p300 is a key protein, which determines acceleration or deceleration of signal transduction. Recently, renal proximal tubular cells have not only been found to be a harboring site for HIV-1 but have also been shown to undergo apoptosis in response to HIV-1 exposure. Both HIV-1 and its envelop glycoprotein, i.e. gp120, triggered tubular cell apoptosis in the same magnitude. In the present study, we evaluated the role of p300 in gp120-induced tubular cell apoptosis and associated downstream signaling. We have demonstrated that by transient transfection assays, p300 significantly increases susceptibility of human proximal renal tubular HK-2 cells to apoptosis triggered by HIV-1 gp120. A mutant p300, missing the E1A/TFIIB binding site, fails to produce such sensitization potential. Smad7 and an anti-TGF-β antibody rescue the p300 sensitization. Furthermore, p300 and HIV-1 gp120 synergistically increase TGF-β, ATF-2 and activating protein-1 (AP-1) expression. In addition, HIV-1 gp120 results in phosphorylation of Smad2 and decreases c-Jun. These findings suggest that p300 acts as a potent transcriptional cofactor in HIV-1 gp120-induced apoptosis via TGF-β and Smad signaling.