Epigenetic regulation of gene expression in keratinocytes.

The nucleus is a complex and highly compartmentalized organelle, which undergoes major organization changes during cell differentiation, allowing cells to become specialized and fulfill their functions. During terminal differentiation of the epidermal keratinocytes, the nucleus undergoes a programmed transformation from active status, associated with execution of the genetic programs of cornification and epidermal barrier formation, to a fully inactive condition and becomes a part of the keratinized cells of the cornified layer. Tremendous progress achieved within the past two decades in understanding the biology of the nucleus and epigenetic mechanisms controlling gene expression allowed defining several levels in the regulation of cell differentiation-associated gene expression programs, including an accessibility of the gene regulatory regions to DNA-protein interactions, covalent DNA and histone modifications, and ATP-dependent chromatin remodeling, as well as higher-order chromatin remodeling and nuclear compartmentalization of the genes and transcription machinery. Here, we integrate our current knowledge of the mechanisms controlling gene expression during terminal keratinocyte differentiation with distinct levels of chromatin organization and remodeling. We also propose directions to further explore the role of epigenetic mechanisms and their interactions with other regulatory systems in the control of keratinocyte differentiation in normal and diseased skin.

Fas signaling is involved in the control of hair follicle response to chemotherapy.

Chemotherapeutic agents induce p53-dependent apoptosis in the hair follicle (HF) resulting in hair loss, a common side effect of cancer therapy. Here, we show that Fas as a p53 target plays important role in the HF response to cyclophosphamide. Specifically, we demonstrate that Fas is up-regulated in HF keratinocytes after cyclophosphamide treatment, Fas ligand-neutralizing antibody partially inhibits HF response to cyclophosphamide in wild-type mice, and Fas knockout mice show significant retardation of cyclophosphamide-induced HF involution associated with reduced Fas-associated death domain and caspase-8 expression. These data raise a possibility to explore blockade of Fas signaling as a part of complex local therapy for inhibiting keratinocyte apoptosis and hair loss induced by chemotherapy.

Noggin overexpression inhibits eyelid opening by altering epidermal apoptosis and differentiation.

Contact of developing sensory organs with the external environment is established via the formation of openings in the skin. During eye development, eyelids first grow, fuse and finally reopen, thus providing access for visual information to the retina. Here, we show that eyelid opening is strongly inhibited in transgenic mice overexpressing the bone morphogenetic protein (BMP) antagonist noggin from the keratin 5 (K5) promoter in the epidermis. In wild-type mice, enhanced expression of the kinase-inactive form of BMPR-IB mediated by an adenovirus vector also inhibits eyelid opening. Noggin overexpression leads to reduction of apoptosis and retardation of cell differentiation in the eyelid epithelium, which is associated with downregulation of expression of the apoptotic receptors (Fas, p55 kDa TNFR), Id3 protein and keratinocyte differentiation markers (loricrin, involucrin). BMP-4, but not EGF or TGF-alpha, accelerates opening of the eyelid explants isolated from K5-Noggin transgenic mice when cultured ex vivo. These data suggest that the BMP signaling pathway plays an important role in regulation of genetic programs of eyelid opening and skin remodeling during the final steps of eye morphogenesis.

Fas and c-kit are involved in the control of hair follicle melanocyte apoptosis and migration in chemotherapy-induced hair loss.

Chemotherapy alters the structure and function of hair follicle melanocytes. Molecular mechanisms controlling melanocyte responses during chemotherapy-induced hair loss, however, remain largely unknown. Using immunohistology and multicolor confocal microscopy, we show here that cyclophosphamide administration to C57BL/6 mice alters the activity and fate of hair follicle melanocytes. After 24-48 h, hair bulb melanocytes expressing Fas undergo apoptosis. The number of apoptotic follicular melanocytes is significantly reduced (p<0.01) in cyclophosphamide-treated Fas knockout mice compared to wild-type controls, suggesting that Fas signaling contributes to chemotherapy-induced melanocyte death. After 3-5 d, surviving hair bulb melanocytes express c-kit receptor, proliferate, and appear to migrate up the outer root sheath. Tyrosinase-positive and melanogenically active cells then appear in the epidermis. By Western blotting and immunohistochemistry, expression levels of the c-kit ligand, stem cell factor, in skin and epidermis are strongly increased after cyclophosphamide treatment. Cyclophosphamide-induced migration of the hair follicle melanocytes into epidermis is completely abrogated by administration of c-kit neutralizing antibody. These data suggest that chemotherapy induces a complex response in the hair follicle melanocytes, which includes apoptosis, proliferation, and migration. Pharmacologic manipulation of Fas and c-kit signaling pathways might be useful for the correction of skin hyperpigmentation as a side-effect of chemotherapy.

Modulation of BMP signaling by noggin is required for induction of the secondary (nontylotrich) hair follicles.

Increasing evidence suggests that morphogenesis of the distinct developmental structures derived from the same organ-committed epithelium is controlled by differential mechanisms. As was recently shown in mice with mutations in the downless (dL) gene, induction of primary or tylotrich hair follicles is strikingly dependent of signaling through the Tnf receptor homologue, Edar. Here, we show that dorsal skin of murine embryos with constitutive deletion of the BMP2/4 antagonist noggin, after transplantation into SCID mice, is characterized by the lack of induction of secondary hair follicles, and by the arrest of primary hair follicle development prior to hair shaft formation. The loss of noggin activity was associated with failure to express genes that specify hair follicle cell fates in the epidermis (Lef-1, beta-catenin, Shh) and dermal papilla (p75 kDa neurotrophin receptor, alkaline phosphatase). This suggests that regulation of BMP2/4 signaling by noggin is essential for the induction of secondary hair follicles, as well as for advanced stages of development in primary hair follicles.