RESUMEN
OBJECTIVES: We sought to determine if a durable bilayer implant composed of trabecular metal with autologous periosteum on top would be suitable to reconstitute large osteochondral defects. This design would allow for secure implant fixation, subsequent integration and remodeling. MATERIALS AND METHODS: Adult sheep were randomly assigned to one of three groups (n = 8/group): 1. trabecular metal/periosteal graft (TMPG), 2. trabecular metal (TM), 3. empty defect (ED). Cartilage and bone healing were assessed macroscopically, biochemically (type II collagen, sulfated glycosaminoglycan (sGAG) and double-stranded DNA (dsDNA) content) and histologically. RESULTS: At 16 weeks post-operatively, histological scores amongst treatment groups were not statistically different (TMPG: overall 12.7, cartilage 8.6, bone 4.1; TM: overall 14.2, cartilage 9.5, bone 4.9; ED: overall 13.6, cartilage 9.1, bone 4.5). Metal scaffolds were incorporated into the surrounding bone, both in TM and TMPG. The sGAG yield was lower in the neo-cartilage regions compared with the articular cartilage (AC) controls (TMPG 20.8/AC 39.5, TM 25.6/AC 33.3, ED 32.2/AC 40.2 µg sGAG/1 mg respectively), with statistical significance being achieved for the TMPG group (p < 0.05). Hypercellularity of the neo-cartilage was found in TM and ED, as the dsDNA content was significantly higher (p < 0.05) compared with contralateral AC controls (TM 126.7/AC 71.1, ED 99.3/AC 62.8 ng dsDNA/1 mg). The highest type II collagen content was found in neo-cartilage after TM compared with TMPG and ED (TM 60%/TMPG 40%/ED 39%). Inter-treatment differences were not significant. CONCLUSIONS: TM is a highly suitable material for the reconstitution of osseous defects. TM enables excellent bony ingrowth and fast integration. However, combined with autologous periosteum, such a biocomposite failed to promote satisfactory neo-cartilage formation.Cite this article: E. H. Mrosek, H-W. Chung, J. S. Fitzsimmons, S. W. O'Driscoll, G. G. Reinholz, J. C. Schagemann. Porous tantalum biocomposites for osteochondral defect repair: A follow-up study in a sheep model. Bone Joint J 2016;5:403-411. DOI: 10.1302/2046-3758.59.BJR-2016-0070.R1.
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The objective of this study was to develop a scaffold for mesenchymal stromal cell (MSC) recruitment, proliferation, and chondrogenic differentiation. The concept behind the design is to mimic the cartilage matrix and contain stimulatory agents that make continuous supply of inductive factors redundant. Nanofibrous (N: ~400 nm) and microfibrous (M: ~10 µm) poly-ε-caprolactone (PCL) scaffolds were combined with 1% high-molecular-weight sodium hyaluronate (NHA/MHA), 1% hyaluronan (HA) and 200 ng transforming growth factor-beta 1 (TGF-ß1; NTGF/MTGF), or 0.1% bovine serum albumin (N/M). Scaffolds were seeded with MSCs from bone marrow and cultured without growth factors in vitro. Cultures with chondrogenic medium supplemented with TGF-ß1 served as controls. Proliferation, migration, and release of TGF-ß1 were investigated. Cell differentiation was evaluated by polymerase chain reaction (PCR) and real-time PCR. NTGF and MTGF exhibited primarily an initial release of TGF-ß1. None of the factors released by the scaffolds recruited MSCs. The expression of aggrecan was dependent on the scaffold ultrastructure with nanofibers promoting increasing and microfibers decreasing expression levels. Composites containing HA demonstrated elevated seeding efficiency and lower type I collagen expression. Expression of type II collagen was dependent on continuous or late supply of TGF-ß1, which was not provided by our scaffold design. The initial release of TGF-ß1 induced an expression of type I collagen and osteogenic marker genes. In conclusion, nanofibrous PCL scaffolds with or without augmentation are suitable for chondrogenic initiation of MSCs. Initial release of HA is sufficient in terms of directing the implanted MSCs toward a chondrogenic end, whereas a late release of TGF-ß1 is preferred to foster type II and avoid type I collagen expression.
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Materiales Biomiméticos/farmacología , Diferenciación Celular/efectos de los fármacos , Condrogénesis/efectos de los fármacos , Células Madre Mesenquimatosas/citología , Poliésteres/farmacología , Andamios del Tejido/química , Anciano , Anciano de 80 o más Años , Agrecanos/genética , Agrecanos/metabolismo , Fosfatasa Alcalina/genética , Fosfatasa Alcalina/metabolismo , Animales , Biomarcadores/metabolismo , Bovinos , Diferenciación Celular/genética , Condrogénesis/genética , Colágeno Tipo I/genética , Colágeno Tipo I/metabolismo , Colágeno Tipo II/genética , Colágeno Tipo II/metabolismo , ADN/metabolismo , Regulación de la Expresión Génica/efectos de los fármacos , Humanos , Células Madre Mesenquimatosas/efectos de los fármacos , Células Madre Mesenquimatosas/metabolismo , Persona de Mediana Edad , Nanofibras/ultraestructura , Osteocalcina/genética , Osteocalcina/metabolismo , Osteopontina/genética , Osteopontina/metabolismo , Factor de Crecimiento Transformador beta1/metabolismoRESUMEN
OBJECTIVE: To compare the efficacy of in situ transforming growth factor-beta1 (TGF-beta1)-pretreated periosteum to untreated periosteum for regeneration of osteochondral tissue in rabbits. METHODS: In the pretreatment group, 12 month-old New Zealand white rabbits received subperiosteal injections of 200 ng of TGF-beta1 percutaneously in the medial side of the proximal tibia, 7 days prior to surgery. Control rabbits received no treatment prior surgery. Osteochondral transverse defects measuring 5mm proximal to distal and spanning the entire width of the patellar groove were created and repaired with untreated or TGF-beta1-pretreated periosteal grafts. Post-operatively the rabbits resumed normal cage activity for 6 weeks. RESULTS: Complete filling of the defects with regenerated tissue was observed in both the TGF-beta1-pretreated and control groups with reformation of the original contours of the patellar groove. The total histological score (modified O'Driscoll) in the TGF-beta1-pretreated group, 20 (95% Confidence Interval (CI), 19-21), was significantly higher (P=0.0001) than the control group, 18 (16-19). The most notable improvements were in structural integrity and subchondral bone regeneration. No significant differences in glycosaminoglycan or type II collagen content, or equilibrium modulus were found between the surgical groups. The cambium of the periosteum regenerated at the graft harvest site was significantly thicker (P=0.0065) in the TGF-beta1-pretreated rabbits, 121 microm (94-149), compared to controls, 74 microm (52-96), after 6 weeks. CONCLUSIONS: This study demonstrates that in situ pretreatment of periosteum with TGF-beta1 improves osteochondral tissue regeneration at 6-weeks post-op compared to untreated periosteum in 12 month-old rabbits.
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Cartílago Articular/efectos de los fármacos , Cartílago Articular/lesiones , Condrogénesis/efectos de los fármacos , Periostio/efectos de los fármacos , Periostio/patología , Ingeniería de Tejidos/métodos , Factor de Crecimiento Transformador beta1/farmacología , Animales , Cartílago Articular/fisiopatología , Condrogénesis/fisiología , Modelos Animales de Enfermedad , Conejos , Trasplante de Tejidos , Trasplante Autólogo , Cicatrización de Heridas/efectos de los fármacos , Cicatrización de Heridas/fisiologíaRESUMEN
OBJECTIVE: To determine the potential of periosteal cells to infiltrate poly-epsilon-caprolactone (PCL) nanofiber scaffolds in vivo and subsequently produce cartilage in vitro. DESIGN: PCL nanofiber scaffolds, with or without chitosan-coating were implanted under periosteum in 6-month-old rabbits. Transforming growth factor-beta1 (TGF-beta1) or vehicle was injected into each implant site. After 1, 3, 5 or 7 days, scaffolds were removed, separated from the periosteum, and the scaffolds and periosteum were cultured separately for 6 weeks under chondrogenic conditions. Sulfated glycosaminoglycan (GAG), type II collagen, DNA content, cartilage yield, and calcium deposition were then analyzed. RESULTS: Cell infiltration was observed in all scaffolds. Cartilage formation in the uncoated scaffolds increased with duration of implantation (maximum at 7 days). Cells in the uncoated scaffolds implanted for 7 days produced significantly higher levels of both GAG [560 (95% confidence interval (CI), 107-1013) vs 228 (95% CI, 177-278) microg GAG/microg DNA] and cartilage yield [9% (95% CI, 3-14%) vs 0.02% (95% CI, 0-0.22%)] compared to chitosan-coated scaffolds (P=0.006 or less). There was no significant difference in GAG content or cartilage yield between the TGF-beta1-injected and vehicle-injected scaffolds. However, significantly more mineral deposition was detected in TGF-beta1-injected scaffolds compared to vehicle-injected scaffolds (P<0.0001). Cartilage yield from the periosteum, moreover, was significantly increased by subperiosteal TGF-beta1 injections (P<0.001). However, this response was reduced when chitosan-coated scaffolds were implanted. CONCLUSIONS: This study demonstrates that it is possible to seed PCL nanofiber scaffolds with periosteal cells in vivo and subsequently produce engineered cartilage in vitro.
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Caproatos/metabolismo , Cartílago/efectos de los fármacos , Condrogénesis/efectos de los fármacos , Lactonas/metabolismo , Periostio/efectos de los fármacos , Ingeniería de Tejidos/métodos , Andamios del Tejido , Factor de Crecimiento Transformador beta/metabolismo , Animales , Cartílago/crecimiento & desarrollo , Nanofibras , Periostio/citología , ConejosRESUMEN
The aim of this study was to determine the suitability of hybrid scaffolds composed of naturally derived biopolymer gels and macroporous poly-epsilon-caprolactone (PCL) scaffolds for neocartilage formation in vitro. Rabbit articular chondrocytes were seeded into PCL/HA (1 wt % hyaluronan), PCL/CS (0.5 wt % chitosan), PCL/F (1:3 fibrin sealant plus aprotinin), and PCL/COL1 (0.24% type I collagen) hybrids and cultured statically for up to 50 days. Growth characteristics were evaluated by histological analysis, scanning electron microscopy, and confocal laser scanning microscopy. Neocartilage was quantified using a dimethyl-methylene blue assay for sulfated glycosaminoglycans (sGAG) and an enzyme-linked immunosorbent assay for type II collagen (COL2), normalized to dsDNA content by fluorescent PicoGreen assay. Chondrocytes were homogenously distributed throughout the entire scaffold and exhibited a predominantly spheroidal shape 1 h after being seeded into scaffolds. Immunofluorescence depicted expanding proteoglycan deposition with time. The sGAG per dsDNA increased in all hybrids between days 25 and 50. PCL/HA scaffolds consistently promoted highest yields. In contrast, total sGAG and total COL2 decreased in all hybrids except PCL/CS, which favored increasing values and a significantly higher total COL2 at day 50. Overall, dsDNA content decreased significantly with time, and particularly between days 3 and 6. The PCL/HA hybrid displayed two proliferation peaks at days 3 and 25, and PCL/COL1 displayed one proliferation peak at day 12. The developed hybrids provided distinct short-term environments for implanted chondrocytes, with not all of them being explicitly beneficial (PCL/F, PCL/COL1). The PCL/HA and PCL/CS hybrids, however, promoted specific neocartilage formation and initial cell retention and are thus promising for cartilage tissue engineering.
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Cartílago , Geles/química , Poliésteres/química , Ingeniería de Tejidos/instrumentación , Andamios del Tejido/química , Animales , Materiales Biocompatibles/química , Cartílago/citología , Cartílago/fisiología , Técnicas de Cultivo de Célula/instrumentación , Técnicas de Cultivo de Célula/métodos , Condrocitos/metabolismo , Condrocitos/ultraestructura , ADN/análisis , Glicosaminoglicanos/química , Glicosaminoglicanos/metabolismo , Ensayo de Materiales , Conejos , Ingeniería de Tejidos/métodosRESUMEN
OBJECTIVE: To examine the potential for rejuvenation of aged periosteum by local injection of transforming growth factor-beta1 (TGF-beta1) and insulin-like growth factor-1 (IGF-1) alone or in combination to induce cambium cell proliferation and enhance in vitro periosteal cartilage formation. METHODS: A total of 367 New Zealand white rabbits (6, 12, and 24+ month-old) received subperiosteal injections of TGF-beta1 and/or IGF-1 percutaneously. After 1, 3, 5, or 7 days, the rabbits were sacrificed and cambium cellularity or in vitro cartilage forming capacity was determined. RESULTS: A significant increase in cambium cellularity and thickness, and in vitro cartilage formation was observed after injection of TGF-beta1 alone or in combination with IGF-1. In 12 month-old rabbits, mean cambium cellularity increased 5-fold from 49 to 237 cells/mm and in vitro cartilage production increased 12-fold from 0.8 to 9.7 mg 7 days after TGF-beta1 (200 ng) injection compared to vehicle controls (P<0.0001). A correlation was observed between cambium cellularity and in vitro cartilage production (R2=0.98). An added benefit of IGF-1 plus TGF-beta1 on in vitro cartilage production compared to TGF-beta1 alone was observed in the 2 year-old rabbits. IGF-1 alone generally had no effect on either cambium cellularity or in vitro cartilage production in any of the age groups. CONCLUSIONS: These results clearly demonstrate that it is possible to increase cambium cellularity and in vitro cartilage production in aged rabbit periosteum, to levels comparable to younger rabbits, using local injection of TGF-beta1 alone or in combination with IGF-1, thereby rejuvenating aged periosteum.
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Cartílago Articular/efectos de los fármacos , Proliferación Celular/efectos de los fármacos , Factor I del Crecimiento Similar a la Insulina/administración & dosificación , Periostio/efectos de los fármacos , Rejuvenecimiento/fisiología , Factor de Crecimiento Transformador beta1/administración & dosificación , Animales , Cartílago Articular/fisiopatología , Condrogénesis/efectos de los fármacos , Periostio/fisiopatología , ConejosRESUMEN
Animal models are widely used to develop and evaluate tissue-engineering techniques for the reconstruction of damaged human articular cartilage. For the purpose of this review, these model systems will include in vitro culture of animal cells and explants, heterotopic models of chondrogenesis, and articular cartilage defect models. The objectives for these preclinical studies are to engineer articular cartilage for the functional restoration of a joint surface that appears anatomically, histologically, biologically, biochemically, and mechanically to resemble the original joint surface. While no animal model permits direct application to humans, each is capable of yielding principles on which decisions can be made that might eventually translate into a human application. Clearly, the use of animal models has and will continue to play a significant role in the advancement of this field. Each animal model has specific advantages and disadvantages. The key issue in the selection of an appropriate animal model is to match the model to the question being investigated and the hypothesis to be tested. The purpose of this review is to discuss issues regarding animal model selection, the benefits and limitations of these model systems, scaffold selection with emphasis on polymers, and evaluation of the tissue-engineered articular cartilage.
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Enfermedades de los Cartílagos/cirugía , Cartílago Articular/crecimiento & desarrollo , Técnicas de Cultivo de Célula/métodos , Condrogénesis , Modelos Animales de Enfermedad , Procedimientos de Cirugía Plástica/métodos , Ingeniería de Tejidos/métodos , Animales , Enfermedades de los Cartílagos/clasificación , Enfermedades de los Cartílagos/patología , Enfermedades de los Cartílagos/fisiopatología , Cartílago Articular/lesiones , Cartílago Articular/patología , Cartílago Articular/cirugía , Técnicas de Cultivo de Célula/instrumentación , Humanos , Polímeros , Prótesis e Implantes , Procedimientos de Cirugía Plástica/instrumentación , Ingeniería de Tejidos/instrumentación , Resultado del TratamientoRESUMEN
OBJECTIVE: Periosteum contains undifferentiated mesenchymal stem cells that have both chondrogenic and osteogenic potential, and has been used to repair articular cartilage defects. During this process, the role of growth factors that stimulate the periosteal mesenchymal cells toward chondrogenesis to regenerate articular cartilage and maintain its phenotype is not yet fully understood. In this study, we examined the effects of insulin-like growth factor-1 (IGF-1) and transforming growth factor-beta1 (TGF-beta1), alone and in combination, on periosteal chondrogenesis using an in vitro organ culture model. METHODS: Periosteal explants from the medial proximal tibia of 2-month-old rabbits were cultured in agarose under serum free conditions for up to 6 weeks. After culture the explants were weighed, assayed for cartilage production via Safranin O staining and histomorphometry, assessed for proliferation via proliferative cell nuclear antigen (PCNA) immunostaining, and assessed for type II collagen mRNA expression via in situ hybridization. RESULTS: IGF-1 significantly increased chondrogenesis in a dose-dependent manner when administered continuously throughout the culture period. Continuous IGF-1, in combination with TGF-beta1 for the first 2 days, further enhanced overall total cartilage growth. Immunohistochemistry for PCNA revealed that combining IGF-1 with TGF-beta1 gave the strongest proliferative stimulus early during chondrogenesis. In situ hybridization for type II collagen showed that continuous IGF-1 maintained type II collagen mRNA expression throughout the cambium layer from 2 to 6 weeks. CONCLUSION: The results of this study demonstrate that IGF-1 and TGF-beta1 can act in combination to regulate proliferation and differentiation of periosteal mesenchymal cells during chondrogenesis.
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Cartílago Articular/metabolismo , Condrogénesis/efectos de los fármacos , Factor I del Crecimiento Similar a la Insulina/farmacología , Factor de Crecimiento Transformador beta/farmacología , Animales , Colágeno Tipo II/metabolismo , Miembro Posterior , Técnicas In Vitro , Factor I del Crecimiento Similar a la Insulina/administración & dosificación , Periostio/metabolismo , Conejos , Factor de Crecimiento Transformador beta/administración & dosificación , Factor de Crecimiento Transformador beta1RESUMEN
Bisphosphonates are widely used clinically to treat bone diseases in which bone resorption is in excess. However, the mechanism of bisphosphonate action on bone is not fully understood. Studies of direct action of bisphosphonates on bone have been limited mainly to their effects on bone-resorbing osteoclast cells, with implications that some activity may be mediated indirectly through paracrine factors produced by the bone-forming osteoblast cells. Little is known about the direct effects of bisphosphonates on osteoblasts. In this report, the direct actions of several bisphosphonates on cell proliferation, gene expression, and bone formation by cultured human fetal osteoblasts were examined. Osteoblast cell proliferation was decreased, and cytodifferentiation was increased in a dose-dependent manner in cultures treated with the bisphosphonate pamidronate. In addition, pamidronate treatment increased total cellular protein, alkaline phosphatase activity, and type I collagen secretion in osteoblasts. Consistent with the above-mentioned findings, the rate of bone formation was also increased in osteoblasts cultured with pamidronate. The actions of two other bisphosphonates, the weak-acting etidronate and the potent new analogue zoledronate, were also compared with the action of pamidronate on proliferation of immortalized human fetal osteoblast (hFOB) cells and rate of bone formation. Pamidronate and zoledronate decreased hFOB cell proliferation with equal potency, whereas etidronate decreased proliferation only at much higher concentrations. Studies comparing EDTA and etidronate indicate that etidronate may act indirectly on the hFOB cells by reducing free divalent ion concentrations, whereas pamidronate and zoledronate appear to act on the hFOB cells by a direct action. Both pamidronate and zoledronate increase hFOB cell bone formation, whereas no increase is observed with etidronate and EDTA. Taken together, these observations strongly suggest that treatment with pamidronate or zoledronate enhances the differentiation and bone-forming activities of osteoblasts.
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Difosfonatos/farmacología , Osteoblastos/efectos de los fármacos , Fosfatasa Alcalina/metabolismo , Antiinflamatorios/farmacología , Antineoplásicos/farmacología , Desarrollo Óseo/efectos de los fármacos , Calcificación Fisiológica/efectos de los fármacos , Diferenciación Celular/efectos de los fármacos , División Celular/efectos de los fármacos , Células Cultivadas , Quelantes/farmacología , Colágeno/metabolismo , Ácido Edético/farmacología , Ácido Etidrónico/farmacología , Feto , Expresión Génica/efectos de los fármacos , Humanos , Imidazoles/farmacología , Osteoblastos/citología , Osteoblastos/fisiología , Pamidronato , Proteínas/metabolismo , Ácido ZoledrónicoRESUMEN
The 24-hydroxylase is the enzyme responsible for the first step in the catabolism of 1,25-dihydroxyvitamin D3, the active form of vitamin D. This enzyme was shown to be upregulated by 1,25-dihydroxyvitamin D3 itself and downregulated by parathyroid hormone (PTH). Upregulation of 24-hydroxylase by 1,25-dihydroxyvitamin D3 has been characterized; however, the mechanism by which PTH acts to downregulate 24-hydroxylase expression remains unknown. Here we report the cloning of the porcine 24-hydroxylase, and show that 1,25-dihydroxyvitamin D3-stimulated 24-hydroxylase mRNA and activity are repressed by PTH in AOK-B50 cells, a porcine kidney proximal tubule cell line with stably transfected opossum PTH receptors. Forskolin mimicked the effects of PTH consistent with in vivo data, and suppression by PTH was not due to changes in VDR levels. The first 1400 bp of the 24-hydroxylase promoter were not able to mediate the effects of PTH on a reporter gene. In view of the above findings we concluded that AOK-B50 cells are a suitable model for further studying the mechanism of action of PTH on 24-hydroxylase mRNA.
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Calcitriol/farmacología , Sistema Enzimático del Citocromo P-450/metabolismo , Hormona Paratiroidea/farmacología , Esteroide Hidroxilasas/metabolismo , Secuencia de Aminoácidos , Animales , Secuencia de Bases , Línea Celular , Clonación Molecular , Sistema Enzimático del Citocromo P-450/genética , Cartilla de ADN/genética , ADN Complementario/genética , Expresión Génica/efectos de los fármacos , Túbulos Renales Proximales/efectos de los fármacos , Túbulos Renales Proximales/enzimología , Datos de Secuencia Molecular , Zarigüeyas , ARN Mensajero/genética , ARN Mensajero/metabolismo , Receptores de Calcitriol/metabolismo , Receptores de Hormona Paratiroidea/genética , Receptores de Hormona Paratiroidea/metabolismo , Esteroide Hidroxilasas/genética , Porcinos , Transfección , Vitamina D3 24-HidroxilasaRESUMEN
Although transforming growth factor-beta (TGF-beta) is a growth factor with many known regulatory activities in many different cell types, its intracellular signaling pathway is still not fully understood. A TGF-beta-inducible early gene (TIEG) was discovered and shown by this laboratory to be a 3-zinc finger transcription factor family member; its expression is rapidly induced in cells treated with TGF-beta. To ascertain whether TIEG plays a major role in the TGF-beta pathway, human osteosarcoma MG-63 cells were stably transfected either with an expression vector containing a TIEG cDNA or with the vector alone. Clones that contain only the vector express normal levels of TIEG mRNA and protein and display the same patterns of gene expression and levels of cell proliferation as the nontransfected, non-TGF-beta-treated parental cells. However, transfected cells that overexpress TIEG mRNA and protein (TIEG-6 and TIEG-7) display changes that mimic those of MG-63 cells treated with TGF-beta, i.e. increased alkaline phosphatase activity, decreased levels of osteocalcin mRNA and protein, and decreased cell proliferation. The degree of these changes correlated with the level of TIEG expressed in the cell lines. TGF-beta treatment of the overexpressed cells showed no added effects. These findings and other published reports support a primary role of TIEG as a transcription factor in the TGF-beta signaling pathway.
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Proteínas de Unión al ADN/genética , Osteoblastos/metabolismo , Factores de Transcripción/genética , Factor de Crecimiento Transformador beta/metabolismo , Fosfatasa Alcalina/metabolismo , Proteínas de Unión al ADN/metabolismo , Factores de Transcripción de la Respuesta de Crecimiento Precoz , Regulación de la Expresión Génica/efectos de los fármacos , Humanos , Factores de Transcripción de Tipo Kruppel , Proteínas Nucleares/genética , Proteínas Nucleares/metabolismo , Osteoblastos/efectos de los fármacos , Osteocalcina/genética , Osteocalcina/metabolismo , ARN Mensajero/metabolismo , Transducción de Señal , Factores de Transcripción/metabolismo , Transfección , Células Tumorales Cultivadas , Dedos de Zinc/genéticaRESUMEN
The regulation of the hepatic vitamin D3 25-hydroxylase enzyme system by 1,25-dihydroxyvitamin D3 was examined using male rats. Circulating 25-hydroxyvitamin D3 levels decreased in response to increasing doses of 1,25-dihydroxyvitamin D3 as determined by HPLC and serum protein binding assay. A maximum reduction in serum 25-hydroxyvitamin D3 of 57.7% was achieved by the administration of 1,25-dihydroxyvitamin D3 and higher doses produced no further reduction. The in vitro rate of vitamin D 25-hydroxylation by liver homogenates from rats given 1,25-dihydroxyvitamin D3 was also reduced by 60.4%. Whole rat extracts from animals given [3H]vitamin D3 contained 50.4% less 25-[3H]hydroxyvitamin D3 in treated rats given 1,25-dihydroxyvitamin D3 than in those given only the vehicle. Further, 1,25-dihydroxyvitamin D3 treatment had no effect on in vivo disappearance of 25-[3H]hydroxyvitamin D3. Taken together, these results demonstrate that the decrease in 25-hydroxyvitamin D3 levels observed in 1,25-dihydroxyvitamin D3-treated rats results from a decrease in production and not an increase in the metabolic clearance of 25-[3H]hydroxyvitamin D3.