RECIPROCAL CHANGES IN P27(KIP1) AND P21(CIP1) IN GROWTH-INHIBITION MEDIATED BY BLOCKADE OR OVERSTIMULATION OF EPIDERMAL GROWTH-FACTOR RECEPTORS

Many human epithelial tumors express high levels of epidermal growth f actor (EGF) receptors. A human-mouse chimeric version of anti-EGF rece ptor monoclonal antibody (mAb) C225, which blocks receptor activation and produces inhibition of cell proliferation, is currently being inve stigated in clinical trials. When cells bear high numbers of EGF recep tors, either complete blockade of receptors with mAb 225 or full activ ation of receptors with EGF results in inhibition of proliferation, In the present study, we have explored the molecular mechanisms explaini ng how a receptor inhibitor, mAb 225, and a receptor activator, EGF, c an both produce growth inhibition of A431 human squamous epithelial ca rcinoma cells. We reported previously that inhibition of A431 cells by EGF is associated with up-regulation of p21(Cip1). We now demonstrate that mAb 255-mediated inhibition is associated with up-regulation of p27(Kip1), which binds to and inactivates cyclin-dependent kinase-2 ac tivity and produces cell cycle arrest in G(1). Furthermore, inhibition by mAb 225 can be overcome by titrating the cultures with increasing concentrations of EGF, which is accompanied by a concurrent fall in th e level of p27(Kip1). At properly titrated concentrations of mAb 225 a nd EGF, the inhibitory activities of both mAb 225 and EGF are counterb alanced and abolished. When EGF concentrations reach levels high enoug h to compete with mAb to produce near-saturating levels of receptor ac tivation, p27(Kip1) falls below basal levels; however, the concomitant marked rise in the level of p21(Cip1) results in growth inhibition. O ur data suggest that although p27(Kip1) and p21(Cip1) are induced and act independently, they play reciprocal roles in mediating inhibition of A431 cell growth by blockade of EGF receptors with mAb 225 and by a ctivation of receptors with saturating concentrations of EGF.