The Effect Of HIV-1 Tat On KSHV Viral IL-6-induced Tumorigenesis And Its Molecular Mechanism

Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), is a herpesvirus, which was implicated in the development of Kaposi's sarcoma (KS), primary effusion lymphoma (PEL) and Multicentric Castleman's disease (MCD). KSHV infection is necessary but not sufficient to cause KS. Infection with human immunodeficiency virus type 1 (HIV-1) significantly elevated KS incidence in KSHV-infected individuals. To investigate the effect of HIV-1 transactivative transcription protein (Tat) on tumorigenesis in nude mice induced by NIH3T3 cells transformed with KSHV oncogene K2 (encoding viral IL-6, vIL-6) and the molecular mechanisms lied behind, eukaryotic expression plasmids pK2F containing KSHV K2 genewas constructed with the molecular cloning techniques. Then two cell strains named N/K2 and N/K2+Tat expressing vIL-6 and vIL-6 plus Tat respectively were obtained by stable transfection NIH3T3 cells with the plasmid pK2F alone or together with pTat, respectively. The results of MTT assay and flow cytometric analysis indicated that Tat slightly inhibited N/K2 cells proliferation and significantly blocked cell-cycle progression of N/K2 cells resulting in a great portion of N/K2+Tat cells arrested in the S phase in vitro. Further Western blot analysis demonstrated that HIV-1 Tat blocked vIL-6 signaling specifically and inhibited vIL-6-induced phosphorylation of signal transducers and activators of transcription 3 (STAT3) and AKT protein kinase in vitro. Tumors induced by N/K2 and N/K2+Tat in nude mice exhibited a striking histological resemblance to KS including intricately arrayed spindle cell, a high degree of vascularization and brisk mitotic activity, but the tumors induced by N/K2+Tat cells manifested obviously smaller in mean size and lower in expression level of Cyclin D1 and vessel endothelium growth factor (VEGF) than those induced by N/K2 cells. Furthermore, the mean size of tumors induced by N/K2 cells became smaller and the expression level of Cyclin D1 and VEGF appeared to be lower also when NIH3T3 cells expressing Tat were injected in the same mouse with N/K2 cells separately and simultaneously. These data suggested that both intracellular and intercellular Tat could inhibit tumorigenesis induced by vIL-6-expressing NIH3T3 cells in nude mice.