Ubiquitin‐immunoreactive degradation products of cytokeratin 8/18 correlate with aggressive breast cancer

Decreased amounts of cytokeratin (CK) 8/18 in the cytoplasm of breast cancer cells correlate with a poor prognosis. Although such decreases have been attributed to suppressed gene expression, accelerated protein degradation may also be responsible. In order to investigate whether selective degradation via the ubiquitin (Ub)‐dependent proteasome pathway occurs in breast cancer, one‐ and two‐dimensional (1‐D and 2‐D) immunoblot analysis was performed on cancerous and normal breast tissue from 50 breast cancer patients using the anti‐Ub monoclonal antibodies (mAbs) KM691 and KM690. On 1‐D gel electrophoresis, one broad band or two bands were detected at about 43 kDa; these were detected only in cancer tissue. Immunoreactive bands at 43 kDa were significantly associated with aggressive morphology ( P =0.011), nuclear p53 accumulation ( P =0.015) and overexpression of Her2/ neu protein ( P =0.012). On 2‐D gel electrophoresis, these bands were fractionated into a group of several spots that formed a staircase pattern at 40–45 kDa. Partial amino acid sequencing analysis demonstrated that these Ub‐immunoreactive spots corresponded to CK8 and CK18; however, since they did not have an amino‐terminal domain, and were located at lower molecular weight positions than intact CK8 and CK18 on the 2‐D gel, they were regarded as degradation products. CK18 degradation was confirmed by confocal microscopy as loss of the frame‐like network that forms the luminal structure. These results indicate that CK 8/18 degradation products are detected specifically in breast cancer and may determine its aggressiveness.