Hair follicle expression of 1,25-dihydroxyvitamin D3 receptors during the murine hair cycle

Role of hormones in pilosebaceous unit development

Androgens are required for sexual hair and sebaceous gland development. However, pilosebaceous unit (PSU) growth and differentiation require the interaction of androgen with numerous other biological factors. The pattern of PSU responsiveness to androgen is determined in the embryo. Hair follicle growth involves close reciprocal epithelial-stromal interactions that recapitulate ontogeny; these interactions are necessary for optimal hair growth in culture. Peroxisome proliferator-activated receptors (PPARs) and retinoids have recently been found to specifically affect sebaceous cell growth and differentiation. Many other hormones such as GH, insulin-like growth factors, insulin, glucocorticoids, estrogen, and thyroid hormone play important roles in PSU growth and development. The biological and endocrinological basis of PSU development and the hormonal treatment of the PSU disorders hirsutism, acne vulgaris, and pattern alopecia are reviewed. Improved understanding of the multiplicity of factors involved in normal PSU growth and differentiation will be necessary to provide optimal treatment approaches for these disorders.

Evaluation of keratinocyte proliferation and differentiation in vitamin D receptor knockout mice

The biological effects of 1,25-dihydroxyvitamin D3 are mediated by a nuclear receptor, the vitamin D receptor (VDR). Targeted ablation of the VDR in mice results in hypocalcemia, hypophosphatemia, hyperparathyroidism, rickets, osteomalacia, and alopecia. Normalization of mineral ion homeostasis prevents these abnormalities with the exception of the alopecia. Because 1,25(OH)2D3 has been shown to play a role in keratinocyte proliferation and differentiation, we undertook studies in primary keratinocytes and skin isolated from VDR null mice to determine if a keratinocyte abnormality could explain the alopecia observed. The basal proliferation rate of the VDR null and wild-type keratinocytes was identical both under proliferating and differentiating conditions. Assessment of in vivo keratinocyte proliferation at 4 days of age confirmed that VDR ablation did not have a significant effect. There was no difference in the basal expression of markers of keratinocyte differentiation (keratin 1, involucrin, and loricrin) in the keratinocytes isolated from VDR-ablated mice when compared with those isolated from control littermates. Similarly, in vivo expression of these genes was not altered at 4 days of age. When anagen was induced by depilation at 18 days of age, the VDR null mice had a profound impairment in initiation of the hair cycle. These data suggest that the alopecia in the VDR null mice is not attributable to an intrinsic defect in keratinocyte proliferation or differentiation, but rather to an abnormality in initiation of the hair cycle.

Effects of plant-induced hypervitaminosis D on cutaneous structure, cell differentiation and cell proliferation in cattle

Solanum glaucophyllum (Sg) (synonym S. malacoxylon) is a plant toxic to cattle due to its high levels of 1,25-dihydroxyvitamin D3 as glycoside derivatives. Sg causes a disease characterized by wasting and calcification of soft tissues. The effects of vitamin D are not only important in calcium homeostasis, but also in immune regulation, cell growth and cell differentiation. Skin samples in Sg-intoxicated and control heifers were studied histologically. Cellular differentiation and proliferation were analysed by immunohistochemical expression of cytokeratins, involucrin and proliferating cell nuclear antigen (PCNA). The results were obtained by image processing and analysis and were statistically evaluated. Sg-intoxicated cattle showed atrophy of epidermis and severe involution of hair follicles and of sebaceous and sweat glands. As judged by PCNA expression, cellular proliferation was reduced, even though the reduction was not statistically significant. The analysed markers of differentiation, e.g. involucrin and cytokeratins 10 and 11, changed in relation to Sg-poisoning. The possible pathogenesis of the skin lesions is discussed.

Differential regulation of direct repeat 3 vitamin D3 and direct repeat 4 thyroid hormone signaling pathways by the human TR4 orphan receptor

In situ hybridization analysis demonstrated that abundant testicular orphan receptor (TR4) transcripts were detected in kidney, intestine, and bone, which are vitamin D3 target organs. Cell transfection studies also demonstrated that the expression of the vitamin D3 target gene, 25-hydroxyvitamin D3 24-hydroxylase, can be repressed by TR4 through high affinity binding (Kd = 1.32 nM) to the direct repeat 3 vitamin D3 receptor response element (DR3VDRE). This TR4-mediated repression of DR3VDRE is in contrast to our earlier report that TR4 could induce thyroid hormone target genes containing a direct repeat 4 thyroid hormone response element (DR4T3RE). Electrophoretic mobility shift assay using several TR4 monoclonal antibodies when combined with either TR4-DR3VDRE or TR4-DR4T3RE showed a distinct supershifted pattern, and proteolytic analysis further demonstrated distinct digested peptides with either TR4-DR3VDRE or TR4-DR4T3RE. These results may therefore suggest that TR4 can adapt to different conformations once bound to DR3VDRE or DR4T3RE. The consequence of these different conformations of TR4-DR3VDRE and TR4-DR4T3RE may allow each of them to recruit different coregulators. The differential repression of TR4-mediated DR3VDRE and DR4T3RE transactivation by the receptor interacting protein 140, a TR4 coregulator, further strengthens our hypothesis that the specificity of gene regulation by TR4 can be modulated by protein-DNA and protein-protein interactions.

Reduction of intrafollicular apoptosis in chemotherapy-induced alopecia by topical calcitriol-analogs

Chemotherapy-induced alopecia is thought to result from cytotoxic and apoptosis-related damage to the hair follicle. This study was designed to confirm whether keratinocyte apoptosis is indeed induced in growing (= anagen) hair follicles of C57 BL/6 mice after the injection of cyclophosphamide, using improved methods for histologic detection of apoptotic cells in murine skin. More importantly, we asked whether topical calcitriol-analogs are able to modulate cyclophosphamide-induced apoptosis in vivo, because there are conflicting reports on the effects of calcitriols on apoptosis in vitro. Anagen was induced in telogen mice on day 0 by depilation. Starting on day 5 post-depilation, the back skin of mice was topically treated with either 0.2 microg 1,25-dihydroxyvitamin D3, 2.0 microg calcipotriol, 0.02 microg KH 1060, or vehicle (ethanol) only. On the last day of treatment (i.e., day 9 post-depilation), all mice received 150 mg cyclophosphamide i.p. per kg as a single dose to induce alopecia, or vehicle (aqua dist.). Analysis of the treated skin by in situ-end labeling (using a modified terminal UTP nucleotide end labeling technique suitable for murine skin), by Hoechst 33342 stain, and by DNA electrophoresis on days 10 and 14, revealed the induction of massive apoptosis in cyclophosphamide-treated anagen hair bulbs, which was most prominent on day 10, whereas controls showed no follicular apoptosis. The calcitriol-pretreated groups demonstrated a significant reduction of apoptosis, with a maximal inhibition seen on day 14. This confirms that cyclophosphamide indeed induces massive keratinocyte apoptosis in anagen hair follicles, and provides evidence that topical calcitriol-analogs can suppress epithelial cell apoptosis in vivo. The mouse model employed here offers an excellent tool for dissecting the as yet poorly understood controls of keratinocyte apoptosis in situ and its pharmacologic manipulation.

Targeted ablation of the vitamin D receptor: an animal model of vitamin D-dependent rickets type II with alopecia

Vitamin D, the major steroid hormone that controls mineral ion homeostasis, exerts its actions through the vitamin D receptor (VDR). The VDR is expressed in many tissues, including several tissues not thought to play a role in mineral metabolism. Studies in kindreds with VDR mutations (vitamin D-dependent rickets type II, VDDR II) have demonstrated hypocalcemia, hyperparathyroidism, rickets, and osteomalacia. Alopecia, which is not a feature of vitamin D deficiency, is seen in some kindreds. We have generated a mouse model of VDDR II by targeted ablation of the second zinc finger of the VDR DNA-binding domain. Despite known expression of the VDR in fetal life, homozygous mice are phenotypically normal at birth and demonstrate normal survival at least until 6 months. They become hypocalcemic at 21 days of age, at which time their parathyroid hormone (PTH) levels begin to rise. Hyperparathyroidism is accompanied by an increase in the size of the parathyroid gland as well as an increase in PTH mRNA levels. Rickets and osteomalacia are seen by day 35; however, as early as day 15, there is an expansion in the zone of hypertrophic chondrocytes in the growth plate. In contrast to animals made vitamin D deficient by dietary means, and like some patients with VDDR II, these mice develop progressive alopecia from the age of 4 weeks.

Hair follicle growth controls

Research in hair biology has embarked in the pursuit for molecules that control hair growth. Many molecules already have been associated with the controls of hair patterning, hair maturation, and hair cycling and differentiation. Knowing how these molecules work gives us the tools for understanding and treating patients with hair disorders.

Noncalcemic actions of 1,25-dihydroxyvitamin D3 and clinical applications

Vitamin D is absolutely essential for the maintenance of a healthy skeleton. Without vitamin D, children develop rickets and adults exacerbate their osteoporosis and develop osteomalacia. Casual exposure to sunlight is the major source of vitamin D for most people. During exposure to sunlight, ultraviolet B photons photolyze cutaneous stores of 7-dehydrocholesterol to previtamin D3. Previtamin D3 undergoes a thermal isomerization to form vitamin D3. Increased skin pigmentation, changes in latitude, time of day, sunscreen use, and aging can have a marked influence on the cutaneous production of vitamin D3. Once vitamin D3 is formed in the skin or ingested in the diet, it must be hydroxylated in the liver and kidney to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. It is now recognized that a wide variety of tissues and cells, both related to calcium metabolism and unrelated to calcium metabolism, are target sites for 1,25(OH)2D3. 1,25(OH)2D3 stimulates intestinal calcium absorption and mobilizes stem cells to mobilize calcium stores from bone. Noncalcemic tissues that possess receptors for 1,25(OH)2D3 respond to the hormone in a variety of ways. Of great interest is that 1,25(OH)2D3 is a potent antiproliferative and prodifferentiation mediator. As a result, 1,25(OH)2D3 and its analogs have wide clinical application in such diverse clinical disorders as rheumatoid and psoriatic arthritis; diabetes mellitus type I; hypertension; cardiac arrhythmias; seizure disorders; cancers of the breast, prostate, and colon; some leukemias and myeloproliferative disorders; chemotherapy-induced hair loss; and skin rejuvenation as well as skin diseases like psoriasis and ichthyosis.