Expression and activity of the CDK inhibitor p57Kip2 in chondrocytes undergoing hypertrophic differentiation

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

Functional Versatility of the CDK Inhibitor p57Kip2

The cyclin/CDK inhibitor p57^Kip2 belongs to the Cip/Kip family, with p21^Cip1 and p27^Kip1, and is the least studied member of the family. Unlike the other family members, p57^Kip2 has a unique role during embryogenesis and is the only CDK inhibitor required for embryonic development. p57^Kip2 is encoded by the imprinted gene CDKN1C, which is the gene most frequently silenced or mutated in the genetic disorder Beckwith-Wiedemann syndrome (BWS), characterized by multiple developmental anomalies. Although initially identified as a cell cycle inhibitor based on its homology to other Cip/Kip family proteins, multiple novel functions have been ascribed to p57^Kip2 in recent years that participate in the control of various cellular processes, including apoptosis, migration and transcription. Here, we will review our current knowledge on p57^Kip2 structure, regulation, and its diverse functions during development and homeostasis, as well as its potential implication in the development of various pathologies, including cancer.

Notch signaling indirectly promotes chondrocyte hypertrophy via regulation of BMP signaling and cell cycle arrest

Cell cycle regulation is critical for chondrocyte differentiation and hypertrophy. Recently we identified the Notch signaling pathway as an important regulator of chondrocyte proliferation and differentiation during mouse cartilage development. To investigate the underlying mechanisms, we assessed the role for Notch signaling regulation of the cell cycle during chondrocyte differentiation. Real-time RT-PCR data showed that over-expression of the Notch Intracellular Domain (NICD) significantly induced the expression of p57, a cell cycle inhibitor, in chondrocytes. Flow cytometric analyses further confirmed that over-expression of NICD in chondrocytes enhances the G0/G1 cell cycle transition and cell cycle arrest. In contrast, treatment of chondrocytes with the Notch inhibitor, DAPT, decreased both endogenous and BMP2-induced SMAD 1/5/8 phosphorylation and knockdown of SMAD 1/5/8 impaired NICD-induced chondrocyte differentiation and p57 expression. Co-immunoprecipitation using p-SMAD 1/5/8 and NICD antibodies further showed a strong interaction of these proteins during chondrocyte maturation. Finally, RT-PCR and Western blot results revealed a significant reduction in the expression of the SMAD-related phosphatase, PPM1A, following NICD over-expression. Taken together, our results demonstrate that Notch signaling induces cell cycle arrest and thereby initiates chondrocyte hypertrophy via BMP/SMAD-mediated up-regulation of p57.

mTORC1 regulates PTHrP to coordinate chondrocyte growth, proliferation and differentiation

Precise coordination of cell growth, proliferation and differentiation is essential for the development of multicellular organisms. Here, we report that although the mechanistic target of rapamycin complex 1 (mTORC1) activity is required for chondrocyte growth and proliferation, its inactivation is essential for chondrocyte differentiation. Hyperactivation of mTORC1 via TSC1 gene deletion in chondrocytes causes uncoupling of the normal proliferation and differentiation programme within the growth plate, resulting in uncontrolled cell proliferation, and blockage of differentiation and chondrodysplasia in mice. Rapamycin promotes chondrocyte differentiation and restores these defects in mutant mice. Mechanistically, mTORC1 downstream kinase S6K1 interacts with and phosphorylates Gli2, and releases Gli2 from SuFu binding, resulting in nuclear translocation of Gli2 and transcription of parathyroid hormone-related peptide (PTHrP), a key regulator of bone development. Our findings demonstrate that dynamically controlled mTORC1 activity is crucial to coordinate chondrocyte proliferation and differentiation partially through regulating Gli2/PTHrP during endochondral bone development.

Promyelocytic leukemia zinc finger mediates glucocorticoid-induced cell cycle arrest in the chondroprogenitor cell line ATDC5

Glucocorticoids (GCs) affect the proliferation of growth plate chondrocytes. In this study, we investigated the role of the GC-inducible promyelocytic leukemia zinc finger (PLZF) gene in chondrocyte differentiation by using the chondrogenic cell line ATDC5. PLZF overexpression suppressed cell cycle progression (p < 0.01) and promoted differentiation into hypertrophic chondrocytes by inducing mRNA expression of alkaline phosphatase (p < 0.01), and the cyclin-dependent kinase (CDK) inhibitor p21 (p < 0.01). In contrast, PLZF knockdown impaired differentiation into hypertrophic chondrocytes and promoted cell cycle progression (p < 0.01). Treatment with the GC analogue dexamethasone (10(-6) M) suppressed cell cycle progression in ATDC5 cells. PLZF shRNA attenuated dexamethasone-induced cell cycle arrest (p < 0.01) by downregulating the mRNA expression of the CDK inhibitors p21 and p57 (p < 0.01). These results clearly indicated that PLZF promoted differentiation into hypertrophic chondrocytes and mediated dexamethasone-induced cell cycle arrest by regulating CDK inhibitors.

Liver X Receptor activation delays chondrocyte hypertrophy during endochondral bone growth

OBJECTIVE

Activation of the Liver X Receptor (LXR) has recently been identified as a therapeutic strategy for osteoarthritis (OA). Human OA articular cartilage explants show decreased LXR expression, and LXRβ-null mice display OA-like symptoms. LXR agonist administration to OA articular cartilage explants suppresses proteoglycan degradation and restores LXR-activated transcription. We aimed to investigate the effect of LXR activation on chondrocyte differentiation to elucidate the molecular mechanisms behind its protection against OA.

METHOD

The specific LXR agonist, GW3965, was used to examine the effect of LXR activation on chondrocyte differentiation. Tibia organ cultures were used to examine the effect of LXR activation on bone growth and growth plate morphology, followed by immunohistochemical analysis. In ATDC5 and micromass cultures, chondrocyte differentiation was examined through cellular staining and proliferation assays. Various chondrogenic markers were analyzed by real-time reverse-transcription polymerase chain reaction (qRT-PCR) in micromass RNA.

RESULTS

Chondrocyte hypertrophy was suppressed by GW3965 treatment, as shown by decreased hypertrophic zone length in the tibial growth plate, decreased alkaline phosphatase staining in ATDC5 and micromass cultures, and down regulation of Col10a1, Mmp13 and Runx2 expression. Increased proliferation in treated ATDC5 cells and up-regulation of Col2a1 expression in treated micromass cultures suggest hypertrophy is suppressed secondary to prolonged proliferation. Decreased p57 levels in treated growth plates suggest this to be due to cell-cycle exit delay.

CONCLUSION

Our findings regarding LXR's role in cartilage development provide insight into how LXR activation prevents cartilage breakdown, further solidifying its potential as a therapeutic target of OA.

Interleukin-1β promotes proliferation and inhibits differentiation of chondrocytes through a mechanism involving down-regulation of FGFR-3 and p21

The proinflammatory cytokine IL-1β is elevated in many childhood chronic inflammatory diseases as well as obesity and can be associated with growth retardation. Here we show that IL-1β affects bone growth by directly disturbing the normal sequence of events in the growth plate, resulting in increased proliferation and widening of the proliferative zone, whereas the hypertrophic zone becomes disorganized, with impaired matrix structure and increased apoptosis and osteoclast activity. This was also evident in vitro: IL-1β increased proliferation and caused a G1-to-S phase shift in the cell cycle in ATDC5 chondrocytes, accompanied by a reduction in fibroblast growth factor receptor-3 (FGFR-3) and its downstream gene, the cell-cycle inhibitor p21 and its family member p57, whereas the cell-cycle promoter E2F-2 was increased. The reduction in FGFR-3, p21, and p57 was followed by delayed cell differentiation, manifested by decreases in proteoglycan synthesis, mineralization, alkaline phosphatase activity, and the expression of Sox9, RunX2, collagen type II, collagen type X, and other matrix proteins. Taken together, we suggest that IL-1β alters normal chondrogenesis and bone growth through a mechanism involving down-regulation of FGFR-3 and p21.

p57(Kip2) and cancer: time for a critical appraisal

p57(Kip2) is a cyclin-dependent kinase inhibitor belonging to the Cip/Kip family, which also includes p21(Cip1) and p27(Kip1). So far, p57(Kip2) is the least-studied Cip/Kip protein, and for a long time its relevance has been related mainly to its unique role in embryogenesis. Moreover, genetic and molecular studies on animal models and patients with Beckwith-Wiedemann syndrome have shown that alterations in CDKN1C (the p57(Kip2) encoding gene) have functional relevance in the pathogenesis of this disease. Recently, a number of investigations have identified and characterized heretofore unexpected roles for p57(Kip2). The protein appears to be critically involved in initial steps of cell and tissue differentiation, and particularly in neuronal development and erythropoiesis. Intriguingly, p27(Kip1), the Cip/Kip member that is most homologous to p57(Kip2), is primarily involved in the process of cell cycle exit. p57(Kip2) also plays a critical role in controlling cytoskeletal organization and cell migration through its interaction with LIMK-1. Furthermore, p57(Kip2) appears to modulate genome expression. Finally, accumulating evidence indicates that p57(Kip2) protein is frequently downregulated in different types of human epithelial and nonepithelial cancers as a consequence of genetic and epigenetic events. In summary, the emerging picture is that several aspects of p57(Kip2)'s functions are only poorly clarified. This review represents an appraisal of the data available on the p57(Kip2) gene and protein structure, and its role in human physiology and pathology. We particularly focus our attention on p57(Kip2) changes in cancers and pharmacological approaches for modulating p57(Kip2) levels.

Deletion of glycogen synthase kinase-3β in cartilage results in up-regulation of glycogen synthase kinase-3α protein expression

The rate of endochondral bone growth determines final height in humans and is tightly controlled. Glycogen synthase kinase-3 (GSK-3) is a negative regulator of several signaling pathways that govern bone growth, such as insulin/IGF and Wnt/β-catenin. The two GSK-3 proteins, GSK-3α and GSK-3β, display both overlapping and distinct roles in different tissues. Here we show that pharmacological inhibition of GSK-3 signaling in a mouse tibia organ culture system results in enhanced bone growth, accompanied by increased proliferation of growth plate chondrocytes and faster turnover of hypertrophic cartilage to bone. GSK-3 inhibition rescues some, but not all, effects of phosphatidylinositide 3-kinase inhibition in this system, in agreement with the antagonistic role of these two kinases in response to signals such as IGF. However, cartilage-specific deletion of the Gsk3b gene in mice has minimal effects on skeletal growth or development. Molecular analyses demonstrated that compensatory up-regulation of GSK-3α protein levels in cartilage is the likely cause for this lack of effect. To our knowledge, this is the first tissue in which such a compensatory mechanism is described. Thus, our study provides important new insights into both skeletal development and the biology of GSK-3 proteins.

Developmental mechanisms in articular cartilage degradation in osteoarthritis

Osteoarthritis is the most common arthritic condition, which involves progressive degeneration of articular cartilage. The most recent accomplishments have significantly advanced our understanding on the mechanisms of the disease development and progression. The most intriguing is the growing evidence indicating that extracellular matrix destruction in osteoarthritic articular cartilage resembles that in the hypertrophic zone of fetal growth plate during endochondral ossification. This suggests common regulatory mechanisms of matrix degradation in OA and in the development and can provide new approaches for the treatment of the disease by targeting reparation of chondrocyte phenotype.

Microtopographically patterned surfaces promote the alignment of tenocytes and extracellular collagen

This paper investigates the role of microtopographical features on the cytomorphology, alignment, proliferation and gene expression of tenocytes. We made use of simple microfabrication approaches to create surfaces patterned with topographical features suitable for in vitro studies of tenocytes. These surfaces were composed of glass substrates patterned with polymeric ridges spaced from 50 to 250 microm apart. Our studies demonstrate that the microgrooves differentially impact tenocyte shape, alignment and matrix organization along the direction of grooves. Groove widths significantly influenced cellular alignment, with 50 microm grooved patterns affecting alignment most substantially. Polarized light microscopy demonstrated that mature collagen fibers were denser and more oriented within 50 microm patterns. None of the patterns had a significant effect on the expression of genes linked to proliferation or extracellular matrix synthesis, although time in culture profoundly influenced both gene groups. COMP mRNA expression was moderately increased in tenocytes seeded onto 250 microm grooves, but there was no overall beneficial phenotypic effect of aligned growth. The results of this study indicate that microtopography affects cell density and alignment of tenocytes and leads to the deposition of an aligned collagen matrix, but does not significantly impact matrix gene expression or cell phenotype. These outcomes provide insights into the biology of tendon regeneration, thus providing guidance in the design of clinical procedures for tendon repair.

A new technique for seeding chondrocytes onto solvent-preserved human meniscus using the chemokinetic effect of recombinant human bone morphogenetic protein-2

Many investigators are currently studying the use of decellularized tissue allografts from human cadavers as scaffolds onto which patients' cells could be seeded, or as carriers for genetically engineered cells to aid cell transplantation. However, it is difficult to seed cells onto very dense regular connective tissue which has few interstitial spaces. Here, we discuss the development of a chemotactic cell seeding technique using solvent-preserved human meniscus. A chemokinetic response to recombinant human bone morphogenetic protein-2 (rhBMP-2) was observed in a monolayer culture of primary chondrocytes derived from femoral epiphyseal cartilage of 2-day-old rats. The rhBMP-2 significantly increased their migration upto 10 ng/ml in a dose-dependent manner. When tested with solvent-preserved human meniscus as a scaffold, which has few interstitial spaces, rhBMP-2 was able to induce chondrocytes to migrate into the meniscus. After a 3-week incubation, newly-formed cartilaginous extracellular matrix was synthesized by migrated chondrocytes throughout the meniscus, down to a depth of 3 mm. These findings demonstrate that rhBMP-2 may be a natural chemokinetic factor in vivo, which induces migration of proliferative chondrocytes into the narrow interfibrous spaces. Our results suggest a potential application of rhBMP-2 for the designed distribution of chondrocytes into a scaffold to be used for tissue engineering.

CDKN1C negatively regulates RNA polymerase II C-terminal domain phosphorylation in an E2F1-dependent manner

CDKN1C is a cyclin-dependent kinase inhibitor and is a candidate tumor suppressor gene. We previously found that the CDKN1C protein represses E2F1-driven transcription in an apparent negative feedback loop. Herein, we explore the mechanism by which CDKN1C represses transcription. We find that adenoviral-mediated overexpression of CDKN1C leads to a dramatic reduction in phosphorylation of the RNA polymerase II (pol II) C-terminal domain (CTD). RNA interference studies demonstrate that this activity is not an artifact of CDKN1C overexpression, because endogenous CDKN1C mediates an inhibition of RNA pol II CTD phosphorylation in HeLa cells upon treatment with dexamethasone. Surprisingly, we find that CDKN1C-mediated repression of RNA pol II phosphorylation is E2F1-dependent, suggesting that E2F1 may direct CDKN1C to chromatin. Chromatin immunoprecipitation assays demonstrate that CDKN1C is associated with E2F1-regulated promoters in vivo and that this association can dramatically reduce the level of RNA pol II CTD phosphorylation at both Ser-2 and Ser-5 of the C-terminal domain repeat. In addition, we show that CDKN1C interacts with both CDK7 and CDK9 (putative RNA pol II CTD kinases) and that CDKN1C blocks their ability to phosphorylate a glutathione S-transferase-CTD fusion protein in vitro. E2F1 and CDKN1C are found to form stable complexes both in vivo and in vitro. Molecular studies demonstrate that the E2F1-CDKN1C interaction is mediated by two E2F domains. A central E2F1 domain interacts directly with CDKN1C, whereas a C-terminal E2F1 domain interacts with CDKN1C via interaction with Rb. The results presented in this report highlight a novel mechanism of tumor suppression by CDKN1C.

p57KIP2: "Kip"ing the cell under control

p57(KIP2) is an imprinted gene located at the chromosomal locus 11p15.5. It is a cyclin-dependent kinase inhibitor belonging to the CIP/KIP family, which includes additionally p21(CIP1/WAF1) and p27(KIP1). It is the least studied CIP/KIP member and has a unique role in embryogenesis. p57(KIP2) regulates the cell cycle, although novel functions have been attributed to this protein including cytoskeletal organization. Molecular analysis of animal models and patients with Beckwith-Wiedemann Syndrome have shown its nodal implication in the pathogenesis of this syndrome. p57(KIP2) is frequently down-regulated in many common human malignancies through several mechanisms, denoting its anti-oncogenic function. This review is a thorough analysis of data available on p57(KIP2), in relation to p21(CIP1/WAF1) and p27(KIP1), on gene and protein structure, its transcriptional and translational regulation, and its role in human physiology and pathology, focusing on cancer development.

Chondrocyte p21(WAF1/CIP1) expression is increased by dexamethasone but does not contribute to dexamethasone-induced growth retardation in vivo

It has been shown that cell cycle genes play an important role in the coordination of chondrocyte proliferation and differentiation. The inhibitory effects of glucocorticoids (GCs) on chondrocyte proliferation are consistent with GCs disrupting cell cycle progression and promoting cell cycle exit. Cyclin-dependent kinase inhibitors (CDKIs) force cells to exit the cell cycle and differentiate, and studies have shown that expression of the CDKI p21(CIP1/WAF1) is increased in terminally differentiated cells. In this study, p21 mRNA and protein expression was increased during chondrocyte differentiation and after exposure to dexamethasone (Dex, 10(-6 )M) in murine chondrogenic ATDC5 cells. In 4-week-old mice lacking a functional p21 gene, Dex caused a reduction in body weight compared to saline control null mice, but this was consistent with the reduction in body weight observed in Dex-treated wild-type littermates. In addition, p21 ablation had no effect on the reduction in width of the growth plate or reduced mineral apposition rate in Dex-treated mice. However, an alteration in growth rate and epiphyseal structure is evident when comparing p21(-/-) and wild-type mice. These findings suggest that p21 does not directly contribute to GC-induced growth retardation in vivo but is involved in the maintenance of the growth plate.

Cip/Kip cyclin-dependent protein kinase inhibitors and the road to polyploidy

Cyclin-dependent kinases (CDKs) play a central role in the orderly transition from one phase of the eukaryotic mitotic cell division cycle to the next. In this context, p27Kip1 (one of the CIP/KIP family of CDK specific inhibitors in mammals) or its functional analogue in other eukarya prevents a premature transition from G1 to S-phase. Recent studies have revealed that expression of a second member of this family, p57Kip2, is induced as trophoblast stem (TS) cells differentiate into trophoblast giant (TG) cells. p57 then inhibits CDK1 activity, an enzyme essential for initiating mitosis, thereby triggering genome endoreduplication (multiple S-phases without an intervening mitosis). Expression of p21Cip1, the third member of this family, is also induced in during differentiation of TS cells into TG cells where it appears to play a role in suppressing the DNA damage response pathway. Given the fact that p21 and p57 are unique to mammals, the question arises as to whether one or both of these proteins are responsible for the induction and maintenance of polyploidy during mammalian development.

The PI3K pathway regulates endochondral bone growth through control of hypertrophic chondrocyte differentiation

Background

The majority of our bones develop through the process of endochondral ossification that involves chondrocyte proliferation and hypertrophic differentiation in the cartilage growth plate. A large number of growth factors and hormones have been implicated in the regulation of growth plate biology, however, less is known about the intracellular signaling pathways involved. PI3K/Akt has been identified as a major regulator of cellular proliferation, differentiation and death in multiple cell types.

Results and Discussion

Employing an organ culture system of embryonic mouse tibiae and LY294002, a pharmacological inhibitor of PI3K, we show that inhibition of the pathway results in significant growth reduction, demonstrating that PI3K is required for normal endochondral bone growth in vitro. PI3K inhibition reduces the length of the proliferating and particularly of the hypertrophic zone. Studies with organ cultures and primary chondrocytes in micromass culture show delayed hypertrophic differentiation of chondrocytes and increased apoptosis in the presence of LY294002. Surprisingly, PI3K inhibition had no strong effect on IGF1-induced bone growth, but partially blocked the anabolic effects of C-type natriuretic peptide.

Conclusion

Our data demonstrate an essential role of PI3K signaling in chondrocyte differentiation and as a consequence of this, in the endochondral bone growth process.

Cooperation between p27 and p107 during endochondral ossification suggests a genetic pathway controlled by p27 and p130

Pocket proteins and cyclin-dependent kinase (CDK) inhibitors negatively regulate cell proliferation and can promote differentiation. However, which members of these gene families, which cell type they interact in, and what they do to promote differentiation in that cell type during mouse development are largely unknown. To identify the cell types in which p107 and p27 interact, we generated compound mutant mice. These mice were null for p107 and had a deletion in p27 that prevented its binding to cyclin-CDK complexes. Although a fraction of these animals survived into adulthood and looked similar to single p27 mutant mice, a larger number of animals died at birth or within a few weeks thereafter. These animals displayed defects in chondrocyte maturation and endochondral bone formation. Proliferation of chondrocytes was increased, and ectopic ossification was observed. Uncommitted mouse embryo fibroblasts could be induced into the chondrocytic lineage ex vivo, but these cells failed to mature normally. These results demonstrate that p27 carries out overlapping functions with p107 in controlling cell cycle exit during chondrocyte maturation. The phenotypic similarities between p107(-/-) p27(D51/D51) and p107(-/-) p130(-/-) mice and the cells derived from them suggest that p27 and p130 act in an analogous pathway during chondrocyte maturation.

Transcriptional upregulation of p57 (Kip2) by the cyclin-dependent kinase inhibitor BMS-387032 is E2F dependent and serves as a negative feedback loop limiting cytotoxicity

In spite of the fact that cyclin-dependent kinase (cdk) inhibiting drugs are potent transcriptional repressors, we discover that p57 (Kip2, CDKN1C) transcription is significantly upregulated by three small molecule cdk inhibitors, including BMS-387032. Treatment of MDA-MB-231 breast cancer cells with BMS-387032 led to a stabilization of the E2F1 protein that was accompanied by significant increases in the p57 mRNA and protein. This increase did not occur in an E2F1-deficient cell line. An E2F1-estrogen receptor fusion protein activated the endogenous p57 promoter in response to hydroxytamoxifen treatment in the presence of cycloheximide. Luciferase constructs driven by the p57 promoter verified that upregulation of p57 mRNA by BMS-387032 is transcriptional and dependent on E2F-binding sites in the promoter. Expression of exogenous p57 significantly decreased the fraction of cells in S phase. Furthermore, p57-deficient MDA-MB-231 cell lines were significantly more sensitive to BMS-387032-induced apoptosis than controls. The results presented in this manuscript demonstrate that small molecule cdk inhibitors transcriptionally activate p57 dependent upon E2F1 and that this activation in turn serves to limit E2F1's death-inducing activity.

Expression of p57-Kip2 in monocytes and macrophages

The p57-Kip2 gene encodes a cyclin-dependent kinase inhibitor and hence this gene has received much attention in the study of malignancy. We have analysed expression of this gene in human monocytes and macrophages. In comparison to CD14++ monocytes, p57-Kip2 expression was higher in both CD14+16+ monocytes and alveolar macrophages. p57-Kip2 expression decreased in CD14++ monocytes after stimulation with lipopolysaccharide but increased after incubation with methylprednisolone. The results indicate that p57-Kip2 may be involved in regulating the inflammatory response of monocytic cells.

ADAMTS5-mediated aggrecanolysis in murine epiphyseal chondrocyte cultures

OBJECTIVE

Aggrecan degradation by aggrecanases [a disintegrin and metalloproteinase with thrombospondin-like motifs (ADAMTS) 1, 4, 5, 8, 9, 15] is considered to initiate much of the cartilage pathology seen in human arthritis, however, the proteinase responsible and its mode of control is unclear. The present work was done to examine mechanisms of aggrecanase control in a novel murine epiphyseal cell system and to determine whether ADAMTS5 alone is responsible for aggrecanolysis by these cells.

METHODS

Epiphyseal cells from 4-day-old mice (wild type, TS-5 (-/-), CD44(-/-), syndecan-1(-/-), membrane type-4 matrix metalloproteinase [MT4MMP(-/-)]) were maintained in non-adherent aggregate cultures and aggrecanolysis studied by biochemical and histochemical methods. Confocal immunolocalization analyses were done with specific probes for ADAMTS5, hyaluronan (HA) and aggrecanase-generated fragments of aggrecan.

RESULTS

Aggrecanolysis by these cells was specifically aggrecanase-mediated and it occurred spontaneously without the need for addition of catabolic stimulators. Chondrocytes from ADAMTS5-null mice were aggrecanase-inactive whereas all other mutant cells behaved as wild type in this regard suggesting that ADAMTS5 activity is not controlled by CD44, syndecan-1 or MT4MMP in this system. Immunohistochemical analysis supported the central role for ADAMTS5 in the degradative pathway and indicated that aggrecanolysis occurs primarily in the HA-poor pericellular region in these cultures.

CONCLUSION

These findings are consistent with published in vivo studies showing that single-gene ADAMTS5 ablation confers significant protection on cartilage in murine arthritis. We propose that this culture system and the analytical approaches described provide a valuable framework to further delineate the expression, activity and control of ADAMTS-mediated aggrecanolysis in human arthritis.

Downregulation of cyclin-dependent kinase inhibitor; p57(kip2), is involved in the cell cycle progression of vascular smooth muscle cells

Immature vascular smooth muscle cells (VSMCs) proliferate responding to extrinsic mitogens and accumulate in neointima after arterial injuries. Cell proliferation is positively regulated by cyclin/cyclin-dependent kinase (CDK) complex and negatively controlled by CDK inhibitors; CKIs such as p27(kip1) and p57(kip2). In this study, embryonic rat thoracic aorta VSMCs; A10 were G0/G1 arrested by serum starvation, re-stimulated with serum, and harvested every four hours. Both CKIs co-expressed in quiescent VSMCs and rapidly diminished by stimulation. Protein level of p27(kip1) was regulated by both transcription and post-transcription, but that of p57(kip2) was mainly by post-transcription. Supplemental overexpression of p57(kip2) inhibited the activations of G1 cyclin/CDKs and subsequent hyperphosphorylations of all three retinoblastoma pocket proteins as well as G1/S transition of cell cycle. Our findings suggest that the downregulations of not only p27(kip1), but also p57(kip2) responding to mitogenic stimulation, play key roles in the cell cycle progression of VSMCs.

Quantitative transcriptional profiling of ATDC5 mouse progenitor cells during chondrogenesis

During the differentiation of a mouse chondroprogenitor cell line, ATDC5, an analysis of the transcription cartilage-related genes was carried out using real-time RT-PCR in a semiquantitative fashion. A total number of 104 genes both previously linked to chondrogenesis and hitherto not associated with the development of cartilage were analyzed. Parametric statistics, and unsupervised hierarchical and K-medians clustering approaches were used to analyze the gene expression during the sequential processes of proliferation, condensation, differentiation, maturation, and hypertrophic conversion of ATDC5 cells. The obtained data provided a robust determination of expression patterns that make possible an accurate assessment of the molecular events along the chondrogenic differentiation pathway. In addition, time-course expression profiles were described for eight highly regulated genes that have not been associated with chondrogenesis as yet. These included Cryab, Rcor2, Hig1, Bnip3, Mst4, Calml4, Gng2, and Islr.

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.

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.

Cell-cycle control and the cartilage growth plate

The longitudinal growth of endochondral bones is governed by proliferation and hypertrophic differentiation of growth plate chondrocytes. Numerous growth factors and hormones have been implicated in the regulation of these processes, but the intracellular mechanisms involved remain much less understood. We had suggested a role of cell-cycle genes in the integration of these diverse extracellular signals and their translation into coordinated proliferation and differentiation of chondrocytes. Numerous recent studies have provided support for such a scenario and provide novel insights into the regulation and function of cell-cycle genes in chondrocytes. This review article summarizes recent progress in the field.