Background:  The HIV-1 viral protein R (Vpr) causes cell cycle arrest and apoptosis in dividing cells. The cellular pathway that leads to Vpr-induced arrest and apoptosis is active in all tested neoplastic cell lines and is independent of p53 status.  Several recent reports have indicated that the effects of Vpr may be similar to those of genotoxic agents. In this study we examined the hypothesis that the cytostatic effect of Vpr is similar to effects observed in response to UV irradiation. 

Methods:  We used defective viruses encoding wild-type or truncated Vpr to infect target cells.  To probe for the role of various cellular enzymes in the biology of Vpr, infected cells were exposed to various inhibitory drugs and transdominant proteins.  The effects of Vpr on the transcriptional activity of the viral long terminal repeat (LTR) were also examined under the above set of conditions.

Results:  We show that Vpr acts through an ATM-related protein of the PI3K family, ATR, and activates both the p38 MAP kinase and the Chk1 kinase.  Abrogation of ATR function either genetically or via treatment with PI3K inhibitors led to dramatic reduction in the Vpr-induced cell cycle arrest. Similarly treatment of cells with p38 inhibitors also abrogated Vpr-induced arrest.  Vpr expression also leads to the phosphorylation and activation of Chk1 protein kinase, which in turn induces inactivation of Cdc25C and inactivation of Cdc2. 

Conclusions: When the transcriptional activity of the viral LTR was analyzed, we found that any treatments that interfered with the activity of ATR, p38 or Chk1 also abrogated the ability of Vpr to induce transcriptional transactivation.