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

Glycogen synthase kinase-3 regulates cytoskeleton and translocation of Rac1 in long cellular extensions of human keratinocytes

Wound keratinocytes form long cellular extensions that facilitate their migration from the wound edge into provisional matrix. We have previously shown that similar extensions can be induced by a long-term exposure to EGF or rapidly by staurosporine in cultured cells. This morphological change depends on the activity of glycogen synthase kinase-3 (GSK-3). Here, we have characterized the cytoskeletal changes involved in formation of these extended lamellipodia (E-lam) in human HaCaT keratinocytes. E-lams contained actin filaments, stable microtubules and keratin intermediate filaments. E-lam formation was prevented by cytochalasin D, colchicine and low concentrations of taxol and nocodazole, suggesting that actin and microtubule organization and dynamics are essential for E-lam formation. Staurosporine induced recruitment of filamentous actin (F-actin), cortactin, filamin, Arp2/3 complex, Rac1 GTPase and phospholipase C-gamma1 (PLC-gamma1) to lamellipodia. Treatment of cells with the GSK-3 inhibitors SB-415286 and LiCl(2) inhibited E-lam formation and prevented the accumulation of Rac1 and Arp2/3 complex at lamellipodia. The formation of E-lams was dependent on fibronectin-binding integrins and normally regulated Rac1, and expression of either dominant-negative or constitutively active forms of Rac1 prevented E-lam formation. Overexpression of either RhoA or Cdc42 GTPases suppressed E-lam formation. We conclude that extended lamellipodia formation in keratinocytes requires actin and tubulin assembly at the leading edge, and this process is regulated by Rac1 downstream of GSK-3.

Aryl hydrocarbon receptor-mediated posttranscriptional regulation of IL-1beta

TCDD stimulated IL-1beta gene expression in differentiating human keratinocyte cell lines in a time- and dose-dependent manner. Increases in prointerleukin-1beta (pIL-1beta) protein and IL-1beta steady state mRNA levels were observed in both SCC-12F and HaCaT cells following TCDD treatment. When pretreated with alpha-naphthoflavone, an AhR antagonist, TCDD-mediated increases in IL-1beta gene expression were attenuated, demonstrating for the first time that the environmental toxin, TCDD, can stimulate cytokine (IL-1beta) gene expression in an AhR-dependent manner. Nuclear run-on experiments were performed in SCC-12 cells to determine if the AhR-dependent increases in IL-1beta expression were due to transcriptional activation of the IL-1beta gene. Results showed high constitutive levels of IL-1beta transcriptional activity, however, TCDD treatment, which stimulated IL-1beta steady state mRNA levels, failed to potentiate IL-1beta transcription. Taken together, these results demonstrate that AhR-mediated IL-1beta regulation is occurring posttranscriptionally.

Protein kinase C isozymes regulate proliferation and high cell density-mediated differentiation in HaCaT keratinocytes

Protein kinase C (PKC) isoforms play pivotal roles in the regulation of differentiation of normal human epidermal keratinocytes (NHEK). In this study, we investigated the participation of the PKC system in the proliferation and high cell density-induced differentiation of the human immortalized keratinocyte line HaCaT. HaCaT keratinocytes possessed a characteristic PKC isoform pattern (PKC alpha, beta, gamma, delta, epsilon, eta, theta, zeta), which altered during proliferation and differentiation. The GF109203X compound, a selective PKC inhibitor, suppressed the expressions of the lat (granular cell) differentiation markers involucrin (INV) and filaggrin (FIL), and the terminal marker keratinocyte-specific transglutaminase-1 (TG), but did not affect the level of the early (spinous cell) marker keratin 10 (K10) and cellular proliferation. Phorbol 12-myristate 13-acetate (PMA), an activator of PKC, inhibited proliferation, elevated intracellular calcium concentration, decreased the expression of K10, and increased the expressions of INV, FIL, and TG. These data indicate that the endogenous activation of PKC regulates the expressions of the late differentiation markers, and that the exogenous activation of PKC by PMA results in the induction of terminal differentiation. Because the cellular effects of PMA were accompanied by differential down-regulations of the sensitive PKC isoforms in proliferating and differentiating cultures, our findings argue for the differential roles of the existing PKC isoforms in the regulation of cellular proliferation and high cell density-induced differentiation of HaCaT cells.

Epidermal tissue regeneration and stromal interaction in HaCaT cells is initiated by TGF-alpha

The human keratinocyte cell line HaCaT expresses essentially all epidermal differentiation markers but exhibits deficiencies in tissue organization as surface transplants in nude mice and even more so in organotypic co-cultures with fibroblasts. Whereas tissue differentiation by normal keratinocytes (NEKs) is regulated by stromal interactions, this mechanism is impaired in HaCaT cells. This regulatory process is initiated by interleukin-1 (IL-1) release in keratinocytes, which induces expression of keratinocyte growth factor (KGF/FGF-7) and granulocyte macrophage-colony stimulating factor (GM-CSF) in fibroblasts. Production and release of IL-1 is very low and, consequently, expression of the fibroblast-derived growth factors KGF/FGF-7 and GM-CSF is absent in HaCaT-fibroblast co-cultures. However, addition of KGF and GMCSF, respectively, is inefficient to improve stratification and differentiation by HaCaT cells due to the low expression of their cognate receptors. More importantly, expression and release of the autocrine keratinocyte growth factor TGF-alpha is dramatically decreased in HaCaT cells. Addition of TGF- alpha or EGF stimulated HaCaT cell proliferation but, even more effectively, suppressed apoptosis, thus facilitating the formation of a regularly stratified epithelium. Furthermore, TGF-alpha enhanced the expression of the receptors for KGF and GM-CSF so that addition of these growth factors, or of their inducer IL-1, further improved epidermal tissue differentiation leading to in vitro skin equivalents comparable with cultures of NEKs. Thus, supplementing TGF-alpha normalized epidermal tissue regeneration by immortal HaCaT keratinocytes and their interaction with stromal cells so that regular skin equivalents are produced as standardized in vitro models.

Role of intracellular calcium mobilization and cell-density-dependent signaling in oxidative-stress-induced cytotoxicity in HaCaT keratinocytes

Peroxynitrite is a nitric-oxide-derived cytotoxic mediator produced in a broad range of inflammatory conditions, ranging from sunburn erythema to contact hypersensitivity. Our previous work has shown that in HaCaT cells the cytotoxic activity of peroxynitrite involves both apoptotic and necrotic routes with poly(ADP-ribose) polymerase activation serving as a mol-ecular switch diverting the default apoptotic pathway toward necrosis. Nonetheless, keratinocytes are regarded as highly resistant toward environmental noxa including oxidative stress. We set out to investigate the possible role of two parameters, intracellular calcium mobilization and high cell density, in protecting HaCaT cells from peroxynitrite/oxidative-stress-induced cytotoxicity. First we characterized the effect of peroxynitrite on the calcium homeostasis of HaCaT cells and demonstrated that both authentic peroxynitrite and the peroxynitrite generating compound 3-morpholino-sydnonimine triggered an elevation in intracellular calcium levels. Moreover, we established that treatment of cells with the cell-permeable calcium chelator BAPTA-AM provided significant cytoprotection against peroxynitrite- and hydrogen-peroxide-induced cytotoxicity. Furthermore, when cells reached confluence they were highly resistant to the toxic effects of peroxynitrite, hydrogen peroxide, and superoxide. The resistance to oxidative stress provided by calcium chelation and high cell density involved inhibiting the activation of both poly(ADP-ribose) polymerase and caspases. Our data may provide an explanation for the resistance to oxidative stress of superficial, highly differentiated keratinocytes and indicate that basal proliferative keratinocytes are sensitive in vivo targets of oxidative stress injury.

Discriminating expression of differentiation markers evolves in transplants of benign and malignant human skin keratinocytes through stromal interactions

Accumulating evidence indicates a decisive role for the adjacent stroma in tumour growth and dissemination. However, it is not clear how far altered differentiation such as expression of aberrant keratins and vimentin, common in invasive human carcinomas, may reflect intrinsic cell properties or a response to the tumour environment. We have addressed this by transplanting benign and malignant human HaCaT-ras keratinocytes, seeded on collagen matrix, onto nude mice. Initially, epithelia derived from benign and malignant cells, being separated from host stroma by collagen, were poorly organized and exhibited the same differentiation markers, as identified by immunofluorescence and in situ hybridization. Epidermal basal and suprabasal keratins were expressed persistently even upon contact with newly formed stroma and malignant cell invasion. In contrast, non-epidermal keratins (K4/K13, K8/18, K19), which were similarly synthesized by benign and malignant cells in culture and in early transplants, were differentially regulated with increasing stromal vicinity. While both proteins and mRNAs were downregulated in benign epithelia, the malignant, invasive tumour cells continuously expressed these non-epidermal keratins throughout (K19), suprabasally (K4/13) or at invasive sites (K8/18). Furthermore, the mesenchymal protein vimentin was expressed de novo in invasive areas confronting tumour stroma. Thus, atypical tissue markers, similarly synthesized in isolated cells in vitro, are downregulated in benign but maintained and upregulated in malignant epithelia. This is presumably caused by the neighbouring stroma being permanently activated by malignant epithelia.

Confocal ratiometric voltage imaging of cultured human keratinocytes reveals layer-specific responses to ATP

Recent evidence suggests that changes in membrane potential influence the proliferation and differentiation of keratinocytes. To further elucidate the role of changes in membrane potential for their biological fate, the electrical behavior of keratinocytes needs to be studied under complex conditions such as multilayered cultures. However, electrophysiological recordings from cells in the various layers of a complex culture would be extremely difficult. Given the high spatial resolution of confocal imaging and the availability of novel voltage-sensitive dyes, we combined these methods in an attempt to develop a viable alternative for recording membrane potentials in more complex tissue systems. As a first step, we used confocal ratiometric imaging of fluorescence resonance energy transfer (FRET)-based voltage-sensitive dyes. We then validated this approach by comparing the optically recorded voltage signals in HaCaT keratinocytes with the electrophysiological signals obtained by whole cell recordings of the same preparation. We demonstrate 1) that optical recordings allow precise multisite measurements of voltage changes evoked by the extracellular signaling molecules ATP and bradykinin and 2) that responsiveness to ATP differs in various layers of cultured keratinocytes.

Human keratinocytes express multiple P2Y-receptors: evidence for functional P2Y1, P2Y2, and P2Y4 receptors

Extracellular nucleotides are agonists at the family of receptors known as the P2 receptors, and in keratinocytes the P2Y2 subtype is known to elevate the intracellular free calcium concentration (Cai) and stimulate proliferation. In this study, we have investigated the presence of other functional members of the P2Y subgroup in both normal human keratinocytes and the HaCaT cell line. Using reverse transcription polymerase chain reaction, the expression of mRNA for P2Y1, P2Y2, P2Y4, and P2Y6 receptors was demonstrated in HaCaT cells and differentiated and undifferentiated normal human keratinocytes. Cai was monitored in response to a panel of P2Y receptor agonists. To couple mobilized Cai to a downstream cellular response, cell proliferation was also addressed. In both cell types, adenosine 5'-triphosphate and uridine 5'-triphosphate induced Cai transients of approximately equal duration, magnitude, and shape, confirming the presence of functional P2Y2 receptors. In HaCaT cells, additional characteristic responses were observed in a subpopulation of cells; adenosine 5'-triphosphate failed to elevate Cai in some cells responding to uridine 5'-triphosphate, indicating the presence of P2Y4 receptors, whereas the P2Y1-specific agonist 2-methylthio-5'-adenosine diphosphate was, again, only effective in a small subpopulation. Uridine 5'-diphosphate was ineffective, indicating the absence of functional P2Y6 receptors. Adenosine 5'-triphosphate and uridine 5'-triphosphate equally promoted cell growth in normal human keratinocytes in comparison with the control. In HaCaT cells, adenosine 5'-triphosphate, uridine 5'-triphosphate, and adenosine 5'-diphosphate significantly increased proliferation in comparison to the controls, with a 30% higher response to uridine 5'-triphosphate than with adenosine 5'-triphosphate. These data demonstrate that multiple P2Y receptors (P2Y1, P2Y2, and P2Y4 subtypes) are differentially involved in the regulation of proliferation in human keratinocytes and therefore may be important in wound healing.

Unexpected down-regulation of the hIK1 Ca2+-activated K+ channel by its opener 1-ethyl-2-benzimidazolinone in HaCaT keratinocytes. Inverse effects on cell growth and proliferation

We used a combination of electrophysiological and cell and molecular biological techniques to study the regulation and functional role of the intermediate conductance Ca(2+)-activated K(+) channel, hIK1, in HaCaT keratinocytes. When we incubated cells with the hIK1 opener, 1-ethyl-2-benzimidazolinone (1-EBIO), to investigate the cellular consequences of prolonged channel activity, an unexpected down-regulation of channels occurred within a few hours. The same effect was produced by the hIK1 openers chlorzoxazone and zoxazolamine and was also observed in a different cell line (C6 glioma cells). After 3 days of treatment with 1-EBIO, mRNA levels of hIK1 were substantially diminished and no channel activity was detected. Down-regulation of hIK1 was accompanied by a loss of mitogenic activity and a strong increase in cell size. After withdrawal of 1-EBIO, hIK1 mRNA and channel activity fully recovered and the cells resumed mitogenic activity. Our data present evidence for a novel feedback mechanism of hIK1 expression that appears to result from the paradoxical action of its pharmacological activator during prolonged application. Because the down-regulation of hIK1 bears immediate significance on the biological fate of keratinocytes, 1-EBIO and related compounds might emerge as potent tools to influence the proliferation of various non-excitable cells endowed with IK channels.

Tumor-stroma interactions directing phenotype and progression of epithelial skin tumor cells

Tumor-stroma interactions play a significant role in tumor development and progression. Alterations in the stromal microenvironment, including enhanced vasculature (angiogenesis), modified extracellular matrix composition, inflammatory cells, and dys-balanced protease activity, are essential regulatory factors of tumor growth and invasion. Differential modulation of stromal characteristics is induced by epithelial skin tumor cells depending on their transformation stage when grown as surface transplants in vivo. Tumor cells can regulate the development of a "tumor-stroma" via the aberrant expression of growth factors or induction of growth factor receptors in the stromal compartment. In this context, secretion of the hematopoietic growth factors G-CSF and GM-CSF, constituitively expressed in enhanced malignant tumors, may be good candidates for induction of a tumor stroma through their effect on inflammatory cells. Upon its induction, the tumor stroma will reciprocally influence the differentiation status of tumor cells resulting in a normalization of benign tumor epithelia and the maintenance of a malignant phenotype, respectively. In the HaCaT model for squamous cell carcinoma of the skin, stromal activation and angiogenesis are transient in pre-malignant transplants, however they remain persistent in malignant transplants where progressive angiogenesis is closely correlated with tumor invasion. While continued expression of VEGF and PDGF are associated with benign tumor phenotypes, activation of VEGFR-2 is a hallmark of malignant tumors and accompanies ongoing angiogenesis and tumor invasion. As a consequence the inhibition of ongoing angiogenesis by blocking VEGFR-2 signalling resulted in dramatically impaired malignant tumor expansion and invasion. Comparably, tumor vascularization and invasion was blocked by disturbing the balance of matrix protease activity caused by a lack of PAI-1 in the stromal cells of the knockout mouse hosts. A similar inhibition of tumor vascularization was caused by TSP-1 over-expression in skin carcinoma cells, which also blocked tumor invasion and expansion. On the other hand, when granulation tissue and angiogenesis were only transiently activated as a result of stable transfection of PDGF into non-tumorigenic HaCaT cells, the target cells formed benign, but not malignant, tumors. Collectively, these data show that tumor vascularization, providing intimate association of blood vessels with tumor cells, is a prerequisite for tumor invasion. A potential mechanism for this interrelationship may be the differential regulation of MMP-expression in tumors of different grades of malignancy. In vitro MMP expression did not discriminate between benign and malignant tumor cells unless they were co-cultured with stromal fibroblasts. However, in vivo regulation of MMP expression was clearly dependent on tumor phenotype. While MMP-1 and MMP-13 were down-regulated in benign transplants, they were persistently up-regulated in malignant ones. A tight balance between proteases and their inhibitors is crucial for both the formation and infiltration of blood vessels and for tumor cell invasion, thus again emphasizing the importance of the stromal compartment for the development and progression of carcinomas.

Tumor progression of skin carcinoma cells in vivo promoted by clonal selection, mutagenesis, and autocrine growth regulation by granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor

Tumor microenvironment is crucial for cancer growth and progression as evidenced by reports on the significance of tumor angiogenesis and stromal cells. Using the HaCaT/HaCaT-ras human skin carcinogenesis model, we studied tumor progression from benign tumors to highly malignant squamous cell carcinomas. Progression of tumorigenic HaCaT-ras clones to more aggressive and eventually metastatic phenotypes was reproducibly achieved by their in vivo growth as subcutaneous tumors in nude mice. Their enhanced malignant phenotype was stably maintained in recultured tumor cells that represented, identified by chromosomal analysis, a distinct subpopulation of the parental line. Additional mutagenic effects were apparent in genetic alterations involving chromosomes 11 and 2, and in amplification and overexpression of the H-ras oncogene. Importantly, in vitro clonal selection of benign and malignant cell lines never resulted in late-stage malignant clones, indicating the importance of the in vivo environment in promoting an enhanced malignant phenotype. Independently of their H-ras status, all in vivo-progressed tumor cell lines (five of five) exhibited a constitutive and stable expression of the hematopoietic growth factors granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor, which may function as autocrine/paracrine mediators of tumor progression in vivo. Thus, malignant progression favored by the in vivo microenvironment requires both clonal selection of subpopulations adapted to in vivo growth and mutational events leading to stable functional alterations.

The Ca2+-binding proteins S100A8 and S100A9 are encoded by novel injury-regulated genes

To gain insight into the molecular mechanisms underlying cutaneous wound repair, we performed a large scale screen to identify novel injury-regulated genes. Here we show a strong up-regulation of the RNA and protein levels of the two Ca(2+)-binding proteins S100A8 and S100A9 in the hyperthickened epidermis of acute murine and human wounds and of human ulcers. Furthermore, both genes were expressed by inflammatory cells in the wound. The increased expression of S100A8 and S100A9 in wound keratinocytes is most likely related to the activated state of the keratinocytes and not secondary to the inflammation of the skin, since we also found up-regulation of S100A8 and S100A9 in the epidermis of activin-overexpressing mice, which develop a hyperproliferative and abnormally differentiated epidermis in the absence of inflammation. Furthermore, S100A8 and S100A9 expression was found to be associated with partially differentiated keratinocytes in vitro. Using confocal microscopy, both proteins were shown to be at least partially associated with the keratin cytoskeleton. In addition, cultured keratinocytes efficiently secreted the S100A8/A9 dimer. These results together with previously published data suggest that S100A8 and S100A9 are novel players in wound repair, where they might be involved in the reorganization of the keratin cytoskeleton in the wounded epidermis, in the chemoattraction of inflammatory cells, and/or in the defense against microorganisms.

Human calmodulin-like protein is an epithelial-specific protein regulated during keratinocyte differentiation

Human calmodulin-like protein (CLP) is a calcium-binding protein down-regulated in a cell culture model of mammary tumorigenesis as well as in a majority of breast cancers in vivo. CLP down-regulation may be a result of the poorly differentiated state of these cell lines and tumors, or CLP expression may be incompatible with the uncontrolled cell growth associated with tumorigenesis. To learn more about CLP expression and regulation, we determined the distribution of CLP in various human tissues by immunohistochemistry. CLP was expressed exclusively in the epithelium of the tissues surveyed and was most abundant in thyroid, breast, prostate, kidney, and skin. CLP expression appears to increase in stratified epithelium during differentiation, as illustrated in the skin where CLP staining intensified from the basal through the spinous to the granular layers. Using a normal human keratinocyte culture model, we examined CLP expression in response to various agents known to affect keratinocyte differentiation. Agents that inhibit (epidermal growth factor, EGF) or permit (keratinocyte growth factor) terminal differentiation correspondingly regulate CLP expression. Factors modulating the EGF receptor signaling pathway were particularly potent in regulating CLP expression. CLP expression correlated with an agent's ability to promote terminal differentiation regardless of the agent's effect on keratinocyte proliferation. These studies show that CLP expression is coordinately regulated by, and may be involved in, the program of terminal differentiation in human keratinocytes and, likely, other differentiating epithelial cell types.

Modulation of keratinocyte gene expression and differentiation by PPAR-selective ligands and tetradecylthioacetic acid

Peroxisome proliferator-activated receptors (PPARs) are pleiotropic regulators of growth and differentiation of many cell types. We have performed a comprehensive analysis of the expression of PPARs, transcriptional cofactors, and marker genes during differentiation of normal human keratinocytes using a combination of reverse transcriptase polymerase chain reaction, Northern and Western blotting, and immunohistochemistry. PPARdelta was the predominant PPAR subtype in human keratinocytes and highly expressed in basal cells and suprabasal cells. Induction of PPARalpha and PPARgamma expression was linked to differentiation, and accordingly, expression of PPARalpha and PPARgamma was in essence confined to suprabasal cells. Differentiation was not accompanied by significant changes in the expression of the coactivators CREB-binding protein, p300, steroid receptor coactivator 1, or the corepressors nuclear receptor corepressor and silence mediator for retinoid and thyroid hormone receptors. We critically evaluated the effects of selective PPAR ligands and a synthetic fatty acid analog, tetradecylthioacetic acid. Tetradecylthioacetic acid activated all human PPAR subtypes in the ranking order PPARdelta >> PPARalpha > PPARgamma. All selective PPAR ligands marginally induced transglutaminase-1 expression with the PPARdelta-selective ligand L165041 being the most potent. The PPARalpha- and PPARgamma-selective ligands Wy14643 and BRL49653 had negligible effect on involucrin expression, whereas a dose-dependent induction was observed with L165041. Simultaneous addition of L165041 and BRL49653 synergistically induced strong involucrin expression. Additionally, L165041 potently induced CD36 mRNA expression. Administration of tetradecylthioacetic acid resulted in a dramatic decrease in proliferation and a robust upregulation of the expression of involucrin and transglutaminase. Our results indicate that tetradecylthioacetic acid may affect keratinocyte gene expression and differentiation via PPAR-dependent and PPAR-independent pathways, and that the latter play an important role.

The alphaVbeta6 integrin regulates its own expression with cell crowding: implications for tumour progression

Expression of the growth-promoting integrin alphavbeta6 in colon cancer cells induces gelatinase B secretion and activation, the inhibition of which abolishes alphavbeta6-mediated tumour cell growth within a collagen matrix. Herein, we show that high cell density selectively enhances alphavbeta6 expression in a protein kinase C (PKC)-dependent manner in preference to other beta integrin subunits, resulting in a marked increase in gelatinase B secretion as cells reach confluence. Moreover, PKC activity increases with cell confluence, and the rise in PKC activity is much greater for alphavbeta6-expressing cells than for colon cancer cells which lack alphavbeta6. We propose a self-perpetuating system of colon cancer progression in which the integrin alphavbeta6 provides a means of sustaining tumour cell proliferation. In this model, alphavbeta6 regulates its own expression via a PKC-mediated signalling pathway as tumour cells become crowded and quiescent. The alphavbeta6-mediated induction of gelatinase B secretion facilitates peri-cellular matrix degradation, which helps overcome crowding and restores cell proliferation.

Impact of Bcl-2 and Ha-ras on keratinocytes in organotypic culture

In this study, the role of specific molecular alterations associated with multistep skin carcinogenesis was assessed using in vitro organotypic cultures of the spontaneously immortalized, nontumorigenic HaCaT keratinocyte cell line. HaCaT vector control clones and clones expressing bcl-2, activated Ha-ras, or both genes were generated. Clones were induced to stratify and differentiate by culturing on dermal equivalents for 2 wk at the air-medium interface. In parental and vector control HaCaT rafts the expression and distribution of cytokeratin K1, K14, involucrin, proliferating cell nuclear antigen, and p21cip1/waf1 were assessed using immunohistochemistry and immunoblotting and were similar to normal epidermis. Apoptosis was also examined using the TUNEL technique. HaCaT-bcl-2 rafts were similar to control rafts but exhibited lower spontaneous rates of apoptosis and a moderate increase in the rate of proliferation. Differentiation was significantly inhibited in HaCaT-ras organotypic cultures and was associated with high rates of proliferation and lower rates of spontaneous apoptosis. Additionally, HaCaT-ras rafts exhibited significantly higher rates of apoptosis following ultraviolet irradiation compared with vector control or HaCaT-bcl-2 rafts. Bcl-2 was able to largely restore normal differentiation, proliferation, and apoptosis in HaCaT-ras/bcl-2 organotypic cultures. Bcl-2 also abrogated apoptosis induction following ultraviolet irradiation in HaCaT-ras/bcl-2 organotypic cultures. Organotypic keratinocyte culture represents a valuable in vitro system to evaluate the impact of individual molecular genetic alterations on the coordinate regulation of cell proliferation, differentiation, and cell death.

Expression and biological significance of Ca2+-activated ion channels in human keratinocytes

In whole-cell recordings from HaCaT keratinocytes, ATP, bradykinin, and histamine caused a biphasic change of the membrane potential consisting of an initial transient depolarization, followed by a pronounced and long-lasting hyperpolarization. Flash photolysis of caged IP3 mimicked the agonist-induced voltage response, suggesting that intracellular Ca2+ release and subsequent opening of Ca2+-activated ion channels serve as the common transduction mechanism. In contrast, cAMP- and PKC-dependent pathways were not involved in the electrophysiological effects of the extracellular signaling molecules. The depolarization was predominantly mediated by a DIDS- and niflumic acid-sensitive Cl- current, whereas a charybdotoxin- and clotrimazole-sensitive K+ current underlay the prominent hyperpolarization. Consistent with the electrophysiological data, RT-PCR showed that HaCaT keratinocytes express two types of Ca2+-activated Cl- channels, CaCC2 and CaCC3 (CLCA2), as well as the Ca2+-activated K+ channel hSK4. That the pronounced hSK4-mediated hyperpolarization bears significance on the growth and differentiation properties of keratinocytes is suggested by RNase protection assays showing that hSK4 mRNA expression is strongly down-regulated under conditions that allow keratinocyte differentiation. hSK4 might thus play a role in linking changes in membrane potential to the biological fate of keratinocytes.

K15 expression implies lateral differentiation within stratified epithelial basal cells

Keratins are intermediate filament proteins whose expression in epithelial tissues is closely linked to their differentiated state. The greatest complexity of this expression is seen in the epidermis and associated structures. The critical basal (proliferative) cell layer expresses the major keratin pair, K5 and K14, but it also expresses an additional type I keratin, K15, about which far less is known. We have compared the expression of K15 with K14 in normal, pathological, and tissue culture contexts; distinct differences in their expression patterns have been observed that imply different regulation and function for these two genes. K15 appears to be preferentially expressed in stable or slowly turning over basal cells. In steady-state epidermis, K15 is present in higher amounts in basal cells of thin skin but in lower amounts in the rapidly turning over thick plantar skin. Although remaining high in basal cell carcinomas (noninvasive) it is suppressed in squamous cell carcinomas (which frequently metastasize). Wounding-stimulated epidermis loses K15 expression, whereas K14 is unchanged. In cultured keratinocytes, K15 levels are suppressed until the culture stratifies, whereas K14 is constitutively expressed throughout. Therefore, unlike K14, which appears to be a fundamental component of all keratinocytes, K15 expression appears to be more tightly coupled to a mature basal keratinocyte phenotype.

Activation of the small GTPase Rac is sufficient to disrupt cadherin-dependent cell-cell adhesion in normal human keratinocytes

To achieve strong adhesion to their neighbors and sustain stress and tension, epithelial cells develop many different specialized adhesive structures. Breakdown of these structures occurs during tumor progression, with the development of a fibroblastic morphology characteristic of metastatic cells. During Ras transformation, Rac-signaling pathways participate in the disruption of cadherin-dependent adhesion. We show that sustained Rac activation per se is sufficient to disassemble cadherin-mediated contacts in keratinocytes, in a concentration- and time-dependent manner. Cadherin receptors are removed from junctions before integrin receptors, suggesting that pathways activated by Rac can specifically interfere with cadherin function. We mapped an important region for disruption of junctions to the putative second effector domain of the Rac protein. Interestingly, although this region overlaps the domain necessary to induce lamellipodia, we demonstrate that the disassembly of cadherin complexes is a new Rac activity, distinct from Rac-dependent lamellipodia formation. Because Rac activity is also necessary for migration, Rac is a good candidate to coordinately regulate cell-cell and cell-substratum adhesion during tumorigenesis.

Organotypic cultures of gingival cells: an epithelial model to assess putative local effects of orthodontic plate and occlusal splint materials under more tissue-like conditions

This article explores whether organotypic cultures of immortalized gingival keratinocytes constitute a suitable model for assessing the epithelial cell compatibility of two groups of dental resins, each of them representing one group used in orthodontics and temporo-mandibular disorders (TMD) therapy under conditions more closely resembling the actual tissue situation. The resins were tested with the agar diffusion assay (ADA) in conventional monolayer and organotypic cultures. Compared to the control exhibiting a neutral red destaining index of 3, the index of 4 obtained after exposure of monolayers to one soft permanent resin (Durabase) indicated the presence of a non-lytic but physiologically active substance. In contrast, the adaptation of the ADA to organotypic cultures revealed no apparent lesions at the epithelial surface by performing scanning electron microscopy, while histoarchitecture indicated the development of stratified surface epithelia. This was substantiated by undamaged cells in the uppermost cell layers and by the preservation of cell-to-cell contacts. Furthermore, indirect immunofluorescence for Ki-67 and the cytokeratins ck 14 and ck4 revealed that cell proliferation and epithelial structure were maintained, while differentiation was enhanced, possibly increasing epithelial resistance. The results obtained from the organotypic cultures suggest that (i) cell-affecting effects of materials visible in monolayer cultures may not be seen in epithelia resembling that in vivo and that (ii) enhanced differentiation may be associated with increased stability of the epithelial cells. Thus, organotypic cultures of gingival cells constitute a tissue model allowing short-term tissue compatibility studies of dental materials and rendering a potential candidate also for long-term studies.

Modeling of diffusion and concurrent metabolism in cutaneous tissue

Clearance by cutaneous metabolism can shield the body from penetration of environmental and therapeutic xenobiotics. Here we report on a physical model to relate Fickian diffusion and concurrent Michaelis-Menten metabolism of drugs in the viable epidermis of human skin. For this purpose, we numerically generated substrate concentration profiles within the metabolizing tissue and the resulting donor-to-receiver substrate fluxes through the tissue for various mass transport and metabolism parameters. To validate the model, permeation and concurrent metabolism of a peptidomimetic compound, L -Ala-4-methoxy-2-naphthylamide (Ala-MNA), across both stripped human skin and HaCaT cell culture sheets were compared to numerical simulations. Parameter estimates for those calculations were extracted from independent experiments. Experimental data and numerical predictions were in excellent agreement. Also, numerical fits and independently validated parameters correlated closely, indicating the principal validity of the physical model. Numerical simulations and theoretical derivations illustrate the kinetic impact of the factors involved, i.e. the diffusion coefficient D, substrate donor concentration C(S,D), substrate partition coefficient P, tissue thickness L and maximum metabolic rate V(max), on drug permeation, with L having the strongest effect. In the steady state, the coefficient 2 alpha, i.e. the dimensionless ratio of the residence time term (L(2)/D) of a substrate in the tissue to the metabolic half-life term (C(S,D)P/2 V(max)), allows to estimate concentration gradients within the tissue and the extent of metabolism. High 2 alpha values represent practically complete metabolic cleavage upon penetration. Epidermis ( approximately 40 microm thick) of stripped human skin and HaCaT sheets ( approximately 10 microm) had 2 alpha values of 43 and 2.7, respectively, indicating that intact Ala-MNA could only permeate HaCaT sheets, but not skin. Independent permeation experiments confirmed this outcome. This physical model may be applicable to other metabolizing tissues as well.

Suppression of keratin 15 expression by transforming growth factor beta in vitro and by cutaneous injury in vivo

Transforming growth factor beta (TGF-beta) is a multifunctional cytokine which plays an important role in cutaneous wound repair. To gain insight into the mechanisms of action of this growth and differentiation factor in the skin, we searched for genes which are regulated by TGF-beta1 in cultured HaCaT keratinocytes. Using the differential display RT-PCR technology we identified a gene which was strongly downregulated by TGF-beta1. The identified cDNA includes sequences of the keratin 15 (K15) gene which encodes a component of the cytoskeleton of basal cells in stratified epithelia. Surprisingly, our cDNA also included an unknown sequence. Since this cDNA lacks an open reading frame, the corresponding mRNA is likely to be nonfunctional. However, we also demonstrate a strong negative regulation of the expression of the published, functional K15 variant. Expression of K15 was also suppressed by tumor necrosis factor alpha (TNF-alpha) and to a lesser extent by epidermal growth factor (EGF) and keratinocyte growth factor (KGF). By contrast, the major basal type I keratin, K14, was upregulated by TGF-beta1, whereas TNF-alpha, EGF, and KGF had no effect. Consistent with the in vitro data, we found a significant reduction of the K15 mRNA levels after skin injury, whereas K14 expression increased during the wound healing process. Immunostaining revealed the presence of K15 in all basal cells of the epidermis adjacent to the wound, but not in the hyperproliferative epithelium above the granulation tissue. These data demonstrate that K15 is excluded from the activated keratinocytes of the hyperthickened wound epidermis, possibly as a result of increased growth factor expression in injured skin.

Examination of early interactions between Haemophilus ducreyi and host cells by using cocultured HaCaT keratinocytes and foreskin fibroblasts

Haemophilus ducreyi is the etiologic agent of chancroid, a sexually transmitted genital ulcer disease. Keratinocytes are likely the first cell type encountered by H. ducreyi upon infection of human skin; thus, the interaction between H. ducreyi and keratinocytes is probably important for the ability of H. ducreyi to establish infection. We have used the HaCaT keratinocyte cell line grown in monolayers and in cocultures with HS27 fibroblasts to investigate H. ducreyi interactions with keratinocytes and the host-cell response to H. ducreyi infection. Using quantitative adherence and gentamicin protection assays, we determined that approximately 13% of H. ducreyi adhered to HaCaT cell monolayers, while only a small proportion (0.0052%) was intracellular. By transmission electron microscopy, we observed numerous H. ducreyi organisms adherent to but rarely within HaCaT cells cocultured with fibroblasts. Both live H. ducreyi and purified H. ducreyi lipooligosaccharide (LOS) induced significant interleukin 8 (IL-8) expression from HaCaT cell-HS27 cell cocultures. However, the level of IL-8 expression in response to LOS alone was not as pronounced. H. ducreyi LOS was a more potent inducer of IL-8 from cocultures than Escherichia coli lipopolysaccharide (LPS) at the same concentration, suggesting a unique effect of H. ducreyi LOS on cocultures. Neither live H. ducreyi nor purified H. ducreyi LOS or E. coli LPS induced tumor necrosis factor alpha expression from cocultures. H. ducreyi induced drastically different cytokine profiles from cocultures than from HS27 or HaCaT cells cultured separately. IL-8 expression by skin cells in response to H. ducreyi infection in vivo may be responsible for the massive influx of polymorphonuclear leukocytes and other inflammatory cells to the site of infection. This influx of inflammatory cells may be partly responsible for the tissue destruction characteristic of chancroid.

Arsenic disrupts cellular levels of p53 and mdm2: a potential mechanism of carcinogenesis

The antitumor protein p53 plays a critical role in DNA repair. Inorganic arsenic exposure is associated with a wide variety of human tumors, particularly of the skin. To investigate how inorganic arsenic might interfere with DNA repair and lead to greater incidence of hyperkeratosis and skin tumors, we exposed human keratinocytes (HaCaT) to environmentally relevant concentrations of arsenite for 14 days. Arsenite reduced p53 levels while concomitantly increasing the p53 regulatory protein mdm2 levels in a dose- and time-dependent manner. We propose the disruption of the p53-mdm2 loop regulating cell cycle arrest as a model for arsenic-related skin carcinogenesis and it may be important in tumors with elevated mdm2 levels.

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.

IL-1 beta and IFN-gamma induce the regenerative epidermal phenotype of psoriasis in the transwell skin organ culture system. IFN-gamma up-regulates the expression of keratin 17 and keratinocyte transglutaminase via endogenous IL-1 production

Skin biopsies from healthy human skin and non-lesional skin from patients with psoriasis were cultured for 24 h and stimulated with interleukin-1 beta (IL-1 beta) and interferon-gamma (IFN-gamma) in a skin organ culture model and the induction of the psoriasiform regenerative epidermal phenotype was analysed using immunostaining. In the presence of IL-1 beta, the psoriasiform regenerative epidermal phenotype was clearly induced. This involved strong up-regulation of the expression of keratin 16, keratin 17, and keratinocyte transglutaminase (TGk) in the suprabasal layers, strong up-regulation and a shift of the expression of keratin 5 and integrin beta 1 from the basal to suprabasal keratinocytes, and induction of the expression of ICAM-1 and HLA-DR on basal keratinocytes. The effects of IL-1 beta in the organ cultures of normal skin could be completely neutralized by anti-IL-1 polyclonal antibodies. The effects of IFN-gamma in healthy and non-lesional psoriatic skin were qualitatively similar to those of IL-1 beta. The IFN-gamma-induced epidermal expression of keratin 17 and TGk could be completely blocked by culturing the biopsies in the presence of IL-1ra or anti-IL-1 antibodies, while the induction of HLA-DR and ICAM-1 was not inhibited. The induction of the psoriasiform regenerative epidermal phenotype by IFN-gamma is partially mediated via endogenous epidermal IL-1.

Proteinase inhibitor 6 (PI-6) expression in human skin: induction of PI-6 and a PI-6/proteinase complex during keratinocyte differentiation

Proteinase inhibitor 6 (PI-6) is a 42-kDa intracellular protein present in epithelial cells and endothelial cells. It is capable of inhibiting a number of serine proteinases, including trypsin and chymotrypsin. In this study we examined PI-6 expression in human skin and its primary cell type, the keratinocyte. By immunohistochemical analysis, PI-6 staining is absent from the basal cells, weak in the spinous layer, and strongest in the granulosa layer of human epidermis. Immunoblotting of cultured primary keratinocytes revealed that PI-6 production increases 24-fold on differentiation. Analysis of an immortalized keratinocyte cell line, HaCat, showed a 5-fold increase in PI-6 mRNA and a 7-fold increase in PI-6 protein upon differentiation, and indirect immunofluorescence revealed that this is due to an increase in the number of differentiated cells expressing high levels of PI-6. Of particular interest is the appearance of a preformed complex between PI-6 and an endogenous serine proteinase in differentiating HaCat cells, which was detected by a monoclonal antibody demonstrated to preferentially recognize PI-6 in complex with a proteinase. This identification of a PI-6/proteinase complex is the first example of a serpin bound to a proteinase in keratinocytes. We postulate that a physiological role of PI-6 is to regulate a serine proteinase associated with keratinocyte differentiation.

Upregulation of the calcium-dependent protease, calpain, during keratinocyte differentiation

Calpain is a ubiquitous neutral calcium-activated thiol protease that is implicated in various cellular functions including exocytosis, cell fusion, apoptosis and proliferation. The calpain system is composed of the enzymes mu-calpain and m-calpain and their endogenous inhibitor, calpastatin. We employed the spontaneously immortalized human HaCaT keratinocytes, which retain their ability to differentiate in vitro and in vivo, to study the modulation of the calpain system during keratinocyte differentiation. The cellular levels of keratinocyte differentiation markers and of the components of the calpain system were monitored by immunoblotting. Three established differentiation stimuli: increase in cell density as a function of time in culture, elevation of extracellular calcium concentration and exposure to 1,25-dihydroxyvitamin D3 enhanced the expression of the three keratinocyte differentiation markers keratin 10, involucrin and transglutaminase. The differentiation of HaCaT cells was accompanied by elevation of the components of the calpain system, although the pattern of increase varied according to the specific differentiation stimulus. A higher increase in calpains as compared with the increase in calpastatin suggests an increase in net calpain activity during differentiation. Such an increase may play a part in the differentiation process itself and/or in the regulation of key events in differentiating keratinocyte metabolism.

cDNA cloning, expression, and assembly characteristics of mouse keratin 16

There has been speculation as to the existence of the mouse equivalent of human type I keratin 16 (K16). The function of this keratin is particularly intriguing because, in normal epidermis, it is usually confined to hair follicles and only becomes expressed in the suprabasal intrafollicular regions when the epidermis is traumatized. Previous studies suggested that K16 is highly expressed in the skin of mice carrying a truncated K10 gene. We therefore used the skin of heterozygous and homozygous mice to create a cDNA library, and we report here the successful cloning and sequencing of mouse K16. Recent in vitro studies suggested that filaments formed by human K16 are shorter than those formed by other type I keratins. One hypothesis put forward was that a proline residue in the 1B subdomain of the helical domain was responsible. The data presented here demonstrate that this proline is not conserved between mouse and human, casting doubt on the proposed function of this proline residue in filament assembly. In vitro assembly studies showed that mouse K16 produced long filaments in vitro. Also, in contrast to previous observations, transfection studies of PtK2 cells showed that mouse K16 (without the proline) and also human K16 (with the proline) can incorporate into the endogenous K8/K18 network without detrimental effect. In addition, K16 from both species can form filaments de novo when transfected with human K5 into immortalized human lens epithelial cells, which do not express keratins. These results suggest that reduced assembly capabilities due to unusual sequence characteristics in helix 1B are not the key to the unique function of K16. Rather, these data implicate the tail domain of K16 as the more likely protein domain that determines the unique functions.

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.

Human gap junction protein connexin31: molecular cloning and expression analysis

We have isolated and characterized a human genomic clone containing the complete coding region of connexin31 (Cx31). Similar to rodent Cx31, the coding region of human Cx31 is completely contained within the second exon and consists of 810 nucleotides. The deduced human Cx31 polypeptide consists of 270 amino acids with a predicted molecular mass of 30.818 kDa. Its sequence is most similar to mouse Cx31 (82.6% identical amino acids) and rat (83.0% identical amino acids), but shows considerably fewer potential sites of phosphorylation. After Northern blot hybridization, two Cx31 transcripts of 2.2 and 1.8 kb were detected in total RNA of the human keratinocyte cell line HaCaT and two transcripts of 2.2 and 1.9 kb in total RNA of E6/E7 transfected human keratinocytes (HEK cells). Using affinity-purified rabbit antibodies to mouse Cx31, immunofluorescence analysis demonstrated relatively weak expression of human Cx31 in HaCaT and HEK cells. The Cx31 gene exists as a single copy gene in the human genome and was mapped to the chromosomal region 1p34-p36 by analyzing human-mouse somatic cell hybrids.

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

The immortal human keratinocyte line HaCaT has been employed in many studies as paradigm for epidermal keratinocytes. In order to demonstrate its potential to form stable epidermal structures in response to connective tissue, this was challenged in surface transplants on nude mice, where normal keratinocytes rebuild a typical epidermis within two weeks. During the initial regeneration phase (day 1-4) multilayered but poorly organized epithelia formed with proliferating cells in all layers in analogy to normal keratinocytes. Similarly, with tissue consolidation (around day 7) proliferation was reduced and restricted to cells in basal position marked by keratin K14 and beta1-integrin immunostaining. The strong suprabasal reaction for K1 and K10, the appearance of the late markers K2e, filaggrin and loricrin as well as the polarized distribution of alpha2beta1 and alpha3beta1 indicated advancing tissue normalization (day 14). Keratinization further improved at around three weeks switching from the initial parakeratotic to the regular orthokeratotic type which was prominent at six weeks. Accordingly, most ultrastructural features typical for epidermis or normal keratinocyte grafts were detectable including a complete basement membrane (BM) with regular attachment structures. Matrix- and BM-components appeared sequentially with marked linear deposition of laminin-5 (day 4) followed by accumulation of collagen-IV and 'classical' BM-laminin between one and two weeks. With the general codistribution of integrin alpha6beta4 and BM-molecules (day 14) collagen-VII lining of BM became prominent, while epithelium and host connective tissue were still separated by the collagen matrix. In accordance with the delayed orthokeratinization, wound-matrix molecules (fibronectin, tenascin) persisted longer than in normal keratinocyte transplants. Finally, grafts of long-term passaged (no. 310) cells demonstrated a remarkable stability in the expression of epidermal markers. Thus, the immortalized HaCaT cells reveal a generally high competence to realize an epidermal phenotype in a natural environment and appear therefore qualified for in vitro studies on structural and regulatory aspects of keratinocyte physiology and pathology.

Intracellular calcium oscillations in cell populations of ras-transfected I-7 subline of human HaCaT keratinocytes

We have observed oscillations of intracellular Ca2+ (Ca[i]) concentration in populations of ras-transfected HaCaT keratinocytes of I-7 subline. In postconfluent monolayers of I-7 keratinocytes, an increase in extracellular Ca2+ (Ca[o]) concentration to 0.25-0.5 mM induced sinusoidal Ca(i) oscillations, which persisted longer than 1 h with amplitudes of 50-150 nM and periods of 5-10 min. Thapsigargin, which depletes internal Ca2+ stores, did not prevent Ca(o)-induced Ca(i) oscillations, and it also induced Ca(i) oscillations in the ras-transfected I-7 line. Removal of extracellular Ca2+ or addition of Ca2+-entry blocker La3+ or SK&F 96365 inhibited Ca(i) oscillations, suggesting that Ca(i) oscillations in ras-transfected HaCaT keratinocytes were dependent on Ca2+ influx across the plasma membrane. Because the Ca(o)-induced Ca(i) oscillations have been observed only in ras-transfected I-7 subline and not in its nontransfected parental HaCaT line, this may provide a partial explanation for the divergent responses of ras-transfected and nontransfected keratinocytes to Ca(o) signal for control of growth and differentiation.

PTHrP and cell division: expression and localization of PTHrP in a keratinocyte cell line (HaCaT) during the cell cycle

Parathyroid hormone-related protein (PTHrP) is highly expressed in normal skin keratinocytes, and its involvement in growth and differentiation processes in these cells has been implicated by several lines of evidence which include the use of antisense PTHrP (Kaiser et al., 1994, Mol. Endocrinol., 8:139-147). In this study, we have investigated whether PTHrP expression and its subcellular localization is linked to cell cycle progression in a human keratinocyte cell line (HaCat), which constitutively expresses and secretes PTHrP. PTHrP mRNA and immunoreactive PTHrP were assessed in asynchronous dividing cells and in cells blocked at G1 or G2 + M phases of the cell cycle using several different protocols. The response of PTHrP mRNA expression was examined following readdition of serum in the continued presence of cycle blockers, and after release from cell cycle block, or from cell synchronization by serum deprivation. PTHrP expression was greatest in actively dividing cells when cells were in S and G2 + M phases of the cell cycle and were lowest in quiescent G1 cells. Most notable were the high levels of PTHrP mRNA and protein in cells at G2 + M phase of the cell cycle at division. Furthermore, PTHrP was localized to the nucleolus in quiescent cells, but redistributed to the cytoplasm when cells were actively dividing. Taken together, these results support a role for PTHrP in cell division in keratinocytes. In asynchronously growing cells, PTHrP expression fell as cells became confluent at a time when cell growth is inhibited and cells begin to differentiate. Mitogen stimulation of HaCaT cells resulted in a rapid increase in PTHrP mRNA expression, but was dependent upon cells being in the G1 phase of the cell cycle. Cells blocked in G1 responded to mitogen both in the continued presence of aphidicolin or when released from block. Cells blocked at G2 + M with colcemid expressed high levels of PTHrP mRNA and protein, and PTHrP mRNA did not respond further to mitogen in the continued presence of blocker. However, in cells released from block at G2 + M by addition of serum, an increase in PTHrP expression was seen coincident with the progression of cells into G1. In contrast, in a squamous cancer cell line (COLO16), basal PTHrP expression was high and was not altered during the cell cycle or by cell cycle block, consistent with association of its dysregulated expression in malignant cells. The results of this study suggest that PTHrP may have two roles in the cell cycle; one in G1 in response to mitogen, and a second at cell division when its expression is high and it is relocated from the nucleolus to the cytoplasm.

Sustained nontumorigenic phenotype correlates with a largely stable chromosome content during long-term culture of the human keratinocyte line HaCaT

Altered growth and differentiation and a highly abnormal karyotype are generally believed to be indicators for tumorigenic conversion of human cells. Inactivation of TP53 is supposedly one possible mechanism for accelerated genetic aberrations via reduced control of the genetic integrity. To examine the significance of this functional relationship, we investigated the long-term development of the spontaneously immortalized human skin keratinocyte line HaCaT, carrying UV-specific mutations in both alleles of the TP53 tumor suppressor gene. During > 300 passages, proliferation, clonogenicity, and serum-independent growth potential increased. In addition, HaCaT cells gained anchorage independence and at late passages showed reduced differentiation. Karyotypic analysis up to passage 225 revealed a high frequency of translocations and deletions, with a particular increase during passages 30 and 50. Nevertheless, the HaCaT cells remained nontumorigenic when injected subcutaneously, and noninvasive in surface transplants in nude mice. By comparative genomic hybridization, we confirmed the karyotypically identified phase of increased chromosomal aberrations between passages 30 and 50. However, before and thereafter, the CGH profiles of the individual chromosomes were largely unchanged, demonstrating that those translocations-also maintained in later passages-did not cause a gross chromosomal imbalance. Thus, our data suggest that multiple changes often correlated with a "transformed phenotype," including extensive karyotypic alterations and mutational inactivation of TP53, are well compatible with a nontumorigenic phenotype of the HaCaT cells, and that preserved chromosomal balance may be crucial for this stability during long-term propagation.

Ex vivo expansion of human pancreatic endocrine cells

Cell transplantation as a therapy for type 1 diabetes is facilitated by ex vivo cell expansion of pancreatic beta-cells without loss of differentiative characteristics. The aim of this study was to determine the optimal conditions for in vitro growth of functional human pancreatic endocrine tissue. We examined the mitogenicity of matrixes from a variety of cell lines; proliferation was greater in cells growing on matrixes from bladder carcinoma cell lines, especially in monolayers grown on matrix from the human cell line HTB-9. After 14-day culture, there was a more than 100-fold proliferative increase, which was augmented to a more than 200-fold when hepatocyte growth factor/scatter factor was added; however, hepatocyte growth factor/scatter factor induced a rapid decrease in insulin content. Without the growth factor, fetal cell monolayers expanded 4-fold with no insulin loss; however, after 12-fold expansion, the insulin levels decreased to 40% of those in unexpanded cells. Adult islet cells expanded 3-fold without insulin loss. After 5-fold expansion, insulin levels decreased by 25% compared to those in free floating islets while retaining a normal response to secretagogues. Together, these results indicate that HTB-9 matrix provides the best stimulatory effect on replication of human endocrine cells, with little loss of in vitro function.

Integrin and basement membrane normalization in mouse grafts of human keratinocytes--implications for epidermal homeostasis

Integrin patterns and formation of basement membrane (BM) were investigated in correlation to epidermal growth and differentiation during skin regeneration in human keratinocyte transplants on nude mice. Immuno-fluorescence and transmission electron microscopy (TEM) showed that different stages of tissue reconstruction were characterized by a sequence of coordinated events. Features of the initial tissue activation, with rapid keratinocyte proliferation around day 4, including cells in a suprabasal position, were: (1) a marked increase in and extended distribution of the integrin chains alpha 2, alpha 3, beta 1 and alpha 6, while beta 4 already showed a preferential basal location; (2) de novo expression of alpha 5 and alpha v; and (3) marked deposition of laminin-5 and nidogen but low levels of other BM components. Tissue normalization during the 2nd week, initiated by a drastic decrease in the number of proliferating cells after day 4, now strictly in basal position, was signified: by (1) orthotopic staining for basal-type keratins (K5, K14) together with a regular pericellular alpha 2 beta 1 and alpha 3 beta 1 distribution, (2) linear, balanced deposition of BM components (e.g. laminin-1, type IV collagen) and (3) colocalization of integrin alpha 6 beta 4 and bullous pemphigoid antigen. Simultaneously at 7 days hemidesmosomes and a defined BM had developed (TEM), becoming continuous at 14 days. This coincided with the regular distribution of suprabasal keratins (K1, K10) as well as intermediate (involucrin) and late differentiation markers (filaggrin, loricrin). Type-VII collagen deposition, still irregular at 14 days, became continuous at 22 days together with developing BM-anchoring fibrils indicating final tissue consolidation. This model mimics principal stages of epidermal wound healing in human skin and implies a linkage between BM assembly, integrin distribution and the compartment of proliferation competent cells, which in turn determines the onset of differentiation. Thus, apart from the balance of diffusible growth regulators, this positional control of keratinocytes, largely accomplished by integrin-matrix interactions, seems to be prerequisite to establishment and maintenance of tissue homeostasis.

Immunological demonstration of protein kinase C mu in murine tissues and various cell lines. Differential recognition of phosphorylated forms and lack of down-regulation upon 12-O-tetradecanoylphorphol-13-acetate treatment of cells

13 murine tissues and 12 cell lines were tested for the expression of the novel protein kinase C (PKC) isoenzyme mu. Using two different PKC mu antibodies (sc-639 and P26720), PKC mu was detected in all tissues and cells and thus proved to be an ubiquitous PKC isotype. However, in some tissues, PKC mu was recognized only by the antibody P26720 and not by sc-639. Thymus, lung and peripheral blood mononuclear cells expressed the greatest amount of PKC mu. Recognition of PKC mu by the antibody sc-639 was drastically impaired when treating keratinocytes or mouse skin in vivo with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), thus mimicking down-regulation of PKC mu. The lack of a decrease in the PKC mu amount and, thus, the lack of down-regulation could be proved using the antibody P26720. This antibody was able to recognize PKC mu in extracts of untreated as well as TPA-treated tissues or cells. Phosphorylation of proteins in a cell-free system (cell or tissue extracts) in the presence and absence of TPA or other PKC activators and various protein kinase inhibitors indicated that phosphorylation of activated PKC mu caused its reduced interaction with the antibody sc-639. Therefore, this antibody might present a well suited tool for the detection of activated PKC mu in vivo. Moreover, our results clearly show that some antibodies, such as sc-639, might be able to selectively detect non-phosphorylated or phosphorylated forms of a protein, and that such properties of an antibody have to be studied carefully before the latter can be used for reliable quantitative determination of this protein. We consider this information important to avoid misinterpretation of data concerning the immunological quantification of proteins such as PKC mu.

Increased expression of a high molecular weight (130 KD) protein kinase C isoform in a differentiation-defective ras-transfected keratinocyte line

The role of ras on protein kinase C (PKC) signaling was examined in two keratinocyte cell lines. Increasing the level of extracellular calcium from 0.15 mM to 1.0 mM induces some features of differentiation in the spontaneously immortalized HaCaT line, but fails to do so in a c-H-ras-transfected subline (ras-HaCaT). Raising extracellular calcium also induced a transient increase in membrane-associated PKC activity 5 min after calcium addition, in HaCaT, but not in the ras-HaCaT cells. Partial purification of PKC from the membrane/particulate fraction revealed the major isoform expressed in HaCaT to be an 80 KD species recognized by the anti-PKC alpha antibody. In ras-HaCaT, the major expressed isoform is a 130 KD species recognized by the PKC beta antibody. The kinase activity of the partially purified high molecular weight PKC is phospholipid dependent but calcium independent. Further evaluation of PKC in the HaCaT and ras-HaCaT membrane/particulate cell fraction by immunoblotting using affinity-purified antibodies against PKC alpha, beta, delta, epsilon and zeta revealed a 130 KD band reacting with the PKC delta antibody. Increased expression of this high molecular weight protein was observed in ras-HaCaT. Immunoprecipitation of PKC in ras-HaCaT using the PKC delta antibody also revealed a 130 KD species. Analysis of the PKC delta immunoprecipitate demonstrated a phospholipid, but not calcium-dependent kinase which autophosphorylated. These results suggest that the 130 KD protein may be a novel (calcium-independent) PKC (nPKC) isoform and increased expression in the ras-transfected HaCaT may be a consequence of oncogenic ras expression. This 130 KD species may also play a role in the ras-associated inhibition of differentiation in HaCaT.

Dermal metabolism of topically applied drugs: pathways and models reconsidered

The study of skin metabolism is of prime importance not only in the field of transdermal drug delivery but also for the safe and efficient local skin treatment with topically applied substances. Since it has become clear that even peptides may be delivered across the permeation barrier of the stratum corneum, e.g. by means of iontophoresis, phonophoresis or electroporation, the enzymatic barrier of the epidermis deserves more attention as another important limiting factor for the dermal delivery of drugs. The purpose of this review is to give a survey of the major aspects concerning the assessment of the metabolism of xenobiotics in the skin. First, this review will focus on the localization of enzyme activity within the skin. Further, important aspects to be considered for the planning and evaluation of skin metabolism studies will be discussed: differences among species and requirements of skin quality. Among the various metabolic pathways within the skin broad interest will be concentrated on the metabolism of peptide drugs. Therefore, exopeptidases in the skin are the subject of an additional section. Also models for skin metabolism studies will be especially considered including the development of suitable cell culture models. Finally, major gap areas in the investigation of skin metabolism will be identified and summarized, namely; classes of xenobiotics, proteolytic enzyme activity in the skin, validity of experimental models, localization of enzyme activity and physical model development.

Differentiation and tumor progression

Clinical and experimental experience indicate that differentiation and malignancy are inversely correlated. However, more recent experimental studies using mouse and human keratinocyte systems have demonstrated that complete or even substantial loss in overall epithelial differentiation is not a prerequisite for malignant growth of cancer cells. Major defects in differentiation are also not a prerequisite for premalignant stages, in particular for cell immortalization, which is considered an early and essential step in the transformation process. Moreover, progressive dedifferentiation, often associated with advanced tumor stages, is also found in immortalized cell lines which are, however, nontumorigenic. On the other hand, malignant cell lines may have maintained a high degree of their normal differentiation program and sensitivity to differentiation modulators. However, to date no transformed keratinocyte cell lines with completely normal differentiation have been observed. Since epidermal keratinization is a very complex process involving many different parameters and is fully expressed only under in vivo conditions, an exact and quantitative comparison of such ill-defined phenomena (differentiation and malignancy) is still problematic. Obviously, both phenomena are under separate control and not causally linked. Nevertheless, a better understanding of factors and mechanisms regulating differentiation and of their disturbance in carcinogenesis would offer new possibilities to design novel tumor therapeutic strategies in the field of differentiation therapy.

A keratinocyte cell line synthesizes a predominant insulin-like growth factor-binding protein (IGFBP-3) that modulates insulin-like growth factor-I action

Insulin-like growth factor-I (IGF-I) is an important regulator of epidermal proliferation and has been shown in vitro to be a powerful stimulator of keratinocyte growth. It is synthesized by fibroblasts in the dermis, along with several IGF-binding proteins (IGFBPs), which are known to modulate IGF-I responsiveness of virtually all tissues studied. Because it was not known how or in what form IGF-I produced in the dermis acts on epidermal keratinocytes in vivo, we investigated the possible role of IGFBPs in modulating the response of epidermal keratinocytes to IGF-1. We show here that tIGF-I, a non-IGFBP-binding analogue of IGF-1, is a more potent mitogenic stimulator of the keratinocyte cell line HaCaT than IGF-I, suggesting that keratinocytes produce IGFBPs that modulate their response to IGF-I. To confirm this and to identify which IGFBPs were produced, we analyzed HaCaT cell-conditioned medium and mRNA, with the following findings: HaCaT cells produce a major IGFBP, identified as IGFBP-3, and a minor 24-kD IGFBP, likely to be IGFBP-4. Northern analysis revealed a 2.6-kb IGFBP-3 mRNA; however, IGFBP-4 mRNA was not detectable. We conclude that production of predominantly IGFBP-3 by the HaCaT cell line modulates its sensitivity to IGF-I stimulation. Epidermal IGFBPs thus have a potential role in vivo in the interaction of dermis derived IGF-I with epidermal keratinocytes.

Disruption of epithelial cell-matrix interactions induces apoptosis

Cell-matrix interactions have major effects upon phenotypic features such as gene regulation, cytoskeletal structure, differentiation, and aspects of cell growth control. Programmed cell death (apoptosis) is crucial for maintaining appropriate cell number and tissue organization. It was therefore of interest to determine whether cell-matrix interactions affect apoptosis. The present report demonstrates that apoptosis was induced by disruption of the interactions between normal epithelial cells and extracellular matrix. We have termed this phenomenon "anoikis." Overexpression of bcl-2 protected cells against anoikis. Cellular sensitivity to anoikis was apparently regulated: (a) anoikis did not occur in normal fibroblasts; (b) it was abrogated in epithelial cells by transformation with v-Ha-ras, v-src, or treatment with phorbol ester; (c) sensitivity to anoikis was conferred upon HT1080 cells or v-Ha-ras-transformed MDCK cells by reverse-transformation with adenovirus E1a; (d) anoikis in MDCK cells was alleviated by the motility factor, scatter factor. The results suggest that the circumvention of anoikis accompanies the acquisition of anchorage independence or cell motility.

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.