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‐de‐pendent 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 anti‐sense 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. (Cancer Sci 2003; 94: 344–349)