Adipose tissue and serum CCDC80 in obesity and its association with related metabolic disease

Coiled-coil domain-containing 80 (CCDC80) is an adipocyte-secreted protein that modulates glucose homeostasis in response to diet-induced obesity in mice. The objective of this study is to analyze the link between human CCDC80 and obesity. CCDC80 protein expression was assessed in paired visceral (VAT) and subcutaneous (SAT) adipose tissue from 10 subjects (BMI range 22.4-38.8 kg/m²). Circulating CCDC80 levels were quantified in serum samples from two independent cross-sectional cohorts comprising 33 lean and 15 obese (cohort 1) and 32 morbid obese (cohort 2) male subjects. Insulin sensitivity, insulin secretion and blood neutrophil count were quantified in serum samples from both cohorts. Additionally, circulating free IGF-1 levels and oral glucose tolerance tests (OGTT) were assessed in cohort 1 whereas C-reactive protein levels and degree of atherosclerosis and hepatic steatosis were studied in cohort 2. In lean subjects, total CCDC80 protein content assessed by immunoblotting was lower in VAT than in SAT. In obese patients, CCDC80 was increased in VAT (P<0.05), but equivalent in SAT compared with lean counterparts. In cohort 1, serum CCDC80 correlated negatively with the acute insulin response to glucose and IGF1 levels, and positively with blood neutrophil count, independently of BMI, but not with insulin sensitivity. In cohort 2, serum CCDC80 was positively linked to the inflammatory biomarker C-reactive protein (r=0.46; P=0.009), atherosclerosis (carotid intima-media thickness, r=0.62; P<0.001) and hepatic steatosis (ANOVA P=0.025). Overall, these results suggest for the first time that CCDC80 may be a component of the obesity-altered secretome in VAT and could act as an adipokine whose circulant levels are linked to glucose tolerance derangements and related to inflammation-associated chronic complications.

New emerging role of protein-tyrosine phosphatase 1B in the regulation of glycogen metabolism in basal and TNF-α-induced insulin-resistant conditions in an immortalised muscle cell line isolated from mice

AIMS/HYPOTHESIS

Protein-tyrosine phosphatase 1B (PTP1B) negatively regulates insulin action, promoting attenuation of the insulin signalling pathway. The production of this phosphatase is enhanced in insulin-resistant states, such as obesity and type 2 diabetes, where high levels of proinflammatory cytokines (TNF-α, IL-6) are found. In these metabolic conditions, insulin action on glycogen metabolism in skeletal muscle is greatly impaired. We addressed the role of PTP1B on glycogen metabolism in basal and insulin-resistant conditions promoted by TNF-α.

METHODS

We studied the effect of TNF-α in the presence and absence of insulin on glycogen content and synthesis, glycogen synthase (GS) and glycogen phosphorylase (GP) activities and on glycogen synthesis and degradation signalling pathways. For this purpose we used immortalised cell lines isolated from skeletal muscle from mice lacking PTP1B.

RESULTS

Absence of PTP1B caused activation of GS and GP with a net glycogenolytic effect, reflected in lower amounts of glycogen and activation of the glycogenolytic signalling pathway, with higher rates of phosphorylation of cyclic adenosine monophosphate-dependent kinase (PKA), phosphorylase kinase (PhK) and GP phosphorylation. Nevertheless, insulin action was strongly enhanced in Ptp1b (also known as Ptpn1)(-/-) cells in terms of glycogen content, synthesis, GS activation rates and GS Ser641 dephosphorylation. Treatment with TNF-α augmented the activity ratios of both GS and GP, and impaired insulin stimulation of glycogen synthesis in wild-type myocytes, whereas Ptp1b (-/-) myocytes restored this inhibitory effect. We report a glycogenolytic effect of TNF-α, as demonstrated by greater activation of the degradation signalling cascade PKA/PhK/GP. In our model, this effect is mediated by the activation of PKA.

CONCLUSIONS/INTERPRETATION

We provide new data about the role of PTP1B in glycogen metabolism and confirm the beneficial effect that absence of the phosphatase confers against an insulin-resistant condition.

Adenovirus-mediated introduction of DNA into pig sperm and offspring

The ability of adenoviral vectors to transfer DNA into boar spermatozoa and to offspring was tested. Exposure of spermatozoa to adenovirus bearing the E. coli lacZ gene resulted in the transfer of the gene to the head of the spermatozoa. Treatment did not affect either viability or acrosomal integrity of boar sperm. Of the 2-to 8-cell embryos obtained after in vitro fertilization with adenovirus-exposed sperm, 21.7% expressed the LacZ product. Four out of 56 piglets (about 7%) obtained after artificial insemination with adenovirus-exposed spermatozoa were positive in PCR analyses, even though none of the piglets showed the LacZ gene after southern blot analysis. RT-PCR analysis performed in tissues from two positive stillborn piglets showed the presence of the LacZ mRNA in all of the tissues tested. The offspring obtained after mating two positive animals did not show LacZ gene presence. Our results indicate that adenovirus could be a feasible mechanism for the delivery of DNA into spermatozoa, even though the transfer of the transgene may be limited to the first generation.

Evidence for a role of glucose-induced translocation of glucokinase in the control of hepatic glycogen synthesis

Glucokinase reversibly partitions between a bound and a free state in the hepatocyte in response to the metabolic status of the cell. Maximum binding occurs at low [glucose] (<5 mM) and minimum binding at high [glucose] or in the presence of sorbitol or fructose. In this study we determined the binding characteristics of glucokinase in the hepatocyte in situ, by adenovirus-mediated glucokinase overexpression combined with the digitonin-permeabilization technique. We also determined the sensitivity of glycogen synthesis to changes in either total glucokinase overexpression or in free glucokinase activity. Glucokinase overexpression is associated with an increase in both free and bound activity, with an overall decrease in the proportion of bound activity. In hepatocytes incubated at low [glucose] (0-5 mM), glucokinase binding involves a high-affinity binding site with a Kd of approximately 0.1 microM and a binding capacity of approximately 3 pmol/mg total cell protein and low-affinity binding with a Kd of approximately 1.6 microM. Increasing glucose concentration to 20 mM causes a dose-dependent increase in the Kd of the high- affinity site to approximately 0.6 microM, and this effect was mimicked by 50 microM sorbitol, a precursor of fructose 1-P, confirming that this site is the regulatory protein of glucokinase. Glycogen synthesis determined from the incorporation of [2-3H,U-14C]glucose into glycogen at 5 mM or 10 mM glucose was very sensitive to small increases in total glucokinase activity and correlated more closely with the increase in free glucokinase activity. The relation between glycogenic flux and glucokinase activity is sigmoidal. Expression of the sensitivity of glycogen synthesis to glucokinase activity as the control coefficient reveals that the coefficient is greater for the incorporation of 2-tritium (which occurs exclusively by the direct pathway) than for incorporation of 14C label (which involves direct and indirect pathways) and is greater at 5 mM glucose (when glucokinase is maximally sequestered at its high-affinity site) than at 10 mM glucose. The results support the hypothesis that compartmentation of glucokinase in the hepatocyte increases the sensitivity of glycogen synthesis to small changes in total glucokinase activity and that glucose-induced translocation of glucokinase has a major role in the acute control of glycogen synthesis.

Lithium inhibits hepatic gluconeogenesis and phosphoenolpyruvate carboxykinase gene expression

Incubation of isolated hepatocytes from fasted rats with 20 mM LiCl for 1 h decreased glucose production from lactate, pyruvate, and alanine. In addition, phosphoenolpyruvate carboxykinase (PEPCK) gene expression in FTO-2B rat hepatoma cells was inhibited by treatment with LiCl. Lithium was also able to counteract the increased PEPCK mRNA levels caused by both Bt2cAMP and dexamethasone, in a concentration-dependent manner. A chimeric gene containing the PEPCK promoter (-550 to +73) linked to the amino-3-glycosyl phosphotransferase (neo) structural gene was transduced into FTO-2B cells using a Moloney murine leukemia virus-based retrovirus. In these infected cells, 20 mM LiCl decreased both the concentration of neo mRNA transcribed from the PEPCK-neo chimeric gene and mRNA from the endogenous PEPCK gene. Lithium also inhibited the stimulatory effect of Bt2cAMP and dexamethasone on both genes. The stability of neo mRNA was not altered by lithium, since in cells infected with retrovirus containing only the neo gene transcribed via the retroviral 5'-LTR and treated with 20 mM LiCl, no change in neo mRNA levels was observed. The intraperitoneal administration of LiCl to rats caused a decrease in hepatic PEPCK mRNA, indicating that lithium could also modify gene expression in vivo. The effects of lithium were not due to an increase in the concentration of insulin in the blood but were correlated with an increase in hepatic glycogen and fructose 2,6-bisphosphate levels. These results indicate that lithium ions, at concentrations normally used therapeutically for depression in humans, can inhibit glucose synthesis in the liver by a mechanism which can selectively modify the expression of hepatic phosphoenolpyruvate carboxykinase.

Insulin controls key steps of carbohydrate metabolism in cultured HT29 colon cancer cells

Effects of insulin on key steps of carbohydrate metabolism were investigated in cultured HT29 colon cancer cells by two different approaches, i.e. incubation of the cells either in the absence or in the presence of glucose in the medium. In glucose-deprived cells, insulin decreased glycogen breakdown, but did not affect polysaccharide levels when glucose was present. Glycogen synthase became activated after insulin treatment in both conditions, even though the activation was more evident when glucose was omitted. No effect on glycogen phosphorylase activity was evident under our experimental conditions. In cells incubated with glucose, the hormone stimulated in a dose-dependent manner the rates of glucose uptake and lactate release. Concomitantly with the increase in glycolytic rate, insulin caused a strong increase in fructose 2,6-bisphosphate. This effect was not observed in the absence of glucose. It is concluded that the carbohydrate metabolism of cultured HT29 cells responds to insulin, making this biological model suitable for investigations in vitro on the mechanism of insulin action.