The role of caspase 3 in producing cytokeratin 19 fragment (CYFRA21‐1) in human lung cancer cell lines

The CYFRA 21-1 assay, which detects cytokeratin 19 (CK19) fragment, is widely used as a tumor marker for lung cancer, particularly non-small cell lung cancer. However, the reason that some lung cancer cell lines release CYFRA 21-1 in culture supernatants and others do not remains unclear. We hypothesized that the release of CYFRA 21-1 might be related to the expression of CK19 and caspase 3. In order to prove this, the quantities of mRNA for CK19 were evaluated by the competitive reverse transcriptase-polymerase chain reaction (RT-PCR). CK19 protein synthesis was also evaluated by Western blotting and immunohistochemistry, and the levels of CYFRA 21-1 in the culture supernatant were measured by an immunoradiometric assay. The expression of mRNA for caspase 3 was evaluated by the RT-PCR, and caspase 3 protein synthesis was also evaluated by immunohistochemistry. In 13 lung cancer cell lines, the amounts of mRNA for CK19 correlated with the levels of CYFRA 21-1 in culture supernatants, results of Western blotting for CK19, and positivities of immunohistochemistry for CK19. In 5 cell lines that produced a significant amount of CYFRA 21-1, the level of CYFRA 21-1 correlated with the positivity of RT-PCR for caspase 3 and immunohistochmistry for caspase 3. This suggests that caspase 3 played a role in the formation of CYFRA 21-1. In addition, the specific inhibitor of caspase 3 significantly inhibited the release of CYFRA 21-1 in culture supernatants. In conclusion, we demonstrate that caspase 3, which cleaves several intermediate filaments and carries out cell apoptosis, played an important role in producing CYFRA 21-1 in human lung cancer cell lines. © 2001 Wiley-Liss, Inc.