CD34 Antigen: Determination of Specific Sites of Phosphorylation In Vitro and In Vivo.

CD34, a type I transmembrane glycoprotein, is a surface antigen which is expressed on several cell types, including hematopoietic progenitors, endothelial cells, as well as mast cells. Recently, CD34 has been described as a marker for epidermal stem cells in mouse hair follicles, and is expressed in outer root sheath cells of the human hair follicle. Although the biological function and regulation of CD34 is not well understood, it is thought to be involved in cell adhesion as well as possibly having a role in signal transduction. In addition, CD34 was shown to be critical for skin tumor development in mice, although the exact mechanism remains unknown.Many proteins' functions and biological activities are regulated through post-translational modifications. The extracellular domain of CD34 is heavily glycosylated but the role of these glycans in CD34 function is unknown. Additionally, two sites of tyrosine phosphorylation have been reported on human CD34 and it is known that CD34 is phosphorylated, at least in part, by protein kinase C; however, the precise location of the sites of phosphorylation has not been reported. In an effort to identify specific phosphorylation sites in CD34 and delineate the possible role of protein kinase C, we undertook the identification of the in vitro sites of phosphorylation on the intracellular domain of mouse CD34 (aa 309-382) following PKC treatment. For this work, we are using a combination of enzymatic proteolysis and peptide sequencing by mass spectrometry. After which the in vivo sites of phosphorylation of full-length mouse CD34 expressed from HEK293F cells were determined. The observed in vivo sites of phosphorylation, however, are not consensus PKC sites, but our data indicate that one of these sites may possibly be phosphorylated by AKT2. These results suggest that other kinases, as well as PKC, may have important signaling functions in CD34.

A novel role for the T-box transcription factor Tbx1 as a negative regulator of tumor cell growth in mice.

The T-box transcription factor, Tbx1, an important regulatory gene in development, is highly expressed in hair follicle (HF) stem cells in adult mice. Because mouse models of skin carcinogenesis have demonstrated that HF stem cells are a carcinogen target population and contribute significantly to tumor development, we investigated whether Tbx1 plays a role in skin carcinogenesis. We first assessed Tbx1 expression levels in mouse skin tumors, and found down-regulation in all tumors examined. To study the effect of Tbx1 expression on growth and tumorigenic potential of carcinoma cells, we transfected mouse Tbx1 cDNA into a mouse spindle cell carcinoma cell line that did not express endogenous Tbx1. Following transfection, two cell lines expressing different levels of the Tbx1/V5 fusion protein were selected for further study. Intradermal injection of the cell lines into mice revealed that Tbx1 expression significantly suppressed tumor growth, albeit with no change in tumor morphology. In culture, ectopic Tbx1 expression resulted in decreased cell growth and reduced development into multilayered colonies, compared to control cells. Tbx1-transfectants exhibited a reduced proliferative rate compared to control cells, with fewer cells in S and G2/M phases. The Tbx1 transfectants developed significantly fewer colonies in soft agar, demonstrating loss of anchorage-independent growth. Taken together, our data show that ectopic expression of Tbx1 restored contact inhibition to the skin tumor cells, suggesting that this developmentally important transcription factor may have a novel dual role as a negative regulator of tumor growth. © 2011 Wiley Periodicals, Inc.

Erbb2 suppresses DNA damage-induced checkpoint activation and UV-induced mouse skin tumorigenesis.

The Erbb2 receptor is activated by UV irradiation, the primary cause of non-melanoma skin cancer. We hypothesized that Erbb2 activation contributes to UV-induced skin tumorigenesis by suppressing cell cycle arrest. Consistent with this hypothesis, inhibition of Erbb2 in v-ras(Ha) transgenic mice before UV exposure resulted in both 56% fewer skin tumors and tumors that were 70% smaller. Inhibition of the UV-induced activation of Erbb2 also resulted in milder epidermal hyperplasia, S-phase accumulation, and decreased levels of the cell cycle regulator Cdc25a, suggesting altered cell cycle regulation on inhibition of Erbb2. Further investigation using inhibition or genetic deletion of Erbb2 in vitro revealed reduced Cdc25a levels and increased S-phase arrest in UV-irradiated cells lacking Erbb2 activity. Ectopic expression of Cdc25a prevented UV-induced S-phase arrest in keratinocytes lacking Erbb2 activity, demonstrating that maintenance of Cdc25a by Erbb2 suppresses cell cycle arrest. Examination of checkpoint pathway activation upstream of Cdc25a revealed Erbb2 activation did not alter Ataxia Telangiectasia and Rad3-related/Ataxia Telangiectasia Mutated activity but increased inhibitory phosphorylation of Chk1-Ser(280). Since Akt phosphorylates Chk1-Ser(280), the effect of Erbb2 on phosphatidyl inositol-3-kinase (PI3K)/Akt signaling during UV-induced cell cycle arrest was determined. Erbb2 ablation reduced the UV-induced activation of PI3K while inhibition of PI3K/Akt increased UV-induced S-phase arrest. Thus, UV-induced Erbb2 activation increases skin tumorigenesis through inhibitory phosphorylation of Chk1, Cdc25a maintenance, and suppression of S-phase arrest via a PI3K/Akt-dependent mechanism.

Arsenic exposure in utero exacerbates skin cancer response in adulthood with contemporaneous distortion of tumor stem cell dynamics.

Arsenic is a carcinogen with transplacental activity that can affect human skin stem cell population dynamics in vitro by blocking exit into differentiation pathways. Keratinocyte stem cells (KSC) are probably a key target in skin carcinogenesis. Thus, we tested the effects of fetal arsenic exposure in Tg.AC mice, a strain sensitive to skin carcinogenesis via activation of the v-Ha-ras transgene likely in KSCs. After fetal arsenic treatment, offspring received topical 12-O-tetradecanoyl phorbol-13-acetate (TPA) through adulthood. Arsenic alone had no effect, whereas TPA alone induced papillomas and squamous cell carcinomas (SCC). However, fetal arsenic treatment before TPA increased SCC multiplicity 3-fold more than TPA alone, and these SCCs were much more aggressive (invasive, etc.). Tumor v-Ha-ras levels were 3-fold higher with arsenic plus TPA than TPA alone, and v-Ha-ras was overexpressed early on in arsenic-treated fetal skin. CD34, considered a marker for both KSCs and skin cancer stem cells, and Rac1, a key gene stimulating KSC self-renewal, were greatly increased in tumors produced by arsenic plus TPA exposure versus TPA alone, and both were elevated in arsenic-treated fetal skin. Greatly increased numbers of CD34-positive probable cancer stem cells and marked overexpression of RAC1 protein occurred in tumors induced by arsenic plus TPA compared with TPA alone. Thus, fetal arsenic exposure, although by itself oncogenically inactive in skin, facilitated cancer response in association with distorted skin tumor stem cell signaling and population dynamics, implicating stem cells as a target of arsenic in the fetal basis of skin cancer in adulthood.

Identification of a specific motif of the DSS1 protein required for proteasome interaction and p53 protein degradation.

Deleted in Split hand/Split foot 1 (DSS1) was previously identified as a novel 12-O-tetradecanoylphorbol-13-acetate (TPA)-inducible gene with possible involvement in early event of mouse skin carcinogenesis. The mechanisms by which human DSS1 (HsDSS1) exerts its biological effects via regulation of the ubiquitin-proteasome system (UPS) are currently unknown. Here, we demonstrated that HsDSS1 regulates the human proteasome by associating with it in the cytosol and nucleus via the RPN3/S3 subunit of the 19S regulatory particle (RP). Molecular anatomy of HsDSS1 revealed an RPN3/S3-interacting motif (R3IM), located at amino acid residues 15 to 21 of the NH(2) terminus. Importantly, negative charges of the R3IM motif were demonstrated to be required for proteasome interaction and binding to poly-ubiquitinated substrates. Indeed, the R3IM motif of HsDSS1 protein alone was sufficient to replace the ability of intact HsDSS1 protein to pull down proteasome complexes and protein substrates with high-molecular mass ubiquitin conjugates. Interestingly, this interaction is highly conserved throughout evolution from humans to nematodes. Functional study, lowering the levels of the endogenous HsDSS1 using siRNA, indicates that the R3IM/proteasome complex binds and targets p53 for ubiquitin-mediated degradation via gankyrin-MDM2/HDM2 pathway. Most significantly, this work indicates that the R3IM motif of HsDSS1, in conjunction with the complexes of 19S RP and 20S core particle (CP), regulates proteasome interaction through RPN3/S3 molecule, and utilizes a specific subset of poly-ubiquitinated p53 as a substrate.

Gene expression response in target organ and whole blood varies as a function of target organ injury phenotype.

This report details the standardized experimental design and the different data streams that were collected (histopathology, clinical chemistry, hematology and gene expression from the target tissue (liver) and a bio-available tissue (blood)) after treatment with eight known hepatotoxicants (at multiple time points and doses with multiple biological replicates). The results of the study demonstrate the classification of histopathological differences, likely reflecting differences in mechanisms of cell-specific toxicity, using either liver tissue or blood transcriptomic data.

CD34 expression by hair follicle stem cells is required for skin tumor development in mice.

The cell surface marker CD34 marks mouse hair follicle bulge cells, which have attributes of stem cells, including quiescence and multipotency. Using a CD34 knockout (KO) mouse, we tested the hypothesis that CD34 may participate in tumor development in mice because hair follicle stem cells are thought to be a major target of carcinogens in the two-stage model of mouse skin carcinogenesis. Following initiation with 200 nmol 7,12-dimethylbenz(a)anthracene (DMBA), mice were promoted with 12-O-tetradecanoylphorbol-13-acetate (TPA) for 20 weeks. Under these conditions, CD34KO mice failed to develop papillomas. Increasing the initiating dose of DMBA to 400 nmol resulted in tumor development in the CD34KO mice, albeit with an increased latency and lower tumor yield compared with the wild-type (WT) strain. DNA adduct analysis of keratinocytes from DMBA-initiated CD34KO mice revealed that DMBA was metabolically activated into carcinogenic diol epoxides at both 200 and 400 nmol. Chronic exposure to TPA revealed that CD34KO skin developed and sustained epidermal hyperplasia. However, CD34KO hair follicles typically remained in telogen rather than transitioning into anagen growth, confirmed by retention of bromodeoxyuridine-labeled bulge stem cells within the hair follicle. Unique localization of the hair follicle progenitor cell marker MTS24 was found in interfollicular basal cells in TPA-treated WT mice, whereas staining remained restricted to the hair follicles of CD34KO mice, suggesting that progenitor cells migrate into epidermis differently between strains. These data show that CD34 is required for TPA-induced hair follicle stem cell activation and tumor formation in mice.

Cadmium-induced toxicity in rat primary mid-brain neuroglia cultures: role of oxidative stress from microglia.

This study examined the role of oxidative stress in neurotoxic effects of cadmium chloride (Cd) in rat primary mid-brain neuron-glia cultures. Cd accumulated in neuron-glia cultures and produced cytotoxicity in a dose-dependent manner, with IC(50) of 2.5microM 24 h after exposure. (3)H-dopamine uptake into neuron-glia cultures was decreased 7 days after Cd exposure, with IC(50) of 0.9microM, indicative of the sensitivity of dopaminergic neurons to Cd toxicity. To investigate the role of microglia in Cd-induced toxicity to neurons, microglia-enriched cultures were prepared. Cd significantly increased intracellular reactive oxygen species production in microglia-enriched cultures, as evidenced by threefold increases in 2',7'-dichlorofluorescein signals. Using 5,5-dimethyl-1-pyrroline N-oxide as a spin-trapping agent, Cd increased electron spin resonance signals by 3.5-fold in microglia-enriched cultures. Cd-induced oxidative stress to microglia-enriched cultures was further evidenced by activation of redox-sensitive transcription factor nuclear factor kappa B and activator protein-1 (AP-1), and the increased expression of oxidative stress-related genes, such as metallothionein, heme oxygenase-1, glutathione S-transferase pi, and metal transport protein-1, as determined by gel-shift assays and real-time reverse transcription-PCR, respectively, in microglia-enriched cultures. In conclusion, Cd is toxic to neuron-glia cultures, and the oxidative stress from microglia may play important roles in Cd-induced damage to dopaminergic neurons.

Identification of genes and gene ontology processes critical to skin papilloma development in Tg.AC transgenic mice.

This study analyzes gene expression associated with papilloma development in Tg.AC v-Ha-ras transgenic mice and identifies novel genes and biological processes that may be critical to skin carcinogenesis in these mice. Epidermal abrasion was used to synchronously induce epidermal regeneration in FVB/N wild type and transgenic Tg.AC mice. Skin papillomagenesis was uniquely induced in Tg.AC mice, and gene expression profiling was carried out using a 22,000 element mouse DNA microarray. Histological analysis showed that papillomas developed at a high rate by d 30 after abrasion in transgenic animals, while no papilloma developed in wild type mice. Transgene-specific differentially expressed genes were identified at d 30 postabrasion and these genes were annotated using EASE software and literature mining. Annotated and non-annotated genes associated with papilloma development were identified and clustering analysis revealed groups of genes that are coordinately expressed. A number of genes associated with differentiation and development were also physically clustered on mouse chromosome 16, including 16B3 that contains several Stefins and stefin-like genes, and 16A1 containing a number of keratin associated protein genes. Additional analyses presented here yield novel insights into the genes and processes involved in papilloma development in Tg.AC mice.

Biological, cellular, and molecular characteristics of an inducible transgenic skin tumor model: a review.

The genetically initiated Tg.AC transgenic mouse carries a transgene consisting of an oncogenic v-Ha-ras coding region flanked 5' by a mouse zeta-globin promoter and 3' by an SV-40 polyadenylation sequence. Located on chromosome 11, the transgene is transcriptionally silent until activated by chemical carcinogens, UV light, or full-thickness wounding. Expression of the transgene is an early event that drives cellular proliferation resulting in clonal expansion and tumor formation, the unique characteristics now associated with the Tg.AC mouse. This ras-dependent phenotype has resulted in the widespread interest and use of the Tg.AC mouse in experimental skin carcinogenesis and as an alternative carcinogenesis assay. This review examines the general biology of the tumorigenic responses observed in Tg.AC mice, the genetic interactions of the ras transgene, and explores the cellular and molecular regulation of zeta-globin promoted transgene expression. As a prototype alternative model to the current long-term rodent bioassays, the Tg.AC has generated a healthy discussion on the future of transgenic bioassays, and opened the doors for subsequent models for toxicity testing. The further exploration and elucidation of the molecular controls of transgene expression will enhance the usefulness of this mouse and enable a better understanding of the Tg.AC's discriminate response to chemical carcinogens.

Chemoprevention of UV light-induced skin tumorigenesis by inhibition of the epidermal growth factor receptor.

The epidermal growth factor receptor (EGFR) is activated in skin cells following UV irradiation, the primary cause of nonmelanoma skin cancer. The EGFR inhibitor AG1478 prevented the UV-induced activation of EGFR and of downstream signaling pathways through c-Jun NH2-terminal kinases, extracellular signal-regulated kinases, p38 kinase, and phosphatidylinositol 3-kinase in the skin. The extent to which the UV-induced activation of EGFR influences skin tumorigenesis was determined in genetically initiated v-ras(Ha) transgenic Tg.AC mice, which have enhanced susceptibility to skin carcinogenesis. Topical treatment or i.p. injection of AG1478 before UV exposure blocked the UV-induced activation of EGFR in the skin and decreased skin tumorigenesis in Tg.AC mice. AG1478 treatment before each of several UV exposures decreased the number of papillomas arising and the growth of these tumors by approximately 50% and 80%, respectively. Inhibition of EGFR suppressed proliferation, increased apoptotic cell death, and delayed the onset of epidermal hyperplasia following UV irradiation. Genetic ablation of Egfr similarly delayed epidermal hyperplasia in response to UV exposure. Thus, the UV-induced activation of EGFR promotes skin tumorigenesis by suppressing cell death, augmenting cell proliferation, and accelerating epidermal hyperplasia in response to UV. These results suggest that EGFR may be an appropriate target for the chemoprevention of UV-induced skin cancer.

Gene expression profiling of rat livers reveals indicators of potential adverse effects.

This study tested the hypothesis that gene expression profiling can reveal indicators of subtle injury to the liver induced by a low dose of a substance that does not cause overt toxicity as defined by conventional criteria of toxicology (e.g., abnormal clinical chemistry and histopathology). For the purpose of this study we defined this low dose as subtoxic, i.e., a dose that elicits effects which are below the detection of conventional toxicological parameters. Acetaminophen (APAP) was selected as a model hepatotoxicant because (1) considerable information exists concerning the mechanism of APAP hepatotoxicity that can occur following high doses, (2) intoxication with APAP is the leading cause of emergency room visits involving acute liver failure within the United States, and (3) conventional clinical markers have poor predictive value. Rats treated with a single dose of 0, 50, 150, or 1500 mg/kg APAP were examined at 6, 24, or 48 h after exposure for conventional toxicological parameters and for gene expression alterations. Patterns of gene expression were found which indicated cellular energy loss as a consequence of APAP toxicity. Elements of these patterns were apparent even after exposure to subtoxic doses. With increasing dose, the magnitude of changes increased and additional members of the same biological pathways were differentially expressed. The energy loss suggested by gene expression changes was confirmed at the 1500 mg/kg dose exposure by measuring ATP levels. Only by ultrastructural examination could any indication of toxicity be identified after exposure to a subtoxic dose of APAP and that was occasional mitochondrial damage. In conclusion, this study provides evidence that supports the hypothesis that gene expression profiling may be a sensitive means of identifying indicators of potential adverse effects in the absence of the occurrence of overt toxicity.

Strong synergy between mutant ras and HPV16 E6/E7 in the development of primary tumors.

E6/E7 oncogenes of high-risk human papilloma virus (HPV) subtypes are essential for the development of certain types of cancers. However, these oncogenes are insufficient to transform normal cells into an immortalized or malignant state. Mutant Ha-ras cooperates with E6/E7 of HPV subtype 16 in transformation of cells in vitro and may contribute to some HPV-associated cancers in humans. This study investigates whether HPV16 E6/E7 and v-Ha-ras synergize in vivo. FVB/n mice transgenic for v-Ha-ras gene (R+) were crossed with transgenic C57BL/6 mice that harbor E6/E7 of HPV16 (E+). Beginning at about 3 months of age, the bitransgenic E(+)R(+)(C57BL/6 x FVB/n) F1 mice developed mouth, eye and ear tumors. By 6 months, the prevalence of these types of mouth, eye and ear tumors was 100, 71 and 79% respectively in the E(+)R+ mice. Most tumors grew progressively until the mice had to be killed. The median times for the appearance of the first mouth, eye and ear tumor were 3.6, 4.3 and 4.2 months, respectively. For the two singly transgenic groups of mice, the prevalence of mouth, eye and ear tumors was 0, 0 and 6% (E(-)R+) and 0, 0 and 0% (E(+)R-), respectively, and the median time to first tumor was greater than 12 months for singly transgenic mice (E(-)R+, E(+)R-). Thus, a remarkable synergy occurred between the v-Ha-ras and HPV16 E6/E7 oncogenes in the development of primary tumors in mice.

12-O-tetradecanoylphorbol-13-acetate and UV radiation-induced nucleoside diphosphate protein kinase B mediates neoplastic transformation of epidermal cells.

The molecular changes associated with early skin carcinogenesis are largely unknown. We have previously identified 11 genes whose expression was up- or down-regulated by 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse skin keratinocyte progenitor cells (Wei, S.-J., Trempus, C. S., Cannon, R. E., Bortner, C. D., and Tennant, R. W. (2003) J. Biol. Chem. 278, 1758-1768). Here, we show an induction of a nucleoside diphosphate protein kinase B (NDPK-B) gene in response to TPA or UV radiation (UVR). TPA or UVR significantly induced the expression of NDPK-B both in vivo hyperplastic mouse skin and in vitro mouse JB6 Cl 41-5a epidermal cells. Indeed, this gene was also up-regulated in TPA or UVR-mediated skin tumors including papillomas, spindle cell tumors, and squamous cell carcinomas, relative to adjacent normal skins. Functional studies by constitutive expression of nm23-M2/NDPK-B in TPA susceptible JB6 Cl 41-5a and TPA-resistant JB6 Cl 30-7b preneoplastic epidermal cell lines showed a remarkable gene dosage-dependent increase in foci-forming activity, as well as an enhancement in the efficiency of neoplastic transformation of these cells in soft agar but no effect on proliferation in monolayer cultures. Interestingly, stable transfection of the nm23-M2/NDPK-B del-RGD or G106A mutant gene in JB6 Cl 41-5a cells selectively abrogated NDPK-B-induced cellular transformation, implicating a possible Arg105-Gly106-Asp107 regulatory role in early skin carcinogenesis.

Toxicogenomic approach for assessing toxicant-related disease.

The problems of identifying environmental factors involved in the etiology of human disease and performing safety and risk assessments of drugs and chemicals have long been formidable issues. Three principal components for predicting potential human health risks are: (1) the diverse structure and properties of thousands of chemicals and other stressors in the environment; (2) the time and dose parameters that define the relationship between exposure and disease; and (3) the genetic diversity of organisms used as surrogates to determine adverse chemical effects. The global techniques evolving from successful genomics efforts are providing new exciting tools with which to address these intractable problems of environmental health and toxicology. In order to exploit the scientific opportunities, the National Institute of Environmental Health Sciences has created the National Center for Toxicogenomics (NCT). The primary mission of the NCT is to use gene expression technology, proteomics and metabolite profiling to create a reference knowledge base that will allow scientists to understand mechanisms of toxicity and to be able to predict the potential toxicity of new chemical entities and drugs. A principal scientific objective underpinning the use of microarray analysis of chemical exposures is to demonstrate the utility of signature profiling of the action of drugs or chemicals and to utilize microarray methodologies to determine biomarkers of exposure and potential adverse effects. The initial approach of the NCT is to utilize proof-of-principle experiments in an effort to "phenotypically anchor" the altered patterns of gene expression to conventional parameters of toxicity and to define dose and time relationships in which the expression of such signature genes may precede the development of overt toxicity. The microarray approach is used in conjunction with proteomic techniques to identify specific proteins that may serve as signature biomarkers. The longer-range goal of these efforts is to develop a reference relational database of chemical effects in biological systems (CEBS) that can be used to define common mechanisms of toxicity, chemical and drug actions, to define cellular pathways of response, injury and, ultimately, disease. In order to implement this strategy, the NCT has created a consortium of research organizations and private sector companies to actively collaborative in populating the database with high quality primary data. The evolution of discrete databases to a knowledge base of toxicogenomics will be accomplished through establishing relational interfaces with other sources of information on the structure and activity of chemicals such as that of the National Toxicology Program (NTP) and with databases annotating gene identity, sequence, and function.

Molecular structure and characterization of a novel murine ABC transporter, Abca13.

We report the isolation and structural characterization of the full-length gene and cDNA for a novel mouse ATP-binding cassette (ABC) transporter, Abca13. The mRNA, isolated from mouse kidney, is 6.7 kb in size and encodes a protein consisting of 2143 amino acids with a predicted molecular weight of 240 kDa. The Abca13 gene consists of 44 exons which span 360 kb of genomic sequence. Abca13 has been mapped to mouse chromosome 11.a2, revealing the human orthologue highly conserved on a syntenic region of human chromosome 7p12. The deduced mouse Abca13 protein shows highest amino acid sequence homology to human ABCA1 (50%), ABCA4 (50%), and ABCA12 (56%). Analysis of the putative Abca13 promoter region revealed potential transcription factor binding sites associated with myeloid- and lymphoid-derived cell types. mRNA transcript levels were highest in mouse submaxillary gland, epididymus, ovary, and thymus; with lower levels in a variety of other tissues. An alternative transcript was discovered in mouse kidney devoid of exon 11. The removal of exon 11 by post-transcriptional splicing causes a frameshift in the open reading frame and results in a premature termination codon. We hypothesize that the excision of exon 11 may serve as a regulatory mechanism in kidney, and perhaps other tissues as well. The molecular characterization of the mouse Abca13 gene will establish the foundation for future functional studies of the human ABCA13 transporter.

Enrichment for living murine keratinocytes from the hair follicle bulge with the cell surface marker CD34.

It is widely believed that epithelial stem cells reside in the hair follicle bulge region. We investigated the hematopoietic stem and progenitor cell marker, CD34, as a potential marker of hair follicle bulge keratinocytes. Using a CD34-specific antibody, we identified intense membrane staining on keratinocytes in the bulge region of the mouse hair follicle. CD34 expression colocalized with both slowly cycling (label retaining) cells and keratin 15 expression. Live CD34+ keratinocytes were positively selected using antibodies to CD34 and alpha6 integrin in combination with fluorescent activated cell sorting. Sorted cells were analyzed for DNA content, and a staining profile was generated to confirm these cells as keratinocytes. CD34+ keratinocytes were predominantly in Go/G1, in contrast to CD34- cells, which had well defined G2/M and S phases. In addition, CD34+ keratinocytes were found to express alpha6 integrin more intensely than CD34- cells (p<0.05), identifying this population as an alpha6 integrin bright subset. When seeded at clonal density, CD34+ keratinocytes formed larger colonies than CD34- cells (p<0.05), indicating a higher proliferative potential. All flow-sorted cells were positive for keratin 14 expression, and negative for keratin 1, loricrin, vimentin, and CD31. The majority of CD34+ cells (98%) were positive for keratin 6, establishing this population as basal keratinocytes of follicular origin. CD34 message was detected by reverse transcription polymerase chain reaction predominantly in the CD34+ keratinocytes, confirming specificity of the antibody. This work is the first to demonstrate that CD34 is a specific marker of bulge cell keratinocytes in the cutaneous epithelium. Furthermore, the use of this marker facilitates isolation of live epithelial cells with stem and progenitor cell characteristics, potentially providing a tool for the study of carcinogen target cells, gene therapy, and tissue engineering applications.