Down-Regulation of p53 by Double-Stranded RNA Modulates the Antiviral Response

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Abstract
Human cytomegalovirus (HCMV) encodes several proteins that can modulate components of the cell cycle machinery. The UL123 gene product, IE1-72, binds the Rb-related, p107 protein and relieves its repression of E2F-responsive promoters; however, it is unable to induce quiescent cells to enter S phase in wild-type (p53+/+) cells. IE1-72 also induces p53 accumulation through an unknown mechanism. We present here evidence suggesting that IE1-72 may activate the p53 pathway by increasing the levels of p19Arf and by inducing the phosphorylation of p53 at Ser15. Phosphorylation of this residue by IE1-72 expression alone or HCMV infection is found to be dependent on the ataxia-telangiectasia mutated kinase. IE2-86 expression leads to p53 phosphorylation and may contribute to this phenotype in HCMV-infected cells. We also found that IE1-72 promotes p53 nuclear accumulation by abrogating p53 nuclear shuttling. These events result in the stimulation of p53 activity, leading to a p53- and p21-dependent inhibition of cell cycle progression from G1 to S phase in cells transiently expressing IE1-72. Thus, like many of the small DNA tumor viruses, the first protein expressed upon HCMV infection activates a p53 response by the host cell.