N-Lactoyl Phenylalanine Disrupts Insulin Signaling, Induces Inflammation, and Impairs Mitochondrial Respiration in Cell Models

N-lactoyl amino acids (Lac-AAs) are key players that regulate appetite and body weight. The most prominent and well-studied member is N-lactoyl phenylalanine (Lac-Phe), which can be induced by food intake, exercise and metformin treatment. However, its broader metabolic impact remains insufficiently characterized. This study investigates the effects of Lac-Phe on insulin signaling, inflammation, and mitochondrial respiration using HepG2 and differentiated C2C12 cell models, as well as isolated rat brain mitochondria and synaptosomes. Our results demonstrate that Lac-Phe significantly impairs insulin-stimulated phosphorylation of key proteins in the insulin signaling pathway, particularly in skeletal muscle cells, indicating disrupted insulin signaling. Additionally, Lac-Phe exposure increases the secretion of pro-inflammatory cytokines in C2C12 skeletal muscle cells and markedly impairs mitochondrial respiration in HepG2 liver cells and rat brain-derived synaptosomes, but not in isolated mitochondria. These findings highlight potential adverse metabolic effects of Lac-Phe, especially when administered at high concentrations, and underscore the necessity of conducting a comprehensive risk assessment and dose optimization before considering Lac-Phe or related Lac-AAs as therapeutic agents. Our work provides important insights into the molecular liabilities associated with Lac-Phe and calls for further studies to balance its therapeutic promise against possible metabolic risks.