Interferon-γ decreases ATP-binding cassette subfamily G member 1-mediated cholesterol efflux through small ubiquitin-like modifier/ubiquitin-dependent liver X receptor-α degradation in macrophages

The effects of interferon-γ (IFN-γ) on cholesterol accumulation and the development of foam cells are still unclear. In the present study, we found that IFN-γ promoted liver X receptor (LXR)-α degradation through the ubiquitin–proteasome system in macrophages. The process was dependent on its interactions with phosphorylated signal transducer and activator of transcription 1 (p-STAT1) and protein inhibitor of activated STAT 1 (PIAS1) because both fludarabine and PIAS1 shRNA reversed the decrease in LXR-α protein expression induced by IFN-γ. Additionally, IFN-γ enhanced the interactions of ubiquitin-conjugating enzyme 9 (UBC9), small ubiquitin-like modifier (SUMO)-1 and SUMO-2/3 with LXR-α. Moreover, treatment with shRNA specific for them not only reduced LXR-α polyubiquitination but also reversed the IFN-γ-induced decrease in its expression. Two specific sumoylation sites in LXR-α, K22 and K326, were indispensable for its IFN-γ-induced polyubiquitination because the K22R and K326R mutations inhibited the polyubiquitination and degradation of LXR-α in IFN-γ-treated macrophages. In addition, K22R or K326R mutation almost completely restored ATP-binding cassette subfamily G member 1 (ABCG1)-mediated cholesterol efflux in IFN-γ-treated macrophages. Taken together, these findings indicate that IFN-γ promotes LXR-α degradation through a SUMO–ubiquitin-dependent pathway, which may inhibit cholesterol efflux mediated by ABCG1 from macrophages and promote the development of atherosclerosis.