TAR- and Tat-independent replication of human immunodeficiency virus type 1 in human hepatoma cells

The molecular mechanisms involved in the regulation of human immunodeficiency virus type 1 (HIV-1) replication may differ in various cell types and with various exogenous stimuli. TAR/Tat interactions play important roles in HIV-1-long terminal repeat (LTR)-directed transcription, and have become specific targets in molecular therapies for blocking HIV-1 replication. As we previously reported, astrocytic glial cells, which can support HIV-1 replication in cell culture and may be infected in vivo, provide an intracellular milieu in which TAR mutant HIV-1 viruses may replicate. In further studies of this molecular model, several divergent human cell types were analyzed for both TAR- and Tat-independent HIV-1 replication. Human hepatoma cell lines, which can be productively infected by HIV-1 after the hepatoma cells are transduced with the human CD4 receptor gene, were found to support high levels of HIV-1 replication. In these studies, utilizing a transient transfection system with wild-type and various TAR, Tat, or combined TAR/Tat mutant HIV-1 proviral constructs, we demonstrate TAR-independent replication in unstimulated human hepatoma cells. Remarkably, in human hepatoma cells, HIV-1 replication is not only independent of TAR but also can be independent of Tat expression. It is further demonstrated, using electrophoretic mobility shift assays (EMSAs) and an in situ UV cross-linking system, that human hepatoma cells contain novel endogenous cellular proteins that bind to the proviral HIV-1 5' LTR in the downstream region, between nucleotides +38 to +125 on proviral DNA. This alternative regulatory pathway of TAR- and Tat-independent viral production may provide a new system to dissect further the interactions of Tat/TAR and determine the role of the TAR element, in its DNA form, in HIV-1 replication.