Function and regulation of AP-1 subunits in skin physiology and pathology

Reconstituted skin from murine embryonic stem cells

Embryonic stem (ES) cell lines can be expanded indefinitely in culture while maintaining their potential to differentiate into any cell type. During embryonic development, the skin forms as a result of reciprocal interactions between mesoderm and ectoderm. Here, we report the in vitro differentiation and enrichment of keratinocytes from murine ES cells seeded on extracellular matrix (ECM) in the presence of Bone Morphogenic Protein-4 (BMP-4) or ascorbate. The enriched preparation of keratinocytes was able to form an epidermal equivalent composed of a stratified epithelium when cultured at the air-liquid interface on a collagen-coated acellular substratum. Interestingly, an underlying cellular compartment that belongs to the fibroblast lineage was systematically formed between the reconstituted epidermis and the inert membrane. The resulting tissue displayed morphological patterns similar to normal embryonic skin, as evidenced by light and transmission electron microscopy. Immunohistochemical studies revealed expression patterns of cytokeratins, basement membrane (BM) proteins and late differentiation markers of epidermis, as well as fibroblast markers, similar to native skin. The results demonstrate the capacity of ES cells to reconstitute in vitro a fully differentiated skin. This ES-derived bioengineered skin provides a powerful tool for studying the molecular mechanisms controlling epidermal and dermal commitments.

Profile of gene expression induced by the tumour promotor TPA in murine epithelial cells

Malignant transformation of mouse skin by chemical carcinogens and tumour promoters, such as the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), is a multistage process that leads to squamous cell carcinoma (SCC) formation. In an effort to identify tumour-associated genes, we studied the influence of short-term TPA-treatment on the gene expression profile of murine skin. A comprehensive microarray with some 5,000 murine gene specific cDNA fragments was established and hybridised with pooled RNA derived from control and TPA-treated dorsal skin samples. Of these genes, 54 were up- and 35 were down-regulated upon TPA application. Additionally, we performed suppression subtractive hybridisation (SSH) with respective RNA pools to generate and analyse a cDNA library enriched for TPA-inducible genes. Expression data of selected genes were confirmed by quantitative real-time PCR and Northern blot analysis. Comparison of microarray and SSH data revealed that 26% of up-regulated genes identified by expression profiling matched with those present in the SSH library. Besides numerous known genes, we identified a large set of unknown cDNAs that represent previously unrecognised TPA-regulated genes in murine skin with potential function in tumour promotion. Additionally, some TPA-induced genes, such as Sprr1A, Saa3, JunB, Il4ralpha, Gp38, RalGDS and Slpi exhibit high basal level in advanced stages of skin carcinogenesis, suggesting that at least a subgroup of the identified TPA-regulated genes may contribute to tumour progression and metastasis.

p38 Mitogen-activated protein kinases on the body surface--a function for p38 delta

The p38 family of mitogen-activated protein kinases includes p38 alpha (SAPK2a, CSBP), p38 beta (SAPK2b), p38 delta (SAPK4), and p38 gamma (SAPK3/ERK6). p38 alpha and p38 beta are widely expressed p38 isoforms that are involved in regulation of cell proliferation, differentiation, development, and response to stress. Relatively less is known regarding the function of the p38 delta isoform. In this review, we discuss the role of the p38 alpha, p38 beta, and p38 gamma isoforms and then present recent findings that define a role for p38 delta as a regulator of differentiation-dependent gene expression in keratinocytes.

Determination of leptin signaling pathways in human and murine keratinocytes

Recently, we have determined the role of leptin as a keratinocyte mitogen in vitro and during skin repair in vivo. In this study, we assessed leptin-stimulated signal transduction in the human keratinocyte cell line HaCaT and the murine keratinocyte cell line PAM 212. HaCaT keratinocytes were characterized by a constitutive phosphorylation of janus kinase (JAK)-2. By contrast, PAM 212 keratinocytes responded to leptin with a rapid phosphorylation of JAK-2. However, we could determine a cytoplasmic activation of signal transducer and activator of transcription (STAT)-3 by phosphorylation of tyrosine 705 (Y705) within minutes only upon leptin stimulation in both keratinocyte cell lines. Subsequently, STAT-3 translocated to the nucleus where serine 727 (S727) was phosphorylated, establishing a transcriptionally active STAT-3 transcription factor. In a model of cutaneous wound healing, treatment of leptin-deficient obese/obese (ob/ob) mice strongly augmented phosphorylation of STAT-3 (Y705) in wound keratinocytes also in vivo.

The structural determinants responsible for c-Fos protein proteasomal degradation differ according to the conditions of expression

c-fos gene is expressed constitutively in a number of tissues as well as in certain tumor cells and is inducible, in general rapidly and transiently, in virtually all other cell types by a variety of stimuli. Its protein product, c-Fos, is a short-lived transcription factor that heterodimerizes with various protein partners within the AP-1 transcription complex via leucine zipper/leucine zipper interactions for binding to specific DNA sequences. It is mostly, if not exclusively, degraded by the proteasome. To localize the determinant(s) responsible for its instability, we have conducted a genetic analysis in which the half-lives of c-Fos mutants and chimeras made with the stable EGFP reporter protein were compared under two experimental conditions taken as example of continous and inducible expression. Those were constitutive expression in asynchronously growing Balb/C 3T3 mouse embryo fibroblasts and transient induction in the same cells undergoing the G0/G1 phase transition upon stimulation by serum. Our work shows that c-Fos is degraded faster in synchronous- than in asynchronous cells. This difference in turnover is primarily accounted for by several mechanisms. First, in asynchronous cells, a unique C-terminal destabilizer is active whereas, in serum-stimulated cells two destabilizers located at both extremities of the protein are functional. Second, heterodimerization and/or binding to DNA accelerates protein degradation only during the G0/G1 phase transition. Adding another level of complexity to turnover control, phosphorylation at serines 362 and 374, which are c-Fos phosphorylation sites largely modified during the G0/G1 phase transition, stabilizes c-Fos much more efficiently in asynchronous than in serum-stimulated cells. In both cases, the reduced degradation rate is due to inhibition of the activity of the C-terminal destabilizer. However, in serum-stimulated cells, this effect is partially masked by the activation of the N-terminal destabilizer and basic domain/leucine zipper-dependent mechanisms. Taken together, our data show that multiple degradation mechanisms, differing according to the conditions of expression, may operate on c-Fos to ensure a proper level and/or timing of expression. Moreover, they also indicate that the half-life of c-Fos during the G0/G1 phase transition is determined by a delicate balance between opposing stabilizing and destabilizing mechanisms operating at the same time.

Psoriasin (S100A7) expression is altered during skin tumorigenesis

BACKGROUND

Psoriasin (S100A7) expression has previously been associated with psoriasiform hyperplasia as well as with tumor progression in breast cancer. Its expression profile for different stages of skin lesions is unknown. The aim of this study was to determine the relationship between psoriasin (S100A7) and tumor progression in skin.

METHODS

Psoriasin was assessed by immunohistochemistry and levels of expression determined by semi-quantitative scoring in skin biopsies from 50 patients. The cohort included normal skin, actinic keratosis, squamous carcinoma in-situ, invasive squamous cell carcinoma, and basal cell carcinoma.

RESULTS

In normal skin, psoriasin was rarely detected in epidermis but was expressed in underlying adnexae. In abnormal epidermis psoriasin was frequently expressed in abnormal keratinocytes in actinic keratosis, in-situ and invasive squamous cell carcinoma, but was rarely observed in the basal epidermal layer or in superficial or invasive basal cell carcinoma. The highest levels of expression were seen within squamous carcinoma in-situ. Significantly reduced levels of expression were observed in both unmatched (p = 0.0001) and matched (p < 0.004) invasive squamous cell carcinoma. Psoriasin expression within abnormal squamous lesions correlated with mitotic count (r = 0.54, p = 0.0036), however no significant relation was found with the intensity of dermal inflammatory cell infiltrates assessed within each pathology.

CONCLUSION

These results suggest that altered psoriasin expression occurs in abnormal epidermis and that downregulation may be related to the onset of invasion in squamous cell carcinoma in skin.

Amplification and overexpression of JUNB is associated with primary cutaneous T-cell lymphomas

Primary cutaneous lymphomas (PCLs) represent a heterogeneous group of extranodal T- and B-cell malignancies. The underlying molecular pathogenesis of this malignancy remains unclear. This study aimed to characterize oncogene abnormalities in PCLs. Using genomic microarray, we detected oncogene copy number gains of RAF1 (3p25), CTSB (8p22), PAK1 (11q13), and JUNB (19p13) in 5 of 7 cases of mycosis fungoides (MF)/Sezary syndrome (SS) (71%), gains of FGFR1 (8p11), PTPN (20q13), and BCR (22q11) in 4 cases (57%), and gains of MYCL1 (1p34), PIK3CA (3q26), HRAS (11p15), MYBL2 (20q13), and ZNF217 (20q13) in 3 cases (43%). Amplification of JUNB was studied in 104 DNA samples from 78 PCL cases using real-time polymerase chain reaction. Twenty-four percent of cases, including 7 of 10 cases of primary cutaneous CD30(+) anaplastic large-cell lymphoma (C-ALCL), 4 of 14 MF, 4 of 22 SS, and 2 of 23 primary cutaneous B-cell lymphoma (PCBCL) showed amplification of JUNB, and high-level amplification of this oncogene was present in 3 C-ALCL and 2 MF cases. JUNB protein expression was analyzed in tissue sections from 69 PCL cases, and 44% of cases, consisting of 21 of 23 SS, 6 of 8 C-ALCL, 5 of 10 MF, and 9 of 21 PCBCL, demonstrated nuclear expression of JUNB by tumor cells. Overexpression of JUNB also was detected in 5 C-ALCL and 2 SS cases. These results have revealed, for the first time, amplification and expression patterns of JUNB in PCL, suggesting that JUNB may be critical in the pathogenesis of primary cutaneous T-cell lymphomas.

FosB is highly expressed in normal mammary epithelia, but down-regulated in poorly differentiated breast carcinomas

FosB is a member of the AP-1 family of transcription factors which represent important regulators of cell proliferation and differentiation. Based on prior results which indicated a role of FosB in breast cancer, we studied FosB protein and mRNA expression by immunohistochemistry and, partly, in situ hybridization in 68 mammary carcinomas and normal breast tissues. We found strong nuclear FosB immunoreactivity in epithelial cells of normal lobules and ducts, whereas carcinomas frequently showed loss of FosB expression (n = 8) or weak immunostaining (n = 24). Reduced FosB protein expression in tumors correlated with high grading (p = 0.005), negative estrogen and progesterone receptor status (p < 0.001), and strong HER2/neu expression (p = 0.025). Comparison with expression of seven cell-cycle regulators revealed an association of low/absent FosB staining with p16MTS1 overexpression (p = 0.005). RT-PCR showed expression of full-length FosB and the smaller splice variant FosB2 in most carcinomas and cell lines with and without FosB protein expression, indicating that both proteins are differentially regulated mainly at a post-transcriptional level. By sequence analysis of the coding region in four cell lines and 17 carcinomas we detected a mutation in HBL-100 cells. Our results indicate that high FosB expression might be necessary for normal proliferation and differentiation of mammary epithelial cells, and reduced FosB protein levels might be involved in dedifferentiation during breast tumorigenesis.

Role and regulation of activator protein-1 in toxicant-induced responses of the lung

Aberrant cell proliferation and differentiation after toxic injury to airway epithelium can lead to the development of various lung diseases including cancer. The activator protein-1 (AP-1) transcription factor, composed of mainly Jun-Jun and Jun-Fos protein dimers, acts as an environmental biosensor to various external toxic stimuli and regulates gene expression involved in various biological processes. Gene disruption studies indicate that the AP-1 family members c-jun, junB, and fra1 are essential for embryonic development, whereas junD, c-fos, and fosB are required for normal postnatal growth. However, broad or target-specific transgenic overexpression of the some of these proteins gives very distinct phenotype(s), including tumor formation. This implies that, although they are required for normal cellular processes, their abnormal activation after toxic injury can lead to the pathogenesis of the lung disease. Consistent with this view, various environmental toxicants and carcinogens differentially regulate Jun and Fos expression in cells of the lung both in vivo and in vitro. Moreover, Jun and Fos proteins distinctly bind to the promoter regions of a wide variety of genes to differentially regulate their expression in epithelial injury, repair, and differentiation. Importantly, lung tumors induced by various carcinogens display a sustained expression of certain AP-1 family members. Therefore a better understanding of the mechanisms of regulation and functional role(s), as well as identification of target genes of members of the AP-1 family in airway epithelial cells, will provide additional insight into toxicant-induced lung diseases. These studies might offer a unique opportunity to use AP-1 family members and transactivation as potential diagnostic markers or drug targets for early detection and/or prevention of various lung diseases.

Function of AP-1 target genes in mesenchymal-epithelial cross-talk in skin

An increasing number of examples on the importance of mesenchymal-epithelial interactions in physiological (e.g. embryonic development) and pathological (tumourigenesis) processes have been described. This is best illustrated in the skin, where the well-controlled balance of keratinocyte proliferation and differentiation forms the basis for a proper histoarchitecture of the epidermis. Here, a double paracrine loop of cytokines, which are synthesised and secreted by cells of the epidermis (keratinocytes) and the underlying dermis (fibroblasts) seems to play a major role. The aim of this commentary is to review research that has investigated the role of specific subunits of transcription factor AP-1 (Jun/Fos) in this regulatory network. Using an in vitro skin equivalent model strong evidence was provided for a critical and specific function of c-Jun and JunB in mesenchymal-epithelial interaction in the skin by regulating the expression of interleukin-1 (IL-1)-induced keratinocyte growth factor (KGF) and GM-CSF in fibroblasts. These factors, in turn, adjust the balance between proliferation and differentiation of keratinocytes ensuring proper architecture of the epidermis. This commentary will summarise our current knowledge on the molecular mechanisms underlying AP-1-dependent mesenchymal-epithelial interactions and discuss the physiological relevance of these in vitro findings in skin physiology and pathology.

Tumour necrosis factor-alpha mediates tumour promotion via a PKC alpha- and AP-1-dependent pathway

Tumour necrosis factor-alpha (TNF-alpha) deficient mice (TNF-alpha(-/-) mice) are resistant to skin carcinogenesis. Cellular signalling via the transcription factor complex AP-1 is thought to play a key role in tumour promotion. The induction of a specific subset of AP-1 responsive genes thought to be important for tumour development, namely GM-CSF, MMP-9 and MMP-3, was suppressed in TNF-alpha(-/-) compared to wild-type mouse skin in response to the tumour promotor TPA. The differential induction of these genes correlated with a temporal shift in AP-1 activation and c-Jun expression in TNF-alpha(-/-) compared to wild-type epidermis. The major receptor for TPA-induced signalling in basal keratinocytes, PKC alpha, was also differentially regulated in wild-type compared with TNF-alpha(-/-) epidermis. A marked delay in TPA-induced intracellular translocation and downregulation of PKC alpha was observed in TNF-alpha(-/-) epidermis, which correlated with the deregulated TPA-induced AP-1 activation and c-Jun expression. The frequency of DNA adduct formation and c-Ha-ras mutations was the same in wild-type and TNF-alpha(-/-) epidermis after DMBA treatment, suggesting that TNF-alpha was not involved in tumour initiation. These data suggest that the pro-inflammatory cytokine TNF-alpha is a critical mediator of tumour promotion, acting via a PKC alpha- and AP-1-dependent pathway. This may be one mechanism by which chronic inflammation increases susceptibility to cancer.

Dynamic characterization of the molecular events during in vitro epidermal wound healing

The aim of this study was to characterize some of the molecular events stimulated in vitro in response to injury within a confluent culture of normal epidermal keratinocytes as a model to understand the mechanisms of wound healing. To this end, an original device was developed specifically designed to perform calibrated injuries of great lengths within mono-stratified or pluri-stratified keratinocyte cultures. The experiments performed in this study validate this device as an appropriate tool for studying epidermal wound healing; this is because it performs mechanical injuries that stimulate the expression of multiple healing markers also known to be upregulated during wound healing in vivo (growth factors, cytokines, proteinases, extracellular matrix proteins). Using this device, it was demonstrated in human keratinocytes: mechanical injuries (i) immediately stimulate the tyrosine phosphorylation of numerous cellular proteins; (ii) induce molecular cascades leading to the activation of p21ras, mitogen-activated protein kinases, extracellular signal-regulated kinases 1/2, c-Jun NH2 terminal kinase, and p38 mitogen-activated protein kinase; and (iii) increase the phosphorylation of their respective substrates, c-jun and activator transcription factor 1. Wounding of these cells also results in increases in the DNA binding activities of several jun/fos activator protein-1 transcription factor complexes. It is important to note that the development of an appropriate wounding system was essential for performing this study, as use of a classical wounding procedure did not enable the detection of the biologic parameters reported above. In conclusion, these data indicate that using the appropriate system, it is possible to identify the signaling pathways activated in normal human keratinocyte cells after injury. In this study, it was shown that the mitogen-activated protein kinase pathways and activator protein-1 are stimulated in response to physical injury, and may be involved in regulating the expression of healing markers.

Calgranulins S100A8 and S100A9 are negatively regulated by glucocorticoids in a c-Fos-dependent manner and overexpressed throughout skin carcinogenesis

The two calgranulins S100A8 and S100A9 were found to be differentially expressed at sites of acute and chronic inflammation. Here we have employed the phorbol ester-induced multistage skin carcinogenesis protocol in mice to determine the expression of both genes in inflamed skin and in skin tumors. We show that expression is coordinately induced by the phorbol ester TPA in epithelial cells as well as infiltrating leukocytes. By comparing S100A8 and S100A9 mRNA levels in wild type and c-Fos deficient mice (c-fos(-/-)) we found that expression is negatively regulated by c-Fos/AP-1. Glucocorticoids, which exhibit potent anti-inflammatory and anti-tumor promoting activities repressed TPA-mediated S100A8 and S100A9 induction in wild type, but not in c-fos(-/-) mice, thus identifying both genes as the first examples of AP-1 target genes whose repression of TPA-induced transcription by glucocorticoids depends on c-Fos. Finally, we show that enhanced expression is not restricted to the initial TPA-induced inflammatory response but is observed at all stages of skin carcinogenesis. These data identify S100A8 and S100A9 as novel, tumor-associated genes and may point to an as yet unrecognized function of both genes in the development of epithelial skin tumors.

Gene Expression of Genital Human Papillomaviruses and Considerations on Potential Antiviral Approaches

Genital human papillomaviruses (HPVs) are carcinogenic to humans and are associated with most cases of cervical cancer, genital and laryngeal warts, and certain cutaneous neoplastic lesions. Five of the more than 50 known genital HPV types, HPV-6, -11, -16, -18 and -31, have become the models to study gene expression. The comparison of the studies of these five viruses and analyses of the genomic sequences of those genital HPV types that have not been transcriptionally studied make it likely that genital HPVs share most strategies for regulating their transcription. These strategies are quite different from those of unrelated human and animal papillomaviruses. Among these common properties are (i) a specific promoter structure allowing for fine-tuned negative feedback, (ii) a transcriptional enhancer that is specific for epithelial cells, (iii) regulation by progesterone and glucocorticoid hormones, (iv) silencers, whose principal function appears to be transcriptional repression in the basal layer of infected epithelia, (v) specifically positioned nucleosomes that mediate the functions of some enhancer and the silencer factors, (vi) nuclear matrix attachment regions that can, under different conditions, repress or stimulate transcription, and (vii) as yet poorly understood late promoters positioned very remote from the late genes. Most of these properties are controlled by cellular proteins that, due to their simultaneous importance for cellular processes, may not be useful as HPV-specific drug targets. It should be possible, however, to target complex cis-responsive elements unique to these HPV genomes by nucleotide sequence-specific molecules, such as antisense RNA, polyamides and artificial transcription factors. The application of small molecule-based drugs may be restricted to target proteins encoded by the HPV DNA, such as the replication factor E1 and the transcription/replication factor E2.

AP-1 transrepressing retinoic acid does not deplete coactivators or AP-1 monomers but may target specific Jun or Fos containing dimers

Retinoic acid (RA) inhibits tumor promotion in many models in vivo and in vitro, among them mouse epidermal JB6 cells. RA treatment suppresses 12-O-tetradecanoylphorbol-13-acetate (TPA) induced AP-1 activity, an activity that is required for transformation of JB6 P+ cells. The molecular mechanism of AP-1 transrepression by retinoids is unclear, especially as related to inhibition of transformation. Overexpression of AP-1 components did not rescue TPA induced AP-1 activation nor did a GST pull down experiment implicate direct binding, thus rendering unlikely both a Jun/Fos-RA-RAR direct interaction and a Jun/Fos sequestration mechanism. Overexpression of p300, SRC-1 or pCAF did not abrogate AP-1 suppression by RA, thus arguing against coactivator competition. Overexpression of the corepressor silencing mediator for retinoic acid and thyroid hormone receptors (SMRT) suppressed AP-1 activity. However, SMRT but not RA inhibited cJun transactivation, suggesting SMRT does not mediate RA transrepression. RA treatment also did not block TPA induced ERK phosphorylation, Jun/Fos family protein expression except for cFos, or DNA binding of the AP-1 complex. The transcriptional activities of full-length JunB and full-length Fra-1, but not the transactivation domain fusions, were increased by TPA treatment and suppressed by RA. Since these full-length fusions have bzip domains, the results suggest that JunB and/or Fra-1-containing dimers may constitute one target of RA for transrepression of AP-1.

JAC, a direct target of oncogenic transcription factor Jun, is involved in cell transformation and tumorigenesis

Using subtractive hybridization techniques, we have isolated a gene termed JAC that is strongly and specifically activated in avian fibroblasts transformed by the v-jun oncogene of avian sarcoma virus 17 (ASV17), but not in cells transformed by other oncogenic agents. Furthermore, JAC is highly expressed in cell lines derived from jun-induced avian fibrosarcomas. Kinetic analysis using a doxycycline-controlled conditional cell transformation system showed that expression of the 0.8-kb JAC mRNA is induced rapidly upon activation of the oncogenic v-jun allele. Nucleotide sequence analysis and transcriptional mapping revealed that the JAC gene contains two exons, with the longest ORF confined to exon 2. The deduced 68-amino acid chicken JAC protein is rich in cysteine residues and displays 37% sequence identity to mammalian high-sulfur keratin-associated proteins. The promoter region of JAC contains a consensus (5'-TGACTCA-3') and a nonconsensus (5'-TGAGTAA-3') AP-1 binding site in tandem, which are both specifically bound by the Gag-Jun hybrid protein encoded by ASV17. Mutational analysis revealed that the two AP-1 sites confer strong transcriptional activation by Gag-Jun in a synergistic manner. Ectopic expression of JAC in avian fibroblasts leads to anchorage-independent growth, strongly suggesting that deregulation of JAC is an essential event in jun-induced cell transformation and tumorigenesis.

TOJ3, a target of the v-Jun transcription factor, encodes a protein with transforming activity related to human microspherule protein 1 (MCRS1)

Using the established quail cell line Q/d3 conditionally transformed by the v-jun oncogene, cDNA clones (TOJ2, TOJ3, TOJ5, TOJ6) were isolated by representational difference analysis (RDA) that correspond to genes which were induced immediately upon conditional activation of v-jun. One of these genes, TOJ3, is immediately and specifically activated after doxycycline-mediated v-jun induction, with kinetics similar to the induction of well characterized direct AP-1 target genes. TOJ3 is neither activated upon conditional activation of v-myc, nor in cells or cell lines non-conditionally transformed by oncogenes other than v-jun. Sequence analysis revealed that the TOJ3-specific cDNA encodes a 530-amino acid protein with significant sequence similarities to the murine or human microspherule protein 1 (MCRS1, MSP58), a nucleolar protein that directly interacts with the ICP22 regulatory protein from herpes simplex virus 1 or with p120, a proliferation-related protein expressed at high levels in most human malignant tumor cells. Similar to its mammalian counterparts, the TOJ3 protein contains a bipartite nuclear localization motif and a forkhead associated domain (FHA). Using polyclonal antibodies directed against a recombinant amino-terminal TOJ3 protein segment, the activation of TOJ3 in jun-transformed fibroblasts was also demonstrated at the protein level by specific detection of a polypeptide with an apparent M(r) of 65 000. Retroviral expression of the TOJ3 gene in quail or chicken embryo fibroblasts induces anchorage-independent growth, indicating that the immediate activation of TOJ3 in fibroblasts transformed by the v-jun oncogene contributes to cell transformation.