Expression of MUC5AC apomucin in transitional cell carcinomas of the urinary bladder and its possible role in the development of mucus-secreting adenocarcinomas.

The histogenesis of primary nonurachal mucus-producing adenocarcinomas of the urinary bladder including signet ring cell carcinomas remains to be elucidated, since the normal bladder contains neither columnar nor mucus-secreting glandular epithelium. Based upon the assumption that adenocarcinomas may develop secondarily from pre-existent transitional cell carcinomas (TCC) by a metaplastic process, it was the purpose of the current immunohistochemical study to analyze whether urothelial carcinomas are capable of secreting MUC5AC apomucin, using the monoclonal antibody 45MI. This antibody has been initially demonstrated to strongly react with the mucus-producing columnar cells of the surface gastric epithelium, recognizing a specific epitope located on the peptide core of glycoproteins as major components of mucins. Nine of 64 uniformly differentiated papillary (14.1%) and 5 of 66 nonpapillary (solid) TCC with a uniform urothelial differentiation (7.6%) expressed the MUC5AC antigen, yielding an overall incidence of 10.8%. Transitional cell carcinomas with a focally altered cellular and structural differentiation (squamous cell, pseudoglandular, true glandular and mixed differentiation) stained positively in a substantially higher percentage of 43.8% (21 of 48 cases). A positive immunoreactivity was also observed in 3 of 19 mixed transitional cell and nonurothelial carcinomas. The tumor-associated resurgence of normally cryptic MUC5AC antigenic determinants in transitional cell carcinomas is considered as a re-expression of oncofetal antigenicity, probably as a result of the embryologic origin of the urinary bladder from the pluripotent tissues of the cloacal endoderm and the mesodermal wolffian ducts. Our findings may help to better understand the histogenetic development of mucus-secreting vesical adenocarcinomas from pre-existent urothelial carcinomas.