Peroxisome Proliferator-Activated Receptor-γ Agonist Attenuates Vocal Fold Fibrosis in Rats via Regulation of Macrophage Activation

Macrophages aid in wound healing by changing their phenotype and can be a key driver of fibrosis. However, the contribution of macrophage phenotype to fibrosis following vocal fold injury remains unclear. Peroxisome proliferator-activated receptor-γ (PPARγ) is expressed mainly by macrophages during early wound healing and regulates the macrophage phenotype. This study aimed to evaluate the effects of pioglitazone, a PPARγ agonist, on the macrophage phenotype and fibrosis following vocal fold injury in rats. Pioglitazone was injected into the rats' vocal folds on days 1, 3, 5, and 7 after injury, and the vocal fold lamina propria was evaluated on days 4 and 56 after injury. Moreover, THP-1-derived macrophages were treated with pioglitazone, and the expression of pro-inflammatory cytokines under lipopolysaccharide/interferon-γ stimulation was analyzed. The results revealed that pioglitazone reduced the expression of Ccl2 both in vivo and in vitro. Furthermore, pioglitazone decreased the density of inducible nitric oxide synthase+ CD68+ macrophages and inhibited the expression of fibrosis-related factors on day 4 after injury. On day 56 after injury, pioglitazone inhibited fibrosis, tissue contracture, and hyaluronic acid loss in a PPARγ-dependent manner. These results indicate that PPARγ activation could inhibit accumulation of inflammatory macrophages and improve tissue repair. Considered together, these findings imply that inflammatory macrophages play a key role in vocal fold fibrosis.