Role for WNT16B in human epidermal keratinocyte proliferation and differentiation

Upregulation of FOXM1 induces genomic instability in human epidermal keratinocytes

Background

The human cell cycle transcription factor FOXM1 is known to play a key role in regulating timely mitotic progression and accurate chromosomal segregation during cell division. Deregulation of FOXM1 has been linked to a majority of human cancers. We previously showed that FOXM1 was upregulated in basal cell carcinoma and recently reported that upregulation of FOXM1 precedes malignancy in a number of solid human cancer types including oral, oesophagus, lung, breast, kidney, bladder and uterus. This indicates that upregulation of FOXM1 may be an early molecular signal required for aberrant cell cycle and cancer initiation.

Results

The present study investigated the putative early mechanism of UVB and FOXM1 in skin cancer initiation. We have demonstrated that UVB dose-dependently increased FOXM1 protein levels through protein stabilisation and accumulation rather than de novo mRNA expression in human epidermal keratinocytes. FOXM1 upregulation in primary human keratinocytes triggered pro-apoptotic/DNA-damage checkpoint response genes such as p21, p38 MAPK, p53 and PARP, however, without causing significant cell cycle arrest or cell death. Using a high-resolution Affymetrix genome-wide single nucleotide polymorphism (SNP) mapping technique, we provided the evidence that FOXM1 upregulation in epidermal keratinocytes is sufficient to induce genomic instability, in the form of loss of heterozygosity (LOH) and copy number variations (CNV). FOXM1-induced genomic instability was significantly enhanced and accumulated with increasing cell passage and this instability was increased even further upon exposure to UVB resulting in whole chromosomal gain (7p21.3-7q36.3) and segmental LOH (6q25.1-6q25.3).

Conclusion

We hypothesise that prolonged and repeated UVB exposure selects for skin cells bearing stable FOXM1 protein causes aberrant cell cycle checkpoint thereby allowing ectopic cell cycle entry and subsequent genomic instability. The aberrant upregulation of FOXM1 serves as a 'first hit' where cells acquire genomic instability which in turn predisposes cells to a 'second hit' whereby DNA-damage checkpoint response (eg. p53 or p16) is abolished to allow damaged cells to proliferate and accumulate genetic aberrations/mutations required for cancer initiation.

WNT16B is a new marker of cellular senescence that regulates p53 activity and the phosphoinositide 3-kinase/AKT pathway

Senescence is a tumor suppression mechanism that is induced by several stimuli, including oncogenic signaling and telomere shortening, and controlled by the p53/p21(WAF1) signaling pathway. Recently, a critical role for secreted factors has emerged, suggesting that extracellular signals are necessary for the onset and maintenance of senescence. Conversely, factors secreted by senescent cells may promote tumor growth. By using expression profiling techniques, we searched for secreted factors that were overexpressed in fibroblasts undergoing replicative senescence. We identified WNT16B, a member of the WNT family of secreted proteins. We found that WNT16B is overexpressed in cells undergoing stress-induced premature senescence and oncogene-induced senescence in both MRC5 cell line and the in vivo murine model of K-Ras(V12)-induced senescence. By small interfering RNA experiments, we observed that both p53 and WNT16B are necessary for the onset of replicative senescence. WNT16B expression is required for the full transcriptional activation of p21(WAF1). Moreover, WNT16B regulates activation of the phosphoinositide 3-kinase (PI3K)/AKT pathway. Overall, we identified WNT16B as a new marker of senescence that regulates p53 activity and the PI3K/AKT pathway and is necessary for the onset of replicative senescence.

Localization and quantification of intact, undamaged right-handed double-stranded B-DNA, and denatured single-stranded DNA in normal human epidermis and its effects on apoptosis and terminal differentiation (denucleation)

Quantification of two types of nucleic acids [double-stranded (ds-) and single-stranded (ss-) DNA] was performed to understand the distribution of DNA within the epidermal strata and to examine the effects of DNA structure on gene expression, viz., apoptosis and terminal differentiation. In addition, we examined the precise starting point of cell death within the epidermis (suprabasal layer); examined how DNA structure affects gene expression of melanocytes; and characterized the "transitional cells" located between the stratum granulosum and stratum corneum, viz., epidermal phase transition zone (EPTZ). Ultrasensitive anti-DNA antibody probes (ds-DNA, ss-DNA), the Feulgen reaction, histological stains (morphological characterization) and the terminal deoxyribonucleotidyl transferase (TUNEL) assay (apoptosis) were used to characterize cell death in normal human epidermis. This study characterized, for the first time, the deterioration of right-handed ds-B-DNA and the increase in denatured ss-DNA during epidermal maturation. For the first time, this approach also allowed for the quantitative and qualitative characterization of DNA content and structure in all epidermal strata, using anti-ds-B-DNA and anti-ss-DNA antibodies. In order to improve the retention and quality of DNA, a novel histotechnological processing procedure was used. The results indicate that the largest decline in DNA occurred within the stratum granulosum, followed by the EPTZ, and the stratum spinosum. Not all epidermal nuclei lost DNA, indicating two differentiating keratinocyte pathways, viz., apoptotic and non-apoptotic. Both pathways united in the stratum granulosum. These results suggest that keratinocyte terminal differentiation and apoptosis are distinct cellular events, cell death begins earlier than expected, and molecular epidermal events take place in a gradual and orderly manner within keratinocytes. During maturation, ds-B-DNA decreases as ss-DNA increases. Therefore, during differentiation of keratinocytes, both DNA content and DNA structure are altered.

FOXM1 upregulation is an early event in human squamous cell carcinoma and it is enhanced by nicotine during malignant transformation

BACKGROUND

Cancer associated with smoking and drinking remains a serious health problem worldwide. The survival of patients is very poor due to the lack of effective early biomarkers. FOXM1 overexpression is linked to the majority of human cancers but its mechanism remains unclear in head and neck squamous cell carcinoma (HNSCC).

METHODOLOGY/PRINCIPAL FINDINGS

FOXM1 mRNA and protein expressions were investigated in four independent cohorts (total 75 patients) consisting of normal, premalignant and HNSCC tissues and cells using quantitative PCR (qPCR), expression microarray, immunohistochemistry and immunocytochemistry. Effect of putative oral carcinogens on FOXM1 transcriptional activity was dose-dependently assayed and confirmed using a FOXM1-specific luciferase reporter system, qPCR, immunoblotting and short-hairpin RNA interference. Genome-wide single nucleotide polymorphism (SNP) array was used to 'trace' the genomic instability signature pattern in 8 clonal lines of FOXM1-induced malignant human oral keratinocytes. Furthermore, acute FOXM1 upregulation in primary oral keratinocytes directly induced genomic instability. We have shown for the first time that overexpression of FOXM1 precedes HNSCC malignancy. Screening putative carcinogens in human oral keratinocytes surprisingly showed that nicotine, which is not perceived to be a human carcinogen, directly induced FOXM1 mRNA, protein stabilisation and transcriptional activity at concentrations relevant to tobacco chewers. Importantly, nicotine also augmented FOXM1-induced transformation of human oral keratinocytes. A centrosomal protein CEP55 and a DNA helicase/putative stem cell marker HELLS, both located within a consensus loci (10q23), were found to be novel targets of FOXM1 and their expression correlated tightly with HNSCC progression.

CONCLUSIONS/SIGNIFICANCE

This study cautions the potential co-carcinogenic effect of nicotine in tobacco replacement therapies. We hypothesise that aberrant upregulation of FOXM1 may be inducing genomic instability through a program of malignant transformation involving the activation of CEP55 and HELLS which may facilitate aberrant mitosis and epigenetic modifications. Our finding that FOXM1 is upregulated early during oral cancer progression renders FOXM1 an attractive diagnostic biomarker for early cancer detection and its candidate mechanistic targets, CEP55 and HELLS, as indicators of malignant conversion and progression.

beta-catenin is involved in N-cadherin-dependent adhesion, but not in canonical Wnt signaling in E2A-PBX1-positive B acute lymphoblastic leukemia cells

OBJECTIVE

The t(1;19)(q23;13) translocation, resulting in the production of the E2A-PBX1 chimeric protein, is a common nonrandom translocation in pediatric B-lineage acute lymphoblastic leukemia (B-ALL). The E2A-PBX1 chimeric protein activates expression of several genes, including Wnt16. In the present study, we explored the role of Wnt16 and beta-catenin in t(1;19) B-ALL cells.

MATERIALS AND METHODS

Canonical Wnt signaling was measured by TOPflash activity. Localization of beta-catenin in the cell membrane and its involvement in leukemia-stroma interaction were studied by confocal microscopy. Adhesion to N-cadherin was analyzed by adding (3)H-thymidin-labeled cells to N-cadherin-coated wells.

RESULTS

In contrast to previous reports, we detected no effects on cell viability or proliferation upon modulation of the Wnt16 levels. Moreover, despite high levels of Wnt16 and beta-catenin, the cells had very low levels of canonical Wnt signaling. Instead, beta-catenin was located in the cell membrane along with N-cadherin. E2A-PBX1-positive leukemia cells adhered strongly to bone marrow stroma cells, and we showed that adherence junctions stained strongly for both proteins. Moreover, knockdown of beta-catenin reduced the adhesion of E2A-PBX1-positive leukemia cells to N-cadherin, suggesting that beta-catenin and N-cadherin play a central role in homotypic cell-to-cell adhesion and in leukemia-stroma adhesion. Interestingly, knockdown of Wnt16 by small interfering RNA reduced the level of N-cadherin.

CONCLUSION

Wnt16 does not activate canonical Wnt signaling in E2A-PBX1-positive cells. Instead, beta-catenin is involved in N-cadherin-dependent adherence junctions, suggesting for the first time that leukemia-stroma interactions may be mediated via an N-cadherin-dependent mechanism.

Secreted Frizzled related protein-4 (sFRP4) promotes epidermal differentiation and apoptosis

The skin provides vital protection from infection and dehydration. Maintenance of the skin is through a constant program of proliferation, differentiation and apoptosis of epidermal cells, whereby proliferating cells in the basal layer differentiating to form the keratinized, anucleated stratum corneum. The WNT signalling pathway is known to be important in the skin. WNT signalling has been shown to be important both in epidermal development and in the maintenance and cycling of hair follicles and epidermal stem cells. However, the precise role for this pathway in epidermal differentiation remains unknown. We investigated the role of the WNT signalling inhibitor sFRP4 in epidermal differentiation. sFRP4 is expressed in both normal skin and keratinocytes in culture. Expression of sFRP4 mRNA and protein increases with keratinocyte differentiation and apoptosis, whilst exposure of keratinocytes to exogenous sFRP4 promotes apoptosis and expression of the terminal differentiation marker Involucrin. These data suggest sFRP4 promotes epidermal differentiation.