NS1-and minute virus of mice-induced cell cycle arrest: Involvement of p53and p21(cip1)

The nonstructural protein NS1 of the autonomous parvovirus minute virus of mice (MVMp) is cytolytic when expressed in transformed cells. Before causin g extensive cell lysis, NS1 induces a multistep cell cycle arrest in G(1) S , and G(2) well reproducing the arrest in S and G(2) observed upon MVMp inf ection. In this work we investigated the molecular mechanisms of growth inh ibition mediated by NS1 and MVMp. We show that NS1-mediated cell cycle arre st correlates with the accumulation of the cyclin-dependent kinase (Cdk) in hibitor p21(cip1) associated with both the cyclin A/Cdk and cyclin E/Cdk2 c omplexes but in the absence of accumulation of p53, a potent transcriptiona l activator of p21(cip1). By comparison, MVMp infection induced the accumul ation of both p53 and p21(cip). We demonstrate that p53 plays an essential role in the MVMp-induced cell cycle arrest in both S and G(2) by using p53 wild-type (+/+) and null (-/-) cells. Furthermore, only the G(2) arrest was abrogated in p21(cip1) null (-/-) cells. Together these results show that the MVMp-induced cell cycle arrest in S is p53 dependent but p21(cip1) inde pendent, whereas the arrest in G(2) depends on both p53 and its downstream effector p21(cip1). They also suggest that induction of p21(cip1) by the vi ral protein NS1 arrests cells in G(2) through inhibition of cyclin A-depend ent kinase activity.