ABSTRACT
BACKGROUND/AIMS: Osteoprotegerin (OPG) is a profibrotic mediator produced by myofibro-blasts under influence of transforming growth factor ß (TGFß). Its expression in experimental models of liver fibrosis correlates well with disease severity and treatment responses. The regulation of OPG in liver tissue is largely unknown and we therefore set out to elucidate which growth factors/interleukins associated with fibrosis induce OPG and through which pathways. METHODS: Precision-cut liver slices of wild type and STAT6-deficient mice and 3T3 fibroblasts were used to investigate the effects of TGFß, interleukin (IL) 13 (IL13), IL1ß, and platelet-derived growth factor BB (PDGF-BB) on expression of OPG. OPG protein was measure by ELISA, whereas OPG mRNA and expression of other relevant genes was measured by qPCR. RESULTS: In addition to TGFß, only IL13 and not PDGF-BB or IL1ß could induce OPG expression in 3T3 fibroblasts and liver slices. This IL13-dependent induction was not shown in liver slices of STAT6-deficient mice and when wild type slices were cotreated with TGFß receptor 1 kinase inhibitor galunisertib, STAT6 inhibitor AS1517499, or AP1 inhibitor T5224. This suggests that the OPG-inducing effect of IL13 is mediated through IL13 receptor α1-activation and subsequent STAT6-dependent upregulation of IL13 receptor α2, which in turn activates AP1 and induces production of TGFß and subsequent production of OPG. CONCLUSION: We have shown that IL13 induces OPG release by liver tissue through a TGFß-dependent pathway involving both the α1 and the α2 receptor of IL13 and transcription factors STAT6 and AP1. OPG may therefore be a novel target for the treatment liver fibrosis as it is mechanistically linked to two important regulators of fibrosis in liver, namely IL13 and TGFß1.
Subject(s)
Gene Expression Regulation , Interleukin-13/metabolism , Liver Cirrhosis/metabolism , Liver/metabolism , Osteoprotegerin/biosynthesis , Signal Transduction , Transforming Growth Factor beta/metabolism , Animals , Female , Male , MiceABSTRACT
Osteoprotegerin (OPG) serum levels are associated with liver fibrogenesis and have been proposed as a biomarker for diagnosis. However, the source and role of OPG in liver fibrosis are unknown, as is the question of whether OPG expression responds to treatment. Therefore, we aimed to elucidate the fibrotic regulation of OPG production and its possible function in human and mouse livers. OPG levels were significantly higher in lysates of human and mouse fibrotic livers compared to healthy livers. Hepatic OPG expression localized in cirrhotic collagenous bands in and around myofibroblasts. Single cell sequencing of murine liver cells showed hepatic stellate cells (HSC) to be the main producers of OPG in healthy livers. Using mouse precision-cut liver slices, we found OPG production induced by transforming growth factor ß1 (TGFß1) stimulation. Moreover, OPG itself stimulated expression of genes associated with fibrogenesis in liver slices through TGFß1, suggesting profibrotic activity of OPG. Resolution of fibrosis in mice was associated with decreased production of OPG compared to ongoing fibrosis. OPG may stimulate fibrogenesis through TGFß1 and is associated with the degree of fibrogenesis. It should therefore be investigated further as a possible drug target for liver fibrosis or biomarker for treatment success of novel antifibrotics.