An immunofluorescence study of the calcium-induced coordinated reorganization of microfilaments, keratin intermediate filaments, and microtubules in cultured human epidermal keratinocytes

Keratin Promotes Differentiation of Keratinocytes Seeded on Collagen/Keratin Hydrogels

Keratinocytes undergo a complex process of differentiation to form the stratified stratum corneum layer of the skin. In most biomimetic skin models, a 3D hydrogel fabricated out of collagen type I is used to mimic human skin. However, native skin also contains keratin, which makes up 90% of the epidermis and is produced by the keratinocytes present. We hypothesized that the addition of keratin (KTN) in our collagen hydrogel may aid in the process of keratinocyte differentiation compared to a pure collagen hydrogel. Keratinocytes were seeded on top of a 100% collagen or 50/50 C/KTN hydrogel cultured in either calcium-free (Ca-free) or calcium+ (Ca+) media. Our study demonstrates that the addition of keratin and calcium in the media increased lysosomal activity by measuring the glucocerebrosidase (GBA) activity and lysosomal distribution length, an indication of greater keratinocyte differentiation. We also found that the presence of KTN in the hydrogel also increased the expression of involucrin, a differentiation marker, compared to a pure collagen hydrogel. We demonstrate that a combination (i.e., containing both collagen and kerateine or “C/KTN”) hydrogel was able to increase keratinocyte differentiation compared to a pure collagen hydrogel, and the addition of calcium further increased the differentiation of keratinocytes. This multi-protein hydrogel shows promise in future models or treatments to increase keratinocyte differentiation into the stratum corneum.

Substrate deformations induce directed keratinocyte migration

Cell migration is an essential part of many (patho)physiological processes, including keratinocyte re-epithelialization of healing wounds. Physical forces and mechanical cues from the wound bed (in addition to biochemical signals) may also play an important role in the healing process. Previously, we explored this possibility and found that polyacrylamide (PA) gel stiffness affected human keratinocyte behaviour and that mechanical deformations in soft (approx. 1.2 kPa) PA gels produced by neighbouring cells appeared to influence the process of de novo epithelial sheet formation. To clearly demonstrate that keratinocytes do respond to such deformations, we conducted a series of experiments where we observed the response of single keratinocytes to a prescribed local substrate deformation that mimicked a neighbouring cell or evolving multicellular aggregate via a servo-controlled microneedle. We also examined the effect of adding either Y27632 or blebbistatin on cell response. Our results indicate that keratinocytes do sense and respond to mechanical signals comparable to those that originate from substrate deformations imposed by neighbouring cells, a finding that could have important implications for the process of keratinocyte re-epithelialization that takes place during wound healing. Furthermore, the Rho/ROCK pathway and the engagement of NM II are both essential to substrate deformation-directed keratinocyte migration.

Mouse Keratinocytes Without Keratin Intermediate Filaments Demonstrate Substrate Stiffness Dependent Behaviors

INTRODUCTION

Traditionally thought to serve active vs. passive mechanical functions, respectively, a growing body of evidence suggests that actin microfilament and keratin intermediate filament (IF) networks, together with their associated cell-cell and cell-matrix anchoring junctions, may have a large degree of functional interdependence. Therefore, we hypothesized that the loss of keratin IFs in a knockout mouse keratinocyte model would affect the kinematics of colony formation, i.e., the spatiotemporal process by which individual cells join to form colonies and eventually a nascent epithelial sheet.

METHODS

Time-lapse imaging and deformation tracking microscopy was used to observe colony formation for both wild type (WT) and keratin-deficient knockout (KO) mouse keratinocytes over 24 h. Cells were cultured under high calcium conditions on collagen-coated substrates with nominal stiffnesses of ~ 1.2 kPa (soft) and 24 kPa (stiff). Immunofluorescent staining of actin and selected adhesion proteins was also performed.

RESULTS

The absence of keratin IFs markedly affected cell morphology, spread area, and cytoskeleton and adhesion protein organization on both soft and stiff substrates. Strikingly, an absence of keratin IFs also significantly reduced the ability of mouse keratinocytes to mechanically deform the soft substrate. Furthermore, KO cells formed colonies more efficiently on stiff vs. soft substrates, a behavior opposite to that observed for WT keratinocytes.

CONCLUSIONS

Collectively, these data are strongly supportive of the idea that an interdependence between actin microfilaments and keratin IFs does exist, while further suggesting that keratin IFs may represent an important and under-recognized component of keratinocyte mechanosensation and the force generation apparatus.

Substrate Stiffness Affects Human Keratinocyte Colony Formation

Restoration of epidermal organization and function in response to a variety of pathophysiological insults is critically dependent on coordinated keratinocyte migration, proliferation, and stratification during the process of wound healing. These processes are mediated by the reconfiguration of both cell-cell (desmosomes, adherens junctions) and cell-matrix (focal adhesions, hemidesmosomes) junctions and the cytoskeletal filament networks that they serve to interconnect. In this study, we investigated the role of substrate elasticity (stiffness) on keratinocyte colony formation in vitro during the process of nascent epithelial sheet formation as triggered by the calcium switch model of keratinocyte culture. Keratinocytes cultured on pepsin digested type I collagen coated soft (nominal E = 1.2 kPa) polyacrylamide gels embedded with fluorescent microspheres exhibited (i) smaller spread contact areas, (ii) increased migration velocities, and (iii) increased rates of colony formation with more cells per colony than did keratinocytes cultured on stiff (nominal E = 24 kPa) polyacrylamide gels. As assessed by tracking of embedded microsphere displacements, keratinocytes cultured on soft substrates generated large local substrate deformations that appeared to recruit adjacent keratinocytes into joining an evolving colony. Together with the observed differences in keratinocyte kinematics and substrate deformations, we developed two ad hoc analyses, termed distance rank (DR) and radius of cooperativity (RC), that help to objectively ascribe what we perceive as increasingly cooperative behavior of keratinocytes cultured on soft versus stiff gels during the process of colony formation. We hypothesize that the differences in keratinocyte colony formation observed in our experiments could be due to cell-cell mechanical signaling generated via local substrate deformations that appear to be correlated with the increased expression of β4 integrin within keratinocytes positioned along the periphery of an evolving cell colony.

Using 3D culture to investigate the role of mechanical signaling in keratinocyte stem cells

The ability to grow keratinocyte stem cells (KSCs) in 3D culture is an important step forward for investigating the physiological properties of these cells. In the epidermis, KSCs are subject to various types of mechanical stress. To study the effects of mechanical stress on KSCs, monolayer cultures are limited as the KSCs can only form cell-cell contacts in one plane and to prevent differentiation, KSCs are grown in low (0.05 mM) calcium, which impairs formation of calcium-dependent adhesion structures such as desmosomes. This is in contrast to how KSCs are found in the epidermis in vivo, where they are connected on all sides by other cells, allowing them to form a more organized cytoskeleton. The cytoskeleton is essential for transducing mechanical signals between cells, and this cannot be accurately reproduced in monolayer cultures, where the cells do not have the same level of organization or connections. We describe a technique which allows the generation of large numbers of uniformly sized cell aggregates using cultured murine KSCs. These aggregates are produced using physiological calcium concentrations (1.2 mM), allowing the cells within the aggregates to form calcium-dependent contacts with other cells on all sides, resulting in the reorganization of the cytoskeleton, integrating the cells within each aggregate. Within the aggregates, KSCs retain stem cell properties, such as p63 expression, despite the increased calcium concentration and show activation of the mitogen-activated protein kinase ERK upon stretch. KSC aggregates can be manipulated further and provide a more physiologically relevant model for studying mechanical signaling in KSCs.

Calcium regulation of keratinocyte differentiation

Calcium is the major regulator of keratinocyte differentiation in vivo and in vitro. A calcium gradient within the epidermis promotes the sequential differentiation of keratinocytes as they traverse the different layers of the epidermis to form the permeability barrier of the stratum corneum. Calcium promotes differentiation by both outside-in and inside-out signaling. A number of signaling pathways involved with differentiation are regulated by calcium, including the formation of desmosomes, adherens junctions and tight junctions, which maintain cell-cell adhesion and play an important intracellular signaling role through their activation of various kinases and phospholipases that produce second messengers that regulate intracellular free calcium and PKC activity, critical for the differentiation process. The calcium receptor plays a central role by initiating the intracellular signaling events that drive differentiation in response to extracellular calcium. This review will discuss these mechanisms.

Plakophilin 2 couples actomyosin remodeling to desmosomal plaque assembly via RhoA

The desmosomal armadillo protein plakophilin 2 (PKP2) regulates cell contact-initiated cortical actin remodeling through the regulation of RhoA localization and activity to couple adherens junction maturation with desmosomal plaque assembly.

Plakophilin 2 (PKP2), an armadillo family member closely related to p120 catenin (p120ctn), is a constituent of the intercellular adhesive junction, the desmosome. We previously showed that PKP2 loss prevents the incorporation of desmosome precursors enriched in the plaque protein desmoplakin (DP) into newly forming desmosomes, in part by disrupting PKC-dependent regulation of DP assembly competence. On the basis of the observation that DP incorporation into junctions is cytochalasin D–sensitive, here we ask whether PKP2 may also contribute to actin-dependent regulation of desmosome assembly. We demonstrate that PKP2 knockdown impairs cortical actin remodeling after cadherin ligation, without affecting p120ctn expression or localization. Our data suggest that these defects result from the failure of activated RhoA to localize at intercellular interfaces after cell–cell contact and an elevation of cellular RhoA, stress fibers, and other indicators of contractile signaling in squamous cell lines and atrial cardiomyocytes. Consistent with these observations, RhoA activation accelerated DP redistribution to desmosomes during the first hour of junction assembly, whereas sustained RhoA activity compromised desmosome plaque maturation. Together with our previous findings, these data suggest that PKP2 may functionally link RhoA- and PKC-dependent pathways to drive actin reorganization and regulate DP–IF interactions required for normal desmosome assembly.

A phase I study of paclitaxel and continuous daily CAI in patients with refractory solid tumors

BACKGROUND

Carboxyamido-triazole (CAI) is a calcium influx inhibitor with anti-angiogenic and anti-invasive properties and stabilizes tumor progression in patients. We hypothesized daily oral micronized CAI with q3 week paclitaxel would be well-tolerated and active.

RESULTS

Twenty-nine heavily pretreated patients [median 3 [0-7]] were enrolled on five dose levels. No additive or cumulative toxicity was observed, and grade III nonhematological toxicity was rare. Neutropenia was the most common hematologic toxicity, seen in 79% of patients, with a trend towards increasing grade with higher paclitaxel doses. The recommended phase II dose defined by the maximum tolerated dose (MTD) was CAI 250 mg daily and paclitaxel 200 mg/m(2) q3weeks. Pharmacokinetic analysis revealed paclitaxel increases CAI trough concentration at all dose levels by over 100% (p < 0.0001). A trend towards higher steady-state CAI trough concentrations was found in patients with a partial response (PR; p = 0.09). Six patients had confirmed PR (24%; 4-67 cycles, median 10); two patients had minor responses.

PATIENTS AND METHODS

Eligible patients with solid tumors received micronized CAI daily (150-250 mg PO) and paclitaxel intravenously q3weeks (175-250 mg/m(2)), sequentially escalating each drug. CAI preceded paclitaxel by one week to permit pharmacokinetic analysis. Patients were assessed for toxicity, pharmacokinetics and disease outcome.

CONCLUSIONS

The MTD of the combination of CAI and paclitaxel is 250 mg daily and 200 mg/m(2) q3weeks, respectively. The combination is tolerable and has potential antitumor activity.

Actin at cell-cell junctions is composed of two dynamic and functional populations

The ability of epithelial cells to polarize requires cell-cell adhesion mediated by cadherin receptors. During cell-cell contact, the mechanism via which a flat, spread cell shape is changed into a tall, cuboidal epithelial morphology is not known. We found that cadherin-dependent adhesion modulates actin dynamics by triggering changes in actin organization both locally at junctions and within the rest of the cell. Upon induction of cell-cell contacts, two spatial actin populations are distinguishable: junctional actin and peripheral thin bundles. With time, the relative position of these two populations changes and becomes indistinguishable to form a cortical actin ring that is characteristic of mature, fully polarized epithelial cells. Junctional actin and thin actin bundles differ in their actin dynamics and mechanism of formation, and interestingly, have distinct roles during epithelial polarization. Whereas junctional actin stabilizes clustered cadherin receptors at cell-cell contacts, contraction of peripheral actin bundle is essential for an increase in the maximum height at the lateral domain during polarization (cuboidal morphology). Thus, both junctional actin and thin bundles are necessary, and cooperate with each other to generate a polarized epithelial morphology.

Medium calcium concentration determines keratin intermediate filament density and distribution in immortalized cultured thymic epithelial cells (TECs)

Isolation and culture of thymic epithelial cells (TECs) using conventional primary tissue culture techniques under conditions employing supplemented low calcium medium yielded an immortalized cell line derived from the LDA rat (Lewis [Rt1l] cross DA [Rt1a]) that could be manipulated in vitro. Thymi were harvested from 4-5-day-old neonates, enzymically digested using collagenase (1 mg/ml, 37 degrees C, 1 h) and cultured in low calcium WAJC404A medium containing cholera toxin (20 ng/ml), dexamethasone (10 nM), epidermal growth factor (10 ng/ml), insulin (10 mug/ml), transferrin (10 mug/ml), 2% calf serum, 2.5% Dulbecco's Modified Eagle's Medium (DMEM), and 1% antibiotic/antimycotic. TECs cultured in low calcium displayed round to spindle-shaped morphology, distinct intercellular spaces (even at confluence), and dense reticular-like keratin patterns. In high calcium (0.188 mM), TECs formed cobblestone-like confluent monolayers that were resistant to trypsinization (0.05%) and displayed keratin intermediate filaments concentrated at desmosomal junctions between contiguous cells. Changes in cultured TEC morphology were quantified by an analysis of desmosome/membrane relationships in high and low calcium media. Desmosomes were significantly increased in the high calcium medium. These studies may have value when considering the growth conditions of cultured primary cell lines like TECs.

Vitamin D and skin cancer

Skin cancer is the most common cancer afflicting humans. These cancers include melanomas and 2 types of malignant keratinocytes: basal-cell carcinomas (BCC) and squamous-cell carcinomas (SCC). UV light exposure is linked to the incidence of these cancers. On the other hand, the skin is the major source of vitamin D-3 (cholecalciferol) and UV light is critical for its formation. Keratinocytes can convert vitamin D-3 to its hormonal form, 1,25 dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] (calcitriol). 1,25(OH)(2)D(3) in turn stimulates the differentiation of keratinocytes, raising the hope that 1,25(OH)(2)D(3) may prevent the development of malignancies in these cells. We identified a number of mechanisms by which 1,25(OH)(2)D(3) regulates the differentiation of keratinocytes and explored where this regulation breaks down in SCCs. 1,25(OH)(2)D(3) regulates gene expression by activating the vitamin D receptor (VDR). When activated, the VDR binds to one of two coactivator complexes: DRIP or p160/SRC. Binding to DRIP occurs in the undifferentiated keratinocyte, but, as the cell differentiates, DRIP(205) levels fall and p160/SRC binding takes over as SRC3 expression increases. SCCs fail to respond to the prodifferentiating actions of 1,25(OH)(2)D(3). These cells have normal levels of VDR and normal binding of VDR to vitamin D response elements. However, they overexpress DRIP(205) such that the p160/SRC complex is blocked from binding to VDR. We hypothesize that failure of 1,25(OH)(2)D(3) to induce differentiation in SCCs lies at least in part with its failure to induce the replacement of the DRIP complex with the SRC complex in the promoters of genes required for differentiation.

Vitamin D regulated keratinocyte differentiation

The epidermis is the largest organ in the body. It is comprised primarily of keratinocytes which are arranged in layers that recapitulates their programmed life cycle. Proliferating keratinocytes are on the bottom-the stratum basale. As keratinocytes leave the stratum basale they begin to differentiate, culminating in the enucleated stratum corneum which has the major role of permeability barrier. Calcium and the active metabolite of vitamin D, 1,25(OH)(2)D(3), play important roles in this differentiation process. The epidermis has a gradient of calcium with lowest concentrations in the stratum basale, and highest concentrations in the stratum granulosum where proteins critical for barrier function are produced. Vitamin D is made in different layers of the epidermis, but 1,25(OH)(2)D(3) is made primarily in the stratum basale. Together calcium and 1,25(OH)(2)D(3) regulate the ordered differentiation process by the sequential turning on and off the genes producing the elements required for differentiation as well as activating those enzymes involved in differentiation. Animal models in which the sensing mechanism for calcium, the receptor for 1,25(OH)(2)D(3), or the enzyme producing 1,25(OH)(2)D(3) have been rendered inoperative demonstrate the importance of these mechanisms for the differentiation process, although each animal model has its own phenotype. This review will examine the mechanisms by which calcium and 1,25(OH)(2)D(3) interact to control epidermal differentiation.

Actin reorganization is abnormal and cellular ATP is decreased in Hailey-Hailey keratinocytes

Actin reorganization and the formation of adherens junctions are necessary for normal cell-to-cell adhesion in keratinocytes. Hailey-Hailey disease (HHD) is blistering skin disease, resulting from mutations in the Ca2+ ATPase ATP2C1, which controls Ca2+ concentrations in the cytoplasm and Golgi of human keratinocytes. Because actin reorganization is among the first responses to raised cytoplasmic Ca2+, we examined Ca2+-induced actin reorganization in normal and HHD keratinocytes. Even though HHD keratinocytes display raised baseline cytoplasmic Ca2+, we found that actin reorganization in response to Ca2+ was impaired in HHD keratinocytes. Defects in actin reorganization were linked to a marked decrease in cellular ATP in HHD keratinocytes, which persists, in vivo, in HHD epidermis. Defective actin reorganization was reproduced in normal keratinocytes in which the intracellular ATP concentration had been lowered pharmacologically. ATP concentrations in undifferentiated keratinocytes markedly declined after extracellular Ca2+ was increased, but then recovered to a new baseline that was approximately 150% of the previous baseline. In contrast, ATP concentrations in HHD keratinocytes did not change in response to increased extracellular Ca2+. This report provides new insights into how the ATP2C1-controlled ATP metabolism mediates Ca2+-induced cell-to-cell adhesion in normal keratinocytes. In addition, these findings implicate inadequate ATP stores as an additional cause in the pathogenesis of HHD and suggest novel therapeutic options.

Cell-cell adhesion and signalling

Signalling pathways activated by Rho small GTPases have recently been identified that coordinate junction assembly, stability and function, as well as interactions of adhesive complexes with the underlying cortical cytoskeleton. Particularly exciting is the interplay between adherens junctions, activation of Rho proteins and the dynamics of microtubule, actin and intermediate filaments. This interplay has important implications for functional regulation of cell-cell adhesion, and points to a more integrated view of signalling processes.

Human hepatocyte culture systems for the in vitro evaluation of cytochrome P450 expression and regulation

Primary cultures of human hepatocytes have been used extensively by both academic and industrial laboratories for evaluating the hepatic disposition of drugs and other xenobiotics. Their primary utility has been for assessing the induction potential of new chemical entities (NCEs) and they continue to serve as the gold standard. Primary considerations for conducting in vitro drug testing utilizing cultures of human hepatocytes, such as the effects of culture and study conditions, are discussed. The maintenance of normal cellular physiology and intercellular contacts in vitro is of particular importance for optimal phenotypic gene expression and response to drugs and other xenobiotics. Significant advances in our understanding of cytochrome P450 (CYP450) enzyme regulation have been made with the recent identification of the nuclear receptors mediating the induction of CYP2B and CYP3A enzymes. In particular, the activation of pregnane X receptor (PXR) by prototypical inducers of CYP3A has been found to correlate well with the species-specific modulation of CYP3A by various drugs and other xenobiotics. Concomitant with the discovery of PXR has been the identification of compounds that may act synergistically or antagonistically on multiple receptors (e.g., co-repressors and/or co-activators of the receptor) introducing novel mechanisms of drug-drug interactions. Differential expression of the individual isoforms of the major CYP450 enzymes over time in culture suggest that this model system is not reflective of in vivo profiles and, therefore, may be limited in its application for drug metabolism studies. Overall, primary cultures of human hepatocytes can serve as a sensitive and selective model for predicting the regulation of CYP450 modulation by drugs and other xenobiotics. Considerations and recommendations for standardizing testing conditions and choosing relevant endpoint(s) are presented.

Upregulation and redistribution of E-MAP-115 (epithelial microtubule-associated protein of 115 kDa) in terminally differentiating keratinocytes is coincident with the formation of intercellular contacts

Microtubules are involved in the positioning and movement of organelles and vesicles and therefore play fundamental roles in cell polarization and differentiation. Their organization and properties are cell-type specific and are controlled by microtubule-associated proteins (MAP). E-MAP-115 (epithelial microtubule-associated protein of 115 kDa) has been identified as a microtubule-stabilizing protein predominantly expressed in epithelial cells. We have used human skin and primary keratinocytes as a model to assess a putative function of E-MAP-115 in stabilizing and reorganizing the microtubule network during epithelial cell differentiation. Immunolabeling of skin sections indicated that E-MAP-115 is predominantly expressed in the suprabasal layers of the normal epidermis and, in agreement with this observation, is relatively abundant in squamous cell carcinomas but barely detectable in basal cell carcinomas. In primary keratinocytes whose terminal differentiation was induced by increasing the Ca2+ concentration of the medium, E-MAP-115 expression significantly increased during the first day, as observed by northern and western blot analysis. Parallel immunofluorescence studies showed an early redistribution of E-MAP-115 from microtubules with a paranuclear localization to cortical microtubules organized in spike-like bundles facing intercellular contacts. This phenomenon is transient and can be reversed by Ca2+ depletion. Treatment of cells with cytoskeleton-active drugs after raising the Ca2+ concentration indicated that E-MAP-115 is associated with a subset of stable microtubules and that the cortical localization of these microtubules is dependent on other microtubules but not on strong interactions with the actin cytoskeleton or the plasma membrane. The mechanism whereby E-MAP-115 would redistribute to and stabilize cortical microtubules used for the polarized transport of vesicles towards the plasma membrane, where important reorganizations take place upon stratification, is discussed.

The small GTPases Rho and Rac are required for the establishment of cadherin-dependent cell-cell contacts

Cadherins are calcium-dependent cell-cell adhesion molecules that require the interaction of the cytoplasmic tail with the actin cytoskeleton for adhesive activity. Because of the functional relationship between cadherin receptors and actin filament organization, we investigated whether members of the Rho family of small GTPases are necessary for cadherin adhesion. In fibroblasts, the Rho family members Rho and Rac regulate actin polymerization to produce stress fibers and lamellipodia, respectively. In epithelial cells, we demonstrate that Rho and Rac are required for the establishment of cadherin-mediated cell-cell adhesion and the actin reorganization necessary to stabilize the receptors at sites of intercellular junctions. Blocking endogenous Rho or Rac selectively removed cadherin complexes from junctions induced for up to 3 h, while desmosomes were not perturbed. In addition, withdrawal of cadherins from intercellular junctions temporally precedes the removal of CD44 and integrins, other microfilament-associated receptors. Our data showed that the concerted action of Rho and Rac modulate the establishment of cadherin adhesion: a constitutively active form of Rac was not sufficient to stabilize cadherindependent cell-cell contacts when endogenous Rho was inhibited. Upon induction of calcium-dependent intercellular adhesion, there was a rapid accumulation of actin at sites of cell-cell contacts, which was prevented by blocking cadherin function, Rho or Rac activity. However, if cadherin complexes are clustered by specific antibodies attached to beads, actin recruitment to the receptors was perturbed by inhibiting Rac but not Rho. Our results provide new insights into the role of the small GTPases in the cadherin-dependent cell- cell contact formation and the remodelling of actin filaments in epithelial cells.

Hyperphosphorylation of beta-catenin on serine-threonine residues and loss of cell-cell contacts induced by calyculin A and okadaic acid in human epidermal cells

Phosphorylation and dephosphorylation events may critically control junction assembly and stability, as well as regulate the formation of the cadherin-cytoskeleton complex, thus influencing the adhesive function of cells. In the present study, we have used specific activators and inhibitors of protein kinases and phosphatases to analyze the role of protein phosphorylation in the maintenance of epithelial architecture. Okadaic acid and calyculin A cell treatments induced two major effects: a dramatic alteration of the keratin network of epidermal cells and a complete disruption of cell-cell contacts. This loss in cell-cell contacts was not tissue and species restricted and the interactions of keratinocytes with the matrix were not involved. The observed changes were highly specific for these drugs and were obtained in the range of concentrations corresponding to the inhibition of protein phosphatase 1 (PP1). They were time- and dose-dependent, and reversible, excluding a cytotoxic effect of the drugs. A decrease in electrophoretic mobility of beta-catenin, a major protein involved in the regulation of intercellular adherens junctions, was observed in keratinocytes and fibroblasts treated with okadaic acid and calyculin A, suggesting a change in the protein phosphorylation level and/or protein conformation. Data from beta-catenin immunocomplex autoradiography performed after 32P in vivo incorporation in untreated and okadaic acid or calyculin A-treated HaCaT cells, demonstrated a higher level of phosphorylation of beta-catenin in treated cells compared to untreated ones. Analysis of 32P-labeled phosphoaminoacids demonstrated that beta-catenin was exclusively phosphorylated on serine-threonine residues but not on tyrosine residues. Immunoprecipitations and Western blotting using anti-phosphoserine and anti-phosphotyrosine antibodies confirmed these data. The change in beta-catenin phosphorylation on serine-threonine residues may play a role in the control of the cohesion between epithelial cells and may be involved in the regulation of the transduction signal.

The presence of h2-calponin in human keratinocyte

Calponin (h1 isoform) was characterized as a smooth muscle specific, actin-, tropomyosin-, calmodulin-binding protein and described as a factor which inhibits contraction. H2-calponin, encoded by a different gene from h1-calponin, was identified from the smooth muscles of mouse and pig. However, non-muscle calponin analogues have recently been reported in rat and pig brains. Here we show the presence of calponin expressed in human skin tissue and in cultured human keratinocytes using polyclonal antibodies to bovine aortic smooth muscle calponin. Western blot analysis demonstrated that calponin with a molecular weight around 36,000 existed in extracts of keratinocytes. Immunofluorescence microscopy displayed the localization of calponin in the cytoplasm of the basal cells in situ, and along the cell-to-cell borders in cultured human keratinocytes maintained in standard calcium medium. Furthermore, according to RT-PCR analysis using human h1- and h2-calponin-specific primers, calponin expressed by cultured human keratinocytes was identified as the h2 isoform. We demonstrated the presence of h2-calponin in human keratinocytes, and it might play a role in the structural organization of actin cytoskeleton at the cytoplasmic region of cell-to-cell junctions of keratinocytes.

Increase of calcium levels in epithelial cells induces translocation of calcium-binding proteins migration inhibitory factor-related protein 8 (MRP8) and MRP14 to keratin intermediate filaments

Migration inhibitory factor-related protein 8 (MRP8) and MRP14, two S-100-like Ca(2+)-binding proteins, have been described in cells of the epithelial lineage where they are either expressed constitutively (e.g. by mucosal squamous epithelium) or induced during disease (e.g. in keratinocytes during the course of psoriasis). Their biological function, however, is not yet clear. Recent studies have provided evidence that S-100-like proteins may interact with cytoskeletal components; we have therefore studied the biochemical properties and subcellular distribution of MRP8 and MRP14 in epithelial cells. TR146 human squamous carcinoma cells, which were found to express MRP8 and MRP14 in Northern and Western blot studies, were chosen for analysis. Cross-linking experiments using bis(sulphosuccinimidyl)suberate followed by SDS/PAGE and Western blot analysis revealed formation of heteromeric MRP8-MRP14 complexes. On subjecting TR146 cell lysates to two-dimensional gel electrophoresis and Western blotting, four distinct MRP14 isoforms could be identified resembling those described earlier in macrophages. A differential centrifugation technique revealed a Ca(2+)-dependent translocation of MRP8-MRP14 from the cytoplasm to the membrane and the Nonidet P40-insoluble cytoskeletal fraction. Double-label immunofluorescence microscopy of Ca2+ ionophore A23187-stimulated TR146 cells and cytochalasin B and demecolcine cytoskeleton disruption studies identified these structures as keratin intermediate filaments. Ca(2+)-dependent binding of MRP8-MRP14 to keratin filaments was additionally confirmed by an in vitro binding assay. In conclusion, our data suggest that MRP8 and MRP14 may be involved in Ca(2+)-dependent reorganization of cytoskeletal filaments in epithelial cells, which could be of importance for events associated with differentiation and inflammatory activation.

Influence of short-term hydrostatic pressure on organization of stress fibers in cultured chondrocytes

The present study describes changes in the organization of stress fibers that occur in articular cartilage chondrocytes subjected to hydrostatic pressure. Primary cultures of chondrocytes from bovine articular cartilage, grown on coverslips, were subjected to 5, 15, or 30 MPa hydrostatic pressure at 37 degrees C. The pressure was applied continuously or cyclically at two frequencies: 0.125 Hz (4 seconds of pressure and 4 seconds of no pressure) or 0.05 Hz (1 second of pressure and 19 seconds of no pressure) for a period of 2 hours. Control chondrocytes showed a polygonal form with prominent stress fibers extending across the cells. The exposure of cells to 30 MPa pressure caused a nearly total disappearance of stress fibers and retraction of the cells from each other. With pressure at 15 MPa or cyclic pressure, the number of cells with stress fibers was decreased. In cells subjected to 5 MPa pressure, the stress fibers resembled those in control chondrocytes. The pressure effects were reversible after 2 hours. Pressure had no effect on the staining pattern of vinculin, which suggests that microfilaments are more vulnerable to pressure than vinculin. The results indicate that cytoskeletal changes may be an integral part of the response of chondrocytes to hydrostatic pressure.

E-cadherin mediates adherens junction organization through protein kinase C

Cultured human keratinocytes maintained in 30 microM Ca2+ do not form adherens junctions; however, when the extracellular Ca2+ concentration is raised to 1 mM, adherens junctions form very rapidly. The formation of a junction involves the coordinate organization of intracellular and extracellular components. Cadherins have been shown to mediate this coordinate organization. In this report we show that E-cadherin organizes the various junctional components by signalling through protein kinase C.

Cadherin function is required for human keratinocytes to assemble desmosomes and stratify in response to calcium

Elevation of the calcium concentration in keratinocyte culture induces the rapid organization of adherens junctions and desmosomes. Formation of these intercellular junctions is accompanied by reorganization of the cytoskeleton and, with a more delayed timecourse, stratification of the culture into a multilayered epithelial cell sheet. Keratinocytes express two cadherins, known as E- and P-cadherin, which are the cell-cell adhesion molecules of the adherens junction. Antibody that blocks E-cadherin function delays the calcium-induced formation of both adherens junctions and desmosomes and leads to an abnormally stratified cultured. In the present study, we show that anti-E-cadherin plus anti-P-cadherin antibodies inhibit the formation of adherens junctions and desmosomes, prevent reorganization of the cytoskeleton, and block stratification. These studies indicate that cadherin function is required for the calcium-induced intercellular junction organization and stratification of human keratinocytes in vitro.

Improved barrier structure formation in air-exposed human keratinocyte culture systems

The epidermis (including stratum corneum) of human keratinocytes cultured at the air-liquid interface attached to an appropriate substrate shows a morphology closely mimicking that of its in vivo counterpart. In spite of the histologic similarities, the barrier function seems to be impaired. The aim of the present study was to characterize development and structure of the epidermal permeability barrier in two human skin recombinants using electron microscopy (including ruthenium tetroxide-post fixation technique) and analysis of lipid composition. The epidermis was reconstructed by growing human keratinocytes either on de-epidermized dermis or on a bovine collagen-containing matrix with active fibroblasts (Living Skin Equivalent). Ultrastructurally both culture systems showed a) an abnormal lamellar body delivery system, b) disturbance of transformation into lamellar lipid bilayers, c) an impaired structural organization and distribution of the epidermal lipids in the intercellular spaces. In either of the systems used, prolongation of the culture period did not induce any significant improvement in the stratum corneum lipid organization. Whereas the Living Skin Equivalent showed only sparse lamellar bodies, the number of lamellar bodies in the human keratinocyte culture on de-epidermized dermis grown in regular medium seemed to be comparable to native skin. Contrary to the Living Skin Equivalent, the keratinocyte culture on de-epidermized dermis contained a higher number of intracorneocytic lipid droplets correlating with a higher triglyceride content in the lipid analyses. By reconstructing the keratinocyte culture on de-epidermized dermis with the same medium as used for the Living Skin Equivalent, both lipid composition (lower triglyceride, higher ceramide contents) and structural organization were improved, and regular lamellar lipid bilayers comparable to those of native skin appeared.

An immunofluorescence study of the effects of ultraviolet radiation on the organization of microfilaments, keratin intermediate filaments, and microtubules in human keratinocytes

Indirect immunofluorescence microscopy has been used to investigate the ultraviolet (UV) radiation induced disruption of the organization of microfilaments, keratin intermediate filaments, and microtubules in cultured human epidermal keratinocytes. Following irradiation, concurrent changes in the organization of the three major cytoskeletal components were observed in cells incubated under low Ca2+ (0.15 mM) conditions. UV irradiation induced a dose-dependent condensation of keratin filaments into the perinuclear region. This collapse of the keratin network was accompanied by the reorganization of microfilaments into rings and a restricted distribution of microtubules, responses normally elicited by exposure to high Ca2+ (1.05 mM) medium. The UV induced alteration of the keratin network appears to disrupt the interactions between keratin and actin, permitting the reorganization of actin filaments in the absence of Ca2+ stimulation. In addition to the perinuclear condensation of keratin filaments, UV irradiation inhibits the Ca2+ induced formation of keratin alignments at the membrane of apposed cells if UV treatment precedes exposure to high Ca2+ medium. Incubation of keratinocytes in high Ca2+ medium for 24 hours prior to irradiation results in the stabilization of membrane associated keratin alignments and a reduced susceptibility of cytoplasmic keratin filaments to UV induced disruption. Unlike results from investigations with isogenic skin fibroblasts, no UV induced disassembly of microtubules was discernible in irradiated human keratinocytes.