Aging disrupts hepatocyte zonation homeostasis in mice and humans.

Aging-induced degenerative changes in the liver are not inherently pathologic but pose an increased risk for liver diseases. However, the molecular mechanisms underlying aging-induced hepatic dyshomeostasis remain incompletely characterized. Here, we investigate how aging alters liver architecture, cellular communication, and hepatocyte zonation. Histological analyses of aged (>24-month-old) wild-type mouse livers showed no fibrosis, but a uniform cellular enlargement compared to young (2-month-old) mouse livers. For an unbiased characterization of aging-driven changes, we used single-nucleus RNA sequencing and found that aged livers had altered cell-cell interactions and hepatocyte zonation with zone-specific transcriptomic changes. Immunostaining confirmed aging-induced expansion of ASS1 + , CYP2E1 + , and GS + hepatic zones, and an aberrant expression of ASS1 + -GS + "bi-zonal" hepatocytes, causing loss of distinct zonation. Mechanistically, this breakdown was associated with downregulation of key zonation regulators ( Ctnnb1 , Foxo1 , Tcf7l2 ) and compensatory alterations in Wnt and Rspo3 signaling from non-parenchymal cells. To assess translational relevance, liver biopsies from young (≤25YO) and aged (>60YO) human donors were analyzed, revealing comparable zonal alterations and supporting the conservation of these aging-associated phenotypes across species. These findings reveal that aging causes loss of distinct hepatic zonation and alters intercellular communication through widespread transcriptional and architectural remodeling of liver cell types. The emergence of bi-zonal hepatocytes and expansion of hepatic zones in aged livers represent key hallmarks of hepatic aging. Our study provides new insights into mechanisms of liver aging and may inform therapeutic strategies targeting age-associated liver dysfunction.