Background: It is known that recognition of bacterial lipopolysaccharide (LPS) and various endogenous ligands by Toll-like receptor 4 (TLR4) induces inflammatory reactions. However, the role of TLR4 activation in mesangial inflammation remains to be elucidated. Retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5) are putative RNA helicases and are involved in immune and inflammatory reactions. The purpose of the present study was to investigate the implication of RIG-I and MDA5 in TLR4 signaling in mesangial cells.
Methods: Normal human mesangial cells in culture were treated with LPS. Expression of RIG-I, MDA5, interferon-β (IFN-β), CXCL10 and CXCL8 was examined using real-time RT-PCR, Western blotting and ELISA. The cells were also subjected to RNA interference against TLR4, IFN-β, RIG-I or MDA5.
Results: LPS induced the expression of IFN-β, RIG-I, MDA5, CXCL8 and CXCL10 in human mesangial cells. RNA interference against either TLR4 or IFN-β inhibited LPS-induced RIG-I and MDA5 expression. Knockdown of TLR4, IFN-β, RIG-I or MDA5 resulted in decreased induction of CXCL10, while only TLR4 knockdown inhibited CXCL8 induction.
Conclusions: TLR4 signaling induces the expression of RIG-I and MDA5 in mesangial cells. RIG-I and MDA5 may be involved in inflammatory reactions by regulating CXCL10 expression in the downstream of TLR4 signaling in human mesangial cells.