The role of the PI3K/AKT/mTOR pathway in mediating PD-L1 upregulation during fibroblast transdifferentiation

Silicosis is a progressive interstitial lung disease characterized by diffuse pulmonary fibrosis. The transdifferentiation of lung fibroblasts into myofibroblasts is a key cellular event driving the progression of silicosis fibrosis. Recent studies have shown that PD-L1 expression is significantly upregulated in activated fibroblasts, and PD-L1 plays a crucial role in mediating fibroblast transdifferentiation. This study aims to elucidate the molecular mechanisms regulating PD-L1 expression in fibroblasts and analyze the functional significance of PD-L1 upregulation in fibroblast activity and silicosis fibrosis. In this research, an in vitro model of TGF-β1-induced NIH-3 T3 fibroblast transdifferentiation was established. Small molecule inhibitors, siRNA, and plasmids were used to interfere with the PI3K/AKT/mTOR signaling pathway and PD-L1 expression. It was found that TGF-β1 stimulation increased PD-L1 expression in fibroblasts, while blocking the PI3K/AKT/mTOR pathway inhibited this upregulation. Knockdown of PD-L1 significantly inhibited fibroblast transdifferentiation and impeded TGF-β1-induced activation of the PI3K/AKT/mTOR pathway, whereas PD-L1 overexpression had the opposite effect. Additionally, PD-L1 protein in fibroblasts undergoes ubiquitin-proteasome-mediated degradation, negatively regulating PD-L1 upregulation. In vivo, adeno-associated virus was used to specifically knockdown PD-L1 in mouse lung fibroblasts, resulting in significantly reduced lung tissue damage and fibrosis in silicosis mice. This effect was associated with the involvement of the PI3K/AKT/mTOR pathway. In summary, PD-L1 expression in fibroblasts is upregulated during transdifferentiation, a process regulated by the PI3K/AKT/mTOR pathway. Upregulated PD-L1 enhances PI3K/AKT/mTOR signaling through positive feedback, sustaining fibroblast activation. Ubiquitin-proteasome-mediated protein degradation may serve as a negative feedback mechanism maintaining PD-L1 homeostasis.