E3 ligase RNF145 regulates airway smooth muscle cell proliferation via the PP2A-p70S6K signaling axis

Airway smooth muscle (ASM) is a key determinant of airway caliber and a major contributor to structural remodeling in obstructive lung diseases, including asthma and COPD. ASM proliferation is regulated by transcriptional and post-transcriptional mechanisms, including ubiquitin-dependent protein turnover mediated by E3 ubiquitin ligases. Here, we investigated the role of the RING-type E3 ligase RNF145 in mitogenic signaling and ASM cell proliferation. Human ASM cells were transfected with RNF145 shRNA or treated with the E3 ligase inhibitor SMER3, followed by stimulation with fetal bovine serum (FBS) or platelet-derived growth factor (PDGF). RNF145 knockdown or SMER3 treatment dose-dependently inhibited mitogen-induced ASM cell proliferation without inducing cytotoxicity, supporting a pro-mitogenic role for RNF145. Both interventions reduced phosphorylation of p70S6K at Thr421/Ser424 and Thr389 without affecting ERK MAPK signaling. Inhibition of PP2A and PP1 with okadaic acid or calyculin A restored p70S6K phosphorylation in RNF145-deficient cells, while SMER3 increased serine/threonine phosphatase activity. Mechanistically, RNF145 promoted K48-linked ubiquitination and proteasomal degradation of the PP2A scaffold subunit Aα, thereby limiting PP2A holoenzyme assembly during mitogenic signaling. Re-expression of RNF145 in RNF145-deficient cells reduced PP2A Aα abundance and restored p70S6K activation and ASM cell proliferation. These findings identify RNF145 as a positive regulator of ASM cell proliferation that sustains p70S6K signaling by suppressing PP2A. The RNF145-PP2A-p70S6K axis may therefore represent a therapeutic target for airway remodeling in chronic obstructive airway diseases.