PPARs in molecular pathogenesis and drug treatment of type 2 diabetes-related MASLD

Peroxisome proliferator-activated receptors (PPARs) comprise three isoforms: PPARα, PPARβ/δ, and PPARγ, which regulate the expression of genes involved in fatty acid uptake, β-oxidation, adipogenesis, gluconeogenesis, and insulin sensitivity. Type 2 diabetes (T2D), often accompanied by other features of metabolic syndrome, contributes to vasculopathy, end-stage organ failure, and cancer. Metabolic dysfunction-associated steatotic liver disease (MASLD) refers to steatotic liver disease in the presence of cardiometabolic risk factor(s) and without excessive alcohol consumption. MASLD is prevalent among adults with T2D and carries a high risk of liver fibrosis, metabolic dysfunction-associated steatohepatitis (MASH), cirrhosis and incident T2D. In MASLD, the liver becomes a hub of lipid toxicity, oxidative stress, and fibrotic signalling whenever T2D disrupts hormonal and adipokine signalling, increases free fatty acid flux, and promotes chronic inflammation. MASLD, therefore, results from an impairment of the protection physiologically offered by PPARs through fatty acid oxidation, lipid storage in the adipose tissue, and mitigation of insulin resistance and pro-inflammatory cascades. By examining the molecular mechanisms of PPARα, PPARβ/δ, and PPARγ, as well as their interactions with cofactors like PGC-1α, and their crosstalk with pathways like sterol regulatory element-binding protein (SREBP), NF-κB, AMP-activated protein kinase (AMPK), and adipokines, researchers and clinicians can better understand how T2D-related MASLD can be prevented or treated. Single PPAR agonists, such as fibrates and glitazones, have limited clinical efficacy in achieving hard liver histology endpoints like MASH resolution and fibrosis regression in humans. However, the Pan-PPAR agonist Lanifibranor at the highest doses shows promise in ameliorating these outcomes in subjects with non-cirrhotic MASH. This suggests that activating all three PPAR isoforms together enhances their therapeutic effects on various cells and target organs, restoring insulin resistance, improving gluco-lipidic homeostasis, while inhibiting pro-inflammatory and pro-fibrogenic pathways. Analysis of unresolved issues should dictate the research agenda.