Changes in the expression of the putative p53-inducible inhibitor p21 of cyclin-dependent kinase (CDK) and p21-associated CDK activity were examined in relation to the DNA synthesis-associated nuclear exclusion of p53 after stimulating growth arrested human breast cancer MCF7 cells with serum. Following stimulation, p53 was coincidentally excluded from the nucleus to the cytoplasm with a major increase in DNA synthesis that occurred 24 h later. The p53 protein in the cytoplasm formed a complex with a 44-kDa protein (p44). Northern blots and reverse transcription-polymerase chain reaction (PCR) revealed that the mRNA levels of p21 were unchanged immediately before and after stimulation, but were reduced significantly 24 h later. Immunoprecipitation showed that newly synthesized p21 coprecipitated with CDK2 but not with CDK4. Synthesis of p21, the level of which was high in arrested cells, decreased after stimulation and remained at low levels thereafter. The CDK2 activity as assessed by histone H1 kinase activity in vitro was low in arrested cells and changed in parallel with DNA synthesis after stimulation. The present results suggest that the nuclear exclusion and complex formation of p53 with p44 in the cytoplasm control DNA synthesis by reducing p21 expression, thereby leading to the activation of CDK2.

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