Demonstration and characterization of 1,25-dihydroxyvitamin D3 receptors in basal cells of epidermis of neonatal and adult mice

Differential activity of 2-methylene-19-nor vitamin D analogs on growth factor gene expression in rhino mouse skin and comparison to all-trans retinoic acid

While all 2-methylene-19-nor analogs of 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃) tested produce an increase in epidermal thickness in the rhino mouse, only a subset reduce utricle size (comedolysis). All-trans retinoic acid (atRA) also causes epidermal thickening and a reduction in utricle size in the rhino mouse. We now report that 2-methylene-19-nor-(20S)-1α-hydroxybishomopregnacalciferol (2MbisP), a comedolytic analog, increases epidermal thickening more rapidly than does atRA, while both reduce utricle area at an equal rate. Whereas unlike atRA, 2MbisP does not alter the epidermal growth factor receptor ligand, heparin-binding epidermal growth factor-like growth factor, it does increase the expression of both amphiregulin and epigen mRNA, even after a single dose. In situ hybridization reveals an increase in these transcripts throughout the closing utricle as well as in the interfollicular epidermis. The mRNAs for other EGFR ligands including betacellulin and transforming growth factor-α, as well as the epidermal growth factor receptor are largely unaffected by 2MbisP. Another analog, 2-methylene-19-nor-(20S)-26,27-dimethylene-1α,25-dihydroxyvitamin D3 (CAGE-3), produces epidermal thickening but fails to reduce utricle size or increase AREG mRNA levels. CAGE-3 modestly increases epigen mRNA levels, but only after 5 days of dosing. Thus, 2-MbisP produces unique changes in epidermal growth factor receptor ligand mRNAs that may be responsible for both epidermal proliferation and a reduction in utricle size.

Sequential regulation of keratinocyte differentiation by 1,25(OH)2D3, VDR, and its coregulators

Keratinocyte differentiation requires the sequential regulation of gene expression. We have explored the role of 1,25(OH)(2)D(3) and its receptor (VDR) in this process. VDR sequentially binds to coactivator complexes such as Vitamin D receptor interacting protein (DRIP) and steroid receptor coactivator (SRC) during differentiation. Different genes respond differently to the VDR/coactivator complexes as determined by knockdown studies. The binding of DRIP205 and SRC to VDR is ligand (i.e. 1,25(OH)(2)D(3)) dependent. LXXLL motifs in these coactivators are critical for this binding; however, the affinity for VDR of the different LXXLL motifs in these coactivators varies. Hairless is an inhibitor of 1,25(OH)(2)D(3) dependent gene transcription. A phiXXphiphi motif in hairless is crucial for hairless binding to VDR, and its binding is ligand independent. 1,25(OH)(2)D(3) displaces hairless and recruits the coactivators to VDREs. Hsp90 and p23 are chaperone proteins recruited to the DRIP/VDR complex, where they block the binding of the complex to VDREs and block 1,25(OH)(2)D(3) stimulated transcription. Thus four mechanisms explain the ability of 1,25(OH)(2)D(3) to sequentially regulate gene transcription during differentiation: changes in coregulator levels, their differential binding to VDR, differential gene responsiveness to the VDR/coregulator complexes, and chaperone proteins facilitating the cycling of VDR/coregulator complexes on and off the VDREs.

25 Hydroxyvitamin D 1 alpha-hydroxylase is required for optimal epidermal differentiation and permeability barrier homeostasis

Keratinocytes express high levels of 25OHD 1alpha-hydroxylase (1OHase). The product of this enzyme, 1,25-dihydroxyvitamin D (1,25(OH)(2)D), promotes the differentiation of keratinocytes in vitro suggesting an important role for this enzyme in epidermal differentiation. To test whether 1OHase activity is essential for keratinocyte differentiation in vivo we examined the differentiation process in mice null for the expression of the 1alphaOHase gene (1alphaOHase(-/-)). Heterozygotes for the null allele were bred, and the progeny genotyped by PCR. The epidermis of the 1alphaOHase(-/-) animals and their wild-type littermates (1alphaOHase(+/+)) were examined by histology at the light and electron microscopic level, by immunocytochemistry for markers of differentiation, and by function examining the permeability barrier using transepidermal water loss (TEWL). No gross epidermal phenotype was observed; however, immunocytochemical assessment of the epidermis revealed a reduction in involucrin, filaggrin, and loricrin-markers of differentiation in the keratinocyte and critical for the formation of the cornified envelope. These observations were confirmed at the electron microscopic level, which showed a reduction in the F (containing filaggrin) and L (containing loricrin) granules and a reduced calcium gradient. The functional significance of these observations was tested using TEWL to evaluate the permeability barrier function of the epidermis. Although TEWL was normal in the basal state, following disruption of the barrier using tape stripping, the 1alphaOHase(-/-) animals displayed a markedly delayed recovery of normal barrier function. This delay was associated with a reduction in lamellar body secretion and a failure to reform the epidermal calcium gradient. Thus, the 25OHD 1OHase is essential for normal epidermal differentiation, most likely by producing the vitamin D metabolite, 1,25(OH)(2)D, responsible for inducing the proteins regulating calcium levels in the epidermis that are critical for the generation and maintenance of the barrier.

Inhibition of 1,25-dihydroxyvitamin-D-induced keratinocyte differentiation by blocking the expression of phospholipase C-gamma1

Keratinocytes produce vitamin D3 and convert it to the most active form, 1,25-dihydroxyvitamin D3, which regulates keratinocyte proliferation and differentiation. Phospholipase C-gamma1 is the most abundant member of the phospholipase C family in keratinocytes and is induced by 1,25-dihydroxyvitamin D3. Therefore, phospholipase C-gamma1 might be important in the signaling pathway mediating 1,25-dihydroxyvitamin-D3-induced keratinocyte differentiation. To test this hypothesis, phospholipase C-gamma1 expression in human keratinocytes was reduced by transfecting the cells with an antisense phospholipase C-gamma1 construct and then evaluating the response of the keratinocyte differentiation markers involucrin and transglutaminase to 1,25-dihydroxyvitamin D3. The results showed that involucrin and transglutaminase protein and mRNA levels were markedly reduced in keratinocytes transfected by the antisense phospholipase C-gamma1 construct. Cotransfection of keratinocytes with the involucrin or transglutaminase promoter construct and the antisense phospholipase C-gamma1 construct showed decreased involucrin or transglutaminase promoter activity in response to 1,25-dihydroxyvitamin D3. To further investigate the mechanism by which phospholipase C-gamma1 regulates keratinocyte differentiation, the calcium and inositol triphosphate levels in keratinocytes transfected by the antisense phospholipase C-gamma1 construct were measured following 1,25-dihydroxyvitamin D3 administration. The increase in keratinocyte intracellular free calcium and inositol triphosphate levels following 1,25-dihydroxyvitamin D3 administration were markedly reduced by the transfection of the antisense phospholipase C-gamma1 construct. These studies indicate that phospholipase C-gamma1 plays a critical role in the signal transduction pathway mediating 1,25-dihydroxyvitamin-D3-induced keratinocyte differentiation at least in part by mediating the increase in inositol triphosphate production and intracellular calcium mobilization following 1,25-dihydroxyvitamin D3 administration.

The role of phospholipase C-gamma1 in 1alpha,25-dihydroxyvitamin D(3) regulated keratinocyte differentiation

Phospholipase C-gamma1 (PLC-gamma1) is the most abundant member of the phospholipase C family expressed in human keratinocytes. PLC-gamma1 is induced by 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) in normal keratinocytes via a DR6-type vitamin D responsive element. This regulation is not observed in transformed keratinocytes. The role of PLC-gamma1 in mediating 1alpha,25(OH)(2)D(3) and calcium-regulated differentiation was then tested. Both specific PLC inhibitors and antisense constructs which selectively block PLC-gamma1 production prevented 1alpha,25(OH)(2)D(3) and calcium from inducing markers of differentiation such as involucrin and transglutaminase. These studies demonstrate that PLC-gamma1 induction by 1alpha,25(OH)(2)D(3) is critical to the ability of this hormone to regulate keratinocyte differentiation.

Hexafluoro-1,25-dihydroxyvitamin D3 has markedly increased potency in inhibiting proliferation of cultured human keratinocytes compared with 1,25-dihydroxyvitamin D3

BACKGROUND

Although topical 1,25-dihydroxyvitamin D3 (calcitriol, 1, 25(OH)2D3) and its analogues, calcipotriol and tacalcitol, are effective for patients with psoriasis, some patients show little or no response. There is a need to develop more potent analogues of 1, 25(OH)2D3. Hexafluoro-1,25(OH)2D3 (F6-1,25(OH)2D3) is at least 10 times more potent than 1,25(OH)2D3 on calcium metabolism.

OBJECTIVES

We were interested in whether F6-1,25(OH)2D3 was also more potent than 1,25(OH)2D3 in inhibiting normal human and psoriatic keratinocyte proliferation.

METHODS

The antiproliferative activity of F6-1,25(OH)2D3 and 1,25(OH)2D3 was determined by 3H-thymidine incorporation into keratinocyte DNA and by counting basal cells.

RESULTS

F6-1,25(OH)2D3 was approximately 10-fold more active and had a longer lasting antiproliferative effect than 1,25(OH)2D3 on normal human keratinocytes, and was about 100-fold more potent than 1, 25(OH)2D3 on human psoriatic keratinocytes as determined by 3H-thymidine incorporation. F6-1,25(OH)2D3 also caused a dose-dependent decrease in the number of basal cells and was 100-fold more active than 1,25(OH)2D3.

CONCLUSIONS

The increased potency and the long-lasting effects of F6-1,25(OH)2D3 suggest that F6-1,25(OH)2D3 may be a potent candidate agent for treating psoriasis.

Suppression of vitamin D receptor and induction of retinoid X receptor alpha expression during squamous differentiation of cultured keratinocytes

To gain more insight in the role of the vitamin D system in epidermal differentiation, we studied the expression of the vitamin D receptor and its heterodimeric partner retinoid X receptor alpha in cultured normal human keratinocytes during squamous differentiation, as triggered by different approaches. Northern and western blot analysis allowed us to investigate mRNA and protein levels of these nuclear receptors and of markers for growth control (c-myc, cyclin D1, p21WAF1) and differentiation (keratinocyte transglutaminase, small proline rich proteins). Growing cells to postconfluence was a potent stimulus for growth arrest and differentiation with concomitant suppression of vitamin D receptor and induction of retinoid X receptor alpha, at both the mRNA and the protein level. These changes could be prevented by concomitant treatment with epidermal growth factor or keratinocyte growth factor. Subjecting the cells to a calcium switch leading to stratification and differentiation lowered vitamin D receptor protein levels without affecting vitamin D receptor mRNA and induced both retinoid X receptor alpha mRNA and protein. Interferon-gamma and the phorbolester 12-O-tetradecanoyl phorbol 13-acetate, two well-known inducers of keratinocyte differentiation, both inhibited vitamin D receptor expression but only interferon-gamma induced retinoid X receptor alpha. The decreased vitamin D receptor expression was accompanied by reduced vitamin D responsiveness (as assessed by 24-hydroxylase mRNA induction) in postconfluent, high calcium, and 12-O-tetradecanoyl phorbol 13-acetate treated keratinocytes but not with interferon-gamma treatment. Taken together, our results associate vitamin D receptor expression with undifferentiated, proliferating keratinocytes, whereas retinoid X receptor alpha expression appears to be related to the differentiated phenotype. Therefore, proliferating and differentiating keratinocytes may be differentially targeted by active vitamin D metabolites.

Anchorage-dependent expression of the vitamin D receptor in normal human keratinocytes

Although the nuclear vitamin D receptor (VDR) is involved in the control of keratinocyte proliferation and differentiation by its ligand 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], its role in epidermal physiology remains poorly understood. Because VDR abundance reflects cellular responsiveness to 1,25(OH)2D3, we investigated VDR expression in cultured human keratinocytes and identified cell anchorage and cytoskeletal integrity as essential requirements for the maintenance of VDR levels. Suspension culture rapidly suppressed VDR expression and 1,25(OH)2D3 responsiveness (as estimated by induction of 24-hydroxylase mRNA), due to decreased transcription of the VDR gene. Concomitantly, overt growth arrest with p21WAF1 induction and cyclin D1 and c-myc suppression occurred, together with induction of differentiation markers and retinoid X receptor alpha, the heterodimeric partner for VDR. Reattachment of suspended keratinocytes to fibronectin led to a rapid restoration of VDR expression, which could be blocked by RGD peptides or a blocking anti-beta1 integrin antibody. VDR expression was also reduced by disruption of the actin cytoskeleton with cytochalasin D. Malignant keratinocytes (SCC12B2 and A431), characterized by, anchorage-independent growth, displayed a profound resistance to suspension-induced suppression of VDR, cyclin D1, and c-myc. Taken together, our results associate VDR expression [and 1,25(OH)2D3 responsiveness] with cell adhesion and an organized cytoskeleton, which are also required for cell growth of primary cells.

Interaction of IGF-I and 1 alpha, 25(OH)2D3 on receptor expression and growth stimulation in rat growth plate chondrocytes

Growth plate cartilage cell express receptors for, and are affected by both IGF-I and 1 alpha, 25(OH)2D3. The studies were undertaken to investigate interaction between these two hormone systems, that is, (i) to study effects of 1 alpha, 25(OH)2D3 on IGF-type 1 receptors (IGFIR), on IGF-I stimulated cell replication, colony formation, and on alkaline phosphatase activity (AP), and conversely, (ii) to study the effect of IGF-I on vitamin D receptor (VDR) expression on 1 alpha, 25(OH)2D3 stimulated growth parameters and on AP activity. Freshly isolated rat tibial chondrocytes were grown in monolayer cultures, (serum-free) or in agarose stabilized suspension cultures (0.1% FCS). Vitamin D receptor and IGFIR were visualized by immunostaining with the monoclonal antibody (mAb) 9A7 gamma and mAb alpha IR3, respectively, and quantitated by RT-PCR for mRNA and by Scatchard analysis using [3H]-1,25(OH)2D3 and [125I]-alpha IR3. Cell proliferation was measured by [3H]-thymidine incorporation, growth curves in monolayer cultures, and by colony formation in agarose-stabilized suspension cultures. IGF-I dose-dependently increased [3H]-thymidine incorporation. 1 alpha, 25(OH)2D3, but not 1 beta, 25(OH)2D3 was stimulatory at low ((10-12 M) and slightly inhibitory at high (10-8 M) concentrations. The effect of IGF-I was additive to that of 1 alpha, 25 (OH)2D3 [IGF-I 60 ng/ml, 181 +/- 12.7; 1 alpha, 25(OH)2D3 10(-12) M, 181 +/- 9.8%, IGF-I + 1 alpha, 25(OH)2D3, 247 +/- 16.7%, P < 0.05 by ANOVA] and specifically obliterated by polyclonal IGF-I antibody (AB-1). Interaction could also be confirmed in suspension cultures. IGFIR mRNA and [125I]-alphaIR3 binding was increased by low (10(-12) m) but not by high (10(-8) M) concentrations of 1 alpha, 25(OH)2D3. Homologous up-regulation by IGF-I (60 ng/ml) was specifically inhibited by AB-1 and markedly amplified by coincubation with 1 alpha, 25(OH)2D3 (10(-12)m). Immunostaining with alpha IR3 showed specific IGFIR expression in rat growth cartilage, but not liver tissue. Stimulation of chondrocytes with 1 alpha, 25(OH)2D3 or IGF-I suggested some increase of receptor expression in single cells, but the predominant effect was increased recruitment of receptor positive cells, Vitamin D receptor expression was markedly stimulated (fourfold) by IGF-I (60 ng/ml), but not IGF-II and inhibited by actinomycin D. This study shows that IGF-I and 1 alpha, 25(OH)2D3 mutually up-regulate their respective receptors in growth plate chondrocytes. In parallel, they have additive effects on cell proliferation and colony formation suggesting independent effector pathways.

Distinct, tissue-specific regulation of vitamin D receptor in the intestine, kidney, and skin by dietary calcium and vitamin D

We studied the effects of vitamin D deficiency and its correction by vitamin D or calcium-lactose supplementation on vitamin D receptor (VDR) expression in skin keratinocytes, kidney, and duodenum of adult rats. VDR messenger RNA (mRNA) was assayed by Northern blot, and VDR protein was determined immunocytochemically. In addition, four subpopulations of keratinocytes were isolated, characterized for their stages of differentiation, and analyzed for VDR expression. Vitamin D deficiency decreased VDR mRNA in all three tissues. Treatment with vitamin D or calcium-lactose reestablished the VDR mRNA content of the epidermis, but not that of the kidneys, and only the calcium-lactose diet increased duodenal VDR mRNA. The regulation of VDR mRNA in the epidermis was independent of cell differentiation, whereas VDR protein varied with differentiation. The VDR-positive cells in the control rats were at early and advanced states of differentiation. The expression of VDR was decreased by vitamin D deficiency and returned to control values after vitamin D or calcium supplementation. Vitamin D treatment, but not calcium, induced VDR expression in the normally immature population. Vitamin D and calcium, therefore, have distinct, tissue-specific effects on VDR. In epidermis, the posttranscriptional regulation of VDR expression is linked to cell differentiation. Calcium may be a key factor for VDR transcription, whereas both vitamin D and calcium seem to contribute to its posttranscriptional regulation.

Ontogeny of the 1,25-dihydroxyvitamin D3 receptor in fetal rat bone

To gain insights into 1,25-dihydroxyvitamin D3 receptor (VDR) function during fetal bone development, we examined fetal rat tissues from gestational days 13-21 for the presence and distribution of VDR using immunohistochemistry. Prior to ossification, VDR epitopes were observed in the mesenchyme condensing to form skeletal tissues, on day 13 in the developing vertebral column and limbs, and on day 17 of gestation in developing calvaria. Immunostaining for VDR was seen in proliferating and hypertrophic chondrocytes and in osteoblasts of limb buds and the vertebral column by day 17 of gestation. In calvaria, VDR epitopes were observed in osteoblasts by gestational day 19. VDR immunostaining was also evident in the skin of fetal limbs at all gestational ages examined. We show for the first time that the VDR appears very early in the developing fetal rat skeleton, suggesting that the VDR, in concert with its ligand, 1,25-dihydroxyvitamin D3, may play a role in the differentiation of mesenchymal precursors into bone tissue.

The mRNA expression of the human 1,25-dihydroxyvitamin D3 receptor and the c-myc protooncogene in cultured human keratinocytes

The human vitamin D receptor mRNA expression in preconfluent human cultured keratinocytes was upregulated by treatment of these cells with 10(-8) M 1,25(OH)2D3 for 24 hours. Additionally, human c-myc mRNA expression was decreased in a dose dependent manner by 1,25(OH)2D3 in both preconfluent and confluent cultured human keratinocytes.

Intermittent and continuous exposure to 1,25(OH)2D3 have different effects on growth plate chondrocytes in vitro

Intermittent 1,25(OH)2D3 administration is widely used to suppress parathyroid glands in secondary (renal) hyperparathyroidism. It is unknown whether the effects of continuous and intermittent 1,25(OH)2D3 differ on vitamin D target organs other than parathyroids. Using primary cultures of rat chondrocytes (tibia) we compared the effects of continuous versus intermittent exposure to physiologic concentrations of 1 alpha,25(OH)2D3 on proliferation (radiothymidine incorporation), cell count, protein synthesis ([3H]-leucine incorporation), alkaline phosphatase activity (as a marker of differentiation) and 1 alpha,25(OH)2D3 receptor (VDR) regulation. Cells were synchronized and then exposed for variable periods to a medium containing 10% delipidated FCS and 10(-8) M to 10(-12) M 1 alpha,25(OH)2D3 (or 1 beta,25(OH)2D3 as specificity control). Intermittent (8 hr exposure every 48 hr) as well as continuous (sham washing) administration of 1 alpha,25(OH)2D3 had a biphasic effect on proliferation, that is, stimulation at low (10(-12) M) and inhibition at high (10(-8) M) concentrations. At 10(-12) M intermittent 1 alpha,25(OH)2D3 yielded higher cell counts than continuous 1,25(OH)2D3. This was seen in the log phase, which was day 3 (continuous 141 +/- 2.3% of solvent control; intermittent 185 +/- 2.0%) and in the plateau phase of growth, which was day 6 (128 +/- 2.6 vs. 169 +/- 2.7% of solvent control). Dependence on extracellular Ca is suggested by the effects of varying nominal Ca concentrations in the medium and of Ca channel blockers. Even two hours of exposure to 1 alpha,25(OH)2D3 (10(-12) M) yielded maximal activation of AP during postincubation.(ABSTRACT TRUNCATED AT 250 WORDS)

1,25(OH)2D3 receptor regulation and 1,25(OH)2D3 effects in primary cultures of growth cartilage cells of the rat

Vitamin D deficiency leads to disturbed calcification of growth cartilage and enlargement of growth plate, illustrating that chondrocytes are a target for vitamin D. This observation prompted an investigation of 1,25(OH)2D3 receptor expression and action of vitamin D metabolites on chondrocyte proliferation. In primary cultures of tibial growth cartilage of male SD rats (80 g), specific binding of [3H]-1,25(OH)2D3 is noted in both the logarithmic growth phase and at confluence (Nmax 12780 molecules/cell versus 4368 molecules/cell). Scatchard analysis revealed the presence of a single class of noninteracting binding sites. KD was 10(-11) M irrespective of growth phase. The binding macromolecule had a sedimentation coefficient of 3.5 S. Interaction with DNA was demonstrated by DNA cellulose affinity chromatography. In immunohistology, growth cartilage cells (rabbit tibia) expressed nuclear 1,25(OH)2D3 receptors most prominently in the proliferative and hypertrophic zone. This corresponds to binding data which showed highest Nmax in the proliferating cartilage. 1,25(OH)2D3 in the presence of delipidated fetal calf serum (FCS) had a biphasic effect on cell proliferation and density, i.e., stimulation at 10(-12) M and dose-dependent inhibition at 10(-10) M and below. Inhibition was specific and not seen with 24,25(OH)2D3 or dexamethasone. Growth phase-dependent 1,25(OH)2D3 receptor expression and effects of 1,25(OH)2D3 on chondrocyte proliferation point to a role of vitamin D in the homeostasis of growth cartilage.

Squamous carcinoma cell lines produce 1,25 dihydroxyvitamin D, but fail to respond to its prodifferentiating effect

The active metabolite of vitamin D3, 1,25 dihydroxyvitamin D3 [1,25(OH)2D], is produced by normal human keratinocytes (NKC) and regulates their differentiation. Squamous carcinoma cell (SCC) lines lack the ability to differentiate in vitro, which might involve defective 1,25(OH)2D synthesis or response. To address this possibility we obtained four SCC lines (12F2, 12B2, 25, and A431) and first determined whether they could produce 1,25(OH)2D from its substrate 25 hydroxyvitamin D3 (250HD). All could (12F2 greater than NKC greater than 25 greater than 12B2 greater than A431). Furthermore, exogenously added 1,25(OH)2D inhibited 1,25(OH)2D production and stimulated 24,25 dihydroxyvitamin D3 [24,25(OH)2D] production in all cell lines but with different potency (25 = A431 greater than NKC greater than 12B2 greater than 12F2). Cellular binding studies suggested that the high-affinity binding site for 1,25(OH)2D in NKC is not found in 12F2 and 12B2. When the effect of 1,25(OH)2D on differentiation was determined, only NKC responded with an increase in cornified envelope formation, although some of the cell lines responded to the proliferative [at low 1,25(OH)2D concentration] or antiproliferative [at high 1,25(OH)2D concentration] effect of 1,25(OH)2D. Thus, although SCC lines synthesize 1,25(OH)2D and respond to exogenous 1,25(OH)2D with respect to appropriate regulation of endogenous 250HD metabolism, these cell lines fail to respond to the differentiating influence of this vitamin D metabolite.

Effects of 1,25(OH)2D3 on compensatory renal growth in the growing rat

Renal compensatory growth after uninephrectomy (UNX) was examined in vitamin D replete male 100 g Sprague-Dawley rats. Five days after UNX, the contralateral kidney wet weight increased by 25% with the kidney weight/body weight ratio reaching a plateau by day 7 after UNX. The early weight increase was primarily due to an increased cell number, as evaluated by a stereological technique in perfusion-fixed kidneys. Twenty pmol 1,25(OH)2D3 by daily s.c. injection increased time-averaged 1,25(OH)2D3 concentrations 3.3-fold and reduced the increment in the kidney weight of UNX pairfed rats compared to solvent UNX controls. The number of mitoses (whole kidney and different nephron segments) were significantly reduced by giving 1,25(OH)2D3 to UNX animals at different levels of food intake. The effect was also demonstrable in PTX animals on a constant infusion of exogenous PTH (100 ng/kg/hr 1,34 bPTH by osmotic minipump). The data suggest that changes of 1,25(OH)2D3 concentration within a physiologically relevant range modulate compensatory (and possibly basal) growth of the kidney.

Effect of vitamin D on growth cartilage cell proliferation in vitro

Disturbed calcification of the growth plate and stunting is a frequent finding in vitamin D-deficiency rickets, vitamin D-dependency rickets and renal osteodystrophy, illustrating that chondrocytes are a target for vitamin D. This observation prompted an investigation of Ic, 25-dihydroxy vitamin D3 [1,25(OH)2D3] receptor expression and action of vitamin D metabolites on chondrocyte proliferation. In tibial growth plates and in primary cultures of tibial growth cartilage of male Sprague-Dawley rats (80 g) specific binding of [3H]-1,25(OH)2D3 was noted. Scatchard analysis revealed the presence of a single class of non-interacting binding sites. Kd was 10(-11) M irrespective of growth phase. The binding macromolecule had a sedimentation coefficient of 3.5 S. Interaction with DNA was demonstrated by DNA-cellulose affinity chromatography. By immunohistology, growth cartilage cells (rabbit tibia) were shown to express nuclear 1,25(OH)2D3 receptors, most prominently in the proliferative and early hypertrophic zone. This corresponds to binding data which showed highest binding of 1,25(OH)2D3 in the logarithmic growth phase (12,780 molecules/cell versus 4,538 molecules/cell in confluent cells) in primary cultures of growth plate chondrocytes. In the presence of delipidated fetal calf serum 1,25(OH)2D3 had a biphasic effect on cell proliferation and density, i.e. stimulation at 10(-12) M and dose-dependent inhibition at 10(-10) M and below. Inhibition was specific and not seen with 24 (R), 25-dihydroxy-vitamin D3 or dexamethasone. Growth phase-dependent 1,25(OH)2D3 receptor expression and specific effects of 1,25(OH)2D3 on chondrocyte proliferation in vitro point to a role for vitamin D in the homeostasis of growth cartilage of the rat.

Suppression of proliferation of human epidermal keratinocytes by 1,25-dihydroxyvitamin D3. Analysis of its effect on psoriatic lesion and of its mechanism using human keratinocytes in culture

Psoriasis is a genetically determined chronic disease of the skin, and an accelerated proliferation of epidermal keratinocytes is one of the pathophysiological characteristics of psoriatic lesion. Recently, it was reported that topical administration of 1,25-dihydroxyvitamin D3 was effective for psoriasis. Our study was done to investigate the effect of 1,25-(OH)2-D3 on cell kinetics on human epidermal keratinocytes as a possible mechanism of its effect on psoriasis. After 24-h application of 1 microgram g-1 1,25-(OH)2-D3 ointment to psoriatic lesion, the number of mitotic keratinocytes decreased. When the cultured human keratinocytes were exposed to 10(-8) mol l-1 1,25-(OH)2-D3 for more than 9 days, inhibition of cell proliferation was noted. DNA distribution analysis by flow cytometry showed a decrease in cells in the S phase, and increase in 2c cells. This indicates blockade of the cell cycle in the G1 phase. The cell cycle time was not extended as a result of 1,25-(OH)2-D3-treatment.

Effect of 1,25-dihydroxyvitamin D3 on human keratinocytes grown under different culture conditions

1,25-Dihydroxyvitamin D3 (1,25-(OH)2-D3) is known to decrease the proliferation and increase the differentiation of different cell types including human keratinocytes. The growth and differentiation of keratinocytes in the presence of 1,25-(OH)2-D3 using serum-free media formulations has been described previously. This investigation extends these studies to describe various culture conditions with human foreskin keratinocytes to determine the optimal antiproliferative activity of 1,25-(OH)2-D3. Keratinocytes were plated onto tissue culture dishes using one of three basic serum-free media protocols; a) with no feeder layer in keratinocyte growth medium (KGM); b) onto mitomycin C-treated 3T3 mouse embryo fibroblasts; or c) onto mitomycin C-treated dermal human fibroblasts. The last two protocols utilized Dulbecco's modified Eagle's Medium (DMEM) supplemented with growth factors. Keratinocyte cell growth was greatest in the KGM medium. Although the growth of keratinocytes on either feeder layer was similar, there were differences in the ability of the cells to form envelopes in the presence of 1,25-(OH)2-D3. The addition of hydrocortisone and cholera toxin to the medium also affected the response of the keratinocytes to 1,25-(OH)2-D3. The antiproliferative effect of 1,25-(OH)2-D3 was not altered by varying the extracellular calcium levels from 0.25 to 3 mM. The antiproliferative activity of 1,25-(OH)2-D3 is attenuated in cells at low density. Our results suggest that an optimal condition to investigate the ability of 1,25-(OH)2-D3 to inhibit keratinocyte proliferation is at preconfluent cell density in the presence of KGM supplemented with 1.5 mM calcium without a feeder layer. These conditions are not appropriate for investigating the enhancement of differentiation by 1,25-(OH)2-D3, but can be used to assay other agents that modulate keratinocyte proliferation.

Identification and regulation of 1,25-dihydroxyvitamin D3 receptor activity and biosynthesis of 1,25-dihydroxyvitamin D3. Studies in cultured bovine aortic endothelial cells and human dermal capillaries

Because 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) has been shown to play roles in both proliferation and differentiation of novel target cells, the potential expression of 1,25(OH)2D3 receptor (VDR) activity was investigated in cultured bovine aortic endothelial cells (BAEC). Receptor binding assays performed on nuclear extracts of BAEC revealed a single class of specific, high-affinity VDR that displayed a 4.5-fold increase in maximal ligand binding (Nmax) in rapidly proliferating BAEC compared with confluent, density-arrested cells. When confluent BAEC were incubated with activators of protein kinase C (PKC), Nmax increased 2.5-fold within 6-24 h and this upregulation was prevented by sphingosine, an inhibitor of PKC, as well as by actinomycin D or cycloheximide. Immunohistochemical visualization using a specific MAb disclosed nuclear localized VDR in venular and capillary endothelial cells of human skin biopsies, documenting the expression of VDR, in vivo, and validating the BAEC model. Finally, additional experiments indicated that BAEC formed the 1,25(OH)2D3 hormonal metabolite from 25(OH)D3 substrate, in vitro, and growth curves of BAEC maintained in the presence of 10(-8) M 1,25(OH)2D3 showed a 36% decrease in saturation density. These data provide evidence for the presence of a vitamin D microendocrine system in endothelial cells, consisting of the VDR and a 1 alpha-hydroxylase enzyme capable of producing 1,25(OH)2D3. That both components of this system are coordinately regulated, and that BAEC respond to the 1,25(OH)2D3 hormone by modulating growth kinetics, suggests the existence of a vitamin D autocrine loop in endothelium that may play a role in the development and/or functions of this pathophysiologically significant cell population.

1,25(OH)2 vitamin D3 inhibits parathyroid cell proliferation in experimental uremia

Parathyroid cell proliferation and parathyroid hyperplasia are features of renal secondary hyperparathyroidism. Since parathyroids have recently been recognized as an important target for 1,25(OH)2D3, the effects of administration of variable doses of 1,25(OH)2D3 on ex vivo radiothymidine incorporation in the parathyroid glands, on parathyroid cell mitoses, on parathyroid weight, morphometric indices and on parathyroid protein/DNA ratio were examined in rats with uremia (subtotal nephrectomy; NX) or with calcium deficiency. 3H-thymidine incorporation (3 hr; 37 degrees C; PBS with 10 mmol glucose) was elevated in NX animals, that is, 204 +/- 51 dpm/micrograms DNA versus 96 +/- 28 in controls. In vivo pretreatment with 1,25(OH)2D3, either by intermittent i.p. injection or by osmotic minipump, dose-dependently decreased 3H-thymidine incorporation and parathyroid cell mitoses without affecting morphometric indices of parathyroid cells. Prophylactic administration (i.p.) of 1,25(OH)2D3, starting on the day of nephrectomy, prevented parathyroid hyperplasia (NX + 1,25(OH)2D3 0.84 micrograms tissue/g body wt vs. 1.25 micrograms in untreated NX and 0.54 in ad libitum fed controls), but 10 days of treatment beginning on the 21st day of uremia did not reverse existing hyperplasia (NX + 1,25(OH)2D3 1.5 micrograms/g body wt vs. 1.37 micrograms in untreated NX and 0.56 micrograms in ad libitum fed controls). The inhibitory effect was specific for 1,25(OH)2D3 and not imitated by Dexamethason. However, the effect was not specific for parathyroid hyperplasia of uremia, since similar inhibition of 3H-thymidine incorporation by 1,25(OH)2D3 was also observed in rats on low calcium diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Diminished parathyroid 1,25(OH)2D3 receptors in experimental uremia

In Sprague Dawley rats, six days after subtotal nephrectomy, serum 1,25(OH)2D3 concentration was diminished (59.8 +/- 17.5 pg/ml vs. 121 +/- 48; P less than 0.01). Despite low circulating 1,25(OH)2D3 levels, maximal specific binding capacity for 1,25(OH)2D3 in parathyroid glands was diminished (Nmax 87.5 fmol/mg protein and 3.52 fmol/mg DNA vs. 143 fmol/mg protein and 4.75 fmol/mg DNA, respectively). There was no change of KD, apparent molecular size (sucrose density gradient) and DNA binding affinity (DNA cellulose chromatography) pointing to intactness of the receptor. Since 1,25(OH)2D3 is a potent negative feedback signal for parathyroids, the data are potentially relevant for the genesis of secondary renal hyperparathyroidism.

1,25(OH)2D3 receptors and endorgan response in experimental aluminium intoxication

Severe aluminium-induced osteomalacia is refractory to treatment with 1,25(OH)2D3 which frequently causes hypercalcemia. To further explore the mechanisms involved, we have utilized a model of short-term aluminium intoxication in the rat (total: 11 mg elemental aluminium in 3 weeks) to study (a) 1,25(OH)2D3 receptor status in a variety of classical and non-classical target organs for 1,25(OH)2D3; (b) circulating 1,25(OH)2D3 levels; (c) baseline duodenal calcium transport, utilising the Ussing chamber, to investigate the functional significance of receptor status in a classical target organ; and (d) duodenal calcium transport response to exogenously administered 1,25(OH)2D3. Both in the three week model and in the 16 week model (total: 41 mg elemental calcium) increased maximal specific binding capacity for 1,25(OH)2D3 (Nmax), that is, number of unoccupied receptors, was observed in nuclear fractions of all tissues studied. Receptor affinity, the apparent dissociation constant KD, was unchanged. Total binding capacity, measured after displacement of endogenous ligand by Mersalyl, that is, the sum of occupied plus non-occupied receptors, was also increased. Both circulating 1,25(OH)2D3, mucosa-to-serosa calcium flux (Jms) and net calcium flux (Jnet) were reduced under baseline conditions, suggesting the lack of a direct relationship between receptor expression and endorgan response. Following exogenous 1,25(OH)2D3 administration, calcium Jms and Jnet were significantly lower in the aluminium intoxicated animals, with the increment induced in Jnet in aluminium intoxicated animals being 63% of that induced in controls. Our data suggest that resistance to the action of 1,25(OH)2D3 in aluminium intoxication is postreceptor in nature.

No 1,25-dihydroxyvitamin D3 receptors on osteoclasts of calcium-deficient chicken despite demonstrable receptors on circulating monocytes

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is known to stimulate osteoclastic bone resorption in vivo and whole organ bone culture systems in vitro. It has not been established whether 1,25(OH)2D3 acts directly on osteoclasts or whether its action on osteoclasts is mediated via other bone cells (e.g., osteoblasts) or recruitment of osteoclast precursor cells. Circulating monocytes have been characterized as osteoclast precursors. In the present study, vitamin D3-replete chicken on a calcium-deficient diet were studied. Circulating monocytes, whole bone cell preparations, and isolated osteoclasts (differential sedimentation) were examined for presence of 1,25(OH)2D3 receptors. Reversible, specific, and saturable binding of [3H]-1,25(OH)2D3 to a 3.5 S macromolecule was demonstrated in nuclear fractions of monocytes (maximal binding capacity, 48 fmol/mg protein; dissociation constant, 1.3 X 10(-10) M) and of whole bone cell preparations. 1,25(OH)2D3 receptors were not demonstrable in osteoclast preparations (70% pure; detection threshold, 2 fmol/mg protein). Data are consistent with indirect action of 1,25(OH)2D3 on osteoclastic bone resorption.