDM of a different etiology would share similar alterations in G protein expression. Plasma membranes from adipocytes, brain, heart, liver, and testes were probed with alpha-subunit-specific antisera, and the level of G protein expression in each model was compared with that in its lean littermate control. Adipose, heart, and liver cell membranes from ob/ob mice contained significantly less alpha-subunit of stimulatory G protein (Gs alpha) than those from their lean littermates. As compared with the lean littermates, heart alpha-subunit-2 of inhibitory G protein (Gi alpha-2), liver Gi alpha-3, and adipocyte G1 alpha-1 and Gi alpha-3 were also reduced in ob/ob mice. In contrast, Gi alpha-2 and Go alpha were increased over lean-control levels in brain tissue from ob/ob mice, whereas Gs alpha was unchanged. G protein expression in the testes did not differ between lean and ob/ob mice. In the diet-induced model of NIDDM, Gs alpha expression in the liver was twofold greater in obese/diabetic mice as compared with lean controls. However, G protein expression in all other tissues examined did not differ between obese/diabetic animals and lean littermates.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of two splice variant forms of type-IVB cyclic AMP phosphodiesterase, DPD (rPDE-IVB1) and PDE-4 (rPDE-IVB2) in brain: selective localization in membrane and cytosolic compartments and differential expression in various brain regions

In order to detect the two splice variant forms of type-IVB cyclic AMP phosphodiesterase (PDE) activity, DPD (type-IVB1) and PDE-4 (type-IVB2), anti-peptide antisera were generated. One set ('DPD/PDE-4-common'), generated against a peptide sequence found at the common C-terminus of these two PDEs, detected both PDEs. A second set was PDE-4 specific, being directed against a peptide sequence found within the unique N-terminal region of PDE-4. In brain, DPD was found exclusively in the cytosol and PDE-4 exclusively associated with membranes. Both brain DPD and PDE-4 activities, isolated by immunoprecipitation, were cyclic AMP-specific (KmcyclicAMP: approximately 5 microM for DPD; approximately 4 microM for PDE-4) and were inhibited by low rolipram concentrations (K1rolipram approximately 1 microM for both). Transient expression of DPD in COS-1 cells allowed identification of an approx. 64 kDa species which co-migrated on SDS/PAGE with the immunoreactive species identified in both brain cytosol and membrane fractions using the DPD/PDE-4-common antisera. The subunit size observed for PDE-4 (approx. 64 kDa) in brain membranes was similar to that predicted from the cDNA sequence, but that observed for DPD was approx. 4 kDa greater. Type-IV, rolipram-inhibited PDE activity was found in all brain regions except the pituitary, where it formed between 30 and 70% of the PDE activity in membrane and cytosolic fractions when assayed with 1 microM cyclic AMP, PDE-4 formed 40-50% of the membrane type-IV activity in all brain regions save the midbrain (approx. 20%). DPD distribution was highly restricted to certain regions, providing approx. 35% of the type-IV cytosolic activity in hippocampus and 13-21% in cortex, hypothalamus and striatum with no presence in brain stem, cerebellum, midbrain and pituitary. The combined type-IVB PDE activities of DPD and PDE-4 contributed approx. 10% of the total PDE activity in most brain regions except for the pituitary (zero) and the mid-brain (approx. 3%. The isolated cDNAs for DPD and PDE-4 appear to reflect transcription products which are expressed in vivo in brain. The unique N-terminal domain of PDE-4 is suggested to target this PDE to membranes in brain. Type-IVB PDEs are differentially expressed in various brain regions, indicating that there are tissue-specific controls on both the expression of the gene and the splicing of its products.

Evidence from studies with hepatocyte suspensions that store-operated Ca2+ inflow requires a pertussis toxin-sensitive trimeric G-protein

The role of heterotrimeric GTP-binding proteins in the process of store-operated Ca2+ inflow in hepatocytes was investigated by testing the ability of pertussis toxin to inhibit thapsigargin- and 2,5-di-tert-butylhydroquinone (DBHQ)-induced bivalent cation inflow. Hepatocytes isolated from rats treated with pertussis toxin for 24 h exhibited markedly inhibited rates of both Ca2+ and Mn2+ inflow when these were stimulated by vasopressin, angiotension II, epidermal growth factor, thapsigargin and DBHQ. Pertussis toxin had little effect on the basal intracellular free Ca2+ concentration ([Ca2+]i), basal rates of Ca2+ and Mn2+ inflow, the abilities of vasopressin, angiotensin II, thapsigargin and DBHQ to induce the release of Ca2+ from intracellular stores, and the maximum value of [Ca2+]i reached following agonist-induced release of Ca2+ from intracellular stores. It is concluded that store-operated Ca2+ inflow in hepatocytes employs a slowly ADP-ribosylated trimeric GTP-binding protein and is the physiological mechanism, or one of the physiological mechanisms, by which vasopressin and angiotensin stimulate plasma membrane Ca2+ inflow in this cell type.

Bromocriptine treatment increases lipolysis and steady-state levels of G proteins in adipocytes from lactating rats

An increase in the rate of lipogenesis and a decrease in the lipolytic response to catecholamines can be observed in adipocytes after weaning or litter removal. Bromocriptine treatment of lactating rats also produces an increase in lipogenesis but fails to decrease the lipolytic response of adipocytes to catecholamines seen after weaning or litter removal. No changes in total number or affinity of beta-adrenergic receptors or adenosine A1 receptors were detected by bromocriptine treatment using radioligand binding assays. However, we observed an increase in the amount of radioactivity from [32P]NAD+ incorporated into alpha-Gs (192 +/- 26%) and alpha-Gi (178 +/- 33%) by cholera- and pertussis-toxin-catalyzed ADP-ribosylation, respectively, with the same treatment, Immunoblotting using RM/1 and AS/7 antisera, which specifically recognize alpha-Gs and alpha-Gi 1,2, respectively, confirmed the increase in the steady-state levels of these G-protein alpha-subunits. The increase in the steady-state levels of alpha-Gs may account, at least in part, for the increased lipolytic response of adipocytes to catecholamines in bromocriptine-treated rats.

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.

A role for a pertussis toxin-sensitive trimeric G-protein in store-operated Ca2+ inflow in hepatocytes

The mechanism of store-operated Ca2+ inflow in hepatocytes was investigated using fluo-3 and fura-2 to monitor changes in the concentration of intracellular free Ca2+ in single cells, and 1-(alpha-glycerophosphoryl)-myo-inositol 4,5-diphosphate, P4(5)-1-(2-nitrophenyl)ethyl ester ('caged' GPIP2) and 'caged' guanosine 5'-[gamma thio]triphosphate (GTP gamma S) (introduced into the cytoplasmic space by microinjection), thapsigargin and 2,5-di-tert- butylhydroquinone (DBHQ) to stimulate Ca2+ inflow. Photolysis of 'caged' GPIP2 or 'caged' GTP gamma S stimulated Ca2+ inflow. The abilities of GPIP2, thapsigargin and DBHQ to stimulate Ca2+ inflow were inhibited by the pre-treatment of hepatocytes with pertussis toxin in vivo for 36 h. Thapsigargin-stimulated Ca2+ inflow was also inhibited by guanosine 5'-[beta-thio]diphosphate (GDP beta S) (introduced by microinjection). It is concluded that, in hepatocytes, store-operated Ca2+ inflow induced by the actions of either inositol 1,4,5-trisphosphate, thapsigargin or DBHQ requires a pertussis toxin-sensitive trimeric G-protein.

A slowly ADP-ribosylated pertussis-toxin-sensitive GTP-binding regulatory protein is required for vasopressin-stimulated Ca2+ inflow in hepatocytes

The roles of heterotrimeric GTP-binding regulatory proteins (G-proteins) and inositol polyphosphates in the mechanism by which vasopressin stimulates Ca2+ inflow in hepatocytes were investigated by using single cells loaded with fura2 by microinjection. Vasopressin-stimulated Ca2+ inflow was mimicked by microinjection of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) or guanosine 5'-[beta gamma-imido]triphosphate to the cells, but not adenosine 5'-[gamma-thio]triphosphate (ATP[S]) or guanosine 5'-[beta-thio]diphosphate (GDP[S]). Extracellular Gd3+ (5 microM) inhibited both vasopressin- and GTP[S]-stimulated Ca2+ inflow. GDP[S], but not GMP, administered to hepatocytes by microinjection, completely inhibited vasopressin-stimulated Ca2+ inflow and partially inhibited vasopressin-induced release of Ca2+ from intracellular stores. The microinjection of pertussis toxin had no effect either on the release of Ca2+ from intracellular stores or on Ca2+ inflow induced by vasopressin, but completely inhibited changes in these processes induced by epidermal growth factor (EGF). Hepatocytes isolated from rats treated with pertussis toxin for 24 h exhibited no vasopressin- or GTP[S]-stimulated Ca2+ inflow, whereas the vasopressin-stimulated release of Ca2+ from intracellular stores was similar to that observed for control cells. Heparin or ATP[S] inhibited, or delayed the onset of, both vasopressin-induced release of Ca2+ from intracellular stores and vasopressin-stimulated Ca2+ inflow. Vasopressin-induced oscillations in intracellular [Ca2+] were observed in some heparin-treated cells. It is concluded that the stimulation by vasopressin of Ca2+ inflow to hepatocytes requires inositol 1,4,5-trisphosphate (InsP3) and, by implication, the pertussis-toxin-insensitive G-protein required for the activation of phospholipase C beta [Taylor, Chae, Rhee and Exton (1991) Nature (London) 350, 516-518], and another G-protein which is slowly ADP-ribosylated by pertussis toxin and acts between InsP3 and the putative plasma-membrane Ca2+ channel. EGF-stimulated Ca2+ inflow involves at least one G-protein which is rapidly ADP-ribosylated and is most likely required for InsP3 formation.

Guanine nucleotide regulatory protein alterations in young Milan hypertensive strain rats

Vascular smooth muscle cell membranes from prehypertensive rats of the Milan hypertensive strain (MHS) were used to examine adenylyl cyclase activity and its regulation by guanine nucleotide regulatory proteins (G-proteins). Basal adenylyl cyclase activity was similar in MHS and Milan normotensive strain (MNS) membranes. Forskolin (10(-4) M) produced a significantly greater stimulatory response in MHS membranes, but this was not observed with NaF (10(-2) M). Isoproterenol (10(-4) M) caused a significantly decreased stimulation of adenylyl cyclase activity in MHS membranes, while prostaglandin E1 (10(-5) M) produced similar responses in the two strains. Gi function and GTP responses, as observed by biphasic effects of GTP on isoproterenol-stimulated membranes, were similar in both strains. The levels of Gi2 alpha and Gq alpha/G11 alpha were similar in the two strains, while the levels of Gs alpha (44 and 42 kDa forms) and the beta-subunit were significantly reduced by approximately 20% in MHS membranes. The alpha-subunit of Gi3 was dramatically reduced by approximately 80% in MHS membranes. The affinities of beta-adrenergic receptors for the antagonist, cyanopindolol, were similar in the two strains; however, the number of beta-adrenoceptors was substantially reduced in MHS membranes. These findings may be of relevance to altered vascular reactivity and transmembrane ion distribution observed in the MHS.

Regulation of hepatocyte adenylate cyclase by amylin and CGRP: a single receptor displaying apparent negative cooperatively towards CGRP and simple saturation kinetics for amylin, a requirement for phosphodiesterase inhibition to observe elevated hepatocyte cyclic AMP levels and the phosphorylation of Gi-2

Challenge of intact hepatocytes with amylin only succeeded in elevating intracellular cyclic AMP levels and activating phosphorylase in the presence of the cAMP phosphodiesterase inhibitor IBMX. Both amylin and CGRP similarly activated adenylate cyclase, around 5-fold, although approximately 400-fold higher levels of amylin were required to elicit half maximal activation. Amylin activated adenylate cyclase though apparently simple Michaelien kinetics whereas CGRP elicited activation by kinetics indicative of apparent negative co-operativity. Use of the antagonist CGPP(8-37) showed that both CGRP and amylin activated hepatocyte adenylate cyclase through a common receptor by a mnemonical mechanism where it was proposed that the receptor co-existed in interconvertible high and low affinity states for CGRP. It is suggested that this model may serve as a paradigm for G-protein linked receptors in general. Amylin failed to both stimulate inositol phospholipid metabolism in hepatocytes and to elicit the desensitization of glucagon-stimulated adenylate cyclase. Amylin did, however, elicit the phosphorylation of the inhibitory guanine nucleotide regulatory protein Gi-2 in hepatocytes and prevented the action of insulin in reducing the level of phosphorylation of this G-protein.

Bradykinin-dependent activation of adenylate cyclase activity and cyclic AMP accumulation in tracheal smooth muscle occurs via protein kinase C-dependent and -independent pathways

Treatment of cultured tracheal smooth-muscle cells (TSM) with phorbol 12-myristate 13-acetate (PMA) (100 nM) or bradykinin (100 nM) elicited enhanced basal and guanosine 5'-[beta gamma-imido]-triphosphate-stimulated adenylate cyclase activities in subsequently isolated membranes. Combined stimulation of cells was non-additive, indicating that both agents activate adenylate cyclase via similar routes. Both PMA (100 nM) and bradykinin (100 nM) allowed the alpha subunit of Gs to act as a more favourable substrate for its cholera-toxin-catalysed ADP-ribosylation in vitro. PMA was without effect on intracellular cyclic AMP in control cells. However, constitutive activation of Gs by treatment in vivo with cholera toxin (0.5 ng/ml, 18 h) sensitized the cells to PMA stimulation, resulting in a concentration-dependent increase in intracellular cyclic AMP accumulation (EC50 = 7.3 +/- 2.5 nM, n = 5). Bradykinin also elicited a concentration-dependent increase in intracellular cyclic AMP (EC50 = 63.3 +/- 14.5 nM, n = 3). Constitutive activation of Gs resulted in an increased maximal response (10-fold) and potency (EC50 = 6.17 +/- 1.6 nM, n = 3) to bradykinin. This response was not affected by the B2-receptor antagonist, NPC567 [which selectively blocks bradykinin-stimulated phospholipase C (PLC), with minor activity against phospholipase D (PLD) activity]. Des-Arg9-bradykinin (a B1-receptor agonist) was without activity. These results suggest that the receptor sub-type capable of activating PLD may also be stimulatory for cyclic AMP accumulation. Furthermore, pre-treatment of the cells with butan-l-ol (0.3%, v/v), which traps phosphatidate derived from PLD reactions, blocked the bradykinin-stimulated increase in intracellular cyclic AMP. These studies suggest that there may be a causal link between PLD-derived phosphatidate and the positive modulation of adenylate cyclase activity. In support of this, the concentration-dependence for bradykinin-stimulated adenylate cyclase activity was identical with that of bradykinin-stimulated phospholipase D activity (EC50 = 5 nM). Bradykinin, but not PMA, was also capable of eliciting the inhibition of cyclic AMP phosphodiesterase activity in TSM cells (EC50 > 100 nM) via an unidentified mechanism. These studies indicate that cross-regulation between the cyclic AMP pathway and phospholipid-derived second messengers in TSM cells does not occur as a consequence of PLC-catalysed PtdIns(4,5)P2 hydrolysis, but may involve, in part, PLD-catalysed phosphatidylcholine hydrolysis.

GTP-binding protein-activator sequences in the insulin receptor

Some functions of the insulin receptor (insR) are assumed to be mediated by pertussis toxin-sensitive Gi/G(o) proteins. Here we have located G-protein-activator domains in the cytoplasmic region of the human insR. We searched the sequence of insR and found three candidate regions at residues 1039-1061, 1147-1168 and 1325-1345, referred to as ISRP1, ISRP2 and ISRP3, respectively. Among them, the Gi/G(o)-activating function was observed only in peptide ISRP3. ISRP1 specifically activated Gs, whereas ISRP2 had no effect on G proteins. ISRP2 and ISRP3 contained five of six autophosphorylated tyrosine residues in insR. After tyrosine phosphorylation, ISRP2 showed specific Gi-activating function, and ISRP3 potentiated its ability and became capable of activating G proteins generally. This is the first study that specifies G-protein-activator domains in insR and describes their modification by autophosphorylation.

Age-dependent changes in transmembrane signalling: identification of G proteins in human lymphocytes and polymorphonuclear leukocytes

In human neutrophils (PMNLs) we found that in the elderly IP3 formation was significantly decreased compared to that of young subjects. For FMLP receptor binding affinity and number no measurable differences occurred upon ageing, studying both the low or the high affinity receptors. The amount of ADP-ribosylated G proteins, catalysed by pertussis toxin (PT) or cholera toxin (CT), was significantly increased in PMNLs of the elderly. In lymphocytes, the PT-catalysed ADP ribosylation of G proteins was also increased with ageing, while the CT-catalysed ribosylation was decreased. The autoradiogram of [32P]ADP-ribosylated proteins by CT in lymphocytes of young individuals showed a major polypeptide of 40,000 M(r). In contrast, in lymphocytes of the elderly, the major polypeptide was 45,000 M(r). In PMNLs, CT labelled quite strongly the 45,000 M(r) band, mainly in the elderly. When PT was used, no age-related pattern changes could be demonstrated, while differences could be observed between the two types of cells. The use of antiserum P680 (G alpha common) showed no age-related pattern changes, while the intensity of the labelled proteins varies with age and cell type. The antiserum U46 (Go alpha) could identify in lymphocytes of young subjects two polypeptides 68,000 and 41,000 M(r). The prominent polypeptide in lymphocytes of the elderly was the 70,000 M(r) and no other polypeptides could be recognized. In PMNLs of young subjects the U46 and serum identified a range of species. In PMNLs of the elderly all these bands were weakly labelled. The present data indicate changes in the pattern and the quantity of G proteins in lymphocytes and PMNLs of elderly subjects.

Diabetes induces selective alterations in the expression of protein kinase C isoforms in hepatocytes

Membrane and cytosol fractions from hepatocytes of both normal and streptozotocin-induced diabetic animals were probed with a panel of polyclonal anti-peptide antisera in order to identify protein kinase C (PKC) isoforms. Immunoreactive species were noted with antisera specific for alpha (approximately 81 kDa), beta-II (approximately 82 kDA), epsilon (approximately 95 kDa) and epsilon (approximately 79 kDa). In addition, a species migrating with an apparent size of approximately 94 kDa was also detected in cytosol fractions using an antiserum specific for PKC-alpha. Each of these species was specifically displaced when the PKC-isoform specific peptide was included in the immunodetection system. No immunoreactive species consistent with the presence of the beta-I, gamma, delta and eta isoforms of protein kinase C was observed. Induction of diabetes using streptozotocin invoked selective alterations in the expression of PKC isoforms which were reversed upon insulin therapy. In the cytosol fraction, marked increases of approximately 3-fold occurred in levels of the beta-II isoform and the approximately 90 kDa (upper) form of PKC-alpha, with no apparent/little change in the levels of the approximately 81 kDa (lower) form of PKC-alpha and those of PKC-zeta. Diabetes induction also appeared to have elicited the translocation of PKC-beta-II and the approximately 81 kDa (lower) form of PKC-alpha to the membrane fraction where immunoreactivity for these species was now apparent. The level of PKC-epsilon, which was noted only in membrane fractions, was also increased upon induction of diabetes. It is suggested that the selective alterations in the expression of PKC isoforms occurring upon streptozotocin-induced diabetes may lead to altered cellular functioning and underly defects in inhibitory G-protein functioning and insulin action which characterise this animal model of diabetes.

Adenylyl cyclase in lung from hypersensitive guinea pig displays increased responsiveness to guanine nucleotides and isoprenaline: the role of the G proteins Gs and Gi

Basal adenylyl cyclase activity in lung membranes isolated from hypersensitive guinea pigs was increased and more sensitive to stimulation by isoprenaline, GTP and GppNHp when compared to adenylyl cyclase in lung membranes isolated from normal healthy guinea pigs. Maximal forskolin-stimulated adenylyl cyclase activity was unaltered. There was no change in the immunological quantitative amounts of either alpha subunits of the G proteins GiII and Gs (G(o), GiI and GiIII were not present). Maximal pertussis-toxin- and cholera-toxin-catalyzed ADP-ribosylation of Gi alpha and Gs alpha respectively were not significantly altered. The addition of purified protein kinase C to isolated lung membranes resulted in the phosphorylation of the alpha subunit of Gs (stoichiometry was 0.53 mol of 32P incorporated/mol of Gs alpha). Addition of protein kinase C to lung membranes isolated from hypersensitive guinea pigs was equally effective at catalysing the phosphorylation of the alpha subunit of Gs. GppNHp-stimulated and basal adenylyl cyclase activity was also enhanced in isolated tracheal smooth-muscle membranes from hypersensitive guinea pigs. These results suggest that hypersensitive reactions are associated with the improved coupling of the stimulatory G protein (Gs) with adenylyl cyclase.

Altered cellular signalling and decreased platelet sensitivity to adenosine in insulin-dependent diabetic patients with proliferative retinopathy

Platelets from patients with insulin-dependent diabetes with proliferative retinopathy showed the same reactivity to ADP as those from control subjects. Responsiveness of platelets to the aggregation inhibitor adenosine and to the analogue N-ethylcarboxamidoadenosine was decreased in diabetes. In contrast, responsiveness to the anti-aggregatory effects of prostaglandin I2 was not significantly altered in diabetes. Platelets from diabetic patients exhibited decreased formation of cyclic AMP in response to N-ethylcarboxamidoadenosine compared with those from control subjects. In contrast, when adenylyl cyclase was stimulated by prostaglandin I2 or by forskolin, no differences in cyclic AMP formation were observed between control and diabetic platelets. Diabetes was associated with an apparent loss of high-affinity binding of [3H]N-ethylcarboxamidoadenosine to platelet membranes. Possible mechanisms that could contribute to this diabetes-induced change in signalling through the platelet A2 adenosine receptor are discussed.

Guanine nucleotide regulatory proteins in the spontaneously hypertensive rat

We compared guanine nucleotide regulatory protein (G protein) levels and function in plasma membranes from resistance vessels (mesenteric arteries) isolated from spontaneously hypertensive (SHR) and normotensive Wistar rats. G protein function was deduced from studies of adenylate cyclase activity. Although the basal level of adenylate cyclase activity (+/- Mn2+ ions) was significantly greater in SHR membranes, addition of agents that function via the stimulatory G protein--i.e., NaF (10(-2) M), (-)-isoproterenol (10(-4) M), and prostaglandin E1 (10(-5) M)--resulted in a significantly lower stimulatory response in SHR membranes. Ligands that function via the inhibitory G protein--i.e., adrenaline (10(-5) M)/propranolol (10(-5) M) (this combination being equivalent to an alpha 2-receptor agonist), carbachol (10(-3) M), and serotonin (10(-5) M)--were responsible for only slight inhibitory responses in both SHR and Wistar rat membranes, which were not significantly different. Western blotting identified the presence of Gs, Gi2, and Gi3 alpha-subunits in rat vascular smooth muscle, but there were no differences in the levels of these G protein alpha-subunits found in SHR and Wistar rat plasma membranes. The levels of the beta-subunit in the two sets of membranes were also similar. In conclusion, there is a reduced response in adenylate cyclase activity to agents that function via the stimulatory G protein in SHR membranes. However, this is not a consequence of altered levels of the different G protein subunits.

Diabetes-induced alterations of central nervous system G proteins. ADP-ribosylation, immunoreactivity, and gene-expression studies in rat striatum

Previous studies from our laboratory have suggested that diabetes-associated central nervous system abnormalities are characterized by progressive alterations of neurotransmitters and of transductional Gi/Go proteins. In this study, we have further characterized these abnormalities in the striatum of alloxan-diabetic rats by means of adenosine 5'-diphosphate (ADP)-ribosylation, and Western and Northern blotting techniques. Fourteen weeks after diabetes induction, pertussis-toxin (PTX) catalyzed ADP-ribosylation of Gi/Go proteins was markedly reduced in diabetic animals, as shown by a clear decrease of 32P-ADPribose incorporation into G protein alpha subunits. In agreement with our previous pharmacological studies that showed a reduction of Gi-mediated modulation of adenylate cyclase activity only at this stage of diabetes, no changes in PTX-mediated ADP-ribosylation were observed earlier (5-wk diabetes). Immunoblotting studies performed by using antibodies selectively raised against Gi-2, Go, and Gs proteins did not reveal any differences between control and diabetic animals at any stage of diabetes. Similarly, the mRNAs corresponding to the alpha subunits of Gi-2, Go, and Gs proteins did not show any marked changes in chronic diabetic rats with respect to control animals. It is therefore concluded that diabetes is associated with development of a time-related alteration of cerebral Gi/Go proteins and that this defect is not owing to gross changes in either content of G proteins or mRNA level, but probably reflects modifications of G protein's structure or physiological status affecting the coupling with membrane effector systems and the sensitivity to PTX.

Ethanol increases receptor-dependent cyclic AMP production in cultured hepatocytes by decreasing G(i)-mediated inhibition

Increasing evidence suggests that ethanol-induced changes in cyclic AMP (cAMP) signal transduction play a critical role in the acute and chronic effects of ethanol. Here we have investigated the effects of ethanol on cAMP signal transduction in primary cultures of rat hepatocytes. Acute exposure to ethanol had a biphasic effect on glucagon-receptor-dependent cAMP production in intact cells: 25-50 mM-ethanol decreased cAMP, whereas treatment with 100-200 mM-ethanol increased cAMP. After chronic exposure to 50-200 mM-ethanol for 48 h in culture, glucagon-receptor-dependent cAMP levels were increased, but no change in glucagon receptor number was observed. These effects of ethanol were independent of ethanol oxidation. Chronic ethanol treatment also increased adenosine-receptor- and forskolin-stimulated cAMP production. Increased cAMP production was also observed upon stimulation of adenylate cyclase with glucagon, forskolin and F- in membranes isolated from cells cultured with 100 mM-ethanol for 48 h. However, no differences were observed in basal and MnCl2-stimulated adenylate cyclase activity. The quantity of alpha i protein was decreased by 35% after chronic ethanol treatment, but no change in the quantity of alpha s protein was detected. Decreased alpha i protein was associated with a decrease in G(i) function, as assessed by the ability of 0.1 nM-guanosine 5'-[beta gamma-imido]triphosphate and 1 microM-somatostatin to inhibit forskolin-stimulated adenylate cyclase activity. Taken together, these results suggest that chronic exposure to ethanol increases receptor-dependent cAMP production in hepatocytes by decreasing the quantity of alpha i protein at the plasma membrane and thereby decreasing the inhibitory effects of G(i) on adenylate cyclase activity.

Guanine nucleotide regulatory protein levels and function in spontaneously hypertensive rat vascular smooth-muscle cells

We compared G-protein levels and function in membranes from vascular smooth-muscle cells (VSMC) derived from mesenteric arteries from SHR, WKY and Wistar rats. Basal adenylyl cyclase activity was significantly reduced in SHR membranes compared with Wistar, but was similar to WKY. Isoproterenol stimulation (10(-4) M) was significantly lower in SHR membranes compared to WKY, but was similar to that in Wistar, which was also significantly lower than WKY. Forskolin (10(-4) M) and NaF (10(-2) M), resulted in a higher stimulatory response in SHR membranes. Biphasic effects of GTP on isoproterenol-stimulated membranes demonstrated unaltered Gi function in SHR membranes. No significant differences were seen in the levels of Gs alpha (44- and 42-kDa forms), Gi2 alpha and the beta-subunit in immunoblotting studies of the membranes. Amounts of Gq alpha/G11 alpha and Gi3 alpha were also unchanged. In conclusion, there are differences in adenylyl cyclase responses in SHR VSMC membranes which are not a consequence of altered levels of G-proteins, but may reflect genetic differences rather than effects of hypertension.