Establishment of an osseous cell line from fetal rat calvaria using an immunocytolytic method of cell selection: characterization of the cell line and of derived clones

Quantification of glycosaminoglycans by reversed-phase HPLC separation of fluorescent isoindole derivatives

Glycosaminoglycans (GAGs) are linear polysaccharides made by all animal cells. GAGs bind to hundreds of proteins, such as growth factors, cytokines, chemokines, extracellular matrix components, protease inhibitors, proteases, and lipoprotein lipase, through carbohydrate and protein interactions. These interactions control many multicellular processes. The increased use of GAGs isolated from cells and small tissue samples in bioassays and binding experiments demands a sensitive and robust quantification method. We have developed such a method, which is based on a popular assay for amino acid analysis. We have refined it to enhance GAG quantification. It allows the quantification of glucosamine- and galactosamine-containing GAGs after the reversed-phase separation of their fluorescent isoindole derivatives. The derivatives are created by the reaction of o-phthaldialdehyde and 3-mercaptopropionic acid (3MPA) with the amino group of hexosaminitol monosaccharides generated from GAG acid hydrolysis and sodium borohydride reduction. The advantages of our method include automatic derivitization, a simple chromatograph with clean separation of glucosaminitol and galactosaminitol derivatives from contaminating amino acids, excellent sensitivity with 0.04 pmol detection, and linearity from 2.5 to 1280 pmol. A major advantage is that it can be readily implemented in any laboratory with typical reversed-phase high performance liquid chromatography (HPLC) equipment.

Regulation of Indian hedgehog mRNA levels in chondrocytic cells by ERK1/2 and p38 mitogen-activated protein kinases

Indian hedgehog (Ihh) is produced by growth plate pre-hypertrophic chondrocytes, and is an important regulator of endochondral ossification. However, little is known about the regulation of Ihh in chondrocytes. We have examined the role of integrins and mitogen-activated protein (MAP) kinases in Ihh mRNA regulation in CFK-2 chondrocytic cells. Cells incubated with the beta1-integrin blocking antibody had decreased Ihh mRNA levels, which was accompanied by decreases of activated extracellular signal-regulated kinases (ERK1/2) and activated p38 MAPK. Ihh mRNA levels were also inhibited by U0126, a specific MEK1/2 inhibitor, or SB203580, a specific p38 MAPK inhibitor. Cells transfected with constitutively active MEK1 or MKK3 had increased Ihh mRNA levels, which were diminished by dominant-negative MEK1, p38alpha or p38beta. Stimulation of the PTH1R with 10(-8) M rPTH (1-34) resulted in dephosphorylation of ERK1/2 that was evident within 15 min and sustained for 1 h, as well as transient dephosphorylation of p38 MAPK that was maximal after 25 min. PTH stimulation decreased Ihh mRNA levels, and this effect was blocked by transfecting the cells with constitutively active MEK1 but not by MKK3. These studies demonstrated that activation of ERK1/2 or p38 MAPK increased Ihh mRNA levels. Stimulation of the PTH1R or blocking of beta1-integrin resulted in inhibition of ERK1/2 and p38 MAPK and decreased levels of Ihh mRNA. Our data demonstrate the central role of MAPK in the regulation of Ihh in CFK-2 cells.

Fibroblast Growth Factor Receptor 3 Gene: Regulation by Serum Response Factor

We have previously identified a cis-acting sequence in the proximal promoter of the fibroblast growth factor receptor 3 (FGFR3) gene that strongly activates transcription in chondrocytic cells. Here we report that the transcriptional activity of this sequence (FRE3) requires serum response factor and its cognate recognition motif, serum response element. Although the FRE3 contains consensus sequence motifs for several transcription factors, the serum response element is paramount for the transcriptional activity of the FRE3. Additionally, the transcriptional activity of the proximal promoter of the FGFR3 gene is suppressed by mutation of the serum response element. Serum response factor binds to the FRE3 as evidenced by gel shift experiments and antibody supershift experiments and expression of a dominant negative form of serum response factor suppresses the activity of FRE3. Additionally, serum response factor binds to the FGFR3 gene in vivo, as demonstrated by chromatin immunoprecipitation. Serum response factor is an important regulator of cardiac, skeletal, and smooth muscle gene expression; these data suggest that serum response factor is also an important determinant of chondrocyte gene expression.

TNFα suppresses link protein and type II collagen expression in chondrocytes: Role of MEK1/2 and NF-κB signaling pathways

Tumor necrosis factor alpha (TNFalpha) inhibits matrix synthesis by chondrocytes in rheumatoid arthritis and osteoarthritis; however, the underlying signaling pathways are poorly characterized. This study investigated the TNFalpha-activated pathways regulating expression of two key components of the cartilage matrix-link protein and type II collagen. In rat articular chondrocytes, TNFalpha decreased link protein and type II collagen mRNA to undetectable levels within 48 h. Levels of link protein mRNA recovered more readily than type II collagen mRNA following removal of the cytokine. TNFalpha-mediated reduction in mRNA of both matrix molecules occurred at the level of transcription and, for link protein, mRNA stability. Turnover of type II collagen and link protein mRNA was dependent on new protein synthesis. In both prechondrocytes and articular chondrocytes, TNFalpha induced concentration-dependent activation of MEK1/2 and NF-kappaB, but not p38 or JNK. Sustained activation of NF-kappaB was observed for up to 72 h following continuous or transient exposure to TNFalpha. Using pharmacological and molecular approaches, the MEK1/2 and NF-kappaB pathways were found to mediate inhibition of type II collagen and link protein gene expression by TNFalpha. Both prechondrocytes and articular chondrocytes are targets of TNFalpha. This study identifies pathways through which TNFalpha perturbs the synthesis and organization of articular cartilage matrix during inflammation.

1Alpha,25-dihydroxyvitamin D3 promotes vascularization of the chondro-osseous junction by stimulating expression of vascular endothelial growth factor and matrix metalloproteinase 9

Vitamin D deficiency results in defects in endochondral bone development characteristic of rickets, which include elongation of the cartilaginous growth plates and disorganization of the primary spongiosa. These defects are caused in part by impaired cartilage mineralization and vascularization of the chondro-osseous junction. Blood vessel invasion of mineralized cartilage is an essential step in endochondral ossification, providing access for cells that degrade cartilage as well as those that form bone. Vascular endothelial growth factor (VEGF) was shown to be a key regulator of this process when infusion of a dominant negative VEGF receptor effectively blocked vascular invasion and endochondral ossification in the growth plates of juvenile mice. Here, we show that the active metabolite of vitamin D 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] directly stimulates transcription of mRNAs encoding VEGF121 and -165 isoforms in the CFK2 chondrogenic cell line. Enhanced VEGF expression also was evident in growth plate chondrocytes and osteoblasts in the tibia of juvenile mice treated systemically with 1alpha,25(OH)2D3. This was seen in conjunction with enhanced expression of matrix metalloproteinase (MMP) 9, which activates VEGF stored in the cartilage matrix, in osteoclastic cells adjacent to the chondro-osseous junction. The alterations in VEGF and MMP-9 expression were accompanied by enhanced vascular invasion of mineralized cartilage, as assessed by CD31 immunoreactivity. These results provide evidence that 1alpha,25(OH)2D3 signaling stimulates VEGF and MMP-9 gene expression and promotes neovascularization of the epiphyseal growth plate in vivo through increased availability of active growth factor.

Ihh enhances differentiation of CFK-2 chondrocytic cells and antagonizes PTHrP-mediated activation of PKA

Indian Hedgehog (Ihh), a member of the hedgehog (HH) family of secreted morphogens, and parathyroid hormone-related peptide (PTHrP) are key regulators of cartilage cell (chondrocyte) differentiation. We have investigated, in vitro, the actions of HH signalling and its possible interplay with PTHrP using rat CFK-2 chondrocytic cells. Markers of chondrocyte differentiation[alkaline phosphatase (ALP) activity, and type II (Col2a1) and type X collagen (Col10a1) expression] were enhanced by overexpression of Ihh or its N-terminal domain (N-Ihh), effects mimicked by exogenous administration of recombinant N-terminal HH peptide. Moreover, a missense mutation mapping to the N-terminal domain of Ihh (W160G) reduces the capacity of N-Ihh to induce differentiation. Prolonged exposure of CFK-2 cells to exogenous N-Shh(5×10-9 M) in the presence of PTHrP (10-8 M) or forskolin (10-7 M) resulted in perturbation of HH-mediated differentiation. In addition, overexpression of a constitutively active form of the PTHrP receptor (PTHR1 H223R) inhibited Ihh-mediated differentiation,implicating activation of protein kinase A (PKA) by PTHR1 as a probable mediator of the antagonistic effects of PTHrP. Conversely, overexpression of Ihh/N-Ihh or exogenous treatment with N-Shh led to dampening of PTHrP-mediated activation of PKA. Taken together, our data suggest that Ihh harbors the capacity to induce rather than inhibit chondrogenic differentiation, that PTHrP antagonizes HH-mediated differentiation through a PKA-dependent mechanism and that HH signalling, in turn, modulates PTHrP action through functional inhibition of signalling by PTHR1 to PKA.

Alterations in the sensing and transport of phosphate and calcium by differentiating chondrocytes

During endochondral bone formation and fracture healing, cells committed to chondrogenesis undergo a temporally restricted program of differentiation that is characterized by sequential changes in their phenotype and gene expression. This results in the manufacture, remodeling, and mineralization of a cartilage template on which bone is laid down. Articular chondrocytes undergo a similar but restricted differentiation program that does not proceed to mineralization, except in pathologic conditions such as osteoarthritis. The pathogenesis of disorders of cartilage development and metabolism, including osteochondrodysplasia, fracture non-union, and osteoarthritis remain poorly defined. We used the CFK2 model to examine the potential roles of phosphate and calcium ions in the regulatory pathways that mediate chondrogenesis and cartilage maturation. Differentiation was monitored over a 4-week period using a combination of morphological, biochemical, and molecular markers that have been characterized in vivo and in vitro. CFK2 cells expressed the type III sodium-dependent phosphate transporters Glvr-1 and Ram-1, as well as a calcium-sensing mechanism. Regulated expression and activity of Glvr-1 by extracellular phosphate and parathyroid hormone-related protein was restricted to an early stage of CFK2 differentiation, as evidenced by expression of type II collagen, proteoglycan, and Ihh. On the other hand, regulated expression and activity of a calcium-sensing receptor by extracellular calcium was most evident after 2 weeks of differentiation, concomitant with an increase in type X collagen expression, alkaline phosphatase activity and parathyroid hormone/parathyroid hormone-related protein receptor expression. On the basis of these temporally restricted changes in the sensing and transport of phosphate and calcium, we predict that extracellular phosphate plays a role in the commitment of chondrogenic cells to differentiation, whereas extracellular calcium plays a role at a later stage in their differentiation program.

Molecular markers predictive of the capacity of expanded human articular chondrocytes to form stable cartilage in vivo

OBJECTIVE

To establish a model and associated molecular markers for monitoring the capacity of in vitro-expanded chondrocytes to generate stable cartilage in vivo.

METHODS

Adult human articular chondrocytes (AHAC) were prepared by collagenase digestion of samples obtained postmortem and were expanded in monolayer. Upon passaging, aliquots of chondrocyte suspensions were either injected intramuscularly into nude mice, cultured in agarose, or used for gene expression analysis. Cartilage formation in vivo was documented by histology, histochemistry, immunofluorescence for type II collagen, and proteoglycan analysis by 35S-sulfate incorporation and molecular sieve chromatography of the radiolabeled macromolecules. In situ hybridization for species-specific genomic repeats was used to discriminate human-derived from mouse-derived cells. Gene expression dynamics were analyzed by semiquantitative reverse transcription-polymerase chain reaction.

RESULTS

Intramuscular injection of freshly isolated AHAC into nude mice resulted in stable cartilage implants that were resistant to mineralization, vascular invasion, and replacement by bone. In vitro expansion of AHAC resulted in the loss of in vivo cartilage formation. This capacity was positively associated with the expression of fibroblast growth factor receptor 3, bone morphogenetic protein 2, and alpha1(II) collagen (COL2A1), and its loss was marked by the up-regulation of activin receptor-like kinase 1 messenger RNA. Anchorage-independent growth and the reexpression of COL2A1 in agarose culture were insufficient to predict cartilage formation in vivo.

CONCLUSION

AHAC have a finite capacity to form stable cartilage in vivo; this capacity is lost throughout passaging and can be monitored using a nude mouse model and associated molecular markers. This cartilage-forming ability in vivo may be pivotal for successful cell-based joint surface defect repair protocols.

Establishment and characterization of chondrocyte cell lines from the costal cartilage of SV40 large T antigen transgenic mice

Complete understanding of the physiology and pathology of the cartilage is essential to establish treatments for a variety of cartilage disorders and defects such as rheumatoid arthritis, congenital malformations, and tumors of cartilage. Although synthetic materials have been used in many cases, they possess inherent problems including wear of the materials and low mechanical strength. Autograft has been considered very effective to overcome these problems. However, the limitation of the transplant volume is a major problem in autograft to be overcome. The costal cartilage is the most serious candidate for donor site transplantation, since it is the largest permanent hyaline cartilage in the body. To investigate the possibility using the costal cartilage as a transplant source, we have established and characterized three mouse chondrocyte cell lines (MCC-2, MCC-5, and MCC-35) derived from the costal cartilage of 8-week-old male SV40 large T-antigen transgenic mice. At confluence, all the cell lines formed nodules that could be positively stained with alcian blue (pH 2.5). The size of nodules gradually increased during culturing time. After 2 and 6 weeks of culture, RT-PCR analysis demonstrated that all three cell lines expressed mRNA from the cartilage-specific genes for type II collagen, type XI collagen, aggrecan, and link protein. Furthermore, type X collagen expression was detected in MCC-5 and MCC-35 but not in MCC-2. Any phenotypic changes were not observed over 31 cell divisions. Immunocytochemistry showed further that MCC-2, MCC-5, and MCC-35 produced cartilage-specific proteins type II collagen and type XI collagen, while in addition MCC-5 and MCC-35 produced type X collagen. Treatment with 1alpha, 25-dihydroxyvitamin D(3) inhibited cell proliferation and differentiation of the three cell lines in a dose-dependent manner. These phenotypic characteristics have been found consistent with chondrocyte cell lines established from cartilage tissues other than costal cartilage. In conclusion, costal cartilage shows phenotypic similarities to other cartilages, i.e., articular cartilage and embryonic limbs, suggesting that costal cartilage may be very useful as the donor transplantation site for the treatment of cartilage disorders. Furthermore, the cell lines established in this study are also beneficial in basic research of cartilage physiology and pathology.

Wnts differentially regulate colony growth and differentiation of chondrogenic rat calvaria cells

The wingless- and int-related proteins (Wnts) have an important role during embryonic development and limb patterning. To investigate their function during chondrocyte differentiation, we used NIH3T3 cells producing seven members of the Wnt family and secreted frizzled-related protein (sFRP-2) for co-culture experiments with the rat chondrogenic cell line pColl(II)-EGFP-5. Pilot experiments showed a negative effect of Wnt-7a on the proliferation of three rodent chondrogenic cell lines, RCJ3.1(C5.18), CFK-2, and C1. To establish a reporter system for chondrogenic differentiation we then produced a stably transfected chondrogenic cell line based on RCJ3.1(C5.18) for further experiments, which expresses green fluorescence protein (EGFP) under the collagen type II promoter (pColl(II)-EGFP-5). This cell line permits convenient observation of green fluorescence as a marker for differentiation in life cultures. The colony size of this cell line in agarose suspension cultures was reduced to 20-40% of control, when exposed to Wnt-1, 3a, 4, 7a, and 7b for 14 days. Similarly, reporter gene expression and the synthesis of cartilage-specific proteoglycans were inhibited by this group of Wnts. In contrast, pColl(II)-EGFP-5 cells exposed to Wnt-5a and Wnt-11 reached 140% of control, and reporter gene expression and proteoglycan synthesis were stimulated. The effects of Wnt-7a and Wnt-5a were additive in pColl(II)-EGFP-5 cells and some but not all Wnt effects were antagonized by the inhibition of proteoglycan sulfation with chlorate, by sFRP-2, which may modulate Wnt receptor binding, or by inhibitors of protein kinase C. These results suggest two functional Wnt subclasses that differentially regulate proliferation and chondrogenic differentiation in vitro which may have implications for cartilage differentiation in vivo. Since some, but not all Wnt effects were sensitive to inhibitors of proteoglycan synthesis or protein kinase C, multiple modes of signal transduction may be involved.

Expression of FGFR3 with the G380R achondroplasia mutation inhibits proliferation and maturation of CFK2 chondrocytic cells

A G380R substitution in the transmembrane-spanning region of FGFR3 (FGFR3Ach) results in constitutive receptor kinase activity and is the most common cause of achondroplastic dwarfism in humans. The epiphyseal growth plates of affected individuals are disorganized and hypocellular and show aberrant chondrocyte maturation. To examine the molecular basis of these abnormalities, we used a chondrocytic cell line, CFK2, to stably express the b variant of wild-type FGFR3 or the the constitutively active FGFR3Ach. Overexpression of FGFR3 had minimal effects on CFK2 proliferation and maturation compared with the severe growth retardation found in cells expressing FGFR3Ach. Cells expressing the mutant receptor also showed an abnormal apoptotic response to serum deprivation and failed to undergo differentiation under appropriate culture conditions. These changes were associated with altered expression of integrin subunits, which effectively led to a switch in substrate preference of the immature cell from fibronectin to type II collagen. These in vitro observations support those from in vivo studies indicating that FGFR3 mediates an inhibitory influence on chondrocyte proliferation. We now suggest that the mechanism is related to altered integrin expression.

Fibroblast growth factor receptor 3 gene transcription is suppressed by cyclic adenosine 3',5'-monophosphate. Identification of a chondrocytic regulatory element

Signaling through fibroblast growth factor receptors (FGFRs) is critical for the development and patterning of the vertebrate skeleton. Gain-of-function alleles of fgfr2 and fgfr3 have been linked to several dominant skeletal disorders in humans, while null mutations in fgfr3 result in the overgrowth of long bones in a mouse model system. Interestingly, the expression pattern of fgfr3 in growth plate chondrocytes overlaps that of the parathyroid hormone (PTH)-related peptide (PTHrP) receptor, a signaling molecule that also regulates endochondral ossification. The coincident expression of these two receptors suggests that their signaling pathways may also interact. To gain insight into the regulatory mechanism(s) that govern the expression of the fgfr3 gene in chondrocytes, we have identified a cell-specific transcriptional regulatory element (CSRh) by measuring the activity of various promoter fragments in FGFR3-expressing (CFK2) and nonexpressing (RCJ) chondrocyte-like cell lines. Furthermore, we demonstrate that activation of PTH/PTHrP receptors, either by stimulation with PTH or through the introduction of activating mutations, represses CSRh-mediated transcriptional activity. Finally, the transcriptional repression of the CSRh element was mimicked by treatment with forskolin, 8-bromo-cAMP, and 3-isobutyl-1-methylxanthine or by overexpression of the catalytic subunit of protein kinase A. Together, these data suggest that protein kinase A activity is a critical factor that regulates fgfr3 gene expression in the proliferative or prehypertrophic compartment of the epiphyseal growth plate. Furthermore, these results provide a possible link between PTHrP signaling and fgfr3 gene expression during the process of endochondral ossification.

Progress toward understanding craniofacial malformations

Significant advances in the study of the human face have revealed the genetic and gene-environment bases of numerous common and rare craniofacial disorders. Classification of craniofacial malformations based on clinical phenotypes is sometimes quite different from the genetic findings of patients. Different mutations in a single gene can cause distinct syndromes, and mutations in different genes can cause the same syndrome. The extracellular signaling molecule SHH, fibroblast growth factor receptors, and transcription factors GLI3, MSX2, and TWIST are discussed as examples of molecules involved in interrelated signal transduction networks regulating craniofacial development. Progress in the understanding of normal and abnormal craniofacial development, through the study of morphoregulatory signaling pathways, has benefited from multifactorial approaches recommended 40 years ago at the National Institute of Dental Research-sponsored landmark Gatlinburg Conference. The utilization of biochemistry, protein structure analyses, tissue culture, and animal model systems for developmental genetics has resulted in remarkable scientific advances. The evolutionary conservation of morphoregulatory pathways has revealed the homology of genes associated with human craniofacial malformations and their counterparts that regulate the morphogenesis of fruit flies. The continued investments in basic, translational, and patient-oriented research regarding normal and abnormal craniofacial development will translate into substantial improvements in the prevention, diagnosis, and treatment of craniofacial diseases and disorders.

Combinatorial expression patterns of individual TLE proteins during cell determination and differentiation suggest non-redundant functions for mammalian homologs of Drosophila Groucho

The Drosophila protein Groucho is involved in the regulation of cell-determination events during insect neurogenesis and segmentation. A group of mammalian proteins, referred to as transducin-like Enhancer of split (TLE) 1 through 4, share with Groucho identical structures and molecular properties. The aim was to determine whether individual TLE proteins participate in the regulation of cell determination in mammals like their Drosophila counterpart. It is here reported that TLE family members are expressed in combinatorial ways during the in vitro differentiation of mouse P19 embryonic carcinoma cells (a model for neural determination) and rat CFK2 cells (a model for chondrocytic determination). TLE1 is up-regulated and TLE2 and TLE4 are down-regulated to different extents during early stages of differentiation. In contrast, later stages correlate with up-regulation of TLE2 and TLE4, and decreased expression of TLE1. Individual TLE proteins are also expressed in combinatorial as well as complementary patterns during the development of the cerebral cortex and spinal cord of mouse embryos. In particular, TLE1 is robustly expressed in both neural progenitor cells and postmitotic neurons of the outer layers of the cortical plate, whereas TLE4 expression marks preferentially postmitotic neurons of the inner layers. Taken together, these results strongly suggest non-redundant roles for individual TLE proteins during both cell-determination and cell-differentiation events.

Constitutive expression of parathyroid hormone-related peptide (PTHrP) stimulates growth and inhibits differentiation of CFK2 chondrocytes

We have examined the effects of constitutive expression of PTHrP on the growth and differentiation of populations of cells derived from a clonal chondrocytic cell line, CFK2. Cells were stably transfected with cDNA encoding either full-length, secretory PTHrP (CFK2P) or nonsecretory PTHrP (CFK2P-SS). In cultures of cells plated at low density, secretory PTHrP acted as a potent mitogen compared with nonsecretory PTHrP or exogenous PTHrP-(1-34), both of which stimulated only a minor increase in proliferation. In populations of control cells maintained postconfluent for several weeks, there was a dramatic increase in expression of mRNA for type II collagen, aggrecan, and link protein. Addition of exogenous PTHrP-(1-34) at a concentration of 10(-8) M to these cultures was ineffective in inhibiting this time-dependent increase in expression of matrix proteins. In contrast, populations of cells producing either secretory or nonsecretory forms of PTHrP, maintained over the same time period, demonstrated an almost complete inhibition of mRNA expression for matrix proteins. These observations demonstrate that PTHrP acts as a bifunctional modulator of chondrogenesis and that some of its biological activity is exerted via a mechanism distinct from the recognised signal transduction pathways linked to the PTH/PTHrP receptor.

Identification of a minimum enhancer sequence for the type II collagen gene reveals several core sequence motifs in common with the link protein gene

The type II collagen gene (Col2a1) is expressed primarily in chondrocytes. Transcription of Col2a1 is mediated by cell-specific regulatory elements located within the promoter and first intron. Here, we map a minimal enhancer and identify elements that determine cartilage-specific Col2a1 expression by analyzing the activity of a series of chimeric genes consisting of rat Col2a1 first intron deletion mutants ligated to the chloramphenicol acetyltransferase reporter gene. We show that a 100-base pair (bp) segment within the first intron is the minimum size necessary for high level, cell type-specific expression of Col2a1. Sequence analysis of this 100-bp Col2a1 enhancer revealed several sequence motifs similar to motifs present within the regulatory region of the link protein gene, another cartilage gene. These motifs include an AT-rich element, a C1 motif and a C3 motif. Deletion of any of these elements reduced Col2a1 enhancer activity in chick embryo chondrocytes. We also tested enhancer-mediated activity in CFK2 cells which differentiate to a chondrogenic phenotype and begin to express type II collagen mRNA after extended culture. In stably transfected CFK2 cells, constructs containing the 100-bp enhancer were activated during the transition from prechondrogenic to chondrogenic cell populations and deletions within the enhancer strongly down-regulated activity. Chondrocyte-specific DNA-protein complexes were identified using nuclear extracts prepared from chick embryo chondrocytes and 32P-labeled oligonucleotides from these regions of the first intron. These results suggest that interaction of chondrocyte specific nuclear factors with multiple core elements from a small region within the first intron are important for cell-type specific Col2a1 enhancer activity.

Nucleolar localization of parathyroid hormone-related peptide enhances survival of chondrocytes under conditions that promote apoptotic cell death

Parathyroid hormone-related peptide (PTHrP) is a mediator of cellular growth and differentiation as well as a cause of malignancy-induced hypercalcemia. Most of the actions of PTHrP have been attributed to its interaction with a specific cell surface receptor that binds the N-terminal domain of the protein. Here we present evidence that PTHrP promotes some of its cellular effects by translocating to the nucleolus. Localization of transiently expressed PTHrP to the nucleolus was dependent on the presence of a highly basic region at the carboxyl terminus of the molecule that bears homology to nucleolar targeting sequences identified within human retroviral (human immunodeficiency virus type 1 and human T-cell leukemia virus type 1) regulatory proteins. Endogenous PTHrP also localized to the nucleolus in osseous cells in vitro and in vivo. Moreover, expression of PTHrP in chondrocytic cells (CFK2) delayed apoptosis induced by serum deprivation, and this effect depended on the presence of an intact nucleolar targeting signal. The present findings demonstrate a unique intracellular mode of PTHrP action and a novel mechanism by which this peptide growth factor may modulate programmed cell death.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

Responsiveness of clonal limb bud cell lines to bone morphogenetic protein 2 reveals a sequential relationship between cartilage and bone cell phenotypes

There is growing evidence to suggest that BMPs are among the signals necessary to create the embryonic skeleton, but how these regulatory molecules enter the pathways of embryonic bone formation remains to be defined. The earliest steps of endochondral bone formation, consisting of mesenchymal condensation and chondrogenesis, have been shown to result directly from BMP-2 action. To determine whether the transition from chondrogenesis to osteogenesis occurring later in endochondral bone formation is also the result of BMP activity, we tested the effects of BMP-2 on immortalized endochondral skeletal progenitor cells derived from mouse limb bud. The cell lines established by this process were found to fall into three general categories: undifferentiated skeletal progenitor cells, which in the presence of BMP-2 first express cartilage matrix proteins and then switch to production of bone matrix proteins; prechondroblast-like cells that constitutively express a subset of markers associated with chondrogenesis and, in the presence of BMP-2, shut off synthesis of these molecules and are induced to produce bone matrix molecules; and osteoblast-like cells that are not significantly affected by BMP-2 treatment. These data suggest that BMP-2 initiates the differentiation of limb bud cells into cells of both the cartilage and bone lineages in a sequential manner, making BMP-2 a potent regulator of skeletal cell differentiation.

Colorimetric evaluation of cultured osteoblast-like cells (ROS 17/2.8)

We have developed a colorimetric method for evaluating the number of osteoblastic cells in culture without destroying the cells. This assay is based on the staining of basophilic cellular compounds with methylene blue. The dye bound by the cells is released at low pH and measured in a spectrophotometer at 662 nm. Linear correlations exist between the absorbance measured by the methylene blue assay and the number of cells seeded, the total cellular protein content, and thymidine labeling. This colorimetric method has the advantage of preserving cell integrity. After destaining, scanning electron microscopy can be performed on well preserved cell morphology.

Human osteogenic protein-1 induces both chondroblastic and osteoblastic differentiation of osteoprogenitor cells derived from newborn rat calvaria

Osteogenetic protein-1 (OP-1), a member of the TGF-beta superfamily, induces endochondrial bone formation at subcutaneous sites in vivo and stimulates osteoblastic phenotypic expression in vitro. Primary cultures of newborn rat calvarial cells contain a spectrum of osteogenic phenotypes ranging from undifferentiated mesenchymal osteoprogenitor cells to parathyroid hormone (PTH)-responsive osteoblasts. We examined whether treatment of this cell population with recombinant human osteogenic protein-1 could induce chondrogenesis in vitro. Markers of chondroblastic versus osteoblastic differentiation included alcian blue staining at pH 1, alkaline phosphatase-specific activity, osteocalcin radioimmunoassay, and expression of collagen mRNAs. 6 d of treatment (culture days 1-7) with 4-100 ng OP-1/ml caused dose-dependent increases in alcian blue staining intensity and alkaline phosphatase activity (4.7- and 3.4-fold, respectively, at 40 ng/ml), while osteocalcin production decreased twofold. Clusters of round, refractile, alcian blue-stained cells appeared by day 3, increased in number until day 7, and then became hypertrophic and gradually became less distinct. Histochemically, the day 7 clusters were associated with high alkaline phosphatase activity and became mineralized. mRNA transcripts for collagen types II and IX were increased by OP-1, peaking at day 4, while type X collagen mRNA was detectable only on day 7 in OP-1-treated cultures. Delay of OP-1 exposure until confluence (day 7) amplifies expression of the normal osteoblastic phenotype and accelerates its developmental maturation. In contrast, early OP-1 treatment commencing on day 1 strongly amplifies chondroblastic differentiation. In the same protocol, TGF-beta 1 alone at 0.01-40 ng/ml fails to induce any hypertrophic chondrocytes, and in combination with OP-1, TGF-beta 1 blocks OP-1-dependent chondroinduction. OP-1 is believed to act on a subpopulation of primitive osteoprogenitor cells to induce endochondrial ossification, but does not appear to reverse committed osteoblasts to the chondrocyte phenotype.

Regulation of expression of the chondrocytic phenotype in a skeletal cell line (CFK2) in vitro

We have examined in vitro the spontaneous and regulated expression of phenotypic characteristics associated with differentiated chondrocytes in an established skeletal cell line (CFK2) derived from fetal rat calvariae. Extended culture of CFK2 cells resulted in the appearance of glycosaminoglycans and type II collagen in the cell layer in association with the formation of focal nodes of cells. In addition, induction of mRNA-encoding link protein, cartilage-specific proteoglycan core protein, and thrombospondin was observed in the differentiated population (dCFK2 cells). The expression of these mRNAs was present for at least two passages after subculturing the dCFK2 cells. The dCFK2 cells also demonstrated enhanced parathyroid hormone (PTH)-stimulated adenylate cyclase activity. Proliferation of CFK2 cells was stimulated by the peptide regulatory factors EGF and PTH and inhibited by the steroidal agents dexamethasone and retinoic acid. EGF and retinoic acid inhibited the formation of cell foci and glycosaminoglycan deposition and the expression of mRNA-encoding link protein. In contrast, PTH and dexamethasone enhanced the formation of focal cellular nodes and augmented matrix deposition and link protein mRNA expression. These studies therefore show that the CFK2 cell line can serve as a nontransformed model of rat chondrocytic cells in which both induction and regulation of the expression of cartilaginous matrix components can be observed. This line thereby provides a unique renewable source of chondrocytic precursor cells and an excellent in vitro model for evaluating temporal and environmental control of chondrocyte differentiation and cartilage matrix production.

Effect of protein and steroidal osteotropic agents on differentiation and epidermal growth factor-mediated growth of the CFK1 osseous cell line

The effects of factors known to influence bone metabolism were examined using the osseous cell line CFK1. Parathyroid hormone (PTH) and dexamethasone (DEX) appeared to enhance the formation of cell foci of CFK1 cells in culture whereas retinoic acid (RA) caused a marked alteration in individual cell morphology. Bone morphogenetic protein (BMP-2) and PTH increased alkaline phosphatase activity, however, this index of differentiation was suppressed by epidermal growth factor (EGF), DEX, and RA. BMP-2 and EGF each stimulated DNA synthesis in a dose-dependent manner and enhanced cell numbers, but, no synergistic response of EGF and BMP-2 was observed. PTH and DEX failed to significantly alter cell number or EGF-stimulated DNA synthesis or cell proliferation. Although RA treatment of CFK1 cells resulted in a reduction in cell number compared to control, pretreatment with RA enhanced EGF-stimulated DNA synthesis and proliferative effects. At least part of this effect was by increasing the EGF receptor binding capacity of the cells. Furthermore, using cell cycle analysis, addition of EGF stimulated the progression of RA-treated cells into the DNA synthesis (S) phase with a reduced lag time. EGF and BMP-2, therefore, appear to exert a role in the expansion dynamics of the CFK1 population although BMP-2 may also enhance differentiation. PTH and DEX may act primarily to modulate the differentiated function of the CFK1 cells. RA inhibited cell proliferation and may mediate differentiation towards a less established cell population with upregulation of EGF receptors. The CFK1 cell model may, therefore, provide insight into microenvironmental control of growth and differentiation of precursor osseous cells.

Concepts of osteoblast growth and differentiation: basis for modulation of bone cell development and tissue formation

The combined application of molecular, biochemical, histochemical, and ultrastructural approaches has defined a temporal sequence of gene expression associated with development of the bone cell phenotype in primary osteoblast cultures. The peak levels of expressed genes reflect a developmental sequence of bone cell differentiation characterized by three principal periods: proliferation, extracellular matrix maturation and mineralization, and two restriction points to which the cells can progress but cannot pass without further signals. The regulation of cell growth and bone-specific gene expression has been examined during this developmental sequence and is discussed within the context of several unique concepts. These are (1) that oncogene expression in proliferating osteoblasts contributes to the suppression of genes expressed postproliferatively, (2) that hormone modulation of a gene is dependent upon the maturational state of the osteoblast, and (3) that chromatin structure and the presence of nucleosomes contribute to three-dimensional organization of gene promoters that support synergistic and/or antagonistic activities of physiologic mediators of bone cell growth and differentiation.

An amino-terminal fragment of urokinase isolated from a prostate cancer cell line (PC-3) is mitogenic for osteoblast-like cells

A peptide mitogen for cultured osteoblast-like cells was purified from serum-free conditioned culture medium of a human prostatic cancer cell line, PC-3. Based on amino acid sequencing and estimated molecular weight, this peptide was identified as an NH2-terminal fragment of urokinase-type plasminogen activator (uPA). Recombinant high molecular weight (HMW) uPA and the NH2-terminal growth factor domain (GFD) of uPA, but not low molecular weight (LMW) uPA (lacking the NH2-terminal region) stimulated [3H] thymidine incorporation and proliferation in osteoblast-like cells, and specific, competitive binding sites for HMW, but not LMW, uPA were demonstrable. These studies demonstrate the production of a mitogenic NH2-terminal fragment of uPA by a human prostatic cancer cell line which may be of importance in the pathogenesis of osteoblastic metastases.