Epidermal differentiation and basement membrane formation by HaCaT cells in surface transplants

Cell interactions control the fate of malignant keratinocytes in an organotypic model of early neoplasia

The role of cell interactions during early neoplastic progression in human skin is not well understood. We report that the fate and behavior of low-grade malignant cells in stratified epithelium is dependent on their interactions with neighboring cells and with extracellular matrix during the early events in neoplastic progression. We utilized an organotypic tissue model which mimics premalignancy to monitor malignant cells (II-4) genetically marked with beta-gal and grown in the context of either normal human keratinocytes or the immortalized cell line HaCaT. HaCaT cells were permissive for clonal expansion of II-4 cells at ratios of 4:1, 12:1, and 50:1 (HAC:II-4) when compared with coculture with normal human keratinocytes. This II-4 cell expansion was associated with the failure of neighboring HaCaT cells to induce differentiation and cell cycle withdrawal of II-4, as had been seen in the context of normal human keratinocytes. When 12:1 mixtures (NHK:II-4) were stripped of all suprabasal cells and regrown, all beta-gal cells were lost showing that these normal human keratinocyte-suppressed II-4 cells had been actively sorted to a suprabasal position where their clonal expansion was limited. These growth-suppressive effects of normal human keratinocytes were found to be conditional on direct cell-cell contact, as II-4 formed colonies when trypsinized from 12:1 (NHK:II-4) mixtures and grown at clonal density in submerged culture. The distribution and behavior of low-grade malignant cells was therefore dependent on the state of transformation of adjacent keratinocytes and on cell-matrix interactions. These results demonstrate that alterations in the cellular microenvironment are central to the induction of clonal expansion and early neoplastic progression in stratified epithelium.

Protein C inhibitor is expressed in keratinocytes of human skin

Protein C inhibitor is a member of the serpin family that inhibits a variety of serine proteases. Protein C inhibitor is present in numerous body fluids and is produced in the liver and by various epithelial cells. To determine if this epithelial serpin is present in skin, immunohistochemical studies were performed that showed strong staining for protein C inhibitor antigen in the epidermis. Protein C inhibitor mRNA was detected in the keratinocyte cell line HaCaT and the epidermoid carcinoma cell line A431 using reverse transcription-polymerase chain reaction suggesting that also in normal skin protein C inhibitor is derived from keratinocytes. Conditioned media from these cell lines were analyzed on immunoblots, which revealed a protein C inhibitor-antigen band that comigrated with protein C inhibitor derived from the hepatoma cell line HepG2. Using an enzyme-linked immunosorbent assay specific for total protein C inhibitor antigen the accumulation of protein C inhibitor in the cell culture supernatants of HaCaT keratinocytes was found to be 0.3 ng per h per 1 million cells. This is similar to the amount of plasminogen activator inhibitor-1 produced by these cells, which also produce tissue plasminogen activator and urokinase. Fluorescence-activated cell sorter analysis revealed similar expression of intracellular protein C inhibitor antigen in proliferating and confluent HaCaT cells. These findings demonstrate that protein C inhibitor antigen is present in the normal epidermis and that protein C inhibitor is constitutively expressed by keratinocytes in culture. Therefore, protein C inhibitor may provide protease inhibitory activity not only to internal, but also to the external surface of the body. Additionally, protein C inhibitor could contribute to the regulation of retinoid supply in the epidermis, as we have shown recently that retinoic acid binds specifically to protein C inhibitor.

Defects of basement membrane and hemidesmosome structure correlate with malignant phenotype and stromal interactions in HaCaT-Ras xenografts

Benign and malignant HaCaT-ras clones, derived from immortalized HaCaT cells were grown as nude mouse surface transplants rendering a human tumor progression model. Searching for malignancy-related alterations, the deposition, localization and mRNA of basement membrane and hemidesmosome components were analysed by immunofluorescence, in situ hybridization and electron microscopy. Initially, at 1 week epithelia of benign and malignant cells revealed a similarly low polarity and an enlarged 'activated basal' compartment, reflected by partial dislocation and extended pericellular staining of the hemidesmosome constituent integrin alpha 6 beta 4 seen by immunofluorescence. Whereas benign grafts eventually normalized, closely resembling grafts of HaCaT cells, malignant growth was correlated with a persisting epithelial activation state and continuing higher expression of alpha 6 (by immunofluorescence and in situ hybridization). The basement membrane components bullous pemphigoid antigen 1, laminin-5 and collagen IV exhibited a largely linear distribution at 1 week. However, in the malignant cell transplants initially minor basement membrane discontinuities became more severe at around 2 weeks, associated with close stromal cell contacts, angiogenesis and invasion. Most striking were basement membrane alterations seen by electron microscopy. At 1 week stretches of basement membrane had developed in malignant transplants, though to a much lesser extent than in benign specimens. With invasion these basement membrane structures mostly disappeared despite persistent although variable immunofluorescence, suggesting high turnover without ultrastructural assembly. The hemidesmosome structures were defective throughout, completely lacking anchoring plaques with keratin filaments, whereas they were still associated with basement membrane deposits. Thus, malignant HaCaT-ras transplants, while initially resembling regenerating wounds, revealed an increasing loss of tissue polarity and basement membrane structures, which seemed to be accelerated upon stromal cell contacts.

Reconstruction of a human skin equivalent using a spontaneously transformed keratinocyte cell line (HaCaT)

Reconstruction of a skin equivalent using an immortalized human keratinocyte line, HaCaT, was investigated in an attempt to generate an in vitro system representative for human skin. Three different substrates were used to establish air-exposed cultures of HaCaT cells: de-epidermized dermis, collagen gels, and filter inserts. Effects of variations in culture conditions on tissue morphology, on the expression of proliferation-specific and differentiation-specific protein markers, and on lipid profiles were investigated. When grown at the air-liquid interface HaCaT cells initially developed a multilayered epithelium, but during the course of culture marked alterations in tissue architecture were observed. Ultrastructurally, a disordered tissue organization was evident as judged from the presence of rounded cells with abnormally shaped nuclei. Keratins K1 and K10 were irregularly expressed in all cell layers, including stratum basale. Staining of K6/K16 was evident in all cell layers. Locally, basal and suprabasal cells were positive for K4 and additionally expressed K13 and K19. The cornified envelope precursors were expressed only in older cultures (>2 wk after air exposure), except for transglutaminase and small proline rich protein 1, which were irregularly expressed in both early and older cultures. In addition, HaCaT cells showed an impaired capacity to synthesize lipids that are necessary for a proper barrier formation as indicated by the absence of free fatty acids and a very low content and incomplete profile of ceramides. Our data demonstrate that the ultimate steps of terminal differentiation in HaCaT cells do not occur irrespective of the type of substrate or the culture conditions.

Epidermal organization and differentiation of HaCaT keratinocytes in organotypic coculture with human dermal fibroblasts

The immortal human keratinocyte line HaCaT is frequently used as a paradigm for skin keratinocytes in vitro because of its highly preserved differentiation capacity. HaCaT cells form a nearly regular epidermal architecture when transplanted onto subcutaneous tissue of athymic mice. In order to analyze further their differentiation capacity in vitro, HaCaT cells were studied in organotypic cocultures on top of collagen gels containing human dermal fibroblasts. Within 1 wk HaCaT cells formed a still dysplastic epithelium, the thickness of which correlated with the number of fibroblasts in the collagen gel. With further culture time of up to 3 wk a remarkably well structured and differentiated squamous epithelium developed. After 1 wk, keratins 10 and 16, involucrin, and transglutaminase I were expressed in suprabasal layers, whereas filaggrin, keratin 2e, and loricrin appeared after 2-3 wk. Within this time, a nearly complete basement membrane had formed including hemidesmosomes and anchoring fibrils. Epithelial cell proliferation became restricted to the basal layer after 2 and 3 wk. Using the TdT-mediated dUTP nick end labeling assay, fragmentation of DNA was detectable in nuclei of the parakeratotic stratum corneum. Ultrastructurally, many features of keratinization accumulated after 2 and 3 wk, though an orthokeratotic keratinization was not achieved, in contrast to HaCaT transplants. This differentiation deficiency - as compared with normal keratinocytes -- might be due to a lack of paracrine factors important for keratinocyte differentiation or to a reduced sensitivity of these cells. Nevertheless, this high degree of differentiation under organotypic conditions qualifies this cell line as an appropriate model for elucidation of the molecular mechanisms regulating keratinocyte growth and differentiation and for use in pharmacotoxicology.

Multiple stages and genetic alterations in immortalization, malignant transformation, and tumor progression of human skin keratinocytes

An in vitro carcinogenesis model of human skin keratinocytes has been developed based on the spontaneously immortalized keratinocyte cell line HaCaT. Immortalization, the initial stage in human carcinogenesis in vitro, was induced by ultraviolet-type mutations in the p53 gene followed by further genetic alterations leading to the loss of senescence genes, in particular on chromosome 3p. Despite multiple genetic changes, the HaCaT cell line sustained its genomic balance up to high passage levels and maintained a non-tumorigenic phenotype. Tumorigenic transformation was induced by ras oncogene transfection but also by culture stress and elevated temperature, resulting in benign and malignant tumorigenic clones. Malignant conversion was associated with the loss of a copy of chromosome 15, leading to a decrease in thrombospondin-1 (TSP-1) expression. Heat-induced malignant conversion was associated with a gain of material on chromosome 11, including the cyclin D1 gene. The microenvironment plays a major role in tumorigenic transformation and the control of malignant cells. Overexpression of platelet-derived growth factor in HaCaT cells caused mesenchyme activation and formation of benign tumors. Halting tumor angiogenesis completely prevented invasion of malignant cells and induced a benign tumor phenotype. Transfer of a normal chromosome 15 or TSP-1 transfection into a skin carcinoma line resulted in tumor suppression due to TSP-1-blocked tumor vascularization. Because of the reduced TSP-1 expression, blood vessels infiltrated the tumor, and it expanded. Progression to more aggressive tumor phenotypes required the in vivo environment and was caused by selection of a subpopulation and further genetic modifications. The improved autonomous growth of these cells was associated with new expression of granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor, which acted in an autocrine manner to stimulate proliferation and migration. With this in vitro skin carcinogenesis model we were able to demonstrate multiple stages in the transformation process that were associated with different genetic and phenotypic characteristics. In addition, we documented that modulation of the tumor stroma plays an important and decisive role in tumor development and progression. From this we hypothesize that the growth restraints of the microenvironment are increasingly lost with advancing stages of carcinogenesis but can be restored by modulation of the tumor stroma.