The Multifaceted Roles of Proline in Cell Behavior.

Herein, we review the multifaceted roles of proline in cell biology. This peculiar cyclic imino acid is: A main precursor of extracellular collagens (the most abundant human proteins), antimicrobial peptides (involved in innate immunity), salivary proteins (astringency, teeth health) and cornifins (skin permeability); an energy source for pathogenic bacteria, protozoan parasites, and metastatic cancer cells, which engage in extracellular-protein degradation to invade their host; an antistress molecule (an osmolyte and chemical chaperone) helpful against various potential harms (UV radiation, drought/salinity, heavy metals, reactive oxygen species); a neural metabotoxin associated with schizophrenia; a modulator of cell signaling pathways such as the amino acid stress response and extracellular signal-related kinase pathway; an epigenetic modifier able to promote DNA and histone hypermethylation; an inducer of proliferation of stem and tumor cells; and a modulator of cell morphology and migration/invasiveness. We highlight how proline metabolism impacts beneficial tissue regeneration, but also contributes to the progression of devastating pathologies such as fibrosis and metastatic cancer.