Human keratinocyte culture. Identification and staging of epidermal cell subpopulations

Gene expression analysis of skin grafts and cultured keratinocytes using synthetic RNA normalization reveals insights into differentiation and growth control

Background

Keratinocytes (KCs) are the most frequent cells in the epidermis, and they are often isolated and cultured in vitro to study the molecular biology of the skin. Cultured primary cells and various immortalized cells have been frequently used as skin models but their comparability to intact skin has been questioned. Moreover, when analyzing KC transcriptomes, fluctuation of polyA+ RNA content during the KCs’ lifecycle has been omitted.

Results

We performed STRT RNA sequencing on 10 ng samples of total RNA from three different sample types: i) epidermal tissue (split-thickness skin grafts), ii) cultured primary KCs, and iii) HaCaT cell line. We observed significant variation in cellular polyA+ RNA content between tissue and cell culture samples of KCs. The use of synthetic RNAs and SAMstrt in normalization enabled comparison of gene expression levels in the highly heterogenous samples and facilitated discovery of differences between the tissue samples and cultured cells. The transcriptome analysis sensitively revealed genes involved in KC differentiation in skin grafts and cell cycle regulation related genes in cultured KCs and emphasized the fluctuation of transcription factors and non-coding RNAs associated to sample types.

Conclusions

The epidermal keratinocytes derived from tissue and cell culture samples showed highly different polyA+ RNA contents. The use of SAMstrt and synthetic RNA based normalization allowed the comparison between tissue and cell culture samples and thus proved to be valuable tools for RNA-seq analysis with translational approach. Transciptomics revealed clear difference both between tissue and cell culture samples and between primary KCs and immortalized HaCaT cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1671-5) contains supplementary material, which is available to authorized users.

Human Wound-Healing Research: Issues and Perspectives for Studies Using Wide-Scale Analytic Platforms

Significance: Revealing the basic mechanisms in the healing process and then regulating these processes for faster healing or to avoid negative outcomes such as infection or scarring are fundamental to wound research. The normal healing process is basically known, but to thoroughly understand the very complex aspects involved, it is necessary to characterize the course of events at a higher resolution with the latest molecular techniques and methodologies. Recent Advances: Various animal models are used in wound-healing research. Rodent and pig models are the ones most often used, probably because of pre-existing sophisticated research methodologies and as the proper care and ethical use of these species are highly developed and organized to serve science throughout the world. Critical Issues: Since several animal models are used, their anatomical and physiological differences varyingly affect the translation of results on healing mechanisms. Hence, to avoid species-specific misinformation, more ways to study wound healing directly in humans are needed. Future Directions: Fortunately, novel techniques have enabled high-end molecular-level research even from small samples of tissue. Since these methods require only a small amount of patient skin, they make it possible to study wound healing directly in humans.

Label retaining cells and cutaneous stem cells

This is a comprehensive review on label retaining cells (LRC) in epidermal development and homeostasis. The precise in vivo identification and location of epidermal stem cells is a crucial issue in cutaneous biology. We discuss here the following problems: (1) Identification and location of LRC in the interfollicular epithelium and hair follicle; (2) The proliferative potential of LRC and their role in cutaneous homeostasis (3); LRC phenomenon and the Immortal Strand Hypothesis, which suggests an alternative mechanism for retention of genetic information; (4) Significance of LRC studies for development of stem cell concept. Now, it seems evident that LRC are a frequent feature of stem cell niches and revealing highly dormant LRC may be used for identification of stem cell niches in different tissues. LRC were used for screening specific markers of epidermal stem cells. Within a given tissue stem cells have different proliferative characteristics. There are more frequently cycling stem cells which function primarily in homeostasis, while LRC form a reserve of dormant, may be ultimate, stem cells, which are set aside for regeneration of injury or unforeseen need. The authors suggest that LRC dormancy described in Mammalia has much in common with developmental quiescence found in some other animals. For example in C. elegans reproductive system, vulval precursor cells have developmentally programmed cell-cycle arrest in the first larval stage, and then undergo an extended period of quiescence before resuming proliferation. Another example of developmental quiescence is the diapause, a widespread phenomenon exhibited by animals ranging from nematodes to mammals, often occurring at genetically predetermined life history stage.

Comparison of two cellular harvesting methods for primary human oral culture of keratinocytes

The possibility of obtaining transplantable oral epithelia opens new perspectives for oral treatments. Most of them are surgical, resulting in mucosal failures. As reconstructive material this in vitro epithelia would be also useful for other parts of the human body. Many researchers still use controversial methods; therefore it was evaluated and compared the efficiency of the enzymatic and direct explant methods to obtain oral keratinocytes. To this project oral epithelia fragments were used. This work compared: time needed for cell obtainment, best cell amount, life-span and epithelia forming cell capacity. The results showed the possibility to obtain keratinocytes from a small oral fragment and we could verify the advantages and peculiar restrictions. We concluded that under our conditions the enzymatic method showed the best results: in the cells obtaining time needed, cell amount and life-span. Both methods showed the same capacity to form in vitro epithelia.

PAI-1 transcriptional regulation during the G0 --> G1 transition in human epidermal keratinocytes

Plasminogen activator inhibitor type-1 (PAI-1) is the major negative regulator of the plasmin-dependent pericellular proteolytic cascade. PAI-1 gene expression is normally growth state regulated but frequently elevated in chronic fibroproliferative and neoplastic diseases affecting both stromal restructuring and cellular migratory activities. Kinetic modeling of cell cycle transit in synchronized human keratinocytes (HaCaT cells) indicated that PAI-1 transcription occurred early after serum stimulation of quiescent (G0) cells and prior to entry into a cycling G1 condition. PAI-1 repression (in G0) was associated with upstream stimulatory factor-1 (USF-1) occupancy of two consensus E box motifs (5'-CACGTG-3') at the PE1 and PE2 domains in the PF1 region (nucleotides -794 to -532) of the PAI-1 promoter. Chromatin immunoprecipitation (ChIP) analysis established that the PE1 and PE2 site E boxes were occupied by USF-1 in quiescent cells and by USF-2 in serum-activated, PAI-1-expressing keratinocytes. This reciprocal and growth state-dependent residence of USF family members (USF-1 vs. USF-2) at PE1/PE2 region chromatin characterized the G0 --> G1 transition period and the transcriptional status of the PAI-1 gene. A consensus E box motif was required for USF/E box interactions, as a CG --> AT substitution at the two central nucleotides inhibited formation of USF/probe complexes. The 5' flanking sites (AAT or AGAC) in the PE2 segment were not necessary for USF binding. USF recognition of the PE1/PE2 region E box sites required phosphorylation with several potential involved residues, including T153, maping to the USF-specific region (USR). A T153A substitution in USF-1 did not repress serum-induced PAI-1 expression whereas the T153D mutant was an effective suppressor. As anticipated from the ChIP results, transfection of wild-type USF-2 failed to inhibit PAI-1 induction. Collectively, these data suggest that USF family members are important regulators of PAI-1 gene control during serum-stimulated recruitment of quiescent human epithelial cells into the growth cycle.

A high yield method for growing primary canine keratinocytes

From a small amount of starting material, a large quantity of canine keratinocytes can be generated for experimental purposes using a refined method of explant culture to initiate the growth of basal cells with a high proliferative potential. The dividing capacity of cultures was promoted by a system selecting clonogenic cells onto an i3T3 feeder layer in combination with carefully monitoring cell morphology and passaging to select out excessive numbers of differentiated keratinocytes. Levels of contaminating dermal fibroblasts, which if left unchecked will overgrow keratinocytes, were kept to a minimum by a combination of careful explant micro-dissection to remove dermis, eliminating explants with signs of fibroblast growth as well as using cholera toxin, EGF and i3T3 feeder layers. The advantage of the method described is that it does not rely on the provision of large quantities of starting material thereby reducing the need for repeated tissue sampling, and passage numbers of five or six can be routinely achieved. This technique can therefore be useful to experimenters who require a regular and reliable source of cells for their studies.

Evidence for local control of gene expression in the epidermal differentiation complex

The epidermal differentiation complex (EDC), located on chromosomal band 1q21, consists of at least 43 genes that are expressed during keratinocyte differentiation. Indicative of a role for chromatin structure in tissue specificity of EDC gene expression, we identified an inverse correlation between expression and DNA methylation for two EDC genes (S100A2 and S00A6) in human keratinocytes and fibroblasts. 5-azacytidine (5AC) and sodium butyrate (NaB) are two agents known to promote 'open' chromatin structure. To explore the relationship between chromatin structure and keratinocyte differentiation, we treated normal human keratinocytes (NHK) with 5AC or NaB, or with protocols known to promote their terminal differentiation. We then measured the steady-state mRNA levels for several S100 genes, small proline rich region-1, -2, and -3, loricrin, and involucrin by Northern blotting. 5AC and NaB each markedly increased expression of SPRR1/2 and involucrin in NHK. In contrast, expression of S100A2 was reduced by both agents, and by induction of keratinocyte differentiation. Moreover, while the clustered EDC genes displayed a general tendency to be expressed in epithelial cells, they displayed different patterns of cell type-specific expression. These results indicate that local, gene-specific factors play an important role in the regulation of EDC gene expression in the keratinocyte lineage and during keratinocyte terminal differentiation.

In vitro construction of a potential skin substitute through direct human keratinocyte plating onto decellularized glycerol-preserved allodermis

This work demonstrates that glycerol-preserved acellular allodermis can be used as support for the proliferation of human keratinocytes and that the characteristics of this bioengineered tissue suggest its possible use as a permanent skin substitute for therapeutic challenges such as extensive burns as well as its possible use as an in vitro model for pharmacological studies. The removal of all basal membrane components during preparation of the dermal support also provides an original in vitro situation that allows observation of the reorganization of the dermal-epidermal junction. The tissue composite obtained is constituted of dermis covered by a well attached, multistratified epithelium with morphological characteristics that resemble human epidermis as evidenced by light and transmission electron microscopy, including the neoformation, albeit incomplete, of the dermal-epidermal junction. Assessment of involucrin and cytokeratin 14 expression by immunohistochemical assays established differentiation patterns. Both immerse and air-liquid interface culture systems were tested.

Changes in the ultrastructure of cytoskeleton and nuclear matrix during HaCaT keratinocyte differentiation

The cellular scaffold that comprises nuclear matrix and cytoskeleton provides mechanical support for the cell and plays a crucial role in motility, cellular signaling, regulation of gene transcription and DNA replication. In this study we examined the structure of cytoskeleton and nuclear matrix in the keratinocyte cell line HaCaT using a recently developed technique, embedment-free electron microscopy. With this method the three-dimensional structure of cellular scaffold is visualized in the cells extracted from soluble proteins and the chromatin. In actively proliferating cells the cytoskeleton appeared to consist of a continuous meshwork of 10--15 nm filaments with a smaller amount of thin (5 nm) and ultrathin (1--2 nm) filaments. In contrast to what could be expected from earlier immunofluorescence and electron microscopy studies, the cytoskeleton in HaCaT keratinocytes did not consist of superposed autonomous networks of different filaments but was a highly integrated, continuous structure filling whole cytoplasmic territory. Moreover, cytoskeletons of adjacent cells were in a direct physical contact. Nuclear matrix consisted of globular ribonucleoprotein aggregates attached to the meshwork of 20--40 nm filaments. Nuclear envelope was firmly fastened to the cytoskeleton. In keratinocytes induced to differentiation by calcium switch both the cytoskeleton and nuclear matrix were drastically rearranged and comprised a monomorphic, dense and regular meshwork of 10--15 nm filaments. Our data underscore the fact that in HaCaT keratinocyte monolayer in vitro, and probably also in the epidermis in vivo, the nuclear matrices and the cytoskeletons of adjacent cells seemed to form a continuous, highly ordered structure which is rapidly rearranged during cell differentiation. This feature may be crucial for the understanding of how the signal initiated by, e.g. mechanical forces generated through the cell--cell and cell--matrix interaction is transmitted to the nucleus producing ultimately changes in the pattern of gene expression.

Retroviral transduction of murine epidermal stem cells demonstrates clonal units of epidermal structure

It has been suggested that the number and position of epidermal stem cells are related to the units of columnar structure in the upper epidermal strata and that the cells of each unit are derived from a single stem cell. Studies of cell lineage in developing tissues have been facilitated by the use of retroviral transduction to provide inherited expression of a histochemically demonstrable foreign gene product. To provide direct evidence about the clonal nature of epidermal units, murine epidermal keratinocytes were transduced with a replication-deficient retroviral vector carrying the beta-galactosidase gene. Subepidermal injection of virus in vivo led to infrequent transduction with only transient presence of beta-gal-staining keratinocytes within the epidermis. Transduction of keratinocytes in vitro and transplantation back to in vivo sites permitted demonstration of the transduced gene in clusters of cells within the reformed epidermis throughout a 12-wk period. The epidermis redeveloped an ordered columnar structure with restriction of transduced cells to individual columnar units. This clonal appearance is compatible with derivation of each epidermal unit from a single stem cell but is not compatible with a random pattern of cell proliferation. Transduced epidermal sheets that were recombined with oral mucosal connective tissue also redeveloped normal columnar structure with restriction of beta-gal staining to individual columnar units. These data suggest that the establishment of an epidermal stem cell pattern related to units of structure is an intrinsic property of the epithelium and is not dependent on regionally-specific connective tissue influences.

FK506 and cyclosporin A inhibit growth factor-stimulated human keratinocyte proliferation by blocking cells in the G0/G1 phases of the cell cycle

FK506, a new immunosuppressive agent, is effective in treating patients with psoriasis. A major feature of psoriasis vulgaris is the hyperproliferation of keratinocytes together with inflammation. To determine the effect of FK506 or cyclosporin A (CsA) on the keratinocyte cell cycle, flow cytometry and the growth factor free normal human keratinocyte-arrested system were used to assess keratinocyte proliferation. FK506 and CsA inhibit keratinocyte proliferation induced by EGF, TGF-alpha or IL-6. The antiproliferative effects of FK506 and CsA directly correlated with blockade of the keratinocyte cell cycle at the G0/G1 phases. These findings might indicate that the effects of FK506 and CsA on proliferation of cultured normal human keratinocytes are probably related to direct effects on growth regulation of keratinocytes via EGF, TGF-alpha or IL-6 stimulation.

In vivo effects of vitamin D on the proliferation and differentiation of rat keratinocytes

The hormonal form of vitamin D appears to be a physiological regulator of the epidermogenesis. While its differentiation-promoting effect is well accepted, there are conflicting reports of its action on keratinocyte proliferation. This study evaluates the specific changes induced by vitamin D treatment in the epidermis of rats nutritionally deprived of vitamin D by cell size analysis, acridine orange flowcytometry, and the immunohistochemical detection of proteins related to the different stages of differentiation (epidermal calcium binding protein and suprabasal keratins recognized by KL1 antibody) The total keratinocyte and isolated keratinocyte subpopulations were studied. Vitamin D deficiency was associated in the total population with a lower percentage of actively proliferating cells and with a lack of differentiation markers. Study of the isolated cell populations demonstrated, however, that small cells were actively proliferating, whereas they were mainly in the resting stage in the normal epidermis. Treatment with vitamin D dramatically increased cell proliferation and stimulated the appearance of differentiation markers. Some of the observed effects, such as an increase in proliferation and the appearance of epidermal calcium binding protein, were due to the normalisation of the vitamin D deficiency-induced hypocalcemia, whereas the expression of suprabasal keratins was directly dependent on vitamin D. We conclude that the action of vitamin D on the epidermis is associated with increases in both proliferation and differentiation of keratinocytes. Vitamin D itself and its resulting action on calcium homeostasis appear to contribute to the observed effects.

Transplantation of cryopreserved cultured epidermal allografts

OBJECTIVE

To optimize cryopreservation methods for cultured epidermal allografts (CEAs) for transplantation onto wounds.

DESIGN

Conditions were determined to optimize the cryopreservation of CEAs. Cryopreserved CEAs were then grafted onto 16 donor sites in a double blind randomized trial.

MATERIALS AND METHODS

CEAs were grown in culture. Viability of cryopreserved CEAs was determined by: 1) trypan blue dye exclusion; 2) histology; 3) flow cytometry; and 4) acid lipase activity. Cryopreserved CEAs were grafted onto 16 donor sites. Wound healing assessments included 1) visual assessment of healing; 2) histologic assessment of re-epithelialization and differentiation; and 3) visual scar assessment.

MEASUREMENTS AND MAIN RESULTS

CEAs were cryopreserved using a controlled rate freezer with an experimentally determined setting of -7 degrees C for the CEA freezing point. Such freezing conditions resulted in retention of approximately 92% of the original viability, no loss in basal keratinocytes as determined by flow cytometry, and no change in acid lipase activity. CEAs cryopreserved according to this method were grafted onto 16 donor sites in a double blind randomized trial. All donor sites underwent complete healing. Histologic examination of biopsies taken from the center of the wound beds showed that CEA-treated wounds significantly accelerated the rate of re-epithelialization (7.8 +/- 0.6 days vs. 9.2 +/- 0.9 days for CEA- and control-treated wounds, respectively; p = 0.039) and epithelial differentiation (p = 0.023) compared with control sites. Longterm results showed that CEA-treated wounds were comparable to control sites with regard to: 1) pigmentation; 2) scar height; 3) scar pliability; 4) vascularity; and 5) pain. Wounds treated with cryopreserved CEAs remained durable and not prone to blistering after healing.

CONCLUSIONS

CEAs can be successfully cryopreserved for long-term storage. Upon retrieval from storage, CEAs may be used to treat partial thickness wounds.

The effects of burn blister fluid on cultured keratinocytes

OBJECTIVE

Previous studies have suggested that burn blister fluid (BBF) may be detrimental to the healing of the underlying wound bed. In this study, the effects of burn blister fluid on cultured keratinocyte proliferation and differentiation were examined and quantitated using various techniques.

METHODS

At three different concentrations (2%, 10%, 20% in 20% fetal bovine serum/complete culture medium), 19 BBFs were tested in triplicate using 12 populations of cultured keratinocytes. All BBFs were collected from partial thickness burns within 72 hours of injury,. BBF was added on day 4 of the keratinocyte culture. The effect on proliferation and viability was assessed using trypan blue dye exclusion. Multiparameter flow cytometric analysis was used to quantitate population kinetics and cell size distribution. Keratinocyte differentiation was determined using immunohistochemical staining of differentiation markers and quantitation of cornified envelope formation.

RESULTS

Relative to control fluid, the BBF caused a variable effect on proliferation, ranging from 67% inhibition to 103% stimulation with an overall 4% inhibition. The range of keratinocyte viability was narrower, with a similar overall 4% reduction. Using flow cytometry to analyze RNA/DNA content and cell size, the BBF caused a subtle shift in keratinocyte population kinetics and cell size distribution toward larger, less rapidly dividing cells. The BBF had no significant effect on expression of the differentiation markers, filaggrin and involucrin. Finally, the BBF did not alter terminal differentiation as it did not influence formation of cornified envelopes (BBF = 9.1 +/- 4.8%, control = 9.9 +/- 6.6%).

CONCLUSION

Previous biochemical analysis has shown that BBF consists primarily of human serum filtrate with locally produced acute reactants. Our study suggests that BBF is biologically similar to serum and does not significantly alter keratinocyte proliferation or differentiation in vitro.

Cell-shape-dependent modulation of p52(PAI-1) gene expression involves a secondary response pathway

Expression of the rat p52(PAI-1) gene is positively regulated by agents that influence cellular microfilament organization and/or cell-to-substrate adhesion [e.g. cytochalasin D (CD) and sodium n-butyrate (NaB)] [Higgins, Chaudhari and Ryan (1991) Biochem. J. 273, 651-658; Higgins, Ryan and Providence (1994) J. Cell. Physiol. 159, 187-195]. As shape-responsive genes may be subject to inducer-specific controls, the biochemical mechanisms underlying the shape-dependent pathway of p52(PAI-1) gene regulation were examined in v-ras-transformed rat kidney (KNRK) cells. NaB and/or CD effectively stimulated p52(PAI-1) run-off transcription and augmented de novo p52(PAI-1) mRNA and protein synthesis in KNRK cells; induction at both the mRNA and protein levels was inhibited by actinomycin D. Pretreatment with cycloheximide (CX) markedly attenuated NaB- and/or CD-stimulated p52(PAI-1) expression. CX alone, however, induced low levels of p52(PAI-1) mRNA; increased p52(PAI-1) protein synthesis was evident after release of KNRK cells from CX blockade. Such CX-mediated induction was also sensitive to actinomycin D. Full stimulation of p52(PAI-1) expression in KNRK cells in response to the shape modulators NaB and/or CD involves transcriptional activation of the p52(PAI-1) gene, requires de novo RNA synthesis and occurs through a secondary-response (i.e. protein-synthesis-dependent) pathway.

Synthesis of and response to 1,25 dihydroxycholecalciferol by subpopulations of murine epidermal keratinocytes: existence of a paracrine system for 1,25 dihydroxycholecalciferol

The epidermis is both a target tissue for and a source of 1,25 dihydroxycholecalciferol. The present study determines which of the epidermal cell populations synthesizes 1,25 dihydroxycholecalciferol and which responds to this hormone. Epidermal keratinocytes from new born rat epidermis were separated by unit gravity sedimentation into poorly differentiated cells, slow-cycling more differentiated cells, actively proliferating cells, and terminally differentiating subpopulations. The keratinocyte populations were characterized by cell size analysis, cell morphology, and DNA and RNA contents (acridine orange flow cytometry). 1,25(OH)2D3 synthesis was studied by measuring the conversion of [3H] 25(OH)D3 to [3H] 1,25(OH)2D3. The purified product was tested for its ability to compete with synthetic [3H] 1,25(OH)2D3 for binding to chick intestinal cytosol. The responses of the keratinocyte subpopulations to exogenous 1,25(OH)2D3 were evaluated by the increase in 25(OH)D3-24 hydroxylase activity. Furthermore the expression of 1,25(OH)2D3 receptors (VDR) was examined in these cell populations. The results show that only the least differentiated cells produced 1,25(OH)2D3. In contrast, immunocytochemical detection of VDR, the VDR mRNA, and a 25(OH)D3-24 hydroxylase response to 1,25(OH)2D3 were mainly found in the more differentiated cells. Thus, the ability of epidermis to synthesize 1,25(OH)2D3 and be simultaneously sensitive to it depends on the state of cell differentiation. This suggests that the mammalian epidermis contains a paracrine system in which the more differentiated keratinocytes are sensitive to the 1,25(OH)2D3 produced locally by neighboring immature ones.

The growth and differentiation of human keratinocytes in vitro: a combined immunohistochemical and flow cytometric study

In this study we performed a cell kinetic characterization of the growth and differentiation of human keratinocytes. Using a combination of immunohistochemical and flow cytometric techniques it was possible to obtain a detailed description of these processes. The proliferative activity of the cell cultures was analysed with flow cytometric techniques, measuring relative DNA content, iododeoxyuridine incorporation and the expression of the antigen recognized by Ki-67. In addition to a standard monolayer culture technique, cells were maintained in suspension. Under these conditions these cells were not capable of dividing, started to lose their nuclei, and the expression of differentiation-related proteins such as involucrin and filaggrin was induced, suggesting that the cells changed towards a differentiated phenotype. Binding of the antibody Ks8.12, recognizing keratins 13 and 16, occurred under all culture conditions, independent of cell density, and also in suspension, suggesting that it is a marker for abnormal differentiation rather than for hyperproliferation.

Upper human hair follicle contains a subpopulation of keratinocytes with superior in vitro proliferative potential

We and others have shown previously that corneal keratinocyte stem cells can proliferate in vitro better than their progeny cells. In this paper, we applied this approach to the identification of hair follicular stem cells. When human scalp hair follicles were placed in explant culture, the bulge area yielded best outgrowths. In another experiment, we isolated different subpopulations of human follicular keratinocytes by micro-dissection, dispersed them by trypsin/EDTA into single cells, and grew them in the presence of 3T3 feeder cells. The keratinocytes were then subcultured under identical conditions to compare their in vitro life span. Our results indicate that the life span of keratinocytes of the upper follicle (containing mainly the isthmus area) > sebaceous gland > lower follicle (between the bulge and bulb) > bulb (containing the matrix cells). The cultured upper follicular keratinocytes tend to be small and relatively uniform in size. The poor in vitro growth of matrix cells may reflect their non-stem cell nature and/or special growth requirement(s) satisfied in vivo by the neighboring dermal papilla cells. Unexpectedly, we found that the upper follicular keratinocytes grow even better than epidermal keratinocytes. The existence of a subpopulation of keratinocytes with an in vitro growth potential superior than other known keratinocytes of the skin supports the hypothesis that follicular stem cells reside in the upper follicle. Our data also raise the possibility that putative follicular stem cells are involved not only in forming the follicle, but also in the long-term maintenance of the epidermis. Finally, we discuss the possibility that keratinocyte stem cells, as defined by their in vivo slow-cycling nature, are absent in culture.

Characteristics of oligonucleotide uptake in human keratinocyte cultures

Oligodeoxyribonucleotides have the potential to interfere selectively with cellular protein synthesis by sequence-specific hybridization to DNA or RNA molecules. We have investigated the properties of uptake and intracellular localization of fluorescently labeled oligonucleotides in cultured human keratinocytes using confocal laser scanning microscopy. Unlike many other cell types studied, keratinocytes can internalize oligonucleotides without apparent sequestration in endosomes or cell surface accumulation. Uptake is primarily nuclear and unaltered by sodium azide, monensin, or chloroquine pretreatment. We have verified our results with two different fluorophores, fluorescein and Bodipy, and found similar uptake and distribution patterns in both live and fixed cell populations. Surprisingly, we have found uptake to be heterogeneous within a population, with 15-30% of cells internalizing the oligonucleotides. This percentage is drastically increased to roughly 80% at cell population margins, and after release from M phase arrest. These results on uptake and intracellular localization suggest that keratinocytes may have increased sensitivity as target cells for oligonucleotide based gene regulation strategies.

Flow cytometric identification of proliferative subpopulations within normal human epidermis and the localization of the primary hyperproliferative population in psoriasis

In this study we define the proliferative compartments of in vivo human epidermis, using specific antibodies related to cell differentiation (beta 1 and beta 4 integrins and K1/K10 differentiation keratins) and cell cycle (proliferating cell nuclear antigen [PCNA]) in combination with flow cytometric quantitation of the DNA content and optical characteristics of the cells. The beta 1 integrin (CD29) marked both of the potentially proliferative subsets in normal epidermis. One subset of normal epidermis is CD29+ K1/K10-, which was predominantly basal, and found to be comprised of slow cycling, small cells with primitive cytoplasmic organization. The vast majority (95.5%) of these cells were in a quiescent state (G0/early G1) as indicated by their lack of the cyclin, PCNA. The other proliferative subset of normal epidermis was CD29+ K1/K10+, which was suprabasal and occasional basal, highly proliferative, larger in size, and which exhibited a more complex cytoplasmic structure. Because early differentiation (K1/K10 expression) has begun in the CD29+ K1/K10+ subset, it is highly likely that they represent the proliferative population which is capable of transiently amplifying itself before terminal differentiation. Within lesional psoriatic epidermis, similar proliferative cell populations were present as in normal epidermis, and the hyperproliferative defect was localized to the beta 1 and beta 4 integrin+, K1/K10- populations, which in normal epidermis is basally located and quiescent with regard to cell cycle. In psoriatic epidermis, a six- to sevenfold increase in the number of cells in the S/G2+M phase of cell cycle was found among CD29+ K1/K10- cells (p < 0.05). Furthermore, all lesional K1/K10- cells showed high PCNA positivity, indicating that all these cells had been recently induced into cell cycle. By contrast, the proportion of cycling cells among lesional psoriatic CD29+ K1/K10+ keratinocytes was similar to normals. Anti-HLA-DR, CD45, and vimentin antibodies were used to concomitantly track the proliferative states of Langerhans cell, melanocyte, and infiltrating leukocyte populations. In normal epidermis, the cycling fractions (cells in S/G2/M phase) of these cells were similar to the CD29+K1/K10- keratinocytes, whereas in lesional epidermis their cycling pools were increased relative to normal, but not so much as the proliferative fractions of psoriatic CD29+ K1/K10- keratinocytes. These data demonstrate the use of simultaneous analysis of integrin expression, differentiation keratins, cyclin, cell cycle status, and optical characteristics of freshly isolated human epidermal cells. Such analysis allowed the physical identification and quantification of cy cling populations in normal human skin, and has enabled the precise location of the primary epidermal proliferative defect in psoriasis.

Heterogeneity in the mouse epidermal cell cycle analysed by computer simulations

Different sets of cell kinetic data obtained over many years from hairless mouse epidermis have been simulated by a mathematical model including circadian variations. Simulating several independent sets of data with the same mathematical model strengthens the validity of the results obtained. The data simulated in this investigation were all obtained with the experimental system in a state of natural synchrony. The data include cell cycle phase distributions measured by DNA flow cytometry of isolated epidermal basal cells, fractions of tritiated thymidine ([3H]TdR) labelled cells within the cell cycle phases measured by cell sorting at intervals after [3H]TdR pulse labelling, bivariate bromodeoxyuridine (BrdUrd)/DNA data from epidermal basal cells isolated at intervals after pulse labelling with BrdUrd, mitotic rate and per cent labelled mitosis (PLM) data from histologic sections. The following main new findings were made from the simulations: the second PLM peak observed at about 35 h after pulse labelling is hardly influenced by circadian variations; the peak is mainly determined by persisting synchrony of a rapidly cycling population with a G1-duration (TG1) of 20 h to 30 h; and there is a highly significant population of slowly cycling G1-cells (G1 sigma). However, no significant circadian variations were found in the number of these cells.

Epidermal calcium-binding protein: a marker of early differentiation of basal layer keratinocytes of rats

Epidermal calcium-binding protein (ECaBP) is present in the cells of the basal layer of the epidermis and other stratified epithelia. Since the basal layer compartment contains at least two types of cells: slow-cycling, poorly-differentiated, and actively proliferating, more differentiated cells, it was of interest to determine whether they both contained ECaBP. Basal and nearly suprabasal layer keratinocytes from newborn rat epidermis were fractionated into three fractions on the basis of cell size, using low-gravity sedimentation. The cell differentiation in each subgroup was estimated by cell size, morphology, cell cycle stage, RNA/DNA content, and the presence of specific keratins. The presence of ECaBP in these fractions was detected by immunocytochemistry and immunoblotting. Double staining with ECaBP antibodies and propidium iodide followed by flow cytometry was used to correlate ECaBP production and the stage of cell cycle. The relative cell size, measured by the light scattering was used to study the relationship between cell size and ECaBP production. The results show that small keratinocytes with low DNA and RNA content (G0 cells) do not express ECaBP. ECaBP was found only in intermediate size basal keratinocytes with higher DNA and RNA contents, corresponding to actively proliferating S phase cells. Large keratinocytes, which express suprabasal keratin and have low DNA and high RNA content, cease to express ECaBP. ECaBP may, therefore, be a useful marker for assessing the movement of cells from poorly differentiated reserve compartment towards proliferation and further differentiation in both physiological and pathological situations.

Flow cytometric analysis of pig epidermal keratinocytes: effects of tape stripping

Tape stripping is a dynamic in vivo model for the induction of synchronized keratinocyte proliferation. We investigated the cell kinetics of pig epidermis by DNA-flow cytometric analysis, which was compared with [3H]thymidine incorporation mitotic counts and 2-[3H]-deoxy-D-glucose uptake. The stripping was standardized and confirmed histologically by the observation of complete removal of horny layer. Following the stripping, the proportion of cells in S-phase showed no remarkable change until 12 h. This was followed by a spike-like increase in the S-phase cells, the peak of which was reached at 24 h. This gradually decreased and returned to basal levels by 48 h. The cells in G2/M fraction initially decreased; the lowest value was obtained at 12 h. This was followed by a marked increase in the G2/M fraction, the peak of which was at 36 h. The keratinocytes in G2/M fraction gradually returned to basal levels by 96 h. [3H]Thymidine uptake and mitotic counts were mostly parallel with the data of the flow cytometric analysis, suggesting the latter as being a reliable system for cell kinetic analysis. The glucose uptake initially decreased (at 6 h following the stripping) and then increased at 24 h. Histologically the stripped epidermis regained its horny layer almost completely by 72 h following the stripping; this was occasionally accompanied by a moderate acanthotic change thereafter.

Cell shape changes during transition of basal keratinocytes to mature enucleate-cornified envelopes: modulation of terminal differentiation by fibronectin

Normal human keratinocytes (NHK) in submerged culture were used to assess mechanisms associated with in vitro exposure to known stimulators (sodium butyrate; NaB) and inhibitors (fibronectin; FN) of NHK maturation. A multiparameter approach was used to define cell types generated under a range of growth conditions. Differentiation induced in response to NaB occurred through a series of morphologically distinct phenotypes and culminated in the formation of enucleate-cornified envelopes. Two-dimensional electrophoresis provided a limited database to evaluate global changes in cellular protein composition as a function of induced differentiation. Proteins were identified that characterized predominantly basal cell cultures, NaB-treated cells, and fully differentiated NHKs. Growth of NHKs on FN suppressed both spontaneous and NaB-directed maturation and inhibited maximal expression of protein changes associated with the differentiated state. Thus, the composition of the extracellular matrix can modulate (at both the morphologic and protein levels) the response of basal NHKs to a potent differentiation-inducing agent. Abrogation of NHK maturation by FN was not due to adverse effects on cellular metabolism, abortive differentiation, or altered timing of induced differentiation. FN appears to exert its suppressive effect by either maintaining an early stem cell phenotype which is poorly competent for terminal maturation or attenuating an as yet unknown aspect of the NaB-initiated differentiation cascade.

Anthralin decreases keratinocyte TGF-alpha expression and EGF-receptor binding in vitro

Anthralin is an effective topical treatment for active psoriasis; however, its mechanism of action is unknown. Both TGF-alpha and its receptor, the EGF receptor, are overexpressed in active psoriatic plaques and might, therefore, play a role in psoriatic epidermal hyperplasia. In order to assess whether anthralin might act via alteration of this growth factor pathway, we examined the in vitro effects of pharmacologic concentrations of anthralin on cultured normal human keratinocytes. Keratinocyte proliferation was inhibited by 98% at an anthralin concentration of 10 ng/ml. In contrast, lymphocyte proliferation was inhibited by only 50% at an anthralin concentration of 10 micrograms/ml. Anthralin treatment did not induce cell-cycle-specific growth arrest as assessed by flow-cytometric analysis of acridine-orange-stained keratinocytes. Northern analysis of anthralin-treated keratinocytes demonstrated a marked decrease in TGF-alpha mRNA expression. Anthralin-treated keratinocytes showed decreased binding of 125I-EGF and 125I-IGF-I to their respective receptors, but EGF receptor binding was inhibited to a greater extent. Anthralin decreased ligand-binding affinity and cell-surface numbers of EGF receptors as assessed by Scatchard analysis of 125I-EGF binding to anthralin-treated keratinocytes. These results indicate that anthralin alters components of the EGF receptor pathway in cultured keratinocytes and that these effects might contribute to the clinical efficacy of anthralin in the treatment of active psoriasis.

Transforming growth factor-beta 1 acts cooperatively with sodium n-butyrate to induce differentiation of normal human keratinocytes

Growth factors with established biological activity toward cultured normal human epidermal keratinocytes (NHEKs) (e.g., transforming growth factor-beta, TGF-beta; retinoic acid, RA) initiate programmed changes in cellular maturation which differ with regard to the specific differentiation pathway (normal or abnormal) analyzed. Sodium butyrate (NaB) initiates one form of epidermal differentiation leading to enhanced cornified envelope (CE) formation which involves abrogation of the normally inhibitory effect of RA on NHEK maturation. NaB also induces TGF-beta mRNA in the maturing suprabasal compartment, suggesting that TGF-beta may play a role in NaB-initiated NHEK differentiation. Treatment with TGF-beta 1 alone, however, only marginally increased (by twofold) the number of detergent-resistant CEs compared to control NHEKs and did not alter the prevalence of fully mature enucleated CEs. TGF-beta 1 was quite effective in inducing significant levels of CE expression when used simultaneously with suboptimal concentrations of NaB. The cooperative action of suboptimal NaB and TGF-beta 1 generated numbers of CEs which, in fact, exceeded the incidence of mature CEs formed in response to optimal levels of NaB alone. Neutralizing antibodies to TGF-beta, moreover, effectively reduced the incidence of CE formation in cultures treated with optimal NaB concentrations, further implicating endogenous TGF-beta activity in the NaB-initiated NHEK differentiation model. It is suggested, therefore, that within the NaB-induced pathway of NHEK differentiation, TGF-beta can positively modulate expression of the differentiated phenotype but alone is insufficient for generation of mature CEs.

Multivariate flow cytometry of epidermal regeneration provoked by a skin irritant and a tumor promoter

The DNA content and the changes in cellular and nuclear size of isolated regenerating mouse epidermal basal cells were studied after topical application of the skin irritant cantharidin and the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) to the back skin of hairless mice. The DNA and protein contents of isolated basal cells were stained with propidium iodide and fluorescein isothiocyanate, respectively, and analysed by flow cytometry using the total protein fluorescence as an estimate of cell size and the DNA fluorescence pulse width as an estimate of nuclear size. Transmission electron microscopy was used to identify cells sorted from regions in the bivariate DNA/protein distributions. The results showed that both chemicals induced an increase in cellular as well as nuclear size of the basal cells. The increase in size was higher in TPA treated than in cantharidin treated animals, and the bivariate DNA/protein distributions of TPA treated cells differed from those of cantharidin treated cells in that two subpopulations of cycling keratinocytes could be identified. These deviations are probably related to the higher proliferative response observed after TPA treatment and the possibility that proliferative subpopulations in epidermis respond differently to TPA. It may reflect mechanisms providing for a growth advantage of initiated cells, important in tumor promotion. About 8% of the cells in the suspensions from treated animals were non-cycling non-keratinocytes, probably infiltrating leukocytes. The results indicate a strong correlation between rapid regenerative cell cycle progression, i.e., reduced G1 transit time and increased cellular and nuclear size.(ABSTRACT TRUNCATED AT 250 WORDS)

TGF-alpha and TGF-beta expression during sodium-N-butyrate-induced differentiation of human keratinocytes: evidence for subpopulation-specific up-regulation of TGF-beta mRNA in suprabasal cells

Sodium-N-butyrate (NaB) induces terminal differentiation and cornified envelope formation in cultured human keratinocytes. In the present study we explored the question of whether NaB-induced maturation could be mediated through changes in TGF-alpha and TGF-beta expression in normal keratinocytes. NaB induced a four-fold increase in TGF-beta mRNA transcript levels. This increase in TGF-beta mRNA abundance occurred only within the nonbasal keratinocyte subpopulation which maximally responds to NaB treatment by progression to cornified envelopes. Basal keratinocytes, which are relatively refractive to cornified envelope formation, did not show any increase in TGF-beta mRNA abundance after NaB treatment. By comparison, TGF-alpha mRNA transcript and extracellular TGF-alpha protein levels were unaffected by NaB treatment. A 50% decrease in EGF receptor binding was observed in NaB-treated keratinocyte cultures, rendering the cells less responsive to proliferation induction.

Epidermal cell-shape regulation and subpopulation kinetics during butyrate-induced terminal maturation of normal and SV40-transformed human keratinocytes: epithelial models of differentiation therapy

Recent data indicate that malignant human epidermal cells may be appropriate targets for sodium butyrate (NaB)-mediated differentiation therapy. The response of pre- and post-crisis populations of SV40-transformed human keratinocytes (SVKs) to this differentiation-inducing agent was assessed, therefore, within the framework of NaB-directed normal human keratinocyte (NHK) maturation. NaB augmented cornified envelope (CE) production in NHK and pre-crisis SVK cultures; the time-course and efficiency of induced maturation were similar in the 2 cell systems. In NHKs, the percentage of amplifying ("B" substate) cells decreased with time in NaB correlating with increases in both "C" stage keratinocytes and CEs. The latter formed over one or 2 layers of nucleated basal-like cells. Inductions were accompanied by immediate cell cycle blocks (in both the G1 and G2/M phases), reorganization within the actin cytoskeleton, and transient early increases in cellular actin content. Increased NHK and pre-crisis SVK cytoskeletal-associated actin reached a maximum approximately 48 hr after NaB addition and preceded development of CEs. The CE precursors, thus, probably reside in the "B" substate. Post-crisis SVKs, in contrast, were refractive to NaB-induced terminal maturation or cell-cycle perturbation, failed to initiate actin filament rearrangements, and retained a basal cell-like phenotype. Stable transformation of human SVKs in post-crisis phase, therefore, appears to be associated with loss of maturation "competence" within the "B" keratinocyte subpopulation.

Mathematical model analysis of mouse epidermal cell kinetics measured by bivariate DNA/anti-bromodeoxyuridine flow cytometry and continuous [3H]-thymidine labelling

In a previous study the epidermal cell kinetics of hairless mice were investigated with bivariate DNA/anti-bromodeoxyuridine (BrdU) flow cytometry of isolated basal cells after BrdU pulse labelling. The results confirmed our previous observations of two kinetically distinct sub-populations in the G2 phase. However, the results also showed that almost all BrdU-positive cells had left S phase 6-12 h after pulse labelling, contradicting our previous assumption of a distinct, slowly cycling, major sub-population in S phase. The latter study was based on an experiment combining continuous tritiated thymidine [( 3H]TdR) labelling and cell sorting. The purpose of the present study was to use a mathematical model to analyse epidermal cell kinetics by simulating bivariate DNA/BrdU data in order to get more details about the kinetic organization and cell cycle parameter values. We also wanted to re-evaluate our assumption of slowly cycling cells in S phase. The mathematical model shows a good fit to the experimental BrdU data initiated either at 08.00 hours or 20.00 hours. Simultaneously, it was also possible to obtain a good fit to our previous continuous labelling data without including a sub-population of slowly cycling cells in S phase. This was achieved by improving the way in which the continuous [3H]TdR labelling was simulated. The presence of two distinct subpopulations in G2 phase was confirmed and a similar kinetic organization with rapidly and slowly cycling cells in G1 phase is suggested. The sizes of the slowly cycling fractions in G1 and G2 showed the same distinct circadian dependency. The model analysis indicates that a small fraction of BrdU labelled cells (3-5%) was arrested in G2 phase due to BrdU toxicity. This is insignificant compared with the total number of labelled cells and has a negligible effect on the average cell cycle data. However, it comprises 1/3 to 1/2 of the BrdU positive G2 cells after the pulse labelled cells have been distributed among the cell cycle compartments.

Cell size and RNA content correlate with cell differentiation and proliferative capacity of rat keratinocytes

Keratinocytes from rat skin were separated according to their size in a specially designed unit-gravity sedimentation chamber. The fractions obtained with this technique showed clear morphological differences, and analysis of size distribution confirmed that size was the criterion for separation. Simultaneous DNA and RNA staining of the fractions with acridine orange and subsequent flow cytometric analysis enabled one to classify cells into resting, proliferating, and differentiating stages. Cell size was not directly correlated with proliferation in situ as determined with acridine orange flow cytometry, nor with proliferative capacity in culture as assayed by BrdU/Hoechst flow cytometry. The smallest cells, exhibiting low DNA and RNA content, which do not proliferate in vivo, required a prolonged period of serum stimulation in vitro to initiate RNA and DNA synthesis. Cells of intermediate size exhibited early RNA synthesis and maximal proliferative capacity, whereas the largest cell population displayed no RNA synthesis in culture and the least proliferative capacity. In conclusion, these results suggest that RNA synthesis early after serum stimulation, in addition to a specific, optimal cell size, correlates with the proliferative capacity of keratinocytes in cell culture.

Occlusive hydrocolloid dressings decrease keratinocyte population growth fraction and clinical scale and skin thickness in active psoriatic plaques

Clinical studies suggest a therapeutic role for occlusion in the treatment of psoriasis. Previous studies, using multiparameter RNA/DNA flow cytometric analysis of epidermal suspensions obtained from active plaques, demonstrated increased keratinocyte growth fraction which reversed with successful medical treatment. Because keratinocyte growth fraction reflected disease activity, it was used in this study in addition to clinical evaluations in order to determine the efficacy of occlusion in the treatment of psoriatic plaques. In each of 9 patients, scale, skin thickness and erythema were compared in one occluded and one control plaque using an analog scale. Both scale and skin thickness, but not erythema, were decreased after 2 weeks of occlusion. However after 10 weeks, no additional differences were seen when compared with assessments made after 2 weeks, suggesting that the benefits of occlusive therapy occurred early. After 10 weeks of occlusion, the keratinocyte growth fraction was significantly decreased in occluded plaques. This study demonstrates that occlusion plays a synergistic role with other therapeutic modalities in ameliorating psoriatic plaques.

Cell kinetics in mouse epidermis studied by bivariate DNA/bromodeoxyuridine and DNA/keratin flow cytometry

Hairless mice were injected intraperitoneally with bromodeoxyuridine (Brd-Urd). Basal cells were isolated from epidermis, fixed in 70% ethanol, and prepared for bivariate BrdUrd/DNA flow cytometric (FCM) analysis. Optimum detection of incorporated BrdUrd in DNA was obtained by combining pepsin digestion and acid denaturation. The cell loss was reduced to a minimum by using phosphate-buffered saline containing Ca2+ and Mg2+ to neutralize the acid. The percentage of cells in S phase and the average uptake of BrdUrd per labelled cell in eight consecutive windows throughout the S phase were measured after pulse labelling at intervals during a 24 h period. Furthermore, the cell cycle progression of a pulse-labelled cohort of cells was followed up to 96 h after BrdUrd injection. In general the results from both experiments were in good agreement with previous data from 3H-thymidine labelling studies. The percentage of cells in S phase was highest at night and lowest in the afternoon, whereas the average uptake of BrdUrd per labelled cell showed only minor circadian variations. There were no indications that BrdUrd significantly perturbed normal epidermal growth kinetics. A cell cycle time of about 36 h was observed for the labelled cohort. Indications of heterogeneity in traverse through G1 phase were found, and the existence of slowly cycling or temporarily resting cells in G2 phase was confirmed. There was, however, no evidence of a significant population of temporarily resting cells in the S phase. Bivariate DNA/keratin FCM analysis revealed a high purity of basal cells in the suspensions and indicated that the synthesis of the differentiation-keratin K10 was turned on only in G1 phase and after the last division.

Sodium-N-butyrate induces cytoskeletal rearrangements and formation of cornified envelopes in cultured adult human keratinocytes

The technique developed in our laboratory allows us to culture multilayered, stratified sheets of human keratinocytes, which can be used to cover the burn wounds of patients. Organization of cells in these cultures resembles stratum germinativum and stratum spinosum but there are only a few fully keratinized cells and the stratum corneum is not developed. Since the fully differentiated sheets may offer additional advantages as epidermal transplants, attempts were made to enhance the degree of differentiation in vitro. In the present study sodium-N-butyrate (NaB) was used as a differentiating agent and its effect on the cell cycle and cytoarchitecture of epidermal cells was investigated. Incubation of keratinocytes in the presence of 2.5 mM NaB induced the appearance of enucleated cornified envelopes, covering approximately 70-80% of the surface of the cultures. Their appearance correlated with a decrease in expression of keratin K13, previously shown to be inhibited during terminal differentiation of human keratinocytes. An increase in transglutaminase transferase activity was also observed. The induction of cornified layers also correlated with an increase in the amount of microfilament (MF)-associated actin. NaB also induced changes in the cell cycle distribution of the keratinocyte cultures. A decrease in the proportion of S and G1B phase cells was paralleled by an increase in G1A cells, maximally expressed 30-48 h following addition of the inducer. Interestingly, NaB also induced a cell arrest in G2 phase. These cell cycle perturbations preceded the onset of keratinocyte differentiation. The results indicate that the enhanced differentiation of human keratinocytes in the presence of NaB may serve as a means to produce epidermal sheets with improved properties for transplantation in a clinical setting. It also serves as an in vitro model system to study the interrelationships between biochemical events and cell cycle changes accompanying differentiation.

Cultured human keratinocytes: discrimination of different cell cycle compartments based upon measurement of nuclear RNA or total cellular RNA content

Correlated measurements of total cellular RNA and DNA of cultured human keratinocytes by flow cytometry, followed by multivariate analysis, discriminate three distinct subpopulations of cells differing in RNA content. The first subpopulation is comprised of small cells resembling basal cells of epidermis, with low RNA content and long (100-300 h) generation times. The second subpopulation consists of keratinocytes resembling cells in the spinous layer of epidermis, characterized by increased RNA content and shorter (35-40 h) generation times. The third subpopulation consists of the largest, keratinohyalin-containing cells which remain in G1 and undergo terminal differentiation. In contrast to total cellular RNA, correlated measurements of DNA and nuclear RNA reveal that: (1) entrance of all cultured cells from G1 into S phase occurs only after accumulation of the same, threshold amount of nuclear RNA; hence there is only a single population of S + G2 + M-phase cells; (2) there are two distinct subpopulations in G1, one with minimal nuclear RNA content and another with increased RNA. Stathmokinetic experiments indicate that the G1-phase cells with low nuclear RNA have distinctly longer residence times in G1 compared to cells with high nuclear RNA content. Thus, measurements of the total cellular RNA versus nuclear RNA content reveal kinetically distinct cell subpopulations. Whereas total cellular RNA content correlates more with differentiation, nuclear RNA content reflects primarily the kinetic properties of the cell.

Cell subpopulations within proliferative and differentiating compartments of epidermis

The heterogeneous population of newborn rat keratinocytes was separated into different subgroups according to their cell size. The relation between cell size, position in the cell cycle, RNA content, and proliferative potential in culture was examined. A reserve stem cell population of Go/G1 cells, low in RNA, giving rise to colonies of undifferentiated phenotype in cell culture, has been separated from more differentiated transit basal cells. In the fractions of the larger cells, several subgroups, probably corresponding to different stages of differentiation, were identified: G2M cells with low RNA content, large S-phase cells rich in RNA, and small Go/G1 cells low in RNA. The clonogenic cells from these fractions have limited growth potential and give rise to moderately or terminally differentiated colonies. The selective sorting of stem cell populations may be useful for elucidating the mechanism of carcinogenesis in epidermis and other proliferative tissues. Analysis of the relative proportions of cell subpopulations represents a novel approach leading to the refinement of the concepts of epidermal structure in physiological and pathological states. It also could, by extension, shed new light on the behavior of other proliferative tissues.

Marked increase in the frequency of psoriatic arthritis in psoriasis patients with HLA-DR+ keratinocytes

Immunocytochemical studies with a monoclonal anti-HLA-DR antibody were performed on skin sections and keratinocyte (KTC) suspensions obtained from suction blisters of active psoriatic plaques. HLA-DR+ KTCs were found in the plaques of 23 of 38 patients with active psoriasis. Of these 23, 16 had clinical findings typical of psoriatic arthritis (PA); none of the 15 patients who lacked HLA-DR+ KTCs had PA. Although KTC HLA-DR expression was more prevalent in patients with severe skin disease, 7 of the 23 patients with HLA-DR+ KTCs in active psoriatic plaques had mild skin disease; 4 of these 7 had PA. Nail pitting or duration of skin disease did not account for increased incidence of PA in patients with HLA-DR+ KTCs. All psoriasis patients with arthritis received nonsteroidal antiinflammatory drug therapy; 14 received additional therapy directed primarily to the cutaneous manifestations of psoriasis. Nine of these noted arthritis improvement with concurrent skin response; however, in 5 patients, arthritis activity increased, despite improvement of the cutaneous disease. Two other patients, treated with methotrexate, also had concurrent skin and joint improvement. These data suggest that psoriasis patients with HLA-DR+ KTCs are at increased risk for the development of associated arthritis.

RNA, DNA, and cell surface characteristics of lesional and nonlesional psoriatic skin

We have measured the RNA and DNA content and examined cell surface characteristics of human epidermal cells derived from normal skin, and lesional and nonlesional areas of psoriatic skin prior to and following treatment on a modified Goeckerman protocol. Our results show that cells from active psoriatic lesions contain greater numbers of basal keratinocytes when compared with either nonlesional skin from the same patients or skin from healthy volunteers and individuals with other inflammatory skin lesions. Follow-up measurements 2-3 weeks after the initiation of therapy showed that the numbers of basal keratinocytes in resolving psoriatic lesions had decreased and approached normal levels. Multiparameter RNA/DNA flow cytometric analysis on parallel samples from the same psoriasis patients revealed an increased growth fraction and proportion of cycling cells in both the nonlesional and lesional skin compared with controls. Furthermore, the cellular RNA content was elevated in lesional psoriatic skin when compared with either nonlesional or normal skin. Flow cytometric examination of nonlesional and lesional epidermal cells obtained 2-3 weeks after the commencement of therapy revealed that the growth fraction and mean RNA content of the keratinocytes from resolving psoriatic plaques decreased in response to therapy. In contrast, the proportion of keratinocytes within the S + G2 + M phases of the cell cycle remained elevated. These data indicate that "uninvolved" psoriatic skin exhibits characteristics more closely resembling lesional psoriatic skin than normal skin. The results further suggest that quantitation of cellular RNA content and basal cell number might be sensitive indicators of early treatment response in psoriasis.

Expression of HLA-DR molecules by keratinocytes, and presence of Langerhans cells in the dermal infiltrate of active psoriatic plaques

Immunoperoxidase staining of skin sections and immunofluorescence analysis of keratinocyte suspensions obtained from suction blisters of psoriatic plaques were performed using an mAb, Josh 524.4.1, and Fab'2 fragments of a rabbit antiserum, both of which are directed against nonpolymorphic determinants of HLA-DR molecules. HLA-DR+ keratinocytes were present in plaques, but not normal-appearing skin, from a significant portion of patients with active psoriasis. Double-labelling immunofluorescence experiments with either the monoclonal or polyclonal anti-HLA-DR antibody, in conjunction with the mAb OKT6, which identifies DR+ Langerhans cells, demonstrated that HLA-DR molecules were present on OKT6- keratinocytes. The dermal infiltrate of psoriatic plaques contained T cells expressing the activation antigens, IL-2 receptor (Tac) and HLA-DR, as well as macrophages and OKT6+ cells. There was little difference in the characteristics of the dermal infiltrate between the lesions with or without HLA-DR+ keratinocytes. OKT6+ presumptive Langerhans cells were also found in the dermal infiltrates of patients with lichen planus, contact dermatitis, spongiotic dermatitis, erythema multiforme, basal and squamous cell carcinoma. Studies of keratinocyte suspensions showed that 7-84% of keratinocytes were HLA-DR+. Flow cytometry experiments showed that keratinocytes at all stages of differentiation were HLA-DR+. However, the stem cell-enriched population contained the highest proportion of HLA-DR+ cells. HLA-DR expression by keratinocytes correlated with disease activity. The expression was reversible with successful medical therapy. HLA-DR+ keratinocytes may activate T cells directly or may present an as yet unknown antigen to T cells. These studies provide further support for the hypothesis that immunological mechanisms play an important role in the pathogenesis of psoriasis.

Stathmokinetic analysis of human epidermal cells in vitro

Proliferation kinetics of cultured human epidermal cells is characterized in quantitative terms. Three distinct subpopulations of keratinocytes, two of which are cycling, have been discriminated by two parameter DNA/RNA flow cytometry. Based on mathematical modelling, the cell cycle parameters of the cycling subpopulations have been assessed from stathmokinetic data collected at different time points after initiation of cultures (7-15 days). The first subpopulation is composed of low-RNA cells which resemble basal keratinocytes of epidermis and which show some characteristics of stem cells; these cells have a mean generation time of approximately 100 hr. The second subpopulation consists of high-RNA cells, resembling stratum spinosum cells of epidermis, which have an average generation time of approximately 40 hr. The third subpopulation consists of non-cycling cells with G0/G1 DNA content, with cytochemical features similar to those of cells in granular layer of epidermis. The results based on modelling can reproduce with acceptable accuracy the actual growth curve of the cultured cell population. Analysis of kinetics and differentiation of human keratinocytes is of interest in view of the recent application of cultured epidermal cell sheets for transplantation onto burn wounds. The results of this study also reveal the existence of regulatory mechanisms associated with proliferation and differentiation in the cultured epidermal cell population.