4 Now joining this group of entry factors are RTKs, which Lupberger et al. have demonstrated in vitro and selleck in vivo to specifically cooperate with CD81 and CLDN1 to facilitate the intricate process of HCV entry. Using a large-scale short interfering RNA (siRNA) screen against 691 known human kinases, Lupberger et al. revealed 58 kinases that appear to have a role in the HCV life cycle. The investigators focused on two RTKs: epidermal

growth factor receptor (EGFR) and ephrin receptor A2 (EphA2). Focus was placed on these two RTKs because their functions have been extensively documented. Furthermore, they are highly expressed in the human liver, and protein kinase inhibitors (PKIs) specific for EGFR and EphA2 are approved Trichostatin A clinically for use in the treatment of other conditions.5-7 RTKs are activated after growth factor(s) bind to their extracellular

ligand-binding domain, resulting in receptor dimerization and subsequent activation of intracellular signaling pathways.8 Perhaps, it is not surprising that RTKs are involved in the HCV life cycle, given that they are known to regulate a vast number of cellular processes, namely proliferation, differentiation, survival, metabolism, migration, and cell-cycle control.9 A number of elegant techniques were employed by the investigators to demonstrate that EGFR and EphA2 are necessary for HCV entry. Inhibition of EGFR and EphA2 with the PKIs, erlotinib or dasatinib, respectively, inhibited HCV entry into 上海皓元医药股份有限公司 hepatoma cells and primary human hepatocytes without affecting HCV-RNA replication. Similarly, the blocking of these RTKs with specific antibodies and siRNA-mediated knockdown markedly decreased HCV entry. Mechanistically, the investigators showed that activation of EGFR and EphA2 promote an association between the HCV coreceptors, CD81 and CLDN1. This association and trafficking of these receptors is perturbed by treatment with PKIs erlotinib and dasatinib, and, in turn, HCV entry is blocked. Interestingly, PKI treatment did not appear to alter expression levels of CD81, CLDN1, or the other

HCV entry factors, SR-BI and OCLN. Furthermore, using cell-fusion assays it was shown that EGFR potentially plays a functional role in late steps of HCV entry, specifically via facilitating the fusion of the viral envelope to host cell membranes. To this end, treatment of the hepatocyte-derived cell lines, Huh-7.5.1, polarized HepG2 cells (expressing CD81), and primary human hepatocytes with EGF and transforming growth factor alpha (TGF-α), ligands of EGFR, appeared to increase the association between CD81 and CLDN1 and enhance the fusion of viral and host membranes, leading to increased uptake of HCV (Fig. 1). These extensive in vitro investigations were substantiated with the use of the well-characterized chimeric urokinase plasminogen activator/severe combined immunodeficiency (uPA-SCID) mouse model.