Expression of specific keratin markers by rabbit corneal, conjunctival, and esophageal epithelia during vitamin A deficiency

New approaches and concepts in the study of differentiation of oral epithelia

Epithelial structural proteins, the keratins and keratin-associated proteins, are useful as markers of differentiation because their expression is both region-specific and differentiation-specific. In general, basal cells in all stratified oral epithelia express similar keratins, while the suprabasal cells express a specific set of markers indicating commitment to a distinct program of differentiation. Critical factors in the regulation of epithelial protein expression are now under investigation. The promoter regions of keratin genes are being characterized to determine what sequences within the genes are responsible for differential expression. One important extracellular factor that influences epithelial protein expression is retinol (vitamin A), which exerts its effects via a group of nuclear receptor proteins that may also be expressed in a region-specific manner. These molecular biological approaches enhance our understanding of the mechanisms regulating differentiation of oral epithelia and its regional complexity.

Inhibition of growth and squamous-cell differentiation markers in cultured human head and neck squamous carcinoma cells by beta-all-trans retinoic acid

Vitamin A and some of its metabolites such as beta-all-trans retinoic acid (RA) have been implicated in the regulation of differentiation of normal and malignant epithelial cells in vivo and in vitro. In the present study the effects of RA on the growth and differentiation of 7 cell lines derived from human head and neck squamous-cell carcinomas (HNSCCs) were examined. RA (greater than 0.01 microM) inhibited the proliferation in monolayer culture of 6 of 7 HNSCC cell lines. One cell line (UMSCC-35) was very sensitive, 5 (UMSCC-10A, -19, -30, -22B and HNSCC 1483) were moderately sensitive, and 1 (HNSCC 183) was insensitive. Three of the cell lines (UMSCC-22B, -30, and HNSCC 1483) were capable of forming colonies in semisolid medium--a capability that was suppressed by RA. The HNSCC cell lines expressed various levels of the squamous-cell differentiation markers type I (particulate, epidermal) transglutaminase (TGase) and cholesterol sulfate (CS). RA treatment (I microM, 6 days) decreased TGase activity by more than 50% in 3 (UMSCC-10A, -22B and 1483) of the 7 cell lines, and the effect on UMSCC-22B was dose-dependent. Type II TGase (soluble, tissue type) activity was detected in 3 cell lines, and after RA treatment its activity increased in HNSCC 1483 and 183 cells and decreased in UMSCC-19. Following RA treatment, CS levels decreased by 20, 25, 70, 76, 89 and 91% in cell lines UMSCC-30, -10A, 183, UMSCC-35, -22B, and HNSCC 1483, respectively. The suppression by RA of CS accumulation in the 1483 cells was dose-dependent. Cholesterol sulfotransferase activity, which is responsible for CS synthesis, was suppressed by 40-97% after RA treatment of UMSCC-19, -22B, and HNSCC 1483. Our results demonstrate that RA inhibits the growth and decreases the level of 2 squamous differentiation markers in HNSCC cells.

The pancornulins: a group of basic low molecular weight proteins in mammalian epidermis and epithelium that may function as cornified envelope precursors

1. A monoclonal antibody (HCE-2) to human epidermal and epithelial cornified envelopes identified a group of soluble basic protein precursors. 2. Using HCE-2, envelope-like staining was observed in the epidermis and stratified squamous epithelium of a number of mammalian species. 3. Basic polypeptides reactive to HCE-2 varied in size and number among the different animals. 4. In those species studied, HCE-2-reactive peptides were substrates for transglutaminase and protease treatment of cornified envelopes released HCE-2-reactive degradation products. 5. These results suggest a new family of proteins in mammalian epidermis that may function as cornified envelope precursors.

Extensive changes in cytokeratin expression patterns in pathologically affected human gingiva

The stratified squamous epithelium of the oral gingiva and the hard palate is characterized by a tissue architecture and a cytoskeletal composition similar to, although not identical with, that of the epidermis and fundamentally different from that of the adjacent non-masticatory oral mucosa. Using immunocytochemistry with antibodies specific for individual cytokeratins, in situ hybridization and Northern blots of RNA with riboprobes specific for individual cytokeratin mRNAs, and gel electrophoresis of cytoskeletal proteins of microdissected biopsy tissue samples, we show changes in the pattern of expression of cytokeratins and their corresponding mRNAs in pathologically altered oral gingiva. Besides a frequently, although not consistently, observed increase in the number of cells producing cytokeratins 4 and 13 (which are normally found as abundant components in the sulcular epithelium and the alveolar mucosa but not in the oral gingiva) and a reduction in the number of cells producing cytokeratins 1, 10 and 11, the most extensive change was noted for cytokeratin 19, a frequent cytokeratin in diverse one-layered and complex epithelia. While in normal oral gingiva cytokeratin 19 is restricted to certain, sparsely scattered cells of --or near--the basal cell layer, probably neuroendocrine (Merkel) cells, in altered tissue of inflamed samples it can appear in larger regions of the basal cell layer(s) and, in apparently more advanced stages, also in a variable number of suprabasal cells. Specifically, our in situ hybridization experiments show that this altered suprabasal cytokeratin 19 expression is more extended at the mRNA than at the protein level, indicating that cytokeratin 19 mRNA synthesis may be a relatively early event during the alteration. These changes in cytokeratin expression under an external pathological influence are discussed in relation to other factors known to contribute to the expression of certain cytokeratins and with respect to changes occurring during dysplasia and malignant transformation of oral epithelia.

Transient synthesis of K6 and K16 keratins in regenerating rabbit corneal epithelium: keratin markers for an alternative pathway of keratinocyte differentiation

Cultured rabbit corneal epithelial cells undergo three distinct stages of growth and differentiation characterized by the sequential appearance of K5/K14 keratin markers for basal keratinocytes, K6/K16 keratin markers for "hyperproliferative" keratinocytes, and K3/K12 keratin markers for corneal-type differentiation. Analyses of [35S]methionine-labeled, newly synthesized keratins revealed that K6/K16 are synthesized only briefly when the cells undergo exponential growth, and their synthesis is suppressed when the cells reach confluence and switch to synthesizing K3/K12. Transient synthesis of K6/K16 was also observed in vivo during corneal epithelial regeneration. Although K6/K16 expression in general correlates well with cellular growth, drug-induced inhibition of corneal epithelial growth and related data on human epidermal keratinocytes indicate that these two events are dissociable. These results establish clearly for the first time a reciprocal relationship, on a protein level, between the synthesis of K6/K16 and a differentiation-related keratin pair, K3/K12. Such a relationship strongly suggests a competitive mechanism controlling the synthesis of these two major classes of keratins in the suprabasal compartment. Our results also indicate that although hyperproliferation is usually accompanied by K6/K16 expression, the reverse is not always true. Taken together, the data suggest that K6/K16 are synthesized, perhaps by default, as an alternative suprabasal keratin pair under conditions that are nonpermissive for keratinocytes to express their normal, differentiation-related keratin pairs.

Keratin protein expression during the development of conducting airway epithelium in nonhuman primates

Keratin protein expression during the development of Rhesus monkey conducting airway epithelium was investigated by both biochemical and immunohistochemical methods. Keratin proteins were extracted from tracheal and intrapulmonary airway tissues of fetal (at 80- and 140-day gestational ages), neonatal, and adult animals. Using immunoblot analyses and immunohistochemistry with various monoclonal (AE1, AE3, AE8, 6.01 and 6.11) and monospecific antibodies (anti-50/55 and anti-40 kDa), the presence of keratins 5, 6, 8, 13, 14, and 19 in adult airway epithelium were demonstrated. Except for keratin 13 (51 kDa), the remaining keratins could be immunologically detected in fetal and neonatal tissues. To further understand the nature of the synthesis of keratin 13 during development, airway epithelial cells from different ages were isolated and cultured in vitro. Cultured cells were labeled with 35S-methionine, and the patterns of keratin protein were analyzed by one- and two-dimensional sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Results indicated that the cultured airway cells synthesized additional keratins including 7, 15, 16, 17, and 18. However, consistent with the in vivo finding, fetal cells synthesized less or no keratin 13. These in vivo and in vitro studies strongly suggest that the synthesis of the keratin 13 in monkey conducting airway epithelium is developmentally regulated.

Expression of hair-related keratins in a soft epithelium: subpopulations of human and mouse dorsal tongue keratinocytes express keratin markers for hair-, skin- and esophageal-types of differentiation

The dorsal surfaces of mammalian tongues are covered with numerous projections known as filiform papillae whose morphology varies in different species. Using a panel of monoclonal antibodies to keratins as probes, we have established that, in both human and mouse, the interpapillary epithelia express mainly the "esophageal-type" keratins, while the papillary epithelia express "skin-type" keratins as well as some keratins reacting with a monoclonal antibody (AE13) to hair keratins. The AE13-reactive proteins of the mouse were found to be very similar to those of authentic mouse hair keratins. However, the corresponding protein of human tongue appears to be different from all known human keratins. This protein has a MW of 51K; it is relatively acidic; it is sulfhydryl-rich, as revealed by iodoacetic acid-induced charge and apparent size shift; it shares an epitope with all the known acidic human hair keratins; and it is associated with keratin fibrils in vivo. This protein may therefore be regarded as a novel type I "hard" keratin. These data establish that mammalian dorsal tongue epithelia can be divided into at least three compartments that undergo mainly "esophageal-", "skin-" and "hair"-types of differentiation. Different keratin filaments, e.g., those of the esophageal- and hair-types, exhibit strikingly different degrees of lateral aggregation, which can potentially account for the different physical strength and rigidity of various cellular compartments. Our data also suggest the possibility that variations in papillary structure in human and mouse may arise from different spatial arrangements of specific keratinocytes, and/or from the expression of specialized hair-related keratins.

Density-dependent modulation of synthesis of keratins 1 and 10 in the human keratinocyte line HACAT and in ras-transfected tumorigenic clones

The spontaneous human keratinocyte line HaCaT and c-Ha-ras oncogene-transfected cell clones are capable of expressing an unusually broad spectrum of keratins, not observed so far in epithelial cells. This expression is, however, strongly modulated by environmental conditions, including cell density. Both cells of the nontumorigenic HaCaT line and the tumorigenic HaCaT-ras clones, I-7 and II-3 (giving rise to benign and malignant tumors, respectively), constitutively expressed the keratins K5, K6, K14, K16 and K17, which are also common in cultures of normal keratinocytes. In addition keratins K7, K8, K18 and K19, generally associated with simple epithelia, were synthesized (to a most pronounced extent in sparse cultures), while keratins K4, K13 and K15 appeared at confluence, presumably with the onset of stratification. Moreover, in both HaCaT and HaCaT-ras clones the epidermal "suprabasal" keratins, K1 and K10, were expressed in conventional submerged cultures (at normal vitamin A levels), markedly rising with cell density, but not strictly correlated with the degree of stratification. This property was maintained in HaCaT cells up to the highest passages. According to immunofluorescence, this was due to increasing numbers of strongly stained cells, and not due to a gradual increase in all cells. Most strikingly, there was a significant delay in the appearance of K10 compared to K1, and this dissociation of expression was most evident in dispase-detached cell sheets (submerged cultures) and organotypic cultures of the ras clones (grown at the air-liquid interface). While on frozen sections bright staining for K1 was seen in some basal and virtually all suprabasal cell layers, K10 was largely restricted to the uppermost layers. Thus, obviously synthesis of K1 and K10 can be regulated independently, although generally in this given sequence. The apparent compatibility of K1 synthesis with proliferation and particularly the extended delay of K10 expression (as a postmitotic event) might be causally related to altered growth control and as such imply the significance of this disturbance. Finally, the highly preserved epidermal characteristics, in terms of expression of keratins (and other differentiation markers [5]) and their regulation, makes these cell lines excellent candidates for studying external modulators of differentiation and also underlying molecular mechanisms.

Concept and application of limbal stem cells

Cumulative reported evidence indicates that some fraction of limbal basal epithelial cells are the stem cells for corneal epithelial cell proliferation and differentiation. Limbal epithelium is therefore crucial in maintaining the cell mass of corneal epithelium under normal conditions and plays an important role in corneal epithelial wound healing. Deficiency or absence of limbal stem cells explains well the pathogenesis of several ocular surface disorders characterised by defective conjunctival transdifferentiation or conjunctivalisation of cornea. This paper reviews and updates the basic concept of stem cells, the reported findings of limbal stem cells for corneal epithelium, and their therapeutic applications. Through this review, one hopes to gain a more complete understanding and increase proficiency in treating these diseases.

Identification of an orthologous mammalian cytokeratin gene. High degree of intron sequence conservation during evolution of human cytokeratin 10

Among the human acidic (type I) cytokeratins, components 10 and 11 are especially interesting, as they are under various kinds of expression control. They are synthesized in the suprabasal cell layers of certain stratified epithelia, notably epidermis, in an endogenous differentiation program; they are expressed in certain epithelial tumours but not in others; they can appear de novo in certain pathological situations such as in squamous metaplasias; and their expression in vivo and in vitro is under positive influence of extracellular calcium concentrations and is reduced in the presence of vitamin A or other retinoids. To provide a basis for studies of the various regulatory elements, we have isolated the human gene encoding cytokeratin 10, using a cDNA probe derived from the corresponding bovine gene, and have sequenced the mRNA coding region as well as adjacent regions approximately 1500 bases 5' upstream and 1000 bases 3' downstream. The eight exons encode a polypeptide 59,535 Mr, i.e. somewhat larger than the corresponding bovine and murine proteins. The deduced amino acid sequences display a high degree of homology, which is not restricted to the exons and the 5' and 3' adjacent regions but, surprisingly, is also evident in the seven introns, some of which contain extended sequence elements with 70% identical nucleotides and more, i.e. similar to the homology in the adjacent exons. This exceptionally high level of conservation of intron sequences is discussed in relation to the recently accumulating evidence of the occurrence of intron sequences important in the regulation of the expression of members of other multigene families during development.

Effect of retinoid deficiency on keratin expression in mouse bladder

Twelve to sixteen weeks following treatment of CF-1 mice with a vitamin A-deficient diet, characteristic signs of retinoid deficiency including body wasting, poor hair coat, altered gait, decreased mobility, and xerophthalmia were observed. Histological examination of tissue sections from these mice revealed dramatic changes in the urinary tract epithelium. The normal transitional epithelium was replaced by a stratified squamous epithelium that resembled hyperproliferative epidermis. Using two-dimensional gel electrophoresis, a number of new proteins were found to be synthesized in vitamin A-deficient bladder when compared to tissue from control bladders. Using antikeratin antibodies in immunoblot experiments, we found that at least some of the newly synthesized proteins were keratins. These proteins, which comprise the intermediate filaments of the cytoskeleton, are known to be specific markers of epithelial differentiation. Of particular interest was the appearance of a Mr 67,000 basic and Mr 61,000 acidic keratin pair, characteristic of terminally differentiating murine epidermal cells. Unexpectedly, several other keratins, previously associated only with hyperproliferative epidermis, were also expressed in the tissue. These results demonstrate that vitamin A deficiency in the mouse leads to the appearance of a squamous metaplasia in the urinary tract epithelium that is characterized by the expression of distinct epidermal keratins.

Induction of conjunctival transdifferentiation on vascularized corneas by photothrombotic occlusion of corneal neovascularization

Previous studies have established that conjunctival transdifferentiation (transformation into cornea-like morphology) is inhibited by corneal vascularization. Conversely, occlusion of corneal vessels may induce conjunctival transdifferentiation on vascularized corneas. To test this hypothesis, the corneal epithelia of New Zealand albino rabbits were debrided 3 mm beyond the limbus with n-heptanol. Sixteen corneas healed by conjunctival epithelium, with vascularization persisting for 20 months, were used in this study. Photochemically induced occlusion of the corneal vessels was achieved by intravenous administration of rose bengal-saline solution (40 mg/kg body weight) with subsequent argon laser irradiation of the vessels (514.5 nm, 130 mW, 63 micron and 0.2 sec). The treated vessels remained occluded in an 18-week study, as confirmed by corneal fluorescein angiography. Corneal clarity and epithelial integrity were improved after treatment. Goblet cell loss and morphologic transformation into a cornea-like epithelium were verified by flat-mount preparations, histology, impression cytology, and immunofluorescence studies using a mucin-specific monoclonal antibody. These results indicate that conjunctival transdifferentiation can be induced on vascularized corneas after occlusion of corneal vessels by photothrombosis.

Selective suppression of two postnatally acquired 70 kD and 65 kD keratin proteins during continuous treatment of adult mouse tail epidermis with vitamin A

Using mouse tail epidermis as a model system we have studied the morphologic and biochemical effects of continuous topical treatment with vitamin A acid. Normal tail epidermis shows a regular pattern of parakeratotic scale regions and orthokeratotic interscale regions which arise postnatally from a uniformly orthokeratinizing neonatal epidermis. Daily treatment of tail epidermis with vitamin A acid for 14 days results in the induction of hyperplasia and the orthokeratotic conversion of the scale regions. The degree of these alterations is dose-dependent and maximally brought about by repetitive 30-microgram doses of the vitamin. To correlate morphologic with biochemical alterations, we have analyzed the keratin patterns of normal and vitamin A acid-treated epidermis by one- and two-dimensional gel electrophoresis. The results indicate that repetitive vitamin A treatment leads to the selective suppression of two postnatally acquired 70 kD and 65 kD type II keratin proteins. Again the minimum repetitive dose required for their complete suppression is 30 micrograms vitamin A acid. Kinetic studies reveal an initial lag phase of 6 days of apparent nonresponsiveness, followed by a 5-day period during which the adult pattern is gradually replaced by the neonatal pattern. Repetitive treatment of tail epidermis with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate leads to a strong hyperplasia; however, it strictly maintains the scale parakeratosis. Under these conditions only the 70 kD keratin subunit is suppressed. This indicates that the suppression of the 70 kD keratin is generally linked to the induction of hyperproliferation, whereas the suppression of the scale-associated 65 kD subunit is due to the metaplastic potency of vitamin A. We provide evidence that this vitamin A-specific in vivo effect can be used to determine the biologic activity of synthetic retinoids relative to vitamin A acid.

An unusual type-II 70-kilodalton keratin protein of mouse epidermis exhibiting postnatal body-site specificity and sensitivity to hyperproliferation

Keratin extracts from the epidermis of adult mouse ears, footpads, and tail contain large amounts of a 70-kilodalton (kDa) protein which has not been detected in any other body site of the adult mouse or in the epidermis of neonatal mice. Two-dimensional immunoblotting using an antiserum which recognizes both type-I and type-II murine keratins revealed that the 70-kDa protein is indeed a keratin belonging to the type-II subfamily. Its postnatal induction occurs during the first 2 weeks after birth, being first observed in tail epidermis, then in footpad epidermis, and only rather late in ear epidermis. Although in vitro translation experiments with polyA+-RNA from adult tail and footpad epidermis consistently failed to reveal the 70-kDa protein among the translation products, we obtained evidence using a specific cDNA clone that, in vivo, the protein is encoded by a discrete mRNA. This clone, termed pke70, was isolated from a cDNA library of footpad epidermal mRNA. Homology comparisons with a variety of known keratin cDNAs indicated that pke70 contains sequence information for a type-II keratin that is substantially larger than the mouse 67-kDa keratin protein. Northern-blot analysis with a specific 3'-fragment of pke70 demonstrated a single 2.8 +/- 0.1 kb mRNA species exclusively in adult ear, footpad, and tail epidermis. In situ hybridization with the same fragment revealed the presence of the pke70-hybridizing mRNA in both basal and suprabasal cells of ear and footpad epidermis as well as in the orthokeratinizing parts of the tail epidermis; however in the epidermis covering the balls of the feet, labeling was restricted to suprabasal cells at the base of these nodular elevations. Continuous treatment of adult tail or ear epidermis with hyperplasiogenic agents, e.g., vitamin A acid and the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), leads to a gradual disappearance of the 70-kDa protein. We obtained evidence using in situ hybridization that the loss of the 70-kDa keratin is preceded by a specific suppression of the transcription of its putative mRNA in basal cells, whereas initially suprabasal cells are apparently still able to complete their original commitment. The particular properties of the 70-kDa keratin protein, i.e., its topological restriction, its postnatal and time-dependent acquisition, and its pronounced sensitivity to hyperplasiogenic stimuli, make this keratin subunit an especially suitable candidate for studies concerning the regulation of keratin expression and morphogenesis in general, as well as for studies of the factors that control its expression so specifically.

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)

Characterization of two monoclonal antibodies to human epidermal keratohyalin: reactivity with filaggrin and related proteins

Two monoclonal antibodies (AKH1 and AKH2) were elicited with partially purified human filaggrin and characterized by immunohistochemistry on normal and abnormal skin biopsies, immunoblotting techniques, and antigen purification. Both antibodies react strongly with the granular cell layer consistent with the distribution of keratohyalin and show a more diffuse reaction with the stratum corneum in normal skin biopsies. Reaction in cultured human keratinocytes is limited to immunofluorescent granules in flattened, well-differentiated cells in confluent cultures, in which we have previously demonstrated keratohyalin. On immunoblots AKH1 reacts with filaggrin (37 kD) and profilaggrin (400 kD), while AKH2 primarily stains bands of 150 and 300 kD. The AKH2 antigens were identified in the cationic protein fraction used for immunization and were purified by gel permeation and high-performance liquid chromatography. Amino acid composition of these proteins differs only slightly from filaggrin. Immunohistochemical staining patterns of the two antibodies are very similar in the genetic disorders of keratinization tested, except for ichthyosis vulgaris, and reflect the presence and distribution of keratohyalin. In ichthyosis vulgaris, AKH1 staining is weak, consistent with the morphology and with biochemical absence of profilaggrin/filaggrin; however, AKH2 staining is positive, although weaker than normal, suggesting the presence of the AKH2 antigens even when keratohyalin is absent or abnormal. Antibodies AKH1 and AKH2 may be useful as differentiation markers for keratinization in tissues and for cells in culture. Antibody AKH1 can be used specifically for detection of profilaggrin/filaggrin in tissues, cultured keratinocytes, and extracts.

Acidic and basic hair/nail ("hard") keratins: their colocalization in upper cortical and cuticle cells of the human hair follicle and their relationship to "soft" keratins

Although numerous hair proteins have been studied biochemically and many have been sequenced, relatively little is known about their in situ distribution and differential expression in the hair follicle. To study this problem, we have prepared several mouse monoclonal antibodies that recognize different classes of human hair proteins. Our AE14 antibody recognizes a group of 10-25K hair proteins which most likely corresponds to the high sulfur proteins, our AE12 and AE13 antibodies define a doublet of 44K/46K proteins which are relatively acidic and correspond to the type I low sulfur keratins, and our previously described AE3 antibody recognizes a triplet of 56K/59K/60K proteins which are relatively basic and correspond to the type II low sulfur keratins. Using these and other immunological probes, we demonstrate the following. The acidic 44K/46K and basic 56-60K hair keratins appear coordinately in upper corticle and cuticle cells. The 10-25K, AE14-reactive antigens are expressed only later in more matured corticle cells that are in the upper elongation zone, but these antigens are absent from cuticle cells. The 10-nm filaments of the inner root sheath cells fail to react with any of our monoclonal antibodies and are therefore immunologically distinguishable from the cortex and cuticle filaments. Nail plate contains 10-20% soft keratins in addition to large amounts of hair keratins; these soft keratins have been identified as the 50K/58K and 48K/56K keratin pairs. Taken together, these results suggest that the precursor cells of hair cortex and nail plate share a major pathway of epithelial differentiation, and that the acidic 44K/46K and basic 56-60K hard keratins represent a co-expressed keratin pair which can serve as a marker for hair/nail-type epithelial differentiation.

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."

The role of keratin subfamilies and keratin pairs in the formation of human epidermal intermediate filaments

The four major keratins of normal human epidermis (molecular mass 50, 56.5, 58, and 65-67 kD) can be subdivided on the basis of charge into two subfamilies (acidic 50-kD and 56.5-kD keratins vs. relatively basic 58-kD and 65-67-kD keratins) or subdivided on the basis of co-expression into two "pairs" (50-kD/58-kD keratin pair synthesized by basal cells vs. 56.5-kD/65-67-kD keratin pair expressed in suprabasal cells). Acidic and basic subfamilies were separated by ion exchange chromatography in 8.5 M urea and tested for their ability to reassemble into 10-nm filaments in vitro. The two keratins in either subfamily did not reassemble into 10-nm filaments unless combined with members of the other subfamily. While electron microscopy of acidic and basic keratins equilibrated in 4.5 M urea showed that keratins within each subfamily formed distinct oligomeric structures, possibly representing precursors in filament assembly, chemical cross-linking followed by gel analysis revealed dimers and larger oligomers only when subfamilies were combined. In addition, among the four major keratins, the acidic 50-kD and basic 58-kD keratins showed preferential association even in 8.5 M urea, enabling us to isolate a 50-kD/58-kD keratin complex by gel filtration. This isolated 50-kD/58-kD keratin pair readily formed 10-nm filaments in vitro. These results demonstrate that in tissues containing multiple keratins, two keratins are sufficient for filament assembly, but one keratin from each subfamily is required. More importantly, these data provide the first evidence for the structural significance of specific co-expressed acidic/basic keratin pairs in the formation of epithelial 10-nm filaments.

Vitamin A deficiency and keratin biosynthesis in cultured hamster trachea

Tracheas from vitamin A-deficient hamsters in organ culture in vitamin A-free medium developed squamous metaplasia. Addition of retinyl acetate to the medium prevented squamous metaplasia and a mucociliary epithelium was maintained. Indirect immunofluorescent staining with antikeratin antibodies AE1 and AE3 indicated positive reactions with epithelium of tracheas either cultured in vitamin A-free or retinyl acetate (RAc)-containing medium. The "stratum corneum"-like squames in metaplastic tracheas were strongly stained by AE3. Immunoprecipitation of cytoskeletal extracts from [35S]methionine labeled tracheas with a multivalent keratin antiserum indicated that the concentration of keratins synthesized in tracheas cultured in vitamin A-free medium was greater than that observed in tracheas cultured in the presence of RAc. In addition, new species of keratin were expressed in tracheas cultured in RAc-free medium. Alterations in the program of keratin synthesis were clearly detectable after 1 d in vitamin A-free medium, even though squamous metaplasia was not yet obvious. Squamous tracheas were shown by immunoblot analysis to contain keratins of 50, 48, 46.5, and 45 kilodalton (kd) detected with AE1; and 58, 56, and 52 kd detected with AE3. Immunoblot analysis with monospecific antimouse keratin sera also demonstrated the presence of 60, 55, and 50 kd keratins in the metaplastic tracheas. All these various species of keratins were either absent or present in much reduced quantity in mucociliary tracheas in RAc-containing medium. Interestingly, the induction of squamous metaplasia in tracheal epithelium did not result in the expression of the 59 and 67 kd keratins which are characteristically expressed in the differentiated layers of the epidermis. Therefore, this study shows that squamous metaplasia of tracheas due to vitamin A-free cultivation is accompanied by an increase in keratin synthesis as well as by the appearance of keratin species not normally present in mucociliary tracheal epithelium.

Effects of B-retinyl acetate on human breast epithelium in explant culture

Explants of human breast (obtained aseptically from ten women ages 18-37) were examined and the vitamin A compound B-retinyl acetate was tested (at 3 X 10(-5) and 3 X 10(-6) M) for its effects on the fine structure and growth of the epithelium. In the absence of B-retinyl acetate, cells growing out from the explant (outgrowth cells) underwent squamous metaplasia, began to accumulate many intermediate filaments (tonofilaments), and revealed large desmosomes after 2 weeks in culture. In the presence of either concentration of B-retinyl acetate, the epithelial cells were largely prevented from undergoing squamous metaplasia. The glandular epithelium inside the explant maintained a typical secretory appearance for 2 weeks in control cultures (without the retinoid) and then began to show increased numbers of lysosomes and a loss of secretory granules. These glandular epithelial cells did not undergo squamous metaplasia, but they contained increased numbers of intermediate filaments. In contrast, glandular epithelium cultured in either concentrations of B-retinyl acetate appeared secretory for as long 6 weeks in culture, and intermediate filaments were not obvious. Autoradiographs demonstrated that both concentrations of B-retinyl acetate inhibited cell division in the outgrowth epithelium and in the (internal) glandular epithelium. The ability of B-retinyl acetate to reverse squamous metaplasia in the outgrowth cells was also tested. Outgrowth cells reversed to a more normal ductlike appearance after 6 weeks culture in standard medium followed by only 1 week in 20 micrograms/ml B-retinyl acetate. After 7 weeks in standard medium and 1 week in 10 micrograms/ml B-retinyl acetate, the cells showed a partial reversal of the squamous metaplasia.

Clinical, electron microscopic, and monoclonal antibody studies of intraocular epithelial downgrowth

Epithelial downgrowth developed in three patients following cataract extraction and keratoplasty. Light and electron microscopy of the downgrowth tissue disclosed stratified squamous epithelium, but could not determine whether they were derived from conjunctival or corneal epithelium. The epithelial downgrowth contained cells that were connected laterally by desmosomal junctions and displayed well-defined basement membranes. Surface epithelial cells exhibited myriad microvillous processes scant to moderate mitochondria. In one case, prominent hemidesmosomal junctions were present. Immunohistochemical staining with monoclonal antikeratin antibodies revealed immunoreactivity with AE1, AE3, and AE11 in all cases, but with a cornea-specific antibody, AE5, in only one case. We concluded that in this last case, the epithelial downgrowth appears to have originated from the corneal epithelium.

Keratin proteins in the epithelium of the uterine cervix

Keratin proteins in human cervical squamous epithelium were investigated by the immunofluorescence technique, using the monoclonal keratin antibodies AE1 and AE2. Although the exocervic is covered by a non-keratinizing, histologically homogenous, squamous epithelium, different staining patterns were found within a single histological section. This indicates a regional variation in the maturation pattern of stratified exocervical epithelium. Detection of morphological versus biochemical keratinization in the diagnosis of pathological cervical conditions is discussed.

In vivo metabolism of topically applied retinol and all-trans retinoic acid by the rabbit cornea

Corneas of normal and vitamin A-deficient rabbits were treated topically with [11, 12-3H] retinol or [11, 12-3H] all-trans retinoic acid. Methanol extracts of these corneas were analyzed by high pressure liquid chromatography. Radiolabeled compounds were extracted from the corneas which co-migrated chromatographically with known retinoid standards. In agreement with studies on other tissues and organs, retinol was metabolized to retinoic acid and more polar compounds by corneas of normal and vitamin A-deficient rabbits. All-trans retinoic acid was isomerized to 13-cis retinoic acid in normal rabbit corneas; however, this trans-cis isomerization did not occur in vitamin A-deficient, xerophthalmic corneas.