SKP2-Promoted Ubiquitination of FOXO3 Promotes the Development of Asthma.

Asthma is a respiratory disease with a dramatically increasing incidence globally. The present study explored the roles of S-phase kinase-associated protein 2 (SKP2) and forkhead box O3 (FOXO3) in asthma and their involvement in the Krüppel-like factor 15-lipoprotein receptor-related protein 5 (KLF15-LRP5) axis. SKP2 expression in patients with asthma and OVA-induced asthmatic Sprague Dawley rats was detected by reverse transcription quantitative PCR and Western blot assays. Alterations in SKP2 and LRP5 expression were evaluated in OVA-induced asthmatic rats, followed by measurement of inflammatory cytokines using ELISA and airway resistance using a methacholine challenge test. We applied TGF-β1 to establish the airway smooth muscle cell (ASMC) proliferation model of asthma. The FOXO3 ubiquitination and changes in cell biological behaviors were detected using immunoprecipitation, MTT, and Annexin V/propidium iodide assays. Flow cytometry was adopted to detect cell cycle, and ELISA was used to measure the concentrations of IL-4, IL-5, IL-13, and IgE in rat bronchoalveolar lavage fluid. SKP2 was highly expressed and FOXO3 was poorly expressed in patients with asthma and in OVA-induced asthmatic rats. SKP2 silencing decreased IL-4, IL-5, IL-13, and IgE expression in rat bronchoalveolar lavage fluid, whereas SKP2 enhanced FOXO3 ubiquitination to upregulate KLF15, which bound to the LRP5 promoter in TGF-β1-induced ASMCs and increased LRP5 expression. SKP2 enhanced airway hyperresponsiveness and inflammation in the OVA-induced rat model and augmented TGF-β1-induced ASMC proliferation by inhibiting the FOXO3/KLF15/LRP5 axis. Additionally, overexpressed SKP2 resulted in reduced numbers of ASMCs in the G1 phase but increased numbers in the G2/M phase. Collectively, we show that SKP2 promotes FOXO3 ubiquitination to suppress the KLF15-LRP5 axis, thereby exacerbating asthma.