MD-2 is involved in the stimulation of matrix metalloproteinase-1 expression by interferon-γ and high glucose in mononuclear cells - a potential role of MD-2 in Toll-like receptor 4-independent signalling.

We reported recently that treatment of diabetic apolipoprotein E-deficient mice with the Toll-like receptor 4 (TLR4) antagonist Rs-LPS, a lipopolysaccharide isolated from Rhodobacter sphaeroides, inhibited atherosclerosis. Since it is known that Rs-LPS antagonizes TLR4 by targeting TLR4 co-receptor MD-2, this finding indicates that MD-2 is a potential target for the treatment of atherosclerosis. In this study, we determined if MD-2 is involved in the gene expression regulated by signalling pathways independent of TLR4. Given that interferon- (IFN) and hyperglycaemia play key roles in atherosclerosis, we determined if MD-2 is involved in IFN- and high-glucose-regulated gene expression in mononuclear cells. Results showed that IFN- and high glucose synergistically stimulated matrix metalloproteinase 1 (MMP-1), a proteinase essential for vascular tissue remodelling and atherosclerosis, in U937 mononuclear cells, but Rs-LPS inhibited the MMP-1 stimulation. To provide more evidence for a role of MD-2 in IFN--stimulated MMP-1, studies using antibodies and small interfering RNA demonstrated that MD-2 blockade or knockdown attenuated the effect of IFN- on MMP-1. Furthermore, studies using PCR arrays showed that MD-2 blockade had a similar effect as IFN- receptor blockade on the inhibition of IFN--stimulated pro-inflammatory molecules. Although these findings indicate the involvement of MD-2 in IFN- signalling, we also observed that MD-2 was up-regulated by IFN- and high glucose. We found that MD-2 up-regulation by IFN- played an essential role in the synergistic effect of IFN- and LPS on MMP-1 expression. Taken together, these findings indicate that MD-2 is involved in IFN- signalling and IFN--augmented MMP-1 up-regulation by LPS.