Insulin-like growth factors sustain insulin-like growth factor-binding protein-5 expression in osteoblasts

Insulin-like growth factors (IGFs) I and II are considered to be autocrine regulators of bone cell function. Recently, we demonstrated that IGF-I induces IGF-binding protein-5 (IGFBP-5) expression in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). In the present study, we postulated that IGFs play an autocrine role in the maintenance of IGFBP-5 basal expression in Ob cells. IGFBP-2 and -3, at concentrations that bind endogenous IGFs, decreased IGFBP-5 mRNA levels, as determined by Northern blot analysis, and protein levels, as determined by Western immunoblots of extracellular matrix extracts of Ob cells. IGFBP-2 and -3 in excess inhibited IGFBP-5 heterogeneous nuclear RNA levels, as determined by RT-PCR, and did not alter the half-life of IGFBP-5 mRNA in transcriptionally arrested Ob cells. In conclusion, blocking endogenous IGFs in Ob cells represses IGFBP-5 expression, suggesting that IGFs are autocrine inducers of IGFBP-5 synthesis in osteoblasts.

Insulin-like growth factor II promoter expression in cultured rodent osteoblasts and adult rat bone

Insulin-like growth factor (IGF)-II stimulates bone formation by increasing the replication of cells of the osteoblastic lineage and by enhancing the differentiated function of the osteoblast. Although IGF-II is synthesized by skeletal cells, little is known about the mechanisms involved and its regulation by growth factors. IGF-II expression is tissue specific and is developmentally regulated. In the present study, we examined the expression of IGF-II in fetal rat, newborn mouse and MC3T3-E1 osteoblastic (Ob) cells, and in adult rat calvariae. We also determined mechanisms involved in the regulation of IGF-II by platelet-derived growth factor (PDGF) BB, fibroblast growth factor-2 (FGF-2), and transforming growth factor (TGF) beta1. Northern analysis revealed IGF-II transcripts of 3.6 and 1.2 kb in osteoblastic cells and adult rat calvariae. Ribonuclease (RNase) protection assay using probes specific to the three known IGF-II promoters, P1, P2, and P3, demonstrated messenger RNA (mRNA) expression driven by P3 in osteoblasts and adult rat calvariae, but no expression of P1 or P2 transcripts. PDGF BB, FGF-2, and TGF beta1 inhibited the expression of IGF-II P3 mRNA by 50%. PDGF BB, FGF-2, and TGF beta1 also decreased the rates of IGF-II transcription in rat Ob cells as determined by nuclear run-on assays and did not modify the decay of IGF-II in transcriptionally arrested rat Ob cells. In conclusion, the synthesis of IGF-II in osteoblastic cells and in adult rat calvariae is driven by IGF-II P3 and is regulated by skeletal growth factors acting at the transcriptional level using the IGF-II P3.

Retinoic acid regulates the expression of insulin-like growth factors I and II in osteoblasts

Insulin-like growth factors (IGF) I and II are the most abundant growth factors secreted by skeletal cells, and retinoic acid has many important action on cell differentiation and osteoblastic function. Some of these actions may be mediated by changes in the expression of IGF I and II since IGFs are known to enhance the differentiated function of the osteoblast. We examined the effects of all-transretinoic acid on IGF I and IGF II expression in cultures of osteoblast-enriched cells from 22 day fetal rat calvariae (Ob cells). Retinoic acid caused a transient increase in IGF I and IGF II mRNA levels after 6 h, but after 24 and 48 h of treatment a dose-dependent decrease was observed. Cycloheximide prevented the inhibitory effect of retinoic acid. Retinoic acid treatment for 48 h decreased IGF I polypeptide levels in the culture medium. In contrast, 48 h exposure to retinoic acid increased IGF II polypeptide levels, possible due to increased levels of IGF binding protein-6. The decay of IGF I and II mRNA in transcriptionally arrested Ob cells was similar in control and retinoic acid-treated cells. After 2 h, retinoic acid increased the rates of IGF I and II transcription, as determined by a nuclear run-on assay and heterogeneous nuclear RNA levels, but after 24 h retinoic acid was inhibitory. Retinoic acid had opposite effects to IGFs in osteoblasts and inhibited DNA and collagen synthesis. In conclusion, following a small transient increase, retinoic acid causes a pronounced decrease in IGF I and IGF II mRNA expression in Ob cells. However, treatment with retinoic acid causes a decrease in IGF I and an increase in IGF II polypeptide levels. These changes in the IGF/IGFBP axis may be relevant to the mechanism of action of retinoic acid in bone.

Growth factor regulation of insulin-like growth factor binding protein-6 expression in osteoblasts

Previously we have shown that transforming growth factor beta (TGF beta) 1, basic fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF) BB inhibit the synthesis of insulin-like growth factor (IGF) II, but their effects on IGF binding protein (IGFBP)-6 in osteoblast cultures are not known. IGFBP-6 binds IGF II with high affinity and prevents IGF II-mediated effects, so that a possible mode of regulating the IGF II available to bone cells would be by changing the levels of IGFBP-6. To enhance our understanding of the actions of growth factors on the IGF II axis in bone, we tested the effects of TGF beta 1, basic FGF, PDGF BB, IGF I, and IGF II on the expression of IGFBP-6 in cultures of osteoblast-enriched cells from 22 day fetal rat calvariae (Ob cells). Treatment of Ob cells with TGF beta 1 caused a time- and dose-dependent decrease in IGFBP-6 mRNA levels, as determined by Northern blot analysis. The effect was maximal after 48 h and observed with TGF beta 1 concentrations of 0.04 nM and higher. TGF beta 1 also decreased IGFBP-6 polypeptide levels in the medium, as determined by Western immunoblot analysis. Cycloheximide at 3.6 microM decreased IGFBP-6 transcripts and prevented the effect of TGF beta 1. The decay of IGFBP-6 mRNA in transcriptionally arrested Ob cells was not modified by TGF beta 1. In addition, TGF beta 1 decreased the rates of IGFBP-6 transcription as determined by a nuclear run-on assay. In contrast, basic FGF, PDGF BB, IGF I, and IGF II did not change IGFBP-6 mRNA levels in Ob cells. In conclusion, TGF beta 1 inhibits IGFBP-6 expression in Ob cells by transcriptional mechanisms. Since IGFBP-6 binds IGF II and prevents its effects on bone cells, decreased synthesis of IGFBP-6 induced by TGF beta 1 could be a local feedback mechanism to increase the amount of IGF II available in the bone microenvironment.

Retinoic acid stimulates the transcription of insulin-like growth factor binding protein-6 in skeletal cells

Retinoic acid has important actions on cell differentiation and osteoblastic function, and some of these actions may be mediated by changes in the insulin-like growth factor (IGF) axis. Skeletal cells synthesize IGF I and II and the six known IGF binding proteins (IGFBP). IGFBP-6 binds IGF II with high affinity and prevents IGF II-mediated effects. In fibroblasts, IGFBP-6 levels are regulated by retinoic acid, and we postulated that retinoic acid may regulate IGF II in bone by altering IGFBP-6 synthesis. We examined the effect of retinoic acid on IGFBP-6 expression in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). Retinoic acid caused a time- and dose-dependent increase in IGFBP-6 mRNA levels, as determined by Northern blot analysis. The effect was maximal after 48 h of treatment and observed with retinoic acid at concentrations of 10 nM to 1 microM. Retinoic acid increased IGFBP-6 polypeptide levels in the culture medium, as determined by Western immunoblot analysis. Cycloheximide at 3.6 microM slightly decreased IGFBP-6 transcripts but did not prevent the stimulatory effect of retinoic acid. The decay of IGFBP-6 mRNA in transcriptionally arrested Ob cells was similar in control and retinoic acid-treated cells, and retinoic acid increased the rates of IGFBP-6 transcription, as determined by nuclear run on assays. In conclusion, retinoic acid enhances IGFBP-6 expression in Ob cells by transcriptional mechanisms. Since IGFBP-6 prevents the effects of IGF II, increased synthesis of IGFBP-6 could mediate selected actions of retinoic acid in bone.

Cortisol enhances the transcription of insulin-like growth factor-binding protein-6 in cultured osteoblasts

Previous work indicate that glucocorticoids inhibit the synthesis of insulin-like growth factor I (IGF-I) and IGF-binding protein-3 (IG-FBP-3), -4, and -5, but not IGFBP-6, in osteoblast cultures. IGFBP-6 binds IGF-II with high affinity and prevents IGF-II-mediated effects. As IGF-II is present at high concentrations in bone, we postulate that glucocorticoids may regulate IGF-II by altering IGFBP-6 synthesis. We tested the expression of IGFBP-6 in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells). Treatment of Ob cells with cortisol caused a time- and dose-dependent increase in IGFBP-6 messenger RNA levels, as determined by Northern blot analysis. The effect was maximal after 48 h of treatment and observed with cortisol concentrations of 10 nM to 1 microM. Treatment with cortisol also increased IGFBP-6 polypeptide levels in the medium, as determined by Western immunoblot analysis. Cycloheximide at 3.6 microM decreased IGFBP-6 transcripts and prevented the stimulatory effect of cortisol. Cortisol did not modify the decay of IGFBP-6 messenger RNA in transcriptionally arrested Ob cells. In addition, cortisol increased the rate of IGFBP-6 transcription, as determined by nuclear run-on assays. In conclusion, cortisol stimulates IGFBP-6 expression in Ob cells by transcriptional mechanisms. As IGFBP-6 binds to and prevents the effect of IGF-II, its increased synthesis could be relevant to the inhibitory actions of cortisol in bone.

Cortisol inhibits the synthesis of insulin-like growth factor-binding protein-5 in bone cell cultures by transcriptional mechanisms

Glucocorticoids inhibit the synthesis of insulin-like growth factor-binding protein-5 (IGFBP-5) in osteoblasts, but the mechanisms involved are unknown. IGFBP-5 stimulates bone cell growth, and its inhibition by glucocorticoids may be relevant to the action of this binding protein on bone formation. We tested the effects of cortisol on IGFBP-5 expression in cultures of osteoblast-enriched cells from fetal rat calvariae (Ob cells). Cortisol decreased IGFBP-5 polypeptide levels in the extracellular matrix and caused a time- and dose-dependent decrease in IGFBP-5 mRNA. IGFBP-5 transcripts were markedly decreased by cycloheximide, and further suppressive effects of cortisol could not be determined. Cortisol did not modify the decay of IGFBP-5 mRNA in transcriptionally arrested Ob cells. Cortisol decreased IGFBP-5 hnRNA, the rate of IGFBP-5 transcription, and the activity of the murine IGFBP-5 promoter by 35% in transient transfection experiments. Deletion analysis showed that the region responsive to cortisol is from base pairs -70 to +22, and E-box-binding proteins or c-Myb-related nuclear factors may be involved in its regulation. In conclusion, cortisol inhibits IGFBP-5 transcription in Ob cells through the Myb-binding domain. This effect may be partly responsible for the effect of glucocorticoids on bone formation.

Transcriptional and posttranscriptional regulation of interstitial collagenase by platelet-derived growth factor BB in bone cell cultures

Platelet-derived growth factor (PDGF), a bone cell mitogen, stimulates bone collagen degradation and does not enhance bone matrix apposition rates. The mechanism of the effect on collagen degradation is unknown, and it could involve changes in interstitial collagenase synthesis. We tested the effects of PDGF on interstitial collagenase expression in cultures of osteoblast-enriched cells from fetal rat calvariae (Ob cells). After 4-8 h of treatment, PDGF BB at 0.3 nM increased steady state collagenase messenger RNA (mRNA), whereas PDGF AA had no effect. The effect of PDGF BB on collagenase transcripts was dose dependent. PDGF BB increased the levels of immunoreactive collagenase after 6 h, whereas the levels were decreased after 16 h. Stimulation of collagenase mRNA by PDGF BB was dependent on de novo protein synthesis and activation of protein kinase C. PDGF BB prolonged the half-life of collagenase mRNA in transcriptionally arrested cells. PDGF BB initially increased and subsequently decreased the rate of collagenase gene transcription and the levels of collagenase heterogeneous nuclear RNA. In conclusion, PDGF BB regulates interstitial collagenase in Ob cells by transcriptional and posttranscriptional mechanisms, and this effect may contribute to its stimulatory actions on bone collagen degradation.

Bone morphogenetic protein-2 inhibits the synthesis of insulin-like growth factor-binding protein-5 in bone cell cultures

Previous work from our laboratory indicated that bone morphogenetic protein-2 (BMP-2) enhances the synthesis of insulin-like growth factor-I (IGF-I) and IGF-II by skeletal cells. The activity of IGF-I and -II is regulated by six known IGF-binding proteins (IGFBPs). Although most IGFBP's inhibit the actions of IGF on bone growth, IGFBP-5 is stimulatory, and its synthesis correlates with changes in osteoblast cell growth. We tested the effects of BMP-2 on IGFBP-5 expression in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells). Treatment of Ob cells with BMP-2 caused a time- and dose-dependent decrease in IGFBP-5 messenger RNA (mRNA) levels, as determined by Northern blot analysis. The effect was maximal after 24 h of treatment and occurred at BMP-2 concentrations of 0.03-3.3 nM. Treatment with BMP-2 for 24 h also decreased IGFBP-5 polypeptide levels in the extracellular matrix, as determined by Western blot analysis. The effects of BMP-2 on IGFBP-5 transcripts were independent of cell division, as they were observed in the presence and absence of hydroxyurea (1 mM). IGFBP-5 transcripts were barely detectable in the presence of the protein synthesis inhibitor cycloheximide at 3.6 microM, and further suppressive effects of BMP-2 on IGFBP-5 mRNA could not be determined. BMP-2 did not modify the decay of IGFBP-5 mRNA in transcriptionally arrested Ob cells. In addition, BMP-2 inhibited IGFBP-5 heterogeneous nuclear RNA, determined by reverse transcription-polymerase chain reaction, after 2-6 h of treatment, suggesting an inhibition of IGFBP-5 transcription or processing. In conclusion, BMP-2 inhibits IGFBP-5 expression in Ob cells through pathways that are independent of its mitogenic activity and through mechanisms that may involve decreased transcription or altered RNA processing.

Skeletal growth factors regulate the synthesis of insulin-like growth factor binding protein-5 in bone cell cultures

Skeletal cells secrete insulin-like growth factors (IGFs) I and II and six known IGF binding proteins (IGFBPs). IGFBP-5 stimulates bone formation, and its synthesis correlates with changes in osteoblast cell growth. We tested the effects of basic fibroblast growth factor (bFGF), transforming growth factor beta 1 (TGF beta 1), and platelet-derived growth factor (PDGF) BB on IGFBP-5 expression in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells). Treatment of Ob cells with bFGF, TGF beta 1, and PDGF BB caused a time- and dose-dependent decrease in IGFBP-5 mRNA levels and inhibited IGFBP-5 polypeptide levels in the extracellular matrix. The effects of bFGF, TGF beta 1, and PDGF BB on IGFBP-5 transcripts were independent of cell division and were observed in the presence and absence of hydroxyurea. bFGF, TGF beta 1, and PDGF BB did not modify the decay of IGFBP-5 mRNA in transcriptionally arrested Ob cells, and they inhibited IGFBP-5 heterogeneous nuclear RNA and the rate of IGFBP-5 transcription. In conclusion, bFGF, TGF beta 1, and PDGF BB inhibit IGFBP-5 expression in Ob cells independently of their mitogenic activity and through mechanisms that involve decreased transcription.

Bone morphogenetic protein 2 increases insulin-like growth factor I and II transcripts and polypeptide levels in bone cell cultures

Insulin-like growth factors (IGF) I and II are among the most prevalent growth factors secreted by bone cells and are presumed to act as autocrine regulators of bone formation. Certain growth factors, synthesized by skeletal cells and known to stimulate the replication but not the differentiated function of cells of the osteoblastic lineage, have been shown to inhibit skeletal IGF-I and II synthesis. We postulated that growth factors with limited mitogenic activity and with differentiation-inducing properties, such as bone morphogenetic protein (BMP) 2, have the opposite effect and enhance IGF-I and II synthesis. We tested the effects of BMP-2 on IGF-I and II mRNA expression and polypeptide concentrations in cultures of osteoblast-enriched (OB) cells from 22 day fetal rat calvariae. Steady-state IGF-I and II mRNA levels were determined by northern blot analysis, and IGF-I and II concentrations were determined in acidified and fractionated culture medium by a specific radioimmunoassay. After 24-48 h of treatment, BMP-2 at 3.3 nM increased IGF-I and II transcripts by up to twofold and polypeptide levels by up to fourfold. BMP-2 was a more potent stimulator of IGF-II synthesis, and it was active at doses as low as 0.03 nM for IGF-II mRNA and 0.3 nM for IGF-II protein, whereas a dose of 3.3 nM was required to observe the effect on IGF-I synthesis. The effects of BMP-2 on IGF-I and II transcripts and polypeptide levels were dependent on protein synthesis and decreased in the presence of cycloheximide at 3.6 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of insulin-like growth factor-II synthesis in bone cell cultures by skeletal growth factors

Insulin-like growth factor-II (IGF-II) is a growth factor secreted by bone cells and presumed to act as an autocrine regulator of bone formation. Although hormones and growth factors regulate the synthesis of skeletal IGF-I, hormones do not seem to modify the synthesis of skeletal IGF-II. We postulated that skeletal IGF-II is regulated by growth factors, and we tested the effects of basic fibroblast growth factor (bFGF), transforming growth factor-beta 1 (TGF beta 1), and platelet-derived growth factor-BB (PDGF-BB) on IGF-II messenger RNA (mRNA) expression and polypeptide concentrations in cultures of osteoblast-enriched (Ob) cells from 22-day-old fetal rat calvariae. Steady state IGF-II mRNA levels were determined by Northern blot analysis, and IGF-II concentrations were determined in acidified and fractionated culture medium by a specific RIA. Treatment of Ob cells with bFGF, TGF beta 1, and PDGF-BB decreased IGF-II mRNA levels after 24-48 h. A continuous 48-h treatment with bFGF at 0.6-6 nM, TGF beta 1 at 0.04-1.2 nM, and PDGF-BB at 0.3-3.3 nM caused a dose-dependent decrease in steady state IGF-II mRNA. The effects of bFGF, TGF beta 1, and PDGF-BB on IGF-II mRNA were dependent on protein synthesis and decreased in the presence of cycloheximide at 3.6 microM, but were independent of cell division, because they were observed in the presence and absence of 1 mM hydroxyurea. Treatment with bFGF, TGF beta 1, and PDGF-BB for 24 h did not cause a change in IGF-II polypeptide levels. PDGF-BB at 3.3 nM and TGF beta 1 at 0.04-0.4 nM for 48 h decreased IGF-II polypeptide levels by about 50%, although bFGF had no effect. In conclusion, bFGF, TGF beta 1, and PDGF decrease skeletal IGF-II transcript levels, and this effect may contribute to their actions on selected aspects of Ob cell function.

Growth factors regulate the synthesis of insulin-like growth factor-I in bone cell cultures

Insulin-like growth factor-I (IGF-I), a prevalent growth factor secreted by bone cells, has important effects on bone remodeling. Hormones are known to regulate the synthesis of skeletal IGF-I, but there is limited information about the actions of growth factors on IGF-I synthesis. We tested the effects of basic fibroblast growth factor (bFGF), transforming growth factor-beta 1 (TGF beta 1), and platelet-derived growth factors (PDGF) AA and BB on IGF-I mRNA expression and polypeptide concentrations in cultures of osteoblast-enriched (Ob) cells from 22-day-old fetal rat calvariae. Steady state IGF-I mRNA levels were determined by Northern blot analysis, and IGF-I concentrations were determined in acidified and fractionated culture medium by a specific RIA. Treatment of Ob cells with bFGF at 0.06-6 nM, TGF beta 1 at 0.04-4 nM, and PDGF BB at 0.3-3.3 nM caused a dose-dependent decrease in steady state IGF-I mRNA. A smaller effect was observed with PDGF AA. The effect was initially observed after 6-8 h of treatment and was maximal after 16 h. Treatment with bFGF at 0.6-6 nM, TGF beta 1 at 0.4-4 nM, and PDGF BB at 0.3-3.3 nM for 24 h decreased IGF-I polypeptide concentrations by 40-80%. The effects of bFGF, TGF beta 1, and PDGF BB and AA on IGF-I mRNA were independent of protein synthesis and cell division, as they were observed in the presence and absence of cycloheximide at 3.6 microM or hydroxyurea at 1 mM. Similarly, their inhibitory actions on immunoreactive IGF-I were not prevented by hydroxyurea. In conclusion, bFGF, TGF beta 1, PDGF BB, and, to a lesser extent, PDGF AA decrease skeletal IGF-I synthesis by reducing IGF-I transcript levels, and this effect may contribute to their actions on selected aspects of Ob cell function.