Immunology Enzyme-Linked Signaling

Annalisa M. VanHook

Science Signaling, AAAS, Washington, DC 20005, USA

Bilirubin (BR) acts as a cytoprotective agent by virtue of its antioxidant properties, but its precursor biliverdin (BV) also has anti-inflammatory activity. Wegiel et al. present evidence that the enzymatic conversion of BV to BR by biliverdin reductase (BVR), which also acts as a kinase and a transcription factor, initiates a signaling cascade that suppresses inflammation. BVR was present on the surface of RAW macrophages, oriented with its catalytic domain outside the cell, and lipopolysaccharide (LPS) treatment increased the amount of BVR on the cell surface by stimulating translocation of BVR from the cytoplasm to the cell surface. In vivo LPS treatment also increased the abundance of BVR on the cell surface of hepatic macrophages. Western blotting and coimmunoprecipitation assays indicated that BVR became phosphorylated in response to binding BV and that wild-type BVR, but not kinase-dead or catalytically inactive forms, formed a complex with the p85 subunit of phosphatidylinositol 3-kinase (PI3K) in both RAW macrophages and human embryonic kidney (HEK) cells. BV treatment induced phosphorylation of the kinase Akt, a direct target of PI3K, in both RAW and bone marrow–derived macrophages, and this activation required the BVR-p85 interaction and the kinase activities of both BVR and PI3K. BV treatment also increased production of the anti-inflammatory cytokine interleukin-10 (IL-10) in wild-type RAW cells but not in cells harboring a kinase-dead BVR or a dominant-negative form of Akt. Overexpression of BVR enhanced IL-10 production in response to LPS, and small interfering RNAs (siRNAs) directed against BVR or Akt prevented this. Finally, pretreating mice with BV protected them from a lethal dose of LPS and prevented liver damage induced by injection of LPS plus D-galactosamine; both of these protective effects were prevented by a BVR-specific siRNA. The authors model that BV binding to BVR stimulates tyrosine phosphorylation of BVR, leading to phosphorylation of p85 and subsequent activation of Akt, one effect of which is an increase in IL-10 production. Thus, BV’s cytoprotective activities may be derived not only from its antioxidant properties but also through signaling events.

B. Wegiel, C. J. Baty, D. Gallo, E. Csizmadia, J. R. Scott, A. Akhavan, B. Y. Chin, E. Kaczmarek, J. Alam, F. H. Bach, B. S. Zuckerbraun, L. E. Otterbein, Cell surface biliverdin reductase mediates biliverdin-induced anti-inflammatory effects via phosphatidylinositol 3-kinase and Akt. J. Biol. Chem. 284, 21369–21378 (2009). [Abstract] [Full Text]