The influence of Ca2+ on the effects of glucagon on hepatic glycolysis

Molecular and in vivo phenotyping of missense variants of the human glucagon receptor

Naturally occurring missense variants of G protein–coupled receptors with loss of function have been linked to metabolic disease in case studies and in animal experiments. The glucagon receptor, one such G protein–coupled receptor, is involved in maintaining blood glucose and amino acid homeostasis; however, loss-of-function mutations of this receptor have not been systematically characterized. Here, we observed fewer glucagon receptor missense variants than expected, as well as lower allele diversity and fewer variants with trait associations as compared with other class B1 receptors. We performed molecular pharmacological phenotyping of 38 missense variants located in the receptor extracellular domain, at the glucagon interface, or with previously suggested clinical implications. These variants were characterized in terms of cAMP accumulation to assess glucagon-induced Gα_s coupling, and of recruitment of β-arrestin-1/2. Fifteen variants were impaired in at least one of these downstream functions, with six variants affected in both cAMP accumulation and β-arrestin-1/2 recruitment. For the eight variants with decreased Gα_s signaling (D63^ECDN, P86^ECDS, V96^ECDE, G125^ECDC, R225^3.30H, R308^5.40W, V368^6.59M, and R378^7.35C) binding experiments revealed preserved glucagon affinity, although with significantly reduced binding capacity. Finally, using the UK Biobank, we found that variants with wildtype-like Gα_s signaling did not associate with metabolic phenotypes, whereas carriers of cAMP accumulation-impairing variants displayed a tendency toward increased risk of obesity and increased body mass and blood pressure. These observations are in line with the essential role of the glucagon system in metabolism and support that Gα_s is the main signaling pathway effecting the physiological roles of the glucagon receptor.

Calcium-dependent regulation of glucose homeostasis in the liver

A major role of the liver is to integrate multiple signals to maintain normal blood glucose levels. The balance between glucose storage and mobilization is primarily regulated by the counteracting effects of insulin and glucagon. However, numerous signals converge in the liver to ensure energy demand matches the physiological status of the organism. Many circulating hormones regulate glycogenolysis, gluconeogenesis and mitochondrial metabolism by calcium-dependent signaling mechanisms that manifest as cytosolic Ca(2+) oscillations. Stimulus-strength is encoded in the Ca(2+) oscillation frequency, and also by the range of intercellular Ca(2+) wave propagation in the intact liver. In this article, we describe how Ca(2+) oscillations and waves can regulate glucose output and oxidative metabolism in the intact liver; how multiple stimuli are decoded though Ca(2+) signaling at the organ level, and the implications of Ca(2+) signal dysregulation in diseases such as metabolic syndrome and non-alcoholic fatty liver disease.

Glucagon receptor mediates calcium signaling by coupling to G alpha q/11 and G alpha i/o in HEK293 cells

Glucagon induces intracellular Ca(2+) ([Ca(2+)](i)) elevation by stimulating glucagon receptor (GCGR). Such [Ca(2+)](i) signaling plays important physiological roles, including glycogenolysis and glycolysis in liver cells and the survival of beta-cells. Previous studies indicated that phospholipase C (PLC) might be involved in glucagon-mediated [Ca(2+)](i) response. Other studies also debated whether cAMP accumulation mediated by GCGR/G alpha(s) coupling contributes to [Ca(2+)](i) elevation. But the exact mechanisms remain uncertain. In the present study, we found that glucagon induces [Ca(2+)](i) elevation in HEK293 cells expressing GCGR. Removing extracellular Ca(2+) did not affect glucagon-stimulated [Ca(2+)](i) response. But depleting the intracellular Ca(2+) store by thapsigargin completely inhibited glucagon-induced [Ca(2+)](i) response. Experiments with forskolin and adenylyl cyclase inhibitor revealed that cAMP is not the cause of [Ca(2+)](i) response. Further studies with G alpha(q/11) RNAi and pertussis toxin (PTX) indicated that both G alpha(q/11) and G alpha(i/o) are involved. Combination of G alpha(q/11) RNAi and G alpha(i/o) inhibition almost completely abolished glucagon-induced [Ca(2+)](i) signaling.

Glucose and glycogen catabolism in perfused livers of Walker-256 tumor-bearing rats and the response to hormones

The alterations in hepatic glucose and glycogen catabolism were evaluated in rats bearing the Walker-256 tumor. Food intake was monitored concomitantly with measurements of the in vivo hepatic glycogen levels. Glycogenolysis, glycolysis and oxygen uptake were measured in the isolated perfused liver. The hepatic glucose phosphorylating capacity was measured in the high-speed supernatant fraction of liver homogenates. Food intake was 21.4% reduced in tumor-bearing rats; the glycogen levels were decreased by 63.6%. Initial basal rates of glucose release (glycogenolysis) and lactate+pyruvate production from endogenous glycogen (glycolysis) in the perfused liver were not changed by the tumor-bearing state, resulting in a higher relative rate of glycogen breakdown (% of glycogen degradation per unit time). In absolute terms stimulation of glycogen mobilization by glucagon or norepinephrine was smaller in the tumor-bearing state. The percentage of extra glycogen degradation per unit time caused by both hormones, however, was practically the same in the control and in the tumor-bearing state. The hepatic glucose phosphorylating capacity was reduced from 3.92+/-0.39 nmolmin(-1)(mgprotein)(-1) in normal rats to 2.61+/-0.23 nmolmin(-1)(mgprotein)(-1) in livers from tumor-bearing rats. Glycolysis from exogenous glucose (20 mM) in perfused livers was diminished from 0.136+/-0.023 &mgr;molmin(-1)(gliver)(-1) in normal rats to 0.046+/-0.008 &mgr;molmin(-1)(gliver)(-1) in tumor-bearing rats. It can be concluded that livers from rats bearing the Walker-256 tumor are less able to transform glucose and accumulate glycogen while possessing a greater tendency of releasing glucose from the glycogen stores.

Glycogen levels and glycogen catabolism in livers from arthritic rats

Hepatic glycogen catabolism and glycogen levels in rats with chronic arthritis were investigated. At 9:00 a.m., the hepatic glycogen contents of ad libitum fed arthritic and normal rats were 225.5+/-17.7 and 332.1+/-28.6 micromol glucosyl units x (g liver)(-1), respectively. Food intake of arthritic and normal rats was equal to 100.1+/-6.7 and 105.0+/-3.1 mg x (g body w)(-1) x (per 24 h)(-1), respectively. In isolated perfused livers from normal and arthritic rats the rates of glucose, lactate and pyruvate release were the same when substrate- and hormone-free perfusion was performed. During an infusion period of 20 min glucagon caused an increment in glucose release of 35.3+/-4.7 micromol x (g liver)(-1) in livers from arthritic rats; in the normal condition the corresponding increment was 69.6+/-5.7 micromol x (g liver)(-1). Lactate and pyruvate productions (indicators of glycolysis) were diminished by glucagon in livers from normal rats; in the arthritic condition an initial stimulation was found, followed by a slow decay, which did not result in significant inhibition at the end of the glucagon infusion period (20 min). The actions of cAMP and dibutyryl-cAMP were similar to those of glucagon. It was concluded that livers from arthritic rats show an impaired capacity of releasing glucose under the stimulus of glucagon. This can be partly due to the lower glycogen levels and partly to a smaller capacity of inhibiting glycolysis. Reduction in glycogen levels was not associated with reduction in food intake or failure in the energetic state of the hepatic cells. These changes in glycogen metabolism may be related to reduced gluconeogenic capacity of the livers and/or to production of inflammatory mediators observed in the arthritis disease.