Aberrant in vitro expression of keratin K13 induced by Ca2+ and vitamin A acid in mouse epidermal cell lines

The role of cathepsin E in terminal differentiation of keratinocytes

Cathepsin E (CatE) is predominantly expressed in the rapidly regenerating gastric mucosal cells and epidermal keratinocytes, in addition to the immune system cells. However, the role of CatE in these cells remains unclear. Here we report a crucial role of CatE in keratinocyte terminal differentiation. CatE deficiency in mice induces abnormal keratinocyte differentiation in the epidermis and hair follicle, characterized by the significant expansion of corium and the reduction of subcutaneous tissue and hair follicle. In a model of skin papillomas formed in three different genotypes of syngeneic mice, CatE deficiency results in significantly reduced expression and altered localization of the keratinocyte differentiation induced proteins, keratin 1 and loricrin. Involvement of CatE in the regulation of the expression of epidermal differentiation specific proteins was corroborated by in vitro studies with primary cultures of keratinocytes from the three different genotypes of mice. In wild-type keratinocytes after differentiation inducing stimuli, the CatE expression profile was compatible to those of the terminal differentiation marker genes tested. Overexpression of CatE in mice enhances the keratinocyte terminal differentiation process, whereas CatE deficiency results in delayed differentiation accompanying the reduced expression or the ectopic localization of the differentiation markers. Our findings suggest that in keratinocytes CatE is functionally linked to the expression of terminal differentiation markers, thereby regulating epidermis formation and homeostasis.

Repair characteristics and differentiation propensity of long-term cultures of epidermal keratinocytes derived from normal and NER-deficient mice

Epidermal keratinocytes constitute the most relevant cellular system in terms of DNA damage because of their continuous exposure to UV light and genotoxic chemicals from the environment. Here, we describe the establishment of long-term keratinocyte cultures from the skin of wild-type and nucleotide excision repair (NER) deficient mouse mutants. The use of media with a lowered calcium concentration and the inclusion of keratinocyte growth factor (KGF) permitted repeated passaging of the cultures and resulted in the generation of stable cell lines that proliferated efficiently. The cells retained their normal ability to engage into terminal differentiation when triggered with high calcium concentrations or after suspension in semi-solid medium. The cultures reflected the cellular characteristics (i.e. repair and transcription profiles) of the Xpa(-/-), Xpc(-/-), Csb(-/-) and Xpd(TTD) mouse models from which they were derived. For instance, in line with earlier in vivo results, Xpd(TTD) keratinocytes were disturbed in their ability to terminally differentiate in vitro. This was concluded from a delay in calcium-induced stratification and by reduced transcription of both early (keratin 10) and late (loricrin) terminal differentiation marker genes. UDS measurements in wild-type cells committed to terminal differentiation did not reveal any reduction in global DNA repair that could be indicative of differentiation associated repair (DAR) as found in neurons. UV sensitivity data revealed that in keratinocytes global genome repair contributes more to cell survival than previously concluded from fibroblast studies. It is inferred that these fully controllable in vitro cultures will be a valuable tool to assess critical parameters of genome care-taking systems in cell proliferation and differentiation.

Local administration of antisense phosphorothioate oligonucleotides to the c-kit ligand, stem cell factor, suppresses airway inflammation and IL-4 production in a murine model of asthma

BACKGROUND

The c-kit ligand, stem cell factor (SCF), is an important activating and chemotactic factor for both mast cells and eosinophils. These cells are known to play a fundamental role in the pathogenesis of asthma.

OBJECTIVE

Our goal was to analyze the functional role of SCF in the pathogenesis of asthma.

METHODS

The expression of SCF was targeted in fibroblasts, epithelial cells, and locally in a murine model of asthma in mice induced by ovalbumin sensitization with an antisense DNA strategy.

RESULTS

We could suppress SCF expression in NIH 3T3 fibroblasts and SP1 epithelial cells by a specific antisense phosphorothioate oligonucleotide overlapping the translation start site of SCF, whereas control oligonucleotides were virtually inactive. We then focused on the role of SCF in a murine model of asthma associated with late-phase allergic inflammation in ovalbumin-sensitized mice: Local intranasal administration of FITC-labeled SCF antisense oligonucleotides led to strong DNA uptake in interstitial lung cells associated with a striking reduction of intracellular SCF expression. Such intrapulmonary blockade of SCF expression after repeated allergen challenges suppressed various signs of lung inflammation including IL-4 production and infiltration of eosinophils. SCF antisense DNA treatment was at least as effective as corticosteroid treatment.

CONCLUSION

These data indicate a critical role for SCF in a murine asthma model and suggest that local delivery of SCF antisense oligonucleotides may be a novel approach for the treatment of inflammatory lung disorders such as asthma.

Novel mechanism of steroid action in skin through glucocorticoid receptor monomers

Glucocorticoids (GCs), important regulators of epidermal growth, differentiation, and homeostasis, are used extensively in the treatment of skin diseases. Using keratin gene expression as a paradigm of epidermal physiology and pathology, we have developed a model system to study the molecular mechanism of GCs action in skin. Here we describe a novel mechanism of suppression of transcription by the glucocorticoid receptor (GR) that represents an example of customizing a device for transcriptional regulation to target a specific group of genes within the target tissue, in our case, epidermis. We have shown that GCs repress the expression of the basal-cell-specific keratins K5 and K14 and disease-associated keratins K6, K16, and K17 but not the differentiation-specific keratins K3 and K10 or the simple epithelium-specific keratins K8, K18, and K19. We have identified the negative recognition elements (nGREs) in all five regulated keratin gene promoters. Detailed footprinting revealed that the function of nGREs is to instruct the GR to bind as four monomers. Furthermore, using cotransfection and antisense technology we have found that, unlike SRC-1 and GRIP-1, which are not involved in the GR complex that suppresses keratin genes, histone acetyltransferase and CBP are. In addition, we have found that GR, independently from GREs, blocks the induction of keratin gene expression by AP1. We conclude that GR suppresses keratin gene expression through two independent mechanisms: directly, through interactions of keratin nGREs with four GR monomers, as well as indirectly, by blocking the AP1 induction of keratin gene expression.

Isolation, sequence and expression of the gene encoding human keratin 13

Keratins are a family of highly homologous proteins expressed as pairs of acidic and basic forms which make intermediate filaments in epithelial cells. Keratin 13 (K13) is the major acidic keratin, which together with K4, its basic partner, is expressed in the suprabasal layers of non-cornified stratified epithelia. The mechanism which allows mucosal-specific expression of this keratin remains unknown. To provide insight into the tissue-specific expression, we have isolated the human K13 gene by screening a chromosome 17 library with a specific K13 cRNA probe. Sequence analysis of unidirectional deletions produced by transposon Tn3 has revealed that the gene is 4601 nucleotides long and contains seven introns and eight exons. When driven by the CMV promoter, the gene produced K13 protein in MCF-7 cells, which normally do not express this protein. Two transcription-start sites were identified, the major being at 61 and the minor at 63 nucleotides upstream of ATG. The upstream sequence contained a TATA box and several other putative transcription factor binding sites. A single copy of the K13 gene was detected in the human genome by Southern hybridisation and polymerase chain reaction. K13 mRNA shows differential expression in cultured keratinocytes, and in A431 cells the RNA levels remained independent of calcium concentrations in the culture medium. Characterisation of the human K13 gene will facilitate elucidation of the molecular mechanism regulating K13 expression in mucosal tissues.

Morphological characterization of spindle cell tumors induced in transgenic Tg.AC mouse skin

Transgenic Tg.AC mice carry a v-Ha-ras coding region flanked by a zeta-globin promoter and an SV40 polyadenylation signal sequence. These mice respond to carcinogens by developing epidermal papillomas. In some cases, malignancies develop at the sites of these papillomas. Various patterns of squamous cell differentiation were observed in these malignancies. One malignancy that developed at the site of the papillomas was composed of bundles of spindle cells. This lesion is difficult to distinguish from fibrosarcomas by light microscopy. We characterized 16 of these malignancies (tentatively classified as spindle cell tumors) to determine if they were of epithelial or mesenchymal origin. Papillomas were induced in Tg.AC mice by full thickness wounding, 12-O-tetradecanoyl-13-phorbol acetate treatment, or ultraviolet radiation. With time, some papillomas became broad-based, downwardly invading lesions. These lesions were examined by light microscopy with immunohistochemical analysis for cytokeratins and by electron microscopy. Immunohistochemical examination with a polyclonal anti-cytokeratin antibody demonstrated various degrees of keratin staining in all tumors examined. Attenuated desmosomes were also observed in these lesions by electron microscopy. These results indicate an epithelial origin for these malignancies; therefore, they should be classified as spindle cell carcinomas.

Retinoic acid signaling cascade in differentiating murine epidermal keratinocytes: alterations in papilloma- and carcinoma-derived cell lines

The retinoic acid (RA) signaling pathway was investigated by transient transfection of a chloramphenicol acetyltransferase (CAT) reporter gene construct containing the RA response element (RARE) of the murine (m) RARbeta2 gene into murine primary epidermal keratinocytes (PEK), papilloma-derived SP1 cells, and carcinoma-derived 3P2 cells. Murine PEK transfected in a low-Ca2+ medium (0.05 mM Ca2+) exhibited a strong transactivation of the CATgene after exposure of the cells to 0.1 microM RA. Transactivation of the CATgene could, however, also be achieved by shifting RAREbeta2-transfected low-Ca2+ PEK to high-Ca2+ conditions (0.15-1.2 mM Ca2+). Concomitantly, the Ca2+ raise also led to the induction of both cellular retinol (ROL)-binding protein I (CRBPI) and cellular RA-binding protein II (CRABPII), whereas expression of cellular RA-binding protein I (CRABPI) was not observed. Moreover, induction of in vitro differentiation also activated the ROL-->RA converting enzyme system in PEK. These findings suggest the following sequence of events involved in the high Ca2+-mediated activation of RAREbeta2. First, high Ca2+ induces the synthesis of mCRBPI, which binds ROL released from retinyl ester stores and makes it accessible to the ROL-RA converting enzyme system. Enzymatically generated RA is taken over by mCRABPII and transported to the nucleus, where it acts as ligand for nuclear receptors, which complex with RAREbeta2 to activate the reporter gene. This hypothetical cascade of RA signaling was supported by our findings that inhibition of the ROL-->RA converting enzyme system by citral abolished the Ca2+-mediated transactivation of the CAT gene in a nontoxic manner. Studies in transformed murine cell lines revealed that Ca2+-induced activation of RAREbeta2 was essentially maintained in papilloma-derived SP1 cells, although all parameters of the Ca2+-dependent RAREbeta2 activation cascade were induced to a much lower extent. In contrast, strong RAREbeta2 activity was already observed in low-Ca2+ carcinoma-derived 3P2 cells. Low-Ca2+ 3P2 cells also expressed high levels of both mCRBPI and mCRABPII and possessed a highly active ROL-->RA converting enzyme system. Again, inhibition of the enzyme by citral abolished RAREbeta2 activity in low-Ca2+ 3P2 cells. Our data show that Ca2+-induced differentiation in cultured murine PEK entails a series of events that ultimately lead to the activation of RARE-containing genes. These properties are maintained in transformed epidermal keratinocytes. However, with increasing malignant potential of the cells, the respective signaling pathway becomes independent from a differentiation stimulus and leads to constitutive activation of RARE-controlled genes.

Protein-tyrosine phosphatases expressed in mouse epidermal keratinocytes

The importance of growth factors acting via receptor-type protein-tyrosine kinases in the continuous renewal of the epidermis from the keratinocyte stem cell population has been well established. Protein-tyrosine phosphatases (PTPases), which dephosphorylate phosphotyrosine-containing proteins, may therefore be expected to play an equally important role in the control of epidermal growth and differentiation. In this study, we have made an inventory of the various PTPases that are expressed during mouse keratinocyte proliferation and maturation. A panel of 13 different PTPases probes was obtained by combining a set of PTPase cDNAs previously cloned from mouse brain and a set of PTPase probes obtained from a normalized keratinocyte PTPase cDNA library. This PTPase cDNA panel, spanning probes for receptor-type as well as cytoplasmic-type family members, was used to monitor RNA expression levels in keratinocyte fractions isolated from murine epidermis and in keratinocyte cell cultures. No overt changes were observed in PTPase mRNA levels in all strata of mouse epidermis, but comparison of cultured cells with freshly isolated keratinocytes revealed several conspicuous differences. In the cultured Balb/MK cell line, absence of PTP delta expression and upregulation of PTP kappa and, to a lesser extent, PTP gamma mRNA ratios were observed compared to the freshly isolated cells. These results provide a basis for further research on the impact of PTPase activity on epidermal growth control.

Suppression of the metastatic phenotype of a mouse skin carcinoma cell line independent of E-cadherin expression and correlated with reduced Ha-ras oncogene products

The HaCa4 cell line, derived from a mouse skin carcinoma induced by Harvey murine sarcoma virus, is highly tumorigenic when injected into nude mice and produces multiple metastases in the lungs. HaCa4 cells express high levels of viral Ha-ras oncogene products, anomalously synthesize the embryonic/simple epithelial keratin K8, and have lost the expression of the cell-cell adhesion receptor E-cadherin (E-CD). E-CD(+) cell clones (E62 and E24), obtained by transfection of an exogenous E-CD cDNA into HaCa4 cells, had a decreased ability to migrate through type IV collagen matrices. However, the E-CD (+) E62 clone remained as metastatic as the parental cell line, whereas the E24 clone, which does not take up the exogenous cDNA but spontaneously switches on the endogenous E-CD gene, suppressed the metastatic phenotype although it maintained its tumorigenicity. E24 cells had fivefold to sixfold lower levels of viral Ha-ras mRNA and p21 protein than the other cell lines. In addition, they did not synthesize K8 but rather switched on keratin K19. The comparison of E-CD proteins synthesized by E62 and E24 cell lines revealed no structural or functional differences because both localized at cell-cell contacts and associated with alpha-catenin, beta-catenin, and plakoglobin. Furthermore, E-CD was still expressed in metastatic lung nodules produced by E62 cells. These results suggest that suppression of the metastatic phenotype in E24 cells occurs independently of E-CD expression and correlates with decreased levels of the oncogenic ras p21 protein.

Analysis of v-Ha-ras and v-fos oncogene transduction into a mouse epidermal cell line with "initiated" phenotype in culture but normal skin phenotype in vivo

Cell line SCR722 was derived from adult SENCAR mouse epidermal cells initiated in culture by treatment with the carcinogen N-methyl-N'-nitro-N-nitrosoguanidine and selection for foci proliferating in medium with calcium levels that induce terminal differentiation in normal cells. Expansion of one of these foci and two additional cell clonings produced cell line SCR722, which was near-tetraploid and formed normal skin when grafted to athymic nude mouse hosts. However, unlike normal keratinocytes, SCR722 cells fail to suppress papilloma formation when grafted along with papilloma cell line SP-1. For optimum growth in culture, SCR722 cells required fibroblast-conditioned medium and 0.5 mM Ca2+. SCR722 cells had a wild-type c-Ha-ras gene but had lost their requirement for conditioned medium in culture and produced dysplastic papillomas in grafts when transduced with the v-Ha-ras gene. SCR722 cells stably expressing the v-fos gene produced normal epidermis in grafts, but when these cells were transduced with the v-Ha-ras gene, they produced carcinomas. Clones with greater expression of the transfected v-fos gene had a more invasive phenotype in vivo. These results indicate that carcinogen treatment of epithelial cells can result in an altered but nontumorigenic phenotype that may be at risk for becoming a more advanced neoplastic state with additional genetic alterations.

Distribution of protein kinase C alpha and accumulation of extracellular Ca2+ during early dentin and enamel formation

Activation of the protein kinase C (PKC)-related signal transduction system has been associated with phenotypic expression in a wide variety of cell types. In in vitro studies, it has often been activated by relatively small increases in the Ca2+ concentration ([Ca2+]) in the medium. The studies reported here explored the hypothesis that localized increases in the extracellular [Ca2+] and activation of the PKC-related pathway may be involved in early dentin and enamel formation. Whole-head, freeze-dried sections through the developing molars of 5-day-old rats were evaluated by methods that localized non-crystalline Ca2+. Immunohistochemical methods were adapted for use with the freeze-dried sections, and two monoclonal antibodies were used to localize PKC alpha in the formative cells of the developing teeth. Low concentrations of extracellular Ca2+ were observed in the early, unmineralized dentin in the area of ameloblast differentiation. Increased concentrations occurred at the point of initial dentin mineralization, immediately before the beginning of enamel matrix deposition. PKC alpha was localized in the differentiating odontoblasts, at the beginning of dentin matrix deposition. It was intensely localized in the distal borders of the pre-ameloblasts, and appeared to redistribute in the cells during ameloblast differentiation. These observations suggest that local increases in the extracellular [Ca2+] and the PKC signal transduction pathway may be involved in key inductions in the early stages of dentin and enamel formation.

Differential modulation of epidermal keratinization in immortalized (HaCaT) and tumorigenic human skin keratinocytes (HaCaT-ras) by retinoic acid and extracellular Ca2+

The growth and differentiation response to retinoic acid (RA) was studied in the human keratinocyte line HaCaT and tumorigenic clones transfected with c-Ha-ras oncogene (HaCaT-ras). Differentiation (mainly keratin synthesis) was evaluated and correlated to cell proliferation in vitro but also growth behaviour in vivo (tumorigenicity). Comparable to normal keratinocytes, HaCaT cells and ras clones showed increased expression of the epidermal suprabasal keratins K1 and K10 upon RA depletion of the media (delipidized serum), while simple epithelial type keratins K7, K8 and K18 as well as K19 and K13 (typical of internal stratified epithelia) were almost completely suppressed. The cell density-dependent increase of K1 and K10 at intermediate RA levels (as in regular media with untreated serum) was also observed at Ca2+ levels below 0.1 mM, thus being clearly unrelated to stratification, whereas K13 synthesis was Ca(2+)-dependent and initiated with stratification. The effects on keratins were fully reversed by increasing RA concentrations. There was only mild stimulation of proliferation at RA doses (10(-10) to 10(-8) M) not directly corresponding to suppression of keratinization. Thus, the negative RA influence on K1 and K10, opposed to the effect on simple keratins, substantiates the preserved regulatory capacity rendering these cells appropriate models for biological testing. Among the various tumorigenic HaCaT-ras clones highly and moderately differentiating ones could be distinguished, accordingly induction in vitro led to a comparable spectrum of differentiation markers (K1 and K10 appearing early, and filaggrin late) as growth in vivo. These in vitro results demonstrate that, in spite of some differences in RA sensitivity, virtually all clones possess the epidermal differentiation repertoir which is regulated according to the same principles. Finally, this confirms our in vivo data that differentiation potential is not inversely related to the state of transformation or tumorigenicity.

Low frequency of codon 61 Ha-ras mutations and lack of keratin 13 expression in 7,12-dimethylbenz[a]-anthracene-induced hamster skin tumors

Alterations in the pattern of keratin expression are a common feature of skin-tumor development. In this study, we investigated whether the loss of epidermal keratin 1 (K1) and its replacement by mucosal keratin 13 (K13) is unique to mouse skin tumors induced by 7,12-dimethylbenz[a]anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA), since it has been reported that human epidermal tumors do not exhibit aberrant expression of K13. With that purpose, we analyzed the keratin profiles of 16 DMBA-induced hamster skin tumors using monospecific antibodies against K1 and K13. Although all the tumors expressed K1, they also showed an overall tendency towards loss of this keratin; furthermore, none of the tumors expressed K13. Previous studies have suggested that the induction of K13 in mouse skin is related to the mutation of the Ha-ras gene by the initiating agent DMBA, a mutation consistently found in murine DMBA/TPA-induced tumors and rarely found in human skin tumors. Therefore, we also evaluated the tumors for the presence of codon-61 mutations by direct sequencing of DNA extracted from paraffin-embedded tissue sections. Only three tumors showed an A-->T transversion in the second nucleotide of Ha-ras codon 61. However, presence of the mutation did not correlate with K1 staining. Although hamster skin tumors were induced by the same initiator as were mouse skin tumors, hamster skin tumors did not show the same keratin profile. Moreover, their immunohistochemical expression of K1 and K13 and their codon 61 sequences resembled that of their human counterparts. These results suggest that the aberrant expression of K13 may be unique to murine skin. Furthermore, although codon 61 Ha-ras mutation appears to be related to keratin alterations in the mouse model, this mutation is not sufficient to produce the same biochemical changes in other species.

ras gene activation and aberrant expression of keratin K13 in ultraviolet B radiation-induced epidermal neoplasias of mouse skin

Both papillomas and squamous cell carcinomas (SCC) induced in mouse epidermis by initiation with 7,12-dimethylbenz[a]anthracene (DMBA) and promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA) exhibit aberrant expression of a type I keratin, K13, that is normally characteristic of terminal differentiation of internal stratified epithelia. There is evidence that the aberrant expression of K13 depends on the presence of an activated ras gene in mouse epidermal keratinocytes (Sutter et al., Mol Carcinog 4:467-476, 1991). To assess the general validity of this hypothesis, we investigated both aberrant K13 expression and activation of each of the three members of the ras gene family in epidermal tumors induced in four different mouse strains (SKH-1 hr, SENCAR, BALB/c, and C3H/He) by chronic irradiation with ultraviolet (UV) B. The tumor collection comprised nine papillomas and 30 well or poorly differentiated SCC. Aberrant K13 expression occurred in only five of 39 tumors and was restricted to SCC of both types. This indicates that aberrant K13 expression in UV-induced epidermal tumors was intrinsically different from that in chemically induced tumors. Polymerase chain reaction analysis of the tumors for different point mutations in codons 12, 13, and 61 of the Ha-ras and Ki-ras genes and in codon 61 of the N-ras gene revealed that only one of the well differentiated tumors from a SKH-1 hr mouse exhibited a GGA-->GAA mutation in codon 12 of the Ha-ras gene. Although this tumor was also positive for aberrant K13 expression, such a correlation could not be made for the remaining K13-expressing tumors. This indicates that the activation of one of the members of the ras gene family is not a general prerequisite for the aberrant expression of K13 in mouse epidermal keratinocytes.

v-Ha-ras-induced mouse skin papillomas exhibit aberrant expression of keratin K13 as do their 7,12-dimethylbenz[a]anthracene/12-O-tetradecanoylphorbol-13-acetate -induced analogues

Introduction of the v-Ha-ras gene into primary epidermal keratinocytes, followed by grafting of these cells to animals, leads to the formation of benign epidermal tumors that resemble papillomas induced chemically by a two-stage carcinogenesis protocol. In this study, we investigated v-Ha-ras-induced papillomas for aberrant expression of type I keratin K13, previously described in 7,12-dimethylbenz[a]anthracene/12-O-tetradecanoylphorbol-13- acetate (DMBA/TPA)-induced mouse epidermal tumors. Papillomas produced from three independent infection series were removed 3 wk after grafting concomitant with control grafts originating from mock-, neo-, and v-fos-infected primary keratinocytes. Combined analysis of the grafts by western blotting of extracted keratins and immunofluorescence studies of frozen sections with a K13-monospecific antibody revealed K13 expression in all v-Ha-ras-induced papillomas and absence of this keratin in all control grafts. K13-positive cells in papillomas were restricted to the suprabasal cell layers of the lesions and, at this stage of papilloma development, occurred as foci of varying extensions. Analysis of genomic DNA from v-Ha-ras-induced papillomas for the methylation state of a CpG dinucleotide in the distant promoter region of the K13 gene revealed the occurrence of unmethylated DNA copies that were generated at the expense of methylated DNA copies ubiquitously present in normal epidermis. The ratio of unmethylated to methylated DNA copies correlated with the extent of suprabasal K13 protein expression. Thus, all features of aberrant K13 expression previously described in DMBA/TPA-induced papillomas were shared by v-Ha-ras-induced papillomas.