Sumoylation of PPARγ by apoptotic cells prevents LPS-induced NCoR removal from κB binding sites mediating transrepression of pro-inflammatory cytokines

摘要

Efficient clearance of apoptotic cells (AC) by professional phagocytes is crucial for tissue homeostasis and resolution of inflammation. Macrophages respond to AC with an increase in anti-inflammatory cytokine production, but a diminished release of pro-inflammatory mediators. Mechanisms to explain attenuated pro-inflammatory cytokine formation remain elusive. We provide evidence that peroxisome proliferator-activated receptor γ (PPARγ) coordinates anti-inflammatory responses following its activation by AC. Exposing murine RAW264.7 macrophages to AC prior to LPS-stimulation, reduced NFκB transactivation and lowered target gene expression of e.g. TNFα and IL-6 compared to controls. In macrophages overexpressing a dominant negative (d) mutant of PPARγ, NFκB transactivation in response to LPS was restored, while macrophages from myeloid lineage-specific conditional PPARγ knockout mice proved that PPARγ transmitted an anti-inflammatory response, delivered by AC. Expressing a PPARγ-Δaa32-250 deletion mutant, we observed no inhibition of NFκB. Analyzing the PPARγ domain structures within aa32-250, we anticipated PPARγ sumoylation in mediating the anti-inflammatory effect in response to AC. Interfering with sumoylation of PPARγ by mutating the predicted sumoylation site (K77R), or knockdown of the SUMO E3 ligase PIAS1, eliminated the ability of AC to suppress NFκB. ChIP analysis demonstrated that AC prevented the LPS-induced removal of nuclear receptor co-repressor (NCoR) from the κB site within the TNFα promoter. We conclude that AC induce PPARγ sumoylation to attenuate the removal of NCoR, thereby blocking transactivation of NFκB. This contributes to an anti-inflammatory phenotype shift in macrophages responding to AC, by lowering pro-inflammatory cytokine production.