The hepatitis C virus (HCV) interferes with several cellular signaling pathways to warrant viral replication as well as survival of the infected cell and to protect the infected cell from antiviral efector mechanisms of the host. Thus it counteracts the interferon response to viral infection through non-structural 3/4A (NS3/4A) protease-mediated cleavage of CARDIF and TRIF thereby interrupting an important part of the innate antviral host response. The present study aimed to further clarify the interaction between NS3/4A and growth factor signaling of the host cell and its contribution to viral replication. We report that HCV NS3/4A-dependently suppresses T-cell protein tyrosine phosphatase (TC-PTP) expression through NS3/4A-dependent cleavage of TC-PTP. This down-regulation of TC-PTP leads to enhanced EGF-induced signal-transduction and increased basal as well as induced activity of Akt. Whereas Akt-activity is essential for sufficient viral replication activation of the EGF receptor and of its downstream efector the PI3-kinase are able to transmit enhancing effects of EGF on viral replication but not necesseraly required for viral replication. The enhanced activating phosphorylation of Akt may be due to a suppression of protein phosphatase (PP)2A activity through the enhanced tyrosine phosphorylation at tyrosine residue 307 that occurs in the absence of TC-PTP. Hence, our findings support the notion that therapeutic targeting of NS3/4A may not only disturb viral replication by blocking the processing of the viral polyprotein but also exerts unforeseen indirect antiviral effects further diminishing viral replication. The NS3/4A inhibition reduces the processing of the viral polyprotein but also deprives HCV of modulating EGF-R signaling to its own purposes.
Recently, the epidermal growth factor (EGF) receptor (EGFR), a member of the ErbB receptor family, and its down-stream signalling have been identified as co-factors for HCV entry and replication. Since EGFR also functions as a heterodimer with other ErbB receptor family members, the subject of the present study was to investigate a possible viral interference with these cellular components. By using genotype 1b replicon cells as well as an infection-based system we found that while transcript and protein levels of EGFR and ErbB2 were up-regulated or unaffected, respectively, HCV induced a substantial reduction of ErbB3 and ErbB4 expression. Down-regulation of ErbB3 expression by HCV involves specificity protein (Sp)1-mediated induction of Neuregulin (NRG)1 expression as well as activation of Akt. Consistently, at transcript level disruption of ErbB3 expression by HCV can be prevented by knockdown of NRG1 or Sp1 expression, whereas reconstitution of ErbB3 protein levels requires inhibition of HCV-induced NRG1 expression and of Akt activity. Interestingly, the NRG1-mediated suppression of ErbB3 expression by HCV results in an enhanced expression of EGFR and ErbB2 on the cell surface, which can be mimicked by siRNA-mediated knockdown of ErbB3 expression. These data delineate a novel mechanism enabling HCV to sway the composition of the ErbB family members on the surface of its host cell by an NRG1-driven circuit and unravels a yet unknown cross-regulation between ErbB3 and the two other family members ErbB2 and EGFR. The shift of the receptor surface expression of the ErbB family towards enhanced expression of ErbB2 and EGFR triggered by HCV was found to promote viral RNA replication and infectivity. This suggests that HCV rearranges expression of ErbB family members to adapt the cellular environment to its requirements.
Abstract Background The filamentous fungus Sordaria macrospora forms complex three-dimensional fruiting bodies called perithecia that protect the developing ascospores and ensure their proper discharge. In previous microarray analyses, several genes have been identified that are downregulated in sterile mutants compared to the wild type. Among these genes was tap1 ( t ranscript a ssociated with p erithecial development), a gene encoding a putative lectin homolog. Results Analysis of tap1 transcript levels in the wild type under conditions allowing only vegetative growth compared to conditions that lead to fruiting body development showed that tap1 is not only downregulated in developmental mutants but is also upregulated in the wild type during fruiting body development. We have cloned and sequenced a 3.2 kb fragment of genomic DNA containing the tap1 open reading frame and adjoining sequences. The genomic region comprising tap1 is syntenic to its homologous region in the closely related filamentous fungus Neurospora crassa . To determine whether tap1 is involved in fruiting body development in S. macrospora , a knockout construct was generated in which the tap1 open reading frame was replaced by the hygromycin B resistance gene hph under the control of fungal regulatory regions. Transformation of the S. macrospora wild type with this construct resulted in a tap1 deletion strain where tap1 had been replaced by the hph cassette. The knockout strain displayed no phenotypic differences under conditions of vegetative growth and sexual development when compared to the wild type. Double mutants carrying the Δ tap1 allele in several developmental mutant backgrounds were phenotypically similar to the corresponding developmental mutant strains. Conclusion The tap1 transcript is strongly upregulated during sexual development in S. macrospora ; however, analysis of a tap1 knockout strain shows that tap1 is not essential for fruiting body formation in S. macrospora .
The hepatitis C virus (HCV) is a worldwide major cause of chronic liver disease with a high tendency to establish a persistent infection. To permit persistent replication of viral genomes through the cellular translation machinery without affecting host cell viability, viruses must have developed mechanisms to control cellular cascades required for sufficient viral replication, on the one hand, and to adapt viral replication to the cellular requirements on the other hand. The present study aimed to further elucidate mechanisms by which HCV targets growth factor signaling of the host cell and their implications for viral replication. The study describes a novel mechanism by which HCV influences the activation of the epithelial growth factor receptor/Akt pathway through a nonstructural (NS)3/4A-dependent down-regulation of the ubiquitously expressed tyrosine phosphatase T cell protein tyrosine phosphatase (TC-PTP). NS3/4A is demonstrated to cleave TC-PTP protease-dependently in vitro at two cleavage sites. The in vivo relevance of this finding is supported by the fact that down-regulation of TC-PTP protein expression could also be demonstrated in HCV-infected individuals and in transgenic mice with intrahepatic expression of NS3/4A. Conclusion: This down-regulation of TC-PTP results in an enhancement of epithelial growth factor (EGF)-induced signal transduction and increases basal activity of Akt, which is demonstrated to be essential for the maintenance of sufficient viral replication. Hence, therapeutic targeting of NS3/4A may not only disturb viral replication by blocking the processing of the viral polyprotein but also exerts unforeseen indirect antiviral effects, further diminishing viral replication. (HEPATOLOGY 2009;49:1810–1820.)
