TGF-β and Iron Differently Alter HBV Replication in Human Hepatocytes through TGF-β/BMP Signaling and Cellular MicroRNA Expression

The nature of host-virus interactions in hepatitis B virus infection is incompletely understood. Since soluble factors, e.g., cytokines and metals, may exacerbate liver injury in chronic hepatitis, we considered that defining the effects of receptor-mediated signaling upon viral replication will be significant. Consequently, we studied effects of iron or TGF-beta-induced TGF-beta/BMP signaling in the HepG2 2.2.15 cell model of hepatitis B virus replication. We found iron and TGF-beta increased hepcidin mRNA expression or TGF-beta receptor kinase activity, respectively, which indicated that 2.2.15 cells responded appropriately to these substances. However, iron increased but TGF-beta decreased hepatitis B virus mRNA and DNA expression. TGF-beta-induced expression at the mRNA level of multiple TGF-beta/BMP pathway genes. This change was not observed in iron-treated cells. On the other hand, presence of SMAD proteins in iron or TGF-beta-treated cells, including of SMAD4, did confirm convergence of TGF-beta/BMP signaling pathways under these conditions. Since transcription factors in TGF-beta/BMP signaling pathways could not have directly targeted hepatitis B virus itself, we studied whether iron or TGF-beta exerted their effects through alternative mechanisms, such as by involvement of antiviral cellular microRNAs. We discovered cellular microRNA expression profiles were significantly different in iron or TGF-beta-treated cells compared with untreated control cells. In many cases, exposure to iron or TGF-beta changed microRNA expression in opposite directions. Introduction in cells of sequences representing such differentially expressed microRNAs, e.g., hsa-miR-125a-5p and -151-5p, even reproduced effects on virus replication of iron- or TGF-beta. We surmised that TGF-beta/BMP pathway members, i.e., SMADs, likely governed iron or TGF-beta-induced microRNA expression. Iron may have mediated Drosha/DGCR8/heme-mediated processing of microRNAs. In turn, cellular microRNAs regulated replication of hepatitis B virus in iron or TGF-beta-treated cells. This knowledge should advance studies of mechanisms in viral-host interactions, hepatic injury, and therapeutic developments for hepatitis B.