Suramin increases p53 protein levels but does not activate the p53-dependent G₁ checkpoint

Suramin is an antineoplastic agent which has a cytostatic effect on both normal and tumor-derived cells. We have investigated whether the induction of growth arrest by suramin requires the p53 protein, a tumor suppressor gene product involved in the initiation of growth arrest following DNA damage. Activation of the p53 protein by genotoxic agents causes increased p53 protein levels and p53-dependent transcription of the p21 gene. The p21 protein then inhibits cyclin-dependent kinases, initiating G₁ arrest. Exposure of NIH-3T3 cells to suramin caused a rapid (1-2 h) increase in the level of p53-DNA-binding activity. Flow cytometric analysis indicated that suramin arrested NIH-3T3 cells in G₀-G₁. However, suramin did not increase the p53-dependent transcription of the p21 gene or inhibit cyclin-dependent kinase 2 kinase activity. If NIH-3T3 cells were exposed to radiation or suramin plus radiation, p21 mRNA levels were increased and cyclin-dependent kinase 2 kinase activity was inhibited, indicating that suramin does not block the cells' ability to increase p21 levels. To determine whether the G₀-G₁ arrest induced by suramin required p55, NIH-3T3 cells transfected with a dominant negative mutant p55 gene to eliminate wild-type p53 function (NMP cells) were exposed to suramin. NMP cells still exhibited G₀-G₁ arrest after suramin treatment. Suramin increases p53 protein levels, but fails to increase p21 mRNA levels or to activate the G₁ checkpoint. These data suggest that suramin induces growth arrest in NIH-3T3 cells by a mechanism that is independent of cellular p55 status.