Lung cancer cells express several cytokeratins (CKs) that are subdivided into type I (CK9-23) and type II (CK1-8) subclasses. The functions of CKs in lung cancer cells have not been fully elucidated. The purpose of this study was to investigate the role of CKs in the invasion of lung cancer cells. We investigated the expression levels of CK7, 8, 18 and 19 in 12 non-small cell lung cancer (NSCLC) and seven SCLC cell lines by quantitative immunoblotting. The expression levels of these four CKs were significantly higher in the NSCLC cells. The NSCLC cell line HI1017 expressed CK8 and 18; A549 cells expressed CK7, 8, 18 and 19, respectively. Invasive sublines of HI1017 and A549 were established by repeated selection of invasive cells using a membrane invasion chamber system. The invasive cell lines showed lower expression levels of CKs compared with the parental cells. Exogenous CK19 also resulted in a decrease in invasiveness of the HI1017 cells. Suppression of either CK8 or CK18 by short interfering RNAs led to a decrease in the total CKs and increased invasiveness of both the HI1017 and A549 cells. A549 cells expressed very low levels of CK19. Suppression of CK19 affected neither invasive ability nor total CK amount in the A549 cells. Our observations indicate that CK expression levels were inversely associated with invasiveness of the NSCLC cell lines, and suggest that expression levels of dominant CKs may affect invasive ability.

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