Application of cantharidin or 12-O-tetradecanoylphorbol-13-acetate on mouse epidermis induces a cell population shift that causes altered keratin distribution

WIF1 Suppresses the Generation of Suprabasal Cells in Acanthotic Skin and Growth of Basal Cell Carcinomas upon Forced Overexpression

We analyzed the role of WIF1 in normal and acanthotic epidermis of 12-O-tetradecanoylphorbol-13-acetate (TPA) or all-trans-retinoic acid (ATRA)-treated and basal cell carcinoma (BCC)-bearing mice. WIF1 protein is located in the follicular infundibulum and interfollicular epidermis (IFE) in murine back skin. Within the hyperplastic epidermis of TPA- or ATRA-treated or BCC-bearing murine skin, WIF1 and Keratin 10 overlap in Ki67^⁻ suprabasal layers, while basal epidermal layers expressing Ki67, and BCCs expressing Wif1 mRNA, are free of WIF1 protein. This is similar in human skin, with the exception that WIF1 protein is found in single Ki67^⁻ basal epidermal cells in normal skin and additionally in Ki67⁺ cells in acanthotic skin. Wif1-deficiency enhances acanthosis of the murine BCC-associated epidermis, which is accompanied by an increase of Ki67⁺ and of Sca-1⁺ basal cells. WIF1 overexpression in allografted BCC-derived keratinocytes prevents growth and keratinization, involving enhanced phosphorylation of protein kinase C and extracellular signal-regulated kinase 1 and arguably factors secreted by the in vivo environment. In summary, WIF1 protein marks suprabasal layers in the normal IFE. It is also present in the epidermis overlaying BCCs where it diminishes proliferation of basal cells and production of differentiating suprabasal cells. In addition, WIF1 can prevent proliferation and keratinization of BCC-related keratinocytes.

AP1 factor inactivation in the suprabasal epidermis causes increased epidermal hyperproliferation and hyperkeratosis but reduced carcinogen-dependent tumor formation

Activator protein one (AP1) (jun/fos) factors comprise a family of transcriptional regulators (c-jun, junB, junD, c-fos, FosB, Fra-1 and Fra-2) that are key controllers of epidermal keratinocyte survival and differentiation, and are important drivers of cancer development. Understanding the role of these factors in epidermis is complicated by the fact that each member is expressed in defined cell layers during epidermal differentiation, and because AP1 factors regulate competing processes (that is, proliferation, apoptosis and differentiation). We have proposed that AP1 factors function differently in basal versus suprabasal epidermis. To test this, we inactivated suprabasal AP1 factor function in mouse epidermis by targeted expression of dominant-negative c-jun (TAM67), which inactivates function of all AP1 factors. This produces increased basal keratinocyte proliferation, delayed differentiation and extensive hyperkeratosis. These findings contrast with previous studies showing that basal layer AP1 factor inactivation does not perturb resting epidermis. It is interesting that in spite of extensive keratinocyte hyperproliferation, susceptibility to carcinogen-dependent tumor induction is markedly attenuated. These novel observations strongly suggest that AP1 factors have distinct roles in the basal versus suprabasal epidermis, confirm that AP1 factor function is required for normal terminal differentiation, and suggest that AP1 factors have a different role in normal epidermis versus cancer progression.

Post-transcriptional regulation of the gli1 oncogene by the expression of alternative 5' untranslated regions

The oncogene GLI1 is involved in the formation of basal cell carcinoma and other tumor types as a result of the aberrant signaling of the Sonic hedgehog-Patched pathway. In this study, we have identified alternative GLI1 transcripts that differ in their 5' untranslated regions (UTRs) and are generated by exon skipping. These are denoted alpha-UTR, beta-UTR, and gamma-UTR according to the number of noncoding exons possessed (three, two, and one, respectively). The alpha- and beta-UTR forms represent the major Gli1 transcripts expressed in mouse tissues, whereas the gamma-UTR is present at relatively low levels but is markedly induced in mouse skin treated with 12-O-tetradecanoylphorbol 13-acetate. Transcripts corresponding to the murine beta and gamma forms were identified in human tissues, but significantly, only the gamma-UTR form was present in basal cell carcinomas and in proliferating cultures of a keratinocyte cell line. Flow cytometry analysis determined that the gamma-UTR variant expresses a heterologous reporter gene 14-23-fold higher than the alpha-UTR and 5-13-fold higher than the beta-UTR in a variety of cell types. Because expression of the gamma-UTR variant correlates with proliferation, consistent with a role for GLI1 in growth promotion, up-regulation of GLI1 expression through skipping of 5' noncoding exons may be an important tumorigenic mechanism.

PTHrP regulates epidermal differentiation in adult mice

Emerging evidence suggests that parathyroid hormone-related peptide (PTHrP) serves as a regulator of the development and/or differentiation of a number of organs, including endochondral bone, the tooth, and the mammary gland. Although disruption of the PTHrP gene by homologous recombination results in a lethal chondrodystrophy, PTHrP-knockout mice that have been rescued by the transgenic replacement of the peptide in cartilage display abnormalities in ectodermally derived structures including the skin. At 6-8 wk of age, these rescued PTHrP-knockout mice displayed a markedly thinned epidermis and striking hyperkeratosis, hypoplastic sebaceous glands, and a fibrotic dermis. In contrast, transgenic mice that overexpress PTHrP by virtue of the human keratin-14 promoter displayed a thickened ventral epidermis with marked acanthosis and papillomatosis, hyperplastic sebaceous glands, and a cellular dermis. The absence of PTHrP appeared to result in the reduction of the basal keratinocyte compartment and premature acquisition of suprabasal and granular differentiation markers, whereas overexpression of the peptide generated reciprocal findings. No difference in the epidermal proliferation rate was found in PTHrP-null skin and although an increase was observed in keratin 14-PTHrP transgenic animals, their epidermis did not express the hyperplasia marker K6. Finally, the replacement of PTHrP in the basal keratinocytes of rescued PTHrP-knockout mice under the direction of the keratin 14 promoter reversed the abnormalities seen in PTHrP-null skin. These findings suggest that PTHrP regulates the rate of keratinocyte differentiation in the skin of adult mice.

Topical application of calcitriol alters expression of filaggrin but not keratin K1 in mouse epidermis

Calcitriol (1 alpha,25-dihydroxyvitamin D3) and its analogues are antiproliferative agents which promote epidermal differentiation in vitro, possibly reflecting their modes of action in the treatment of psoriasis. We examined the effect of calcitriol on early and late terminal differentiation in mouse epidermis in vivo using an immunofluorescence assay to detect keratin K1 and filaggrin expression. Pulse labelling with the tymidine analogue 5-bromo-2-deoxyuridine (BrdUrd) was performed by intraperitoneal injection of mice immediately or 16 h after a single topical application of 0.72 nmol calcitriol. The BrdUrd labelling index (LI) and keratin K1 or filaggrin expression of postmitotic cell cohorts were scored by paired immunofluorescence staining for up to 72 h after BrdUrd labelling. Calcitriol induced cell proliferation as shown by a 100% increase in the BrdUrd LI 17 h after application. The onset of keratin K1 expression in the postmitotic period was, however, unchanged in both series after calcitriol treatment. Filaggrin expression appeared earlier after calcitriol treatment than in control epidermis, probably reflecting altered cell kinetics with increased epidermal turnover. The results suggest that calcitriol only influences the later stages of the keratinocyte differentiation programme, possibly secondarily to its hyperproliferative effect.

Topical application of 12-O-tetradecanoylphorbol-13-acetate induces dyssynchronous expression of keratins K1 and K10 in mouse epidermis

12-O-tetradecanoylphorbol-13-acetate (TPA) is a potent tumor promoter that causes severe alterations in the biosynthesis of epidermal keratins. This study shows that TPA induces a dyssynchronous effect on keratin expression in stratified squamous epithelium. The effect of TPA on the separate expression of the maturation-associated keratins K1 and K10 was studied by immunohistochemistry in an unperturbed replicative keratinocyte population of hairless mice epidermis in relation to changes in the cell cycle time during regeneration. Keratinocytes in DNA synthesis were pulse-labeled by intraperitoneal injection of the thymidine analogue 5-bromodeoxyuridine (BrdUrd) 1 h before a single topical application of TPA. The BrdUrd-labeled cell cohort, representing an originally unperturbed replicative keratinocyte population exposed to TPA mainly in the postreplicative period, was followed for up to 97 h. The results suggested unaltered timing of the onset of K1 and K10 expression compared with normal epidermis (18 and 24 h, respectively, following DNA synthesis). This indicates that the synthesis of both keratins was programmed before the keratinocytes entered their last DNA synthesis. A reduction in K10 expression from about 30 h compared with that of K1 expression was observed. Mathematical modeling suggested a delay in K10 expression related to the second and third rounds of cell divisions after pulse-labeling. How TPA induces such dyssynchrony in K1 and K10 regulation remains unknown.

Differential effects of topical retinoic acid application on keratin K1 and filaggrin expression in mouse epidermis

Retinoic acid (RA) modulates epidermal homeostasis and affects differentiation-associated proteins such as keratin K1 and filaggrin. Because results from in vitro and in vivo studies have been conflicting with respect to RA effects on keratinization, we examined the terminal differentiation of epidermal cell cohorts after RA stimulation in vivo. Pulse-labelling with 5-bromo-2-deoxyuridine (BrdU) was performed by intraperitoneal injection of mice immediately or at 16 h after a single topical application of 100 nmol RA. The cell cohort labelled at the time of RA application consisted of previously unperturbed cells exposed to RA after initiation of S-phase, whereas the cohort labelled 16 h after RA application consisted of cells stimulated into the S-phase by RA. These two cohorts of partially synchronized cells were followed for up to 72 h after BrdU labelling. Such labelling combined with keratin K1 or filaggrin expression was scored by paired immunofluorescence staining of skin sections. The onset of keratin K1 expression was unchanged in both series after RA treatment, while filaggrin appeared earlier than in controls. The differential effect of RA on the maturation markers was related to the proliferative activity, the increased cell turnover, and the shortened epidermal transit time. The onset of keratin expression appeared to be regulated before the postmitotic period, whereas filaggrin expression appeared to be regulated during the late phase of the maturation process, thus being influenced by the actual epidermal kinetics and structural alterations. These results suggested that the effect of RA on epidermal differentiation is secondary to its effect on proliferation, as determined by the altered cellular age distribution following regenerative proliferation.