Protein Kinase B/Akt Is Present in Activated Form throughout the Entire Replicative Cycle of ΔU S 3 Mutant Virus but Only at Early Times after Infection with Wild-Type Herpes Simplex Virus 1

Abstract

ABSTRACT The product of the herpes simplex virus 1 (HSV-1) US3 gene is a multifunctional serine-threonine protein kinase that can block apoptosis induced by proapoptotic cellular proteins, exogenous agents, or replication-defective viruses. Earlier studies showed that the U S 3 kinase activates and functionally overlaps cellular protein kinase A (PKA). In this study we examined the status of phosphatidylinositol 3-kinase [PI(3)K] and of its effector, protein kinase B/Akt (PKB/Akt), a component of a major pathway of mammalian antiapoptotic signaling systems. We report the following. (i) Infection of target cells with HSV-1 induces transient phosphorylation of serine 473 of PKB/Akt early in infection, with a mechanism that is dependent on PI(3)K. Inhibition of PI(3)K induced apoptosis in mock-infected or ΔU S 3 mutant-virus-infected but not in wild-type-virus-infected cells and reduced the accumulation of specific viral gene products, including the U S 3 protein kinase, but had a marginal effect on virus yields. (ii) At later times after infection, the total amounts of PKB/Akt decreased and phosphorylated PKB/Akt forms disappeared in a U S 3-dependent and protein phosphatase 2A-independent manner. (iii) Activation of PKA by forskolin did not mediate significant dephosphorylation of PKB/Akt. Our results are consistent with the model that PKB/Akt is activated early in infection and acts to block apoptosis in infected cells prior to the accumulation of U S 3 protein kinase and that it persists and continues to function as an antiapoptotic protein in the absence of U S 3 but becomes redundant or even inimical once U S 3 protein kinase accumulates in effective amounts.