Characterisation of the role of ING5

The tumour suppressor protein p53 stops cell division by activating the expression of p21WAF1/CIP1, whenever it senses that a cells DNA is damaged. Thus, giving the cell a chance to repair the damaged DNA before its errors are duplicated and passed onto daughter cells. When p53 is mutated, it loses its protective function. This allows mutations to accumulate in other genes and leads to more than 50% of all human cancers. p21WAF1/CIP1 inhibits cyclin E/CDK2 complexes to prevent G1-S progression of cell cycle. Though the cyclin/CDK complexes are required for the orderly progression of the cell cycle, overexpression of cyclin E/CDK2 has been reported to lead to poor prognosis in breast cancer. The molecular mechanism underlying this is poorly understood. The current investigation was undertaken to elucidate the role of ING5, in relation to cyclin E/CDK2 activity and p53 function. ING5, which was identified in a novel approach for screening the substrates of cyclin E/CDK2 is a member of the ING family of proteins that inhibit growth. The in vitro and in vivo interaction experiments and the reporter gene assays demonstrate that ING5 interacts with p53 to strongly activate p53-mediated transactivation of the promoters of the p21, bax and mdm2 genes. The extensive mapping of their interaction domains showed that the PH domain of ING5 interacts with the DNA binding domain of p53 indicating that ING5 does not interfere with p53-MDM2 interaction. ING5 mediates apoptosis of breast cancer cells with an intact p53 pathway as demonstrated in MCF-7 cells. It was observed that overexpression of cyclin E/CDK2 negatively regulated ING5-mediated activation of p53 and apoptosis. This work for the first time also illustrates a member of the ING family to be interacting with p63 and p73. ING5 interacts with p63 and p73 both in vitro and in vivo to differentially modulate p63 and p73-mediated transactivation of bax, p21 and pig3 genes. While one important function of p53 is to prevent S phase entry in response to cellular stress by activating the CKI p21, which interferes with the activities of the cyclin /CDKs, the identification of ING5 provides the first example of a reverse regulatory relationship between p53 and CDK activity. The findings suggest that the molecular network of ING5-p53-p21-cyclin E/CDK2 is an example of a positive feed back loop. Therefore ING5 could be useful in therapies for treating cancers with an intact p53 pathway