Human herpesvirus 8 K1-associated nuclear factor-kappa B-dependent promoter activity: role in Kaposi's sarcoma inflammation?

BACKGROUND The growing number of human immunodeficiency virus type 1 (HIV-1) infections worldwide and the increasing use of immunosuppressive modalities for organ transplantation have contributed to an epidemic of Kaposi's sarcoma (KS), which has been etiologically linked to human herpesvirus 8 (HHV8) or KS-associated virus. Since the onset of the acquired immunodeficiency syndrome epidemic, inflammation has been recognized as an essential component of KS pathology. HHV8 bears a gene (K1) encoding a transmembrane protein with an immunoreceptor tyrosine-based activation motif. This motif is present in receptors that mediate inflammation. PURPOSE To dissect the cellular effects of K1 function and the eventual role of K1 in KS, we developed a cell model for studying K1 expression. METHODS K1 was cloned from BC-3 lymphoma cells. To monitor transcriptional activation, K1 was coexpressed with plasmids containing luciferase under control of various promoters. K1 expression was monitored by indirect immunofluorescence and by combined immunoprecipitation/immunoblot analysis. Inflammatory cytokines were measured by enzyme-linked immunosorbent assay. RESULTS Cellular transfection of the K1 gene induced reporter expression under control of nuclear factor-kappa B (NF-kappaB), which controls the transcription of numerous proteins involved in inflammation. Treatment of cells with aspirin, an agent that targets this intracellular pathway and blocks cell inflammatory responses, blocked K1-induced NF-kappaB-dependent promoter activity. When a second KS cofactor, i.e., the HIV-1-transactivating gene tat, was coexpressed with K1, we observed an additive effect on NF-kappaB-dependent transcription. K1 transfection stimulated the secretion of cytokines interleukin (IL) 6, granulocyte-macrophage colony-stimulating factor, and IL-12. Cells treated with the conditioned media of K1 transfectants exhibited similar characteristics of K1 transfectants, indicating that a paracrine loop was being activated. CONCLUSION Thus, K1 may activate cells in which it is expressed, as well as other cells in a paracrine manner. K1 cooperates in signaling with HIV-1 Tat, suggesting that both of the proteins from these viruses converge to reach an enhanced level of inflammation that may underlie progressive KS.