RNF2 mediates pulmonary fibroblasts activation and proliferation by regulating mTOR and p16-CDK4-Rb1 signaling pathway

Pulmonary fibrosis (PF) is a chronic, progressive interstitial lung disease with unknown etiology, associated with increasing morbidity and pessimistic prognosis. Pulmonary fibroblasts (PFbs) are the key effector cells of PF, in which abnormal activation and proliferation is an important pathogenesis of PF. Ring finger protein 2 (RNF2), is identified as the catalytic subunit of poly-comb repressive complex 1, which is closely related to occurrence and development of lung cancer, but its function in PF has not been revealed. In this paper, we sought to identify the regulatory role of RNF2 in lung fibrogenesis and its underlying mechanisms.

The expression of RNF2 in lung fibrosis tissue (human and Bleomycin-induced mouse) and cell model (TGF-β1-induced HFL1 cells) was examined by immunoblotting analysis and immunofluorescence. Western blot, qRT-PCR were performed to evaluate the expression of pro-fibrogenic cytokines (including α-SMA, ECM and MMPs/ TIMPs) induced by TGF-β1 in HFL1 cells. Cell proliferation, cycle progression and apoptosis were examined by fow cytometric. Molecular interactions were tested by Co-IP assays.

RNF2 expression was elevated in PF tissues compared to normal adjacent tissues and in PFbs (HFL1) induced by TGF-β1. Furthermore, knockdown of RNF2 could evidently inhibit the abnormal expression of pro-fibrogenic cytokines (including α-SMA, ECM and MMPs/TIMPs) induced by TGF-β1 in HFL1 cells. Functionally, RNF2 silencing could significantly suppress TGF-β1-induced anomalous proliferation, cell cycle progression, apoptosis and autophagy in HFL1 cells. Mechanistically, RNF2 deficiency could effectively inhibit the abnormal activation of mTOR signaling pathway in TGF-β1-induced HFL1 cells, and mTOR pathway had feedback regulation on the expression of RNF2. Further studies RNF2 could regulate the phosphorylation level of RB1 through interacting with p16 to destroy the binding of p16 and CDK4 competitively. Simultaneously, overexpression of RNF2 could show the opposite results.

These results indicated that RNF2 is a potent pro-fibrogenic molecule for PFbs activation and proliferation through mTOR and p16-CDK4-Rb signaling pathways, and RNF2 inhibition will be a potential therapeutic avenue for treating PF.