Adipose tissue-derived FABP4 mediates glucagon-stimulated hepatic glucose production in gestational diabetes.

AIMS: One of the most common complications of pregnancy is gestational diabetes mellitus (GDM), which may result in significant health threats of the mother, fetus and the newborn. Fatty acid-binding protein 4 (FABP4) is an adipokine that regulates glucose homeostasis by promoting glucose production and liver insulin resistance in mouse models. FABP4 levels are increased in GDM and correlates with maternal indices of insulin resistance, with a rapid decline post-partum. We therefore aimed to determine the tissue origin of elevated circulating FABP4 levels in GDM and to assess its potential contribution in promoting glucagon-induced hepatic glucose production. MATERIALS AND METHODS: FABP4 protein and gene expression was determined in biopsies from placenta, subcutaneous (sWAT) and visceral (vWAT) white adipose tissues from GDM and normoglycaemic pregnant women. FABP4 differential contribution in glucagon-stimulated hepatic glucose production was tested in conditioned media before and after its immune clearance. RESULTS: We showed that FABP4 is expressed in placenta, sWAT and vWAT of pregnant women at term, with a significant increase in its secretion from vWAT of women with GDM compared with normoglycaemic pregnant women. Neutralizing FABP4 from both normoglycaemic pregnant women and GDM vWAT secretome, resulted in a decrease in glucagon-stimulated hepatic glucose production. CONCLUSIONS: This study provides new insights into the role of adipose tissue-derived FABP4 in GDM, highlighting this adipokine, as a potential co-activator of glucagon-stimulated hepatic glucose production during pregnancy.

Fatty acid-binding protein 4: a key regulator of ketoacidosis in new-onset type 1 diabetes.

AIMS/HYPOTHESIS: Fatty acid-binding protein 4 (FABP4) is an adipokine with a key regulatory role in glucose and lipid metabolism. We prospectively evaluated the role of FABP4 in the pathophysiology of diabetic ketoacidosis (DKA) in new-onset type 1 diabetes. METHODS: Clinical and laboratory data were prospectively collected from consecutive children presenting with new-onset type 1 diabetes. In addition to blood chemistry and gases, insulin, C-peptide, serum FABP4 and NEFA were collected upon presentation and 48 h after initiation of insulin treatment. In a mouse model of type 1 diabetes, glucose, insulin, ß-hydroxybutyrate and weight were compared between FABP4 knockout (Fabp4-/-) and wild-type (WT) mice. RESULTS: Included were 33 children (mean age 9.3 ± 3.5 years, 52% male), of whom 14 (42%) presented with DKA. FABP4 levels were higher in the DKA group compared with the non-DKA group (median [IQR] 10.1 [7.9-14.2] ng/ml vs 6.3 [3.9-7] ng/ml, respectively; p = 0.005). The FABP4 level was positively correlated with HbA1c at presentation and inversely correlated with venous blood pH and bicarbonate levels (p < 0.05 for all). Following initiation of insulin therapy, a marked reduction in FABP4 was observed in all children. An FABP4 level of 7.22 ng/ml had a sensitivity of 86% and a specificity of 78% for the diagnosis of DKA, with an area under the receiver operating characteristic curve of 0.78 (95% CI 0.6, 0.95; p = 0.008). In a streptozotocin-induced diabetes mouse model, Fabp4-/- mice exhibited marked hypoinsulinaemia and hyperglycaemia similar to WT mice but displayed no significant increase in ß-hydroxybutyrate and were protected from ketoacidosis. CONCLUSIONS/INTERPRETATION: FABP4 is suggested to be a necessary regulator of ketogenesis in insulin-deficient states.

The adipokine FABP4 is a key regulator of neonatal glucose homeostasis.

During pregnancy, fetal glucose production is suppressed, with rapid activation immediately postpartum. Fatty acid-binding protein 4 (FABP4) was recently demonstrated as a regulator of hepatic glucose production and systemic metabolism in animal models. Here, we studied the role of FABP4 in regulating neonatal glucose hemostasis. Serum samples were collected from pregnant women with normoglycemia or gestational diabetes at term, from the umbilical circulation, and from the newborns within 6 hours of life. The level of FABP4 was higher in the fetal versus maternal circulation, with a further rise in neonates after birth of approximately 3-fold. Neonatal FABP4 inversely correlated with blood glucose, with an approximately 10-fold increase of FABP4 in hypoglycemic neonates. When studied in mice, blood glucose of 12-hour-old WT, Fabp4-/+, and Fabp4-/- littermate mice was 59 ± 13 mg/dL, 50 ± 11 mg/dL, and 43 ± 11 mg/dL, respectively. Similar to our observations in humans, FABP4 levels in WT mouse neonates were approximately 8-fold higher compared with those in adult mice. RNA sequencing of the neonatal liver suggested altered expression of multiple glucagon-regulated pathways in Fabp4-/- mice. Indeed, Fabp4-/- liver glycogen was inappropriately intact, despite a marked hypoglycemia, with rapid restoration of normoglycemia upon injection of recombinant FABP4. Our data suggest an important biological role for the adipokine FABP4 in the orchestrated regulation of postnatal glucose metabolism.

Intercellular Transmission of Hepatic ER Stress in Obesity Disrupts Systemic Metabolism.

Endoplasmic reticulum stress (ERS) has a pathophysiological role in obesity-associated insulin resistance. Yet, the coordinated tissue response to ERS remains unclear. Increased connexin 43 (Cx43)-mediated intercellular communication has been implicated in tissue-adaptive and -maladaptive response to various chronic stresses. Here, we demonstrate that in hepatocytes, ERS results in increased Cx43 expression and cell-cell coupling. Co-culture of ER-stressed "donor" cells resulted in intercellular transmission of ERS and dysfunction to ERS-naive "recipient" cells ("bystander response"), which could be prevented by genetic or pharmacologic suppression of Cx43. Hepatocytes from obese mice were able to transmit ERS to hepatocytes from lean mice, and mice lacking liver Cx43 were protected from diet-induced ERS, insulin resistance, and hepatosteatosis. Taken together, our results indicate that in obesity, the increased Cx43-mediated cell-cell coupling allows intercellular propagation of ERS. This novel maladaptive response to over-nutrition exacerbates the tissue ERS burden, promoting hepatosteatosis and impairing whole-body glucose metabolism.

The short-chain fatty acid propionate increases glucagon and FABP4 production, impairing insulin action in mice and humans.

The short-chain fatty acid propionate is a potent inhibitor of molds that is widely used as a food preservative and endogenously produced by gut microbiota. Although generally recognized as safe by the U.S. Food and Drug Administration, the metabolic effects of propionate consumption in humans are unclear. Here, we report that propionate stimulates glycogenolysis and hyperglycemia in mice by increasing plasma concentrations of glucagon and fatty acid-binding protein 4 (FABP4). Fabp4-deficient mice and mice lacking liver glucagon receptor were protected from the effects of propionate. Although propionate did not directly promote glucagon or FABP4 secretion in ex vivo rodent pancreatic islets and adipose tissue models, respectively, it activated the sympathetic nervous system in mice, leading to secretion of these hormones in vivo. This effect could be blocked by the pharmacological inhibition of norepinephrine, which prevented propionate-induced hyperglycemia in mice. In a randomized, double-blind, placebo-controlled study in humans, consumption of a propionate-containing mixed meal resulted in a postprandial increase in plasma glucagon, FABP4, and norepinephrine, leading to insulin resistance and compensatory hyperinsulinemia. Chronic exposure of mice to a propionate dose equivalent to that used for food preservation resulted in gradual weight gain. In humans, plasma propionate decreased with weight loss in the Dietary Intervention Randomized Controlled Trial (DIRECT) and served as an independent predictor of improved insulin sensitivity. Thus, propionate may activate a catecholamine-mediated increase in insulin counter-regulatory signals, leading to insulin resistance and hyperinsulinemia, which, over time, may promote adiposity and metabolic abnormalities. Further evaluation of the metabolic consequences of propionate consumption is warranted.

The enigma of the E326K mutation in acid ß-glucocerebrosidase.

A large number of mutations, and several polymorphisms, have been characterized in the GBA gene, encoding the lysosomal enzyme glucocerebrosidase, the activity of which is impaired in Gaucher disease. In this communication we summarize published and new data concerning biochemical characterization of the E326K amino acid change (1093G>A in the GBA1 cDNA) in tissue culture and its association with Parkinson disease, suggesting it is a disease causing mutation and not merely a polymorphism in the GBA gene.

Characterization of the ERAD process of the L444P mutant glucocerebrosidase variant.

A large number of mutations in the glucocerebrosidase gene (GBA gene), encoding the lysosomal acid hydrolase glucocerebrosidase (GCase), lead to Gaucher disease (GD). The second most prevalent GD causing mutation, carried by 38% of non-Jewish patients, is L444P, resulting from a T to C transition in nucleotide 6092 of the GBA gene. It is a severe mutation that, in homozygosity, leads to neuropathic type 3 GD. We have previously shown that mutant GCase variants present variable degrees of endoplasmic reticulum (ER) retention and undergo ER associated degradation (ERAD). However, ERAD of the L444P mutant variant of GCase has never been tested. In the current study, we present results indicating that the L444P mutant protein undergoes extensive ERAD. In skin fibroblasts, originated from GD patients homozygous for L444P mutation, the level of GCase is 12%-21% of normal and at least 50% of it is in the ER. The mutant protein undergoes polyubiquitination and proteasome-dependent degradation. Recently Ambroxol, a known expectorant, was identified as a pharmacological chaperone for mutant GCase. We tested the effect of Ambroxol on the L444P mutant GCase and found that it enhances the removal of the mutant enzyme from the ER. In some cases, this removal leads to a concomitant increase in enzymatic activity.

Interaction between parkin and mutant glucocerebrosidase variants: a possible link between Parkinson disease and Gaucher disease.

Gaucher disease (GD), a sphingolipidosis characterized by impaired activity of the lysosomal enzyme glucocerebrosidase (GCase), results from mutations in the GCase-encoding gene, GBA. We have shown that mutant GCase variants present variable degrees of endoplasmic reticulum (ER) retention and undergo ER-associated degradation (ERAD) in the proteasome. Furthermore, the degree of ERAD of mutant GCase variants correlates with and is one of the factors that determine GD severity. An association between GD and Parkinson disease (PD) has been demonstrated by the concurrence of PD in GD patients and the identification of GCase mutations in probands with sporadic PD. One of the genes involved in PD is PARK2, encoding the E3 ubiquitin ligase parkin. Parkin functions in the ERAD of misfolded ER proteins, and it is upregulated by unfolded protein response. Loss of parkin function leads to the accumulation of its substrates, which is deleterious to dopaminergic neurons in PD. We, therefore, tested the possibility that the association between GD and PD reflects the fact that parkin acts as an E3 ligase of mutant GCase variants. Our results showed that mutant GCase variants associate with parkin. Normal parkin, but not its RING finger mutants, affects the stability of mutant GCase variants. Parkin also promotes the accumulation of mutant GCase in aggresome-like structures and is involved in K48-mediated polyubiquitination of GCase mutants, indicating its function as its E3 ligase. We suggest that involvement of parkin in the degradation of mutant GCase explains the concurrence of GD and PD.

Intracellular cholesterol modifies the ERAD of glucocerebrosidase in Gaucher disease patients.

Gaucher disease (GD) is a lysosomal storage disorder resulting from an inborn reduced activity or deficiency of glucocerebrosidase due mainly to mutations in the glucocerebrosidase gene. We have recently shown that mutant glucocerebrosidase variants present variable degrees of endoplasmic reticulum (ER) retention and undergo ER associated degradation (ERAD) in the proteasomes. The degree of ERAD is one of the factors that determine GD severity. In order to define what factors affect the ERAD process of glucocerebrosidase in GD, we focused on two brothers with GD, carrying the same mutations but presenting extremely different clinical manifestations. One is mildly affected while the other developed severe GD with nervous system complications. Our results strongly indicated that both brothers presented variable degrees of ERAD, which was more extensive in the severely affected brother. Measurement of cholesterol demonstrated high intracellular levels in cells that derived from the severely affected brother. Growing the cells in cholesterol depleted medium led to lessening in the degree of ERAD in cells that derived from the severely affected brother and thus to improvement in stabilization, maturation, lysosomal localization and activity of the mutant glucocerebrosidase variants. The same effect was achieved by treating the cells with the HMG CoA reductase inhibitor mevastatin. None of the treatments had a significant effect on glucocerebrosidase properties in normal cells or in cells that derived from the mildly affected brother, indicating that intracellular cholesterol is one of the factors that affect the ERAD process of glucocerebrosidase and may influence the severity of GD.

ER retention and degradation as the molecular basis underlying Gaucher disease heterogeneity.

Gaucher disease (GD), an autosomal recessive disease, is characterized by accumulation of glucosylceramide mainly in cells of the reticuloendothelial system, due to mutations in the acid beta-glucocerebrosidase gene. Some of the patients suffer from neurological symptoms (type 2 and type 3 patients), whereas patients with type 1 GD do not present neurological signs. The disease is heterogeneous even among patients with the same genotype, implicating that a mutation in the glucocerebrosidase gene is required to cause GD but other factors play an important role in the manifestation of the disease. Glucocerebrosidase is a lysosomal enzyme, synthesized on endoplasmic reticulum (ER)-bound polyribosomes and translocated into the ER. Following N-linked glycosylations, it is transported to the Golgi apparatus, from where it is trafficked to the lysosomes. In this study, we tested glucocerebrosidase protein levels, N-glycans processing and intracellular localization in skin fibroblasts derived from patients with GD. Our results strongly suggest that mutant glucocerebrosidase variants present variable levels of ER retention and undergo ER-associated degradation in the proteasomes. The degree of ER retention and proteasomal degradation is one of the factors that determine GD severity.

Use of fluorescent substrates for characterization of Gaucher disease mutations.

Gaucher disease results from impaired activity of the lysosomal enzyme beta-glucocerebrosidase. More than 200 mutations within the glucocerebrosidase gene have been associated with this disease. In this study we tested the effect of several mutations (K157Q, D140H, E326K, D140H+E326K, V394L and R463C) on RNA stability, protein stability and activity toward four different fluorescent substrates (LR-12-GC, Bodipy-12-GC, LR-0-PAP-glucose and 4-MUG), using the vaccinia-derived expression system. The results indicated that the K157Q mutation leads to RNA instability, causing low protein levels and a concomitant reduction in beta-glucocerebrosidase activity. All other tested mutations led to production of glucocerebrosidase RNA and protein with stabilities comparable to those of the normal counterpart. The D140H variant exhibited a high activity toward the tested substrates while the variant enzymes containing either the E326K or D140H and E326k mutations together expressed low beta-glucocerebrosidase activity. The V394L variant exhibited low activity toward the tested substrates, while a higher activity was presented by the R463C containing glucocerebrosidase variant. Our results strongly indicated that the LR-12-GC substrate distinguishes between severities of different mutant glucocerebrosidase variants overexpressed in a heterologous system.