Once written off as nothing more than a waste product of glycolysis, lactic acid is now seen as a key signaling molecule that operates across a wide range of physiological and pathological processes, from immune regulation and tumor metabolism to neural function. But its role goes beyond energy metabolism and cell signaling. Recent studies have uncovered a new type of post-translational modification called histone lactylation, in which lactate itself provides the lactoyl group attached to lysine residues on histones. This modification directly ties a cell's metabolic state to the epigenetic control of gene expression. For example, histone lactylation helps shift macrophages from a pro-inflammatory M1 phenotype to an anti-inflammatory M2 phenotype by fine-tuning gene transcription. In this review, we walk through the discovery and biochemical foundation of histone lactylation; discuss the likely writer and eraser enzymes that manage its dynamic changes; and highlight recent advances in understanding the role of this modification in inflammation, tumorigenesis, neurological disorders, and interactions with gut microbes. We also lay out key unanswered questions and consider why targeting protein lactylation might open up new therapeutic possibilities.
Keywords: gut microbe; histone lactylation; inflammation; lactic acid; neuropathy; tumor.