EPIDERMAL GROWTH-FACTOR REGULATES TOPOISOMERASE-II ACTIVITY AND DRUG-SENSITIVITY IN HUMAN KB CELLS

Because of its unique DNA-cleaving and strand-passing activities, topo isomerase II is involved in many aspects of DNA metabolism, including replication, transcription, recombination, and repair. The cytotoxic p otential of topoisomerase II-targeted drugs, such as etoposide, is rel ated to their ability to stabilize covalently linked enzyme-DNA comple xes, which are intermediates in the enzyme's catalytic cycle. Epiderma l growth factor receptor is expressed on the cell surface of the major ity of squamous cell carcinomas, and epidermal growth factor binding i s known to stimulate a number of cellular transduction pathways, inclu ding tyrosine kinase, protein kinase C, and phospholipase C. Because t opoisomerase II is a proliferation-dependent protein and has been show n to be a high-affinity substrate for many of these cellular transduct ion pathways, the effects of epidermal growth factor on cellular regul ation and sensitivity to etoposide were studied with the human oral ca vity squamous cell line, KB. Topoisomerase II catalytic activity was r apidly and transiently inhibited after the addition of epidermal growt h factor to the cellular growth media. Western blot on nuclear extract s did not demonstrate alterations in topoisomerase II polypeptide leve ls to account for changes in catalytic activity. Epidermal growth fact or treatment also led to the formation of stabilized, covalently linke d enzyme-DNA complexes. Furthermore, epidermal growth factor-induced, topoisomerase II-mediated DNA strand breaks were additive to those Ind uced by etoposide. This study indicates that epidermal growth factor s pecifically regulates the catalytic and DNA-cleaving activities of top oisomerase II in KB cells. This may direct clinical strategies for cir cumventing the intrinsic cellular resistance to chemotherapy commonly observed in squamous cell carcinomas of the head and neck.