MicroRNA-27b Contributes to Lipopolysaccharide-mediated Peroxisome Proliferator-activated Receptor γ (PPARγ) mRNA Destabilization

Peroxisome proliferator-activated receptor gamma (PPAR gamma) gained considerable interest as a therapeutic target during chronic inflammatory diseases. Remarkably, the pathogenesis of diseases such as multiple sclerosis or Alzheimer is associated with impaired PPAR gamma expression. Considering that regulation of PPAR gamma expression during inflammation is largely unknown, we were interested in elucidating underlying mechanisms. To this end, we initiated an inflammatory response by exposing primary human macrophages to lipopolysaccharide (LPS) and observed a rapid decline of PPAR gamma 1 expression. Because promoter activities were not affected by LPS, we focused on mRNA stability and noticed a decreased mRNA half-life. As RNA stability is often regulated via 3'-untranslated regions (UTRs), we analyzed the impact of the PPAR gamma-3'-UTR by reporter assays using specific constructs. LPS significantly reduced luciferase activity of the pGL3-PPAR gamma-3'-UTR, suggesting that PPAR gamma 1 mRNA is destabilized. Deletion or mutation of a potential microRNA-27a/b (miR-27a/b) binding site within the 3'-UTR restored luciferase activity. Moreover, inhibition of miR-27b, which was induced upon LPS exposure, partially reversed PPAR gamma 1 mRNA decay, whereas miR-27b overexpression decreased PPAR gamma 1 mRNA content. In addition, LPS further reduced this decay. The functional relevance of miR-27b-dependent PPAR gamma 1 decrease was proven by inhibition or overexpression of miR-27b, which affected LPS-induced expression of the pro-inflammatory cytokines tumor necrosis factor alpha (TNF alpha) and interleukin (IL)-6. We provide evidence that LPS-induced miR-27b contributes to destabilization of PPAR gamma 1 mRNA. Understanding molecular mechanisms decreasing PPAR gamma might help to better appreciate inflammatory diseases.