Loss of 24-hydroxylated catabolism increases calcitriol and fibroblast growth factor 23 and alters calcium and phosphate metabolism in fetal mice

Calcitriol circulates at low levels in normal human and rodent fetuses, in part due to increased 24-hydroxylation of calcitriol and 25-hydroxyvitamin D by 24-hydroxylase (CYP24A1). Inactivating mutations of CYP24A1 cause high postnatal levels of calcitriol and the human condition of infantile hypercalcemia type 1, but whether the fetus is disturbed by the loss of CYP24A1 is unknown. We hypothesized that loss of Cyp24a1 in fetal mice will cause high calcitriol, hypercalcemia, and increased placental calcium transport. The Cyp24a1+/- mice were mated to create pregnancies with wildtype, Cyp24a1+/-, and Cyp24a1 null fetuses. The null fetuses were hypercalcemic, modestly hypophosphatemic (compared to Cyp24a1+/- fetuses only), with 3.5-fold increased calcitriol, 4-fold increased fibroblast growth factor 23 (FGF23), and unchanged parathyroid hormone. The quantitative RT-PCR confirmed the absence of Cyp24a1 and 2-fold increases in S100g, sodium-calcium exchanger type 1, and calcium-sensing receptor in null placentas but not in fetal kidneys; these changes predicted an increase in placental calcium transport. However, placental 45Ca and 32P transport were unchanged in null fetuses. Fetal ash weight and mineral content, placental weight, crown-rump length, and skeletal morphology did not differ among the genotypes. Serum procollagen 1 intact N-terminal propeptide and bone expression of sclerostin and Blgap were reduced while calcitonin receptor was increased in nulls. In conclusion, loss of Cyp24a1 in fetal mice causes hypercalcemia, modest hypophosphatemia, and increased FGF23, but no alteration in skeletal development. Reduced incorporation of calcium into bone may contribute to the hypercalcemia without causing a detectable decrease in the skeletal mineral content. The results predict that human fetuses bearing homozygous or compound heterozygous inactivating mutations of CYP24A1 will also be hypercalcemic in utero but with normal skeletal development.

Loss of maternal calcitriol reversibly alters early offspring growth and skeletal development in mice

Ablation of Cyp27b1 eliminates calcitriol but does not disturb fetal mineral homeostasis or skeletal development. However, independent of fetal genotypes, maternal loss of Cyp27b1 altered fetal mineral and hormonal levels compared to offspring of WT dams. We hypothesized that these maternal influences would alter postnatal skeletal development. Cyp27b1 null and WT females were mated to bear only Cyp27b1+/- offspring. 48 hrs after birth, pups were cross-fostered to dams of the same or opposite genotype that bore them. Maternal and offspring samples were collected on days 21 (weaning) and 42. Offspring measurements included minerals and hormones, bone mineral content (BMC) by DXA, ash weight and mineral content, gene expression, 3-point bending tests, and microCT. Maternal lactational behavior was evaluated. Milk was analyzed for nutritional content. At day 21, offspring fostered by nulls, independent of birth dam, had ~20% lower weight, BMC, ash weight, and ash calcium than pups fostered by WT dams. Adjustment for body weight accounted for the lower BMC but not the lower ash weight and ash calcium. Hormones and serum/urine minerals did not differ across offspring groups. Offspring fostered by nulls had shorter femurs and lower cortical thickness, mean polar moment of inertia, cortical area, trabecular bone volume, and trabecular number. Dam lactational behaviors and milk nutritional content did not differ between groups. At day 42, body weight, ash weight, lengths, BMC, and tibial bone strength were no longer different between pups fostered by null vs. WT dams. In summary, pups fostered by Cyp27b1 nulls, regardless of birth dam, have proportionately smaller skeletons at 21 days, impaired microstructure, but normal mineral homeostasis. The skeletal effects are largely recovered by day 42 (three weeks after weaning). In conclusion, maternal loss of calcitriol impairs early postnatal cortical bone growth and trabecular bone mass, but affected offspring catch up after weaning.

USP53 Regulates Bone Homeostasis by Controlling Rankl Expression in Osteoblasts and Bone Marrow Adipocytes

In the skeleton, osteoblasts and osteoclasts synchronize their activities to maintain bone homeostasis and integrity. Investigating the molecular mechanisms governing bone remodeling is critical and helps understand the underlying biology of bone disorders. Initially, we have identified the ubiquitin-specific peptidase gene (Usp53) as a target of the parathyroid hormone in osteoblasts and a regulator of mesenchymal stem cell differentiation. Mutations in USP53 have been linked to a constellation of developmental pathologies. However, the role of Usp53 in bone has never been visited. Here we show that Usp53 null mice have a low bone mass phenotype in vivo. Usp53 null mice exhibit a pronounced decrease in trabecular bone indices including trabecular bone volume (36%) and trabecular number (26%) along with an increase in trabecular separation (13%). Cortical bone parameters are also impacted, showing a reduction in cortical bone volume (12%) and cortical bone thickness (15%). As a result, the strength and mechanical bone properties of Usp53 null mice have been compromised. At the cellular level, the ablation of Usp53 perturbs bone remodeling, augments osteoblast-dependent osteoclastogenesis, and increases osteoclast numbers. Bone marrow adipose tissue volume increased significantly with age in Usp53-deficient mice. Usp53 null mice displayed increased serum receptor activator of NF-κB ligand (RANKL) levels, and Usp53-deficient osteoblasts and bone marrow adipocytes have increased expression of Rankl. Mechanistically, USP53 regulates Rankl expression by enhancing the interaction between VDR and SMAD3. This is the first report describing the function of Usp53 during skeletal development. Our results put Usp53 in display as a novel regulator of osteoblast-osteoclast coupling and open the door for investigating the involvement of USP53 in pathologies. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Vitamin D and Diseases of Mineral Homeostasis: A Cyp24a1 R396W Humanized Preclinical Model of Infantile Hypercalcemia Type 1

Infantile hypercalcemia type 1 (HCINF1), previously known as idiopathic infantile hypercalcemia, is caused by mutations in the 25-hydroxyvitamin D 24-hydroxylase gene, CYP24A1. The R396W loss-of-function mutation in CYP24A1 is the second most frequent mutated allele observed in affected HCINF1 patients. We have introduced the site-specific R396W mutation within the murine Cyp24a1 gene in knock-in mice to generate a humanized model of HCINF1. On the C57Bl6 inbred background, homozygous mutant mice exhibited high perinatal lethality with 17% survival past weaning. This was corrected by crossbreeding to the CD1 outbred background. Mutant animals had hypercalcemia in the first week of life, developed nephrolithiasis, and had a very high 25(OH)D₃ to 24,25(OH)₂D₃ ratio which is a diagnostic hallmark of the HCINF1 condition. Expression of the mutant Cyp24a1 allele was highly elevated while Cyp27b1 expression was abrogated. Impaired bone fracture healing was detected in CD1-R396^w/w mutant animals. The augmented lethality of the C57Bl6-R396W strain suggests an influence of distinct genetic backgrounds. Our data point to the utility of unique knock-in mice to probe the physiological ramifications of CYP24A1 variants in isolation from other biological and environmental factors.

NACA and LRP6 Are Part of a Common Genetic Pathway Necessary for Full Anabolic Response to Intermittent PTH

PTH induces phosphorylation of the transcriptional coregulator NACA on serine 99 through Gαs and PKA. This leads to nuclear translocation of NACA and expression of the target gene Lrp6, encoding a coreceptor of the PTH receptor (PTH1R) necessary for full anabolic response to intermittent PTH (iPTH) treatment. We hypothesized that maintaining enough functional PTH1R/LRP6 coreceptor complexes at the plasma membrane through NACA-dependent Lrp6 transcription is important to ensure maximal response to iPTH. To test this model, we generated compound heterozygous mice in which one allele each of Naca and Lrp6 is inactivated in osteoblasts and osteocytes, using a knock-in strain with a Naca⁹⁹ Ser-to-Ala mutation and an Lrp6 floxed strain (test genotype: Naca^99S/A; Lrp6^+/fl;OCN-Cre). Four-month-old females were injected with vehicle or 100 μg/kg PTH(1-34) once daily, 5 days a week for 4 weeks. Control mice showed significant increases in vertebral trabecular bone mass and biomechanical properties that were abolished in compound heterozygotes. Lrp6 expression was reduced in compound heterozygotes vs. controls. The iPTH treatment increased Alpl and Col1a1 mRNA levels in the control but not in the test group. These results confirm that NACA and LRP6 form part of a common genetic pathway that is necessary for the full anabolic effect of iPTH.

Expression and Role of Ubiquitin-Specific Peptidases in Osteoblasts

The ubiquitin-proteasome system regulates biological processes in normal and diseased states. Recent investigations have focused on ubiquitin-dependent modifications and their impacts on cellular function, commitment, and differentiation. Ubiquitination is reversed by deubiquitinases, including ubiquitin-specific peptidases (USPs), whose roles have been widely investigated. In this review, we explore recent findings highlighting the regulatory functions of USPs in osteoblasts and providing insight into the molecular mechanisms governing their actions during bone formation. We also give a brief overview of our work on USP53, a target of PTH in osteoblasts and a regulator of mesenchymal cell lineage fate decisions. Emerging evidence addresses questions pertaining to the complex layers of regulation exerted by USPs on osteoblast signaling. We provide a short overview of our and others' understanding of how USPs modulate osteoblastogenesis. However, further studies using knockout mouse models are needed to fully understand the mechanisms underpinning USPs actions.

Vitamin D Regulates CXCL12/CXCR4 and Epithelial-to-Mesenchymal Transition in a Model of Breast Cancer Metastasis to Lung

Vitamin D deficiency is associated with poor cancer outcome in humans, and administration of vitamin D or its analogs decreases tumor progression and metastasis in animal models. Using the mouse mammary tumor virus-polyoma middle T antigen (MMTV-PyMT) model of mammary cancer, we previously demonstrated a significant acceleration of carcinogenesis in animals on a low vitamin D diet and a reduction in spontaneous lung metastases when mice received vitamin D through perfusion. We investigate here the action mechanism for vitamin D in lung metastasis in the same non-immunodeficient model and demonstrate that it involves the control of epithelial to mesenchymal transition as well as interactions between chemokine C-X-C motif chemokine 12 (CXCL12) and its receptor C-X-C chemokine receptor type 4 (CXCR4). In vitro, 10-9M vitamin D treatment modifies the phenotype of MMTV-PyMT primary mammary tumor cells and significantly decreases their invasiveness and mammosphere formation capacity by 40% and 50%, respectively. Vitamin D treatment also inhibits phospho-signal transducer and activator of transcription 3 (p-STAT3), zinc finger E-box-binding homeobox 1 (Zeb1), and vimentin by 52%, 75%, and 77%, respectively, and increases E-cadherin by 87%. In vivo, dietary vitamin D deficiency maintains high levels of Zeb1 and p-STAT3 in cells from primary mammary tumors and increases CXCL12 expression in lung stroma by 64%. In lung metastases, vitamin D deficiency increases CXCL12/CXCR4 co-localization by a factor of 2.5. These findings indicate an involvement of vitamin D in mammary cancer "seed" (primary tumor cell) and "soil" (metastatic site) and link vitamin D deficiency to epithelial-to-mesenchymal transition (EMT), CXCL12/CXCR4 signaling, and accelerated metastasis, suggesting vitamin D repleteness in breast cancer patients could enhance the efficacy of co-administered therapies in preventing spread to distant organs.

The ER protein TLC domain 3B2 and its enzymatic product lactosylceramide enhance chondrocyte maturation

Purpose/Aim of study: We previously cloned Tlcd3b2 (Tram-Lag1-CLN8 domain 3B2, formerly Fam57b2) from bone fracture repair callus tissue of Cyp24a1 knockout mice and showed that it synthesizes lactosylceramide (LacCer) under allosteric control of the vitamin D metabolite, 24,25-dihydroxyvitamin D3 [24,25(OH)2D3]. Tlcd3b2 was mainly detected in chondrocytes and the 24,25(OH)2D3-TLCD3B2-LacCer signaling cascade was shown to be important for optimal bone fracture repair, suggesting a role for TLCD3B2 in chondrocyte differentiation or maturation. We report the subcellular localization of TLCD3B2 and its effect on chondrocyte differentiation. Materials and Methods: Immunofluorescence detection of epitope-tagged mutants was used to assess localization. ATDC5 chondrogenic cells were transfected with Tlcd3b2 expression vectors to examine effects on chondrocyte differentiation. Results and Conclusions: TLCD3B2 localized to the endoplasmic reticulum, with both the N- and C-termini facing the cytosolic compartment. Chondrogenic ATDC5 cells stably overexpressing Tlcd3b2 showed elevated type 2 (Col2a1) and type 10 (Col10a1) collagen gene expression and increased proteoglycan synthesis, and the effect on Col2a1 was enhanced by treatment with 24,25(OH)2D3. LacCer treatment of ATDC5 cells potentiated Col10a1 expression. Our results show that TLCD3B2 is an ER protein and implicate its expression and enzymatic product in chondrocyte maturation.

Nfil3, a target of the NACA transcriptional coregulator, affects osteoblast and osteocyte gene expression differentially

Intermittent administration of PTH(1-34) has a profound osteoanabolic effect on the skeleton. At the cellular level, osteoblasts and osteocytes are two crucial cell types that respond to PTH stimulation in bone. The transcriptional cofactor Nascent polypeptide Associated Complex and coregulator alpha (NACA) is a downstream target of the PTH-Gαs-PKA axis in osteoblasts. NACA functions as a transcriptional cofactor affecting bZIP factor-mediated transcription of target promoters in osteoblasts, such as Osteocalcin (Bglap2). Here, we used RNA-Seq and ChIP-Seq against NACA in PTH-treated MC3T3-E1 osteoblastic cells to identify novel targets of the PTH-activated NACA. Our approach identified Nuclear factor interleukin-3-regulated (Nfil3) as a target promoter of this pathway. Knockdown of Naca reduced the response of Nfil3 to PTH(1-34) stimulation. In silico analysis of the Nfil3 promoter revealed potential binding sites for NACA (located within the ChIP fragment) and CREB. We show that following PTH stimulation, phosphorylated-CREB binds the proximal promoter of Nfil3 in osteoblasts. The activity of the Nfil3 promoter (-818/+182 bp) is regulated by CREB and this activation relies on the presence of NACA. In addition, we show that knockdown of Nfil3 enhances the expression of osteoblastic differentiation markers in MC3T3-E1 cells while it represses osteocytic marker gene expression in IDG-SW3 cells. These results show that the PTH-induced NACA axis regulates Nfil3 expression and suggest that NFIL3 acts as a transcriptional repressor in osteoblasts while it exhibits differential activity as an activator in osteocytes.

Inhibition of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) stimulates osteoblastogenesis by potentiating bone morphogenetic protein 2 (BMP2) responses

The catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is a pleiotropic enzyme involved in DNA repair, cell cycle control, and transcription regulation. A potential role for DNA-PKcs in the regulation of osteoblastogenesis remains to be established. We show that pharmacological inhibition of DNA-PKcs kinase activity or gene silencing of Prkdc (encoding DNA-PKcs) in murine osteoblastic MC3T3-E1 cells and human adipose-derived mesenchymal stromal cells markedly enhanced osteogenesis and the expression of osteoblast differentiation marker genes. Inhibition of DNA-PKcs inhibited cell cycle progression and increased osteogenesis by significantly enhancing the bone morphogenetic protein 2 response in osteoblasts and other mesenchymal cell types. Importantly, in vivo pharmacological inhibition of the kinase enhanced bone biomechanical properties. Bones from osteoblast-specific conditional Prkdc-knockout mice exhibited a similar phenotype of increased stiffness. In conclusion, DNA-PKcs negatively regulates osteoblast differentiation, and therefore DNA-PKcs inhibitors may have therapeutic potential for bone regeneration and metabolic bone diseases.

Preclinical safety and efficacy of 24R,25-dihydroxyvitamin D3 or lactosylceramide treatment to enhance fracture repair

Background/Objective

Cyp24a1-null mice deficient in 24,25(OH)₂D₃ display impaired callus formation during the endochondral phase of bone fracture repair. The 24,25(OH)₂D₃ metabolite acted by binding to the TLC domain containing 3B isoform 2 (TLCD3B2, previously named FAM57B2) effector protein, which then synthesizes lactosylceramide (LacCer). Treatment with 24,25(OH)₂D₃ or LacCer restored callus size and mechanical properties in Cyp24a1-null mice.

Methods

To assess the safety of these molecules and test their efficacy for bone healing in wild-type, non-genetically modified mice, we treated 12-week-old, osteotomized C57BL/6 female mice with each compound for up to 21 days post-osteotomy. Control cohorts were injected with vehicle.

Results

Neither compound was found to exhibit any nephro- nor hepato-toxicity. Calcemia remained stable throughout the experiment and was unaffected by either treatment. Supplementation with 24,25(OH)₂D₃ increased circulating levels of this metabolite about 8-fold, decreased 1,25(OH)₂D₃ levels, and significantly increased circulating 1,24,25(OH)₃D₃ levels, suggesting 1?-hydroxylation of 24,25(OH)₂D₃. TLCD3B2 was found to be expressed in fracture callus at the surface of unmineralized or pre-mineralized cartilage on day 10 and day 12 post-osteotomy and to progressively recede to become undetectable by day 18. Treatment with 24,25(OH)₂D₃ or LacCer reduced the number of TLCD3B2-positive cells. Both treatments also significantly increased stiffness and elastic modulus of the healing bone callus.

Conclusion

Exogenous administration of 24,25(OH)₂D₃ or LacCer improved the biomechanical properties of repaired bones in wild-type animals without affecting circulating calcium levels or other blood parameters, demonstrating preclinical safety and efficacy.

Translational potential

Our data suggest the use of 24R,25-dihydroxyvitamin D₃ or lactosylceramide for ameliorating fracture healing in clinical practice.

Dephosphorylation of the transcriptional cofactor NACA by the PP1A phosphatase enhances cJUN transcriptional activity and osteoblast differentiation

The transcriptional cofactor nascent polypeptide-associated complex and co-regulator α (NACA) regulates osteoblast maturation and activity. NACA functions, at least in part, by binding to Jun proto-oncogene, AP-1 transcription factor subunit (cJUN) and potentiating the transactivation of AP-1 targets such as osteocalcin (Bglap) and matrix metallopeptidase 9 (Mmp9). NACA activity is modulated by phosphorylation carried out by several kinases, but a phosphatase regulating NACA's activity remains to be identified. Here, we used affinity purification with MS in HEK293T cells to isolate NACA complexes and identified protein phosphatase 1 catalytic subunit α (PP1A) as a NACA-associated Ser/Thr phosphatase. NACA interacted with multiple components of the PP1A holoenzyme complex: the PPP1CA catalytic subunit and the regulatory subunits PPP1R9B, PPP1R12A and PPP1R18. MS analysis revealed that NACA co-expression with PPP1CA causes dephosphorylation of NACA at Thr-89, Ser-151, and Thr-174. NACA Ser/Thr-to-alanine variants displayed increased nuclear localization, and NACA dephosphorylation was associated with specific recruitment of novel NACA interactants, such as basic transcription factor 3 (BTF3) and its homolog BTF3L4. NACA and PP1A cooperatively potentiated cJUN transcriptional activity of the AP-1-responsive MMP9-luciferase reporter, which was abolished when Thr-89, Ser-151, or Thr-174 were substituted with phosphomimetic aspartate residues. We confirmed the NACA-PP1A interaction in MC3T3-E1 osteoblastic cells and observed that NACA phosphorylation status at PP1A-sensitive sites is important for the regulation of AP-1 pathway genes and for osteogenic differentiation and matrix mineralization. These results suggest that PP1A dephosphorylates NACA at specific residues, impacting cJUN transcriptional activity and osteoblast differentiation and function.

Mineral Homeostasis in Murine Fetuses Is Sensitive to Maternal Calcitriol but Not to Absence of Fetal Calcitriol

Vitamin D receptor (VDR) null fetuses have normal serum minerals, parathyroid hormone (PTH), skeletal morphology, and mineralization but increased serum calcitriol, placental calcium transport, and placental expression of Pthrp, Trpv6, and (as reported in this study) Pdia3. We examined Cyp27b1 null fetal mice, which do not make calcitriol, to determine if loss of calcitriol has the same consequences as loss of VDR. Cyp27b1 null and wild-type (WT) females were mated to Cyp27b1^+/- males, which generated Cyp27b1 null and Cyp27b1^+/- fetuses from Cyp27b1 null mothers, and Cyp27b1^+/- and WT fetuses from WT mothers. Cyp27b1 null fetuses had undetectable calcitriol but normal serum calcium and phosphorus, PTH, fibroblast growth factor 23 (FGF23), skeletal mineral content, tibial lengths and morphology, placental calcium transport, and expression of Trpv6 and Pthrp; conversely, placental Pdia3 was downregulated. However, although Cyp27b1^+/- and null fetuses of Cyp27b1 null mothers were indistinguishable, they had higher serum and amniotic fluid calcium, lower amniotic fluid phosphorus, lower FGF23, and higher 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D than in WT and Cyp27b1^+/- fetuses of WT mothers. In summary, loss of fetal calcitriol did not alter mineral or bone homeostasis, but Cyp27b1 null mothers altered mineral homeostasis in their fetuses independent of fetal genotype. Cyp27b1 null fetuses differ from Vdr null fetuses, possibly through high levels of calcitriol acting on Pdia3 in Vdr nulls to upregulate placental calcium transport and expression of Trpv6 and Pthrp. In conclusion, maternal calcitriol influences fetal mineral metabolism, whereas loss of fetal calcitriol does not. © 2018 American Society for Bone and Mineral Research.

Calcioic acid: In vivo detection and quantification of the terminal C24-oxidation product of 25-hydroxyvitamin D3 and related intermediates in serum of mice treated with 24,25-dihydroxyvitamin D3

Calcitroic acid, the excretory form of vitamin D, is the terminal product of a 5-step pathway catalyzed by CYP24A1, commencing with C24-hydroxylation of 1,25-dihydroxyvitamin D₃ (1,25-(OH)₂D₃). Catabolism of 25-hydroxyvitamin D₃ (25-OH-D₃) proceeds via analogous steps culminating in calcioic acid; however this C23-truncated acid has not been reported in the circulation. It has recently been shown that 24,25-dihydroxyvitamin D₃ (24,25-(OH)₂D₃) is an important factor in optimal bone fracture healing acting via an effector molecule FAM57B2 to produce lactosylceramide. Administration of 24,25-(OH)₂D₃ was found to restore normal fracture repair in Cyp24a1^-/- mice devoid of 24,25-(OH)₂D₃. We set out to study the multi-step catabolism of D₃ metabolites in vivo using LC-MS/MS methods in vehicle or 24,25-(OH)₂D₃-treated mice. Vehicle-treated Cyp24a1^+/- mice possessed normal levels of serum 24,25-(OH)₂D₃ (7 ng/mL) and 25-OH-D₃-26,23-lactone (4 ng/mL). We also detected 24-oxo-25-OH-D₃ (3 ng/mL) and 24-oxo-23,25-(OH)₂D₃ (0.4 ng/mL); which were not detectable in vehicle-treated Cyp24a1^-/- mice. In 24,25-(OH)₂D₃-treated Cyp24a1^+/- mice, serum 24,25-(OH)₂D₃ rose to 200 ng/mL while 25-OH-D₃-26,23-lactone remained unchanged in comparison to vehicle-treated Cyp24a1^+/- mice Concentration of serum 24-oxo-25-OH-D₃ and 24-oxo-23,25-(OH)₂D₃ rose by 10-fold, when Cyp24a1^+/- mice were treated with 24,25-(OH)₂D₃ Calcioic acid was increased to 0.030 ng/mL for 24,25-(OH)₂D₃-treated Cyp24a1^+/- mice. In 24,25-(OH)₂D₃-treated Cyp24a1^-/- mice, serum 24,25-(OH)₂D₃ rose further to a striking 830 ng/mL due to lack of catabolism of the 24,25-(OH)₂D₃ dose. Serum 1,25-(OH)₂D₃ levels were suppressed in 24,25-(OH)₂D₃-treated Cyp24a1^+/- and Cyp24a1^-/- mice. Circulating 1,24,25-(OH)₃D₃ rose from 73 pg/mL to 106 pg/mL when Cyp24a1^+/- mice were treated with 24,25-(OH)₂D₃. While undetectable in vehicle-treated Cyp24a1^-/- mice, 1,24,25-(OH)₃D₃ rose unexpectedly to 153 pg/mL in 24,25-(OH)₂D₃-treated nulls suggesting conversion of 24,25-(OH)₂D₃ to 1,24,25-(OH)₃D₃ via 1-hydroxylation. Taken together, amplification of 24,25-(OH)₂D₃ catabolism by exogenous doses of this metabolite have enabled detection of downstream C24-oxidation pathway products in vivo, including calcioic acid; and provides a platform for studying alternative routes of vitamin D metabolism that may occur in pathological states including hypervitaminosis D and idiopathic infantile hypercalcemia caused by mutations of CYP24A1.

An upstream activator sequence regulates the murine Pgk-1 promoter and binds multiple nuclear proteins

The murine Pgk-1 gene is driven by a strong promoter that is regulated by a 304 bp upstream activator sequence (UAS). The activity of the UAS is high in undifferentiated embryonal carcinoma cells but declines when these cells are induced to differentiate with retinoic acid. The effect of the UAS on promoter activity is particularly striking when the activity of the Pgk-1 promoter is assayed following its integration into the genome, suggesting that it may function by regulating chromatin structure in the region of the core promoter. Three sites on the UAS bind nuclear proteins. Two of these sites bind factors present in both embryonal carcinoma cells and their differentiated derivatives whereas one site binds factors present only in differentiated cells. There appears to be both cooperation and antagonism in the binding of proteins to different sites in the UAS, suggesting that the activity of the Pgk-1 promoter is determined by the constellation of proteins assembled upstream of its transcription start site.

Optimal bone fracture repair requires 24R,25-dihydroxyvitamin D3 and its effector molecule FAM57B2

The biological activity of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] remains controversial, but it has been suggested that it contributes to fracture healing. Cyp24a1-/- mice, synthesizing no 24R,25(OH)2D3, show suboptimal endochondral ossification during fracture repair, with smaller callus and reduced stiffness. These defects were corrected by 24R,25(OH)2D3 treatment, but not by 1,25-dihydroxyvitamin D3. Microarrays with Cyp24a1-/- callus mRNA identified FAM57B2 as a mediator of the 24R,25(OH)2D3 effect. FAM57B2 produced lactosylceramide (LacCer) upon specific binding of 24R,25(OH)2D3. Fam57b inactivation in chondrocytes (Col2-Cre Fam57bfl/fl) phenocopied the callus formation defect of Cyp24a1-/- mice. LacCer or 24R,25(OH)2D3 injections restored callus volume, stiffness, and mineralized cartilage area in Cyp24a1-null mice, but only LacCer rescued Col2-Cre Fam57bfl/fl mice. Gene expression in callus tissue suggested that the 24R,25(OH)2D3/FAM57B2 cascade affects cartilage maturation. We describe a previously unrecognized pathway influencing endochondral ossification during bone repair through LacCer production upon binding of 24R,25(OH)2D3 to FAM57B2. Our results identify potential new approaches to ameliorate fracture healing.

Absence of vitamin D receptor in mature osteoclasts results in altered osteoclastic activity and bone loss

Mature osteoclasts express the vitamin D receptor (VDR) and are able to synthesise and respond to 1,25(OH)₂D₃ via CYP27B1 enzyme activity. Whether vitamin D signalling within osteoclasts is necessary for the regulation of osteoclastic bone resorption in an in vivo setting is unclear. To determine the requirement for the VDR- and CYP27B1-mediated activity in mature osteoclasts, conditional deletion mouse models were created whereby either Vdr or Cyp27b1 gene was inactivated by breeding either Vdr^fl/fl or Cyp27b1^fl/fl mice with Cathepsin K-Cre transgenic mice (Cstk^Cre) to generate Ctsk^Cre/Vdr^-/- and Ctsk^Cre/Cyp27b1^-/- mice respectively. To account for potential Ctsk^Cre-meaited off-target deletion of Vdr, Dmp1^Cre were also used determine the effect of Vdr deletion in osteocytes. Furthermore, Ctsk^Cre/Vdr^-/- mice were ovariectomised (OVX) to assess the role of VDR in osteoclasts under bone-loss conditions and bone marrow precursor cells were cultured under osteoclastogenic conditions to assess osteoclast formation. Six-week-old Ctsk^Cre/Vdr^-/- female mice demonstrated a 15% decrease in femoral BV/TV (p<0.05). In contrast, BV/TV remained unchanged in Ctsk^Cre/Cyp27b1^-/- mice as well as in Dmp1^Cre/VDR^-/- mice. When Ctsk^Cre/Vdr^-/- mice were subjected to OVX, the bone loss that occurred in Ctsk^Cre/Vdr^-/- was predominantly due to a diminished volume of thinner trabeculae when compared to control levels. These changes in bone volume in Ctsk^Cre/Vdr^-/- mice occurred without an observable histological change in osteoclast numbers or size. However, while cultured bone marrow-derived osteoclasts from Ctsk^Cre/Vdr^-/- mice were marginally increased when compared to VDR^fl/fl mice, elevated expression of genes such as Cathepsin K, Nfatc1 and VATPase was observed. Collectively, these data indicate that the absence of VDR in mature osteoclasts causes exacerbated bone loss in young mice and during OVX which is associated with enhanced osteoclastic activity and without increased osteoclastogenesis.

Lrp6 is a target of the PTH-activated αNAC transcriptional coregulator

In the nucleus of differentiated osteoblasts, the alpha chain of nascent polypeptide associated complex (αNAC) interacts with cJUN transcription factors to regulate the expression of target genes, including Osteocalcin (Bglap2). PTH induces the phosphorylation of αNAC on serine 99 through a Gαs-PKA dependent pathway. This leads to activation of αNAC and expression of Bglap2. To identify additional target genes regulated by PTH-activated αNAC, we performed ChIP-Seq against αNAC in PTH-treated MC3T3-E1 cells. This identified Low density lipoprotein receptor-Related Protein 6 (Lrp6) as a potential αNAC target. LRP6 acts as a co-receptor for the PTH receptor to allow optimal activation of PTH signaling. PTH increased Lrp6 mRNA levels in primary osteoblasts. Conventional quantitative ChIP confirmed the ChIP-Seq results. To assess whether αNAC plays a critical role in PTH-stimulated Lrp6 expression, we knocked-down Naca expression in MC3T3-E1 cells. Reduction of αNAC levels decreased basal expression of Lrp6 by 30% and blocked the stimulation of Lrp6 expression by PTH. We cloned the proximal mouse Lrp6 promoter (-2523/+120 bp) upstream of the luciferase reporter. Deletion and point mutations analysis in electrophoretic mobility shift assays and transient transfections identified a functional αNAC binding site centered around -343 bp. ChIP and ChIP-reChIP against JUND and αNAC showed that they cohabit on the proximal Lrp6 promoter. Luciferase assays confirmed that PTH-activated αNAC potentiated JUND-mediated Lrp6 transcription and Jund knockdown abolished this response. This study identified a novel αNAC target gene induced downstream of PTH signaling and represents the first characterization of the regulation of Lrp6 transcription in osteoblasts.

Absence of Calcitriol Causes Increased Lactational Bone Loss and Lower Milk Calcium but Does Not Impair Post-lactation Bone Recovery in Cyp27b1 Null Mice

We hypothesized that adaptation to calcium supply demands of pregnancy and lactation do not require calcitriol. Adult Cyp27b1 null mice lack calcitriol and have hypocalcemia, hypophosphatemia, and rickets. We studied wild-type (WT) and null sister pairs raised on a calcium-, phosphorus-, and lactose-enriched "rescue" diet that prevents hypocalcemia and rickets. Bone mineral content (BMC) increased >30% in pregnant nulls, declined 30% during lactation, and increased 30% by 4 weeks post-weaning. WT showed less marked changes. Micro-CT revealed loss of trabecular bone and recovery in both genotypes. In lactating nulls, femoral cortical thickness declined >30%, whereas endocortical perimeter increased; both recovered to baseline after weaning; there were no such changes in WT. Histomorphometry revealed a profound increase in osteoid surface and thickness in lactating nulls, which recovered after weaning. By three-point bend test, nulls had a >50% decline in ultimate load to failure that recovered after weaning. Although nulls showed bone loss during lactation, their milk calcium content was 30% lower compared with WT. Serum parathyroid hormone (PTH) was markedly elevated in nulls at baseline, reduced substantially in pregnancy, but increased again during lactation and remained high post-weaning. In summary, pregnant Cyp27b1 nulls gained BMC with reduced secondary hyperparathyroidism, implying increased intestinal calcium delivery. Lactating nulls lost more bone mass and strength than WT, accompanied by increased osteoid, reduced milk calcium, and worsened secondary hyperparathyroidism. This implies suboptimal intestinal calcium absorption. Post-weaning, bone mass and strength recovered to baseline, whereas BMC exceeded baseline by 40%. In conclusion, calcitriol-independent mechanisms regulate intestinal calcium absorption and trabecular bone metabolism during pregnancy and post-weaning but not during lactation; calcitriol may protect cortical bone during lactation. © 2017 American Society for Bone and Mineral Research.

Endogenous Calcitriol Synthesis Controls the Humoral IgE Response in Mice

The vitamin D receptor participates in the control of IgE class-switch recombination in B cells. The physiologic vitamin D receptor agonist, 1,25(OH)₂D₃ (calcitriol), is synthesized by the essential enzyme 25-hydroxyvitamin D₃-1α-hydroxylase (CYP27B1), which can be expressed by activated immune cells. The role of endogenous calcitriol synthesis for the regulation of IgE has not been proven. In this study, we investigated IgE-responses in Cyp27b1-knockout (KO) mice following sensitization to OVA or intestinal infection with Heligmosomoides polygyrus Specific Igs and plasmablasts were determined by ELISA and ELISpot, Cyp27b1 expression was measured by quantitative PCR. The data show elevated specific IgE and IgG1 concentrations in the blood of OVA-sensitized Cyp27b1-KO mice compared with wild-type littermates (+898 and +219%). Accordingly, more OVA-specific IgG1-secreting cells are present in spleen and fewer in the bone marrow of Cyp27b1-KO mice. Ag-specific mechanisms are suggested as the leucopoiesis is in general unchanged and activated murine B and T lymphocytes express Cyp27b1 Accordingly, elevated specific IgE concentrations in the blood of sensitized T cell-specific Cyp27b1-KO mice support a lymphocyte-driven mechanism. In an independent IgE-inducing model, i.e., intestinal infection with H. polygyrus, we validated the increase of total and specific IgE concentrations of Cyp27b1-KO compared with wild-type mice, but not those of IgG1 or IgA. We conclude that endogenous calcitriol has an impact on the regulation of IgE in vivo. Our data provide genetic evidence supporting previous preclinical and clinical findings and suggest that vitamin D deficiency not only promotes bone diseases but also type I sensitization.

CYP24A1-deficiency does not affect bone regeneration in distraction osteogenesis

The putative biological activity of 24,25-(OH)₂D remains unclear. Previous studies showed an increase in the circulating levels of this metabolite following fracture in chicks. Our laboratory has generated a mouse model deficient for the Cyp24a1 gene for studying the role of 24,25-(OH)₂D. We set out to study the role of CYP24A1 and 24,25-(OH)₂D in intramembranous bone formation during distraction osteogenesis in wild-type and Cyp24a1-deficient mice. Distraction osteogenesis was applied to mouse tibiae using a miniature external fixator apparatus. Histomorphometric parameters and gene expression differences between the mutant mice and the wild-type controls were measured using micro computed tomography and reverse-transcription quantitative PCR.There were no statistically significant differences between genotypes when bone volume/tissue volume ratios were calculated at mid distraction, end of distraction, mid consolidation, or end of consolidation. We measured reduced expression of the Col10a1 gene in the mutant vs. wild-type mice at mid distraction (0.4±0.1 vs. 1.0±0.2 respectively, p=0.01). Similarly, we measured a significantly lower expression of the osteogenic marker Atf4 in mutant vs. wild-type mice at end of distraction (0.7±0.1 vs. 1.0±0.1 respectively, p=0.01) and at mid consolidation (0.6±0.1 vs. 1.0±0.1 respectively, p=0.0003). These results suggest moderate and restricted differences in chondrogenesis and osteogenesis that did not affect the volume of bone produced following distraction. We conclude that CYP24A1 activity is not essential for intramembranous bone formation.

Integrin-Linked Kinase Regulates Bone Formation by Controlling Cytoskeletal Organization and Modulating BMP and Wnt Signaling in Osteoprogenitors

Cell-matrix interactions constitute a fundamental aspect of skeletal cell biology and play essential roles in bone homeostasis. These interactions are primarily mediated by transmembrane integrin receptors, which mediate cell adhesion and transduce signals from the extracellular matrix to intracellular responses via various downstream effectors, including integrin-linked kinase (ILK). ILK functions as adaptor protein at focal adhesion sites, linking integrins to the actin cytoskeleton, and has been reported to act as a kinase phosphorylating signaling molecules such as GSK-3β and Akt. Thereby, ILK plays important roles in cellular attachment, motility, proliferation and survival. To assess the in vivo role of ILK signaling in osteoprogenitors and the osteoblast lineage cells descending thereof, we generated conditional knockout mice using the Osx-Cre:GFP driver strain. Mice lacking functional ILK in osterix-expressing cells and their derivatives showed no apparent developmental or growth phenotype, but by 5 weeks of age they displayed a significantly reduced trabecular bone mass, which persisted into adulthood in male mice. Histomorphometry and serum analysis indicated no alterations in osteoclast formation and activity, but provided evidence that osteoblast function was impaired, resulting in reduced bone mineralization and increased accumulation of unmineralized osteoid. In vitro analyses further substantiated that absence of ILK in osteogenic cells was associated with compromised collagen matrix production and mineralization. Mechanistically, we found evidence for both impaired cytoskeletal functioning and reduced signal transduction in osteoblasts lacking ILK. Indeed, loss of ILK in primary osteogenic cells impaired F-actin organization, cellular adhesion, spreading, and migration, indicative of defective coupling of cell-matrix interactions to the cytoskeleton. In addition, BMP/Smad and Wnt/β-catenin signaling was reduced in the absence of ILK. Taken together, these data demonstrate the importance of integrin-mediated cell-matrix interactions and ILK signaling in osteoprogenitors in the control of osteoblast functioning during juvenile bone mass acquisition and adult bone remodeling and homeostasis. © 2017 American Society for Bone and Mineral Research.

Cyp24a1 Attenuation Limits Progression of BrafV600E -Induced Papillary Thyroid Cancer Cells and Sensitizes Them to BRAFV600E Inhibitor PLX4720

CYP24A1, the primary inactivating enzyme for vitamin D, is often overexpressed in human cancers, potentially neutralizing the antitumor effects of calcitriol, the active form of vitamin D. However, it is unclear whether CYP24A1 expression serves as a functional contributor versus only a biomarker for tumor progression. In this study, we investigated the role of CYP24A1 on malignant progression of a murine model of Braf^V600E -induced papillary thyroid cancer (PTC). Mice harboring wild-type Cyp24a1 (BVE^Cyp24a1-wt) developed PTC at 5 weeks of age. Mice harboring a homozygous deletion of Cyp24a1 (BVE^Cyp24a1-null) exhibited a 4-fold reduction in tumor growth. Notably, we found the tumorigenic potential of BVE^Cyp24a1-null-derived tumor cells to be nearly abolished in immunocompromised nude mice. This phenotype was associated with downregulation of the MAPK, PI3K/Akt, and TGFβ signaling pathways and a loss of epithelial-mesenchymal transition (EMT) in BVE^Cyp24a1-null cells, associated with downregulation of genes involved in EMT, tumor invasion, and metastasis. While calcitriol treatment did not decrease cell proliferation in BVE^Cyp24a1-null cells, it strengthened antitumor responses to the BRAF^V600E inhibitor PLX4720 in both BVE^Cyp24a1-null and BVE^Cyp24a1-wt cells. Our findings offer direct evidence that Cyp24a1 functions as an oncogene in PTC, where its overexpression activates multiple signaling cascades to promote malignant progression and resistance to PLX4720 treatment. Cancer Res; 77(8); 2161-72. ©2017 AACR.

FIAT Deletion Increases Bone Mass But Does Not Prevent High-Fat-Diet-Induced Metabolic Complications

Factor inhibiting activating transcription factor 4 (ATF4)-mediated transcription (FIAT) interacts with ATF4 to repress its transcriptional activity. We performed a phenotypic analysis of Fiat-deficient male mice (Fiat-/Y) at 8 and 16 weeks of age. Microcomputed tomography analysis of the distal femur demonstrated 46% and 13% age-dependent increases in trabecular bone volume and thickness, respectively, in Fiat-/Y mice. Cortical bone measurements at the femoral midshaft revealed a substantial increase in cortical thickness in older Fiat-/Y mice. Bone gain was related to increased mineral apposition rate and increased osteoblast function. Femoral stiffness and strength were substantially increased in Fiat-/Y compared with wild-type (WT) mice. We also investigated whether FIAT contributes to metabolic function. When fed standard mouse chow, Fiat-/Y animals were glucose-tolerant. However, when fed a high-fat diet (HFD) for 8 weeks, Fiat-/Y mice gained more weight than control mice, with a specific increase in white adipose tissue fat mass. The increase in fat mass was due to reduced energy expenditure, which correlated with reduced fatty acid oxidation and lipolysis in the adipose tissue of mutant mice. The expression of the Scd1 gene, involved in lipogenesis, was upregulated in the subcutaneous adipose tissue of Fiat-/Y mice. Moreover, HFD-fed Fiat-/Y mice exhibited increased circulating leptin and insulin levels relative to WT mice, demonstrating that endocrine abnormalities are associated with the disturbance in energy balance. We conclude that Fiat-/Y mice exhibited an anabolic bone phenotype but displayed increased susceptibility to developing metabolic-related disorders when consuming an HFD.

Tumoral Vitamin D Synthesis by CYP27B1 1-α-Hydroxylase Delays Mammary Tumor Progression in the PyMT-MMTV Mouse Model and Its Action Involves NF-κB Modulation

Biologically active vitamin D (1,25-dihydroxycholecalciferol or 1,25(OH)2D) is synthetized from inactive prohormone 25-hydroxycholecalciferol (25(OH)D) by the enzyme CYP27B1 1-α-hydroxylase in kidney and several extrarenal tissues including breast. Although the development of breast cancer has been linked to inadequate vitamin D status, the importance of bioactive vitamin D production within tumors themselves is not fully understood. To investigate the role of tumoral vitamin D production in mammary epithelial cell progression to breast cancer, we conducted a Cre-loxP-mediated Cyp27b1 gene ablation in the mammary epithelium of the polyoma middle T antigen-mouse mammary tumor virus (PyMT-MMTV) mouse breast cancer model. Targeted ablation of Cyp27b1 was accompanied by significant acceleration in initiation of spontaneous mammary tumorigenesis. In vivo, cell proliferation, angiogenesis, cell cycle progression, and survival markers were up-regulated in tumors by Cyp27b1 ablation, and apoptosis was decreased. AK thymoma (AKT) phosphorylation and expression of several components of nuclear factor κB (NF-κB), integrin, and signal transducer and activator of transcription 3 (STAT3) signaling pathways were increased in Cyp27b1-ablated tumors compared with nonablated controls. In vitro, 1,25(OH)2D treatment induced a strong antiproliferative action on tumor cells from both ablated and nonablated mice, accompanied by rapid disappearance of NF-κB p65 from the nucleus and segregation in the cytoplasm. In contrast, treatment with the metabolic precursor 25(OH)D was only effective against cells from nonablated mice. 25(OH)D did not inhibit growth of Cyp27b1-ablated cells, and their nuclear NF-κB p65 remained abundant. Our findings demonstrate that in-tumor CYP27B1 1-α-hydroxylase activity plays a crucial role in controlling early oncogene-mediated mammary carcinogenesis events, at least in part by modulating tumoral cell NF-κB p65 nuclear translocation.

CYP24 inhibition as a therapeutic target in FGF23-mediated renal phosphate wasting disorders

CYP24A1 (hereafter referred to as CYP24) enzymatic activity is pivotal in the inactivation of vitamin D metabolites. Basal renal and extrarenal CYP24 is usually low but is highly induced by its substrate 1,25-dihydroxyvitamin D. Unbalanced high and/or long-lasting CYP24 expression has been proposed to underlie diseases like chronic kidney disease, cancers, and psoriasis that otherwise should favorably respond to supplemental vitamin D. Using genetically modified mice, we have shown that renal phosphate wasting hypophosphatemic states arising from high levels of fibroblast growth factor 23 (FGF23) are also associated with increased renal Cyp24 expression, suggesting that elevated CYP24 activity is pivotal to the pathophysiology of these disorders. We therefore crossed 2 mouse strains, each with distinct etiology for high levels of circulating FGF23, onto a Cyp24-null background. Specifically, we evaluated Cyp24 deficiency in Hyp mice, the murine homolog of X-linked dominant hypophosphatemic rickets, and transgenic mice that overexpress a mutant FGF23 (FGF23R176Q) that is associated with the autosomal dominant form of hypophosphatemic rickets. Loss of Cyp24 in these murine models of human disease resulted in near-complete recovery of rachitic/osteomalacic bony abnormalities in the absence of any improvement in the serum biochemical profile. Moreover, treatment of Hyp and FGF23R1760-transgenic mice with the CYP24 inhibitor CTA102 also ameliorated their rachitic bones. Our results link CYP24 activity to the pathophysiology of FGF23-dependent renal phosphate wasting states and implicate pharmacologic CYP24 inhibition as a therapeutic adjunct for their treatment.

SUMOylated αNAC potentiates transcriptional repression by FIAT

The transcriptional coregulator αNAC (Nascent polypeptide associated complex And Coregulator alpha) and the transcriptional repressor FIAT (Factor Inhibiting ATF4-mediated Transcription) interact but the biological relevance of this interaction remains unclear. The activity of αNAC is extensively modulated by post-translational modifications (PTMs). We identified a novel αNAC PTM through covalent attachment of the Small Ubiquitin-like MOdifier (SUMO1). Recombinant αNAC was a SUMO1 target in in vitro SUMOylation assays and we confirmed that αNAC is conjugated to SUMO1 in cultured osteoblasts and in calvarial tissue. The amino acid sequence of αNAC contains one copy of the composite "phospho-sumoyl switch" motif that couples sequential phosphorylation and SUMOylation. We found that αNAC is selectively SUMOylated at lysine residue 127 within the motif and that SUMOylation is enhanced when a phosphomimetic mutation is introduced at the nearby serine residue 132. SUMOylation did not alter the DNA-binding capacity of αNAC. The S132D, hyper-SUMOylated αNAC mutant specifically interacted with histone deacetylase-2 (HDAC2) and enhanced the inhibitory activity of FIAT on ATF4-mediated transcription from the Osteocalcin gene promoter. This effect required binding of SUMOylated αNAC to the target promoter. We propose that maximal transcriptional repression by FIAT requires its interaction with SUMOylated, HDAC2-interacting αNAC.

Abnormal calcium handling and exaggerated cardiac dysfunction in mice with defective vitamin d signaling

Aim

Altered vitamin D signaling is associated with cardiac dysfunction, but the pathogenic mechanism is not clearly understood. We examine the mechanism and the role of vitamin D signaling in the development of cardiac dysfunction.

Methods and Results

We analyzed 1α-hydroxylase (1α-OHase) knockout (1α-OHase^−/−) mice, which lack 1α-OH enzymes that convert the inactive form to hormonally active form of vitamin D. 1α-OHase^−/− mice showed modest cardiac hypertrophy at baseline. Induction of pressure overload by transverse aortic constriction (TAC) demonstrated exaggerated cardiac dysfunction in 1α-OHase^−/− mice compared to their WT littermates with a significant increase in fibrosis and expression of inflammatory cytokines. Analysis of calcium (Ca²⁺) transient demonstrated profound Ca²⁺ handling abnormalities in 1α-OHase^−/− mouse cardiomyocytes (CMs), and treatment with paricalcitol (PC), an activated vitamin D₃ analog, significantly attenuated defective Ca²⁺ handling in 1α-OHase^−/− CMs. We further delineated the effect of vitamin D deficiency condition to TAC by first correcting the vitamin D deficiency in 1α-OHase^−/− mice, followed then by either a daily maintenance dose of vitamin D or vehicle (to achieve vitamin D deficiency) at the time of sham or TAC. In mice treated with vitamin D, there was a significant attenuation of TAC-induced cardiac hypertrophy, interstitial fibrosis, inflammatory markers, Ca²⁺ handling abnormalities and cardiac function compared to the vehicle treated animals.

Conclusions

Our results provide insight into the mechanism of cardiac dysfunction, which is associated with severely defective Ca²⁺ handling and defective vitamin D signaling in 1α-OHase^−/− mice.

Clinical utility of simultaneous quantitation of 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D by LC-MS/MS involving derivatization with DMEQ-TAD

CONTEXT

The discovery of hypercalcemic diseases due to loss-of-function mutations in 25-hydroxyvitamin D-24-hydroxylase has placed a new demand for sensitive and precise assays for 24,25-dihydroxyvitamin D [24,25-(OH)2D].

OBJECTIVE

We describe a novel liquid chromatography and tandem mass spectrometry-based method involving derivatization with DMEQ-TAD {4-[2-(6,7-dimethoxy-4-methyl-3,4-dihydroquinoxalinyl)ethyl]-1,2,4-triazoline-3,5-dione} to simultaneously assay multiple vitamin D metabolites including 25-hydroxyvitamin D (25-OH-D) and 24,25-(OH)2D using 100 μL of serum with a 5-minute run time.

DESIGN

The assay uses a newly synthesized internal standard d6-24,25-(OH)2D3 enabling the quantitation of 24,25-(OH)2D3 as well as the determination of the ratio of 25-OH-D3 to 24,25-(OH)2D3, a physiologically useful parameter.

SETTING

We report data on more than 1000 normal and disease samples involving vitamin D deficiency or hypercalcemia in addition to studies involving knockout mouse models.

RESULTS

The assay showed good correlation with samples from quality assurance schemes for 25-OH-D (25-OH-D2 and 25-OH-D3) determination (-2% to -5% bias) and exhibited low inter- and intraassay coefficients of variation (4%-7%) and lower limits of quantitation of 0.25-0.45 nmol/L. In clinical studies, we found a strong correlation between serum levels of 25-OH-D3 and 24,25-(OH)2D3 (r(2) = 0.80) in subjects over a broad range of 25-OH-D3 values and a marked lack of production of 24,25-(OH)2D3 below 25 nmol/L of 25-OH-D. The ratio of 25-OH-D3 to 24,25-(OH)2D3, which remained less than 25 in vitamin D-sufficient subjects (serum 25-OH-D < 50 nmol/L) but was greatly elevated (80-100) in patients with idiopathic infantile hypercalcemia.

CONCLUSIONS

The new method showed good utility in clinical settings involving vitamin D deficiency; supplementation with vitamin D and idiopathic infantile hypercalcemia, as well as in animal models with ablation of selected cytochrome P450-containing enzymes involved in vitamin D metabolism.

Altered gene dosage confirms the genetic interaction between FIAT and αNAC

Factor inhibiting ATF4-mediated transcription (FIAT) interacts with Nascent polypeptide associated complex and coregulator alpha (αNAC). In cultured osteoblastic cells, this interaction contributes to maximal FIAT-mediated inhibition of Osteocalcin (Ocn) gene transcription. We set out to demonstrate the physiological relevance of this interaction by altering gene dosage in compound Fiat and Naca (encoding αNAC) heterozygous mice. Compound Naca(+/-); Fiat(+/-) heterozygous animals were viable, developed normally, and exhibited no significant difference in body weight compared with control littermate genotypes. Animals with a single Fiat allele had reduced Fiat mRNA expression without changes in the expression of related family members. Expression of the osteocyte differentiation marker Dmp1 was elevated in compound heterozygotes. Static histomorphometry parameters were assessed at 8weeks of age using microcomputed tomography (μCT). Trabecular measurements were not different between genotypes. Cortical thickness and area were not affected by gene dosage, but we measured a significant increase in cortical porosity in compound heterozygous mice, without changes in biomechanical parameters. The bone phenotype of compound Naca(+/-); Fiat(+/-) heterozygotes confirms that FIAT and αNAC are part of a common genetic pathway and support a role for the FIAT/αNAC interaction in normal bone physiology.

Control of Fiat (factor inhibiting ATF4-mediated transcription) expression by Sp family transcription factors in osteoblasts

FIAT (factor inhibiting ATF4-mediated transcription) represses Osteocalcin gene transcription and inhibits osteoblast activity by heterodimerizing with ATF4 to prevent it from binding DNA. It thus appears important to identify and characterize the molecular mechanisms that control Fiat gene expression in osteoblasts. In silico sequence analysis identified a canonical GC-box within a 1,400 bp region of the proximal Fiat gene promoter. Electrophoretic mobility shift assays (EMSA) with MC3T3-E1 osteoblastic cells nuclear extracts indicated that the transcription factors Sp1 and Sp3, but not Sp7/OSTERIX, bound this proximal GC-box. Chromatin immunoprecipitation confirmed interaction of the two transcription factors with the Fiat promoter GC-element in living osteoblasts. Transient transfection studies showed that Sp1 dose-dependently activated the expression of a Fiat-luciferase reporter construct while both the long or short isoforms of Sp3 dose-dependently inhibited transcription from the Fiat reporter construct. Transfection of an Sp7/OSTERIX expression vector did not affect expression of the Fiat-luciferase reporter. Co-transfection of increasing amounts of the Sp3 expression vector in the context of maximal Sp1-dependent Fiat-luciferase activation led to dose-dependent repression of the expression of the reporter. Using RNA knockdown, we measured a reduction in steady-state Fiat expression when Sp1 was inhibited, and a reciprocal increase upon Sp3 knockdown. In parallel, treatment of osteoblasts with WP631, which prevents Sp1/DNA interactions, strongly inhibited the expression of Fiat and reduced the occupancy of the Fiat promoter proximal GC-box by Sp1. Taken together, our results suggest an interplay between Sp1 and Sp3 as a mechanism involved in the control of Fiat gene expression in osteoblasts.

Inactivation of the integrin-linked kinase (ILK) in osteoblasts increases mineralization

In osteoblasts, Integrin-Linked Kinase (ILK)-dependent phosphorylation of the cJUN transcriptional coactivator, αNAC, induces the nuclear accumulation of the coactivator and potentiates cJUN-dependent transcription. Mutation of the ILK phosphoacceptor site within the αNAC protein leads to cytoplasmic retention of the coactivator and cell-autonomous increases in osteoblastic activity. In order to gain further insight into the ILK-αNAC signaling cascade, we inactivated ILK using RNA knockdown in osteoblastic cells and engineered mice with specific ablation of ILK in osteoblasts. ILK knockdown in MC3T3-E1 osteoblast-like cells reduced phosphorylation of its downstream target glycogen synthase kinase 3β (GSK3β), which led to cytoplasmic retention of αNAC and increased mineralization with augmented expression of the osteoblastic differentiation markers, pro-α1(I) collagen (col1A1), Bone Sialoprotein (Bsp) and Osteocalcin (Ocn). Cultured ILK-deficient primary osteoblasts also showed increased cytoplasmic αNAC levels, and augmented mineralization with higher Runx2, Col1a1 and Bsp expression. Histomorphometric analysis of bones from mutant mice with ILK-deficient osteoblasts (Col1-Cre;Ilk(-/fl)) revealed transient changes, with increased bone volume in newborn animals that was corrected by two weeks of age. Our data suggest that the ILK-αNAC cascade acts to reduce the pace of osteoblast maturation. We propose that in vivo, functional redundancy is able to compensate for the loss of ILK activity, leading to the absence of an obvious phenotype when osteoblast-specific Ilk-deficient mice reach puberty.

Adult cartilage-specific peroxisome proliferator-activated receptor gamma knockout mice exhibit the spontaneous osteoarthritis phenotype

Osteoarthritis (OA) is an age-related progressive degenerative joint disease. Peroxisome proliferator-activated receptor gamma (PPARγ), a transcription factor, is suggested as an attractive therapeutic target to counteract degradative mechanisms associated with OA. Studies suggest that activation of PPARγ by its agonists can reduce the synthesis of OA catabolic and inflammatory factors and the development of cartilage lesions in OA animal models. Because these agonists impart several PPARγ-independent effects, the specific in vivo function of PPARγ in cartilage homeostasis and OA remains largely unknown. Herein, we describe the in vivo role of PPARγ in OA using cartilage-specific PPARγ knockout (KO) mice generated using the Cre-lox system. Adult PPARγ KO mice exhibited a spontaneous OA phenotype associated with enhanced cartilage degradation, hypocellularity, synovial and cartilage fibrosis, synovial inflammation, mononuclear cell influx in the synovium, and increased expression of catabolic factors, including matrix metalloproteinase-13, accompanied by an increase in staining for matrix metalloproteinase-generated aggrecan and type II collagen neoepitopes (VDIPEN and C1-2C). We demonstrate that PPARγ-deficient articular cartilage exhibits elevated expression of the additional catabolic factors hypoxia-inducible factor-2α, syndecan-4, and a disintegrin and metalloproteinase with thrombospondin motifs 5 and of the inflammatory factors cyclooxygenase-2 and inducible nitric oxide synthase. In conclusion, PPARγ is a critical regulator of cartilage health, the lack of which leads to an accelerated spontaneous OA phenotype.

CYP24A1 exacerbated activity during diabetes contributes to kidney tubular apoptosis via caspase-3 increased expression and activation

Decreases in circulating 25,hydroxyl-vitamin D3 (25 OH D3) and 1,25,dihydroxyl-vitamin D3 (1,25 (OH)2 D3) have been extensively documented in patients with type 2 diabetes. Nevertheless, the molecular reasons behind this drop, and whether it is a cause or an effect of disease progression is still poorly understood. With the skin and the liver, the kidney is one of the most important sites for vitamin D metabolism. Previous studies have also shown that CYP24A1 (an enzyme implicated in vitamin D metabolism), might play an important role in furthering the progression of kidney lesions during diabetic nephropathy. In this study we show a link between CYP24A1 increase and senescence followed by apoptosis induction in the renal proximal tubules of diabetic kidneys. We show that CYP24A1 expression was increased during diabetic nephropathy progression. This increase derived from protein kinase C activation and increased H₂O₂ cellular production. CYP24A1 increase had a major impact on cellular phenotype, by pushing cells into senescence, and later into apoptosis. Our data suggest that control of CYP24A1 increase during diabetes has a beneficial effect on senescence induction and caspase-3 increased expression. We concluded that diabetes induces an increase in CYP24A1 expression, destabilizing vitamin D metabolism in the renal proximal tubules, leading to cellular instability and apoptosis, and thereby accelerating tubular injury progression during diabetic nephropathy.

αNAC interacts with histone deacetylase corepressors to control Myogenin and Osteocalcin gene expression

In the nucleus of differentiated osteoblasts, the DNA-binding αNAC protein acts as a transcriptional coactivator of the Osteocalcin gene. Chromatin immunoprecipitation-microarray assays (ChIP-chip) showed that αNAC binds the Osteocalcin promoter but also identified the Myogenin promoter as an αNAC target. Here, we confirm these array data using quantitative ChIP and further detected that αNAC binds to these promoters in myoblasts. Since these genes are differentially regulated during osteoblastogenesis or myogenesis, these results suggest cell- and promoter-context specific functions for αNAC. We hypothesized that αNAC dynamically recruits corepressors to inhibit Myogenin expression in cells committing to the osteoblastic lineage or to inhibit Osteocalcin transcription in differentiating myoblasts. Using co-immunoprecipitation assays, we detected complexes between αNAC and the corepressors HDAC1 and HDAC3, in myoblasts and osteoblasts. Sequential ChIP confirmed HDAC1 recruitment by αNAC at the Osteocalcin and Myogenin promoters. Interaction with the corepressors was detectable in pre-osteoblasts and in myoblasts but disappeared as the cells differentiate. Treatment with an HDAC inhibitor caused de-repression of Osteocalcin expression in myoblasts. Overexpression of αNAC in myoblasts inhibits expression of Myogenin and differentiation. However, overexpression of an N-terminus truncated αNAC mutant allowed myoblasts to express Myogenin and differentiate, and this mutant did not interact with HDAC1 or HDAC3. This study identified an additional DNA-binding target and novel protein-protein interactions for αNAC. We propose that αNAC plays a role in regulating gene transcription during mesenchymal cell differentiation by differentially recruiting corepressors at target promoters.

Association of cartilage-specific deletion of peroxisome proliferator-activated receptor γ with abnormal endochondral ossification and impaired cartilage growth and development in a murine model

OBJECTIVE

Long bones develop through the strictly regulated process of endochondral ossification within the growth plate, resulting in the replacement of cartilage by bone. Defects in this process can result in skeletal abnormalities and a predisposition to degenerative joint diseases such as osteoarthritis (OA). Studies suggest that activation of the transcription factor peroxisome proliferator-activated receptor γ (PPARγ) is an important therapeutic target in OA. To devise PPARγ-related therapies in OA, it is critical to identify the role of this transcription factor in cartilage biology. Therefore, this study sought to determine the in vivo role of PPARγ in endochondral ossification and cartilage development, using cartilage-specific PPARγ-knockout (KO) mice.

METHODS

Cartilage-specific PPARγ-KO mice were generated using the Cre/loxP system. Histomorphometric and immunohistochemical analyses were performed to assess the patterns of ossification, proliferation, differentiation, and hypertrophy of chondrocytes, skeletal organization, bone density, and calcium deposition in the KO mice.

RESULTS

PPARγ-KO mice exhibited reductions in body length, body weight, length of the long bones, skeletal growth, cellularity, bone density, calcium deposition, and trabecular bone thickness, abnormal organization of the growth plate, loss of columnar organization, shorter hypertrophic zones, and delayed primary and secondary ossification. Immunohistochemical analyses for Sox9, 5-bromo-2'-deoxyuridine, p57, type X collagen, and platelet endothelial cell adhesion molecule 1 revealed reductions in the differentiation, proliferation, and hypertrophy of chondrocytes and in vascularization of the growth plate in mutant mice. Isolated chondrocytes and cartilage explants from mutant mice showed aberrant expression of Sox9 and extracellular matrix markers, including aggrecan, type II collagen, and matrix metalloproteinase 13. In addition, chondrocytes from mutant mice exhibited enhanced phosphorylation of p38 and decreased expression of Indian hedgehog.

CONCLUSION

The presence of PPARγ is required for normal endochondral ossification and cartilage development in vivo.

Combinatorial control of ATF4-dependent gene transcription in osteoblasts

Osteoblast-specific gene transcription requires interaction between bone cell-specific transcription factors and more widely expressed transcriptional regulators. This is particularly evident for the basic domain-leucine zipper factor activating transcription factor 4 (ATF4), whose activity can be enhanced or inhibited through interaction with other leucine zipper proteins, intermediate filament proteins, components of the basic transcriptional machinery, nuclear matrix attachment molecules, or ubiquitously expressed transcription factors. We discuss the results supporting the relevance of these interactions and present the first evidence of a functional interaction between ATF4, FIAT (factor-inhibiting ATF4-mediated transcription), and αNAC (nascent polypeptide-associated complex and coactivator alpha), three proteins that have been previously shown to associate using various protein-protein interaction assays.

Vitamin D metabolism, cartilage and bone fracture repair

The 1,25-(OH)(2)D metabolite mediates the endocrine actions of vitamin D by regulating in the small intestine the expression of target genes that play a critical role in intestinal calcium absorption. The major role of the vitamin D hormone on bone is indirect and mediated through its endocrine function on mineral homeostasis. However, genetic manipulation of the expression of Cyp27b1 or the VDR in chondrocytes strongly support a direct role for locally synthesized 1,25(OH)(2)D, acting through the VDR, in vascular invasion and osteoclastogenesis during endochondral bone development. Cells from the growth plate respond to the 24,25-(OH)(2)D and 1,25-(OH)(2)D metabolites in a cell maturation-dependent manner and the effects of 1,25-(OH)(2)D are thought to be mediated through binding to the membrane-associated receptor PDIA3 (protein disulfide isomerase associated 3). The physiological relevance of membrane-mediated 1,25-(OH)(2)D signaling is emerging and is discussed. Finally, preliminary results suggest that mice deficient for Cyp24a1 exhibit a delay in bone fracture healing and support a role for 24,25-(OH)(2)D in mammalian fracture repair.

Differential effects of oral doxercalciferol (Hectorol) or paricalcitol (Zemplar) in the Cyp27b1-null mouse model of uremia

BACKGROUND/AIMS

Kidney disease patients experience declining calcitriol levels and develop secondary hyperparathyroidism (SHPT). Animal models of uremia based on 5/6 nephrectomy (NTX) do not consistently reproduce this calcitriol deficiency. We developed an animal model, the NTX Cyp27b1-null mouse, which completely lacks endogenous calcitriol, and examined the suitability of this model for evaluation of treatment with vitamin D analogs in uremia.

METHODS

NTX was performed at 2 months of age. One week post-NTX, animals were treated for 4 weeks with vehicle; doxercalciferol at 30, 100 or 300 pg/g body weight (b.w.); or paricalcitol at 100, 300 or 1,000 pg/g b.w. by gavage 3 times per week.

RESULTS

Serum blood urea nitrogen and creatinine were elevated. Vehicle-treated NTX null mice had hypocalcemia and SHPT. Doxercalciferol at 100 or 300 pg/g b.w. normalized serum calcium and parathyroid hormone (PTH) levels. Paricalcitol at 300 or 1,000 pg/g normalized serum calcium, but PTH levels remained elevated. Osteomalacia was corrected by 100 pg/g b.w. of doxercalciferol or 1,000 pg/g b.w. of paricalcitol. The highest dose of doxercalciferol, but not of paricalcitol, significantly reduced osteitis fibrosa.

CONCLUSION

Our results reveal the differential efficacy of doxercalciferol and paricalcitol in this novel animal model incorporating both calcitriol deficiency and renal insufficiency.

Osteoclast-specific inactivation of the integrin-linked kinase (ILK) inhibits bone resorption

Bone resorption requires the adhesion of osteoclasts to extracellular matrix (ECM) components, a process mediated by the alpha(v)beta(3) integrin. Following engagement with the ECM, integrin receptors signal via multiple downstream effectors, including the integrin-linked kinase (ILK). In order to characterize the physiological role of ILK in bone resorption, we generated mice with an osteoclast-specific Ilk gene ablation by mating mice with a floxed Ilk allele with TRAP-Cre transgenic mice. The TRAP-Cre mice specifically excised floxed alleles in osteoclasts, as revealed by crossing them with the ROSA26R reporter strain. Osteoclast-specific Ilk mutant mice appeared phenotypically normal, but histomorphometric analysis of the proximal tibia revealed an increase in bone volume and trabecular thickness. Osteoclast-specific Ilk ablation was associated with an increase in osteoclastogenesis both in vitro and in vivo. However, the mutant osteoclasts displayed a decrease in resorption activity as assessed by reduced pit formation on dentin slices in vitro and decreased serum concentrations of the C-terminal telopeptide of collagen in vivo. Interestingly, compound heterozygous mice in which one allele of Ilk and one allele of the beta(3) integrin gene were inactivated (ILK(+/-); beta(3) (+/-)) also had increased trabecular thickness, confirming that beta(3) integrin and Ilk form part of the same genetic cascade. Our results show that ILK is important for the function, but not the differentiation, of osteoclasts.

CYP24A1-deficient mice as a tool to uncover a biological activity for vitamin D metabolites hydroxylated at position 24

The CYP24A1 enzyme (25-hydroxyvitamin D-24-hydroxylase) not only is involved in the catabolic breakdown of 1,25-dihydroxyvitamin D [1,25(OH)2D] but also generates the 24,25-dihydroxyvitamin D [24,25(OH)2D] metabolite. The biological activity of 24,25(OH)2D remains controversial. While in vitro studies suggest that primary cultures of rat rib chondrocytes respond to 24,25(OH)2D in a maturation-specific manner and that the metabolite is necessary for the cells to progress from a proliferating, immature status to a differentiated, 1,25(OH)2D-responsive stage, in vivo evidence to support this putative role remains lacking. Studies in chicken showed increases in serum levels of 24,25(OH)2D and of the renal mRNA levels of Cyp24a1 following fracture, suggesting a role for 24,25(OH)2D in fracture repair. The Cyp24a1-deficient mouse strain represents an invaluable tool to examine the putative role of 24,25(OH)2D in mammalian fracture repair. We have compared fracture repair between Cyp24a1-/- mice and wild-type controls. We have observed a delay in the mineralization of the cartilaginous matrix of the soft callus in Cyp24a1-/- mutant animals, accompanied by reduced expression of chondrocyte marker genes. These results show that Cyp24a1 deficiency delays fracture repair and strongly suggest that vitamin D metabolites hydroxylated at position 24, such as 24,25(OH)2D3, play an important role in the mechanisms leading to normal fracture healing.

FIAT control of osteoblast activity

The basic domain-leucine zipper transcription factor activating transcription factor 4 (ATF4) regulates most functions of the osteoblast. It is therefore not surprising that its activity should be regulated through several mechanisms. Factor inhibiting ATF4-mediated transcription (FIAT) is a leucine zipper nuclear molecule lacking a basic domain for DNA binding that interacts with ATF4 to repress its transcriptional activity. FIAT expression was monitored in parallel with ATF4 during osteoblastogenesis. The mechanism of ATF4 repression by FIAT was investigated through structure-function analysis. The physiological significance of FIAT expression in osteoblasts was studied through silencing FIAT in osteoblasts by RNA interference, as well as through characterization of two genetic mouse models: FIAT transgenic mice which overexpress FIAT in osteoblasts, and FIAT knockout mice. Studies to date show that FIAT and ATF4 are co-expressed in osteoblasts, and that FIAT inhibition of matrix mineralization requires dimerization with ATF4 through the second leucine zipper. Furthermore, transgenic mice overexpressing FIAT exhibit osteopenia. The phenotype of FIAT knockout mice is still under evaluation but the salient aspects are discussed here. Taken together, the results accumulated to date support the hypothesis that FIAT is a transcriptional repressor that modulates osteoblast function.

FIAT, the factor-inhibiting ATF4-mediated transcription, also represses the transcriptional activity of the bZIP factor FRA-1

FIAT is a leucine zipper protein whose name was coined for its interaction with ATF4 and subsequent blockage of ATF4-directed osteocalcin gene transcription. FIAT lacks a basic DNA-binding domain but contains three leucine zippers; it heterodimerizes with ATF4 to prohibit binding to DNA. FIAT could also potentially interact with additional basic domain-leucine zipper transcriptional regulators of osteoblast activity, such as FRA-1. We have found that FIAT inhibits transcriptional activation by a FRA-1/c-JUN heterodimer without affecting transcription mediated by a c-JUN homodimer. The repressor effect of FIAT on FRA-1-dependent transcription was measured using reporter constructs for the natural FRA-1 targets, Mmp-9 and Mgp. The FIAT-FRA-1 interaction is mediated through the second leucine zipper of FIAT. These data confirm an additional target of the FIAT transcriptional repressor activity and suggest that FIAT can both modulate early osteoblast activity by interacting with ATF4 and regulate later osteoblast function through inhibition of FRA-1.

Integrin-linked kinase has a critical role in ErbB2 mammary tumor progression: implications for human breast cancer

Elevated expression of the integrin-linked kinase (ILK) has been observed in a variety of cancers and has been further correlated with poor clinical outcome. Here, we show that mammary epithelial disruption of ILK results in a profound block in mammary tumor induction. Consistent with these observations, inhibition of ILK function in ErbB2-expressing cells with small molecule inhibitor or RNA interference resulted in profound block in their in vitro invasive properties due to the induction of apoptotic cell death. The rare ILK-deficient tumors that eventually arose overcame this block in tumor induction by an upregulation of ErB3 phosphorylation. These observations provide direct evidence that ILK has a critical role in the initiation phase of ErbB2 tumor induction.

Are endogenous BMPs necessary for bone healing during distraction osteogenesis?

Previous reports suggest the application of exogenous BMPs can accelerate bone formation during distraction osteogenesis (DO). However, there are drawbacks associated with the use of exogenous BMPs. A possible alternative to the use of exogenous BMPs is to upregulate the expression of endogenous BMPs. Since DO results in spontaneously generated de novo bone formation in a uniform radiographic, histological, and biomechanical temporal sequence, a genetically engineered model lacking endogenous BMP2 should have measurable deficits in bone formation at different time points. We performed DO on BMP2(fl/+) and BMP2(fl/+ cre) mice using a miniature Ilizarov fixator. Distracted samples were collected at various time points and analyzed using real time-quantitative PCR, lCT, radiology, immunohistochemistry, histology, and biomechanical testing. Immunohistochemical studies of 34-day heterozygous samples showed reduced expression of BMP2, BMP7, BMPR1a, ACTR1, and ACTR2b. lCT analysis of 51-day heterozygous samples revealed a decrease in trabecular number and increase in trabecular separation. Biomechanical testing of 51-day heterozygous samples revealed decreased stiffness and increased ultimate displacement. Radiological analysis showed the heterozygotes contained a decreased bone fill score at 17, 34, and 51 days. These data suggest endogenous BMPs are important for bone healing and manipulating endogenous BMPs may help accelerate bone consolidation during DO.

FIAT is co-expressed with its dimerization target ATF4 in early osteoblasts, but not in osteocytes

FIAT represses osteocalcin gene transcription by heterodimerizing with ATF4 and preventing it from binding to DNA. We report here the expression profiles of FIAT and ATF4 during osteoblastogenesis. Messenger RNA levels for the osteoblast transcriptional regulators Satb2, Runx2, Fiat, and Atf4 were quantified using real-time reverse-transcription PCR (RT-qPCR) and respective protein levels monitored by immunodetection in differentiating primary osteoblast cultures. Satb2, Fiat, and Atf4 mRNA levels remained constant throughout the differentiation sequence, whereas Runx2 transcript levels were significantly increased by 12 days post-confluency. Using immunofluorescence, the SATB2, RUNX2, and ATF4 signals appeared to increase as a function of time in culture. FIAT protein expression was readily detected in early cultures, but signal intensity decreased thereafter. When immunoblotting was used to quantify the relative amounts of FIAT and ATF4 proteins, the expression levels of the two proteins were found to be inversely correlated. The decrease in FIAT protein levels coincided with increased binding of ATF4 to the osteocalcin gene promoter, and with increased osteocalcin expression measured by RT-qPCR or immunoblotting. Immunohistochemistry of long bones from mice at E16.5 and 2 days post-natal revealed that both proteins are initially expressed in osteoblasts. In adult bone, FIAT was detected in osteocytes, while ATF4 expression was observed in active osteoblasts and lining cells, but not in osteocytes. Taken together, these data support the idea that a stoichiometric excess of ATF4 over FIAT in mature osteoblasts releases ATF4 from sequestration by FIAT, thereby allowing ATF4 homodimerization and subsequent transactivation of the osteocalcin gene.

FIAT inhibition increases osteoblast activity by modulating Atf4-dependent functions

The ATF4 transcription factor is a key regulator of osteoblast differentiation that controls osteocalcin gene transcription and type I collagen protein synthesis. We have characterized factor-inhibiting ATF4-mediated transcription (FIAT), a leucine zipper protein that dimerizes with ATF4 to form inactive dimers that cannot bind DNA. Overexpression of FIAT in osteoblasts of transgenic mice inhibited osteocalcin gene transcription and reduced osteoblastic activity, leading to osteopenia (Yu et al. [2005] J Cell Biol 169:591-601). We therefore hypothesized that inhibition of FIAT would enhance ATF4 activity, leading to increased osteocalcin transcription, type I collagen synthesis, and mineralization. We used small interfering RNAs (siRNA) to knockdown FIAT in pools of MC3T3-E1 cells stably transfected with 1.3 kb of the mouse osteocalcin gene promoter driving expression of luciferase. Stable expression of the FIAT siRNA sequence inhibited FIAT expression without significantly affecting the level of total or Ribosomal S6 Kinase-2-phosphorylated ATF4 protein. Occupancy of the osteocalcin proximal promoter by ATF4 was increased and transcription of the osteocalcin-promoter-dependent luciferase reporter showed earlier onset and increased levels. Similarly, endogenous osteocalcin gene expression was enhanced in primary osteoblasts transfected with the FIAT siRNA. FIAT knockdown cells also displayed higher expression of bone sialoprotein, increased type I collagen protein synthesis, and enhanced mineralization. These data suggest that inhibition of FIAT expression increases ATF4 activity and confirm the important role of FIAT in osteoblast function.

FIAT represses bone matrix mineralization by interacting with ATF4 through its second leucine zipper

We have characterized FIAT, a 66 kDa leucine zipper (LZ) protein that dimerizes with activating transcription factor 4 (ATF4) to form inactive dimers that cannot bind DNA. Computer analysis identifies three putative LZ motifs within the FIAT amino acid sequence. We have used deletion- and/or site-specific mutagenesis to individually inactivate these motifs in order to identify the functional LZ that mediates the FIAT-ATF4 interaction. Amino acids 194-222 that encode the FIAT LZ2 were deleted (mutant FIAT ZIP2 DEL). We inactivated each zipper individually by replacing two or three leucine residues within each zipper by alanine residues. The engineered mutations were L142A/L149A (mutant M1, first zipper), L208A/L215A/L222A (mutant M2, second zipper), and L441A/L448A (mutant M3, third zipper). MC3T3-E1 osteoblastic cells with an integrated 1.3 kb mouse osteocalcin gene promoter fragment driving expression of luciferase were transfected with expression vectors for ATF4 and the various FIAT deletion- or site-specific mutants. Inhibition of ATF4-mediated transcription was compared between wild-type (WT) and LZ FIAT mutants. The deletion mutant FIAT ZIP2 DEL and the sequence-specific M2 mutant did not interact with ATF4 and were unable to inhibit ATF4-mediated transcription. The M1 or M3 mutations did not affect the ability of FIAT to contact ATF4 or to inhibit its transcriptional activity. Stable expression of WT FIAT in osteoblastic cells inhibited mineralization, but not expression of the FIAT ZIP2 DEL and M2 mutants. This structure-function analysis reveals that FIAT interacts with ATF4 and modulates its activity through its second leucine zipper motif.