p53- and p73-mediated transcriptional repression of the cyclin B1 promoter is dependent on functional Sp1 DNA binding sites

The p53 protein family, p53, p63 and p73, have an important role in controlling cell growth and differentiation. The p53 tumour suppressor protein is a DNA-binding transcription factor that is activated in response to various DNA damaging agents. Inactivation of the p53 tumour suppressor gene occurs in 40--60% of all human tumours, implying that loss of p53 activity is a critical step in oncogenesis. The primary function of p53 is to protect mammals from tumour growth (neoplasia) by blocking cell cycle progression or by inducing cell death in response to stress and preventing cell cycle transition. The sequence similarity and conservation of functional domains between p73 and p53 is striking. p73 possess both an amino-terminal transactivation domain, a DNA-binding region and a carboxy-terminal oligomerization domain similar to that of p53. Furthermore, ectopic expression of the p73 isoforms, p73alpha and p73beta, can mimic the transcriptional activity and biological function of p53. We have reported that p53 prevents G2/M transition by decreasing intracellular levels of both cyclin B1 mRNA and protein and attenuating the activity of the cyclin B1 promoter. The ability of p53 to control mitotic initiation by regulating intracellular cyclin B1 levels suggests that a cyclin B1-dependent G2 checkpoint has a role in preventing neoplastic transformation. We have recently found that like p53, ectopic expression of p73alpha and p73beta isoforms can decrease the levels of cyclin B1 mRNA and attenuate expression from the cyclin B1 promoter independent of p53. This indicates an important role for p73 in preventing neoplastic transformation by blocking the G2/M transition independently of p53. Several transcription factors are known to interact with the cyclin B1 promoter, among them Sp1, TBP and NF-Y. The p53- and p73-mediated attenuation of the cyclin B1 promoter can be reversed by the transcription factor Spl, and abrogated by mutation of the Spl DNA-binding sites within the cyclin B1 promoter. Moreover, p53- and p73-mediated attenuation of cyclin B1 is independent of the NF-Y binding sites. This suggests that the transcriptional activator Spl is a target for p53- and p73-mediated transcriptional repression. Spl and p53 or Spl and p73alpha/p73beta isoforms can co-immunoprecipitate and can also chromatin immunoprecipitate with the cyclin B1 promoter. This suggests that p53 or p73 mediates transcriptional repression of cyclin B1 through the Spl transcription factor at the site of the cyclin B1 promoter.