Reduced scytonemin isolated from Nostoc commune suppresses LPS/IFNγ-induced NO production in murine macrophage RAW264 cells by inducing hemeoxygenase-1 expression via the Nrf2/ARE pathway

Reduced scytonemin (R-scy) and scytonemin (Scy) isolated from Nostoc commune exhibit anti-tumor and ultraviolet-absorbing properties. In this study, we examined the effects of R-scy and Scy on the induction of nitric oxide (NO) production by lipopolysaccharide (LPS) and interferon-γ (IFNγ) in murine macrophage RAW264 cells. While both R-scy and Scy suppressed LPS/IFNγ-induced NO production, R-scy exhibited a stronger inhibitory effect compared with Scy. To further elucidate the mechanisms underlying the anti-inflammatory effects of R-scy, we examined the changes in the intracellular signaling cascade after LPS/IFNγ stimulation in cells. In addition to the attenuation of LPS/IFNγ-induced upregulation of the inducible isoform of NO synthase, R-scy decreased the activity of nuclear factor-κB, phosphatidylinositol 3-kinase (PI3K)/Akt, and mitogen-activated protein kinases (MAPKs) after LPS/IFNγ stimulation. R-scy treatment increased heme oxygenase-1 (HO-1) expression by increasing the intracellular levels of reactive oxygen species and thereby activating nuclear factor erythroid 2-related factor 2 (Nrf2) and antioxidant response element signaling. The induction of HO-1 by R-scy was inhibited by pretreatment with an antioxidant, N-acetyl-cysteine (NAC), as well as SB203580 and LY294002, inhibitors for p38 MAPK and PI3K/Akt, respectively. Our findings suggest that the anti-inflammatory effects of R-scy could involve both the ROS/PI3K/Akt and the p38 MAPK/Nrf2 signaling pathways.