Metabolism of the antibacterial triclocarban by human epidermal keratinocytes to yield protein adducts

Previous studies of triclocarban suggest that its biotransformation could yield reactive metabolites that form protein adducts. Since the skin is the major route of triclocarban exposure, present work examined this possibility in cultured human keratinocytes. The results provide evidence for considerable biotransformation and protein adduct formation when cytochrome P450 activity is induced in the cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin, a model Ah receptor ligand. Since detecting low adduct levels in cells and tissues is difficult, we utilized the novel approach of accelerator mass spectrometry for this purpose. Exploiting the sensitivity of the method, we demonstrated that a substantial portion of triclocarban forms adducts with keratinocyte protein under the P450 inducing conditions employed.

Proteomic analysis of hair shaft and nail plate

The protein components of living cells in the hair follicle are amenable to study by standard molecular biological techniques, but identifying those in the hair shaft has been problematic until recently. Most of the protein, primarily keratins and keratin associated proteins, can be extracted under denaturing conditions, but 15-20% is intractable due to transglutaminase-mediated cross-linking. Shotgun proteomics now permits identifying >300 constituents of the isopeptide cross-linked proteome and even certain post-translational modifications. The proteins originate from all the intracellular compartments, indicating that the cross-linking process makes effective use of available resources to produce structures with great mechanical stability. Knowing this proteome provides a foundation for correlating defects in hair shaft structure with protein deficiencies. Such investigations can be extended to mouse models of aberrant pelage hair. Thus, inbred mouse strains can be distinguished by their hair proteomes, raising the possibility of similar variation in the human population. The nail plate is also amenable to this shotgun proteomic approach. Providing discrete and noninvasive sampling of the human proteome, these epidermal appendages could have diagnostic utility for certain disease states.

Proteomic analysis of human nail plate

Shotgun proteomic analysis of the human nail plate identified 144 proteins in samples from Causcasian volunteers. The 30 identified proteins solubilized by detergent and reducing agent, 90% of the total nail plate mass, were primarily keratins and keratin associated proteins. Keratins comprised a majority of the detergent-insoluble fraction as well, but numerous cytoplasmic, membrane, and junctional proteins and histones were also identified, indicating broad use by transglutaminases of available proteins as substrates for cross-linking. Two novel membrane proteins were identified, also found in the hair shaft, for which mRNAs were detected only at very low levels by real-time polymerase chain reaction in other tissues. Parallel analyses of nail samples from volunteers from Inner Mongolia, China gave essentially the same protein profiles. Comparison of the profiles of nail plate and hair shaft from the latter volunteers revealed extensive overlap of protein constituents. Analyses of samples from an arsenic-exposed population revealed few proteins whose levels were altered substantially but raised the possibility of detecting sensitive individuals in this way.

Opposing actions of insulin and arsenite converge on PKCdelta to alter keratinocyte proliferative potential and differentiation

When cultured human keratinocytes reach confluence, they undergo a program of changes replicating features of differentiation in vivo, including exit from the proliferative pool, increased cell size, and expression of specialized differentiation marker proteins. Previously, we showed that insulin is required for some of these steps and that arsenite, a human carcinogen in skin and other epithelia, opposes the differentiation process. In present work, we show that insulin signaling, probably through the IGF-I receptor, is required for the increase in cell size accompanying differentiation and that this is opposed by arsenite. We further examine the impact of insulin and arsenite on PKCdelta, a known key regulator of keratinocyte differentiation, and show that insulin increases the amount, tyrosine phosphorylation, and membrane localization of PKCdelta. All these effects are prevented by exposure of cells to arsenite or to inhibitors of downstream effectors of insulin (phosphotidylinositol 3-kinase and mammalian target of rapamycin). Retrovirally mediated expression of activated PKCdelta resulted in increased loss of proliferative potential after confluence and greatly increased formation of cross-linked envelopes, a marker of keratinocyte terminal differentiation. These effects were prevented by removal of insulin, but not by arsenite addition. We further demonstrate a role for src family kinases in regulation of PKCdelta. Finally, inhibiting epidermal growth factor receptor kinase activity diminished the ability of arsenite to prevent cell enlargement and to suppress insulin-dependent PKCdelta amount and tyrosine 311 phosphorylation. Thus suppression of PKCdelta signaling is a critical feature of arsenite action in preventing keratinocyte differentiation and maintaining proliferative capability.

Arsenite suppression of involucrin transcription through AP1 promoter sites in cultured human keratinocytes

While preserving keratinocyte proliferative ability, arsenite suppresses cellular differentiation markers by preventing utilization of AP1 transcriptional response elements. In present experiments, arsenite had a dramatic effect in electrophoretic mobility supershift analysis of proteins binding to an involucrin promoter AP1 response element. Without arsenite treatment, binding of JunB and Fra1 was readily detected in nuclear extracts from preconfluent cultures and was not detected a week after confluence, while c-Fos was detected only after confluence. By contrast, band shift of nuclear extracts from arsenite treated cultures showed only JunB and Fra1 binding in postconfluent as well as preconfluent cultures. Immunoblotting of cell extracts showed that arsenite treatment prevented the loss of Fra1 and the increase in c-Fos proteins that occurred after confluence in untreated cultures. Chromatin immunoprecipitation assays demonstrated substantial reduction of c-Fos and acetylated histone H3 at the proximal and distal AP1 response elements in the involucrin promoter and of coactivator p300 at the proximal element. Alteration of AP1 transcription factors was also examined in response to treatment with four metal containing compounds (chromate, vanadate, hemin, divalent cadmium) that also suppress involucrin transcription. These agents all influenced transcription at AP1 elements in a transcriptional reporter assay, but exhibited less effect than arsenite on binding activity assessed by mobility shift and chromatin immunoprecipitation and displayed variable effects on AP1 protein levels. These findings help trace a mechanism by which transcriptional effects of arsenite become manifest and help rationalize the unique action of arsenite, compared to the other agents, to preserve proliferative ability.

Proteomic analysis of low dose arsenic and ionizing radiation exposure on keratinocytes

Human exposure to arsenic and ionizing radiation (IR) occur environmentally at low levels. While the human health effects of arsenic and IR have been examined separately, there is little information regarding their combined effects at doses approaching environmental levels. Arsenic toxicity may be affected by concurrent IR especially given their known individual carcinogenic actions at higher doses. We found that keratinocytes responded to either low dose arsenic and/or low dose IR exposure, resulting in differential proteomic expression based on 2-DE, immunoblotting and statistical analysis. Seven proteins were identified that passed a rigorous statistical screen for differential expression in 2-DE and also passed a strict statistical screen for follow-up immunoblotting. These included: alpha-enolase, epidermal-fatty acid binding protein, heat shock protein 27, histidine triad nucleotide-binding protein 1, lactate dehydrogenase A, protein disulfide isomerase precursor, and S100A9. Four proteins had combined effects that were different than would be expected based on the response to either individual toxicant. These data demonstrate a possible reaction to the combined insult that is substantially different from that of either separate treatment. Several proteins had different responses than what has been seen from high dose exposures, adding to the growing literature suggesting that the cellular responses to low dose exposures are distinct.

Generation of oxidant response to copper and iron nanoparticles and salts: Stimulation by ascorbate

The present work describes a two-stage approach to analyzing combustion-generated samples for their potential to produce oxidant stress. This approach is illustrated with the two commonly encountered transition metals, copper and iron. First, their abilities to generate hydroxyl radical were measured in a cell-free, phosphate-buffered saline solution containing ascorbate and/or citrate. Second, their abilities to induce heme oxygenase-1 in cultured human epidermal keratinocytes were assessed in cell culture. Combustion-generated copper oxide nanoparticles were active in both assays and were found to be soluble in culture medium. Depletion of glutathione in the cells or loading the cells with ascorbate greatly increased heme oxygenase-1 induction in the presence of copper. By contrast, iron oxide nanoparticles were active in the phosphate-buffered saline but not in cell culture, and they aggregated in culture medium. Soluble salts of copper and iron exhibited the same contrast in activities as the respective combustion-generated particles. The results suggest that the capability of combustion-generated environmental samples to produce oxidant stress can be screened effectively in a two step process, first in phosphate-buffered saline with ascorbate and subsequently in epithelial cell culture for those exhibiting activity initially. The results also point to an unanticipated interaction in cells of oxidant stress-generating metals with an antioxidant (ascorbate) that is usually missing in culture medium formulations. Thus, ascorbate supplementation of cultured human cells is likely to improve their ability to model the in vivo effects of particulate matter containing copper and other redox-active metals.

Founder mutations in the lipase h gene in families with autosomal recessive woolly hair/hypotrichosis

Autosomal-recessive woolly hair (ARWH)/hypotrichosis is a hereditary hair disorder which is characterized by tightly curled hair and is associated with sparse hair. ARWH can be caused by mutations in the P2RY5 or lipase H (LIPH) gene. Disruption of either gene results in phenotypes with features of both wooly hair (WH) and hypotrichosis. In this study, we identified two Guyanese families with ARWH. Both families are of recent Indian descent. Mutation analysis resulted in the identification of mutations in the LIPH gene in both families. Affected individuals in the first family carry compound heterozygous mutations Ex7_8del and 1303_1309dupGAAAACG in the LIPH gene, while those in the second family have a homozygous mutation 659_660delTA in the LIPH gene. The mutations Ex7_8del and 659_660delTA were identified earlier in several Pakistani families with ARWH. Haplotype analysis using microsatellite markers close to the LIPH gene defined a founder haplotype shared in families from Pakistan and Guyana. Proteomic analysis of hair shaft samples from one of the families revealed no substantial changes among the proteins identified, indicating that the syndrome does not involve global alterations in protein expression. Our results further suggest a crucial role of LIPH gene in hair growth.

Cyclin D1 downregulation is important for permanent cell cycle exit and initiation of differentiation induced by anchorage-deprivation in human keratinocytes

To understand the relationship between permanent cell cycle exit and differentiation the immortalized keratinocyte cell line, SIK and the squamous cell carcinoma, SCC9 were compared during differentiation induced by anchorage-deprivation. The SIK cells when placed in suspension culture promptly lost almost all ability to reinitiate growth by 2 days concomitantly expressing the differentiation specific proteins, transglutaminase (TGK) and involucrin. These cells rapidly underwent G1 cell cycle arrest with complete disappearance of phosphorylated RB. In contrast SCC9 cells neither showed TGK expression nor increase in involucrin. They decreased their colony-forming ability much more slowly, which coordinated well with a gradual decrease in phosphorylated RB, demonstrating the significant resistance to loss of colony-forming ability and cell cycle exit. In accordance, cyclin D1, a positive regulator of cyclin-dependent kinase (CDK) 4/6 which phosphorylates RB decreased drastically in anchorage deprived SIK but not in SCC9 cells. Endogenous cyclin D1 knockdown in SCC9 cells by siRNA enhanced loss of the colony-forming ability during anchorage-deprivation. Conversely enforced expression of cyclin D1 in SIK cells and in another immortalized keratinocyte cell line, HaCaT, partly prevented loss of their colony-forming abilities. Cyclin D1 overexpression antagonized Keratin 10 expression in suspended HaCaT cells. The result demonstrates the importance of cyclin D1 down regulation for proper initiation of keratinocyte differentiation.

Assessing probe-specific dye and slide biases in two-color microarray data

BACKGROUND

A primary reason for using two-color microarrays is that the use of two samples labeled with different dyes on the same slide, that bind to probes on the same spot, is supposed to adjust for many factors that introduce noise and errors into the analysis. Most users assume that any differences between the dyes can be adjusted out by standard methods of normalization, so that measures such as log ratios on the same slide are reliable measures of comparative expression. However, even after the normalization, there are still probe specific dye and slide variation among the data. We define a method to quantify the amount of the dye-by-probe and slide-by-probe interaction. This serves as a diagnostic, both visual and numeric, of the existence of probe-specific dye bias. We show how this improved the performance of two-color array analysis for arrays for genomic analysis of biological samples ranging from rice to human tissue.

RESULTS

We develop a procedure for quantifying the extent of probe-specific dye and slide bias in two-color microarrays. The primary output is a graphical diagnostic of the extent of the bias which called ECDF (Empirical Cumulative Distribution Function), though numerical results are also obtained.

CONCLUSION

We show that the dye and slide biases were high for human and rice genomic arrays in two gene expression facilities, even after the standard intensity-based normalization, and describe how this diagnostic allowed the problems causing the probe-specific bias to be addressed, and resulted in important improvements in performance. The R package LMGene which contains the method described in this paper has been available to download from Bioconductor.

Arsenite and insulin exhibit opposing effects on epidermal growth factor receptor and keratinocyte proliferative potential

Previous work has suggested that arsenic exposure contributes to skin carcinogenesis by preserving the proliferative potential of human epidermal keratinocytes, thereby slowing the exit of putative target stem cells into the differentiation pathway. To find a molecular basis for this action, present work has explored the influence of arsenite on keratinocyte responses to epidermal growth factor (EGF). The ability of cultured keratinocytes to found colonies upon passaging several days after confluence was preserved by arsenite and EGF in an additive fashion, but neither was effective when the receptor tyrosine kinase activity was inhibited. Arsenite prevented the loss of EGF receptor protein and phosphorylation of tyrosine 1173, preserving its capability to signal. The level of nuclear beta-catenin was higher in cells treated with arsenite and EGF in parallel to elevated colony forming ability, and expression of a dominant negative beta-catenin suppressed the increase in both colony forming ability and yield of putative stem cells induced by arsenite and EGF. As judged by expression of three genes regulated by beta-catenin, this transcription factor had substantially higher activity in the arsenite/EGF-treated cells. Trivalent antimony exhibited the same effects as arsenite. A novel finding is that insulin in the medium induced the loss of EGF receptor protein, which was largely prevented by arsenite exposure.

Oxygen-dependent differentiation of human keratinocytes

Oxygen is an essential micronutrient. Unlike many internal tissues, human epidermis obtains much of its oxygen supply from the atmosphere (21% oxygen), and it ordinarily experiences higher oxygen levels than internal tissues (estimated approximately 5%). To test whether epidermal cell growth and differentiation depend upon this higher oxygen level, keratinocyte cultures were studied at 21, 5, and 2% oxygen concentrations. Compared to 21% oxygen, culture in 5% had little effect on growth but led to profound suppression of the differentiation program as assessed by expression of differentiation markers and formation of squames in the superficial layers. Culture in 2% oxygen reduced the growth rate as well as stratification and differentiation. In low-oxygen conditions, the cells exhibited increased colony-forming ability, consistent with a lower proportion of differentiated cells, and increased expression of vascular endothelial growth factor and cyclooxygenase-2. Growth in 21% oxygen led to higher levels of glutathione and expression of oxidant-responsive genes. Electrophoretic mobility supershift assay using an involucrin activator protein 1 (AP1) response element sequence revealed altered binding by proteins of the Jun and Fos families in nuclear extracts. The present data thus demonstrate oxygen-dependent differentiation in human keratinocytes, to which altered utilization of AP1 transcriptional response elements may contribute.

Spontaneous immortalization of human epidermal cells with naturally elevated telomerase

This work explores spontaneous immortalization in keratinocytes, derived from two skin samples, that display naturally elevated telomerase activity. Serially passaged with 3T3 feeder layer support, the keratinocytes were examined for colony-forming ability, telomerase activity, telomere length, and finally gene expression using Affymetrix DNA microarrays. The cells initially exhibited normal karyotypes and low colony-forming efficiencies typical of normal epidermal cells, but after 40 passages (approximately 400 generations) colony-forming ability increased markedly, yielding immortalized lines exhibiting a small number of chromosomal aberrations and functionally normal p53. An improved protocol for analysis of microarray data permitted detection of 707 transcriptional changes accompanying immortalization including reduced p16(INK4A) mRNA. Telomerase activity was clearly elevated in cells even at low passage from both samples, and telomerase catalytic subunit mRNA was greatly elevated in those with elevated colony-forming ability. The data raise the possibility of an unusual natural phenotype in which aberrant telomerase regulation extends keratinocyte lifespan until rare variants evade senescence. In addition to revealing a potential tumor-prone syndrome, the findings emphasize the desirability of carefully minimizing the degree or timing of elevated expression of telomerase used to immortalize cells for therapeutic purposes.

Proteome Analysis of Human Hair Shaft

The human hair proteome was investigated using two-dimensional LC-MS/MS. Among the 343 identified proteins, 70 were detected in high relative abundance, including keratin intermediate filament proteins, largely extractable with denaturants. Over 300 proteins were found to constitute the insoluble complex formed by transglutaminase cross-linking. The intracellular distribution of identified proteins is wide from cytoplasm to nucleus, mitochondria, ribosome, and plasma membrane. These results help rationalize ultrastructural features visible in the mature hair. Keratins and several substrates for transglutaminase were found to be posttranslationally modified by methylation and dimethylation. Evidence for ubiquitination of hair proteins was also obtained.

Structural changes in epidermal scale and appendages as indicators of defective TGM1 activity

Defective transglutaminase 1 (TGM1) is a causative factor in some cases of lamellar ichthyosis (LI) and congenital ichthyosiform erythroderma (CIE) despite large differences in the phenotype between these conditions. In some of these individuals, defective cornified envelopes (CEs) have been reported by light or electron microscopic examination in epidermal scale, nail and/or hair. These findings suggest that assessment of such defects could have a diagnostic utility in distinguishing TG1-deficient versus non-deficient cases of autosomal recessive ichthyosis (ARI) . Present work (a) examines the integrity of CEs in epidermal scale and appendages in a case of TGM1-deficient CIE, (b) assesses the utility of hair/nail versus scale analysis in the diagnosis of TGM1 deficiency in vivo and (c) helps characterize the consequences of the V518M mutation in TGM1, about which conflicting reports have appeared. To this end, epidermal scale or callus, nail and hair samples from a patient with TGM1-deficient CIE, his asymptomatic family members and control subjects were extracted vigorously in sodium dodecyl sulfate and dithiothreitol and examined by light (phase contrast) and electron microscopy. Both epidermal scale and nail from the index case lacked the prominent cell borders that were visible by phase contrast microscopy after detergent extraction of control samples. (By contrast, abundant envelope structures were visible in extracted epidermal scale from patients with ichthyosis vulgaris, loricrin keratoderma and epidermolytic hyperkeratosis.) Electron microscopy confirmed the paucity of intact CEs, and revealed further that hair cuticle cells from the same subject also lacked the marginal bands that are visible in control hair samples. Such aberrations were evident neither in the samples from asymptomatic relatives of the index case nor in the hair-cuticle cells of numerous normal individuals, evidence that this defect is not a common polymorphism. These studies extend our prior work on TGM1-deficient LI to the full spectrum of TGM1-deficient patients, showing that the CIE phenotype, when attributable to a V518M heterozygous mutation in TGM1 in combination with an inactive allele, confers a cross-linking deficiency in a variety of keratinizing epithelia, as previously shown for TGM1-negative LI. These results further suggest that a non-invasive assessment of scale, nail and hair could be of diagnostic utility in distinguishing patients across a full range of phenotypes with deficiency in TGM1-encoded transglutaminase activity from other causes of ARI.

Human epidermal cell protein responses to arsenite treatment in culture

Study of the responses of target cells in culture is anticipated to help understand the mechanisms by which inorganic arsenic causes pathological effects in vivo. Treatment of human epidermal cells with arsenic has been shown to produce a myriad of changes in gene transcription. Present work focused on finding the extent of arsenite-induced changes in the protein pattern and whether global effects on protein sulfhydryls were evident. First, examining the profile of protein expression by two-dimensional gel electrophoresis indicated that approximately 40% of the 300 distinct protein spots that were monitored changed by at least two-fold in amount all through a 9-day exposure period. Second, examining soluble extracts of the treated cells by Activated Thiol Sepharose column chromatography gave little indication of change in the overall protein thiol content. Finally, among the 10 proteins identified that showed prominent changes in amount as a result of treatment for 1 or 4 days, enzymes of the glycolytic pathway were seen to be substantially elevated as a result of treatment, suggesting decreased utilization by the cells of oxidative phosphorylation. Since these changes were more conspicuous at the protein level than in previous transcriptional studies, the results emphasize the importance of proteomic analysis to complement transcriptional analysis of cell responses to perturbation by arsenic.

In vivo effects of protein kinase and phosphatase inhibitors on CYP2B induction in rat liver

Effects of inhibiting protein kinases and phosphatases on induction of CYP2B by triphenyldioxane (TPD) and phenobarbital (PB) were investigated. Male Wistar rats were treated with test inhibitors before TPD or PB administration. Inhibitors of phosphatidylinositol-3-kinase (Wortmannin) and protein kinase C (bisindolylmaleimide I) did not have appreciable effects on TPD- or PB-induced pentoxyresorufin O-dealkylase (PROD) activity specific for CYP2B, although bisindolylmaleimide I did give substantial induction alone. W-7, an inhibitor of Ca2+/calmodulin-dependent kinase II, produced a 6-fold increase in the TPD-induced PROD activity and did not lead to a significant increase in basal PROD activity. Treatment of rats with okadaic acid (OA), an inhibitor of protein phosphatases PP1 and PP2A, caused considerable decreases in PROD activity during the induction by TPD and PB (8- and 2.5-fold, respectively). Results of multiplex RT-PCR showed that the increase in enzymatic activity from W7 and OA treatment reflected at least in part increased mRNA levels. CYP2B mRNA level in the liver of rats treated with W-7 and TPD was 1.5 times higher than in the liver of TPD-treated rats. This effect was not observed for PB-induction. OA treatment caused a decrease of the CYP2B mRNA levels of 44% and 33% respectively, for TPD- and PB-induction. Thus, our results are consistent with the hypothesis that phosphorylation/dephosphorylation signaling pathways are involved in regulation of CYP2B induction in rat liver.

Arsenite maintains germinative state in cultured human epidermal cells

Arsenic is a well-known carcinogen for human skin, but its mechanism of action and proximal macromolecular targets remain to be elucidated. In the present study, low micromolar concentrations of sodium arsenite maintained the proliferative potential of epidermal keratinocytes, decreasing their exit from the germinative compartment under conditions that promote differentiation of untreated cells. This effect was observed in suspension and in post-confluent surface cultures as measured by colony-forming ability and by proportion of rapidly adhering colony-forming cells. Arsenite-treated cultures exhibited elevated levels of beta1-integrin and beta-catenin, two proteins enriched in cells with high proliferative potential. Levels of phosphorylated (inactive) glycogen synthase kinase 3beta were higher in the treated cultures, likely accounting for the increased levels of transcriptionally available beta-catenin. These findings suggest that arsenic could have co-carcinogenic and tumor co-promoting activities in the epidermis as a result of increasing the population and persistence of germinative cells targeted by tumor initiators and promoters. These findings also identify a critical signal transduction pathway meriting further exploration in pursuit of this phenomenon.

Biological activity of inorganic arsenic and antimony reflects oxidation state in cultured human keratinocytes

Sodium arsenite is much more potent than sodium arsenate in producing adverse effects in animals and in cultured cells. Although arsenate may exhibit toxicity as a phosphate analogue, its potency in vivo appears to be enhanced by reduction to arsenite. To understand the relative importance of this reduction, which is critical in evaluating the responsiveness of cell culture models to the different oxidation states and thus to elucidating the mechanism of arsenic action, present work has correlated the extent of reduction with biological activity in human keratinocytes. The results show that at biologically relevant concentrations, arsenate reduction to appreciable levels required several days, helping rationalize a previous empirical observation that it was approximately one-third as potent as arsenite. The relatively low conversion rate also emphasizes a limitation of culture; arsenate was nearly as efficacious as arsenite, but the time required for it to reach maximal effect exceeded ordinary medium change intervals. In keratinocytes, an important role for purine nucleoside phosphorylase in the reduction could not be demonstrated, indicating that another pathway is dominant in this cell type. Methylation of inorganic arsenic, uptake of methylated forms, and their reduction were all very slow. These findings suggest that the reduced methylated forms have only a small contribution to skin carcinogenesis unless they are supplied through the circulation. In parallel experiments, trivalent antimony was similar to arsenite in potency and efficacy, whereas pentavalent antimony was virtually without biological effect. Conversion of antimony in the pentavalent to the trivalent oxidation state was not detectable in keratinocytes. These findings emphasize the importance of intracellular reduction of the metalloids for biological effects.

A distal region of the human TGM1 promoter is required for expression in transgenic mice and cultured keratinocytes

Background

TGM1(transglutaminase 1) is an enzyme that crosslinks the cornified envelope of mature keratinocytes. Appropriate expression of the TGM1 gene is crucial for proper keratinocyte function as inactivating mutations lead to the debilitating skin disease, lamellar ichthyosis. TGM1 is also expressed in squamous metaplasia, a consequence in some epithelia of vitamin A deficiency or toxic insult that can lead to neoplasia. An understanding of the regulation of this gene in normal and abnormal differentiation states may contribute to better disease diagnosis and treatment.

Methods

In vivo requirements for expression of the TGM1 gene were studied by fusing various lengths of promoter DNA to a reporter and injecting the DNA into mouse embryos to generate transgenic animals. Expression of the reporter was ascertained by Western blotting and immunohistochemistry. Further delineation of a transcriptionally important distal region was determined by transfections of progressively shortened or mutated promoter DNA into cultured keratinocytes.

Results

In vivo analysis of a reporter transgene driven by the TGM1 promoter revealed that 1.6 kilobases, but not 1.1 kilobases, of DNA was sufficient to confer tissue-specific and cell layer-specific expression. This same region was responsible for reporter expression in tissues undergoing squamous metaplasia as a response to vitamin A deprivation. Mutation of a distal promoter AP1 site or proximal promoter CRE site, both identified as important transcriptional elements in transfection assays, did not prevent appropriate expression. Further searching for transcriptional elements using electrophoretic mobility shift (EMSA) and transfection assays in cultured keratinocytes identified two Sp1 elements in a transcriptionally active region between -1.6 and -1.4 kilobases. While mutation of either Sp1 site or the AP1 site singly had only a small effect, mutation of all three sites eliminated nearly all the transcriptional activity.

Conclusions

A distal region of the TGM1 gene promoter, containing AP1 and Sp1 binding sites, is evolutionarily conserved and responsible for high level expression in transgenic mice and in transfected keratinocyte cultures.

Cross-linked envelopes in nail plate in lamellar ichthyosis

BACKGROUND

Corneocytes of the nail plate, like those of the stratum corneum, generate cornified envelopes (CEs) of cross-linked protein that can be visualized readily after removal of non-cross-linked protein by detergent extraction. Defective CE formation occurs in epidermal scale and hair in transglutaminase 1 (TGM1)-negative lamellar ichthyosis (LI) and has been proposed as a diagnostic aid for this syndrome.

OBJECTIVES

(i) To ascertain whether TGM1 is important for CE formation in nail; (ii) to characterize CE abnormalities occurring in LI that may be distinguished from other types of inherited ichthyosis when nail samples are subjected to detergent extraction; and (iii) to evaluate the utility of nails as a diagnostic aid for LI.

METHODS

Nail samples were provided by nine patients previously classified as having TGM1-negative LI, four with other types of ichthyotic conditions and six normal controls. Samples were extracted extensively in sodium dodecyl sulphate under reducing conditions and examined by light and electron microscopy.

RESULTS

After extraction, defective CE cross-linking was visualized in epidermal corneocytes from seven of nine patients exhibiting TGM1-negative LI, whereas nail samples from patients with the other syndromes were normal. The defects in CE structure resembled those recently reported for LI scale, although in some cases residual CE and CE-associated structures were present.

CONCLUSIONS

Despite the paucity of clinical nail symptoms in LI, TGM1 activity is important for generation of normal CE in nail plate, consistent with its importance in protein cross-linking in interfollicular epidermis and hair. Lack of this activity leads to a strikingly aberrant appearance of CE in LI nail after detergent extraction that is evident ultrastructurally in a large majority of cases. Nail envelopes therefore could provide a useful diagnostic tool in distinguishing LI from other ichthyoses with overlapping clinical features.

Reversible stepwise negative regulation of CYP1A1 in cultured rat epidermal cells

When serially passaged, rat epidermal keratinocytes lose the inducibility of CYP1A1 gene expression in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure. In present experiments, loss of CYP1A1 inducibility occurred in a stepwise fashion, with some keratinocyte lines progressing through a transiently inducible state before becoming completely uninducible. The negative regulation occurred at the level of transcription, but the aryl hydrocarbon receptor (AhR) pathway appeared fully functional. Transient and stable transfection of uninducible cells with reporter constructs containing up to 4.2kb of the CYP1A1 5'-flanking region resulted in a TCDD-inducible increase in luciferase activity, despite no induction of the endogenous gene. Co-treatment with protein synthesis inhibitors and TCDD restored responsiveness of the endogenous CYP1A1 gene, indicating that the negative regulation was reversible and likely mediated by a labile protein. Together, these results demonstrate a novel mechanism of CYP1A1 transcriptional repression that does not involve any previously reported negative regulatory elements for CYP1A1.

Global alteration of gene expression in human keratinocytes by inorganic arsenic

Alteration of gene expression by inorganic arsenic has been studied in cultured human keratinocytes derived from normal epidermis, a premalignant lesion and a malignant tumor. The purpose was to find whether these cells displayed common alterations in gene expression that might elucidate the mechanism of arsenic action. Global analysis of approximately 12 000 genes by microarray showed that approximately 30% were expressed. Of these, transcription of a substantial fraction (up to 12%) was altered, nearly twice as many being suppressed as stimulated by 2-fold or more at 2 micro M sodium arsenite or 6 micro M arsenate, which did not affect cell growth. At 0.67 micro M arsenite (50 p.p.b.), effects on transcription were less pronounced but clearly evident. Genes whose transcription was altered in common among all the treated keratinocytes included those induced by reactive oxygen, of which heme oxygenase-1 displayed the highest fold induction. Genes indicative of reactive oxygen generation were detected at the earliest time examined, raising the possibility this feature drives subsequent cellular responses. Unlike some agents that produced transient induction of heme oxygenase-1, arsenicals produced sustained induction. Comparison with other agents producing reactive oxygen in the cells, as reflected in heme oxygenase-1 induction, suggested cellular differentiation was suppressed by sustained but not transient generation of reactive oxygen. Sustained global changes in gene expression were seen in target cells treated chronically with inorganic arsenic at concentrations consumed by millions of humans in contaminated drinking water.

Gene-specific TCDD suppression of RARalpha- and RXR-mediated induction of tissue transglutaminase

The malignant human keratinocyte line SCC4 provides a model system to study the mechanism by which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) suppresses retinoid induction of the tissue transglutaminase gene (TGM2). The current work explores the nature of TCDD suppression of retinoid action to determine whether it is gene specific, whether it is retinoid receptor isoform-dependent, and whether it requires close proximity of retinoid and TCDD response elements. First, two other retinoid-inducible genes were identified in SCC4 by microarray screening whose induction was unaffected by TCDD, clearly demonstrating the gene specificity of TCDD suppression. Second, the receptor isoform dependence of retinoid responsiveness in SCC4 was tested. TGM2 was found to be inducible by an RARalpha-specific but not by an RARgamma-selective agonist. A lack of responsiveness to RARgamma agonists was found to be characteristic of SCC4, however, inasmuch as transcription driven by a retinoid response element in transfections was also stimulated only by the alpha-specific agonist in these cells. Because SCC4 lacks expression of RARbeta, the gene specificity evidently was not attributable to differential TCDD targeting of retinoid receptor isoforms. Finally, the proximal 5 kb of the TGM2 promoter was found to be retinoid responsive in stable transfections, but the induction was not suppressed by TCDD. These results indicate that the suppressive action of TCDD occurs indirectly and through a separate DNA site likely located outside the 5-kb region, not by direct interference with retinoid action or at retinoid response elements.