P16(INK4) EXPRESSION IS ASSOCIATED WITH THE INCREASED SENSITIVITY OF HUMAN NONSMALL CELL LUNG-CANCER CELLS TO DNA TOPOISOMERASE-I INHIBITORS

Inactivation of p16(INK4), a,inhibitor of cyclin-dependent kinases 4 ( CDK4) and 6 (CDK6), may be essential for oncogenesis in non-small cell lung cancer (NSCLC). We examined the sensitivity of two clones of p16 (INK4)-transfected NSCLC cell line with homozygous deletion of p16(INK 4), A549/p16-1 and 2, to DNA topoisomerase I (topo I) inhibitors. A549 /p16-1 and -2 showed 7.7- and 9.1-fold increases in sensitivity to CPT -11 (11,7-ethyl-10-[4-(1-piperidino)-1-piperidino] respectively, compa red with A549 cells. Ectopic p16(INK4)-expressing cells also showed si milar to 4.0-fold increase in sensitivity to SN-38 (7-ethyl-10-hydroxy camptothecin), the active metabolite of CPT-11, compared to the parent cells. The topo I-mediated DNA relaxation activities of ectopic p16(I NK4)-expressing cells were approximately 5 times higher than those of the parent cells. Northern and western blot analyses indicate that the se increased topo I activities of ectopic p16(INK4)-expressing cells w ere due to an elevated topo I mRNA level and an increase in topo I pro tein. The chemosensitivity to topo I inhibitors, topo I mRNA level, pr otein content and activity of a p16(INK4) revertant, lacking functiona l p16(INK4), tended to be restored toward those of the parental phenot ype to some extent. These results suggest that p16(INK4) expression is closely associated with the increased sensitivity of ectopic p16(INK4 )-expressing NSCLC cells to topo I inhibitors. The up-regulation of to po I mRNA level, protein content and activity may be responsible for t his hypersensitivity.