Glycyrrhizic acid attenuates sepsis-induced acute lung injury by regulating macrophage M2 polarization via PRKCD.

Abstract

Background: Although glycyrrhizic acid (GA) has anti-inflammatory properties and protein kinase C delta (PRKCD) is involved in sepsis, whether GA alleviates sepsis-induced acute lung injury (SI-ALI) by regulating macrophage polarization via PRKCD remains unclear. This study aimed to investigate the protective effects of GA in SI-ALI and explore the underlying molecular mechanisms, with a focus on macrophage polarization mediated by PRKCD.

Methods: A cecal ligation and puncture (CLP) model was used to induce sepsis in mice. GA was administered intraperitoneally at different doses for in vivo intervention, and PRKCD overexpression was employed to validate the mechanism. In vitro, an inflammatory model was established by lipopolysaccharide (LPS) stimulation of RAW 264.7 macrophage cells, followed by GA treatment and PRKCD silencing or overexpression. Hematoxylin and eosin (HE) staining, lung wet-to-dry weight (W/D) ratio measurement, immunohistochemistry, flow cytometry, immunofluorescence, Cell Counting Kit-8 (CCK-8) assay, terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL), Western blot, and enzyme-linked immunosorbent assay (ELISA) were used to assess inflammation, PRKCD expression, and macrophage polarization in lung tissues and cells.

Results: GA significantly alleviated CLP-induced lung tissue injury and pulmonary edema (W/D ratio: 4.5±0.8 vs. 6.7±0.8 in the Model group, P<0.01) and reduced tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β) levels in bronchoalveolar lavage fluid (BALF; from 211±18 to 82±9 pg/mL and from 162±16 to 52±10 pg/mL, respectively, P<0.001). In LPS-stimulated macrophages, GA enhanced cell viability (CCK-8: 83%±6% vs. 55%±4% in the LPS group, P<0.01), reduced apoptosis (TUNEL-positive cells: 20%±3% vs. 64%±6%, P<0.001), suppressed M1 polarization (CD86: 15%±4% vs. 45%±7%, P<0.01), promoted M2 polarization (CD206: 38%±7% vs. 3%±2%, P<0.01), and decreased pro-inflammatory cytokine secretion. PRKCD expression was upregulated under septic conditions. PRKCD silencing mitigated LPS-induced inflammation and modulated macrophage polarization, whereas PRKCD overexpression partially reversed the protective effects of GA.

Conclusions: GA exerts anti-inflammatory and lung-protective effects in SI-ALI, and our findings suggest that these effects are associated with PRKCD inhibition and restoration of macrophage polarization balance. These results identify PRKCD as a candidate target and support further investigation of GA as a potential therapeutic strategy for SI-ALI.