Introduction:
High dietary fat and fructose–induced metabolic dysfunction-associated steatohepatitis (MASH) is a major risk factor for hepatocellular carcinoma (HCC), yet mechanisms linking steatosis to chronic liver inflammation remain poorly defined. ID1, a dominant-negative helix–loop–helix transcriptional regulator, is selectively upregulated in liver-resident macrophages (Kupffer cells; KCs) in murine MASH models and patient biopsies, but not in metabolic dysfunction-associated steatotic liver disease (MAFLD). This study investigates the functional role of KC-specific ID1 in MASH pathogenesis and explores therapeutic KC-localized targeting of ID proteins using nanoparticle-encapsulated protein degraders.

Methods:
KC–specific Id1 knockout mice were generated by introducing iCre into the Clec4f locus and validated via td-Tomato lineage tracing. Mice were maintained on standard chow or high-fat, high-fructose Western diet with fructose-supplemented drinking water for up to 24 weeks to induce MASH. Disease progression was assessed by serum liver enzymes (ALT, AST, ALP) and histological NAS scoring. We employed lipid- or fucoidan-based nanoparticle formulations of the ID degraders AGX51 and AGXA to evaluate the potential for therapeutic intervention in MASH.

Results:
Global Id1 deletion protected mice from diet- and CCl₄-induced MASH, prompting evaluation of KC-specific effects. Clec4f-iCre–mediated Id1 depletion significantly reduced hepatic steatosis, NAS scores, and serum ALT, AST, and ALP following prolonged high-fat, high-fructose feeding. Transcriptomic profiling of KCs identified anti-inflammatory and lipid-regulatory genes, including Serpina1e, Serpin a3k, Abcg5, and Pcsk9, supporting an ID1-dependent inflammatory program and KC–hepatocyte crosstalk. Translationally, Cy5-labeled lipid nanoparticles efficiently localized to Kupffer cells in vivo, supporting the feasibility of nanoparticle-mediated ID1 targeting in inflammatory liver disease.

Conclusion:
These findings identify KC-specific ID1 as a critical regulator of inflammatory pathways driving MASH progression and highlight ID1 as a promising therapeutic target. Nanoparticle-based ID degradation demonstrates translational potential for inflammation-driven liver disease and aggressive cancers.