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This study examines the cytotoxicity potential and the mechanism of toxicity of the HIV-1 gp120 on human neuroblastoma cells.Previous data from our group have suggested that the HIV-1 envelope protein gp120 promotes the secretion of tumor necrosis factor-α and other factors by astrocytes and microglial cells present in primary human brain cell cultures, thereby contributing to the injury of neurons in these cultures. This study investigates the cytotoxicity potential and the mechanism of toxicity of gp120 on human neuroblastoma cells.SK-N-SH cells were treated with HIV-1 gp120, and was followed by in situ DNA fragmentation staining and small molecular weight DNA extraction studies to ascertain the induction of apoptosis by gp120 in these cells. To evaluate a potential role of the growth suppressor gene p53, gp120-treated SK-N-SH cells were subjected to reverse transcription polymerase chain reaction (RT-PCR) and Western blot analyses for the induction of p53. An antisense oligodeoxynucleotide against p53 was used to investigate the role of p53 in the gp120-induced apoptosis in these cells.Data from T7 DNA polymerase staining and small molecular weight DNA extraction studies demonstrated that gp120-induced DNA breakage in SK-N-SH cells with fragmentation patterns characteristic of apoptosis. RT-PCR and Western blot analyses revealed that the gp120-mediated induction of apoptosis was dependent on a gp120-induced and gp120-sustained upregulation of p53. The induction of p53 by gp120 was specific, since an antibody against gp120 prevented both the induction of p53 and subsequent apoptosis in SK-N-SH cells. The critical role of p53 was further illustrated by the effectiveness of a p53 antisense oligodeoxynucleotide to inhibit the gp120-induced apoptosis. As a control, the apoptosis-inducing potential of gp120 on SK-N-SH cells was not seen in the HIV-1 Gag proteins even when used at up to 5 nM.These results established that HIV-1 gp120 is potentially cytotoxic to human neuronal cells through the induction of p53, which may eventually lead to induction of apoptosis.