The hepatokines fetuin-A and fetuin-B are upregulated in the state of hepatic steatosis and may differently impact on glucose homeostasis in humans

The liver is a central regulator of whole body glucose, and lipid homeostasis and hepatokines, like fetuin-A, have been identified as markers and mediators of fatty liver-induced cardiometabolic risk. The closely related protein fetuin-B was shown to be upregulated in the fatty liver and to impact on glucose homeostasis in mice. In the present study we aimed to test the relevance of these findings in humans. In 55 subjects, hepatic mRNA expression of both hepatokines, fetuin-A and fetuin-B, associated positively with liver triglyceride content, whereas only fetuin-A expression associated with the homeostatic model assessment of insulin resistance. In 220 subjects who underwent precise metabolic phenotyping, circulating fetuin-A, but not fetuin-B, associated positively with liver fat content, and negatively with insulin sensitivity, measured during the oral glucose tolerance test (OGTT) and during the euglycemic, hyperinsulinemic clamp. Both circulating fetuin-A and fetuin-B correlated positively with the glucose area under the curve during the OGTT, but after additional adjustment for insulin sensitivity this relationship remained significant only for fetuin-B. In conclusion, despite the fact that the two hepatokines, fetuin-A and fetuin-B, are upregulated in the state of hepatic steatosis in humans, it appears that they differently impact on glucose homeostasis. Our data are in agreement with observations that fetuin-A can alter insulin signaling and that fetuin-B may regulate glucose homeostasis via so far unknown effects, possibly on glucose effectiveness.

Identification and expression analysis of fetuin B (FETUB) in turbot (Scophthalmus maximus L.) mucosal barriers following bacterial challenge

Fetuin B (FETUB), a recently described cysteine proteinase inhibitor, has numerous conserved N-glycosylation sites, species-specific O-glycosylation sites, and two cystatin (CY) domains. FETUB is likely to play regulatory roles in acute inflammation, female infertility, fish organogenesis and tumor suppression. In the present study, transcript of turbot FETUB gene was captured, its protein structure and expression patterns in different tissues with emphasis on mucosal barriers following different bacterial infection were characterized. Turbot FETUB gene showed the closest relationship with Takifugu rubripes in phylogenetic analysis. In addition, FETUB was ubiquitously expressed in all examined tissues with the highest expression level in skin. Finally, FETUB gene showed different expression patterns following both bacterial challenge. The rapidly and significantly differential expression patterns of FETUB in mucosal surfaces against bacterial infections might indicate its key roles to prevent pathogen attachment and entry in turbot mucosal immunity. Functional studies should be carried out to further characterize the FETUB and avail utilization of its function to increase the disease resistance of turbot in maintaining the integrity of the mucosal barriers against infections and to facilitate selection of the fine family/varieties of disease resistance in turbot.

Fetuin B overexpression suppresses proliferation, migration, and invasion in prostate cancer by inhibiting the PI3K/AKT signaling pathway

Fetuin B (FETUB) is a glycoprotein that is a member of the cysteine protease inhibitor family, and it is associated with cancer. However, the role of FETUB in prostate carcinogenesis is unknown. In this study, we overexpressed FETUB in prostate cancer cells by using lentivirus and then studied the impacts on cell apoptosis, migration and invasion. We found that apoptosis was increased and the migration and invasion of prostate cancer cells were significantly inhibited after overexpression. Then, we performed experiments in vivo and found that there were fewer tumors in the overexpression groups than in the control groups. In addition, we demonstrated that overexpression of FETUB inactivates the PI3K/AKT signaling pathway. Rescue assays revealed that intervention of 740Y-P reversed the anti-tumor effect of FETUB on prostate cancer cells. Taken together, our results revealed that FETUB may act as a novel regulator to promote apoptosis and inhibit the migration and invasion of prostate cancer cells and that FETUB is related to the inactivation of the PI3K/AKT signaling pathway.

Population-based meta-analysis and gene-set enrichment identifies FXR/RXR pathway as common to fatty liver disease and serum lipids

Nonalcoholic fatty liver disease (NAFLD) is prevalent worldwide. NAFLD is associated with elevated serum triglycerides (TG), low-density lipoprotein cholesterol (LDL), and reduced high-density lipoprotein cholesterol (HDL). Both NAFLD and blood lipid levels are genetically influenced and may share a common genetic etiology. We used genome-wide association studies (GWAS)-ranked genes and gene-set enrichment analysis to identify pathways that affect serum lipids and NAFLD. We identified credible genes in these pathways and characterized missense variants in these for effects on serum traits. We used MAGENTA to identify 58 enriched pathways from publicly available TG, LDL, and HDL GWAS (n = 99,000). Three of these pathways were also enriched for associations with European-ancestry NAFLD GWAS (n = 7176). One pathway, farnesoid X receptor (FXR)/retinoid X receptor (RXR) activation, was replicated for association in an African-ancestry NAFLD GWAS (n = 3214) and plays a role in serum lipids and NAFLD. Credible genes (proteins) in FXR/RXR activation include those associated with cholesterol/bile/bilirubin transport/absorption (ABCC2 (MRP2) [ATP binding cassette subfamily C member (multidrug resistance-associated protein 2)], ABCG5, ABCG8 [ATP-binding cassette (ABC) transporters G5 and G8], APOB (APOB) [apolipoprotein B], FABP6 (ILBP) [fatty acid binding protein 6 (ileal lipid-binding protein)], MTTP (MTP) [microsomal triglyceride transfer protein], SLC4A2 (AE2) [solute carrier family 4 member 2 (anion exchange protein 2)]), nuclear hormone-mediated control of metabolism (NR0B2 (SHP) [nuclear receptor subfamily 0 group B member 2 (small heterodimer partner)], NR1H4 (FXR) [nuclear receptor subfamily 1 group H member 4 (FXR)], PPARA (PPAR) [peroxisome proliferator activated receptor alpha], FOXO1 (FOXO1A) [forkhead box O1]), or other pathways (FETUB (FETUB) [fetuin B]). Missense variants in ABCC2 (MRP2), ABCG5 (ABCG5), ABCG8 (ABCG8), APOB (APOB), MTTP (MTP), NR0B2 (SHP), NR1H4 (FXR), and PPARA (PPAR) that associate with serum LDL levels also associate with serum liver function tests in UK Biobank. Conclusion: Genetic variants in NR1H4 (FXR) that protect against liver steatosis increase serum LDL cholesterol while variants in other members of the family have congruent effects on these traits. Human genetic pathway enrichment analysis can help guide therapeutic development by identifying effective targets for NAFLD/serum lipid manipulation while minimizing side effects. In addition, missense variants could be used in companion diagnostics to determine their influence on drug effectiveness.

The Central Role of KNG1 Gene as a Genetic Determinant of Coagulation Pathway-Related Traits: Exploring Metaphenotypes

Traditional genetic studies of single traits may be unable to detect the pleiotropic effects involved in complex diseases. To detect the correlation that exists between several phenotypes involved in the same biological process, we introduce an original methodology to analyze sets of correlated phenotypes involved in the coagulation cascade in genome-wide association studies. The methodology consists of a two-stage process. First, we define new phenotypic meta-variables (linear combinations of the original phenotypes), named metaphenotypes, by applying Independent Component Analysis for the multivariate analysis of correlated phenotypes (i.e. the levels of coagulation pathway–related proteins). The resulting metaphenotypes integrate the information regarding the underlying biological process (i.e. thrombus/clot formation). Secondly, we take advantage of a family based Genome Wide Association Study to identify genetic elements influencing these metaphenotypes and consequently thrombosis risk. Our study utilized data from the GAIT Project (Genetic Analysis of Idiopathic Thrombophilia). We obtained 15 metaphenotypes, which showed significant heritabilities, ranging from 0.2 to 0.7. These results indicate the importance of genetic factors in the variability of these traits. We found 4 metaphenotypes that showed significant associations with SNPs. The most relevant were those mapped in a region near the HRG, FETUB and KNG1 genes. Our results are provocative since they show that the KNG1 locus plays a central role as a genetic determinant of the entire coagulation pathway and thrombus/clot formation. Integrating data from multiple correlated measurements through metaphenotypes is a promising approach to elucidate the hidden genetic mechanisms underlying complex diseases.

Fetuin B links vitamin D deficiency and pediatric obesity: Direct negative regulation by vitamin D

Vitamin D (VD) deficiency (VDD) correlates to obesity, with VD a recognized mediator of metabolic diseases. From a previous proteomic study identifying adiponectin as a link between VDD and pediatric obesity, herein we analysed another protein (SSP2301) increased with VDD. A focused 2D-electrophoretic analysis identified 4 corresponding plasma proteins, with one predicted to be fetuin B (FETUB). FETUB was studied due to its emerging role in metabolic diseases and cytogenetic location (3q27.3) with adiponectin. Results were confirmed in obese children, where plasma FETUB was higher with VDD. A direct effect by 1α,25-(OH)2D3 on hepatocellular FETUB synthesis was observed, with a time and dose dependent reduction. Further, we demonstrated the VD-receptor (VDR) is key, with FETUB "released" with VDR silencing. Finally, VD supplementation (6weeks) to juvenile mice fed a standard diet, reduced plasma FETUB. Only at 22weeks did liver FETUB correspond to plasma FETUB, highlighting the contribution of other VD-responsive tissues. Overall, FETUB is a key protein linking VDD to pediatric obesity. With an emerging role in metabolic diseases, we demonstrate that VD/VDR directly regulate FETUB.

Common genetic variants in the FETUB locus, genetically predicted fetuin-B levels, and risk of insulin resistance in obese Chinese adults

Elevated serum fetuin-B is suggested to be associated with insulin resistance, but it is unknown if this association is causal. The aim of this study was to explore the potential causal relationship between fetuin-B and insulin resistance.We used Mendelian randomization analysis by incorporating information of genetic variants in FETUB and serum fetuin-B concentrations with insulin resistance in 1148 obese Chinese adults.Common genetic variants (FETUB rs4686434, rs6785067, and rs3733159) were significantly associated with serum fetuin-B concentrations but not with insulin resistance. Higher serum fetuin-B levels were significantly associated with increased homeostasis model assessment of insulin resistance (HOMA-IR) (0.17 [95%CI: 0.01 to 0.32, P = .037] 10 mol IU L higher per SD). However, Mendelian randomization analysis using 3 single-nucleotide polymorphisms as instrumental variables did not support a significant association between genetically predicted fetuin-B levels and HOMA-IR (-0.09 [95%CI: -0.62 to 0.44, P = .738] 10 mol IU L lower per SD). The regression coefficients for measured and genetically predicted fetuin-B concentrations on HOMA-IR were significantly different (P <.001).This study suggests the association between fetuin-B and insulin resistance may not be causal. Future studies on the nongenetic determinants of serum fetuin-B concentration to assess if such unmeasured factors may confound the association between fetuin-B and insulin resistance as well as more pathway analysis for this association are warranted.

The rs4686434 variant in the fetuin B (FETUB) locus is associated with intrahepatic triglyceride content in obese Chinese adults

BACKGROUND

This study explored associations of genetic variants in the fetuin B (FETUB) locus with intrahepatic triglyceride (IHTG) content.

METHODS

Four tagging single-nucleotide polymorphisms (SNPs) of the FETUB locus and patatin-like phospholipase domain containing 3 (PNPLA3) rs738409 and transmembrane 6 super family member 2 (TM6SF2) rs58542926 were genotyped in 418 obese Chinese adults in whom serum FETUB and IHTG were measured.

RESULTS

Subjects carrying the minor G allele for FETUB rs4686434 (AG/GG) had lower serum FETUB levels (mean [±SD] 3.89 ± 1.36 vs 4.22 ± 1.46 μg/mL; P = 0.021) and IHTG content (12.7 ± 9.4% vs 14.6 ± 9.8%; P = 0.045) than their controls (AA), whereas IHTG content was higher in those carrying the minor G allele for PNPLA3 rs738409 (CG/GG) than in their controls (CC; 14.5 ± 10.1% vs 12.0 ± 8.6%; P = 0.012). After adjusting for potential confounders, IHTG content was lower in carriers of the minor G allele for FETUB rs4686434 (AG/GG vs AA, β -2.27 ± 0.91, P = 0.012), but was higher in carriers of the minor G allele for PNPLA3 rs738409 (CG/GG vs CC, β 2.65 ± 0.97, P = 0.006). There was a significant joint effect between FETUB rs4686434 and PNPLA3 rs738409 on IHTG content, with increasing genetic risk score (counting the risk allele of A in rs4686434 and G in rs738409) being associated with higher IHTG content (β 1.85 ± 0.48, P <0.001).

CONCLUSIONS

Carrying the minor G allele for FETUB rs4686434 was significantly associated with decreased IHTG content and may affect hepatic triglyceride accumulation in individuals at high risk of non-alcoholic fatty liver disease.

The C-terminal region of human plasma fetuin-B is dispensable for the raised-elephant-trunk mechanism of inhibition of astacin metallopeptidases

Human fetuin-B plays a key physiological role in human fertility through its inhibitory action on ovastacin, a member of the astacin family of metallopeptidases. The inhibitor consists of tandem cystatin-like domains (CY1 and CY2), which are connected by a linker containing a "CPDCP-trunk" and followed by a C-terminal region (CTR) void of regular secondary structure. Here, we solved the crystal structure of the complex of the inhibitor with archetypal astacin from crayfish, which is a useful model of human ovastacin. Two hairpins from CY2, the linker, and the tip of the "legumain-binding loop" of CY1 inhibit crayfish astacin following the "raised-elephant-trunk mechanism" recently reported for mouse fetuin-B. This inhibition is exerted by blocking active-site cleft sub-sites upstream and downstream of the catalytic zinc ion, but not those flanking the scissile bond. However, contrary to the mouse complex, which was obtained with fetuin-B nicked at a single site but otherwise intact, most of the CTR was proteolytically removed during crystallization of the human complex. Moreover, the two complexes present in the crystallographic asymmetric unit diverged in the relative arrangement of CY1 and CY2, while the two complexes found for the mouse complex crystal structure were equivalent. Biochemical studies in vitro confirmed the differential cleavage susceptibility of human and mouse fetuin-B in front of crayfish astacin and revealed that the cleaved human inhibitor blocks crayfish astacin and human meprin α and β only slightly less potently than the intact variant. Therefore, the CTR of animal fetuin-B orthologs may have a function in maintaining a particular relative orientation of CY1 and CY2 that nonetheless is dispensable for peptidase inhibition.

Fetuin-B Interacts With Insulin Receptor-β and Promotes Insulin Resistance in Retina Cells

Purpose

The purpose of this study was to investigate the correlation between insulin and Fetuin-B (FETUB) and the influence of FETUB on insulin signaling pathway in diabetic retinopathy (DR).

Methods

Enzyme-linked immunosorbent assay (ELISA) was used to analyze FETUB and insulin levels in the serum and aqueous fluid of patients with DR and healthy controls. Quantitative PCR (q-PCR), Western blotting, and ELISA were used to examine FETUB expression in ARPE-19, BV2, and Müller cells under insulin stimulation. Co-immunoprecipitation was used to investigate the interaction of FETUB with insulin receptor-β (IRβ). Insulin resistance (IR)-BV2 and IR-Müller cells were treated with FETUB recombinant protein or FETUB short hairpin RNA (shRNA) to explore the influence of FETUB on insulin signaling pathway in DR. LY294002 (a PI3K pathway inhibitor) was used to determine whether FETUB affects glucose metabolism via the PI3K/Akt pathway.

Results

In aqueous fluid, FETUB concentrations were positively correlated with insulin levels. FETUB expression increased in Müller and BV2 cells under insulin regulation, and FETUB interacted with IRβ in retinal cells and mice retina. The interaction between IRβ and FETUB increased in BV2 and Müller cells under high-glucose than in controls. Insulin signaling pathway activation was suppressed in FETUB recombinant protein-treated BV2 and Müller cells but increased in FETUB shRNA-transfected cells. FETUB shRNA could not reverse LY294002-mediated inhibition of glucose transporter-4 expression.

Conclusions

Retinal cells are the source of insulin-regulated FETUB. The FETUB interacts with IRβ and affects insulin signaling pathway in BV2 and Müller cells. FETUB may aggravate IR in BV2 and Müller cells via the PI3K/Akt pathway.