Ectopic Expression of the nude Gene Induces Hyperproliferation and Defects in Differentiation: Implications for the Self-Renewal of Cutaneous Epithelia

Abstract

Nude mice are characterized by the absence of visible hair, epidermal defects, and the failure to develop a thymus. This phenotype results from loss-of-function mutations in Whn (Hfh11), a winged-helix transcription factor. In murine epidermis and hair follicles, endogenous whn expression is induced as epithelial cells initiate terminal differentiation. Using the promoter for the differentiation marker involucrin, transgenic mice that ectopically express whn in stratified squamous epithelia, hair follicles, and the transitional epithelium of the urinary tract were generated. Transgenic epidermis and hair follicles displayed impaired terminal differentiation and a subset of hair defects, such as delayed growth, a waved coat, and curly whiskers, correlated with decreased transforming growth factor (TGF)-alpha expression. The exogenous Whn protein also stimulated epithelial cell multiplication. In the epidermis, basal keratinocytes exhibited hyperproliferation, though transgene expression was restricted to suprabasal, postmitotic cells. Hair follicles failed to enter telogen (a resting period) and remained continuously in an abnormal anagen (the growth phase of the hair cycle). Ureter epithelium developed severe hyperplasia, leading to the obstruction of urine outflow and death from hydronephrosis. Though an immune infiltrate was present occasionally in transgenic skin, the infiltrate was not the primary cause of the epithelial hyperproliferation, as the immune reaction was not observed in all affected transgenics, and the transgene induced identical skin and urinary tract abnormalities in immunodeficient Rag1-null mice. Given the effects of the transgene on cell proliferation and TGFalpha expression, the results suggest that Whn modulates growth factor production by differentiating epithelial cells, thereby regulating the balance between proliferative and postmitotic populations in self-renewing epithelia.

Role of Whn in Mammary Gland Development and Tumorigenesis

“…In addition, nude mouse mammary glands fail to develop normally during pregnancy suggesting that Whn is necessary for the mammary epithelial cells to correctly differentiate. These data are consistent with the previous work from our laboratory showing that Whn has roles in both proliferation and differentiation of keratinocytes in the skin Prowse et al, 1999).…”

Severe Defects in Proliferation and Differentiation of Lens Cells in Foxe3 Null Mice

“…The ability of Foxe3/FoxE4 to affect the balance between proliferation and differentiation of cells is not unique among the forkhead genes. For example, in the nude mouse the forkhead transcription factor Foxn1 promotes proliferation of hair follicle epithelial cells and inhibits their differentiation (7,9,38). In Xenopus, the forkhead gene FoxG1 (XBF-1) regulates neurogenesis by controlling the proliferation and differentiation of neuronal cells (6).…”

Transient activation of FOXN1 in keratinocytes induces a transcriptional programme that promotes terminal differentiation: contrasting roles of FOXN1 and Akt

“…Cultured keratinocytes from nude mice express lower levels of the partner keratin, keratin 1 (upregulated 1.8 fold on the arrays) than control keratinocytes (Brissette et al, 1996), and overexpression of FOXN1 under the control of the involucrin promoter in differentiating keratinocytes results in increased levels of keratin 1 (Baxter and Brissette, 2002;Prowse et al, 1999). The most highly induced gene in the microarrays, the β-galactoside binding lectin galectin-7, is also expressed in the suprabasal layers of human epidermis as part of the terminal differentiation programme (data not shown).…”

“…In culture, nude mouse primary keratinocytes have a normal mitogenic response to growth factors but undergo abnormal differentiation, expressing low levels of the early differentiation marker keratin 1 (Brissette et al, 1996). In murine embryonic epidermis and hair follicles, Foxn1 expression is associated with keratinocytes in the early stages of terminal differentiation, while in postpartum mice Foxn1 is predominantly expressed in the anagen (growing) phase of the hair cycle in post-mitotic cells Meier et al, 1999;Prowse et al, 1999).…”