Although its dependence on the target cell type is well established, the cytopathogenicity of parvoviruses has remained elusive to date as far as its mechanism is concerned. However, indirect evidence suggested that parvoviral non‐structural (NS) proteins may be the cytotoxic effectors. In order to test this hypothesis, a molecular clone of parvovirus MVMp was modified, by replacing the P4 promoter of the NS transcription unit by the glucocorticoid‐inducible promoter of the mouse mammary tumour virus. Clones of neoplastic human cells that had incorporated this construct and that were induced to produce NS proteins by dexamethasone, showed a cytopathic effect and eventually died. Our data strongly suggest that the intracellular accumulation of parvoviral NS products jeopardizes the survival of the cells, which cannot be detected unless a threshold protein concentration is reached. Interestingly, a cell variant could be isolated which resisted dexamethasone‐induced killing, although it was fully inducible for the production of NS proteins. This variant was also unusually resistant to infection with MVMp virions, thus confirming the essential role played by the NS proteins in the parvoviral cytotoxicity and indicating that the cytocidal activity of the parvoviral NS products is modulated by cellular factors that may vary from one cell to another.

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The nonstructural protein NS1 of the autonomous parvovirus minute virus of mice (MVMp) is cytolytic when expressed in transformed cells. Before causing 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 infection. In this work we investigated the molecular mechanisms of growth inhibition mediated by NS1 and MVMp. We show that NS1-mediated cell cycle arrest correlates with the accumulation of the cyclin-dependent kinase (Cdk) inhibitor p21 cip1 associated with both the cyclin A/Cdk and cyclin E/Cdk2 complexes but in the absence of accumulation of p53, a potent transcriptional activator of p21 cip1 . By comparison, MVMp infection induced the accumulation of both p53 and p21 cip1 . 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 independent, 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 viral protein NS1 arrests cells in G 2 through inhibition of cyclin A-dependent kinase activity.

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