IDE is not a rate-limiting regulator of plasma insulin levels in vivo.

Background: CEACAM1 regulates insulin sensitivity by promoting insulin clearance. Accordingly, global C57BL/6J.Cc1 −/− null mice display hyperinsulinemia due to impaired insulin clearance at 2 months of age, followed by insulin resistance, steatohepatitis, visceral obesity and leptin resistance at 6 months. The study aimed at investigating the primary role of hepatic CEACAM1 in insulin and lipid homeostasis independently of its metabolic effect in extra-hepatic tissues. Methods: Liver-specific C57BL/6J.AlbCre+Cc1 fl/fl mice were generated and their metabolic phenotype was characterized by comparison to that of their littermate controls at 2-9 months of age, using hyperinsulinemic-euglycemic clamp analysis and indirect calorimetry. The effect of hyperphagia on insulin resistance was assessed by pair-feeding experiments.

Aims/hypothesis The carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) promotes insulin clearance. Mice with global null mutation (Cc1−/−) or with liver-specific inactivation (L-SACC1) of Cc1 (also known as Ceacam1) gene display hyperinsulinaemia resulting from impaired insulin clearance, insulin resistance, steatohepatitis and obesity. Because increased lipolysis contributes to the metabolic phenotype caused by transgenic inactivation of CEACAM1 in the liver, we aimed to further investigate the primary role of hepatic CEACAM1-dependent insulin clearance in insulin and lipid homeostasis. To this end, we examined whether transgenic reconstitution of CEACAM1 in the liver of global Cc1−/− mutant mice reverses their abnormal metabolic phenotype. Methods Insulin response was assessed by hyperinsulinaemic–euglycaemic clamp analysis and energy balance was analysed by indirect calorimetry. Mice were overnight-fasted and refed for 7 h to assess fatty acid synthase activity in the liver and the hypothalamus in response to insulin release during refeeding. Results Liver-based rescuing of CEACAM1 restored insulin clearance, plasma insulin level, insulin sensitivity and steatohepatitis caused by global deletion of Cc1. It also reversed the gain in body weight and total fat mass observed with Cc1 deletion, in parallel to normalising energy balance. Mechanistically, reversal of hyperphagia appeared to result from reducing fatty acid synthase activity and restoring insulin signalling in the hypothalamus. Conclusions/interpretation Despite the potential confounding effects of deleting Cc1 from extrahepatic tissues, liver-based rescuing of CEACAM1 resulted in full normalisation of the metabolic phenotype, underscoring the key role that CEACAM1-dependent hepatic insulin clearance pathways play in regulating systemic insulin sensitivity, lipid homeostasis and energy balance.

The pathogenesis of human non-alcoholic fatty liver disease (NAFLD) remains unclear, in particular in the context of its relationship to insulin resistance and visceral obesity. Work on the carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) in mice has resolved some of the related questions. CEACAM1 promotes insulin clearance by enhancing the rate of uptake of the insulin-receptor complex. It also mediates a negative acute effect of insulin on fatty acid synthase activity. This positions CEACAM1 to coordinate the regulation of insulin and lipid metabolism. Fed a regular chow diet, global null mutation of Ceacam1 manifest hyperinsulinemia, insulin resistance, obesity, and steatohepatitis. They also develop spontaneous chicken-wire fibrosis, characteristic of non-alcoholic steatohepatitis. Reduction of hepatic CEACAM1 expression plays a significant role in the pathogenesis of diet-induced metabolic abnormalities, as bolstered by the protective effect of hepatic CEACAM1 gain-of-function against the metabolic response to dietary fat. Together, this emphasizes that loss of hepatic CEACAM1 links NAFLD to insulin resistance and obesity.

Supplementary key words carcinoembryonic antigen-related cell adhesion molecule 1 • insulin resistance • nicotinic acid • lipolysis • insulin clearanceCirculating insulin levels, in part determined by hepatic insulin clearance, regulate insulin action (1-3). Insulin clearance, which occurs mostly in liver and to a lower extent in kidney, but not in skeletal muscle or white adipose tissue (WAT), plays a pivotal role in promoting insulin sensitivity (4). If impaired, it contributes to mounting hyperinsulinemia in obese humans (5, 6), thus constituting a risk factor for metabolic syndrome (7,8).Our studies on the carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), a transmembrane glycoprotein that is highly expressed in liver and kidney, but not WAT or skeletal muscle (9), support these findings. Upon its phosphorylation by the insulin receptor, CEACAM1 promotes insulin clearance by upregulating receptor-mediated insulin uptake into clathrin-coated pits and degradation in hepatocytes (10). Moreover, it mediates a downregulatory effect on mouse fatty acid synthase Abstract Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) regulates insulin sensitivity by promoting hepatic insulin clearance and mediating suppression of fatty acid synthase activity. Feeding C57BL/6J male mice with a high-fat (HF) diet for 3-4 weeks triggered a >60% decrease in hepatic CEACAM1 levels to subsequently impair insulin clearance and cause systemic insulin resistance and hepatic steatosis. This study aimed at investigating whether lipolysis drives reduction in hepatic CEACAM1 and whether this constitutes a key mechanism leading to diet-induced metabolic abnormalities. Blocking lipolysis with a daily intraperitoneal injection of nicotinic acid in the last two days of a 30-day HF feeding regimen demonstrated that white adipose tissue (WAT)-derived fatty acids repressed hepatic CEACAM1-dependent regulation of insulin and lipid metabolism in 3-month-old male C57BL/6J mice. Adenoviralmediated CEACAM1 redelivery countered the adverse metabolic effect of the HF diet on insulin resistance, hepatic steatosis, visceral obesity, and energy expenditure. It also reversed the effect of HF diet on inflammation and fibrosis in WAT and liver. This assigns a causative role for lipolysis-driven decrease in hepatic CEACAM1 level and its regulation of insulin and lipid metabolism in sustaining systemic insulin resistance, hepatic steatosis, and other abnormalities associated with excessive energy supply.-Russo, L., H. E. Ghadieh, S. S. Ghanem, Q. Y. Al-Share, Z. N. Smiley, C. Gatto-Weis, E. L. Esakov, M. F. McInerney, G. Heinrich, X. Tong, L. Yin, and S. M. Najjar. Role for hepatic CEACAM1 in regulating fatty acid metabolism along the adipocytehepatocyte axis.

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) promotes hepatic insulin clearance. Consistently, mice with null mutation of Ceacam1 (Cc1 ؊/؊ ) exhibit impaired insulin clearance with increased lipid production in liver and redistribution to white adipose tissue, leading to visceral obesity at 2 months of age. When the mutation is propagated on the C57/BL6J genetic background, total fat mass rises significantly with age, and glucose intolerance and systemic insulin resistance develop at 6 months of age. This study was carried out to determine the mechanisms underlying the marked increase in total fat mass in 6-month-old mutants. Indirect calorimetry analysis showed that Cc1 ؊/؊ mice develop hyperphagia and a significant reduction in physical activity, in particular in the early hours of the dark cycle, during which energy expenditure is only slightly lower than in wildtype mice. They also exhibit increased triglyceride accumulation in skeletal muscle, due in part to incomplete fatty acid ␤-oxidation. Mechanistically, hypothalamic leptin signaling is reduced, as demonstrated by blunted STAT3 phosphorylation in coronal sections in response to an intracerebral ventricular injection of leptin. Hypothalamic fatty-acid synthase activity is also elevated in the mutants. Together, the data show that the increase in total fat mass in Cc1 ؊/؊ mice is mainly attributed to hyperphagia and reduced spontaneous physical activity. Although the contribution of the loss of CEACAM1 from anorexigenic proopiomelanocortin neurons in the arcuate nucleus is unclear, leptin resistance and elevated hypothalamic fatty-acid synthase activity could underlie altered energy balance in these mice.

Carcinoembryonic Antigen-related Cell Adhesion Molecule 1 (CEACAM1) has been implicated in physiological processes in the metabolic as well as in the vascular systems, as supported by the cardiometabolic abnormalities associated with insulin resistance that developed in global Ceacam1 null mutants. Given the role of CEACAM1 in maintaining vascular integrity, we aimed at identifying a role for CEACAM1-dependent pathways in endothelial cells in the pathogenesis of hepatic fibrosis. To this end, we generated endothelial cell-specific Ceacam1 knockout mice ( VECadCc1 -/- ) and propagated them on the C57BL/6J genetic background. The mice remain insulin sensitive even at 12-months of age, as determined by hyperinsulinemic-euglycemic clamp analysis at 7 months and by insulin tolerance test. The mice display normoinsulinemia associated with intact insulin clearance and without hepatic steatosis. However, these VECadCc1 -/- mutants display inflammatory foci in liver parenchyma, when stained by H&E staining. This was supported by ~2-fold increase in the mRNA levels of genes involved in inflammation (F4/80, CD68, IL-1β IL-6, and TNF­α) and by immunostaining for macrophage pool (F4/80) and their activation (CD68). Sirius Red stain detected a remarkable chicken-­wire deposition of collagen fibers interstitially and in the perivenular region in livers of VECadCc1 -/- but not controls. The mRNA levels of profibrogenic markers (Tgfβ, collagen 1A1, collagen 6a3, and α-SMA) are ~2-fold higher than controls. Moreover, TGFβ1 is a basally activated in null mice, as shown by increased Smad2/3 phosphorylation compared to their controls. Several mechanisms could contribute to hepatic fibrosis in VECadCc1 -/- mutants. These include: 1) increased endothelial-to-mesenchymal transition, supported by increased expression of mesenchymal markers (Snail, Slug, and [fibroblast-specific protein1 (FSP1)] with a reciprocal decrease in endothelial cell makers (CD31) in mouse liver endothelial cells (MLEC) isolated from VECadCc1 -/- mice; 2) increased capillarization, a liver injury event that precedes fibrogenesis, as shown by the ~2-fold increase in the mRNA levels of markers of capillarization (Gli1, Gli2, Gli3, iNOS, and osteopontin) in hepatic primary endothelial cells; 3) elevated levels of plasma endothelin-1 (ET-1) that promotes fibrogenesis. Mechanistically, the rise in plasma ET-1 results from increased coupling of the Ras/MAPKinase pathway to insulin receptor and VEGFR via Shc, a Src homology 2 (SH2)-containing cytoplasmic adaptor protein that upon its binding to phosphorylated CEACAM1 undergoes sequestration. Thus, endothelial cell CEACAM1 plays a key role in preventing liver injury and resulting fibrogenesis.

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