HIV Type 1 Tat Inhibits Tumor Necrosis Factor α -Induced Repression of Tumor Necrosis Factor Receptor p55 and Amplifies Tumor Necrosis Factor α Activity in Stably tat -Transfected HeLa Cells

The human immunodeficiency virus type 1 (HIV-1) Tat protein is a key regulatory protein in the HIV-1 replication cycle. Tat interacts with cellular transcriptional factors and cytokines, such as tumor necrosis factor (TNF-α), and alters the expression of a variety of genes in HIV-1-infected and noninfected cells. To further elucidate the mechanisms by which HIV-1 Tat amplifies the activity of TNF-α, we transfected the HIV-1 tat gene into an epithelial (HeLa) cell line. We observed that Tat-expressing cells had increased NF-κB-dependent trans-activational activity due to enhanced NF-κB-DNA binding in response to TNF-α treatment. Tumor necrosis factor receptor (TNFR) p55 was the prominent receptor, as neutralizing antibodies to TNFR p55, but not to TNFR p75, blocked TNF-α-mediated NF-κB activation. Furthermore, tat-transfected cells were more sensitive to TNF-α-induced cytotoxicity and only the neutralizing antibodies to TNFR p55 completely protected the cells. To determine whether TNFR p55 was involved in amplification of cellular response to TNF-α by HIV-1 Tat, we investigated the effect of TNF-α on TNFR p55 expression in the tat-transfected cells. TNF-α treatment resulted in a reduction in both TNFR p55 mRNA and protein levels in the control cells but not in the tat-transfected cells as determined with Northern blot and Western blot analyses, respectively. Our results indicate that HIV-1 Tat may inhibit TNF-α-induced repression of TNFR p55 and thereby amplify TNF-α activity in these stably transfected cells.