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J. Biol. Chem., Vol. 282, Issue 16, 11836-11848, April 20, 2007

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p21-activated Kinase 1 Is Activated through the Mammalian Target of Rapamycin/p70 S6 Kinase Pathway and Regulates the Replication of Hepatitis C Virus in Human Hepatoma Cells^*

From the Center for Hepatitis Research, Institute for Human Infections and Immunity, and the Department of Microbiology and Immunology, University of Texas Medical Branch, Galveston, Texas 77555-1018

Cellular mechanisms that regulate the replication of hepatitis C virus (HCV) RNA are poorly understood. p21-activated kinase 1 (PAK1) is a serine/threonine kinase that has been suggested to participate in antiviral signaling. We studied its role in the cellular control of HCV replication. Transfection of PAK1-specific small interfering RNA enhanced viral RNA and protein abundance in established replicon cell lines as well as cells infected with chimeric genotype 1a/2a HCV, despite reducing cellular proliferation, suggesting specific regulation of HCV replication. PAK1 knockdown did not reduce interferon regulatory factor 3-dependent gene expression, indicating that this regulation is independent of the retinoic acid-inducible gene I/interferon regulatory factor 3 pathway. On the other hand, LY294002 and rapamycin abolished PAK1 phosphorylation and enhanced HCV abundance, suggesting that the mammalian target of rapamycin (mTOR) is involved in PAK1 regulation of HCV. Small interfering RNA knockdown of the mTOR substrate p70 S6 kinase abrogated PAK1 phosphorylation and enhanced HCV RNA abundance, whereas overexpression of a constitutively active alternate substrate, eukaryotic translation initiation factor 4E-binding protein 1, increased cap-independent viral translation and viral RNA abundance without influencing PAK1 phosphorylation. Similar data indicated that mTOR is regulated by both phosphatidylinositol 3-kinase/Akt and ERK. Taken together, the data indicate that p70 S6 kinase activates PAK1 and contributes to phosphatidylinositol 3-kinase- and ERK-mediated regulation of HCV RNA replication.

Received for publication, October 30, 2006 , and in revised form, January 22, 2007.

^* This work was supported in part by National Institutes of Health Grants U19-AI40035 (to S. M. L.) and R21-AI063451 (to M. Y.) and R21-DA018054 (to K. L.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Center for Hepatitis Research, 4.104 Blocker Medical Research Bldg., University of Texas Medical Branch at Galveston, 301 University Blvd., Galveston, TX 77555-1019. Tel.: 409-747-7048; Fax: 409-747-7030; E-mail: smlemon{at}utmb.edu.

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