The Hepatitis C Virus NS5A Protein Activates a Phosphoinositide 3-Kinase-dependent Survival Signaling Cascade

Abstract

Hepatitis C virus (HCV) establishes a persistent infection, with up to 80% of infected individuals proceeding to chronic hepatitis, which in many cases may result in liver cirrhosis and hepatocellular carcinoma (HCC); indeed HCV infection is increasingly associated with the development of HCC. The long time period (up to 30 years) between primary infection and the onset of HCC implies that HCV is not directly oncogenic but in some way predisposes patients to develop tumors, though the mechanism for this is unclear as yet. We report here that NS5A binds directly to the Src homology 3 domain of the p85 regulatory subunit of phosphoinositide 3-kinase (PI3K), and this interaction is mediated by a novel (nonproline-rich) motif within NS5A. Coimmunoprecipitation analysis revealed that NS5A bound native heterodimeric PI3K and enhanced the phosphotransferase activity of the catalytic (p110) subunit both in vitro and in human cell lines harboring a subgenomic HCV replicon or expressing NS5A alone. NS5A-mediated activation of PI3K resulted in increased phosphorylation and activity of Akt/protein kinase B and concomitantly provided protection against the induction of apoptosis in both replicon-harboring cells and cells stably expressing NS5A alone. These data suggest that stimulation of PI3K by NS5A may represent an indirect mechanism for development of HCC in HCV-infected patients and further suggests potential therapeutic strategies to counteract the occurrence of HCV-related HCC.

Hepatitis C virus NS5A protein interacts with -catenin and stimulates its transcriptional activity in a phosphoinositide-3 kinase-dependent fashion

“…We have previously shown that NS5A bound to and activated PI3K, an event which led to the downstream phosphorylation and activation of Akt (Street et al, 2004). Furthermore, NS5A induced the stabilization of b-catenin and an increase in b-catenin-dependent transcriptional activity in a manner that was PI3K dependent (Street et al, 2005).…”

“…We have demonstrated previously that NS5A interacts with the p85 regulatory subunit of PI3K (Street et al, 2004). This interaction was further shown to require residues 270-300 of NS5A.…”

“…It functions as a component of an RNA replication complex on cytoplasmic membranes together with the other non-structural proteins (Gosert et al, 2003), and it also plays a role in modulating cellular signalling pathways with implications for viral pathogenesis . Of particular interest is our previous observation (Street et al, 2004), and that of He et al (2002), that NS5A binds to, and activates, phosphoinositide 3-kinase (PI3K), resulting in activation of the downstream effector serine/threonine kinase Akt (PKB). We further showed that this resulted in the stabilization of the proto-oncogene b-catenin and concomitant stimulation of b-catenin-dependent transcription (Street et al, 2005), with implications for the development of HCC.…”

Signal transduction cascades and hepatitis B and C related hepatocellular carcinoma

“…142 This results in activation of downstream Akt, which in turn phosphorylates and inactivates Bad, a pro-apoptotic Bcl-2 homologue, thereby inhibiting apoptosis. 142,143 As shown in Fig. 1, another downstream target of Akt is GSK-3␤.…”

Regulation of c‐Met signaling by the tetraspanin KAI‐1/CD82 affects cancer cell migration

“…6,7 Among these downstream targets, the Ras-and phosphatidylinositol 3-kinase (PI3K)-dependent pathways are required to trigger cell proliferation, motility and survival. 8,9 Tetraspanins, or TM4SF proteins, compose a large group of cell surface transmembrane proteins, some of which can form complexes with integrins. Several tetraspanins appear to be particularly relevant to tumor cell metastasis.…”