To investigate whether rosmarinic acid (RA) confers in intro and vivo protective effects by specifically activating the peroxidase activity of PRDX1.HepG2 cells were pretreated with RA and then treated with H2O2, LPS, or different pro-inflammatory cytokines to assess RA's anti-oxidant and anti-inflammatory activities. Primary hepatocytes were isolated from 6-week-old male PRDX1Cys52Ser mice and pretreated with RA and then treated with LPS to assess RA's anti-oxidant and anti-inflammatory activities in hepatocytes without PRDX1 peroxidase activity. In addition, 8-week-old male PRDX1Cys52Ser mice started to receive western diet (WD) feeding for 20 weeks to induce metabolic dysfunction associated steatohepatitis (MASH). In the meantime, daily injection of RA or Veh was carried out in these animals to evaluate the beneficial effects of RA on MASH and liver fibrosis using a variety of techniques.RA showed anti-oxidant and anti-inflammatory activities in HepG2 cells, while it lost these protective activities in primary PRDX1Cys52Ser mutant hepatocytes. In addition, RA treatment in WD-fed PRDX1Cys52Ser mice did not show any improvements in MASH and liver fibrosis compared with Veh treatment, which was also supported by unaltered expression of hepatic genes related to inflammation and fibrosis, as well as signaling activities of hepatic signal transducer and activator of transcription 1 and 3 (STAT1/3) after RA treatment.RA exerts both in vitro and in vivo beneficial effects by specifically activating the peroxidase activity of PRDX1.Copyright © 2025 Elsevier Inc. All rights reserved.