Aberrant messenger RNA splicing of the cytokeratin 8 in lung cancer.

Cytokeratin 8 (CK8) is one of the cytoskeletal components and shows caspase-mediated degradation when cells undergo apoptosis. We previously reported that CK8 is highly expressed in non-small cell lung cancer (NSCLC) cell lines and increasing values of serum CK8 are significantly associated with tumor progression in patients with NSCLC. In this investigation, reverse transcriptase-polymerase chain reaction (RT-PCR) analysis in lung cancer cell lines, revealed a shorter PCR product, which differed from the wild-type product of CK8. The nucleotide sequence of the shorter PCR products and genomic DNA for CK8 demonstrated that the shorter product was an aberrantly spliced form of CK8 (AS-CK8) which lacked a caspases cleavage site within the linker lesion in exon 5. The putative protein products predicted by the mRNA of AS-CK8 were demonstrated by Western blotting with monoclonal antibodies for CK8. In addition, AS-CK8 mRNA and its protein products were highly expressed in NSCLC cell lines compared with small-cell lung cancer (SCLC) cell lines. Tissue samples obtained from NSCLC patients also expressed mRNA of AS-CK8. In conclusion, we identified aberrantly spliced CK8 (AS-CK8) which lacked a caspases cleavage site in lung cancer cell lines and primary tumors of NSCLC. AS-CK8 was preferentially expressed in NSCLC, rather than SCLC. These findings lead to speculation that cancer cells expressing AS-CK8 may have a resistance to apoptosis and may perturb keratin network formation.