BACKGROUND & AIMS
Obesity is a major driver of metabolic dysfunction-associated steatotic liver disease (MASLD), yet the molecular mechanisms linking excess adiposity to hepatocellular lipid accumulation remain incompletely defined. We investigated whether circulating fatty acid-binding protein 4 (FABP4) mediates adipocyte-hepatocyte lipid crosstalk in obesity.
METHODS
FABP4 expression and localization were analyzed in human liver specimens (n=55) and multiple mouse models of hepatic steatosis. Whole-body and tissue-specific Fabp4 knockout mice (n=7-10/group) were subjected to high-fat diet feeding and assessed for hepatic steatosis. Hepatocyte fatty acid uptake assays were performed in vitro. A high affinity humanized monoclonal antibody targeting FABP4 was generated and evaluated for its effects on hepatocyte binding, lipid uptake, and hepatic steatosis in obese mouse models.
RESULTS
FABP4 protein, but not mRNA, was markedly increased in hepatocytes from steatotic human livers (p<0.05) and obese mice (p<0.0001), suggesting an extrinsic source. Adipocyte-specific deletion of Fabp4 protected mice from diet-induced hepatic steatosis without affecting body weight or circulating lipid levels (p<0.01). Mechanistically, circulating FABP4 directly bound to hepatocytes and facilitated free fatty acid uptake. Neutralization of circulating FABP4 with a humanized monoclonal antibody blocked hepatocyte binding (p<0.001), reduced fatty acid uptake (p<0.01), and significantly attenuated hepatic steatosis across multiple obese mouse models.
CONCLUSIONS
These findings identify circulating FABP4 as a pathogenic lipid chaperone that links adipose tissue dysfunction to hepatocellular lipid accumulation. Targeting circulating FABP4 represents a promising strategy for prevention and treatment of obesity-associated hepatic steatosis.
IMPACT AND IMPLICATIONS
This study identifies circulating FABP4 as an endocrine lipid chaperone that mediates pathogenic adipose-liver crosstalk in obesity by directly promoting hepatocellular fatty acid uptake. The accumulation of FABP4 protein in hepatocytes in the absence of transcriptional induction reveals a previously unrecognized mechanism driving hepatic steatosis independently of systemic lipid levels. Importantly, neutralization of circulating FABP4 markedly attenuates steatosis in obese mouse models, establishing FABP4 as a tractable therapeutic target and supporting adipose-derived lipid carriers as a new intervention axis for MASLD.
Xingshan Jiang, Anthony Avellino, Jianyu Yu et al.· JHEP Reports· 0 citations
ABSTRACT Polymorphic toxins mediate interbacterial antagonism among competitors in the gut microbiome. Nuclease effectors, distantly related to the type VI-secreted Bacteroidales Tde, are enriched in human gut Bacillota. Tde mediates antagonism among Bacillota, and expression of the cognate immunity, Tdi, in recipients is protective. Crystal structures of Tde/Tdi complexes from two Bacillus spp. and Enterococcus quebecensis highlight a conserved mechanism of immunity. Tdi engages Tde with high-affinity, specific binding at an interface that features predominantly polar amino acids. A separate Tdi interface has a very highly conserved P(Φ)4GG motif that structurally mimics and displaces a short helix in Tde’s active site, which contains the critical catalytic residues. An isolated P(Φ)4GG motif peptide is sufficient for Tde nuclease activity inhibition at high concentrations. However, key residues at both the polar interface and P(Φ)4GG are required for complete inhibition of nuclease activity and protection against toxicity. We propose a multivalent Tde/Tdi neutralization mechanism where an initial high-affinity interface increases the local concentration of Tdi’s P(Φ)4GG motif, enabling it to displace the Tde active site through structural mimicry. The resulting conformational rearrangement of Tde increases its flexibility in solution and susceptibility to proteolysis, which may aid in eliminating the toxic effector. IMPORTANCE Bacteria in the gut microbiome compete using toxin secretion systems. Prior research has emphasized the importance of secretion systems in gram-negative bacteria. We describe a class of secreted nuclease effectors (toxins) and protective immunity proteins that are enriched in gram-positive Bacillota in human gut microbiomes. These effector/immunity pairs mediate antagonism among Bacillus and Enterococcus spp. The immunity proteins neutralize the nuclease effector through a unique mechanism of enzymatic active site mimicry. The immunity proteins bind effectors with very high-affinity at an interface with polar residues. The effector undergoes a large conformational change. A very highly conserved motif on the immunity surface competitively displaces an active site short helix and loop that contains the key catalytic residues. This rearrangement of the effector renders it inactive and susceptible to elimination by proteolysis. Bacteria in the gut microbiome compete using toxin secretion systems. Prior research has emphasized the importance of secretion systems in gram-negative bacteria. We describe a class of secreted nuclease effectors (toxins) and protective immunity proteins that are enriched in gram-positive Bacillota in human gut microbiomes. These effector/immunity pairs mediate antagonism among Bacillus and Enterococcus spp. The immunity proteins neutralize the nuclease effector through a unique mechanism of enzymatic active site mimicry. The immunity proteins bind effectors with very high-affinity at an interface with polar residues. The effector undergoes a large conformational change. A very highly conserved motif on the immunity surface competitively displaces an active site short helix and loop that contains the key catalytic residues. This rearrangement of the effector renders it inactive and susceptible to elimination by proteolysis.
