Down-regulation of SKP2 induces apoptosis in lung-cancer cells

S-Phase kinase associated protein 2 (SKP2), an F-box protein constituting the substrate-recognition subunit of the SCF(SKP2) ubiquitin ligase complex, targets cell-cycle regulators, such as the cyclin-dependent kinase inhibitor p27(KIP1), for ubiquitin-mediated degradation. Our earlier studies indicated frequent amplification and over-expression of the SKP2 gene in primary small-cell lung cancers (SCLCs) and cell lines derived from this type of tumor, and showed that down-regulation of SKP2 expression by means of an antisense oligonucleotide inhibited the growth of SCLC cells in culture (Yokoi et al., Am J Pathol, 161, 207-216, 2002). The antisense effect was confirmed in two cell lines of non-small cell lung cancer (NSCLC) that also exhibited over-expression of the gene. In the work reported here, we examined the mechanism(s) responsible for antisense-mediated growth inhibition of SCLC- and NSCLC-derived cultures. SKP2-antisense treatment not only suppressed DNA synthesis, as determined by [(3)H]thymidine incorporation, but also induced spontaneous apoptosis characterized by an increase in the sub-G1 population, fragmentation of nuclei, and activation of caspase-3. Our results suggest that since down-regulation of SKP2 appears to induce apoptosis in lung-cancer cells directly, targeting this molecule could represent a promising new therapeutic approach for this type of cancer, and possibly other tumors that over-express SKP2.