Morphological and functional differentiation in epithelial cultures obtained from human skin explants

Native cellular fluorescence identifies terminal squamous differentiation of normal oral epithelial cells in culture: a potential chemoprevention biomarker

Native cellular fluorescence (NCF) is being investigated as an intermediate endpoint biomarker for chemoprevention. Oral epithelial cells were cultured under three conditions to identify a spectral pattern for epithelial differentiation: cells maintained in serum-free keratinocyte growth medium were the least differentiated (KGM cells); cells switched to DMEM/F12 plus 10% FCS were intermediate in differentiation (DMEM/F12/FCS cells); DMEM/F12/FCS cells switched to serum-free DMEM/F12 plus 0.8 M NaCl to induce cornified envelopes were the most differentiated (DMEM/F12/NaCl cells). The differentiation status was characterized using immunohistochemistry and electron microscopy. NCF analysis was able to distinguish terminally differentiated epithelial cells (DMEM/F12/NaCl) from those less differentiated cells (KGM, DMEM/F12/FCS) in several emission (lambda ex 340 nm, lambda em 360-660 nm; lambda ex 365 nm, lambda em 400-700 nm; lambda ex 420 nm, lambda em 440-800 nm) and excitation scans (lambda ex 200-360 nm; lambda em 380 nm, lambda ex 240-430 nm; lambda em 450 nm, lambda ex 250-460 nm, lambda em 480 nm; lambda ex 270-500 nm, lambda em 520 nm). The ability to discriminate terminal differentiation in this in vitro model supports the concept of using NCF as an intermediate biomarker to monitor in vivo mucosal differentiation.

Expression of monocyte chemotactic and activating factor (MCAF) in skin related cells. A comparative study

A significant proportion of the infiltrating cells in several inflammatory skin diseases, including psoriasis and allergic contact dermatitis, are monocytes. Additionally, it is known that the cytokine monocyte chemotactic and activating factor (MCAF) can be produced by several cell types present in the skin, suggesting a significant role for MCAF in the accumulation of monocytes during immunological and inflammatory skin reactions. We have recently developed a precise method for quantification of the amount of a specific mRNA species in a given sample and have used this technique to compare specific MCAF mRNA amounts in cultures of human keratinocytes, dermal fibroblasts, endothelial cells, and monocytes, after stimulation with interleukin 1 alpha (IL-1 alpha) for 6 h. Endothelial cells produced very high, monocytes and fibroblasts intermediate, and keratinocytes low amounts of MCAF mRNA. We have also performed time course studies of MCAF mRNA levels in the four cell types. Our findings suggest that the regulation of MCAF mRNA expression in these cells parallels the regulation of the lymphocyte and neutrophil chemotactic factor interleukin 8.

Lateral growth and terminal differentiation during repeated epidermal regeneration in vitro

By incubating multilayered primary cultures of human epidermal keratinocytes in a low calcium medium, the suprabasal layers can be stripped off leaving a basal cell monolayer. When this monolayer is refed normal calcium medium a reproducible series of cell kinetic, morphological and biochemical changes take place resulting in the regeneration of a multilayered tissue. The stripping procedure seems to induce the selective proliferation of a cohort of basal cells that is committed to vertical migration and rapid terminal differentiation. In contrast, when the basal cells are allowed to regenerate in the presence of the strong mitogen, cholera toxin, lateral growth and continued proliferation are favoured at the expense of the capacity of the cells to differentiate. Repeated stripping of the same cultures disclosed a considerable heterogeneity in the capacity of the basal cells to regenerate the suprabasal layers. The number of times the basal cells could restore the suprabasal layers after repeated stripping varied from four to nine times. A negative correlation between donor age and regenerative capacity was observed. The experiments with repeated stripping of the same cultures also showed that the capacity to proliferate and to restore the multilayering was fully retained for at least four cycles of stripping-regeneration, whereas the capacity to terminally differentiate was rapidly lost. It is suggested that the present system of regenerating epidermal tissue cultures may serve as an experimental model for the study of epidermal tissue homeostasis and cellular aging.

Quantitative determination of IL-1 alpha-induced IL-8 mRNA levels in cultured human keratinocytes, dermal fibroblasts, endothelial cells, and monocytes

The presence of the leukocyte chemotactic cytokine interleukin 8 (IL-8) in psoriatic scales and in epidermal tissue overlying allergic patch test reactions suggests a role for this cytokine in certain inflammatory skin diseases. IL-8 can be produced by several cell types present in the skin. Their relative potentials for IL-8 expression has, however, not yet been studied, due to the lack of convenient methods for quantitative comparison of specific mRNA amounts in different cell types. Using a new method for quantification, we compared specific IL-8 mRNA amounts in cultures of keratinocytes, dermal fibroblasts, endothelial cells, and monocytes, stimulated with interleukin 1 alpha (IL-1 alpha). Endothelial cells produced very high, fibroblasts and monocytes intermediate, and keratinocytes low amounts of IL-8 mRNA. We also studied the time course of IL-8 mRNA levels in the four cell types following IL-1 alpha stimulation, and found a clear difference both in onset and stability of the response. We discuss the different strength of the response at different time points in the cell types analyzed in relation to their possible role in regulation of the normal response to stimulation.

1,25(OH)2-D3 is a potent regulator of interleukin-1 induced interleukin-8 expression and production

Interleukin 8 (IL-8) is a potent leukocyte chemotactic and activating cytokine produced by keratinocytes, fibroblasts, peripheral blood monocytes (PBMC) and endothelial cells. IL-8 is believed to play an important role in the development of inflammation and is thus an obvious target for therapeutical modulation. We studied the possible effect of an endogenous immune modulator 1,25(OH)2-cholecalciferol (1,25(OH)2-D3) on the IL-1-induced IL-8-production by several types of cells. 1,25(OH)2-D3 inhibited the IL-1-alpha induced IL-8 production and mRNA expression in keratinocytes, fibroblasts and PBMC, but not in endothelial cells. Optimal vitamin concentrations varied between 10(-10) and 10(-11) M. These results suggest a potential role of this hormone in the regulation of chemotactic cytokine production.

Morphological differentiation and changes in polypeptide synthesis pattern during regeneration of human epidermal tissue developed in vitro

By incubating multilayered primary cultures of human keratinocytes in low-calcium medium the suprabasal cell layers can be stripped off leaving a basal cell monolayer. When this monolayer is re-fed normal calcium medium a reproducible series of cell kinetic, morphological, and biochemical changes takes place resulting in the reestablishment of a multilayered tissue. Analysis of cell-cycle-specific proteins indicated that, during regeneration, a large cohort of cells became synchronized undergoing DNA replication after 3 days. Examination of culture morphology at the ultrastructural level confirmed the capacity of the basal cell monolayer to gradually reestablish a multilayered, differentiated epithelium. The ultrastructural appearance at 7 days poststripping was similar to that of unstripped cultures and was indicative of a tissue in steady state. Quantitation of cornified envelope formation at different times during regeneration showed that an increasing proportion of the cells were able to undergo terminal differentiation. In general, the pattern of keratin synthesis in the original epidermal explant labelled in vitro was similar to the pattern observed in human epidermis in vivo; however, in contrast to epidermis in vivo the explant also synthesized the hyperproliferative keratins 6 and 16. The in vitro differentiated keratinocytes showed underexpression of several proteins identified as differentiation markers, whereas several basal cell markers were overexpressed compared to the original explant. In addition, the in vitro differentiated keratinocytes synthesized some new proteins, notably keratins 7, 15 and 19. The basal layer remaining after stripping mainly expressed basal cell markers; however, during recovery, some of the differentiation-specific markers (e.g. keratin 10 and 15) were again expressed together with keratin no. 19, which is also expressed during wound healing in vivo. It is suggested that the present system of regenerating epidermal tissue cultures may serve as an experimental model to investigate certain aspects of the regulation of epidermal tissue homeostasis.

Effects of extra- and intracellular calcium concentration on DNA replication, lateral growth, and differentiation of human epidermal cells in culture

Variation in the extra- and intra-cellular concentration of calcium ([Ca]e and [Ca]i) affected the 3H-thymidine labeling pattern of sorted S-phase cells in human epidermal cultures. A lowering of [Ca]e resulted in retarded lateral growth but, unless [Ca]e was extremely low, caused an increase in the proportion of strongly labelled (rapidly cycling) S-phase cells. An increased desquamation of superficial cells due to a reduced cellular cohesiveness was also observed in low calcium medium. Thus, a lowering of [Ca]e might stimulate the proliferation of a pool of cycling cells destined for rapid terminal differentiation and tissue regeneration, whereas proliferation destined for lateral growth is inhibited. Attempts to decrease the [Ca]i with the calcium chelator quin-2 at low [Ca]e seemed to elevate the proportion of strongly labelled S-phase cells, whereas an increased [Ca]i obtained with the ionophore A23187 caused a dramatic decrease in the proportion of S-phase cells that showed strong 3H-thymidine incorporation. This implies that variation in both [Ca]i and [Ca]e may play a role in the regulation of proliferation and differentiation, in keratinocytes.

High-frequency transfection of cultured human epidermal basal cells that differentiate to form a multilayered tissue

Primary cultures of human keratinocytes form a multilayered tissue. By incubating the tissue cultures in Ca2(+)-free medium the differentiated cell layers can be stripped off leaving a basal cell monolayer. We have developed a method for high-frequency transfection of these epidermal basal cells with genes inserted into Epstein-Barr virus-based expression vectors. Using the Escherichia coli lac z gene as a marker gene, the transient and long-term expression and the fate of the transfected cells were studied. During regeneration of the multilayered tissue most of the transfected basal cells enlarge and undergo differentiation, but a minor population remains as basal cells. Incubation with the tumor promotor 12-O-tetradecanoylphorbol-13-acetate results in an increase in the proportion of transfected keratinocytes that are small, suggesting a relative expansion of the immature cell pool.

Low Ca2+ stripping of differentiating cell layers in human epidermal cultures: an in vitro model of epidermal regeneration

An in vitro model of the epidermal regeneration process is described. Incubation of multilayered, keratinizing cultures of human epidermal cells in Ca2+-free medium for 72 h results in a complete stripping of all suprabasal layers. When the Ca2+ stripped cultures are refed normal Ca2+ medium a reproducible series of morphologic and cell kinetic changes takes place. It is suggested that these changes represent a general pattern of regeneration after epidermal wounding. After an initial lag phase the regenerative response is principally effected by a recruitment to the proliferating pool of cells with a high rate of DNA synthesis. The cells seem to be programmed to rapid differentiation. Studies with cholera toxin suggest that this adenylate cyclase-stimulating agent is able to induce significant changes in the regenerative process causing a prolonged, but less intense, proliferative response leading to lateral growth rather than to rapid differentiation.

Cell cycle progression kinetics of regenerating mouse epidermal cells: an in vivo study combining DNA flow cytometry, cell sorting, and [3H]dThd autoradiography

Cantharidin application to mouse skin induces cell injury followed by a regenerative wave of cells entering S phase in partial synchrony about 16 h after application. After pulse labeling with [3H]dThd the synchronized cohort of cells was traced through subsequent cell cycles during regeneration. This was accomplished by DNA flow cytometry of isolated basal cells combined with sorting from G1, S, and G2 phases followed by autoradiography at intervals after pulse labeling. Successive peaks of labeled cells in S phase at about 12-h intervals, followed by subsequent peaks in G2 and G1 phases were seen. This shows that the peaks of S-phase cells seen at 16 and 28 h after cantharidin application represent mother and daughter cells, respectively, the latter still cycling in partial synchrony. These 2 peaks of S-phase cells, therefore, are not keratinocyte subpopulations with different time lags between the stimulus to regeneration and the subsequent response. It is further shown that the mean cell cycle time is reduced from about 55 h in normal epidermis to 12 h during early regeneration. This is mainly due to a considerably reduced G1 phase duration, but the S and G2 phase durations are also reduced, although still within the range of circadian variations seen in normal animals. It is reasonable to assume a causal relationship between the considerably reduced G1 duration and loss of growth restriction. Cells with a slow progression rate through G2 phase (70% of all G2 cells) in normal mouse epidermis seem to maintain a slow progression rate during regeneration. Normal growth homeostasis seems to be gradually reestablished during the second day of regeneration.

Changes in proliferating cell subpopulations and mitotic activity in human epidermal cultures treated with epithelial growth inhibitors

Labeling of cultured human epidermal cells with [3H]thymidine has revealed a dramatic heterogeneity among sorted S-phase cells. Cell kinetic studies have shown that these differences in labeling intensity most probably reflect differences in the rate of DNA synthesis, and cycling basal cells may be divided into subpopulations on this basis. Studies with growth stimulators have suggested that these subpopulations are involved in cell renewal or population expansion during early differentiation of the keratinocyte. In the present study the effects of an epidermal growth inhibitor purified from an epidermis extract and a kidney epithelial growth inhibitor obtained from conditioned medium of BSC-1 cell cultures were investigated. Both agents were shown to cause a dramatic decrease in mitotic activity in the epidermal cultures and also to diminish the proportion of S-phase cells with a strong thymidine incorporation (high rate of DNA replication). The effect of the BSC-1 growth inhibitor was furthermore shown to be counteracted by epidermal growth factor and cholera toxin.

Changes in basal cell subpopulations and tissue differentiation in human epidermal cultures treated with epidermal growth factor and cholera toxin

Cell kinetic studies on cultured human epidermal cells have indicated that cycling basal cells may be divided into at least two subpopulations that seem to differ with respect to the rate of DNA replication. The present study was undertaken in order to elucidate the biological significance of these subpopulations. The proliferation characteristics of cultured basal cells were changed by the addition of epidermal growth factor (EGF) and cholera toxin to the culture medium. It was shown that EGF and cholera toxin stimulated the growth of human epidermal cells in culture. Simultaneously, the terminal differentiation of the cells was inhibited resulting in a reduced multilayering and a reduced formation of the cornified envelope. However, only minor differences in the protein synthesis pattern were observed between cultures maintained in the presence or absence of the growth stimulators. The effect of EGF and cholera toxin on the basal cell subpopulations was investigated after 3H-thymidine labelling followed by cell sorting and autoradiography. In the presence of EGF and cholera toxin dramatic changes were induced in the labelling pattern of sorted S-phase cells indicating significant alterations in the balance between the subpopulations of cycling basal cells. Our results with these substances are in accord with the hypothesis that the observed cell kinetic subpopulations may be related to regeneration or early events in the differentiation process of the keratinocyte.

Basal-cell subpopulations and cell-cycle kinetics in human epidermal explant cultures

Cultured human epidermal cells were studied by cell sorting and autoradiography after different 3H-thymidine (3H-dThd)-labelling procedures and after labelling with DNA precursors that are incorporated via salvage or de novo pathways. It was shown that 3H-dThd incorporation was the best measure of the rate of DNA replication. Dose-response experiments with pulse and continuous labelling revealed that all S- and G2-phase cells were cycling, whereas some 20% of the cells stayed in G1-phase for long periods of time. Most, if not all of these cells were probably non-proliferating differentiated keratinocytes. At least two subpopulations of S-phase cells could be discriminated on the basis of the rate of incorporation of DNA precursors. The difference in precursor incorporation did not seem to be caused by differences in nucleotide metabolism but rather to reflect true differences in the rate of DNA replication. Continuous labelling experiments showed that these subpopulations also were apparent in the G1- and G2-phases. Studies of the grain-count distribution revealed that cells that appeared to move rapidly through the S-phase moved slowly through the G2-phase, and vice versa. Cells stained with acridine orange were subjected to a two-parameter analysis in the cell sorter by simultaneous measurement of the DNA and RNA fluorescence. Autoradiography of sorted cells revealed that, on average, cells with low RNA contents incorporated 3H-dThd at a higher rate than cells with high RNA contents.