Transient activation of β-catenin signalling in adult mouse epidermis is sufficient to induce new hair follicles but continuous activation is required to maintain hair follicle tumours

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Abstract
When β-catenin signalling is disturbed from mid-gestation onwards lineage commitment is profoundly altered in postnatal mouse epidermis. We have investigated whether adult epidermis has the capacity forβ-catenin-induced lineage conversion without prior embryonic priming. We fused N-terminally truncated, stabilised β-catenin to the ligand-binding domain of a mutant oestrogen receptor (ΔNβ-cateninER).ΔNβ-cateninER was expressed in the epidermis of transgenic mice under the control of the keratin 14 promoter and β-catenin activity was induced in adult epidermis by topical application of 4-hydroxytamoxifen(4OHT). Within 7 days of daily 4OHT treatment resting hair follicles were recruited into the hair growth cycle and epithelial outgrowths formed from existing hair follicles and from interfollicular epidermis. The outgrowths expressed Sonic hedgehog, Patched and markers of hair follicle differentiation, indicative of de novo follicle formation. The interfollicular epidermal differentiation program was largely unaffected but after an initial wave of sebaceous gland duplication sebocyte differentiation was inhibited. A single application of 4OHT was as effective as repeated doses in inducing new follicles and growth of existing follicles. Treatment of epidermis with 4OHT for 21 days resulted in conversion of hair follicles to benign tumours resembling trichofolliculomas. The tumours were dependent on continuous activation of β-catenin and by 28 days after removal of the drug they had largely regressed. We conclude that interfollicular epidermis and sebaceous glands retain the ability to be reprogrammed in adult life and that continuousβ-catenin signalling is required to maintain hair follicle tumours.