p16(INK4) expression is associated with the increased sensitivity of human non-small 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(INK4), 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, compared with A549 cells. Ectopic p16(INK4)-expressing cells also showed similar to 4.0-fold increase in sensitivity to SN-38 (7-ethyl-10-hydroxycamptothecin), the active metabolite of CPT-11, compared to the parent cells. The topo I-mediated DNA relaxation activities of ectopic p16(INK4)-expressing cells were approximately 5 times higher than those of the parent cells. Northern and western blot analyses indicate that these increased topo I activities of ectopic p16(INK4)-expressing cells were due to an elevated topo I mRNA level and an increase in topo I protein. The chemosensitivity to topo I inhibitors, topo I mRNA level, protein content and activity of a p16(INK4) revertant, lacking functional p16(INK4), tended to be restored toward those of the parental phenotype 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 topo I mRNA level, protein content and activity may be responsible for this hypersensitivity.