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Ung-Il Chung

Publications and source records attributed to Ung-Il Chung.

At least 19 recordsLinked to original sources
PubMed2006

Impact of impaired receptor internalization on calcium homeostasis in knock-in mice expressing a phosphorylation-deficient parathyroid hormone (PTH)/PTH-related peptide receptor.

Internalization of G protein-coupled receptors (GPCRs) and desensitization of the hormonal responses are well characterized in vitro for several hormonal systems. The physiological role of internalization for a GPCR receptor involved in homeostatic functions has not been established, although it has been assumed based on in vitro data. We have previously shown that phosphorylation of the PTH/PTHrP receptor is required for its internalization and for the desensitization of the responsiveness to PTH and PTHrP in vitro; the internalization and desensitization response is impaired in a PTH/PTHrP receptor mutant bearing serine to alanine mutations in the phosphate acceptor sites. To understand the physiological role of receptor internalization on calcium homeostasis, we have knocked-in the internalization-impaired PTH/PTHrP receptor mutant using homologous recombination technology. The genetically modified animals exhibited calcium levels no different from control animals, but PTH levels were one third of those in control animals indicating that homeostasis could be maintained only by 3-fold suppression of PTH secretion. We also analyzed the calcemic response to PTH in vivo. Here we show that mice expressing the internalization-impaired PTH/PTHrP receptor mutant have dramatically exaggerated cAMP and calcemic responses to sc PTH administration when compared with control animals given the same dose. These data show for the first time the role of G protein receptor phosphorylation and internalization per se in the regulatory function of an endocrine system controlled by a GPCR.

Animals↗
PubMed2006

PTH/PTHrP receptor delays chondrocyte hypertrophy via both Runx2-dependent and -independent pathways.

The transcription factor, Runx2, promotes chondrocyte hypertrophy, whereas parathyroid hormone-related protein (PTHrP) delays this process. To examine whether PTHrP suppresses chondrocyte hypertrophy via Runx2-dependent or -independent pathways, Runx2 expression and chondrocyte differentiation were analyzed using bones from embryonic limbs of wild type and Runx2(-/-) mice. Treatment of cultured rudiments with PTH dramatically suppresses Runx2 mRNA levels in hypertrophic chondrocytes. PTH-induced delay of chondrocyte hypertrophy was observed in cultured tibiae from both Runx2(-/-) and wild-type embryos. This delay was also seen after PTH administration to limbs from wild type and Runx2(-/-) mice expressing Runx2 in chondrocytes via a collagen 2 promoter-driven transgene. To further explore Runx2-dependent and -independent effects of PTHrP, we examined embryonic tibiae and femurs from littermates null for PTHrP, Runx2, or both genes. Runx2(-/-) femurs exhibited no vascular invasion or chondrocytes expressing collagen type X or osteopontin mRNA. In contrast, Runx2(-/-)/PTHrP(-/-) mice exhibited limited vascular invasion and some chondrocytes expressing collagen X or osteopontin mRNA. In both tibia and femur, Runx2(-/-)/PTHrP(-/-) mice exhibited expanded regions of proliferating chondrocytes when compared to the same regions in PTHrP(-/-) mice. These data indicate that the delayed hypertrophy induced by PTHrP is mediated by both Runx2-dependent and -independent mechanisms.

Animals↗
PubMed2006

[Supramolecular nanocarrier for gene and siRNA delivery].

In this review article, supramolecular nanocarriers for gene and siRNA delivery were introduced. They were formed through the multimolecular assembly of block copolymers, containing the DNA or siRNA inside the core of the micelle structure. Due to the excellent stability under the physiological conditions, low toxic effect, and controllable gene functions, this system should be a promising candidate for in vivo therapeutic applications including the non-lethal diseases and the tissue engineering.

DNA↗
PubMed2005

Suppression of adjuvant-induced arthritic bone destruction by cyclooxygenase-2 selective agents with and without inhibitory potency against carbonic anhydrase II.

UNLABELLED: In vitro assays revealed that COX-2 inhibitors with CA II inhibitory potency suppressed both differentiation and activity of osteoclasts, whereas that without the potency reduced only osteoclast differentiation. However, all COX-2 inhibitors similarly suppressed bone destruction in adjuvant-induced arthritic rats, indicating that suppression of osteoclast differentiation is more effective than that of osteoclast activity for the treatment. INTRODUCTION: Cyclooxygenase (COX)-2 and carbonic anhydrase II (CA II) are known to play important roles in the differentiation of osteoclasts and the activity of mature osteoclasts, respectively. Because several COX-2 selective agents were recently found to possess an inhibitory potency against CA II, this study compared the bone sparing effects of COX-2 selective agents with and without the CA II inhibitory potency. MATERIALS AND METHODS: Osteoclast differentiation was determined by the mouse co-culture system of osteoblasts and bone marrow cells, and mature osteoclast activity was measured by the pit area on a dentine slice resorbed by osteoclasts generated and isolated from bone marrow cells. In vivo effects on arthritic bone destruction were determined by radiological and histological analyses of hind-paws of adjuvant-induced arthritic (AIA) rats. RESULTS: CA II was expressed predominantly in mature osteoclasts, but not in the precursors. CA II activity was inhibited by sulfonamide-type COX-2 selective agents celecoxib and JTE-522 similarly to a CA II inhibitor acetazolamide, but not by a methylsulfone-type COX-2 inhibitor rofecoxib. In vitro assays clearly revealed that celecoxib and JTE-522 suppressed both differentiation and activity of osteoclasts, whereas rofecoxib and acetazolamide suppressed only osteoclast differentiation and activation, respectively. However, bone destruction in AIA rats was potently and similarly suppressed by all COX-2 selective agents whether with or without CA II inhibitory potency, although only moderately by acetazolamide. CONCLUSIONS: Suppression of osteoclast differentiation by COX-2 inhibition is more effective than suppression of mature osteoclast activity by CA II inhibition for the treatment of arthritic bone destruction.

Adjuvants, Immunologic↗
PubMed2005

The canonical Wnt signaling pathway promotes chondrocyte differentiation in a Sox9-dependent manner.

To better understand the role of the canonical Wnt signaling pathway in cartilage development, we adenovirally expressed a constitutively active (ca) or a dominant negative (dn) form of lymphoid enhancer factor-1 (LEF-1), the main nuclear effector of the pathway, in undifferentiated mesenchymal cells, chondrogenic cells, and primary chondrocytes, and examined the expression of markers for chondrogenic differentiation and hypertrophy. caLEF-1 and LiCl, an activator of the canonical pathway, promoted both chondrogenic differentiation and hypertrophy, whereas dnLEF-1 and the gene silencing of beta-catenin suppressed LiCl-promoted effects. To investigate whether these effects were dependent on Sox9, a master regulator of cartilage development, we stimulated Sox9-deficient ES cells with the pathway. caLEF-1 and LiCl promoted both chondrogenic differentiation and hypertrophy in wild-type, but not in Sox9-deficient, cells. The response of Sox9-deficient cells was restored by the adenoviral expression of Sox9. Thus, the canonical Wnt signaling pathway promotes chondrocyte differentiation in a Sox9-dependent manner.

Animals↗
PubMed2005

Insulin receptor substrate-1 is required for bone anabolic function of parathyroid hormone in mice.

Bone anabolic action of PTH has been suggested to be mediated by induction of IGF-I in osteoblasts; however, little is known about the molecular mechanism by which IGF-I leads to bone formation under the PTH stimulation. This study initially confirmed in mouse osteoblast cultures that PTH treatment increased IGF-I mRNA and protein levels and alkaline phosphatase activity, which were accompanied by phosphorylations of IGF-I receptor, insulin receptor substrate (IRS)-1 and IRS-2, essential adaptor molecules for the IGF-I signaling. To learn the involvement of IRS-1 and IRS-2 in the bone anabolic action of PTH in vivo, IRS-1-/- and IRS-2-/- mice and their respective wild-type littermates were given daily injections of PTH (80 mug/kg) or vehicle for 4 wk. In the wild-type mice, the PTH injection increased bone mineral densities of the femur, tibia, and vertebrae by 10-20% without altering the serum IGF-I level. These stimulations were similarly seen in IRS-2-/- mice; however, they were markedly suppressed in IRS-1-/- mice. Although the PTH anabolic effects were stronger on trabecular bones than on cortical bones, the stimulations on both bones were blocked in IRS-1-/- mice but not in IRS-2-/- mice. Histomorphometric and biochemical analyses showed an increased bone turnover by PTH, which was also blunted by the IRS-1 deficiency, though not by the IRS-2 deficiency. These results indicate that the PTH bone anabolic action is mediated by the activation of IRS-1, but not IRS-2, as a downstream signaling of IGF-I that acts locally as an autocrine/paracrine factor.

Animals↗
PubMed2005

Inhibition of Cdk6 expression through p38 MAP kinase is involved in differentiation of mouse prechondrocyte ATDC5.

Because a temporal arrest in the G1-phase of the cell cycle is a prerequisite for cell differentiation, this study investigated the involvement of cell cycle factors in the differentiation of cultured mouse prechondrocyte cell line ATDC5. Among the G1 cell cycle factors examined, both protein and mRNA levels of cyclin-dependent kinase (Cdk6) were downregulated during the culture in a differentiation medium. The protein degradation of Cdk6 was not involved in this downregulation because proteasome inhibitors did not reverse the protein level. When inhibitors of p38 MAPK, ERK-1/2, and PI3K/Akt were added to the culture, only a p38 MAPK inhibitor SB203580 blocked the decrease in the Cdk6 protein level by the differentiation medium, indicating that the Cdk6 inhibition was mediated by p38 MAPK pathway. In fact, p38 MAPK was confirmed to be phosphorylated during differentiation of ATDC5 cells. Enforced expression of Cdk6 in ATDC5 cells blocked the chondrocyte differentiation and inhibited Sox5 and Sox6 expressions. However, the Cdk6 overexpression did not affect the proliferation or the cell cycle progression, suggesting that the inhibitory effect of Cdk6 on the differentiation was exerted by a mechanism largely independent of its cell cycle regulation. These results indicate that Cdk6 may be a regulator of chondrocyte differentiation and that its p38-mediated downregulation is involved in the efficient differentiation.

Animals↗
PubMed2005

Involvement of cyclic guanosine monophosphate-dependent protein kinase II in chondrocyte hypertrophy during endochondral ossification.

During vertebrate skeletal development, the appendicular skeleton forms through endochondral ossification, which involves the intricately regulated multistep differentiation of mesenchymal cells. During this process, mesenchymal condensations initially differentiate into chondrocytes. Then chondrocytes in the center further differentiate into hypertrophic chondrocytes. Hypertrophic chondrocytes express a number of osteogenic factors and induce bone formation. Although numerous studies have provided novel insights into the regulation and function of cartilage development, little is known about the intracellular signaling pathways regulating chondrocyte hypertrophy. Recent study revealed that cyclic guanosine monophosphate (cGMP)-dependent protein kinase II (cGKII) coupled the stop of proliferation and the start of hypertrophic differentiation of chondrocytes. Herein, we review the molecular mechanism of regulation of chondrocyte hypertrophy by cGKII and the interaction between cGKII and other signaling pathways.

Journal Article↗
PubMed2005

Ectopic expression of vasopressin V1b and V2 receptors in the adrenal glands of familial ACTH-independent macronodular adrenal hyperplasia.

OBJECTIVE: ACTH-independent macronodular adrenal hyperplasia (AIMAH) is a rare and unusual cause of Cushing's syndrome, characterized by bilateral nodular adrenocortical hyperplasia and hypersecretion of cortisol. Familial AIMAH has rarely been reported. Recently, the aberrant expression of adrenal receptors for various ligands in AIMAH patients has become important in explaining the pathogenesis of AIMAH. In this study, we present the cases of two sisters who were affected with AIMAH. PATIENTS AND MEASUREMENTS: Two sisters, aged 46 and 58, respectively, at the time of diagnosis, were found to have Cushing's syndrome due to AIMAH. We performed provocation tests with various materials and reverse transcription polymerase chain reaction (RT-PCR) with their resected adrenal tissues to examine the hyper-responsiveness to steroidogenesis and the overexpression of related receptors, respectively. RESULTS: Provocation tests in vivo indicated that arginine vasopressin (AVP) promoted cortisol secretion through vasopressin V1a as well as V1b and V2 receptors. RT-PCR analysis revealed an abnormal cDNA expression of vasopressin V1b and V2 receptors, none of which is known to be normally expressed in the adrenal glands. CONCLUSION: The expression of ectopic vasopressin V1b and V2 receptors may be involved in the aetiology of AIMAH, at least in the case of the sibling patients presented here.

Adrenal Glands↗
PubMed2004

Surface grafting of artificial joints with a biocompatible polymer for preventing periprosthetic osteolysis.

Periprosthetic osteolysis-bone loss in the vicinity of a prosthesis-is the most serious problem limiting the longevity of artificial joints. It is caused by bone-resorptive responses to wear particles originating from the articulating surface. This study investigated the effects of graft polymerization of our original biocompatible phospholipid polymer 2-methacryloyloxyethyl phosphorylcholine (MPC) onto the polyethylene surface. Mechanical studies using a hip-joint simulator revealed that the MPC grafting markedly decreased the friction and the amount of wear. Osteoclastic bone resorption induced by subperiosteal injection of particles onto mouse calvariae was abolished by the MPC grafting on particles. MPC-grafted particles were shown to be biologically inert by culture systems with respect to phagocytosis and resorptive cytokine secretion by macrophages, subsequent expression of receptor activator of NF-kappaB ligand in osteoblasts, and osteoclastogenesis from bone marrow cells. From the mechanical and biological advantages, we believe that our approach will make a major improvement in artificial joints by preventing periprosthetic osteolysis.

Biocompatible Materials↗
PubMed2004

Cyclic GMP-dependent protein kinase II is a molecular switch from proliferation to hypertrophic differentiation of chondrocytes.

The Komeda miniature rat Ishikawa (KMI) is a naturally occurring mutant caused by an autosomal recessive mutation mri, which exhibits longitudinal growth retardation. Here we identified the mri mutation as a deletion in the rat gene encoding cGMP-dependent protein kinase type II (cGKII). KMIs showed an expanded growth plate and impaired bone healing with abnormal accumulation of postmitotic but nonhypertrophic chondrocytes. Ex vivo culture of KMI chondrocytes reproduced the differentiation impairment, which was restored by introducing the adenovirus-mediated cGKII gene. The expression of Sox9, an inhibitory regulator of hypertrophic differentiation, persisted in the nuclei of postmitotic chondrocytes of the KMI growth plate. Transfection experiments in culture systems revealed that cGKII attenuated the Sox9 functions to induce the chondrogenic differentiation and to inhibit the hypertrophic differentiation of chondrocytes. This attenuation of Sox9 was due to the cGKII inhibition of nuclear entry of Sox9. The impaired differentiation of cultured KMI chondrocytes was restored by the silencing of Sox9 through RNA interference. Hence, the present study for the first time shed light on a novel role of cGKII as a molecular switch, coupling the cessation of proliferation and the start of hypertrophic differentiation of chondrocytes through attenuation of Sox9 function.

Animals↗
PubMed2004

Impairment of bone healing by insulin receptor substrate-1 deficiency.

Insulin receptor substrate-1 (IRS-1) is an essential molecule for intracellular signaling of insulin-like growth factor (IGF)-I and insulin, both of which are potent anabolic regulators of bone and cartilage metabolism. To investigate the role of IRS-1 in bone regeneration, fracture was introduced in the tibia, and its healing was compared between wild-type (WT) mice and mice lacking the IRS-1 gene (IRS-1(-/-) mice). Among 15 IRS-1(-/-) mice, 12 remained in a non-union state even at 10 weeks after the operation, whereas all 15 WT mice showed a rigid bone union at 3 weeks. This impairment was because of the suppression of callus formation with a decrease in chondrocyte proliferation and increases in hypertrophic differentiation and apoptosis. Reintroduction of IRS-1 to the IRS-1(-/-) fractured site using an adenovirus vector significantly restored the callus formation. In the culture of chondrocytes isolated from the mouse growth plate, IRS-1(-/-) chondrocytes showed less mitogenic ability and Akt phosphorylation than WT chondrocytes. An Akt inhibitor decreased the IGF-I-stimulated DNA synthesis of chondrocytes more potently in the WT culture than in the IRS-1(-/-) culture. We therefore conclude that IRS-1 deficiency impairs bone healing at least partly by inhibiting chondrocyte proliferation through the phosphatidylinositol 3-kinase/Akt pathway, and we propose that IRS-1 can be a target molecule for bone regenerative medicine.

Adenoviridae↗
PubMed2004

PPARgamma insufficiency enhances osteogenesis through osteoblast formation from bone marrow progenitors.

Based on the fact that aging is associated with a reciprocal decrease of osteogenesis and an increase of adipogenesis in bone marrow and that osteoblasts and adipocytes share a common progenitor, this study investigated the role of PPARgamma, a key regulator of adipocyte differentiation, in bone metabolism. Homozygous PPARgamma-deficient ES cells failed to differentiate into adipocytes, but spontaneously differentiated into osteoblasts, and these were restored by reintroduction of the PPARgamma gene. Heterozygous PPARgamma-deficient mice exhibited high bone mass with increased osteoblastogenesis, but normal osteoblast and osteoclast functions, and this effect was not mediated by insulin or leptin. The osteogenic effect of PPARgamma haploinsufficiency became prominent with aging but was not changed upon ovariectomy. The PPARgamma haploinsufficiency was confirmed to enhance osteoblastogenesis in the bone marrow cell culture but did not affect the cultures of differentiated osteoblasts or osteoclast-lineage cells. This study demonstrates a PPARgamma-dependent regulation of bone metabolism in vivo, in that PPARgamma insufficiency increases bone mass by stimulating osteoblastogenesis from bone marrow progenitors.

Animals↗
PubMed2004

Deficiency of insulin receptor substrate-1 impairs skeletal growth through early closure of epiphyseal cartilage.

UNLABELLED: Morphological analyses in and around the epiphyseal cartilage of mice deficient in insulin receptor substrate-1 (IRS-1) showed IRS-1 signaling to be important for skeletal growth by preventing early closure of the epiphyseal cartilage and maintaining the subsequent bone turnover at the primary spongiosa. INTRODUCTION: IRS-1 is an essential molecule for intracellular signaling by IGF-I and insulin, both of which are potent anabolic regulators of cartilage and bone metabolism. To clarify the role of IRS-1 signaling in the skeletal growth, morphological analyses were performed in and around the epiphyseal cartilage of mice deficient in IRS-1 (IRS-1(-/-)), whose limbs and trunk were 20-30% shorter than wildtype (WT) mice. MATERIALS AND METHODS: The epiphyseal cartilage and the primary spongiosa at proximal tibias of homozygous IRS-1(-/-) and WT male littermates were compared using histological, immunohistochemical, enzyme cytohistochemical, ultrastructural, and bone histomorphometrical analyses. RESULTS: In and around the WT epiphyseal cartilage, IRS-1 and insulin-like growth factor (IGF)-1 receptors were widely expressed, whereas IRS-2 was weakly localized in bone cells. Chronological observation revealed that height of the proliferative zone and the size of hypertrophic chondrocytes were decreased in WT mice as a function of age, and these decreases were accelerated in the IRS-1 (-/-) cartilage, whose findings at 12 weeks were similar to those of WT at 24 weeks. In the IRS-1(-/-) cartilage, proliferating chondrocytes with positive proliferating cell nuclear antigen (PCNA) or parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor immunostaining had almost disappeared by 12 weeks. Contrarily, TUNEL+ apoptotic cells were increased in the hypertrophic zone, at the bottom of which most of the chondrocytes were surrounded by the calcified matrix, suggesting the closure of the cartilage. In the primary spongiosa, bone volume, alkaline phosphatase (ALP)+ osteoblasts, TRACP+ osteoclasts, and the osteopontin-positive cement line were markedly decreased. Bone histomorphometrical parameters for both bone formation and resorption were significantly lower in IRS-1(-/-) mice, indicating the suppression of bone turnover. CONCLUSION: The IRS-1(-/-) epiphyseal cartilage exhibited insufficient proliferation of chondrocytes, calcification of hypertrophic chondrocytes, acceleration of apoptosis, and early closure of the growth plate. Thus, the data strongly suggest that IRS-1 signaling is important for the skeletal growth by preventing early closure of the epiphyseal cartilage and by maintaining the subsequent bone turnover at the primary spongiosa.

Animals↗
PubMed2004

[BMP and bone formation].

Bone Morphogenetic Proteins (BMPs) are secreted proteins that represent a group of the transforming growth factor-beta. BMPs were identified as molecules that induce ectopic bone formation in muscles of mice. The homo- or heterodimetric BMP proteins, which bind to serine/threonine protein-kinase receptor and activate smad and p38MAPK signal transductions, function as potent osteogenic factors. In vivo, it is not clear which BMPs play crucial roles in regulating osteogenesis. Clinical uses of rhBMP-2 and rhBMP-7 have been performed, but we need numerous rhBMPs, which is very expensive, to induce osteogenesis in humans.

Animals↗