Evolution and complexity of the genes encoding the keratins of human epidermal cells

Potential Involvement of Sulfotransferase in the Mechanism of Lamotrigine-induced Skin Rash

The mechanism of drug-induced skin rash is not well understood. Circumstantial evidence suggests that the covalent binding of a reactive metabolite is involved in the mechanism of most idiosyncratic drug reactions. However, there is a limited quantity of drug metabolizing enzymes in the skin, except for sulfotransferases. It is possible that some drugs are metabolized to reactive sulfate metabolites that are responsible for skin rashes. For example, nevirapine-induced skin rash involves metabolism of nevirapine to 12-hydroxy-nevirapine, which is further metabolized by sulfotransferase in the skin to a reactive benzylic sulfate that covalently binds to proteins. The working hypothesis is that lamotrigine, valdecoxib, and sertraline skin rashes involve the formation of reactive sulfate in the skin. Lamotrigine-N-oxide, hydroxy-valdecoxib, and hydroxy-sertraline were tested as substrates with known human sulfotransferases. Hydroxy-valdecoxib and the benzylic alcohol metabolite of sertraline were not substrates for human sulfotransferases. Therefore, this pathway is presumably not involved in the mechanism by which they cause skin rashes. In contrast, lamotrigine-N-oxide is a substrate for several human sulfotransferases and the sulfate is chemically reactive. Furthermore, lamotrigine-N-sulfate not only alkylates proteins as we described previously but also forms the sulfate of tyrosine, suggesting another possible mechanism for protein modification. This study has further added to the understanding of the potential of the sulfotransferase pathways and protein sulfation to play a role in drug-induced skin rash.

A modern baseline for the paired isotopic analysis of skin and bone in terrestrial mammals

We present the isotopic discrimination between paired skin and bone collagen from animals of known life history, providing a modern baseline for the interpretation of archaeological isotopic data. At present, the interpretation of inter-tissue variation (Δ_(skin-bone)) in mummified remains is based on comparisons with other archaeological material, which have attributed divergence to their contrasting turnover rates, with rapidly remodelling skin collagen incorporating alterations in environmental, cultural and physiological conditions in the months prior to death. While plausible, the lack of baseline data from individuals with known life histories has hindered evaluation of the explanations presented. Our analysis of a range of animals raised under a variety of management practices showed a population-wide trend for skin collagen to be depleted in ¹³C by -0.7‰ and enriched in ¹⁵N by +1.0‰ relative to bone collagen, even in stillborn animals. These results are intriguing and difficult to explain using current knowledge; however, on the basis of the findings reported here, we caution any results which interpret simply on differing turnover rates. We hypothesize that there may be a consistent difference in the routing of dietary protein and lipids between skin and bone, with potentially on-site synthesis of non-essential amino acids using carbon and nitrogen that have been sourced via different biochemical pathways.

Investigating the Mechanism of Trimethoprim-Induced Skin Rash and Liver Injury

Trimethoprim (TMP)-induced skin rash and liver injury are likely to involve the formation of reactive metabolites. Analogous to nevirapine-induced skin rash, 1 possible reactive metabolite is the sulfate conjugate of α-hydroxyTMP, a metabolite of TMP. We synthesized this sulfate and found that it reacts with proteins in vitro. We produced a TMP-antiserum and found covalent binding of TMP in the liver of TMP-treated rats. However, we found that α-hydroxyTMP is not a substrate for human sulfotransferases, and we did not detect covalent binding in the skin of TMP-treated rats. Although less reactive than the sulfate, α-hydroxyTMP was found to covalently bind to liver and skin proteins in vitro. Even though there was covalent binding to liver proteins, TMP did not cause liver injury in rats or in our impaired immune tolerance mouse model that has been able to unmask the ability of other drugs to cause immune-mediated liver injury. This is likely because there was much less covalent binding of TMP in the livers of TMP-treated mice than TMP-treated rats. It is possible that some patients have a sulfotransferase that can produce the reactive benzylic sulfate; however, α-hydroxyTMP, itself, has sufficient reactivity to covalently bind to proteins in the skin and may be responsible for TMP-induced skin rash. Interspecies and interindividual differences in TMP metabolism may be 1 factor that determines the risk of TMP-induced skin rash. This study provides important data required to understand the mechanism of TMP-induced skin rash and drug-induced skin rash in general.

The morbid cutaneous anatomy of the human genome revealed by a bioinformatic approach

Computational approaches have been developed to prioritize candidate genes in disease gene identification. They are based on different pieces of evidences associating each gene with the given disease. In this study, 648 genes underlying genodermatoses have been compared to 1808 genes involved in other genetic diseases using a bioinformatic approach. These genes were studied at the structural, evolutionary and functional levels. Results show that genes underlying genodermatoses present longer CDS and have more exons. Significant differences were observed in nucleotide motif and amino-acid compositions. Evolutionary conservation analysis revealed that genodermatoses genes have less paralogs, more orthologs in Mouse and Dog and are less conserved. Functional analysis revealed that genodermatosis genes seem to be involved in immune system and skin layers. The Bayesian network model returned a rate of good classification of around 80%. This computational approach could help investigators working in the field of dermatology by prioritizing positional candidate genes for mutation screening.

Keratin Associations with Synthetic, Biosynthetic and Natural Polymers: An Extensive Review

Among the biopolymers from animal sources, keratin is one the most abundant, with a major contribution from side stream products from cattle, ovine and poultry industry, offering many opportunities to produce cost-effective and sustainable advanced materials. Although many reviews have discussed the application of keratin in polymer-based biomaterials, little attention has been paid to its potential in association with other polymer matrices. Thus, herein, we present an extensive literature review summarizing keratin's compatibility with other synthetic, biosynthetic and natural polymers, and its effect on the materials' final properties in a myriad of applications. First, we revise the historical context of keratin use, describe its structure, chemical toolset and methods of extraction, overview and differentiate keratins obtained from different sources, highlight the main areas where keratin associations have been applied, and describe the possibilities offered by its chemical toolset. Finally, we contextualize keratin's potential for addressing current issues in materials sciences, focusing on the effect of keratin when associated to other polymers' matrices from biomedical to engineering applications, and beyond.

A Unique Expression of Keratin 14 in a Subset of Trophoblast Cells

The placenta, a transient organ in human, is essential for pregnancy maintenance and for fetal growth and development. Trophoblast and stromal cells are the main cell types present in human placenta. Trophoblast cells are present in different subtypes depending on their differentiation state and their temporal and spatial location during pregnancy. The stromal cells are of extraembryonic mesenchymal origin and are important for villous formation and maintenance. Interestingly, many pregnancy–related diseases are associated with defect in trophoblast differentiation and villous integrity. Therefore, it's crucial to specifically identify each type of placental cells using specific markers. Keratins (CK) are widely used as marker of epithelial cells, cancer origin identification and in some cases as marker of stem/progenitor cells. Vimentin is widely used as marker of mesenchymal cells. The aim of this study is to characterize the presence of different keratins in human trophoblast cells and vimentin in stromal cells. Using immunohistochemistry on term placental sections, our results show that vimentin is solely expressed in stromal-mesenchymal cells while keratins 5, 7, 8, 14 and 19 are expressed in trophoblast cells. Interestingly, all keratins tested, except for keratin 14, were evenly expressed in all trophoblast cells. Keratin 14 was expressed in a subset of CK7 positive cells. Moreover, the same results were obtained when using freshly isolated cytotrophoblast cells or BeWo cells. In conclusion, this study is a crucial step in the advancement of our knowledge in placental cell type identification and characterization.

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Using transgenic models to study the pathogenesis of keratin-based inherited skin diseases

In the past decade, the production of transgenic animals whose genome is modified to contain DNA transgenes of interest has significantly contributed to expand our understanding of the molecular etiology and pathobiology of several inherited skin diseases. This technology has led to the discovery that mutations affecting specific keratin genes are responsible for a wide spectrum of inherited bullous diseases, which are collectively characterized by blistering after minor trauma. Type I and type II keratin proteins are restricted to, and very abundant in, epithelial cells, where they occur as a pancytoplasmic network of cytoskeletal filaments. Although it had long been suspected that a primary function of keratin filaments may be to contribute to the physical strength of epithelial sheets, a formal demonstration came from studies of transgenic mouse models and patients suffering from keratin-based blistering diseases. Here we review the basic characteristics of keratin gene and their proteins and relate them to the molecular pathogenesis of relevant inherited skin blistering diseases. A particular emphasis is placed on the role of transgenic mouse models in the past, current, and future studies of these genodermatoses.

The utility of cytokeratin profiles for detecting oral cancer using exfoliative cytology

It is generally agreed that there is a need for a routine, non-invasive screening procedure for oral cancer particularly of high risk groups. Refinements in oral exfoliative cytology now make this technique worthy of consideration for such screening. This study assesses the utility of monitoring cytokeratin expression in smears of oral cancer in comparison with assessing the keratin expression in corresponding biopsies. Smears and biopsies were taken from 34 patients with oral cancer. A panel of antibodies, CAM5.2, LH1, AE8, LP2K and LH8 recognising keratins 8, 10, 13, 19 and a basal cell marker respectively were employed. Keratins were identified using a standard immunocytochemical technique (Vectastain) and assessed on a 3 point scale, for both smears and biopsies. The vast majority of tumours were well differentiated. No particular keratin profile scen within the smear was associated with any particular state of differentiation. Although the sensitivity of K19 was greatest, its specificity was poor. The keratin antibodies with the best positive predictive value were CAM5.2 (K8) and the marker of the basal cell phenotype, LH8. The combination of down regulation of the secondary differentiation markers (K13, K10) coupled with 'simple' keratin expression (K8, K19) would seem to be the most consistent profile. We conclude that for exfoliative cytological screening to be of value as a diagnostic test it remains necessary to employ assays using more than one antikeratin antibody.

Cytokeratin expression in oral cancer and its relationship to tumor differentiation

Although cytokeratin expression is said to alter with the state of tumor differentiation, few studies appear to have confirmed this in fresh tissue biopsies from oral squamous cell carcinomas. Previous studies have been limited by the number of antibodies utilized, small sample size, or lack of information regarding tumor differentiation. A panel of 8 antikeratin antibodies (of which five recognised the simple epithelial keratins, 8, 18 and 19) were applied to fresh tissue biopsies from 24 oral cancer and 15 normal oral mucosal biopsies. A standard immunocytochemical technique was followed (Vectastain ABC method), with keratin expression graded on a 3 point scale. Our analysis indicated that simple epithelial keratins (K8, K18, K19) were not confined to the more poorly differentiated tumors. This may be relevant to tumor prognosis.

Cytokeratin polypeptide expression in a cloacogenic carcinoma and in the normal anal canal epithelium

The aim of the present study was to explore the origin of cloacogenic carcinoma in the anal canal by immunohistochemical methods. We compared cytokeratin polypeptide expression of a cloacogenic carcinoma to normal and epithelia, to anal squamous cell carcinoma and to basal and squamous cell carcinoma of the skin, using a battery of monoclonal anti-cytokeratin, polypeptide-specific antibodies. Our results indicate that cloacogenic carcinoma expresses cytokeratin polypeptides similar to those of the basal layer of anal squamous epithelium, of the anal transitional zone epithelium and of a layer of basal cells in the anal glands. Thus we concluded that each of the above cell types may be the cell of origin of cloacogenic carcinoma.

Epidermal cell-shape regulation and subpopulation kinetics during butyrate-induced terminal maturation of normal and SV40-transformed human keratinocytes: epithelial models of differentiation therapy

Recent data indicate that malignant human epidermal cells may be appropriate targets for sodium butyrate (NaB)-mediated differentiation therapy. The response of pre- and post-crisis populations of SV40-transformed human keratinocytes (SVKs) to this differentiation-inducing agent was assessed, therefore, within the framework of NaB-directed normal human keratinocyte (NHK) maturation. NaB augmented cornified envelope (CE) production in NHK and pre-crisis SVK cultures; the time-course and efficiency of induced maturation were similar in the 2 cell systems. In NHKs, the percentage of amplifying ("B" substate) cells decreased with time in NaB correlating with increases in both "C" stage keratinocytes and CEs. The latter formed over one or 2 layers of nucleated basal-like cells. Inductions were accompanied by immediate cell cycle blocks (in both the G1 and G2/M phases), reorganization within the actin cytoskeleton, and transient early increases in cellular actin content. Increased NHK and pre-crisis SVK cytoskeletal-associated actin reached a maximum approximately 48 hr after NaB addition and preceded development of CEs. The CE precursors, thus, probably reside in the "B" substate. Post-crisis SVKs, in contrast, were refractive to NaB-induced terminal maturation or cell-cycle perturbation, failed to initiate actin filament rearrangements, and retained a basal cell-like phenotype. Stable transformation of human SVKs in post-crisis phase, therefore, appears to be associated with loss of maturation "competence" within the "B" keratinocyte subpopulation.

Sodium-N-butyrate induces cytoskeletal rearrangements and formation of cornified envelopes in cultured adult human keratinocytes

The technique developed in our laboratory allows us to culture multilayered, stratified sheets of human keratinocytes, which can be used to cover the burn wounds of patients. Organization of cells in these cultures resembles stratum germinativum and stratum spinosum but there are only a few fully keratinized cells and the stratum corneum is not developed. Since the fully differentiated sheets may offer additional advantages as epidermal transplants, attempts were made to enhance the degree of differentiation in vitro. In the present study sodium-N-butyrate (NaB) was used as a differentiating agent and its effect on the cell cycle and cytoarchitecture of epidermal cells was investigated. Incubation of keratinocytes in the presence of 2.5 mM NaB induced the appearance of enucleated cornified envelopes, covering approximately 70-80% of the surface of the cultures. Their appearance correlated with a decrease in expression of keratin K13, previously shown to be inhibited during terminal differentiation of human keratinocytes. An increase in transglutaminase transferase activity was also observed. The induction of cornified layers also correlated with an increase in the amount of microfilament (MF)-associated actin. NaB also induced changes in the cell cycle distribution of the keratinocyte cultures. A decrease in the proportion of S and G1B phase cells was paralleled by an increase in G1A cells, maximally expressed 30-48 h following addition of the inducer. Interestingly, NaB also induced a cell arrest in G2 phase. These cell cycle perturbations preceded the onset of keratinocyte differentiation. The results indicate that the enhanced differentiation of human keratinocytes in the presence of NaB may serve as a means to produce epidermal sheets with improved properties for transplantation in a clinical setting. It also serves as an in vitro model system to study the interrelationships between biochemical events and cell cycle changes accompanying differentiation.

Sequence analysis of murine cytokeratin endo A (no. 8) cDNA. Evidence for mRNA species initiated upstream of the normal 5' end in PCC4 cells

Keratin 8 (Endo A) is expressed in simple epithelia, together with keratin 18 (Endo B). Filaments formed by this keratin pair are the first cytoskeletal elements induced during mouse embryogenesis. We have isolated Endo A cDNA clones from lambda gt11 libraries prepared with mRNA isolated from PCC4 embryonal carcinoma (EC) cells. Sequencing of three overlapping cDNAs and of a genomic clone allowed us to determine the complete sequence of the Endo A message. Analysis of the protein sequence deduced showed that the Endo A protein presented all the characteristics of intermediate filaments, including an alpha-helical central rod domain and nonhelical N- and C-termini. In the rod domain, the degree of similarity to the other members of the basic keratin family was high. A high degree of homology to keratin 8 of other species was observed, even in the non-helical domains. During these analyses, we found clones extending upstream of the normal 5' end of the mRNA. Sequence comparison between these cDNAs and the 5' upstream region of the Endo A gene suggested that they corresponded to transcripts initiated at an upstream alternative promoter. These observations supported previous results showing the presence of Endo A transcripts initiated upstream of the normal 5' end in mouse morulae and blastocysts.

Developmental-specific expression and immunoreactivity of keratins during odontogenesis in rat embryos

The enamel organ of the mammalian dental primordium undergoes a precise sequence of differentiation. To correlate this differentiation with tissue-specific markers we analysed the keratin protein composition and immunoreactivity of incisor primordia from the earliest stage of odontogenesis to the neonatal period. Throughout the enamel organ synthesized a characteristic subset of keratin proteins, and the expression of one specific keratin marked the onset of the cap stage. Interestingly, the immunoreactivity of the ameloblastic keratins against polyclonal antibodies increased with progressive odontogenesis, suggesting that cytokeratin filaments may undergo post-translational or conformational alterations during assembly within differentiating enamel-organ cells.

Cytokeratin polypeptides in normal and metaplastic human salivary gland epithelia

Immunofluorescent and immunoperoxidase labelling of normal and metaplastic human submandibular salivary glands with a battery of cytokeratin-specific monoclonal antibodies was carried out. Labelling with a broad spectrum cytokeratin antibody (KG 8.13), as well as with antibody to cytokeratin polypeptide No. 18 (Ks 18.18) was observed in all the epithelial elements of the gland. Polypeptide No. 19, however, was present in ductal cells only, sparing the acini and the associated myoepithelium. Antibody KS 8.58, specific for cytokeratins Nos. 13 and 16, selectively labelled basal cells along the large ducts. Examination of squamous metaplasia associated with chronic suppurative sialadenitis indicated that the metaplastic cells display the same cytokeratin profile as the normal ductal cells and are not labelled with antibodies KS 8.58 and KK 8.60 which usually stain normal and pathological stratified epithelia. The significance of these observations for the histogenesis of normal salivary glands, as well as for the development of the metaplastic process, is discussed.

Suprabasal expression of a 64-kilodalton keratin (no. 3) in developing human corneal epithelium

We have previously shown that a basic 64-kilodalton (no. 3 in the catalog of Moll et al.) and an acidic 55-kilodalton (no. 12) keratin are characteristic of suprabasal cell layers in cultured rabbit corneal epithelial colonies, and therefore may be regarded as markers for an advanced stage of corneal epithelial differentiation. Moreover, using an AE5 mouse monoclonal antibody, we showed that the 64-kilodalton keratin marker is expressed suprabasally in limbal epithelium but uniformly (basal layer included) in central corneal epithelium, suggesting that corneal basal cells are in a more differentiated state than limbal basal cells. In conjunction with previous data implicating the centripetal migration of corneal epithelial cells, our data support a model of corneal epithelial maturation in which corneal epithelial stem cells are located in the limbus, the transitional zone between the cornea and conjunctiva. In the present study, we analyzed the expression of the 64-kilodalton keratin in developing human corneal epithelium by immunohistochemical staining. At 8 weeks of gestation, the presumptive corneal epithelium is composed of a single layer of cuboidal cells with an overlying periderm; neither of these cell layers is AE5 positive. At 12-13 weeks of gestation, some superficial cells of the three- to four-layered epithelium become AE5 positive, providing the earliest sign of overt corneal epithelial differentiation. At 36 weeks, although the epithelium is morphologically mature (four to six layers), AE5 produces a suprabasal staining pattern, this being in contrast to the adult epithelium which exhibits uniform staining.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence of a cDNA encoding human keratin No 10 selected according to structural homologies of keratins and their tissue-specific expression

We present here the nucleotide sequence of a 1700 bp-long cDNA encoding human epidermal keratin No. 10 (56.5 kDa). cDNA clones of the acidic keratin family were first isolated from a pBR322 human epidermal cDNA library by hybridization with a probe coding for keratin No. 14. Differential hybridization using total cDNA probes prepared from poly(A)+ RNA extracted either from epidermis (which contains keratin No. 10) and from squamous carcinoma or hepatoma cell lines (which do not express keratin No. 10) made possible the selection of clones potentially coding for keratin No. 10. The 1.7 kb sequence exhibits the characteristic features of an acidic keratin with a constant central rod domain and C-terminal variable structures. Moreover, the sequence shows extensive homologies with the cDNA of murine keratin No. 10.

Immunohistochemical studies of basal cell carcinomas transplanted into nude mice

Xenografting into nude mice forms a system for analysis of human tissues under experimental conditions. In this study, normal skin samples and basal cell carcinomas were investigated, prior to and after transplantation, using immunofluorescence methods with antibodies against keratins, laminin, and collagen type IV. Three groups of transplants were studied: intact tissue samples, human epithelium (either normal or neoplastic) recombined with normal human dermis and, human epithelium recombined with normal mouse dermis. Transplants recovered after 3 weeks showed the following characteristics. The xenograft system was satisfactory in terms of host survival and rate of successful tissue recovery except for recombinants between human epithelium and mouse dermis. Intact and recombined samples of normal skin retained their preexisting patterns of architecture, cytodifferentiation, and basement membrane staining. Solid nonfibrosing basal cell carcinomas showed altered architecture and differentiation of both the epithelium and the basement membrane zone after transplantation: the solid tumor pattern changed towards spreading of tumor cells, a more squamous differentiation pattern was apparent and was confirmed by reactivity with antibodies against large keratins. Discontinuities of the basement membrane zone were detected with antibodies against laminin and collagen type IV. These changes were seen in both intact and recombined tumor transplants.

Cytokeratin expression in squamous metaplasia of the human uterine cervix

The expression of cytokeratin polypeptides in squamous metaplasia of the human uterine cervix was investigated by immunocytochemical labeling with polypeptide-specific antibodies against cytokeratins. Immunofluorescence microscopic examination of cervical tissues using various monoclonal antibodies indicated that squamous cervical metaplasia expresses a unique set of cytokeratin polypeptides, this being distinctively different from that expressed by all of the normal epithelial elements of the exo- and endocervix. The development of metaplastic foci was accompanied by the expression of cytokeratin polypeptide no. 13, which is commonly detected in stratified epithelia, and by a reduction in the level of polypeptide no. 18, which is typical of simple epithelia. The 40-kilodalton cytokeratin (no. 19) described by Moll et al., which is abundant in the columnar and reserve cells of the endocervix, was found throughout the metaplastic lesions. Only in 'well-differentiated' metaplasias did we detect polarity of cytokeratin expression reminiscent of the staining patterns in the exocervix. This was manifested by the exclusive labeling of the basal cell layer(s) with antibodies KB 8.37 and KM 4.62, which stain the basal cells of the exocervix. Furthermore, a comparison of cervical metaplasia with squamous areas occurring within endometrial adenocarcinomas pointed to a close similarity in the cytokeratin expression of the two. We discuss the use of cytokeratins as specific markers of squamous differentiation, the relationships between squamous metaplasia and cervical neoplasia, and the involvement of reserve cells in the metaplastic process.

Differentiation-related expression of a major 64K corneal keratin in vivo and in culture suggests limbal location of corneal epithelial stem cells

In this paper we present keratin expression data that lend strong support to a model of corneal epithelial maturation in which the stem cells are located in the limbus, the transitional zone between cornea and conjunctiva. Using a new monoclonal antibody, AE5, which is highly specific for a 64,000-mol-wt corneal keratin, designated RK3, we demonstrate that this keratin is localized in all cell layers of rabbit corneal epithelium, but only in the suprabasal layers of the limbal epithelium. Analysis of cultured corneal keratinocytes showed that they express sequentially three major keratin pairs. Early cultures consisting of a monolayer of "basal" cells express mainly the 50/58K keratins, exponentially growing cells synthesize additional 48/56K keratins, and postconfluent, heavily stratified cultures begin to express the 55/64K corneal keratins. Cell separation experiments showed that basal cells isolated from postconfluent cultures contain predominantly the 50/58K pair, whereas suprabasal cells contain additional 55/64K and 48/56K pairs. Basal cells of the older, postconfluent cultures, however, can become AE5 positive, indicating that suprabasal location is not a prerequisite for the expression of the 64K keratin. Taken together, these results suggest that the acidic 55K and basic 64K keratins represent markers for an advanced stage of corneal epithelial differentiation. The fact that epithelial basal cells of central cornea but not those of the limbus possess the 64K keratin therefore indicates that corneal basal cells are in a more differentiated state than limbal basal cells. These findings, coupled with the known centripetal migration of corneal epithelial cells, strongly suggest that corneal epithelial stem cells are located in the limbus, and that corneal basal cells correspond to "transient amplifying cells" in the scheme of "stem cells----transient amplifying cells----terminally differentiated cells."

Alterations in the expression of two epidermal differentiation antigens in human epidermal disorders

The expression of an epidermal keratin subunit and a specific antigen of the keratinocyte membrane, two differentiation antigens in normal human epidermis, was studied in benign and malignant epidermal lesions by use of monoclonal antibodies KL1 (anti 55-57 Kd keratins) and KL3 (anti keratinocyte membrane antigen). In normal human epidermis, KL1 labelled all keratinocytes from the suprabasal layers, KL3 stained the intercellular spaces in all epidermal layers with a fluorescence intensity increasing from the basal to the more upper layers and recognized a keratinocyte membrane antigen as demonstrated in electron microscopy. Frozen or deparaffinized sections of basal cell carcinomas (BCC), squamous cell carcinomas (SCC) malignant melanomas, warts, and skin biopsies from benign lesions (psoriasis, lichen planus, bullous pemphigoid, lupus erythematodes, pemphigus, vasculitis) were tested with either KL1 or KL3 by indirect immunofluorescence and/or immunoperoxidase. Benign and malignant lesions in which modifications of the keratinization process and cell differentiation are known to occur (BCC, SCC, warts, psoriasis) showed the most severe alterations as compared to normal epidermis. With KL1 we observed an irregular staining of basal cells; a reorganization of keratin filaments and variable staining intensities within tumoral cells which did not express high MW keratins. With KL3 drastic alterations in the epidermal intercellular patterns and loss of reactivity of tumoral cells were noted. Conversely, the positivity of epidermal basal cells with KL1, in some cases, was the only modification noted in other skin lesions.

Co-expression of specific acid and basic cytokeratins in teratocarcinoma-derived fibroblasts treated with 5-azacytidine

Epithelial cells always co-express acidic and basic keratin polypeptides. Mesenchymal cells, which do not normally contain keratins, can be induced by the inhibitor of DNA methylation 5-azacytidine to synthesize the basic keratin Endo A. In the present paper we show that the acidic keratins Endo B and Endo C can also be induced by 5-azacytidine in teratocarcinoma-derived fibroblasts. Furthermore, individual cells in which Endo B and/or Endo C keratins are found, always co-express the basic polypeptide Endo A. Other cytokeratins are not or very rarely found. Interestingly, Endo A, B, and C are usually associated in vivo and are known to be the first keratin polypeptides appearing during the development of the mouse embryo.

Intermediates in the conversion of prekeratin into keratin molecules in human epidermis

In order to elucidate the relationship between prekeratin and keratin, we performed pulse-chase experiments using [35S]methionine (35S-Met) in vitro. Of 6 prekeratin molecules (49, 52, 55, 62, 69 and 71 kDa) that incorporated 35S-Met, the 55-kDa prekeratin incorporated the most 35S-Met. In 3 molecules (52, 55 and 62 kDa) incorporation was decreased at 30 min after being chased; however, incorporation of only two molecules (55 and 62 kDa) of the 6 prekeratins was increased at 60 min. From these results and our previous data, we conclude that the initial stage of processing is as follows: 3 prekeratin molecules (52, 55 and 62 kDa) are first cleaved in the N-terminal region, then two prekeratin molecules (55 and 69 kDa) are processed to intermediates (52 and 62 kDa) by some proteolytic enzyme(s).