FXR1 Regulates Inflammatory MRNA Stability and VSMC Proliferation by Modulation of HuR Activity

Vascular smooth muscle cells (VSMC) play a critical role in the etiology and progression of many vascular diseases including atherosclerosis and restenosis. Our laboratory has found that one anti-inflammatory interleukin, IL-19, is atheroprotective and can decrease vascular inflammation by reduction in mRNA stability of inflammatory transcripts by reduction of activity of HuR, an mRNA stability protein. HuR translocates from the nucleus to the cytoplasm where it recognizes AU-rich elements present almost exclusively in the 3’UTR of pro-inflammatory genes. Proteins and pathways which limit HuR translocation are understudied, but may reduce inflammatory mRNA stability. Using MASS SPEC to identify HuR-interacting proteins under different inflammatory conditions, we identified one protein, Fragile X-related protein (FXR1), which interacts with HuR in inflammatory, but not basal conditions, a novel interaction. FXR1 mRNA expression is enhanced in muscle cells, but nothing has been reported on expression of FXR1 in VSMC or function for FXR1 in vascular disease. The FXR1 promoter contains multiple cholesterol-response elements, and in this study we demonstrate that FXR1 expression is increased in injured arteries and TNFα and oxLDL stimulated human VSMC, but also by IL-19. RNA EMSA demonstrates that FXR1 directly interacts with ARE in 3’UTR. SiRNA knock down of FXR1 in VSMC increases stability of inflammatory mRNA and protein abundance as well as VSMC proliferation, while overexpression of FXR1 reduces both their abundance and stability in addition to reducing proliferation. Since FXR1 appears to be a novel repressor of inflammatory proteins, and is also induced by IL-19, our overall hypothesis is that FXR1 expression and HuR interaction is an inflammation responsive, counter-regulatory mechanism to reduce abundance of pro-inflammatory proteins and therefore reduce inflammation.