A role for protein kinase C-mediated phosphorylation in eliciting glucagon desensitization in rat hepatocytes

Glucagon, cyclic AMP, and hepatic glucose mobilization: A half‐century of uncertainty

For at least 50 years, the prevailing view has been that the adenylate cyclase (AC)/cyclic AMP (cAMP)/protein kinase A pathway is the predominant signal mediating the hepatic glucose‐mobilizing actions of glucagon. A wealth of evidence, however, supports the alternative, that the operative signal most of the time is the phospholipase C (PLC)/inositol‐phosphate (IP3)/calcium/calmodulin pathway. The evidence can be summarized as follows: (1) The consensus threshold glucagon concentration for activating AC ex vivo is 100 pM, but the statistical hepatic portal plasma glucagon concentration range, measured by RIA, is between 28 and 60 pM; (2) Within that physiological concentration range, glucagon stimulates the PLC/IP3 pathway and robustly increases glucose output without affecting the AC/cAMP pathway; (3) Activation of a latent, amplified AC/cAMP pathway at concentrations below 60 pM is very unlikely; and (4) Activation of the PLC/IP3 pathway at physiological concentrations produces intracellular effects that are similar to those produced by activation of the AC/cAMP pathway at concentrations above 100 pM, including elevated intracellular calcium and altered activities and expressions of key enzymes involved in glycogenolysis, gluconeogenesis, and glycogen synthesis. Under metabolically stressful conditions, as in the early neonate or exercising adult, plasma glucagon concentrations often exceed 100 pM, recruiting the AC/cAMP pathway and enhancing the activation of PLC/IP3 pathway to boost glucose output, adaptively meeting the elevated systemic glucose demand. Whether the AC/cAMP pathway is consistently activated in starvation or diabetes is not clear. Because the importance of glucagon in the pathogenesis of diabetes is becoming increasingly evident, it is even more urgent now to resolve lingering uncertainties and definitively establish glucagon’s true mechanism of glycemia regulation in health and disease.

Glucagon Receptor Signaling and Glucagon Resistance

Hundred years after the discovery of glucagon, its biology remains enigmatic. Accurate measurement of glucagon has been essential for uncovering its pathological hypersecretion that underlies various metabolic diseases including not only diabetes and liver diseases but also cancers (glucagonomas). The suggested key role of glucagon in the development of diabetes has been termed the bihormonal hypothesis. However, studying tissue-specific knockout of the glucagon receptor has revealed that the physiological role of glucagon may extend beyond blood-glucose regulation. Decades ago, animal and human studies reported an important role of glucagon in amino acid metabolism through ureagenesis. Using modern technologies such as metabolomic profiling, knowledge about the effects of glucagon on amino acid metabolism has been expanded and the mechanisms involved further delineated. Glucagon receptor antagonists have indirectly put focus on glucagon's potential role in lipid metabolism, as individuals treated with these antagonists showed dyslipidemia and increased hepatic fat. One emerging field in glucagon biology now seems to include the concept of hepatic glucagon resistance. Here, we discuss the roles of glucagon in glucose homeostasis, amino acid metabolism, and lipid metabolism and present speculations on the molecular pathways causing and associating with postulated hepatic glucagon resistance.

Characterization of neurosteroid effects on hyperpolarizing current at α4β2δ GABAA receptors

RATIONALE

The neurosteroid 3α,5β-THP (3α-OH-5β-pregnan-20-one, pregnanolone) is a modulator of the GABAA receptor (GABAR), with α4β2δ GABARs the most sensitive. However, the effects of 3α,5β-THP at α4β2δ are polarity-dependent: 3α,5β-THP potentiates depolarizing current, as has been widely reported, but decreases hyperpolarizing current by accelerating desensitization.

OBJECTIVES

The present study further characterized 3α,5β-THP inhibition of hyperpolarizing current at this receptor and compared effects of other related steroids at α4β2δ GABARs.

METHODS

α4β2δ GABARs were expressed in HEK-293 cells, and agonist-gated current recorded with whole cell voltage-clamp techniques using a theta tube to rapidly apply agonist before and after application of neurosteroids.

RESULTS

The GABA-modulatory steroids (30 nM) 3α,5α-THP (3α-OH-5α-pregnan-20-one, allopregnanolone) and THDOC (3α,21-dihydroxy-5α-pregnan-20-one) inhibited hyperpolarizing GABA (10 μM)-gated current at α4β2δ GABARs similar to 3α,5β-THP, while the inactive 3β,5β-THP isomer had no effect. Greater inhibition was seen for current gated by the high efficacy agonist gaboxadol (THIP, 100 μM) than for GABA (0.1-1000 μM), consistent with an effect of 3α,5β-THP on desensitization. Inhibitory effects of the steroid were not seen under low [Cl(-)] conditions or in the presence of calphostin C (500 nM), an inhibitor of protein kinase C. Chimeras swapping the IL (intracellular loop) of α4 with α1, when expressed with β2 and δ, produced receptors (α[414]β2δ) which were not inhibited by 3α,5β-THP when GABA-gated current was hyperpolarizing, while α[141]β2δ exhibited steroid-induced polarity-dependent modulation.

CONCLUSIONS

These findings suggest that numerous neurosteroids exhibit polarity-dependent effects at α4β2δ GABARs, which are dependent upon protein kinase C and the IL of α4.

The metabolic actions of glucagon revisited

The initial identification of glucagon as a counter-regulatory hormone to insulin revealed this hormone to be of largely singular physiological and pharmacological purpose. Glucagon agonism, however, has also been shown to exert effects on lipid metabolism, energy balance, body adipose tissue mass and food intake. The ability of glucagon to stimulate energy expenditure, along with its hypolipidemic and satiating effects, in particular, make this hormone an attractive pharmaceutical agent for the treatment of dyslipidemia and obesity. Studies that describe novel preclinical applications of glucagon, alone and in concert with glucagon-like peptide 1 agonism, have revealed potential benefits of glucagon agonism in the treatment of the metabolic syndrome. Collectively, these observations challenge us to thoroughly investigate the physiology and therapeutic potential of insulin's long-known opponent.

High-fat diet-induced hepatic steatosis reduces glucagon receptor content in rat hepatocytes: potential interaction with acute exercise

Studies have revealed that high-fat (HF) diets promote hyperglycaemia, whole-body insulin resistance and non-alcoholic fatty liver disease (NAFLD). Recently, hepatic glucagon resistance has been shown to occur in rats fed a HF diet. More precisely, diet-induced obesity (DIO) reduces the number of hepatic plasma membrane glucagon receptors (GR), which results in a diminished response to glucagon during a hyperglucagonaemic clamp. The present study was undertaken to test the hypothesis that a HF-DIO is associated with a desensitization and destruction of the hepatic GR. We also hypothesized that a single bout of endurance exercise would modify the GR cellular distribution under our DIO model. Male rats were either fed a standard (SD) or a HF diet for two weeks. Each group was subdivided into a non-exercised (Rest) and an acute exercised (EX) group. The HF diet resulted in a reduction of total hepatic GR (55%) and hepatic plasma membrane GR protein content (20%). These changes were accompanied by a significant increase in endosomal and lysosomal GR content with the feeding of a HF diet. The reduction of GR plasma membrane as well as the increase in endosomal GR was strongly correlated with an increase of PKC-alpha, suggesting a role of PKC-alpha in GR desensitization. EX increased significantly PKC-alpha protein content in both diets, suggesting a role of PKC-alpha in EX-induced GR desensitization. The present results suggest that liver lipid infiltration plays a role in reducing glucagon action in the liver through a reduction in total cellular and plasma membrane GR content. Furthermore, the GR desensitization observed in our in vivo model of HF diet-induced hepatic steatosis and in EX individuals may be regulated by PKC-alpha.

Identification and characterization of a glucagon receptor from the goldfish Carassius auratus: implications for the evolution of the ligand specificity of glucagon receptors in vertebrates

The structural basis of ligand selectivity of G protein-coupled receptors for metabolic hormones has been an area of intense investigation, and yet it remains unresolved. One approach to delineating the mechanism of ligand-receptor interactions is to compare the ligand specificities of receptors expressed in species that emerged at different times within vertebrate evolution. In this paper we describe the isolation, functional, and phylogenetic characterization of the glucagon receptor from the goldfish Carassius auratus (Teleostei, order Cypriniformes), and compare its ligand specificity with that of the homologous rat receptor. Goldfish (gf) glucagon stimulated glucose production in a dose-dependent manner from isolated goldfish hepatocytes, resulting in 5-fold increase at 1 microm. The goldfish glucagon receptor (gfGlucR) shares 56, 51, 50, and 52% amino acid identities with frog Rana tigrina regulosa, mouse, rat, and human glucagon receptors, respectively. In competitive binding experiments, the recombinant gfGlucR displays high affinity toward goldfish, zebrafish, and human glucagons (IC(50) = 0.6, 9, and 13 nm, respectively) but not toward goldfish glucagon-like peptide-1 or human glucagon-like peptide-1 (7-36) amide. Whereas both goldfish and human glucagons stimulated dose-dependent increases in intracellular cAMP through the recombinant gfGlucR, the recombinant rat GlucR interacted only with human glucagon, analogous to the specificity of the previously characterized glucagon receptor from the frog R. tigrina regulosa. Our results demonstrate that the binding pocket of gfGlucR can accommodate a broad range of glucagon structures and that in the frogs and mammals, there is a structural switch to a more restrictive conformation of glucagon receptors.

Glucagon and vasopressin V1a receptor signaling in hepatocytes from aging rats

Glucose tolerance is reduced with age. The relationship between this change in glucose homeostasis and signaling of glucagon and vasopressin V1a receptors was investigated in hepatocytes isolated from 10- and 30-month-old female WAG/Rij rats. Binding capacity of hepatocytes for 125I glucagon and 3H vasopressin increased 2- and 1.8-fold, respectively, between 10 and 30 months. Intracellular cAMP accumulation induced by glucagon was 40% greater in hepatocytes of aging rats than of adults, although EC(50) were similar in the two groups. Conversely, phosphodiesterases activity and nucleotides leakage out of the cells were unchanged with age. The rise in intracellular calcium consecutive to the stimulation of V1a receptor was comparable in adult and senescent animals. Finally, glucose release by hepatocyte suspensions was greater in senescent than in adult animals in absence as in presence of glucagon. These experiments suggest that increase in glucagon receptor expression and cAMP generation would contribute to the impaired glucose tolerance characteristic of the aging process.

Specific features of glycogen metabolism in the liver

Although the general pathways of glycogen synthesis and glycogenolysis are identical in all tissues, the enzymes involved are uniquely adapted to the specific role of glycogen in different cell types. In liver, where glycogen is stored as a reserve of glucose for extrahepatic tissues, the glycogen-metabolizing enzymes have properties that enable the liver to act as a sensor of blood glucose and to store or mobilize glycogen according to the peripheral needs. The prime effector of hepatic glycogen deposition is glucose, which blocks glycogenolysis and promotes glycogen synthesis in various ways. Other glycogenic stimuli for the liver are insulin, glucocorticoids, parasympathetic (vagus) nerve impulses and gluconeogenic precursors such as fructose and amino acids. The phosphorolysis of glycogen is mainly mediated by glucagon and by the orthosympathetic neurotransmitters noradrenaline and ATP. Many glycogenolytic stimuli, e.g. adenosine, nucleotides and NO, also act indirectly, via secretion of eicosanoids from non-parenchymal cells. Effectors often initiate glycogenolysis cooperatively through different mechanisms.

Varying very low-density lipoprotein secretion of rat hepatocytes by altering cellular levels of calcium and the activity of protein kinase C

BACKGROUND

Calcium antagonists lower plasma levels of lipoproteins and suppress hepatic very low-density lipoprotein (VLDL) secretion. Similar effects have been observed with the calcium ionophore A23187. We studied further the effect of calcium on VLDL metabolism.

METHODS

Hepatocytes from male Wistar rats were isolated and cultured in the presence or absence of calcium-mobilizing hormones, or compounds that either stimulate or inhibit the activity of protein kinase C. Secreted VLDL (d < 1.006 g mL-1) was isolated by centrifugation (145,000 x g), and lipids and apolipoprotein B were analysed.

RESULTS

VLDL secretion reached maximum in hepatocytes cultured in medium containing calcium 0.8-2.4 mmolL-1. Depleting the cells of calcium by incubating in calcium-free medium or by treating the cells with the Ca(2+)-ATPase inhibitor thapsigargin (5 x 10-7 molL-1) suppressed lipid secretion to less than 15% of control, and this was accompanied by an increase in cellular levels of triacylglycerol. Calcium loading (medium calcium > 2.4 mmolL-1) suppressed both lipoprotein secretion and cellular levels of lipids, suggesting a reduced overall rate of lipid synthesis. At an extracellular calcium concentration of 0.8 mmolL-1, angiotensin II, vasopressin, endothelin-1 (10(-7) molL-1) or phenylephrine (10(-4) molL-1) suppressed VLDL secretion (maximum to 37% of control), and elevated medium calcium attenuated this effect. The protein kinase C inhibitor chelerythrine (5 x 10(-5) molL-1) and the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) (10(-6) molL-1), suppressed VLDL secretion to 18% and 60% of control, respectively, whereas the protein kinase C-inactive 4 alpha-PMA was without an effect. No effect on ketogenesis was observed by these compounds, indicating that suppressed lipid secretion was not due to an enhanced oxidation of lipids.

CONCLUSIONS

Hepatic VLDL secretion can be related to changes in hepatocyte levels of calcium and the activity of protein kinase C.

Cyclic AMP decreases the availability of 5-phosphoribosyl-1-pyrophosphate and decelerates de novo purine synthesis in rat hepatocytes

Cyclic adenosine monophosphate (cAMP) was found to decrease the availability of 5-phosphoribosyl-1-pyrophosphate (PRPP) and to decelerate the rate of de novo purine synthesis in suspensions of adult rat hepatocytes. Glucagon did not affect these parameters. The glucagon antagonist des-His1[Glu9]glucagon amide (DHGA), and the protein kinase C activator 1,2-dioctanoyl-sn-glycerol (DOG) were also found to lower PRPP availability. Incubation of the hepatocytes with dbcAMP or with DHGA, did not alter the activity of PRPP synthetase in the hepatocyte lysates, indicating that the above effects are not mediated through the activity of this enzyme. The possibility that the decrease in PRPP availability reflects increased consumption associated with accelerated pyrimidine synthesis is discussed. The decelerated rate of de novo purine synthesis is probably secondary to the decreased PRPP availability.

Stimulation of p70S6 kinase via a growth hormone-controlled phosphatidylinositol 3-kinase pathway leads to the activation of a PDE4A cyclic AMP-specific phosphodiesterase in 3T3-F442A preadipocytes

The challenge of 3T3-F442A fibroblasts with growth hormone led to both a decrease in the mobility on SDS/PAGE and activation of the PDE4A cyclic AMP-specific phosphodiesterase isoform PDE4A5. Activation was mediated by a JAK-2-dependent pathway coupled to the activation of phosphatidylinositol 3-kinase and p70S6 kinase. Activation was not dependent on the ability of growth hormone to stimulate ERK2 or protein kinase C or any effect on transcription. Blockade of activation of murine PDE4A5 ablated the ability of growth hormone to decrease intracellular cAMP levels. Antisense depletion of murine PDE4A5 mimicked the ability of rolipram to enhance the growth hormone-stimulated differentiation of 3T3-F442A cells to adipocytes. It is suggested that activation of PDE4A5 by growth hormone serves as a brake on the differentiation processes.

Co-transfection with protein kinase D confers phorbol-ester-mediated inhibition on glucagon-stimulated cAMP accumulation in COS cells transfected to overexpress glucagon receptors

Glucagon elicited a profound increase in the intracellular cAMP concentration of COS-7 cells which had been transiently transfected with a cDNA encoding the rat glucagon receptor and under conditions where cAMP phosphodiesterase activity was fully inhibited. This was achieved in a dose-dependent fashion with an EC50 of 1.8+/-0.4 nM glucagon. In contrast with previous observations made using hepatocytes [Heyworth, Whetton, Kinsella and Houslay (1984) FEBS Lett. 170, 38-42], treatment of transfected COS-7 cells with PMA did not inhibit the ability of glucagon to increase intracellular cAMP levels. PMA-mediated inhibition was not conferred by treatment with okadaic acid, nor by co-transfecting cells with cDNAs encoding various protein kinase C isoforms (PKC-alpha, PKC-betaII and PKC-epsilon) or with the PMA-activated G-protein-receptor kinases GRK2 and GRK3. In contrast, PMA induced the marked inhibition of glucagon-stimulated cAMP production in COS-7 cells that had been co-transfected with a cDNA encoding protein kinase D (PKD). Such inhibition was not due to an action on the catalytic unit of adenylate cyclase, as forskolin-stimulated cAMP production was unchanged by PMA treatment of COS cells that had been co-transfected with both the glucagon receptor and PKD. PKD transcripts were detected in RNA isolated from hepatocytes but not from COS-7 cells. Transcripts for GRK2 were present in hepatocytes but not in COS cells, whereas transcripts for GRK3 were not found in either cell type. It is suggested that PKD may play a role in the regulation of glucagon-stimulated adenylate cyclase.

Pituitary adenylate cyclase-activating polypeptide induces expression of corticosteroid-binding globulin in cultured fetal hepatocytes: synergy with tri-iodothyronine

The purpose of the present study was to determine whether functional receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) are expressed in cultured rat fetal hepatocytes and eventually play a role in regulating gene expression of corticosteroid-binding globulin (CBG). We found PACAP38 and PACAP27 to elevate cAMP levels in hepatocytes in a dose-dependent manner, with a plateau being achieved at 10 nM and EC50 values of about 0.5-1 nM. PACAP failed to alter the turnover of inositol phosphates, whereas PACAP and VIP stimulated cAMP accumulation in an equipotent manner, suggesting the presence in these cells of type II receptor isoforms. As revealed by measurements of both CBG mRNA levels and concentrations of binding sites, long-term treatment of fetal cells with 10 nM PACAP, although resulting in partial desensitization of peptide-induced cAMP accumulation, caused a significant 3-fold elevation in CBG synthesis. This stimulatory influence of PACAP was mimicked by the cell permeant N6,2'-O-dibutyryladenosine 3',5'-phosphate (dbcAMP). Treatment of hepatocytes with tri-iodothyronine (T3) enhanced CBG expression and, most interestingly, appeared to synergize with PACAP to elicit a 2-3-fold amplification of CBG synthesis. This study thus provides first evidence for the up-regulation by PACAP and cAMP of CBG expression in fetal hepatocytes and for T3's playing a synergistic role in enhancing PACAP-induced synthesis of the binder.

Insulin and vasopressin elicit inhibition of cholera-toxin-stimulated adenylate cyclase activity in both hepatocytes and the P9 immortalized hepatocyte cell line through an action involving protein kinase C

Incubation of hepatocytes or the SV40-DNA-immortalized hepatocyte P9 cell line with cholera toxin led to a time-dependent activation of adenylate cyclase activity, which occurred after a defined lag period. When added together with cholera toxin, each of the hormones insulin and vasopressin was capable of attenuating the maximum stimulatory effect achieved by cholera toxin over a period of 60 min through a process which could be blocked by the compounds staurosporine and chelerythrine. Attenuating effects on cholera-toxin-stimulated adenylate cyclase activity could also be elicited by using either the protein kinase C (PKC)-stimulating phorbol ester PMA (phorbol 12-myristate 13-acetate) or the protein phosphatase inhibitor okadaic acid. Alkaline phosphatase treatment of membranes reversed the inhibitory effect of PMA. Cholera toxin also stimulated the adenylate cyclase activity of intact CHO (Chinese-hamster ovary) and NIH-3T3 cells, but this activity was insensitive to the addition of PMA. Overexpression of various PKC isoforms in CHO cell lines did not confer sensitivity to inhibition by PMA upon cholera-toxin-stimulated adenylate cyclase activity. Rather, overexpression of the gamma isoform of PKC allowed PMA to stimulate adenylate cyclase activity in CHO cells. It is suggested that the PKC-mediated phosphorylation of a membrane protein attenuates cholera-toxin-stimulated adenylate cyclase activity in hepatocytes and P9 cells. The cellular selectivity of such an action may be due to the target for this inhibitory action of PKC being a particular isoform of adenylate cyclase which provides the major activity in hepatocytes and P9 cells, but not in either CHO or NIH-3T3 cells.