Metabolic dysfunction-associated steatotic liver disease (MASLD), a spectrum of liver conditions ranging from simple steatosis to steatohepatitis (MASH), fibrosis, and cirrhosis, represents a global health epidemic with no approved pharmacotherapies. While traditionally viewed as a disorder of lipid metabolism, emerging evidence highlights significant perturbations in amino acid metabolism as contributors to MASLD pathogenesis. For example, recent clinical and metabolomics studies have shown that amino acid metabolism is markedly altered in MASLD, with disruptions in glutamine utilization, gut-liver aspartate transport, and circulating levels of arginine, proline, and histidine in patients and animal models. Yet the causal role and underlying mechanisms of proline remained elusive.

To address this question, a recent study published in Life Metabolism unveils a critical role for pyrroline-5-carboxylate synthase (P5CS)-driven proline metabolism in the pathogenesis of MASLD (Figure 1). The research team, led by Drs. Jia Li and Haowen Jiang at Chinese Academy of Sciences, together with Dr. Huihong Jiang at Tongji University and Dr. Xuemei Zhang at Fudan University, embarked on a systematic investigation to decipher the link between proline metabolism and MASLD.

Patient samples and multiple mouse models revealed that P5CS expression and proline levels are positively correlated with serum markers of liver injury (ALT and AST) and the expression of pro-inflammatory and pro-fibrotic genes. Interrogation of public transcriptomic databases (GEO) confirmed that hepatic ALDH18A1 (the gene encoding P5CS) expression escalates with MASLD severity. This pattern was consistently recapitulated in multiple preclinical MASH mouse models (CDAHFD and HFHC diets) and in palmitic acid (PA)-treated hepatocytes in vitro, establishing P5CS upregulation as a hallmark of the diseased liver.

Genetic manipulation in mice established P5CS as a key driver of the disease: liver-specific overexpression of P5CS markedly aggravated diet-induced MASH, evidenced by increased hepatic steatosis, elevated plasma liver enzymes, enhanced inflammatory responses, and exacerbated fibrosis. Conversely, liver-specific P5CS knockdown robustly ameliorated all these pathological features and reduced proline accumulation.

Mechanistically, P5CS-mediated proline accumulation impairs mitochondrial fatty acid oxidation, leading to hepatic lipid accumulation. Supplementing the culture medium with L-proline completely abolished the beneficial effects of P5CS knockdown on mitochondrial function and lipid metabolism in hepatocytes. Mutant forms of P5CS lacking enzyme activity failed to recapitulate the pro-steatotic effects of wild-type P5CS, indicating the pathogenic role of its catalytic function. Importantly, supplementing exogenous proline reversed the protective phenotype conferred by hepatic P5CS knockdown in MASH mice.

By pinpointing P5CS as a druggable upstream regulator of this cascade, the work opens exciting new avenues for therapeutic intervention. Pharmacological inhibition of P5CS activity holds promise for correcting the underlying metabolic defect and halting MASLD progression. Future studies will be needed to validate these findings in female models and primates, and to fully elucidate the precise molecular mechanisms by which proline compromises mitochondrial fitness.
DOI
10.1093/lifemeta/loaf040