Vitamin D Receptor Controls Cell Stemness in Acute Myeloid Leukemia and in Normal Bone Marrow

Vitamin D (VD) is a known differentiating agent, but the role of VD receptor (VDR) is still incompletely described in acute myeloid leukemia (AML), whose treatment is based mostly on antimitotic chemotherapy. Here, we present an unexpected role of VDR in normal hematopoiesis and in leukemogenesis. Limited VDR expression is associated with impaired myeloid progenitor differentiation and is a new prognostic factor in AML. In mice, the lack of Vdr results in increased numbers of hematopoietic and leukemia stem cells and quiescent hematopoietic stem cells. In addition, malignant transformation of Vdr-/- cells results in myeloid differentiation block and increases self-renewal. Vdr promoter is methylated in AML as in CD34+ cells, and demethylating agents induce VDR expression. Association of VDR agonists with hypomethylating agents promotes leukemia stem cell exhaustion and decreases tumor burden in AML mouse models. Thus, Vdr functions as a regulator of stem cell homeostasis and leukemic propagation.

Phosphate-dependent luminal ATP metabolism regulates transcellular calcium transport in intestinal epithelial cells

Extracellular low phosphate strongly enhances intestinal calcium absorption independently of active vitamin D [1,25(OH)₂D₃] signaling, but the underlying mechanisms remain poorly characterized. To elucidate the phosphate-dependent regulation of calcium transport, we investigated part of the enteral environment that is involved in 1,25(OH)₂D₃-independent calcium absorption, which responds to dietary phosphate levels in mice that lack intestinal vitamin D receptor ( Vdr) activity. Impaired calcium absorption in intestinal Vdr-null mice was improved by dietary phosphate restriction. Accordingly, calcium transport in cultured intestinal epithelial cells was increased when the apical side was exposed to low phosphate levels (0.5 mM) compared with normal or high phosphate levels (1.0 or 5.0 mM, respectively). Mechanistically, low phosphate increased ATP in the apical side medium and allowed calcium entry into epithelial cells via the P2X7 purinoreceptor, which results in increased calcium transport. We found that luminal ATP was regulated by the release and degradation of ATP at the epithelium, and phosphate restriction increased ATP release from epithelial cells via connexin-43 hemichannels. Furthermore, ATP degradation by ectonucleotide pyrophosphatase-1 was reduced, which was caused by the reduction of the MAPK cascade. These findings indicate that luminal ATP metabolism regulates transcellular calcium transport in the intestine by an 1,25(OH)₂D₃-independent mechanism in response to dietary phosphate levels.-Uekawa, A., Yamanaka, H., Lieben, L., Kimira, Y., Uehara, M., Yamamoto, Y., Kato, S., Ito, K., Carmeliet, G., Masuyama, R. Phosphate-dependent luminal ATP metabolism regulates transcellular calcium transport in intestinal epithelial cells.

Transgenic Expression of the Vitamin D Receptor Restricted to the Ileum, Cecum, and Colon of Vitamin D Receptor Knockout Mice Rescues Vitamin D Receptor-Dependent Rickets

Although the intestine plays the major role in 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] action on calcium homeostasis, the mechanisms involved remain incompletely understood. The established model of 1,25(OH)2D3-regulated intestinal calcium absorption postulates a critical role for the duodenum. However, the distal intestine is where 70% to 80% of ingested calcium is absorbed. To test directly the role of 1,25(OH)2D3 and the vitamin D receptor (VDR) in the distal intestine, three independent knockout (KO)/transgenic (TG) lines expressing VDR exclusively in the ileum, cecum, and colon were generated by breeding VDR KO mice with TG mice expressing human VDR (hVDR) under the control of the 9.5-kb caudal type homeobox 2 promoter. Mice from one TG line (KO/TG3) showed low VDR expression in the distal intestine (<50% of the levels observed in KO/TG1, KO/TG2, and wild-type mice). In the KO/TG mice, hVDR was not expressed in the duodenum, jejunum, kidney, or other tissues. Growth arrest, elevated parathyroid hormone level, and hypocalcemia of the VDR KO mice were prevented in mice from KO/TG lines 1 and 2. Microcomputed tomography analysis revealed that the expression of hVDR in the distal intestine of KO/TG1 and KO/TG2 mice rescued the bone defects associated with systemic VDR deficiency, including growth plate abnormalities and altered trabecular and cortical parameters. KO/TG3 mice showed rickets, but less severely than VDR KO mice. These findings show that expression of VDR exclusively in the distal intestine can prevent abnormalities in calcium homeostasis and bone mineralization associated with systemic VDR deficiency.

Extra-intestinal calcium handling contributes to normal serum calcium levels when intestinal calcium absorption is suboptimal

The active form of vitamin D, 1,25(OH)2D, is a crucial regulator of calcium homeostasis, especially through stimulation of intestinal calcium transport. Lack of intestinal vitamin D receptor (VDR) signaling does however not result in hypocalcemia, because the increased 1,25(OH)2D levels stimulate calcium handling in extra-intestinal tissues. Systemic VDR deficiency, on the other hand, results in hypocalcemia because calcium handling is impaired not only in the intestine, but also in kidney and bone. It remains however unclear whether low intestinal VDR activity, as observed during aging, is sufficient for intestinal calcium transport and for mineral and bone homeostasis. To this end, we generated mice that expressed the Vdr exclusively in the gut, but at reduced levels. We found that ~15% of intestinal VDR expression greatly prevented the Vdr null phenotype in young-adult mice, including the severe hypocalcemia. Serum calcium levels were, however, in the low-normal range, which may be due to the suboptimal intestinal calcium absorption, renal calcium loss, insufficient increase in bone resorption and normal calcium incorporation in the bone matrix. In conclusion, our results indicate that low intestinal VDR levels improve intestinal calcium absorption compared to Vdr null mice, but also show that 1,25(OH)2D-mediated fine-tuning of renal calcium reabsorption and bone mineralization and resorption is required to maintain fully normal serum calcium levels.

Vitamin B₁₂-dependent taurine synthesis regulates growth and bone mass

Both maternal and offspring-derived factors contribute to lifelong growth and bone mass accrual, although the specific role of maternal deficiencies in the growth and bone mass of offspring is poorly understood. In the present study, we have shown that vitamin B12 (B12) deficiency in a murine genetic model results in severe postweaning growth retardation and osteoporosis, and the severity and time of onset of this phenotype in the offspring depends on the maternal genotype. Using integrated physiological and metabolomic analysis, we determined that B12 deficiency in the offspring decreases liver taurine production and associates with abrogation of a growth hormone/insulin-like growth factor 1 (GH/IGF1) axis. Taurine increased GH-dependent IGF1 synthesis in the liver, which subsequently enhanced osteoblast function, and in B12-deficient offspring, oral administration of taurine rescued their growth retardation and osteoporosis phenotypes. These results identify B12 as an essential vitamin that positively regulates postweaning growth and bone formation through taurine synthesis and suggests potential therapies to increase bone mass.

Vitamin D endocrine system and the intestine

Calcium and phosphate regulate numerous biological processes and they are essential for bone mass and bone quality. The calcium and phosphate balance largely depends on intestinal absorption, and the dietary content of these ions determines the type of transport. High dietary intake of calcium and phosphate enables absorption by passive transport, but often the dietary content of these ions is in the low-normal range, especially for calcium. In this condition, the contribution of active intestinal calcium transport will increase to maintain normal serum levels. This adaptation is mainly regulated by the active form of vitamin D, 1,25 dihydroxyvitamin D, and requires normal concentrations of the precursor 25-hydroxyvitamin D. When intestinal calcium absorption is insufficient, hormonal adaptations will release calcium from bones to secure normocalcemia, not only by increasing bone loss but also by decreasing bone mineralization. These data underline the fact that adequate calcium intake is critical to secure skeletal integrity. Despite the insights that sufficient dietary calcium intake and normal 25-hydroxyvitamin D levels are critical for calcium and bone homeostasis, surprisingly little is known on the proteins that mediate intestinal calcium transport. Also, the interaction between the intestine and the kidney to control serum phosphate levels is still incompletely understood.

Vitamin D and energy homeostasis: of mice and men

The vitamin D endocrine system has many extraskeletal targets, including adipose tissue. 1,25-Dihydroxyvitamin D₃, the active form of vitamin D, not only increases adipogenesis and the expression of typical adipocyte genes but also decreases the expression of uncoupling proteins. Mice with disrupted vitamin D action--owing to gene deletion of the nuclear receptor vitamin D receptor (Vdr) or the gene encoding 1α-hydroxylase (Cyp27b1)--lose fat mass over time owing to an increase in energy expenditure, whereas mice with increased Vdr-mediated signalling in adipose tissue become obese. The resistance to diet-induced obesity in mice with disrupted Vdr signalling is caused at least partially by increased expression of uncoupling proteins in white adipose tissue. However, the bile acid pool is also increased in these animals, and bile acids are known to be potent inducers of energy expenditure through activation of several nuclear receptors, including Vdr, and G-protein-coupled receptors, such as GPBAR1 (also known as TGR5). By contrast, in humans, obesity is strongly associated with poor vitamin D status. A causal link has not been firmly proven, but most intervention studies have failed to demonstrate a beneficial effect of vitamin D supplementation on body weight. The reasons for the major discrepancy between mouse and human data are unclear, but understanding the link between vitamin D status and energy homeostasis could potentially be very important for the human epidemic of obesity and the metabolic syndrome.

Maternal hypervitaminosis D reduces fetal bone mass and mineral acquisition and leads to neonatal lethality

Pregnancy challenges maternal calcium handling because sufficient calcium has to be transferred to the fetus to ensure fetal bone mass acquisition. 1,25(OH)2 vitamin D [1,25(OH)2D] is an important regulator of calcium homeostasis during adulthood, yet its role seems redundant for the maternal adaptations to pregnancy as well as during fetal development. However, not only deficiency but also excess of 1,25(OH)2D can be harmful and we therefore questioned whether high maternal 1,25(OH)2D levels may injure fetal development or neonatal outcome, as maternal-fetal transport of 1,25(OH)2D has been largely disputed. To this end, vitamin D receptor (VDR) null (Vdr(-/-)) females, displaying high 1,25(OH)2D levels, were mated with Vdr(+/-) males to obtain pregnancies with fetuses that are responsive (Vdr(+/-)) or resistant (Vdr(-/-)) to 1,25(OH)2D. Surprisingly, most of the Vdr(+/-) neonates died shortly after birth, whereas none of the Vdr(-/-). Mechanistically, we noticed that in Vdr(+/-) embryos, serum calcium levels were normal, but that skeletal calcium storage was reduced as evidenced by decreased mineralized bone mass as well as bone mineral content. More precisely, bone formation was decreased and the level of bone mineralization inhibitors was increased. This decreased fetal skeletal calcium storage may severely compromise calcium balance and survival at birth. In conclusion, these data indicate that high maternal 1,25(OH)2D levels are transferred across the placental barrier and adversely affect the total amount of calcium stored in fetal bones which is accompanied by neonatal death.

Characterization of the testicular, epididymal and endocrine phenotypes in the Leuven Vdr-deficient mouse model: targeting estrogen signalling

Vitamin D is a key factor for calcium and bone homeostasis, but signalling through the vitamin D receptor (VDR) seems also to be important for testicular function. To test the functional role of vitamin D signalling we examined the male reproductive system of the Leuven Vdr-ablated (Vdr(-/-)) mice, previously established as a model for hereditary vitamin D-resistant rickets. We investigated reproductive hormones, changes in gene expression and histological phenotype of eleven Vdr(-/-), eight Vdr(+/-) and nine Vdr(+/+) mice. Testicular and epididymal histology were grossly normal in Vdr(-/-) mice. Accordingly, no differences were found in serum concentrations of testosterone, estradiol, LH, and FSH or testicular expression of Cyp19a1, Ersα, Cyp17a1, Star, Insl3, Inhbb, and Amh. However, a significantly lower ERβ expression was found in testis of Vdr(+/-) and Vdr(-/-) mice, conversely epididymal expressions of ERα and the estrogen-target gene Aqp9 were higher. In conclusion, vitamin D seems dispensable for murine spermatogenesis and sex hormone production, but aberrant estrogen-signalling may elicit some of the VDR-mediated effects on male reproduction.

High bone turnover persisting after vitamin D repletion: beware of calcium deficiency

Treatment of vitamin D deficiency with vitamin D is a common procedure when taking care of elderly patients, calcium supplementation being added only when calcium dietary intake is insufficient. Here, we report the case of a 58-year-old female who was referred to our unit because of suspicion of Paget's disease of the skull, based on elevated serum alkaline phosphatase and high skull methylene diphosphonate-technetium uptake. She had been prescribed cholecalciferol (100,000 IU/month) and calcium salts for the past 7 months after discovery of severe vitamin D deficiency by her primary care physician. No specific skull bone lesions were observed on both X-ray and computerized tomography. Serum calcium, phosphate and 25(OH) vitamin D levels were normal, while serum C-terminal cross-linked telopeptide, bone alkaline phosphatase and calcitriol were high and daily urinary calcium excretion was low. We found that she had not been compliant with the calcium prescription while vitamin D had been thoroughly taken. We suspected osteomalacia due to calcium deficiency. Both skull uptake and biological abnormalities normalised in few months after adding calcium supplementation to the vitamin D treatment, and spine bone mineral density increased by 9.5 % after 14 months of full treatment. The present case illustrates the necessity for adequate calcium intake during vitamin D repletion to normalise bone mineralisation and turnover and maintain the skeletal integrity.

The delicate balance between vitamin D, calcium and bone homeostasis: lessons learned from intestinal- and osteocyte-specific VDR null mice

The serum calcium levels and the calcium content of the skeleton are highly interdependent. Indeed, bone requires calcium to preserve its strength, but it is at the same time also the predominant calcium storage from which calcium can be mobilized to supply the serum pool. The active form of vitamin D [1,25(OH)2D] plays a crucial role in regulating the transfer of calcium between blood and bone, evidenced by experimental data obtained from systemic, intestinal-specific and osteocyte-specific vitamin D receptor (Vdr) null mice. In fact, 1,25(OH)2D is required to maintain normocalcemia and bone health by enhancing intestinal calcium absorption when dietary calcium intake is normal/low. When, however, insufficient calcium is absorbed via the intestine, 1,25(OH)2D levels will increase and will act on mature osteoblasts and osteocytes to minimize calcium levels in bone tissue in favor of the blood calcium pool. Mechanistically, the high 1,25(OH)2D levels enhance bone remodeling which leads to osteopenia, and suppress bone matrix mineralization by increasing the levels of mineralization inhibitors, which causes hyperosteoidosis and hypomineralization. Thus, depending on the intestinal calcium acquisition, 1,25(OH)2D will target the intestine and/or the skeleton to maintain calcium levels in serum within a normal range.

Vitamin D action: lessons from VDR and Cyp27b1 null mice

Even one century after its discovery, there are still many gaps in the understanding of the vitamin D endocrine system. Inactivation of the vitamin D receptor (VDR) or the enzymes metabolizing its ligand (especially Cyp27bl) in mice has clearly demonstrated that the active form of vitamin D [1,25(OH)2D] is essential to stimulate calcium absorption in the gut during normal/low calcium intake, and as a consequence, that 1,25(OH)2D is required to maintain normal serum calcium, bone and growth plate homeostasis. These findings have resulted in clear clinical guidelines for the treatment of vitamin D-related bone diseases of infants, children and adults. Tissue-specific VDR or Cyp27b1 deletion in mice has also proven to be useful to define the precise role of 1,25(OH)2D action in cells belonging to the intestine, bone, growth plate and also to many non-classical target tissues. Indeed, experimental findings show that 1,25(OH)2D has numerous extraskeletal effects, and observational studies in man demonstrate that disturbances in the vitamin D pathway are associated with major human diseases such as cancer, infections, autoimmune diseases, cardiovascular and metabolic diseases, muscle function, reproduction and neurocognitive disorders. We will compare the findings in VDR- and Cyp27bi-null mice with findings in man to elucidate what is presently understood of the vitamin D endocrine system and to identify the still outstanding questions.

Vitamin D signaling in osteocytes: effects on bone and mineral homeostasis

The active form of vitamin D [1,25(OH)2D] is an important regulator of calcium and bone homeostasis, as evidenced by the consequences of 1,25(OH)2D inactivity in man and mice, which include hypocalcemia, hypophosphatemia, secondary hyperparathyroidism and bone abnormalities. The recent generation of tissue-specific (intestine, osteoblast/osteocyte, chondrocyte) vitamin D receptor (Vdr) null mice has provided mechanistic insight in the cell-specific actions of 1,25(OH)2D and their contribution to the integrative physiology of VDR signaling that controls bone and mineral metabolism. These studies have demonstrated that even with normal dietary calcium intake, 1,25(OH)2D is crucial to maintain normal calcium and bone homeostasis and accomplishes this primarily through stimulation of intestinal calcium transport. When, moreover, insufficient calcium is acquired from the diet (severe dietary calcium restriction, lack of intestinal VDR activity), 1,25(OH)2D levels will increase and will directly act on osteoblasts and osteocytes to enhance bone resorption and to suppress bone matrix mineralization. Although this system is essential to maintain normal calcium levels in blood during a negative calcium balance, the consequences for bone are disastrous and generate an increased fracture risk. These findings evidently demonstrate that preservation of serum calcium levels has priority over skeletal integrity. Since vitamin D supplementation is an essential part of anti-osteoporotic therapy, mechanistic insight in vitamin D actions is required to define the optimal therapeutic regimen, taking into account the amount of dietary calcium supply, in order to maximize the targeted outcome and to avoid side-effects. We will review the current understanding concerning the functions of osteoblastic/osteocytic VDR signaling which not only include the regulation of bone metabolism, but also comprise the control of calcium and phosphate homeostasis via fibroblast growth factor (FGF) 23 secretion and the maintenance of the hematopoeitic stem cell (HSC) niche, with special focus on the experimental data obtained from systemic and osteoblast/osteocyte-specific Vdr null mice.

Normocalcemia is maintained in mice under conditions of calcium malabsorption by vitamin D-induced inhibition of bone mineralization

Serum calcium levels are tightly controlled by an integrated hormone-controlled system that involves active vitamin D [1,25(OH)(2)D], which can elicit calcium mobilization from bone when intestinal calcium absorption is decreased. The skeletal adaptations, however, are still poorly characterized. To gain insight into these issues, we analyzed the consequences of specific vitamin D receptor (Vdr) inactivation in the intestine and in mature osteoblasts on calcium and bone homeostasis. We report here that decreased intestinal calcium absorption in intestine-specific Vdr knockout mice resulted in severely reduced skeletal calcium levels so as to ensure normal levels of calcium in the serum. Furthermore, increased 1,25(OH)(2)D levels not only stimulated bone turnover, leading to osteopenia, but also suppressed bone matrix mineralization. This resulted in extensive hyperosteoidosis, also surrounding the osteocytes, and hypomineralization of the entire bone cortex, which may have contributed to the increase in bone fractures. Mechanistically, osteoblastic VDR signaling suppressed calcium incorporation in bone by directly stimulating the transcription of genes encoding mineralization inhibitors. Ablation of skeletal Vdr signaling precluded this calcium transfer from bone to serum, leading to better preservation of bone mass and mineralization. These findings indicate that in mice, maintaining normocalcemia has priority over skeletal integrity, and that to minimize skeletal calcium storage, 1,25(OH)(2)D not only increases calcium release from bone, but also inhibits calcium incorporation in bone.

The Involvement of TRP Channels in Bone Homeostasis

Calcium and bone homeostasis are intimately related. On the one hand, bone relies on a sufficient supply of calcium to maintain its structural and mechanical properties and thus largely depends on calcium absorption in the intestine and calcium reabsorption in the kidney. On the other hand, bone serves as a calcium reserve from which calcium is mobilized to maintain normal calcium levels in blood. A negative external calcium balance will therefore at all times impair skeletal integrity. In addition to the external calcium balance, skeletal homeostasis also depends on the proper differentiation and functioning of bone cells, which relies for a large part on intracellular Ca(2+) signaling. Members of the transient receptor potential (TRP) family of ion channels affect skeletal homeostasis by mediating processes involved in the extracellular as well as intracellular Ca(2+) balance, including intestinal calcium absorption (TRPV6), renal calcium reabsorption (TRPV5), and differentiation of osteoclasts (TRPV1, TRPV2, TRPV4, TRPV5), chondrocytes (TRPV4), and possibly osteoblasts (TRPV1). In this review, we will give a brief overview of the systemic calcium homeostasis and the intracellular Ca(2+) signaling in bone cells with special focus on the TRP channels involved in these processes.

Calcemic actions of vitamin D: effects on the intestine, kidney and bone

The analysis of mice that lack systemically the actions of the active form of vitamin D, 1,25(OH)₂D, has shown that 1,25(OH)₂D is an essential regulator of calcium homeostasis and that its actions are aimed at maintaining serum calcium levels within narrow limits. Especially the stimulation of intestinal calcium transport by 1,25(OH)₂D is important for calcium and bone homeostasis. The involved transporters are however still elusive. The targeted deletion of 1,25(OH)₂D action in chondrocytes has provided compelling evidence for a paracrine control of bone development and endocrine regulation of phosphate homeostasis by 1,25(OH)₂D. Targeting vitamin D receptor (VDR) function in other tissues will further enhance our understanding of the cell-type specific action of 1,25(OH)₂D. In this review, we will discuss the current understanding and remaining questions concerning the calcemic actions of 1,25(OH)₂D in the intestine, kidney and bone, with special focus on the evidence obtained by the use of transgenic mouse models.

Trpv6 mediates intestinal calcium absorption during calcium restriction and contributes to bone homeostasis

Energy-dependent intestinal calcium absorption is important for the maintenance of calcium and bone homeostasis, especially when dietary calcium supply is restricted. The active form of vitamin D, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], is a crucial regulator of this process and increases the expression of the transient receptor potential vanilloid 6 (Trpv6) calcium channel that mediates calcium transfer across the intestinal apical membrane. Genetic inactivation of Trpv6 in mice (Trpv6(-/-)) showed, however, that TRPV6 is redundant for intestinal calcium absorption when dietary calcium content is normal/high and passive diffusion likely contributes to maintain normal serum calcium levels. On the other hand, Trpv6 inactivation impaired the increase in intestinal calcium transport following calcium restriction, however without resulting in hypocalcemia. A possible explanation is that normocalcemia is maintained at the expense of bone homeostasis, a hypothesis investigated in this study. In this study, we thoroughly analyzed the bone phenotype of Trpv6(-/-) mice receiving a normal (approximately 1%) or low (approximately 0.02%) calcium diet from weaning onwards using micro-computed tomography, histomorphometry and serum parameters. When dietary supply of calcium is normal, Trpv6 inactivation did not affect growth plate morphology, bone mass and remodeling parameters in young adult or aging mice. Restricting dietary calcium had no effect on serum calcium levels and resulted in a comparable reduction in bone mass accrual in Trpv6(+/+) and Trpv6(-/-) mice (-35% and 45% respectively). This decrease in bone mass was associated with a similar increase in bone resorption, whereas serum osteocalcin levels and the amount of unmineralized bone matrix were only significantly increased in Trpv6(-/-) mice. Taken together, our findings indicate that TRPV6 contributes to intestinal calcium transport when dietary calcium supply is limited and in this condition indirectly regulates bone formation and/or mineralization.

Deletion of the vitamin D receptor specifically in the parathyroid demonstrates a limited role for the receptor in parathyroid physiology

1,25(OH)2D3 decreases parathyroid hormone (PTH) gene transcription through the vitamin D receptor (VDR). Total body VDR−/− mice have high PTH levels, hypocalcemia, hypophosphatemia, and bone malformations. To investigate PTH regulation by the VDR specifically in the parathyroid, we generated parathyroid-specific VDR knockout mice ( PT-VDR−/−). In both strains, there was a decrease in parathyroid calcium receptor (CaR) levels. The number of proliferating parathyroid cells was increased in the VDR−/− mice but not in the PT-VDR−/− mice. Serum PTH levels were moderately but significantly increased in the PT-VDR−/− mice with normal serum calcium levels. The sensitivity of the parathyroid glands of the PT-VDR−/− mice to calcium was intact as measured by serum PTH levels after changes in serum calcium. This indicates that the reduced CaR in the PT-VDR−/− mice enables a physiologic response to serum calcium. Serum C-terminal collagen crosslinks, a marker of bone resorption, were increased in the PT-VDR−/− mice with no change in the bone formation marker, serum osteocalcin, consistent with a resorptive effect due to the increased serum PTH levels in the PT-VDR−/− mice. Therefore, deletion of the VDR specifically in the parathyroid decreases parathyroid CaR expression and only moderately increases basal PTH levels, suggesting that the VDR has a limited role in parathyroid physiology.

Bone and metabolism: a complex crosstalk

BACKGROUND

Until recently, communication from metabolism to bone was considered purely unidirectional, involving complex interactions among an adipocyte-derived factor (leptin), the sympathetic nervous system and neuropeptides. However, studies in animal models now show that bone regulates glucose metabolism and fat mass via the uncarboxylated form of an osteoblast-derived factor (osteocalcin). These findings not only demonstrate that energy metabolism regulates bone remodeling through neural relays, but also that the skeleton acts as an endocrine tissue that regulates metabolic homeostasis.

CONCLUSIONS

Further study is needed to understand the physiological role of these complex interactions in man and their implications for human diseases.

Vitamin D and human health: lessons from vitamin D receptor null mice

The vitamin D endocrine system is essential for calcium and bone homeostasis. The precise mode of action and the full spectrum of activities of the vitamin D hormone, 1,25-dihydroxyvitamin D [1,25-(OH)(2)D], can now be better evaluated by critical analysis of mice with engineered deletion of the vitamin D receptor (VDR). Absence of a functional VDR or the key activating enzyme, 25-OHD-1alpha-hydroxylase (CYP27B1), in mice creates a bone and growth plate phenotype that mimics humans with the same congenital disease or severe vitamin D deficiency. The intestine is the key target for the VDR because high calcium intake, or selective VDR rescue in the intestine, restores a normal bone and growth plate phenotype. The VDR is nearly ubiquitously expressed, and almost all cells respond to 1,25-(OH)(2)D exposure; about 3% of the mouse or human genome is regulated, directly and/or indirectly, by the vitamin D endocrine system, suggesting a more widespread function. VDR-deficient mice, but not vitamin D- or 1alpha-hydroxylase-deficient mice, and man develop total alopecia, indicating that the function of the VDR and its ligand is not fully overlapping. The immune system of VDR- or vitamin D-deficient mice is grossly normal but shows increased sensitivity to autoimmune diseases such as inflammatory bowel disease or type 1 diabetes after exposure to predisposing factors. VDR-deficient mice do not have a spontaneous increase in cancer but are more prone to oncogene- or chemocarcinogen-induced tumors. They also develop high renin hypertension, cardiac hypertrophy, and increased thrombogenicity. Vitamin D deficiency in humans is associated with increased prevalence of diseases, as predicted by the VDR null phenotype. Prospective vitamin D supplementation studies with multiple noncalcemic endpoints are needed to define the benefits of an optimal vitamin D status.

TRPV4-mediated calcium influx regulates terminal differentiation of osteoclasts

Calcium signaling controls multiple cellular functions and is regulated by the release from internal stores and entry from extracellular fluid. In bone, osteoclast differentiation is induced by RANKL (receptor activator of NF-kappaB ligand)-evoked intracellular Ca(2+) oscillations, which trigger nuclear factor-activated T cells (NFAT) c1-responsive gene transcription. However, the Ca(2+) channels involved remain largely unidentified. Here we show that genetic ablation in mice of Trpv4, a Ca(2+)-permeable channel of the transient receptor potential (TRP) family, increases bone mass by impairing bone resorption. TRPV4 mediates basolateral Ca(2+) influx specifically in large osteoclasts when Ca(2+) oscillations decline. TRPV4-mediated Ca(2+) influx hereby secures intracellular Ca(2+) concentrations, ensures NFATc1-regulated gene transcription, and regulates the terminal differentiation and activity of osteoclasts. In conclusion, our data indicate that Ca(2+) oscillations and TRPV4-mediated Ca(2+) influx are sequentially required to sustain NFATc1-dependent gene expression throughout osteoclast differentiation, and we propose TRPV4 as a therapeutic target for bone diseases.

Active intestinal calcium transport in the absence of transient receptor potential vanilloid type 6 and calbindin-D9k

To study the role of the epithelial calcium channel transient receptor potential vanilloid type 6 (TRPV6) and the calcium-binding protein calbindin-D9k in intestinal calcium absorption, TRPV6 knockout (KO), calbindin-D9k KO, and TRPV6/calbindin-D(9k) double-KO (DKO) mice were generated. TRPV6 KO, calbindin-D9k KO, and TRPV6/calbindin-D9k DKO mice have serum calcium levels similar to those of wild-type (WT) mice ( approximately 10 mg Ca2+/dl). In the TRPV6 KO and the DKO mice, however, there is a 1.8-fold increase in serum PTH levels (P < 0.05 compared with WT). Active intestinal calcium transport was measured using the everted gut sac method. Under low dietary calcium conditions there was a 4.1-, 2.9-, and 3.9-fold increase in calcium transport in the duodenum of WT, TRPV6 KO, and calbindin-D9k KO mice, respectively (n = 8-22 per group; P > 0.1, WT vs. calbindin-D9k KO, and P < 0.05, WT vs. TRPV6 KO on the low-calcium diet). Duodenal calcium transport was increased 2.1-fold in the TRPV6/calbindin-D9k DKO mice fed the low-calcium diet (P < 0.05, WT vs. DKO). Active calcium transport was not stimulated by low dietary calcium in the ileum of the WT or KO mice. 1,25-Dihydroxyvitamin D3 administration to vitamin D-deficient null mutant and WT mice also resulted in a significant increase in duodenal calcium transport (1.4- to 2.0-fold, P < 0.05 compared with vitamin D-deficient mice). This study provides evidence for the first time using null mutant mice that significant active intestinal calcium transport occurs in the absence of TRPV6 and calbindin-D9k, thus challenging the dogma that TRPV6 and calbindin-D9k are essential for vitamin D-induced active intestinal calcium transport.