Background: Toll-like receptor (TLR)-4 signaling plays a key role in initiating exogenous antigen-independent innate immunity-dominated liver ischemia/reperfusion injury (IRI). Heme oxygenase (HO)-1, a heat-shock protein 32, exerts potent adaptive anti-oxidant and anti-inflammatory functions. Signal transducers and activator of transcription (STAT)-1 activation triggers interferon (IFN)-inducible protein 10 (CXCL-10), one of major products of type-1 IFN pathway downstream of TLR4. This study focuses on the role of type-1 IFN pathway in the mechanism of HO-1 cytoprotection during liver IRI.
Methods and results: Cobalt protoporphyrin (CoPP)-induced HO-1 overexpression ameliorated liver damage in a well-defined mouse model of liver warm IRI, as evidenced by improved hepatic function (serum alanine aminotransferase levels) and liver histology (Suzuki's scores). HO-1 downregulated phospho-STAT-1 and its key product, CXCL-10. In contrast, TLR4 expression remained elevated regardless of the IRI status. To dissect the mechanism of HO-1 upon CXCL-10, we cultured RW 264.7 (macrophage) cells with exogenous rIFN-beta to stimulate CXCL-10 production via TLR4 pathway in vitro. Indeed, CoPP-induced HO-1 suppressed otherwise highly upregulated rIFN-beta-triggered CXCL-10. Moreover, consistent with our in vitro data, CoPP pretreatment diminished rIFN-beta-induced CXCL-10 production in normal mouse livers.
Conclusion: Hepatic IRI activates TLR4 signaling in vivo to elaborate CXCL-10. HO-1 overexpression downregulates activation of STAT1 via type-1 IFN pathway downstream of TLR4, which in turn decreases CXCL-10 production. This study provides evidence for a novel mechanism by which HO-1 exerts adaptive cytoprotective and anti-inflammatory functions in the context of innate TLR4 activation.