Deletion of Gab1 in the liver leads to enhanced glucose tolerance and improved hepatic insulin action

Insulin receptor substrate-1 (IRS-1) and IRS-2 are known to transduce and amplify signals emanating from the insulin receptor. Here we show that Grb2-associated binder 1 (Gab1), despite its structural similarity to IRS proteins, is a negative modulator of hepatic insulin action. Liver-specific Gab1 knockout (LGKO) mice showed enhanced hepatic insulin sensitivity with reduced glycemia and improved glucose tolerance. In LGKO liver, basal and insulin-stimulated tyrosine phosphorylation of IRS-1 and IRS-2 was elevated, accompanied by enhanced Akt/PKB activation. Conversely, Erk activation by insulin was suppressed in LGKO liver, leading to defective IRS-1 Ser612 phosphorylation. Thus, Gab1 acts to attenuate, through promotion of the Erk pathway, insulin-elicited signals flowing through IRS and Akt proteins, which represents a novel balancing mechanism for control of insulin signal strength in the liver.     

Oral glucose decreases hepatic extraction of insulin.

Peripheral venous (plasma) insulin and C-peptide concentrations were measured in eight normal subjects given oral or intravenous glucose sufficient to produce similar plasma glucose concentrations. The expected increased insulin response to oral as compared with intravenous glucose was not matched by a comparable increase in C-peptide concentration. The ratio of insulin to C-peptide concentrations doubled 30 minutes after oral glucose was given; no comparable rise was seen with intravenous glucose (p = 0.01). This finding is interpreted as evidence for decreased hepatic extraction of insulin after administration of oral glucose. Such a decrease could account for at least half of the well known difference in peripheral insulin concentrations after administration of oral as compared with intravenous glucose.     

Effects of oestrogen on adenylate cyclase system and glucose output in rat liver.

Effects of chronic oestrogen treatment on catecholamine- and glucagon-sensitive adenylate cyclase activity and glucose output in hepatocytes of castrated male rats were studied. In hepatocytes from male intact or castrated rats, the beta-adrenergic agonist isoprenaline did not stimulate adenylate cyclase activity and glycogenolysis, but glucagon markedly stimulated all these activities. Treatment of castrated animals with 17 beta-oestradiol for 7 days led to the appearance of beta-adrenergic-stimulated increases in both cyclic AMP generation and glucose output. The basal, glucagon- or fluoride-stimulated activities of adenylate cyclase of hepatic membranes prepared from oestrogen-treated rats were similar to those of control animals. Treatment with oestrogen did not influence the number or affinity of beta-adrenergic receptors. In hepatic plasma membranes from control rats, GTP failed to decrease the affinity of beta-adrenergic receptors for agonists, whereas the GTP-induced shift was apparently observed in those from oestrogen-treated animals. These results suggest that oestrogen is able to facilitate the coupling of hepatic beta-adrenergic receptors to the enzyme by increasing the effectiveness of receptor-guanine nucleotide regulation.     

Dual role of the coactivator TORC2 in modulating hepatic glucose output and insulin signaling

Under fasting conditions, the cAMP-responsive CREB coactivator TORC2 promotes glucose homeostasis by stimulating the gluconeogenic program in liver. Following its nuclear translocation in response to elevations in circulating glucagon, TORC2 regulates hepatic gene expression via an association with CREB on relevant promoters. Here, we show that, in parallel with their effects on glucose output, CREB and TORC2 also enhance insulin signaling in liver by stimulating expression of the insulin receptor substrate 2 (IRS2) gene. The induction of hepatic IRS2 during fasting appears critical for glucose homeostasis; knockdown of hepatic IRS2 expression leads to glucose intolerance, whereas hepatic IRS2 overexpression attenuates the gluconeogenic program and reduces fasting glucose levels. By stimulating the expression of IRS2 in conjunction with gluconeogenic genes, the CREB:TORC2 pathway thus triggers a feedback response that limits glucose output from the liver during fasting.     

Hyperhomocysteinemia stimulates hepatic glucose output and PEPCK expression

Homocysteine is an intermediate in the sulfur amino acid metabolism. Recent studies suggested that there might be links between hyperhomocysteinemia and insulin resistance. In the present study, we investigated the effect of homocysteine on glucose metabolism. We demonstrated that the levels of insulin were significantly higher in mice with hyperhomocysteinemia than those in the normal mice after administration of glucose. The effect of insulin on glucose output was significantly blocked in the homocysteine-treated hepatocytes. In addition, the expression of phosphoenolpyruvate carboxykinase （PEPCK） gene was elevated in the liver of mice with hyperhomocysteinemia and primary mouse hepatocytes treated with homocysteine. The action of homocysteine was suppressed by H89, a protein kinase A （PKA） inhibitor. Thus, hyperhomocysteinemia may be considered as a risk factor that contributes to the development of insulin resistance with respect to elev- ated glucose output and upregulation of PEPCK, probably via the PKA pathway. Our study provides a novel mechanistic explanation for the development of insulin resistance in hyperhomocysteinemia.     

Sensitivity and responsiveness of glucose output to insulin in isolated perfused liver from dexamethasone-treated rats

To elucidate insulin action on hepatic glucose output (glycogenolysis) in the state exposed to an excess glucocorticoid, the fed rat liver was isolated and cyclically perfused with a medium containing 5 mM glucose and various concentrations of insulin. The rat was subcutaneously injected with 1 mg/kg of dexamethasone (Dex) for 7 days. Dex-treated rats showed marked increases of serum insulin and plasma glucose level compared with those in control rats. Hepatic glycogen contents in Dex group were markedly increased compared with those in control (115 +/- 5 and 28 +/- 4 mg/g, respectively). Insulin extraction rate in the perfused liver was not different between control and Dex group. Perfusate glucose level after 60 min perfusion was much higher in the Dex-treated rat liver than that of the control at 0 microU/ml insulin (34.5 +/- 2.5 vs 23.0 +/- 2.0 mM, P less than 0.01), and reduced to the nadir level (19.0 +/- 3.0 and 13.0 +/- 1.5 mM, respectively) at 100 microU/ml insulin in both groups, i.e., the decreasing rate in perfusate glucose level was not different between Dex and control group (43% and 44%, respectively). These results suggest that Dex-treatment augments hepatic glucose output, but does not affect the sensitivity and responsiveness of that to insulin.     

Impact of a single insulin treatment (imprinting) applied during liver regeneration on hepatic insulin receptor development, blood glucose level and liver function parameters in adult rats.

Rats subjected to partial hepatectomy (surgical removal of two thirds of the liver) showed no appreciable change in serum cholesterol, bilirubin, albumin, total protein and A/G values at 2, 5, 12 and 21 days after the intervention. The enzyme activities characteristic of liver damage (GOT, GPT, LDH, AP) were high in the control group and low in the insulin-imprinted group at 2 days, tended to normalize in both groups at 5 days and changed slightly at 12 days. The blood glucose level was markedly decreased in the control group and to a lesser degree also in the experimental group at 2 and 5 days of sampling. Insulin treatment (loading) performed at 2 and 5 days accounted for a drop of blood glucose which was followed by normalization within 2 h. Starving value and response to insulin loading uniformly fell into the physiological range at 21 days, whereas at 12 days no normalization occurred in either group within 2 h of insulin loading, although the starving value was physiological. The binding capacity of the insulin receptor was markedly low in the control group as long as 12 days, and tended to normalize by 21 days. In the insulin-imprinted group the binding capacity increased over the control at 2 and 5 days and normalized by 12 days.     

Effects of anti-insulin serum, insulin, and glucose on output of triglycerides and on ketogenesis by the perfused rat liver.

The rate of change of the concentration of various metabolites in blood in vivo and of the metbolism of free fatty acids by the perfused liver in vitro was sutidied as a function of time after the induction of acute insulin deficiency in rats by administration of guinea pig anti-insulin serum; the rate of reversal of these changes afte treatment of the anti-insulinserum diabetic ratss with insulin was also investigated. The concentrations of blood glucose and ketonebodies, and plasma-free fatty acids increased rapidly after injection of anti-insuli serum, while plasma triglycerides increased more slowly. These alterations were restored rapidly toward normal after treatment of the diabetic animals with insulin...     

Translocation hypothesis of insulin action on glucose transport

This article reviews the experimental data that support the translocation hypothesis of insulin action on glucose transport in adipocytes. According to this hypothesis, 1) most of the glucose transport mechanism in the basal (no insulin) form of fat cells is associated with an unidentified subcellular structure (the storage site), which is separated into the Golgi-rich fraction by centrifugation, and 2) the function of insulin is to induce translocation of the glucose transport mechanism from the above storage site to the plasma membrane. This translocation of the transport mechanism is reversible, dependent on metabolic energy, and independent of protein synthesis.     

Evidence against a physiologic role for acute changes in CNS insulin action in the rapid regulation of hepatic glucose production

This Perspective will discuss the physiologic relevance of data that suggest CNS insulin action is required for the rapid suppression of hepatic glucose production. It will also review data from experiments on the conscious dog, which show that although the canine brain can sense insulin and, thereby, regulate hepatic glucoregulatory enzyme expression, CNS insulin action is not essential for the rapid suppression of glucose production caused by the hormone. Insulin's direct hepatic effects are dominant, thus it appears that insulin's central effects are redundant in the acute regulation of hepatic glucose metabolism.     

Intracellular mechanism of action of sympathetic hepatic nerves on glucose and lactate balance in perfused rat liver

In rat liver perfused in situ stimulation of the nerve plexus around the hepatic artery and the portal vein caused an increase in glucose output and a shift from lactate uptake to output. The effects of nerve stimulation on some key enzymes, metabolites and effectors of carbohydrate metabolism were determined and compared to the actions of glucagon, which led to an increase not only of glucose output but also of lactate uptake. 1. Nerve stimulation caused an enhancement of the activity of glycogen phosphorylase a to 300% and a decrease of the activity of glycogen synthase I to 40%, while it left the activity of pyruvate kinase unaltered. Glucagon, similarly to nerve action, led to a strong increase of glycogen phosphorylase and to a decrease of glycogen synthase; yet in contrast to the nerve effect it lowered pyruvate kinase activity clearly. 2. Nerve stimulation increased the levels of glucose 6-phosphate and of fructose 6-phosphate to 200% and 170%, respectively; glucagon enhanced the levels to about 400% and 230%, respectively. The levels of ATP and ADP were not altered, those of AMP were increased slightly by nerve stimulation. 3. Nerve stimulation enhanced the levels of the effectors fructose 2,6-bisphosphate and cyclic AMP only slightly to 140% and 125%, respectively; glucagon lowered the level of fructose 2,6-bisphosphate to 15% and increased the level of cyclic AMP to 300%. 4. In calcium-free perfusions the metabolic responses to nerve stimulation showed normal kinetics, if calcium was re-added 3 min before, but delayed kinetics, if it was re-added 2 min after the onset of the stimulus. The delay may be due to the time required to refill intracellular calcium stores. The hemodynamic alterations dependent on extracellular calcium were normal in both cases. The activation of glycogen phosphorylase, the inhibition of glycogen synthase and the increase of glucose 6-phosphate can well explain the enhancement of glucose output following nerve stimulation. The unaltered activity of pyruvate kinase and the marginal increase of fructose 2,6-bisphosphate cannot be the cause of the nerve-stimulation-dependent shift from lactate uptake to output. The very slight increase of the level of cyclic AMP after nerve stimulation cannot elicit the observed activation of glycogen phosphorylase.(ABSTRACT TRUNCATED AT 400 WORDS)