Assessment of the free fraction of 25-hydroxyvitamin D in serum and its regulation by albumin and the vitamin D-binding protein

We measured the free fraction of 25-hydroxyvitamin D (25OHD) in human serum and determined that 25OHD bound to a component with an affinity constant of 7 X 10(8) M-1 and a concentration of 4.5 X 10(-6) M. This concentration was equal to that of the vitamin D-binding protein (DBP) in the same serum sample. We removed DBP from the serum using actin affinity columns and found that the affinity for 25OHD of the remaining serum components was equivalent to that of human serum albumin (6 X 10(5) M-1). We then measured the free fractions of 25OHD, DBP, and albumin in normal and cirrhotic subjects. We calculated that 88 +/- 3% (+/- SD) and 83 +/- 8% of the 25OHD were bound to DBP in the serum of normal and cirrhotic subjects, respectively. We compared previously reported data for the free fraction and the free concentration of 1,25-dihydroxyvitamin D in these subjects with the current data for the free fraction and free concentration of 25OHD. The total concentrations and free fractions of both metabolites correlated to each other and to the DBP and albumin concentrations in these subjects, but the free concentrations of these metabolites did not. We conclude that 25OHD, like 1,25-dihydroxyvitamin D, is transported in blood bound primarily to DBP and albumin. Changes in the concentrations of DBP and albumin affected the total and free fractions of 25OHD in serum, but the actual free concentration of 25OHD was independent of such changes.

Hormonal control of the renal conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol

Isolated renal tubules from vitamin D-deficient chicks catalyse the in vitro conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol. This conversion is stimulated by 5 x 10(-10) M bovine parathyroid hormone, or by 10(-6) M cyclic AMP. It is inhibited by 10(-9) M porcine calcitonin. It is concluded that these hormonal controls of the synthesis of the renal hormone 1,25-dihydroxycholecalciferol are of particular physiological significance in coordinating the activities of the various organs involved in extracellular calcium homeostasis.

Calcium regulation of growth and differentiation of normal human keratinocytes: modulation of differentiation competence by stages of growth and extracellular calcium

In this study we examined the different aspects of the pathway leading to the differentiation of keratinocytes as a function of time in culture and calcium concentration of the culture medium. Human neonatal foreskin keratinocytes were grown in a serum-free, defined medium containing 0.07, 1.2, or 2.4 mM calcium and assayed for the rate of growth and protein synthesis, involucrin content, transglutaminase activity, and cornified envelope formation at preconfluent, confluent, and postconfluent stages of growth. We observed that keratinocytes grown to postconfluence in all calcium concentrations showed an increased protein/DNA ratio and an increased rate of membrane-associated protein synthesis. Extracellular calcium concentrations did not have a clear influence on these parameters. However, preconfluent and confluent keratinocytes grown in 0.07 mM calcium showed markedly retarded differentiation at all steps, i.e., involucrin synthesis, transglutaminase activity, and cornified envelope formation. Within 1 week after achieving confluence, these keratinocytes began synthesizing involucrin and transglutaminase and developed the ability to form cornified envelopes. Cells grown in 1.2 and 2.4 mM calcium synthesized involucrin and transglutaminase prior to confluence and were fully competent to form cornified envelopes by confluence. Thus external calcium-regulated keratinocyte differentiation is not an all or none phenomenon, but rather it is the rate at which keratinocytes differentiate that is controlled by calcium. We conclude that either or both higher extracellular calcium concentration and the achievement of cell-cell contacts lead to a coordinate increase of at least two precursors--involucrin content and transglutaminase activity--required for cornified envelope formation. We speculate that a critical level of cytosolic calcium, achieved by increased extracellular calcium or by achievement of intercellular communication established by cell-cell contact, may trigger mechanisms required for initiation of keratinocyte differentiation.

Free 1,25-dihydroxyvitamin D levels in serum from normal subjects, pregnant subjects, and subjects with liver disease

We measured the free concentration of 1,25-dihydroxyvitamin D (1,25[OH]2D) using centrifugal ultrafiltration, and the level of vitamin D-binding protein (DBP) in 24 normal subjects, 17 pregnant subjects, and 25 alcoholic subjects with liver disease. Our objective was to determine whether the increase in total 1,25(OH)2D levels in pregnant women and the reduction in total 1,25(OH)2D levels in subjects with liver disease reflected a true difference in free 1,25(OH)2D levels or whether such differences were due solely to the variations in DBP levels (and thus, the amount of 1,25[OH]2D bound) in these groups. In subjects with liver disease the mean total 1,25(OH)2D concentration (22.6 +/- 12.5 pg/ml) and the mean DBP concentration (188 +/- 105 micrograms/dl) were nearly half the normal values (41.5 +/- 11.5 pg/ml and 404 +/- 124 micrograms/dl, respectively, P less than 0.001), whereas the mean free 1,25(OH)2D level was similar to normal values (209 +/- 91 fg/ml and 174 +/- 46 fg/ml, respectively). In contrast, in pregnant subjects the mean total 1,25(OH)2D level (82 +/- 21 pg/ml) and mean DBP level (576 +/- 128 micrograms/dl) were significantly higher than normal (P less than 0.001). Although the mean percent free 1,25(OH)2D level in pregnant subjects was below normal (0.359 +/- 0.07% vs. 0.424 +/- 0.07%, P less than 0.001), the mean free 1,25(OH)2D level was 69% higher than normal (294 +/- 98 fg/ml vs. 174 +/- 46 fg/ml, P less than 0.001). When data from all three groups were combined, there was a linear correlation between total 1,25(OH)2D and DBP levels but not between DBP and percent free 1,25(OH)2D levels; the increased DBP levels in the pregnant subjects were associated with less of an effect on percent free 1,25(OH)2D than were the reduced DBP levels in the subjects with liver disease. Our data suggest that (a) free 1,25(OH)2D levels appear to be well maintained even in subjects with liver disease and reduced DBP levels, (b) free 1,25(OH)2D levels are increased during pregnancy despite the increase in DBP levels, and (c) free 1,25(OH)2D levels cannot be inferred accurately from measurements of total 1,25(OH)2D and DBP levels alone in subjects with various physiologic and pathophysiologic conditions.

1,25-Dihydroxyvitamin D3 production by human keratinocytes. Kinetics and regulation

Human foreskin keratinocytes in vitro metabolize 25-hydroxyvitamin D3 to a number of metabolites, including 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). This metabolite remains mostly within the cell and does not accumulate in the medium under the conditions of these experiments. With time, 1,25(OH)2D3 is catabolized, and more polar metabolites appear in both the cells and the medium. The production of 1,25(OH)2D3 has an apparent Michaelis constant (Km) for 25-hydroxyvitamin D3 of 5.4 X 10(-8) M. The levels of 1,25(OH)2D3 within the cell are increased both by increased production and decreased catabolism when parathyroid hormone(1-34) and isobutylmethylxanthine are added. Exogenously added 1,25(OH)2D3 at concentrations as low as 10(-12) M reduces endogenous 1,25(OH)2D3 production, increases 1,25(OH)2D3 catabolism, and increases 24,25-dihydroxyvitamin D3 production by an actinomycin D-sensitive process. These data indicate that the regulation of 1,25(OH)2D3 production by keratinocytes is similar to, but not identical to the regulation of 1,25(OH)2D3 by the kidney.

Bone disease in alcohol abuse

We evaluated bone disease in eight white men between the ages of 49 and 61 years who had been abusing alcohol for at least 10 years. The mean density of vertebral cancellous bone was 58% of normal, whereas the mean density of appendicular cortical bone was 90% of normal. Marked reduction in active bone resorption and bone formation was seen without evidence of osteomalacia. Serum levels of calcium and magnesium were in the lower range of normal; serum levels of phosphorus, calcifediol, and calcitriol were normal; and serum levels of parathyroid hormone and nephrogenous cyclic adenosine monophosphate were in the higher range of normal. These data suggest that bone disease in these subjects is not due to inhibition of parathyroid hormone secretion or function, or abnormal vitamin D metabolism, but to an inhibition of bone remodeling by a mechanism independent of the calciotropic hormones.

Serum protein binding of 1,25-dihydroxyvitamin D: a reevaluation by direct measurement of free metabolite levels

Using the technique of centrifugal ultrafiltration isodialysis to measure the free concentration of 1,25-dihydroxyvitamin D [1,25-(OH)2D], we determined the affinity of serum proteins for 1,25-(OH)2D both by Scatchard analysis (increasing ligand concentration at fixed binding site concentrations) and by a novel analysis in which the binding site concentrations were varied (serial dilution) at fixed ligand concentrations. The high affinity binding constant in serum for 1,25-(OH)2D was 3.7 X 10(7) M-1 by Scatchard analysis and 4.2 X 10(7) M-1 by serial dilution analysis. Human serum albumin had a much lower affinity for 1,25-(OH)2D (5.4 X 10(4) M-1). When vitamin D-binding protein (DBP) was selectively removed from serum by an actin affinity column, the affinity of the remaining serum proteins for 1,25-(OH)2D was that of albumin. Postulating a two-site model (DBP and albumin) for transport of 1,25-(OH)2D in serum and incorporating the estimated affinity constants of DBP and albumin for this metabolite, we calculated that 85% of total circulating 1,25-(OH)2D is transported in blood bound to DBP in normal individuals (0.4% is free and 14.6% is bound to albumin). In patients with liver disease, 73% is bound to DBP (1.1% is free and 25.9% is bound to albumin). Using this same two site model, we found a reasonable correlation (r = 0.612; P less than 0.001) between the measured free 1,25-(OH)2D level and the calculated free 1,25-(OH)2D level in serum based on albumin and DBP concentrations in 16 normal subjects and 16 patients with liver disease. These results confirm the concept that although DBP is the principal protein carrier of 1,25-(OH)2D in serum, albumin is a major secondary carrier, especially in patients with low DBP levels.

Changes in calcium responsiveness and handling during keratinocyte differentiation. Potential role of the calcium receptor

Extracellular calcium concentrations (Cao) > 0.1 mM are required for the differentiation of normal human keratinocytes in culture. Increments in Cao result in acute and sustained increases in the intracellular calcium level (Cai), postulated to involve both a release of calcium from intracellular stores and a subsequent increase in calcium influx through nonspecific cation channels. The sustained rise in Cai appears to be necessary for keratinocyte differentiation. To understand the mechanism by which keratinocytes respond to Cao, we measured the acute effects of Cao on Cai and calcium influx in keratinocytes at various stages of differentiation. We then demonstrated the existence of the calcium receptor (CaR) in keratinocytes and determined the effect of calcium-induced differentiation on its mRNA levels. Finally, we examined the role of Cai in regulating both the initial rise in Cai after the switch to higher Cao and the activity of the nonspecific cation channel through which calcium influx occurs. Our data indicate that the acute Cai response to Cao is lost as the cells differentiate and increase their basal Cai. These data correlated with the decrease in CaR mRNA levels in cells grown in low calcium. However, calcium influx as measured by 45Ca uptake increased with differentiation in 1.2mM calcium, consistent with the increase in CaR mRNA in these cells as well as the calcium-induced opening of the nonspecific cation channels. We conclude that the keratinocyte contains a CaR that regulates both the initial release of Cai from intracellular stores and the subsequent increase in calcium flux through nonspecific calcium channels. A rising level of Cai may turn off the release of calcium from intracellular stores while potentiating the influx through the nonspecific cation channels. Differentiation of keratinocytes appears to increase the CaR, which may facilitate the maintenance of the high Cai required for differentiation.

The response of bone to unloading

Skeletal unloading leads to decreased bone formation and decreased bone mass. Bone resorption is uncoupled from bone formation, contributing to the bone loss. During spaceflight bone is lost principally from the bones most loaded in the 1-g environment, and some redistribution of bone from the lower extremities to the head appears to take place. Although changes in calcitropic hormones have been demonstrated during skeletal unloading (PTH and 1,25(OH)2D decrease), it remains unclear whether such changes account for or are in response to the changes in bone formation and resorption. Bed rest studies with human volunteers and hindlimb elevation studies with rats have provided useful data to help explain the changes in bone formation during spaceflight. These models of skeletal unloading reproduce a number of the conditions associated with microgravity, and the findings from such studies confirm many of the observations made during spaceflight. Determining the mechanism(s) by which loading of bone is sensed and translated into a signal(s) controlling bone formation remains the holy grail in this field. Such investigations couple biophysics to biochemistry to cell and molecular biology. Although studies with cell cultures have revealed biochemical responses to mechanical loads comparable to that seen in intact bone, it seems likely that matrix-cell interactions underlie much of the mechanocoupling. The role for systemic hormones such as PTH, GH, and 1,25(OH)2D compared to locally produced factors such as IGF-I, PTHrP, BMPs, and TGF-beta in modulating the cellular response to load remains unclear. As the mechanism(s) by which bone responds to mechanical load with increased bone formation are further elucidated, applications of this knowledge to other etiologies of osteoporosis are likely to develop. Skeletal unloading provides a perturbation in bone mineral homeostasis that can be used to understand the mechanisms by which bone mineral homeostasis is maintained, with the expectation that such understanding will lead to effective treatment for disuse osteoporosis.

Neonatal human foreskin keratinocytes produce 1,25-dihydroxyvitamin D3

Primary cultures of neonatal human foreskin keratinocytes converted 25-hydroxyvitamin D in high yield to a metabolite with the chromatographic behavior of 1,25-dihydroxyvitamin D3. The identity of this metabolite as 1,25-dihydroxyvitamin D3 was confirmed both by its potency in displacing 1,25-dihydroxyvitamin D3 in the chick cytosol receptor assay and by mass spectral analysis. These results suggest that 1,25-dihydroxyvitamin D3 may be formed in the epidermis to regulate vitamin D production by the epidermis and to provide an alternative to 1,25-dihydroxyvitamin D3 production by the kidneys.

The temporal response of bone to unloading

A model of weightlessness in which the hindlimbs of rats are elevated by their tails at a 40 degrees angle to unload the hindlimbs while maintaining normal weight bearing on the forelimbs has been used to simulate certain conditions of space flight. When we used this model in growing rats, we found that growth in bone weight ceased by 1 week in the hindlimbs and lumbar vertebrae, whereas growth in bone weight in the forelimbs and cervical vertebrae remained unaffected. Within 2 weeks, however, the accretion of bone weight in the hindlimbs and lumbar vertebrae returned to normal despite continued skeletal unloading. Since bone weight in the growing rat is primarily determined by bone formation (bone resorption is modest), we investigated the effects of selective skeletal unloading on bone formation during 2 weeks of hindlimb elevation using radioisotope incorporation (with 45Ca and [3H]proline) and histomorphometry (with tetracycline labeling). The studies using radioisotope incorporation showed that bone formation was inhibited by the fifth day of skeletal unloading. By the 10th to 12th day, bone formation had returned toward normal. In comparison with cortical bone, cancellous bone (lumbar vertebrae and proximal tibiae) incorporated more 45Ca and [3H]proline (indicating greater metabolic activity) and had a greater absolute response to skeletal unloading. The results of these studies were confirmed by histomorphometric measurements of bone formation using triple tetracycline labeling. We conclude that this model of simulated weightlessness results in an initial inhibition of bone formation in the unloaded bones. This temporary cessation of bone formation is followed by a cessation in the accretion of bone weight, which then resumes at a normal rate by 14 days despite continued skeletal unloading. We believe that this cycle of inhibition and resumption of bone formation has profound implications for understanding bone dynamics during space flight, immobilization, or bed rest and offers an opportunity to study the hormonal and mechanical factors that regulate bone formation.

A comparison of measured and calculated free 25(OH) vitamin D levels in clinical populations

OBJECTIVE

Our goal was to compare direct quantitation of circulating free 25-hydroxyvitamin D (25(OH)D)levels to calculated free 25(OH)D levels and their relationships to intact PTH (iPTH), a biomarker of 25(OH)D effect, in humans with a range of clinical conditions.

PATIENTS AND METHODS

Serum samples and clinical data were collected from 155 people: 111 without cirrhosis or pregnancy (comparison group), 24 cirrhotic patients with albumin <2.9 g/dL, and 20 pregnant women (second and third trimester). Total 25(OH)D (LC/MS/MS), free 25(OH)D (immunoassay), vitamin D binding protein (DBP) (immunoassay), albumin, and iPTH (immunoassay) were measured.

RESULTS

Total 25(OH)D, DBP, and albumin were lowest in patients with cirrhosis, but measured free 25(OH)D was highest in this group (P < .001). DBP was highest in pregnant women (P < .001), but measured free 25(OH)D did not differ from the comparison group. Calculated free 25(OH)D was positively correlated with measured free 25(OH)D (P < .0001) but explained only 13% of the variability with calculated values higher than measured. African Americans had lower DBP than other ethnic populations within all clinical groups (P < .03), and differences between measured and calculated free 25(OH)D were greatest in African Americans (P < .001). Measured free 25(OH)D was correlated with total 25(OH)D (P < .0001; r(2) = 0.51), but calculated free 25(OH)D was not. Similarly, both measured free 25(OH)D (P < .02) and total 25(OH)D (P < .05) were correlated with iPTH, but calculated free 25(OH)D was not.

CONCLUSIONS

Calculated free 25(OH)D levels varied considerably from direct measurements of free 25(OH)D with discrepancies greatest in the data for African Americans. Differences in DBP binding affinity likely contributed to estimation errors between the races. Directly measured free 25(OH)D concentrations were related to iPTH, but calculated estimates were not. Current algorithms to calculate free 25(OH)D may not be accurate. Further evaluation of directly measured free 25(OH)D levels to determine its role in research and clinical management of patients is needed.

Biochemical and morphological characterization of growth and differentiation of normal human neonatal keratinocytes in a serum-free medium

Growth and differentiation of keratinocytes in a serum-free medium (keratinocyte growth medium or KGM) was studied and compared to that under conditions in which serum and feeder cell layers were used. Cells were grown in KGM containing 0.1 mM calcium (KGM/low calcium), KGM containing 1.2 mM calcium (KGM/normal calcium), or Dulbecco's modified Eagles medium containing 5% fetal calf serum and 1.8 mM calcium in presence of mitomycin treated 3T3 M cells (DMEM/5% FCS). Plating efficiency and rate of growth were similar in the three media till confluence. In postconfluent cultures, protein and DNA content of cells attached to the plate in KGM/low-calcium dishes decreased as an increased number of cells were shed into the medium. Cell shedding was much less evident in the presence of normal calcium. Cells grown in KGM/low calcium had a higher rate of cell proliferation (3H-thymidine incorporation into cellular DNA) than cells grown in normal calcium. Transglutaminase activity, involucrin content, and cornified envelope formation were greatest in cells grown in KGM/normal calcium, intermediate in cells grown in DMEM/5% FCS, and least in cells grown in KGM/low calcium. Keratin profiles from cells grown in KGM/low calcium showed a lower percentage of high molecular weight bands compared to the keratin profiles from cells grown in the presence of normal calcium. Keratinocytes in KGM/low calcium grew as a monolayer of cuboidal cells with few features of differentiation, whereas cells grown in KGM/normal calcium stratified into multilayered islands (3-5 layers) surmounted by 2-4 layers of enucleated cells with thickened cornified envelopes. Cells grown in KGM/normal calcium also contained tonofilaments and lamellar bodies unlike cells grown in KGM/low calcium. Cells grown in DMEM/5% FCS also formed stratified layers comparable to cells grown in KGM/normal calcium but lacked cornified cells, keratohyalin granules, tonofilament bundles, and lamellar bodies. These studies indicate the usefulness of serum-free conditions for the culture of human keratinocytes and confirm the importance of extracellular calcium in keratinocyte differentiation.

Keratinocyte differentiation is stimulated by activators of the nuclear hormone receptor PPARalpha

Peroxisome proliferator activated receptors (PPAR) belong to the superfamily of nuclear hormone receptors that heterodimerize with the retinoid X receptor and regulate transcription of several genes involved in lipid metabolism and adipocyte differentiation. Because of the role of 1,25-dihydroxyvitamin D3 and retinoic acid working through similar receptors (the vitamin D receptor and retinoic acid receptor, respectively) on keratinocyte differentiation, we have examined the effects of activators of PPARalpha on keratinocyte differentiation. The rate of cornified envelope formation was increased 3-fold in keratinocytes maintained in low calcium (0.03 mM) and incubated in the presence of clofibric acid, a potent PPARalpha activator. Involucrin, a cornified envelope precursor, and the cross-linking enzyme transglutaminase, were increased at both the message level (2-7-fold) and the protein level (4-12-fold) by clofibric acid. Furthermore, physiologic doses of the fatty acids oleic acid, linoleic acid, and eicosatetraynoic acid, which are also activators of PPARalpha, also induced involucrin and transglutaminase protein and mRNA. In contrast, the PPARgammaligand prostaglandin J2 had no effect on protein or mRNA levels of involucrin or transglutaminase. Levels of involucrin and transglutaminase mRNA and protein were induced by clofibric acid in keratinocytes incubated in 1.2 mM calcium, a concentration which by itself induces keratinocyte differentiation. Finally, PPARalpha activators inhibit DNA synthesis. This study demonstrates that PPARalpha activators, including putative endogenous ligands such as fatty acids, induce differentiation and inhibit proliferation in keratinocytes, and suggests a regulatory role for the PPARalpha in epidermal homeostasis.

The skeletal structure of insulin-like growth factor I-deficient mice

The importance of insulin-like growth factor I (IGF-I) for growth is well established. However, the lack of IGF-I on the skeleton has not been examined thoroughly. Therefore, we analyzed the structural properties of bone from mice rendered IGF-I deficient by homologous recombination (knockout [k/o]) using histomorphometry, peripheral quantitative computerized tomography (pQCT), and microcomputerized tomography (muCT). The k/o mice were 24% the size of their wild-type littermates at the time of study (4 months). The k/o tibias were 28% and L1 vertebrae were 26% the size of wild-type bones. Bone formation rates (BFR) of k/o tibias were 27% that of the wild-type littermates. The k/o bones responded normally to growth hormone (GH; 1.7-fold increase) and supranormally to IGF-I (5.2-fold increase) with respect to BFR. Cortical thickness of the proximal tibia was reduced 17% in the k/o mouse. However, trabecular bone volume (bone volume/total volume [BV/TV]) was increased 23% (male mice) and 88% (female mice) in the k/o mice compared with wild-type controls as a result of increased connectivity, increased number, and decreased spacing of the trabeculae. These changes were either less or not found in L1. Thus, lack of IGF-I leads to the development of a bone structure, which, although smaller, appears more compact.

Calcium- and vitamin D-regulated keratinocyte differentiation

Calcium and 1,25 dihydroxyvitamin D (1,25(OH)(2)D) regulate the differentiation of keratinocytes. We have examined the mechanisms by which such regulation takes place, focusing primarily on the events leading to cornified envelope (CE) formation, in particular the mechanisms by which calcium and 1,25(OH)(2)D regulate the induction of involucrin, a component of the CE, and transglutaminase, the enzyme cross-linking involucrin and other substrates to form the CE. Both extracellular calcium (Ca(o)) and 1,25(OH)(2)D raise intracellular free calcium (Ca(i)) as a necessary step toward stimulating differentiation. Cells lacking the calcium sensing receptor (CaR) or phospholipase C-gamma 1 (PLC-gamma 1) fail to respond to Ca(o) or 1,25(OH)(2)D with respect to differentiation. Residing in the promoter of involucrin is a region responsive to calcium and 1,25(OH)(2)D, the calcium response element (CaRE). The CaRE contains an AP-1 site, mutations of which result in loss of responsiveness to Ca(o) and 1,25(OH)(2)D, indicating a role for protein kinases C (PKC). PKC alpha is the major PKC isozyme involved at least for calcium-induced differentiation. Thus, the regulation of keratinocyte differentiation by calcium and 1,25(OH)(2)D involves a number of signaling pathways including PLC and PKC activation, leading to the induction of proteins required for the differentiation process.

The calcium sensing receptor and its alternatively spliced form in keratinocyte differentiation

We have recently reported the presence of the calcium sensing receptor (CaR) in keratinocytes and suggested that it signaled calcium-induced differentiation of these cells. cDNA clones encoding the full-length CaR were isolated from human keratinocytes. In addition, an alternatively spliced form that lacks exon 5, encoding a portion of the extracellular domain, also was found. The in frame deletion of 231 nucleotides of exon 5 resulted in the loss of function of the CaR as measured by calcium-stimulated production of inositol phosphates when transfected into HEK293 cells or keratinocytes. This variant produced a smaller CaR protein with an altered glycosylation pattern compared with the full-length CaR. Coexpression of the spliced variant with the full-length CaR reduced the function of the full-length CaR. The full-length CaR was expressed in undifferentiated keratinocytes consistent with their greater response to elevated extracellular calcium in terms of increased intracellular free calcium and production of inositol phosphates. The full-length CaR decreased as the keratinocytes differentiated with an increase in the ratio of the spliced variant to the full-length form. The relative proportions of these two forms of CaR may regulate the calcium responsiveness of keratinocytes during their differentiation.

Stimulation of PPARalpha promotes epidermal keratinocyte differentiation in vivo

Our recent studies have demonstrated that PPARalpha activators stimulate differentiation and inhibit proliferation in cultured human keratinocytes and accelerate epidermal development and permeability barrier formation in fetal rat skin explants. As the role of PPARalpha activation in adult epidermis is not known, the aim of this study was to determine if topically applied PPARalpha ligands regulate keratinocyte differentiation in murine epidermis. Topical treatment with PPARalpha activators resulted in decreased epidermal thickness. Expression of structural proteins of the upper spinous/granular layers (involucrin, profilaggrin-filaggrin, loricrin) increased following topical treatment with PPARalpha activators. Furthermore, topically applied PPARalpha activators also increased apoptosis, decreased cell proliferation, and accelerated recovery of barrier function following acute barrier abrogation. Experiments with PPARalpha-/- knockout mice showed that these effects are specifically mediated via PPARalpha. Compared with the epidermis of PPARalpha+/+ mice, involucrin, profilaggrin-filaggrin, and loricrin expression were slightly decreased in PPARalpha-/- mice. Moreover, topical clofibrate treatment did not increase epidermal differentiation in PPARalpha-/- mice. Furthermore, in cultured human keratinocytes we have demonstrated that PPARalpha activators induce an increase in involucrin mRNA levels. We have also shown that this increase in gene expression requires an intact AP-1 response element at -2117 to -2111 bp. Thus, stimulation of PPARalpha stimulates keratinocyte/epidermal differentiation and inhibits proliferation.

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.

The extracellular calcium-sensing receptor is required for calcium-induced differentiation in human keratinocytes

In cultured keratinocytes, the acute increase of the extracellular calcium concentration above 0.03 mM leads to a rapid increase in intracellular calcium concentration ([Ca(2+)]i) and inositol trisphosphate production and, subsequently, to the expression of differentiation-related genes. Previous studies demonstrated that human keratinocytes express the full-length extracellular calcium-sensing receptor (CaR) and an alternatively spliced variant lacking exon 5 and suggested their involvement in calcium regulation of keratinocyte differentiation. To understand the role of the CaR, we transfected keratinocytes with an antisense human CaR cDNA construct and examined its impact on calcium signaling and calcium-induced differentiation. The antisense CaR cDNA significantly reduced the protein level of endogenous CaRs. These cells displayed a marked reduction in the rise in [Ca(2+)]i in response to extracellular calcium or to NPS R-467, a CaR activator, whereas the ATP-evoked rise in [Ca(2+)]i was not affected. Calcium-induced inhibition of cell proliferation and calcium-stimulated expression of the differentiation markers involucrin and transglutaminase were also blocked by the antisense CaR cDNA. When cotransfected with luciferase reporter vectors containing either the involucrin or transglutaminase promoter, the antisense CaR cDNA suppressed the calcium-stimulated promoter activities. These results indicate that CaR is required for mediating calcium signaling and calcium-induced differentiation in keratinocytes.

Free 25-hydroxyvitamin D levels are normal in subjects with liver disease and reduced total 25-hydroxyvitamin D levels

We determined the free fraction of 25-dihydroxyvitamin D (25OHD) in the serum of subjects with clinical evidence of liver disease and correlated these measurements to the levels of vitamin D binding protein and albumin. These subjects when compared to normal individuals had lower total 25OHD levels, higher percent free 25OHD levels, but equivalent free 25OHD levels. These subjects also had reduced vitamin D binding protein and albumin concentrations. The total concentration of 25OHD correlated positively with both vitamin D binding protein and albumin, whereas the percent free 25OHD correlated negatively with vitamin D binding protein and albumin. The free 25OHD levels did not correlate with either vitamin D binding protein or albumin. We conclude that total vitamin D metabolite measurements may be misleading in the evaluation of the vitamin D status of patients with liver disease, and recommend that free 25OHD levels also be determined before making a diagnosis of vitamin D deficiency.

Role of intracellular-free calcium in the cornified envelope formation of keratinocytes: differences in the mode of action of extracellular calcium and 1,25 dihydroxyvitamin D3

Extracellular calcium (Cao) and the steroid hormone 1,25(OH)2D, induce the differentiation of human epidermal cells in culture. Recent studies suggest that increases in intracellular free calcium (Cai) levels may be an initial signal that triggers keratinocyte differentiation. In the present study, we evaluated cornified envelope formation, the terminal event during keratinocyte differentiation, and correlated it with changes in the Cai levels during differentiation of keratinocytes in culture induced by Cao or 1,25(OH)2D. Keratinocytes were grown in different Cao concentrations (0.1 or 1.2 mM) or in the presence of 1,25(OH)2D (10(-11) to 10(-7) M), and the Cai levels were measured using the fluorescent probe Indo-1. Our results suggest that the induction of cornified envelope formation is associated with an increase in Cai level during calcium-induced differentiation. Cao and the calcium ionophore ionomycin acutely increased Cai and cornified envelope formation. In contrast, the effect of 1,25(OH)2D on increasing Cai levels and stimulating cornified envelope formation was long-term, requiring days of treatment with 1,25(OH)2D. Our data are consistent with other recent studies and support the hypothesis that Cao regulates keratinocyte differentiation primarily by acutely increasing their Cai levels. The role of calcium in the mechanism of action of 1,25(OH)2D on keratinocyte differentiation is less clear. The increase in Cai of keratinocytes during 1,25(OH)2D induced differentiation may be essential for or subsequent to its prodifferentiation effects.

Skeletal unloading causes resistance of osteoprogenitor cells to parathyroid hormone and to insulin-like growth factor-I

Skeletal unloading decreases bone formation and osteoblast number in vivo and decreases the number and proliferation of bone marrow osteoprogenitor (BMOp) cells in vitro. We tested the ability of parathyroid hormone (PTH) to stimulate BMOp cells in vivo by treating Sprague Dawley rats (n = 32) with intermittent PTH(1-34) (1 h/day at 8 microg/100 g of body weight), or with vehicle via osmotic minipumps during 7 days of normal weight bearing or hind limb unloading. Marrow cells were flushed from the femur and cultured at the same initial density for up to 21 days. PTH treatment of normally loaded rats caused a 2.5-fold increase in the number of BMOp cells, with similar increases in alkaline phosphatase (ALP) activity and mineralization, compared with cultures from vehicle-treated rats. PTH treatment of hind limb unloaded rats failed to stimulate BMOp cell number, ALP activity, or mineralization. Hind limb unloading had no significant effect on PTH receptor mRNA or protein levels in the tibia. Direct in vitro PTH challenge of BMOp cells isolated from normally loaded bone failed to stimulate their proliferation and inhibited their differentiation, suggesting that the in vivo anabolic effect of intermittent PTH on BMOp cells was mediated indirectly by a PTH-induced factor. We hypothesize that this factor is insulin-like growth factor-I (IGF-I), which stimulated the in vitro proliferation and differentiation of BMOp cells isolated from normally loaded bone, but not from unloaded bone. These results suggest that IGF-I mediates the ability of PTH to stimulate BMOp cell proliferation in normally loaded bone, and that BMOp cells in unloaded bone are resistant to the anabolic effect of intermittent PTH therapy due to their resistance to IGF-I.

Free, and not total, 1,25-dihydroxyvitamin D regulates 25-hydroxyvitamin D metabolism by keratinocytes

Greater than 99% of the total circulating 1,25-dihydroxyvitamin D [1,25-(OH)2D] is bound to proteins such as the vitamin D-binding protein (DBP) and albumin; in the normal human only 0.4% of the circulating 1,25-(OH)2D is free. Although it is often assumed that only the free concentration of 1,25-(OH)2D is available to cells, this has not been demonstrated. In particular, it is not clear whether the DBPs facilitate 1,25-(OH)2D entry into target cells or serve only to transport these metabolites within the circulation. To address this question, we evaluated one of the best characterized target tissue responses to 1,25-(OH)2D, namely its ability to inhibit its own production and induce that of 24,25-(OH)2D, using one of the most sensitive cells, the human foreskin keratinocyte. We incubated keratinocytes in the presence of 1,25-(OH)2D (from 10(-11)-10(-8) M) in medium containing albumin (from 0.1-10%) or serum (from 0.1-10%) for 4 h [to inhibit the 25-hydroxyvitamin D (250HD) 1 alpha-hydroxylase] or 16 h (to induce the 250HD 24-hydroxylase) before evaluating [3H]250HD metabolism by these cells during a 1-h incubation in serum- and albumin-free medium. The free fraction of 1,25-(OH)2D was determined in each medium by centrifugal ultrafiltration and varied from 36% to 0.57% in direct proportion to the albumin or serum in the medium. Increasing the serum or albumin concentration in the medium increased the concentration of total 1,25-(OH)2D needed to inhibit its own production or stimulate that of 24,25-(OH)2D. In contrast, the concentration of free 1,25-(OH)2D needed to half-maximally inhibit its own production or induce 24,25-(OH)2D production remained constant at approximately 10(-11) M. We conclude that the free, not the total, 1,25-(OH)2D concentration regulates 250HD metabolism by keratinocytes, that DBPs do not facilitate 1,25-(OH)2D entry into the cell, and that these cells sense only free 1,25-(OH)2D.