Abstract. Keratin (K) expression index has been reported to be related to cell invasion activity in adenocarcinoma. In a previous study, we observed a negative correlation between K expression and cell invasion activity; i.e., when many Ks are expressed in the cells, the cell activity is low. To further elucidate the correlation between Ks and invasion activity, RNA interference experiments of K8, K18 and K19 were carried out to clarify the essential role of Ks using T24 and HEC-1 as typical squamous cell carcinoma and adenocarcinoma cells, respectively. K8 small interfering RNA (siRNA) was most effective against K18 and K19 expression and demonstrated the strongest effect on relative invasion activity among the siRNAs used. These results suggest that K8/K18 or K8/K19 filaments may play roles in internal cell structure and invasion activity. Moreover, K18 and K19 were capable of substituting for each other, and K18 or K19 formed filaments with K8. In addition, cells treated with K8 siRNA demonstrated high invasion activity, which was approximately double that observed with control siRNA in HEC-1 cells. The order of effects was K8>K19>K18 in the two cell lines. The above results suggest that K8 may play a signifiant role in invasive functions in epithelial and metastatic cells.
IntroductionAs inferred from the developmental origins of epithelial tissues, keratins (Ks) may be expressed in adenocarcinoma and urinary bladder carcinoma, as well as in squamous cell carcinoma (SCC). In fact, 28 type I Ks and 26 type II Ks as components of intermediate filaments (IFs) are known to be expressed in human epithelial cells (appendage-forming epithelia such as inner hair follicles) and structural epithelial cells (hard structures and appendages such as hair, nail and tongue). Moreover, Ks are differentially expressed as specific pairs of type I (smaller and relatively acidic, 40-63 kDa and pI 4.5-5.5) and type II (larger and more basic, 53-67 kDa and pI 4.5-7.5) proteins, both of which are necessary for filament formation: at least one pair of Ks is always expressed in every epithelial cell (1-3).The type-specific distribution of IF protein has been increasingly used as the basis for the differential histopathological diagnosis of tumors where morphological criteria have proven to be inconclusive. It has been reported that Ks are expressed in epidermal melanocytes derived from neural crest cells and neural ectoderm (4,5). The evidence presented above suggests that K expression may be related to further differentiation in epithelial tissues, and is not derived from original cells or tissues.In a previous study, we observed a negative correlation between K expression (the number of Ks expressed) and cell invasion capability in three cultured human adenocarcinoma cell lines (6). The number of expressed Ks increases inversely with the invasive index as follows: SW-13 (7) (K number, 2.5; invasion index, 1.781), RERF-LC-OK (K number, 6.5; invasion index, 1.219), and HEC-1 (8) (K number, 7.5; invasion index, 0.562). That is, the nu...
