Wild-type p53 up-regulates the tumor suppressor gene p27KIP1 which results in enhanced induction of apoptosis

Background: The tumor suppressor gene p27(KIP1) induces cell cycle arrest and apoptosis. However, many of the regulatory mechanisms are still unknown. We were interested to know whether there is a link to the major cell cycle regulator p53 or not. Methods: We transfected the non-small cell lung cancer cell line H1299 (p53del) and the breast cancer cell line MDA-MB-468 (p53mut) with a p53 expressing adenoviral vector (Ad-p53) with 0.5 and 1 MOI (multiplicity of infection), respectively and studied the expression of p27(KIP1). We combined 30 nM of the anti-EGF-receptor antibody C225 with Ad-p53, which can induce the expression of p27(KIP1) by itself. Results: We found that the combination of Ad-p53 and C225 caused 55% more growth inhibition than Ad-p53 or C225 alone. Western Blot analysis showed that p53 induced p27(KIP1) by 2.7 fold, which was enhanced to 5.1 fold when Ad-p53 was combined to C225. Flow cytometry analysis revealed that the growth inhibition was, at least in part, caused by induction of apoptosis. 33.4% of the H1299 cells underwent apoptosis in the combination group compared to 5% in the single treatment groups. Conclusion: These results show an important novel mechanism of cell cycle control by p53.