Phosphatidylinositol 3-Kinase Regulates Human Immunodeficiency Virus Type 1 Replication following Viral Entry in Primary CD4+ T Lymphocytes and Macrophages

Human immunodeficiency virus type 1 (HIV-1) can activate multiple signaling pathways within a target cell to facilitate viral entry and replication. A number of signal transduction pathways may be activated during engagement of the HIV-1 envelope with CD4 and/or the chemokine coreceptor. Binding to CD4 causes phosphorylation of receptor tyrosine kinases such as p56Lck, which activate the Raf/MEK/ERK and phosphatidylinositol 3-kinase (PI3-kinase) pathways and indirectly activate calcium channels (6, 15, 33, 34, 38). The chemokine receptor is coupled to G-proteins, which, depending on the subunit composition, can activate adenyl cyclase, protein tyrosine kinases such as Pyk2, and phospholipase C, which catalyzes the formation of inositol 1,4,5-triphosphate, subsequently opening calcium channels on the endoplasmic reticulum (reviewed in references 18 and 37). Because these pathways ultimately regulate functions such as cytoskeletal rearrangement, cell survival, differentiation, and activation of transcription, HIV gp120-induced signal transduction may facilitate virus infection. Previously, HIV signaling through the chemokine receptor has been considered dispensable for infection. Under conditions where signaling through the CCR5 receptor is blocked either by mutagenesis or G-protein inactivation with pertussis toxin, tumor cells are still capable of supporting viral entry and replication (3, 21). However, the phenotypes and functions of these tumor cell lines differ from those of the primary targets of HIV infection in vivo, primary macrophages and CD4+ lymphocytes, so the requirements for infection differ substantially. Only a relatively small fraction of T cells are productively infected in HIV-positive patients despite the presence of the relevant receptors, and activation of CD4+ T cells is critical for efficient viral reverse transcription and replication in these cells (29). Although HIV-1 entry can occur in quiescent T cells, there is a preintegration postentry block in replication (8). In addition, signaling through chemokine receptors by their natural ligands can enhance or suppress HIV replication in T cells and macrophages (12, 27, 28). Recent studies suggest that signal transduction by HIV envelope glycoprotein gp120 may affect host cell susceptibility to virus entry and infection in primary cells (2, 4, 10, 11, 23). In the case of HIV-1, only viruses competent to induce signaling through the CCR5 coreceptor are able to establish productive infection within macrophages (4). Primary viral isolates and laboratory-adapted strains with gp120 envelopes that do not induce calcium mobilization enter macrophages but are unable to complete replication. This postentry block can be overcome by stimulating signaling through CCR5 with its natural ligand, MIP-1α. In addition, perturbation of coreceptor signaling with pertussis toxin markedly decreases infection of peripheral blood mononuclear cells with CXCR4-utilizing (X4) and CCR5-utilizing (R5) HIV-1 viruses (2, 23). It has been proposed that coreceptor function is important for both entry and postentry events during HIV infection (9). A role for the Raf/MEK/ERK pathway has been demonstrated for nuclear import of the HIV reverse transcriptase complex (7) and in NF-κB-driven transcription from the HIV long terminal repeat promoter (34). The role of PI3-kinase signal transduction in HIV infection has not been fully studied. PI3-kinases are a cellular family of heterodimeric enzymes that consist of a regulatory subunit (p85) activated by tyrosine phosphorylation, which recruits inositol phospholipids, which are phosphorylated by the catalytic subunit (p110). These lipids serve as second messengers which regulate the phosphorylation of other kinases such as Akt/PKB, cyclic AMP-dependent protein kinase A, some protein kinase C isoforms, and the ribosomal S6 kinases p70 and p85 (reviewed in reference 13). Because PI3-kinase controls the activation of many different pathways, it is a critical mediator of various cellular processes such as cell migration, survival, and changes in morphology. The ability of PI3-kinase to regulate multiple cellular pathways, coupled with the need for HIV to induce an environment favorable for viral replication, prompted us to examine the role of PI3-kinase signaling in HIV-1 infection of CD4+ T cells and macrophages.