Transplantable mouse
squamous cell carcinomas (SCC), originally derived either from back skin or forestomach epithelium, do not synthesize high molecular weight
keratin polypeptides [greater than 60 kilodaltons (kDa)] involved in the process of terminal differentiation in the corresponding normal tissues. The in vivo
tumor keratin spectra consist of only low molecular weight
keratin subunits at 60, 58, 52, 50, 47, and 46 kDa, each encoded by its own
mRNA and encountered also in normal epidermis and forestomach epithelium. In addition, both
tumors express a
mRNA-dependent 40-kDa
protein, whereas a 56-kDa
protein and its
mRNA are selectively found only in the forestomach
tumor. Translation of mRNAs from both
tumors in a cell-free system does not only generate analogues of the in vivo
tumor keratin polypeptides, but also both SCC possess an additional
mRNA coding in vitro for a 67-kDa
keratin subunit that is not expressed, however, in the
carcinomas in vivo. The identity of this in vitro synthesized
keratin member with a 67-kDa
keratin polypeptide of both normal epidermis and forestomach epithelium was confirmed by comparison of charge properties and
peptide mapping. With regard to this particular
keratin polypeptide, the
tumors are obviously able to sequester the
polypeptide's
mRNA in an untranslatable state in the cells.