Keratin 8

Abstract: Three monoclonal antibodies (AE1, AE2, and AE3) were prepared against human epidermal keratins and used to study keratin expression during normal epidermal differentiation Immunofluorescence staining data suggested that the antibodies were specific for keratin-type intermediate filaments The reactivity of these antibodies to individual human epidermal keratin polypeptides (65-67, 58, 56, and 50 kdaltons) was determined by the immunoblot technique AE1 reacted with 56 and 50 kdalton keratins, AE2 with 65-67 and 56-kdalton keratins, and AE3 with 65-67 and 58 kdalton keratins Thus all major epidermal keratins were recognized by at least one of the monoclonal antibodies Moreover, common antigenic determinants were present in subsets of epidermal keratins To correlate the expression of specific keratins with different stages of in vivo epidermal differentiation, the antibodies were used for immunohistochemical staining of frozen skin sections AE1 reacted with epidermal basal cells, AE2 with cells above the basal layer, and AE3 with the entire epidermis The observation that AE1 and AE2 antibodies (which recognized a common 56 kdalton keratin) stained mutually exclusive parts of the epidermis suggested that certain keratin antigens must be masked in situ This was shown to be the case by direct analysis of keratins extracted from serial, horizontal skin sections using the immunoblot technique The results from these immunohistochemical and biochemical approaches suggested that: (a) the 65- to 67-kdalton keratins were present only in cells above the basal layer, (b) the 58-kdalton keratin was detected throughout the entire epidermis including the basal layer, (c) the 56-kdalton keratin was absent in the basal layer and first appeared probably in the upper spinous layer, and (d) the 50-kdalton keratin was the only other major keratin detected in the basal layer and was normally eliminated during s corneum formation The 56 and 65-67-kdalton keratins, which are characteristic of epidermal cells undergoing terminal differentiation, may be regarded as molecular markers for keratinization

Abstract: Keratins are the intermediate filament (IF)-forming proteins of epithelial cells. Since their initial characterization almost 30 years ago, the total number of mammalian keratins has increased to 54, including 28 type I and 26 type II keratins. Keratins are obligate heteropolymers and, similarly to other IFs, they contain a dimeric central α-helical rod domain that is flanked by non-helical head and tail domains. The 10-nm keratin filaments participate in the formation of a proteinaceous structural framework within the cellular cytoplasm and, as such, serve an important role in epithelial cell protection from mechanical and non-mechanical stressors, a property extensively substantiated by the discovery of human keratin mutations predisposing to tissue-specific injury and by studies in keratin knockout and transgenic mice. More recently, keratins have also been recognized as regulators of other cellular properties and functions, including apico-basal polarization, motility, cell size, protein synthesis and membrane traffic and signaling. In cancer, keratins are extensively used as diagnostic tumor markers, as epithelial malignancies largely maintain the specific keratin patterns associated with their respective cells of origin, and, in many occasions, full-length or cleaved keratin expression (or lack there of) in tumors and/or peripheral blood carries prognostic significance for cancer patients. Quite intriguingly, several studies have provided evidence for active keratin involvement in cancer cell invasion and metastasis, as well as in treatment responsiveness, and have set the foundation for further exploration of the role of keratins as multifunctional regulators of epithelial tumorigenesis.