960 Innate immune antimicrobial response by retinoic acid during reactive adipogenesis is dependent on HIF1α

Abstract

Cutaneous bacterial infection by Gram-positive bacteria induces an essential innate immune response in dermal fibroblasts resulting in rapid differentiation of a subset of fibroblasts into adipocytes (reactive adipogenesis). Induction of adipogenesis is accompanied by a large yet transient increase in production of the antimicrobial peptide (AMP) cathelicidin (Camp) by preadipocytes during transition to mature adipocytes. Exposure to retinoic acid (RA) further enhanced Camp expression in 3T3-L1 preadipocytes (129-fold vs. control p < 0.0001) and in vivo in mouse skin (7-fold vs. control, p < 0.001). When RA was present during bacterial infection with S. aureus, Camp expression increased 19-fold, p < 0.001. To understand the mechanism by which RA regulates Camp we examined the role of Hypoxia-inducible factor-1α (HIF-1α). Previous studies have shown that retinoic RA enhances hif1a expression in several cell types, but a connection between RA, HIF-1α and AMPs had not previously been suspected. In concordance with our hypothesis, a HIF-1α transcriptional binding site is present in the Camp promoter, and RA-treated 3T3-L1 cells exhibited ∼2-fold higher hif1a mRNA expression compared to differentiated controls (p < 0.001). Immunostaining and Western blot also showed significantly higher levels of HIF-1α after RA treatment in vitro. In mice, RA treatment resulted in nearly a 2-fold increase in hif1a expression (p < 0.001), while a ∼3-fold, p < 0.0001 increase was seen in mice treated with RA during S. aureus infection. Pharmacological induction of HIF-1α by addition of the agonist (mimosine) in 3T3-L1 cells resulted in enhanced Camp expression comparable to RA (37- and 30- fold respectively vs. control, p < 0.0001) while cotreatment of RA and mimosine was synergistic (112-fold vs. control, p < 0.0001). HIF-1α inhibition by the inhibitor acriflavine abolished Camp expression following RA treatment. These findings link HIF-1α and RA in the capacity of dermal adipogenesis to fight infection.