Identification of a specific receptor for interstitial collagenase on osteoblastic cells

We have previously shown that the rat osteoblastic cell line UMR 106-01 responds to parathyroid hormone treatment by secreting interstitial collagenase. Secreted collagenase reaches a maximal concentration 12-24 h after parathyroid hormone stimulation, but then declines to undetectable levels by 96 h. Neither spontaneous nor cell-mediated extracellular degradation could account for this disappearance, since the enzyme maintained stability in both fresh and conditioned media. Instead, a cell-mediated binding mechanism was suggested by the rapid and saturable removal of exogenous purified rat collagenase at 37 degrees C. Binding studies using 125I-collagenase at 4 degrees C indicated a saturable receptor of a single class and 12,000 receptors per cell (Kd = 5 x 10(-9) M). A time course revealed specific receptor-mediated binding within 10 min and equilibrium by 60 min, while dissociation experiments further demonstrated reversibility. The kinetics of 125I-collagenase binding are characterized by the association (k1 = 4 x 10(6) M-1 min-1) and dissociation (k-1 = 2 x 10(-3) min-1) rate constants. The receptor was shown to be specific for rat collagenase since a host of related and unrelated proteins failed to compete for binding. Internalization studies revealed maximal intracellular accumulation at 30 min and complete degradation by 90 min, suggesting this receptor functions in these osteoblastic cells to eliminate extracellular collagenase.

Prostanoid-induced expression of matrix metalloproteinase-1 messenger ribonucleic acid in rat osteosarcoma cells

Individual prostanoids have distinct potencies in activating intracellular signaling pathways and regulating gene expression in osteoblastic cells. The E-series prostaglandins (PGs) are known to stimulate matrix metalloproteinase-1 (MMP-1) synthesis and secretion in certain rodent and human osteoblastic cells, yet the intracellular events involved remain unclear. To further characterize this response and its signal transduction pathway(s), we examined prostanoid-induced expression of the MMP-1 gene in the rat osteoblastic osteosarcoma cell line UMR 106-01. Northern blot analysis demonstrated that prostaglandin E2 (PGE2) and PGE1 were very potent stimulators (40-fold) of MMP-1 transcript abundance, PGF2 alpha and prostacyclin were weak stimulators (4-fold), and thromboxane-B2 had no effect. The marked increase in MMP-1 transcript abundance after PGE2 treatment was first detected at 2 h, became maximal at 4 h, and persisted beyond 24 h. This response was dose dependent and elicited maximal and half-maximal effects with concentrations of 10(-6) and 0.6 x 10(-7) M, respectively. Cycloheximide, a protein synthesis inhibitor, completely blocked this effect of PGE2, suggesting that the expression of other genes is required. Nuclear run-on experiments demonstrated that PGE2 rapidly activates MMP-1 gene transcription, with a maximal increase at 2-4 h. The second messenger analog, 8-bromo-cAMP, mimicked the effects of PGE2 by stimulating a dose-dependent increase in MMP-1 messenger RNA (mRNA) levels, with a maximal effect quantitatively similar to that observed with PGE2. Thus, in UMR 106-01 cells, different prostanoids have distinct potencies in stimulating MMP-1 mRNA abundance. Our data suggest that PGE2 stimulation of MMP-1 synthesis is due to activation of MMP-1 gene transcription and a subsequent marked increase in MMP-1 mRNA abundance. This effect is dependent on de novo protein synthesis and is mimicked by protein kinase-A activation.

Signal transduction pathways mediating parathyroid hormone regulation of osteoblastic gene expression

Parathyroid hormone (PTH) plays a central role in regulation of calcium metabolism. For example, excessive or inappropriate production of PTH or the related hormone, parathyroid hormone related protein (PTHrP), accounts for the majority of the causes of hypercalcemia. Both hormones act through the same receptor on the osteoblast to elicit enhanced bone resorption by the osteoclast. Thus, the osteoblast mediates the effect of PTH in the resorption process. In this process, PTH causes a change in the function and phenotype of the osteoblast from a cell involved in bone formation to one directing the process of bone resorption. In response to PTH, the osteoblast decreases collagen, alkaline phosphatase, and osteopontin expression and increases production of osteocalcin, cytokines, and neutral proteases. Many of these changes have been shown to be due to effects on mRNA abundance through either transcriptional or post-transcriptional mechanisms. However, the signal transduction pathway for the hormone to cause these changes is not completely elucidated in any case. Binding of PTH and PTHrP to their common receptor has been shown to result in activation of protein kinases A and C and increases in intracellular calcium. The latter has not been implicated in any changes in mRNA of osteoblastic genes. On the other hand activation of PKA can mimic all the effects of PTH; protein kinase C may be involved in some responses. We will discuss possible mechanisms linking PKA and PKC activation to changes in gene expression, particularly at the nuclear level.

Cloning and regulation of rat tissue inhibitor of metalloproteinases-2 in osteoblastic cells

Rat tissue inhibitor of metalloproteinases-2 (TIMP-2) was cloned from a UMR 106-01 rat osteoblastic osteosarcoma cDNA library. The 969-bp full-length clone demonstrates 98 and 86% sequence identity to human TIMP-2 at the amino acid and nucleic acid levels, respectively. Parathyroid hormone (PTH), at 10(-8) M, stimulates an approximately twofold increase in both the 4.2- and 1.0-kb transcripts over basal levels in UMR cells after 24 h of exposure. The PTH stimulation of TIMP-2 transcripts was not affected by the inhibitor of protein synthesis, cycloheximide (10(-5) M), suggesting a primary effect of the hormone. This is in contradistinction to regulation of interstitial collagenase (matrix metalloproteinase-1) by PTH in these same cells. Nuclear run-on assays demonstrate that PTH causes an increase in TIMP-2 transcription that parallels the increase in message levels. Parathyroid hormone, in its stimulation of TIMP-2 mRNA, appears to act through a signal transduction pathway involving protein kinase A (PKA) since the increase in TIMP-2 mRNA is reproduced by treatment with the cAMP analogue, 8-bromo-cAMP (5 x 10(-3) M). The protein kinase C and calcium pathways do not appear to be involved due to the lack of effect of phorbol 12-myristate 13-acetate (2.6 x 10(-6) M) and the calcium ionophore, ionomycin (10(-7) M), on TIMP-2 transcript abundance. In this respect, regulation of TIMP-2 and collagenase in osteoblastic cells by PTH are similar. However, we conclude that since stimulation of TIMP-2 transcription is a primary event, the PKA pathway must be responsible for a direct increase in transcription of this gene.

Parathyroid hormone induces transcription of collagenase in rat osteoblastic cells by a mechanism using cyclic adenosine 3',5'-monophosphate and requiring protein synthesis

Collagenase is synthesized and secreted by rat osteoblastic cells in response to PTH. We have previously demonstrated that this effect involves a substantial increase in collagenase mRNA via transcription. Northern blots and nuclear run-on assays were performed to further investigate the induction of collagenase by PTH in the rat osteoblastic cell line UMR 106-01. Detectable amounts of collagenase mRNA were not apparent until 2 h of PTH treatment, showed the greatest abundance at 4 h, and declined to approximately 30% of maximum by 8 h. The changes in the rate of transcription of the collagenase gene in response to PTH paralleled and preceded the changes in the steady state mRNA levels. After an initial lag period of about 1 h, collagenase transcription rates increased from very low levels to a maximal response at 2 h, returning to about 50% of maximum by 10 h. The increased transcriptional rate of the collagenase gene was found to be dependent on the concentration of PTH, with a half-maximal response at approximately 7 x 10(-10) M rat PTH-(1-34) and a maximal effect with a dose of 10(-8) M. The PTH-mediated induction of collagenase transcriptional activity was completely abolished by cycloheximide, while transcription of the beta-actin gene was unaffected by the translation inhibitor. These data suggest that a protein factor(s) is required for PTH-mediated transcriptional induction of collagenase. Since PTH increases intracellular levels of several potential second messengers, agents that mimic these substances were employed to determine which signal transduction pathway is predominant in the PTH-mediated stimulation of collagenase transcription.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone induces transcription of collagenase in rat osteoblastic cells by a mechanism using cyclic adenosine 3',5'-monophosphate and requiring protein synthesis

Collagenase is synthesized and secreted by rat osteoblastic cells in response to PTH. We have previously demonstrated that this effect involves a substantial increase in collagenase mRNA via transcription. Northern blots and nuclear run-on assays were performed to further investigate the induction of collagenase by PTH in the rat osteoblastic cell line UMR 106-01. Detectable amounts of collagenase mRNA were not apparent until 2 h of PTH treatment, showed the greatest abundance at 4 h, and declined to approximately 30% of maximum by 8 h. The changes in the rate of transcription of the collagenase gene in response to PTH paralleled and preceded the changes in the steady state mRNA levels. After an initial lag period of about 1 h, collagenase transcription rates increased from very low levels to a maximal response at 2 h, returning to about 50% of maximum by 10 h. The increased transcriptional rate of the collagenase gene was found to be dependent on the concentration of PTH, with a half-maximal response at approximately 7 x 10(-10) M rat PTH-(1-34) and a maximal effect with a dose of 10(-8) M. The PTH-mediated induction of collagenase transcriptional activity was completely abolished by cycloheximide, while transcription of the beta-actin gene was unaffected by the translation inhibitor. These data suggest that a protein factor(s) is required for PTH-mediated transcriptional induction of collagenase. Since PTH increases intracellular levels of several potential second messengers, agents that mimic these substances were employed to determine which signal transduction pathway is predominant in the PTH-mediated stimulation of collagenase transcription.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone induces c-fos and c-jun messenger RNA in rat osteoblastic cells

PTH is a potent regulator of osteoblast gene expression, yet the nuclear events that mediate PTH action are poorly understood. We were interested in identifying immediate early genes which may regulate PTH-altered gene expression in the osteoblast. Therefore, we examined the effects of PTH on c-fos and c-jun gene expression in a rat osteoblastic cell line (UMR 106-01). Under control conditions, c-fos and c-jun mRNAs were present at low basal levels. After PTH treatment, c-fos mRNA abundance dramatically increased, with a maximal and transient response at 30 min. PTH also stimulated an increase in c-jun mRNA, but in a biphasic manner, with maximal levels at 30 min and 2 h. These responses were dose dependent, not altered by cotreatment with the protein synthesis inhibitor cycloheximide, and preceded PTH-induced expression of matrix metallo-proteinase-1 mRNA. Nuclear run-on assays demonstrated an increased rate of c-fos and c-jun transcription after PTH exposure. To determine the signal transduction pathways involved, second messenger analogs were tested for their ability to mimic the effects of PTH. 8-Bromo-cAMP and phorbol 12-myristate 13-acetate (PMA) caused increases in the abundance of c-fos and c-jun transcripts. Ionomycin had no effect on the expression of these genes. Pretreatment of the cells with PMA resulted in a decrease in basal c-jun expression, but did not alter the PTH-mediated increase in c-fos, c-jun, or matrix metalloproteinase-1 mRNAs.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of an H-ras-related transcript by parathyroid hormone in rat osteosarcoma cells

The rat osteosarcoma cell line UMR 106-01 is a commonly used model system for the study of osteoblast function. However, it also expresses a phenotype characteristic of transformed cells. To test whether the latter could be accounted for by aberrant oncogene expression, we probed Northern blots of UMR and other osteoblastic cells with a panel of oncogene probes. These blots, when probed with a cDNA specific for v-H-ras, revealed a 7.0-kilobase (kb) H-ras-related transcript (designated HRRT) in UMR 106-01 cells that was not expressed in other osteoblastic cells. Osteoblast-enriched calvarial cells expressed the typical 1.1-kb H-ras mRNA, which was absent in UMR cells. Additionally, Western blots of lysates of UMR cells documented the presence of three proteins immunologically related to H-rasp21. To determine whether HRRT represented a recombinant retrovirus product, Northern blots were probed with a cDNA specific for the highly conserved gag-pol region of Moloney murine leukemia virus. These blots showed parallel cross-reactivity with an apparently identical transcript of 7.0 kb. The 7.0-kb transcripts detected by both v-H-ras and gag-pol probes declined to the same extent after treatment with concentrations of PTH known to inhibit proliferation of these cells. PTH regulated the abundance of HRRT in a time- and dose-dependent manner, with greatest repression of the transcript after 8 h of treatment with 10(-8) M PTH. The decrease in HRRT could not be completely accounted for by changes in transcriptional activity, as determined by nuclear run-on assays.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and sequence analysis of two rat tissue inhibitors of metalloproteinases

Two protein inhibitors of metalloproteinases (TIMP) were isolated from medium conditioned by the clonal rat osteosarcoma line UMR 106-01. Initial purification of both a 30-kDa inhibitor and a 20-kDa inhibitor was accomplished using heparin-Sepharose chromatography with dextran sulfate elution followed by DEAE-Sepharose and CM-Sepharose chromatography. Purification of the 20-kDa inhibitor to homogeneity was completed with reverse-phase high-performance liquid chromatography. The 20-kDa inhibitor was identified as rat TIMP-2. The 30-kDa inhibitor, although not purified to homogeneity, was identified as rat TIMP-1. Amino terminal amino acid sequence analysis of the 30-kDa inhibitor demonstrated 86% identity to human TIMP-1 for the first 22 amino acids while the sequence of the 20-kDa inhibitor was identical to that of human TIMP-2 for the first 22 residues. Treatment with peptide:N-glycosidase F indicated that the 30-kDa rat inhibitor is glycosylated while the 20-kDa inhibitor is apparently unglycosylated. Inhibition of both rat and human interstitial collagenase by rat TIMP-2 was stoichiometric, with a 1:1 molar ratio required for complete inhibition. Exposure of UMR 106-01 cells to 10(-7) M parathyroid hormone resulted in approximately a 40% increase in total inhibitor production over basal levels.

A serum factor promotes collagenase synthesis by an osteoblastic cell line

Regulation of the synthesis of collagenase was investigated in the osteoblastic cell line, UMR 106-01. The cells were stained by the avidin-biotin-complex technique for the presence of the enzyme. By this method, it was possible to identify cells producing collagenase. Synthesis, but not secretion, was found to be constitutive in these cells with the enzyme located intracellularly in cytoplasmic vesicles and the Golgi apparatus. The amount of collagenase contained within UMR cells and the number of cells synthesizing the enzyme were proportional to the concentration of fetal bovine serum in the incubating medium. When serum was withdrawn from the osteosarcoma cells, the content of collagenase decreased with time and the enzyme became undetectable by 48 h of serum depletion. The decrease in collagenase content could be completely reversed by resupplying serum to the cells. The collagenase promoting activity of serum could not be eliminated by adsorption on activated charcoal but was retained by a dialysis membrane with a 12,000 mol wt cutoff. A range of bone-seeking hormones or agents known to affect collagenase secretion was added to the medium in an attempt to mimic the effect of serum on collagenase accumulation. None of these agonists, including parathyroid hormone, could reproduce the effect of serum on these cells, although parathyroid hormone could act as a collagenase secretagogue in the presence or absence of serum. It is concluded that fetal bovine serum contains a yet unidentified factor or factors greater than 12,000 mol wt responsible for the continued synthesis of collagenase by UMR 106-01 cells.

Rat collagenase. Cloning, amino acid sequence comparison, and parathyroid hormone regulation in osteoblastic cells

We have isolated clones for rat collagenase from a rat osteoblastic cell cDNA library. These clones have been sequenced and the amino acids deduced. The calculated molecular weight is 51,352 for the proenzyme and 42,229 for the active enzyme. The deduced amino acid sequence was compared to those previously reported for: 1) human collagenase, 2) rat transin 1 (stromelysin), 3) human stromelysin, and 4) rabbit collagenase. The number of amino acids conserved was 47, 47, 50, and 47%, respectively. We also compared the collagenase mRNA and protein in different rat cells (osteoblast, uterine smooth muscle, synovial fibroblast) and determined that in rat uterine cells the message is slightly larger, although collagenase protein in all three cell types was identical in size. Parathyroid hormone dramatically induces the 2.9-kilobase collagenase mRNA in the rat osteoblastic cells, UMR 106-01. Nuclear run-on studies in UMR 106-01 cells demonstrated a 4-8-fold induction in the rate of synthesis of collagenase mRNA at 2 and 4 after parathyroid hormone treatment, with steady state levels of mRNA increased 100-fold at 4 h. Thus, parathyroid hormone regulation of the collagenase gene in UMR 106-01 cells is in part transcriptional.

Stimulation of collagenase production by rat osteosarcoma cells can occur in a subpopulation of cells

Recent studies have indicated that neutral collagenase can be produced in bones of rats. In addition, it has been demonstrated by in vitro studies that the enzyme is likely secreted by osteoblasts. Cells of the osteoblastic tumor cell line UMR-106 can be stimulated to produce not only collagenase, but also collagenase inhibitor and plasminogen activator. However, it is conceivable that not all osteoblasts produce all of these proteins. In this study, in which UMR cells were maximally stimulated with PTH, only a subpopulation of cells was observed to produce enhanced levels of collagenase but all cells had the ability to synthesize plasminogen activator. Cells of the rat osteosarcoma line UMR-106-01 were stained for the presence of collagenase and tissue plasminogen activator using an immunohistochemical procedure. In many cases, the cells were exposed to monensin for the final 3 h of incubation as well as to the inducing agent PTH. Monensin prevented export of the enzymes, enabling them to be visualized within their cell or origin. Maximal stimulation of collagenase was demonstrated to occur 8 h after exposure to 10(-8) -10(-7) M PTH. Under these conditions, 14-17% of the cells appeared to synthesize elevated amounts of collagenase (as determined by intense staining). Without PTH stimulation, there was a low level of collagenase in all cells, but less than 1% of the cells stained heavily for the enzyme. In contrast, strong staining for plasminogen activator was observed in all cells with or without PTH treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Second messenger signaling in the regulation of collagenase production by osteogenic sarcoma cells

Recent work indicates that PTH can stimulate osteoblastic cells to secrete neutral collagenase, an enzyme thought to be linked to bone matrix turnover. Since recent studies suggest that the calcium/protein kinase-C (PKC) message system is involved in signal transduction stimulated by PTH, we examined the role of these putative second messengers of PTH in the regulation of collagenase production by the osteoblastic tumor cell line UMR 106-01. Immunohistochemical staining of cells exposed to PTH (10(-7) M) revealed that about 20% of the entire population was positive for collagenase, compared to less than 3% staining positively in control untreated cells. Incubation with the cAMP analog 8-bromo-cAMP (8BrcAMP) increased the number of collagenase-staining cells in a dose-dependent manner (ED0.5 = 2.5 x 10(-4) M), but to a lower level than PTH, with the maximal effect producing about 15% positive cells. The calcium ionophore ionomycin (10(-7) M) was ineffective, whereas phorbol 12-myristate 13-acetate (PMA), a PKC activator, increased collagenase-specific staining to about 5%, but only at high concentrations (10(-5) M). Incubation of UMR 106-01 cells with ionomycin and PMA did not change the effect of the latter. When the three agents were used in combination, an additive effect was observed, which fully reproduced that of PTH. Similarly, the amount of collagenase released into the medium by cells stimulated with maximal concentrations of 8BrcAMP (10(-3) M) was only 80% of that induced by maximal doses of PTH (10(-7) M). PMA (10(-5) M) was slightly stimulatory, and ionomycin was ineffective alone, but they were synergistic with submaximal doses of 8BrcAMP (10(-4) M). In agreement with the immunohistochemical results, the full hormonal effect was reproduced when the three second messenger analogs were used in combination. In conclusion, signal transduction from PTH receptor to collagenase production is mediated mainly by cAMP; the Ca2+/PKC system appears to have a contributory role necessary for the full expression of hormonal response. These results support the hypothesis of a dual pathway of target cell activation by PTH.

Parathyroid hormone inhibits collagen synthesis at both ribonucleic acid and protein levels in rat osteogenic sarcoma cells

Cells of the clonal rat osteogenic sarcoma cell line, UMR 106-01, were used to investigate the regulation of collagen synthesis by PTH in osteoblastic cells. Monolayer cultures of cells were labeled with [3H] proline in order to determine both collagen type and rates of production. Analysis of labeled extracellular polypeptides on sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that UMR 106-01 cells synthesized predominantly type I collagen, accounting for 45.48 +/- 2.09% of the radioactivity incorporated into total protein. After 24-h treatment with bovine PTH (1-34, 10(-8) M), collagen synthesis (i.e. collagenase-digestible protein) was decreased to 29.45 +/- 1.39% of total protein production. This decrease was first observed 12 h after addition of hormone and greatest inhibition was achieved at 24 h. The effect of PTH was dose dependent, with half-maximal inhibition of collagen synthesis occurring at 5 x 10(-10) M after 24-h treatment. In contrast, when steady state levels of mRNA for type I collagen chains were examined by Northern blot analysis, the concentration of PTH that reduced collagen synthesis by 35-45% (10(-8) M), caused a net decrease of approximately 80-96% in the number of procollagen transcripts; a small reduction in beta-actin mRNA levels was also observed. The effect of the hormone on procollagen message level was dose dependent, with significant inhibition observed at 10(-10) M PTH and, as with collagen synthesis, maximal after 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)

New concepts in bone remodeling: an expanding role for the osteoblast

Until recently, the prevailing view regarding the function of osteoblasts and osteoclasts was to attribute bone formation to the former and bone resorption to the latter. While the capacity of the osteoclast to degrade bone matrix remains unquestioned, there is now provocative evidence indicating that the osteoblast plays a critical role in regulating osteoclast resorptive activity as well as in contributing directly to matrix dissolution. The first of these points follows from observations indicating that the osteoblast (but not the osteoclast) 1) exhibits receptors and/or responses to resorption-promoting agents (including parathyroid hormone and vitamin D), and 2) releases agents capable of stimulating bone resorption. The second point is derived from studies demonstrating that the osteoblast produces neutral collagenase (an enzyme specialized to degrade type I collagen, the principal organic constituent of bone matrix) and an inhibitor capable of blocking collagenase activity. The synthesis of both of these proteins is, in part, regulated by parathyroid hormone and other resorption-stimulating agents and appears to involve control at the transcriptional, translational, and secretory levels. Thus, in both physiologic bone remodeling and modeling, as well as the altered bone turnover associated with some disease states, it is the osteoblast rather than the osteoclast that may hold the key to understanding the mechanism of tissue form and function.

Hormonal regulation of the production of collagenase and a collagenase inhibitor activity by rat osteogenic sarcoma cells

Collagenases that specifically cleave native collagen at neutral pH have been implicated in the maintenance and turnover of connective tissue. In bone, the origin of neutral collagenase has remained equivocal, although recent studies have indicated that it is synthesized by the osteoblast. In the present work, regulation of secretion of neutral collagenase and a collagenase inhibitory activity was investigated using the osteoblastic tumor cell line UMR 106-01 and a variety of bone-resorbing agents. Under basal conditions, UMR 106-01 cells produced very low levels of collagenase but substantial amounts of the inhibitory activity. Exposure to PTH and, to a lesser extent, 1,25-dihydroxyvitamin D3, prostaglandin E2, retinoic acid, and epidermal growth factor stimulated the release of collagenase, an effect not seen with interleukin-1 or heparin. The stimulation of collagenase by PTH was dose dependent, with a half-maximal response occurring at 10(-8) M. Inclusion of isobutylmethylxanthine decreased the concentration of PTH required to produce half-maximal stimulation to 2 X 10(-10) M, indicating action via cAMP. With respect to the inhibitory activity, PTH and epidermal growth factor were the only agents, among those tested, able to enhance its production. Both hormones caused a 50-100% increase over control levels 72 h after hormone administration. There were notable differences in the time courses of production of collagenase and the inhibitor. After treatment with PTH, the enzyme reached maximal concentrations between 12-48 h, but declined to undetectable levels by 96 h. In contrast, the inhibitory activity was secreted in a linear fashion, with the highest concentrations achieved around 72-96 h. These results suggest a complex pattern of regulation of collagenase and inhibitor secretion by the osteoblastic cell, with the steady accumulation of inhibitor perhaps being responsible for the ultimate curtailment of enzyme activity.

Hormonal influences on bone cells

The methods for establishing osteoblast-rich rat calvarial cell cultures have been described, together with methods for the use of clonal osteogenic sarcoma cells of osteoblast phenotype. The latter clonal lines are useful for several purposes, but all the precautions and quality control measures necessary in the study of clonal lines must be observed. Some of the techniques for studying biochemical responses to hormones in these cells have also been detailed, but clearly others are applicable, including studies of the synthesis of matrix constituents. Osteoclast-like cells have not been considered in this chapter, because osteoclast culture methods have not yet been developed to the degree of purity and reproducibility necessary for this type of biochemical approach.

Inhibitory effects of parathyroid hormone on growth of osteogenic sarcoma cells

The effects of the bone resorbing hormone, parathyroid hormone (PTH), on the growth of malignant osteoblastic cells have been examined. The malignant osteoblastic cells were a clonal line (UMR 106) derived from a transplantable rat osteogenic sarcoma. The predominant effect of PTH at doses above 10(-10) M was an inhibition of replication and DNA synthesis. Replication was decreased by PTH in both the presence or absence of serum and at various cell seeding densities. Both bovine PTH (1-84) and the synthetic hormone, human PTH (1-34), inhibited replication, but with bovine hormone being an order of magnitude more potent. The effects could be observed in as short a time as 6 hours with DNA synthesis and 24 hours with replication.

Regulation of prostaglandin production by osteoblast-rich calvarial cells

The effect of various factors upon prostaglandin (PG) production by the osteoblast was examined using osteoblast-rich populations of cells prepared from newborn rat calvaria. Bradykinin and serum, and to a lesser extent, thrombin, were all shown to stimulate PGE2 and 6-keto-PGF1 alpha (the hydration product of PGI2) secretion by the osteoblastic cells. Several inhibitors of prostanoid synthesis, dexamethasone, indomethacin, dazoxiben and nafazatrom, were tested for their effects on the calvarial cells. All inhibited PGE2 and PGI2 (the major arachidonic acid metabolites of these cells) production with half-maximal inhibition by all four substances occurring at approximately 10(-7) M. For dazoxiben and nafazatrom, this was in contrast to published results from experiments in vivo which have indicated that the compounds stimulated PGI2 production. Finally, since the osteoblast is responsive to bone-resorbing hormones, these were tested. Only epidermal growth factor (EGF) was shown to modify PG production. At early times EGF stimulated PGE2 release, however, the predominant effect of the growth factor was an inhibition of both PGE2 and PGI2 production by the osteoblastic cells. The present results suggest that the bone-resorbing hormones do not act to cause an increase in PG by the osteoblast and that any increase in PG production by these cells may be in response to vascular agents.

Regulation of plasminogen activator production by bone-resorbing hormones in normal and malignant osteoblasts

The plasminogen activator (PA) activity of clonal rat osteogenic sarcoma cell (phenotypically osteoblast) and of osteoblast-rich rat calvarial cells is shown to be increased by treatment with the bone-resorbing hormones, PTH, 1,25-dihydroxyvitamin D3, prostaglandin E2, and epidermal growth factor. Dose-dependent increases were observed, after a lag period of 4 to 8 h. Stimulated and control PA activities were inhibited by actinomycin D and cycloheximide but not by cytosine arabinoside. Glucocorticoid hormones prevented the hormone stimulation, but other steroids did not. Calcitonin had no effect either on basal or on hormone-treated PA activity. Isobutyl-methylxanthine alone increased PA activity and enhanced responsiveness to PTH and to prostaglandin E2. These data point to a common pathway in the actions upon osteoblasts of several hormones with diverse initial cellular actions and raise the possibility that the PA/plasmin system may contribute to cellular mechanisms of bone turnover.

Characterization of an osteoblast-like clonal cell line which responds to both parathyroid hormone and calcitonin

The clonal cell line UMR 106, which was originally derived from a rat transplantable osteogenic sarcoma with an osteoblastic phenotype, was subcloned after the emergence of a calcitonin-responsive adenylate cyclase was noted in late passages. Detailed studies on the stimulation of adenylate cyclase and activation profile of the cyclic AMP-dependent protein kinase isoenzymes in response to parathyroid hormone (PTH) and salmon calcitonin (SCT) were conducted on two subclones (UMR 106-01 and UMR 106-06). Both subclones responded in an identical manner to PTH, which stimulated adenylate cyclase and activated both isoenzyme I and isoenzyme II of cyclic AMP-dependent protein kinase. In contrast, only UMR 106-06 cells responded to calcitonin. At 3 X 10(-8)M SCT, there was a sevenfold stimulation of adenylate cyclase, 84% activation of isoenzyme I, and 44% activation of isoenzyme II. The activation profiles of the isoenzymes to PTH and SCT in UMR 106-06 were similar. Furthermore, their response to SCT correlates with the presence of specific, saturable binding of 125I-labeled SCT. Binding parameters indicate apparent Kd = 0.8 nM and 6,000 receptors/cell. These data point to a significant phenotypic change having taken place in this clonal cell line with prolonged maintenance in culture, with the emergence of a calcitonin receptor linked to adenylate cyclase and protein kinase activation.

Stimulation of plasminogen activator in osteoblast-like cells by bone-resorbing hormones

Hormonal control of plasminogen activator (PA) was studied in clonal rat osteogenic sarcoma cells which are phenotypically osteoblast, and in osteoblast-rich rat bone cell cultures. The bone-resorbing hormones (parathyroid hormone, prostaglandin E2, epidermal growth factor and 1,25-dihydroxyvitamin D3) stimulated PA activity in both cell types. The relative efficacies of vitamin D metabolites and of prostanoids reflect their relative potencies as stimulators of bone resorption.

Morphological and biochemical characterization of four clonal osteogenic sarcoma cell lines of rat origin

The ultrastructural and biochemical properties of four clonal osteogenic sarcoma lines, UMR 104, 105, 106, and 108, have been compared with uncloned osteogenic sarcoma cells and normal osteoblast-rich cells derived from newborn rat calvaria. High alkaline phosphatase activity and activation of adenylate cyclase by parathyroid hormone were used as biochemical markers of osteoblastic cells. Cloning enriched both of these parameters above those of the parent tumor and far higher than that seen in normal cells, suggesting enrichment of the osteoblast phenotype. Both of these properties have been retained through many passages in culture. Morphologically, the clonal lines have also retained the "blast"-like appearance of the uncloned osteogenic sarcoma cells and consist mainly of flat, relatively featureless cells. Many cells with mitotic figures were observed, indicating continuous cell division taking place in the malignant cells. Each clonal line gave rise to characteristic tumors when reinjected into rats. It is concluded that the clonal osteogenic sarcoma lines are highly differentiated tumor lines which have conserved the differentiated properties of the mature osteoblast, making them a suitable model for the study of the effects of hormones on the growth of a differentiated tumor, as well as for the study of hormonal regulation of the osteoblast.

Stimulation of DNA synthesis by epidermal growth factor in osteoblast-like cells

Normal and malignant osteoblast-like cells in culture have been shown to possess specific, high affinity receptors for epidermal growth factor (EGF). In this study, the mitogenic response to EGF was examined in a clonal line of a rat osteogenic sarcoma (UMR 106) and in osteoblast-rich newborn rat calvarial cells. Twenty-four hour treatment of UMR 106 cells with EGF in doses ranging from 10(-12) M to 2 X 10(-8) M stimulated the incorporation of [3H]thymidine and DNA synthesis in a dose-dependent manner. This short-term stimulatory effect was sustained in long-term culture with a dose-dependent increase in cell proliferation by calvarial cells. A lag period of 8 h occurred before significant stimulation of [3H]thymidine incorporation was observed. Commitment to increased incorporation of [3H]thymidine required a minimum of 6 h continuous incubation with EGF. These results establish the osteoblast as a target cell for EGF action on bone.

Epidermal growth factor receptors in clonal lines of a rat osteogenic sarcoma and in osteoblast-rich rat bone cells

Studies were carried out to identify and characterize the receptors for epidermal growth factor (EGF) in osteoblast-rich newborn rat calvarial cells and in 4 clonal lines derived from a transplantable rat osteogenic sarcoma with a well-characterized osteoblast-like phenotype. The cells were grown in monolayer culture in replicate wells; 40,000-50,000 cpm 125I-labeled mouse EGF with a specific activity of 100-120 microCi/micrograms was added to each well. Binding studies were carried out at 37 degrees C. Binding of 125I-labeled EGF was specific, saturable, reversible, and pH dependent. Maximum binding occurred 2 h after addition of the tracer. Thereafter, cell-bound radioactivity decreased to reach a plateau of 15-20% of maximum binding at 24 h. This observation is consistent with internalization and processing of the receptor-hormone complex as has been shown with other EGF target cells. Scatchard analyses revealed a single class of high-affinity binding sites in the normal and malignant osteoblast-like cells. Dissociation constants (KD) in the clonal lines ranged from 2.3 X 10(-10)M to 4.7 X 10(-10)M with receptor number per cell ranging from 25,000 to 33,000. The calvarial cells had a KD of 2.0 X 10(-10)M with 14,000 receptors per cell. In both the normal and malignant cell strains, EGF was found to increase incorporation of 3H-labeled thymidine into acid-precipitable macromolecules. EGF has been shown to stimulate bone resorption; however, studies in organ cultures have not identified the target cell for EGF. The present results point to an interaction of EGF with osteoblasts.