Downregulation of cell growth and cell cycle regulated genes during chick osteoblast differentiation with the reciprocal expression of histone gene variants

Human osteoblastic cells discriminate between 20-kDa amelogenin isoforms

Enamel matrix derivative (EMD) is used to stimulate healing of alveolar bone after destructive marginal periodontitis; however, the roles of the different EMD constituents are unclear. The aim here was to compare the effect of two EMD fractions (A1 and A2) on primary human osteoblasts cultured in the presence of 50 μg ml(-1) of A1, A2, or EMD. SDS-PAGE showed that A1 and A2 were comprised of amelogenins migrating at around 20 kDa. Fourier transform infrared (FTIR) analysis revealed that A1 and A2 had different secondary structures, and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) identified different peptide mass values. Osteoblasts responded differently to A1 and A2. Whereas A1 enhanced the proliferation [measured by the incorporation of 5-bromo-2'-deoxyuridine (BrdU)] of osteoblasts, the expression of runt-related transcription factor-2 (RUNX2) mRNA, and the secretion of interleukin 6 (IL-6) into the cell culture medium, exposure to A2 resulted in increased alkaline phosphatase (ALP) activity, increased expression of CD44 mRNA, and increased secretion of osteoprotegrin (OPG) and receptor activator of nuclear factor-kappaB ligand (RANKL). The level of osteocalcin in the cell culture medium was increased after all treatments, while A2 stimulated the expression of dentin matrix protein 1 (DMP1) mRNA. The results suggest that both A1 and A2 participate in the observed effect of EMD, but have different effects on the expression of osteoblast mRNA and the secretion of osteoblast protein, and thus might facilitate the differentiation of a different phenotype.

Gene expression pattern during osteogenic differentiation of human periodontal ligament cells in vitro

Purpose

Periodontal ligament (PDL) cell differentiation into osteoblasts is important in bone formation. Bone formation is a complex biological process and involves several tightly regulated gene expression patterns of bone-related proteins. The expression patterns of bone related proteins are regulated in a temporal manner both in vivo and in vitro. The aim of this study was to observe the gene expression profile in PDL cell proliferation, differentiation, and mineralization in vitro.

Methods

PDL cells were grown until confluence, which were then designated as day 0, and nodule formation was induced by the addition of 50 µg/mL ascorbic acid, 10 mM β-glycerophosphate, and 100 nM dexamethasone to the medium. The dishes were stained with Alizarin Red S on days 1, 7, 14, and 21. Real-time polymerase chain reaction was performed for the detection of various genes on days 0, 1, 7, 14, and 21.

Results

On day 0 with a confluent monolayer, in the active proliferative stage, c-myc gene expression was observed at its maximal level. On day 7 with a multilayer, alkaline phosphatase, bone morphogenetic protein (BMP)-2, and BMP-4 gene expression had increased and this was followed by maximal expression of osteocalcin on day 14 with the initiation of nodule mineralization. In relationship to apoptosis, c-fos gene expression peaked on day 21 and was characterized by the post-mineralization stage. Here, various genes were regulated in a temporal manner during PDL fibroblast proliferation, extracellular matrix maturation, and mineralization. The gene expression pattern was similar.

Conclusions

We can speculate that the gene expression pattern occurs during PDL cell proliferation, differentiation, and mineralization. On the basis of these results, it might be possible to understand the various factors that influence PDL cell proliferation, extracellular matrix maturation, and mineralization with regard to gene expression patterns.

Role of diabetes- and obesity-related protein in the regulation of osteoblast differentiation

Although thyroid hormone (TH) is known to exert important effects on the skeleton, the nuclear factors constituting the TH receptor coactivator complex and the molecular pathways by which TH mediates its effects on target gene expression in osteoblasts remain poorly understood. A recent study demonstrated that the actions of TH on myoblast differentiation are dependent on diabetes- and obesity-related protein (DOR). However, the role of DOR in osteoblast differentiation is unknown. We found DOR expression increased during in vitro differentiation of bone marrow stromal cells into osteoblasts and also in MC3T3-E1 cells treated with TH. However, DOR expression decreased during cellular proliferation. To determine whether DOR acts as a modulator of TH action during osteoblast differentiation, we examined whether overexpression or knockdown of DOR in MC3T3-E1 cells affects the ability of TH to induce osteoblast differentiation by evaluating alkaline phosphatase (ALP) activity. ALP activity was markedly increased in DOR-overexpressing cells treated with TH. In contrast, loss of DOR dramatically reduced TH stimulation of ALP activity in MC3T3-E1 cells and primary calvaria osteoblasts transduced with lentiviral DOR shRNA. Consistent with reduced ALP activity, mRNA levels of osteocalcin, ALP, and Runx2 were decreased significantly in DOR shRNA cells. In addition, a common single nucleotide polymorphism (SNP), DOR1 found on the promoter of human DOR gene, was associated with circulating osteocalcin levels in nondiabetic subjects. Based on these data, we conclude that DOR plays an important role in TH-mediated osteoblast differentiation, and a DOR SNP associates with plasma osteocalcin in men.

ACE activity is modulated by the enzyme α-galactosidase A

Fabry disease is a multisystem X-linked disorder resulting from α-galactosidase A (α-GalA) gene mutations leading to the accumulation of globotriaosylceramide mainly in endothelium compromising heart, kidney, and brain. In Fabry patients, progressive renal failure is frequently treated with angiotensin I-converting enzyme (ACE) inhibitors. We were interested in the possible interactions between ACE inhibitors therapy and the only causative therapy for Fabry disease, the enzyme replacement therapy (ERT) using recombinant human α-GalA (rhα-GalA). Our results suggest that ACE activity was significantly inhibited in plasma of Fabry patients and the blood pressure level decreased just after ERT (at the end of the rhα-GalA infusion). Interestingly, 2 weeks later, ACE activity was significantly upregulated and the plasma levels of angiotensin II increased in the patients treated with rhα-GalA following the elevations of ACE activity. The same inhibitory effect on ACE activity was also observed in rats after rhα-GalA infusion. Furthermore, ACE activity in CHO cells transfected with the human ACE was inhibited dose and time-dependently by rhα-GalA. In vitro, the incubation of plasma from healthy volunteers with rhα-GalA significantly reduced ACE activity. Finally, rhα-GalA also inhibited ACE activity and released galactose residues from purified rabbit lung ACE dose-dependently. In summary, our results suggest that rhα-GalA interacts with ACE and inhibits its activity, possibly by removing the galactose residues from the enzyme. This modulation might have profound impact on the clinical outcome of Fabry patients treated with rhα-GalA.

Regulation of gene expression in osteoblasts

In recent years, much progress has been made in understanding the factors that regulate the gene expression program that underlies the induction, proliferation, differentiation, and maturation of osteoblasts. A large and growing number of transcription factors make important contributions to the precise control of osteoblast formation and function. It has become increasingly clear that these diverse transcription factors and the signals that regulate their activity cannot be viewed as discrete, separate signaling pathways. Rather, they form a highly interconnected, cooperative network that permits gene expression to be closely regulated. There has also been a substantial increase in our understanding of the mechanistic control of gene expression by cofactors such as acetyltransferases and histone deacetylases. The purpose of this review is to highlight recent progress in understanding the major transcription factors and epigenetic coregulators, including histone deacetylases and microRNAs, involved in osteoblastogenesis and the mechanisms that determine their functions as regulators of gene expression.

Parathyroid Hormone Stimulates Osteoblastic Expression of MCP-1 to Recruit and Increase the Fusion of Pre/Osteoclasts

The clinical findings that alendronate blunted the anabolic effect of human parathyroid hormone (PTH) on bone formation suggest that active resorption is involved and enhances the anabolic effect. PTH signals via its receptor on the osteoblast membrane, and osteoclasts are impacted indirectly via the products of osteoblasts. Microarray with RNA from rats injected with human PTH or vehicle showed a strong association between the stimulation of monocyte chemoattractant protein-1 (MCP-1) and the anabolic effects of PTH. PTH rapidly and dramatically stimulated MCP-1 mRNA in the femora of rats receiving daily injections of PTH or in primary osteoblastic and UMR 106-01 cells. The stimulation of MCP-1 mRNA was dose-dependent and a primary response to PTH signaling via the cAMP-dependent protein kinase pathway in vitro. Studies with the mouse monocyte cell line RAW 264.7 and mouse bone marrow proved that osteoblastic MCP-1 can potently recruit osteoclast monocyte precursors and facilitate receptor activator of NF-kappaB ligand-induced osteoclastogenesis and, in particular, enhanced fusion. Our model suggests that PTH-induced osteoblastic expression of MCP-1 is involved in recruitment and differentiation at the stage of multinucleation of osteoclast precursors. This information provides a rationale for increased osteoclast activity in the anabolic effects of PTH in addition to receptor activator of NF-kappaB ligand stimulation to initiate greater bone remodeling.

Monoclonal Antibodies 1G12 and 6A12 to the N-domain of human angiotensin-converting enzyme: fine epitope mapping and antibody-based detection of ACE inhibitors in human blood

Angiotensin I-converting enzyme (ACE), a key enzyme in cardiovascular pathophysiology, consists of two homologous domains (N- and C-), each bearing a Zn-dependent active site. ACE inhibitors are among the most prescribed drugs in the treatment of hypertension and cardiac failure. Fine epitope mapping of two monoclonal antibodies (mAb), 1G12 and 6A12, against the N-domain of human ACE, was developed using the N-domain 3D-structure and 21 single and double N-domain mutants. The binding of both mAbs to their epitopes on the N-domain of ACE is significantly diminished by the presence of the C-domain in the two-domain somatic tissue ACE and further diminished by the presence of sialic acid residues on the surface of blood ACE. The binding of these mAbs to blood ACE, however, increased dramatically (5-10-fold) in the presence of ACE inhibitors or EDTA, whereas the effect of these compounds on the binding of the mAbs to somatic tissue ACE was less pronounced and even less for truncated N-domain. This implies that the binding of ACE inhibitors or removal of Zn2+ from ACE active centers causes conformational adjustments in the mutual arrangement of N- and C-domains in the two-domain ACE molecule. As a result, the regions of the epitopes for mAb 1G12 and 6A12 on the N-domain, shielded in somatic ACE by the C-domain globule and additionally shielded in blood ACE by sialic acid residues in the oligosaccharide chains localized on Asn289 and Asn416, become unmasked. Therefore, we demonstrated a possibility to employ these mAbs (1G12 or 6A12) for detection and quantification of the presence of ACE inhibitors in human blood. This method should find wide application in monitoring clinical trials with ACE inhibitors as well as in the development of the approach for personalized medicine by these effective drugs.

Localization of an N-domain region of angiotensin-converting enzyme involved in the regulation of ectodomain shedding using monoclonal antibodies

ACE chimeric proteins and N domain monoclonal antibodies (mAbs) were used to determine the influence of the N domain, and particular regions thereof, on the rate of ACE ectodomain shedding. Somatic ACE (having both N and C domains) was shed at a rate of 20%/24 h. Deletion of the C domain of somatic ACE generated an N domain construct (ACEDeltaC) which demonstrated the lowest rate of shedding (12%). However, deletion of the N domain of somatic ACE (ACEDeltaN) dramatically increased shedding (212%). Testicular ACE (tACE) having 36 amino acid residues (heavily O-glycosylated) at the N-terminus of the C domain shows a 4-fold decrease in the rate of shedding (49%) compared to that of ACEDeltaN. When the N-terminal region of the C domain was replaced with the corresponding homologous 141 amino acids of the N domain (N-delACE) the rate of shedding of the ACEDeltaN was only slightly decreased (174%), but shedding was still 3.5-fold more efficient than wild-type testicular ACE. Monoclonal antibodies specific for distinct, but overlapping, N-domain epitopes altered the rate of ACE shedding. The mAb 3G8 decreased the rate of shedding by 30%, whereas mAbs 9B9 and 3A5 stimulated ACE shedding 2- to 4-fold. Epitope mapping of these mAbs in conjunction with a homology model of ACE N domain structure, localized a region in the N-domain that may play a role in determining the relatively low rate of shedding of somatic ACE from the cell surface.

Cell lines and primary cell cultures in the study of bone cell biology

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite and amorphous calcium phosphate crystals deposited in an organic matrix. Bone has two main functions. It forms a rigid skeleton and has a central role in calcium and phosphate homeostasis. The major cell types of bone are osteoblasts, osteoclasts and chondrocytes. In the laboratory, primary cultures or cell lines established from each of these different cell types provide valuable information about the processes of skeletal development, bone formation and bone resorption, leading ultimately, to the formulation of new forms of treatment for common bone diseases such as osteoporosis.

Parathyroid hormone uses multiple mechanisms to arrest the cell cycle progression of osteoblastic cells from G1 to S phase

Parathyroid hormone (PTH) plays a major role in bone remodeling and has the ability to increase bone mass if administered daily. In vitro, PTH inhibits the growth of osteoblastic cell lines, arresting them in G(1) phase. Here, we demonstrate that PTH regulates the expression of at least three genes to achieve the following: inducing expression of MAPK phosphatase 1 (MKP-1) and p21(Cip1) and decreasing expression of cyclin D1 at both mRNA and protein levels. The induction of MKP-1 causes the dephosphorylation of extracellular signal-regulated kinase and therefore the decrease in cyclin D1. Overexpression of MKP-1 arrests UMR cells in G(1) phase. The mechanisms involved in PTH regulation of these genes were studied. Most importantly, PTH administration produces similar effects on expression of these genes in rat femoral metaphyseal primary spongiosa. Analyses of p21(Cip1) expression levels in bone indicate that repeated daily PTH injections make the osteoblast more sensitive to successive PTH treatments, and this might be an important feature for the anabolic functions of PTH. In summary, our data suggest that one mechanism for PTH to exert its anabolic effect is to arrest the cell cycle progression of the osteoblast and hence increase its differentiation.

Gene expression profiles and transcription factors involved in parathyroid hormone signaling in osteoblasts revealed by microarray and bioinformatics

Parathyroid hormone (PTH) binds to its receptor PTH1R (parathyroid hormone 1 receptor) in osteoblastic cells to regulate bone remodeling and calcium homeostasis. While prolonged exposure to PTH causes increased bone resorption, intermittent injections of PTH have an anabolic effect on bone. The molecular mechanisms regulating these processes are still largely unknown. Here, we present our results on gene expression profile changes in the PTH-treated osteoblastic cell line, UMR 106-01, using DNA microarray analysis. A total of 125 known genes and 30 unknown expressed sequence tags (ESTs) were found to have at least 2-fold expression changes after PTH treatment at 4, 12, and 24 h. 14 genes were previously known to be PTH-regulated but many were unknown to be regulated by PTH prior to our experiments. Real-time reverse transcriptase-PCR confirmed that 90 and 50% of the genes are regulated more than 2-fold by PTH in UMR 106-01 and rat primary osteoblastic cells, respectively. Most genes belong to the following protein families: hormones, growth factors, and receptors; signal transduction pathway proteins; transcription factors; proteases; metabolic enzymes; structural and matrix proteins; transporters; etc. These results provide a comprehensive and deeper knowledge about PTH regulation of osteoblastic gene expression. Next, we designed a computational method to extract information about transcription factors likely involved in regulating these genes. These factors include those previously known to be involved in PTH signaling (AP-1 and the cAMP response element-binding protein), those that were identified by microarray data (C/EBP), and some novel transcription factors (AP-2, AP-4, SP1, FoxD3, etc.). Our results suggest that a reliable bioinformatics approach can be easily applied for other systems.

Leptin stimulates human osteoblastic cell proliferation, de novo collagen synthesis, and mineralization: Impact on differentiation markers, apoptosis, and osteoclastic signaling

Anabolic hormones, mechanical loading, and the obese protein leptin play separate roles in maintaining bone mass. We have previously shown that leptin, as well as its receptor, are expressed by normal human osteoblasts. Consequently, we have investigated how leptin affects proliferation, differentiation, and apoptosis of human osteoblasts. Iliac crest osteoblasts, incubated with either leptin (100 ng/ml), calcitriol (1,25(OH)(2)D(3); 10(-9) M) or 1-84 human parathyroid hormone (PTH; 10(-8) M), were cultured for 35 consecutive days and assayed for expression of various differentiation-related marker genes (as estimated by RT-PCR), de novo collagen synthesis, proliferation, in vitro mineralization, and osteoclast signaling. The effects of leptin on protection against retinoic acid (RA; 10(-7) M) induced apoptosis, as well as transition into preosteocytes, were also tested. Leptin exposure enhanced cell proliferation and collagen synthesis over both control condition and PTH exposure. Leptin inhibited in vitro calcified nodule production after 1-2 weeks in culture, however, subsequent to 4-5 weeks, leptin significantly stimulated mineralization. The mineralization profile throughout the entire incubation period was almost undistinguishable from the one induced by PTH. In comparison, 1,25(OH)(2)D(3) generally reduced proliferation and collagen production rates, whereas mineralization was markedly enhanced. Leptin exposure (at 2 and 5 weeks) significantly enhanced the expression of TGFbeta, IGF-I, collagen-Ialpha, ALP, and osteocalcin mRNA. Leptin also protected against RA-induced apoptosis, as estimated by soluble DNA fractions and DNA laddering patterns subsequent to 10 days of culture. The expression profiles of Bax-alpha and Bcl-2 mRNAs indicated that leptin per se significantly protected against apoptosis throughout the entire incubation period. Furthermore, the osteoblast marker OSF-2 was diminished, whereas the CD44 osteocyte marker gene expression was stimulated, indicating a transition into preosteocytes. In terms of osteoclastic signaling, leptin significantly augmented the mRNA levels of both interleukin-6 (IL-6) and osteoprotegerin (OPG). In summary, continuous leptin exposure of iliac crest osteoblasts, promotes collagen synthesis, cell differentiation and in vitro mineralization, as well as cell survival and transition into preosteocytes. Leptin may also facilitate osteoblastic signaling to the osteoclast.

Parathyroid hormone inhibits c-Jun N-terminal kinase activity in rat osteoblastic cells by a protein kinase A-dependent pathway

Treatment of osteoblastic cells with PTH initiates dual signaling cascades resulting in activation of both PKA and PKC. It has been shown that PTH either inhibits or stimulates ERKs depending on dose of the hormone; nevertheless, the ability of PTH to regulate other members of the MAPK family is unknown. Another member of this family, c-Jun-NH(2)-terminal kinase (JNK), is preferentially activated by cytokines and cellular stresses and plays a key role in regulating the activity of various transcription factors. We demonstrate that treatment of UMR 106-01 cells and rat calvarial osteoblasts with PTH (10(-8) M), N-terminal peptides of PTH that selectively activate PKA, or 8-bromo-cAMP (activates PKA) results in the inhibition of JNK activity from high basal levels. Examination of the upstream members of the JNK cascade revealed that both stress-activated protein kinase/extracellular signal-related kinase kinase 1/MAPK kinase 4 and MAPK/extracellular signal-related kinase kinase kinase 1 activities were also inhibited after treatment with PTH (10(-8) M). We conclude that treatment of osteoblastic cells with PTH is sufficient to inhibit high basal JNK activity by activation of the PKA signaling cascade.

Cysteine derivatives as inhibitors for carboxypeptidase A: synthesis and structure-activity relationships

A series of cysteine (Cys) derivatives having an alkyl or arylalkyl moiety on the alpha-amino group of the amino acid have been synthesized as a novel type of inhibitor for carboxypeptidase A. These compounds are readily prepared starting with Cys in an optically active form. The structure-activity relationship study revealed that the inhibitors prepared from D-Cys are much more potent than the corresponding inhibitors obtained from L-Cys, and the most potent inhibitor in the series, (S)-1j with a K(i) value of 55 +/- 4 nM, is obtained by introducing a phenethyl moiety on the amino group of D-Cys. In comparison, the most active inhibitor in the series of 2-substituted 3-mercaptopropanoic acid is found to be 20, in which the phenyl ring is linked to the mercaptocarboxylic acid at the alpha-position with a methylene unit. A proposal that accounts for the different structural requirement for the maximum activity between the two series of inhibitors is provided.

Stimulation of extracellular signal-regulated kinases and proliferation in rat osteoblastic cells by parathyroid hormone is protein kinase C-dependent

Parathyroid hormone (PTH) is known to have both catabolic and anabolic effects on bone. The dual functionality of PTH may stem from its ability to activate two signal transduction mechanisms: adenylate cyclase and phospholipase C. Here, we demonstrate that continuous treatment of UMR 106-01 and primary osteoblasts with PTH peptides, which selectively activate protein kinase C, results in significant increases in DNA synthesis. Given that ERKs are involved in cellular proliferation, we examined the regulation of ERKs in UMR 106-01 and primary rat osteoblasts following PTH treatment. We demonstrate that treatment of osteoblastic cells with very low concentrations of PTH (10(-12) to 10(-11) m) is sufficient for substantial increases in ERK activity. Treatment with PTH-(1-34) (10(-8) m), PTH-(1-31), or 8-bromo-cAMP failed to stimulate ERKs, whereas treatment with phorbol 12-myristate 13-acetate, serum, or PTH peptides lacking the N-terminal amino acids stimulated activity. Furthermore, the activation of ERKs was prevented by pretreatment of osteoblastic cells with inhibitors of protein kinase C (GF 109203X) and MEK (PD 98059). Treatment of UMR cells with epidermal growth factor (EGF), but not PTH, promoted tyrosine phosphorylation of the EGF receptor. Transient transfection of UMR cells with p21(N17Ras) did not block activation of ERKs following treatment with low concentrations of PTH. Thus, activation of ERKs and proliferation by PTH is protein kinase C-dependent, but stimulation occurs independently of the EGF receptor and Ras activation.

Induction of tumor cell differentiation

It is proposed that cell proliferation with reduced individual cell growth (total protein accumulation) is necessary, but not sufficient, for cell differentiation. These conditions may facilitate transcription and accumulation of histones H1 and/or H1o relative to the core histones. This may have a critical role in cell differentiation.

Histone variants of H2A and H3 families are regulated during in vitro aging in the same manner as during differentiation

In a previous communication, we showed that the H2A.1/H2A.2 histone variant ratio decreases in a linear manner during the in vitro aging of human diploid fibroblasts. This ratio is known to decrease in the same manner in progressive stages of development and in the process of differentiation, and is thus considered to be a biochemical marker for differentiation. A detailed analysis of the synthesis of H2A and H3 histone variants as a function of cumulative population doublings in the same in vitro cell system is presented in this study. Quantitative analysis of these variants in the G0 phase, synchronized fibroblasts has shown that their relative amount in chromatin, as well as their biosynthesis rate, change during in vitro aging of human diploid fibroblasts, revealing both up-and down-regulation of certain variants as a function of cumulative population doublings. Furthermore, we show by morphometric studies employing the seven distinct fibroblast morphotypes, as described by the Bayreuther classification, that this regulation is attributable to the replicative sub-populations. These results reveal that histone variants of the H2A and H3 families are regulated during in vitro aging in the same manner as that during differentiation.

Selective expression of specific histone H4 genes reflects distinctions in transcription factor interactions with divergent H4 promoter elements

Expression of many histone H4 genes is stringently controlled during the cell cycle to maintain a functional coupling of histone biosynthesis with DNA replication. The histone H4 multigene family provides a paradigm for understanding cell cycle control of gene transcription. All functional histone H4 gene copies are highly conserved in the mRNA coding region. However, the putative promoter regions of these H4 genes are divergent. We analyzed three representative mouse H4 genes to assess whether variation in H4 promoter sequences has functional consequences for the relative level and temporal control of expression of distinct H4 genes. Using S1 nuclease protection assays with gene-specific probes and RNA from synchronized cells, we show that the mRNA level of each H4 gene is temporally coupled to DNA synthesis. However, there are differences in the relative mRNA levels of these three H4 gene copies in several cell types. Based on gel shift assays, nucleotide variations in the promoters of these H4 genes preclude or reduce binding of several histone gene transcription factors, including IRF2, HiNF-D, SP-1 and/or YY1. Therefore, differential regulation of H4 genes is directly attributable to evolutionary divergence in H4 promoter organization which dictates the potential for regulatory interactions with cognate H4 transcription factors. This regulatory flexibility in H4 promoter organization may maximize options for transcriptional control of histone H4 gene expression in response to the onset of DNA synthesis and cell cycle progression in a broad spectrum of cell types and developmental stages.

Peripheral blood lymphocytes of bipolar affective patients have a histone synthetic profile indicative of an active cell state

1. Although abnormalities of the immune system have been described in depression, no information exists regarding the biochemical parameters which could characterize the physiological state of lymphocytes from patients with bipolar affective disorder. 2. Lymphocytes of normal control subjects are known to be in the Go resting phase of the cell cycle. Histone synthesis is characteristically different during the Go, G1/G2 and the S phases of the cell cycle. As such, it can be used as a biochemical marker with which to distinguish between cycling and noncycling cells. 3. In order to investigate the possibility of whether or not the lymphocytes of patients with bipolar affective disorder are in an activated state, typical of cycling cells, total histone and histone variant synthesis were analysed in peripheral blood lymphocytes of a group of 12 patients with bipolar affective disorder and 7 normal controls. 4. According to the histone variant synthesis pattern, lymphocytes of patients in normothymia have values similar to those of controls, i.e., of noncycling cells, while patients in either the depressed or the manic phase have values intermediate to those of resting and cycling cells. 5. This study shows that histone synthesis can perhaps be used as a biochemical parameter of possible significance in differentiating amongst the three phases of the illness.

Expression patterns of bone-related proteins during osteoblastic differentiation in MC3T3-E1 cells

Bone formation involves several tightly regulated gene expression patterns of bone-related proteins. To determine the expression patterns of bone-related proteins during the MC3T3-E1 osteoblast-like cell differentiation, we used Northern blotting, enzymatic assay, and histochemistry. We found that the expression patterns of bone-related proteins were regulated in a temporal manner during the successive developmental stages including proliferation (days 4-10), bone matrix formation/maturation (days 10-16), and mineralization stages (days 16-30). During the proliferation period (days 4-10), the expression of cell-cycle related genes such as histone H3 and H4, and ribosomal protein S6 was high. During the bone matrix formation/maturation period (days 10-16), type I collagen expression and biosynthesis, fibronectin, TGF-beta 1 and osteonectin expressions were high and maximal around day 16. During this maturation period, we found that the expression patterns of bone matrix proteins were two types: one is the expression pattern of type I collagen and TGF-beta 1, which was higher in the maturation period than that in both the proliferation and mineralization periods. The other is the expression pattern of fibronectin and osteonectin, which was higher in the maturation and mineralization periods than in the proliferation period. Alkaline phosphatase activity was high during the early matrix formation/maturation period (day 10) and was followed by a decrease to a level still significantly above the baseline level seen at day 4. During the mineralization period (days 16-30), the number of nodules and the expression of osteocalcin were high. Osteocalcin gene expression was increased up to 28 days. Our results show that the expression patterns of bone-related proteins are temporally regulated during the MC3T3-E1 cell differentiation and their regulations are unique compared with other systems. Thus, this cell line provides a useful in vitro system to study the developmental regulation of bone-related proteins in relation to the different stages during the osteoblast differentiation.

Cytokine induction of proliferation and expression of CDC2 and cyclin A in FDC-P1 myeloid hematopoietic progenitor cells: regulation of ubiquitous and cell cycle-dependent histone gene transcription factors

To evaluate transcriptional mechanisms during cytokine induction of myeloid progenitor cell proliferation, we examined the expression and activity of transcription factors that control cell cycle-dependent histone genes in interleukin-3 (IL-3)-dependent FDC-P1 cells. Histone genes are transcriptionally upregulated in response to a series of cellular regulatory signals that mediate competency for cell cycle progression of the G1/S-phase transition. We therefore focused on factors that are functionally related to activity of the principal cell cycle regulatory element of the histone H4 promoter: CDC2, cyclin A, as well as RB- and IRF-related proteins. Comparisons were made with activities of ubiquitous transcription factors that influence a broad spectrum of promoters independent of proliferation or expression of tissue-specific phenotypic properties. Northern blot analysis indicates that cellular levels of cyclin A and CDC2 mRNAs increase when DNA synthesis and H4 gene expression are initiated, supporting involvement in cell cycle progression. Using gel-shift assays, incorporating factor-specific antibody and oligonucleotide competition controls, we define three sequential period following cytokine stimulation of FDC-P1 cells when selective upregulation of a subset of transcription factors is observed. In the initial period, the levels of SP1 and HiNF-P are moderately elevated; ATF, AP-1, and HiNF-M/IRF-2 are maximal during the second period; while E2F and HiNF-D, which contain cyclin A as a component, predominate during the third period, coinciding with maximal H4 gene expression and DNA synthesis. Differential regulation of H4 gene transcription factors following growth stimulation is consistent with a principal role of histone gene promoter elements in integrating cues from multiple signaling pathways that control cell cycle induction and progression. Regulation of transcription factors controlling histone gene promoter activity within the context of a staged cascade of responsiveness to cyclins and other physiological mediators of proliferation in FDC-P1 cells provides a paradigm for experimentally addressing interdependent cell cycle and cell growth parameters that are operative in hematopoietic stem cells.

Administration of colony stimulating factor-1 to toothless osteopetrotic rats normalizes osteoblast, but not osteoclast, gene expression

The toothless (tl) osteopetrotic mutation in the rat is characterized by generalized skeletal sclerosis, a severe reduction in the numbers of osteoclasts, monocytes, and macrophages, and absence of tooth eruption. Studies examining gene expression in bone-derived cells of tl rats and their normal littermates have shown that genes related to osteoblast function are aberrantly expressed in tl rats compared to normal littemates. We have previously shown that exogenous administration of colony stimulating factor-1 (CSF-1) to tl rats results in a dramatic reduction of the skeletal sclerosis and significant increases in the number of osteoclasts. Thus, we examined the effects of CSF-1 on osteoblast and osteoclast gene expression in tl rats as demonstrated by Northern blot analysis. While osteoblast-related gene expression as reflected by mRNA levels of alkaline phosphatase, osteocalcin, osteopontin, and type I collagen was normalized, osteoclast-related gene expression, as reflected by mRNA levels of carbonic anhydrase II and tartrate-resistant adenosine triphosphatase, remained significantly lower in CSF-1-treated tl rats compared to untreated normal littermates. Since previous studies have not demonstrated the CSF-1 receptor on osteoblasts, these results suggest that osteoblast abnormalities in tl rats are an effect of the osteopetrotic condition rather than the cause of the disease.

Histone H4 proximal promoter mediates a complex transcriptional response during differentiation of 3T3L1 adipocytes

We have investigated the promoter element(s) required by the cell cycle regulated FO108 human histone H4 gene for control of gene expression during adipocyte proliferation and differentiation. Stable 3T3L1 cell lines were established that express fusion genes in which the histone H4 promoter is joined to chloramphenicol acetyltransferase (cat) as a reporter gene. Expression of the H4CAT fusion genes was monitored in proliferating and confluent 3T3L1 preadipocytes and in differentiating 3T3L1 adipocytes. The results indicate that the H4 cell cycle element (CCE), which mediates S phase-specific stimulation of H4 gene transcription, is not required for transcriptional regulation during differentiation. Instead, a minimal H4 promoter (nucleotides -46 to -11) is sufficient to mediate the complex transcriptional response of H4 gene expression observed during the process of adipocyte differentiation of 3T3L1 cells. In addition, the data suggest that down-regulation of histone gene expression during cellular differentiation may be mediated by passive inactivation of the promoter due to loss of positive regulatory factor(s).

Formation and mineralization of extracellular matrix secreted by an immortal human osteoblastic cell line: modulation by tumor necrosis factor-alpha

Tumor necrosis factor-alpha (TNF-alpha), a 17-kDa cytokine produced by stimulated macrophages/monocytes, modulates the functions of a variety of cells and has been shown to induce bone resorption in vitro. However, the effects that TNF-alpha may have on the process of bone formation are not completely understood. In order to study the effects of TNF-alpha on matrix development and mineralization, we utilized a human osteoblastic cell line, HOS TE85. Our results show that HOS TE85, which has been shown to be responsive to hormones active on normal osteoblasts, forms an extensive extracellular matrix (ECM) that mineralizes during extended culture. Treatment during the development of the matrix with TNF-alpha has little effect on cell number and DNA synthesis, showing thereby that TNF-alpha is not cytotoxic to the cells. However, TNF-alpha inhibits the formation of alkaline phosphatase (AP)-positive foci in a dose-dependent manner at concentrations of 0.1-10 ng/ml. TNF-alpha treatment caused a significant decrease in the incorporation of collagen into the developing matrix. In addition, TNF-alpha treatment resulted in a significant decrease in the synthesis of AP by HOS TE85 cells during the process of ECM formation and resulted in a pronounced lack of mineralization of the ECM. These results indicate that TNF-alpha may be acting as an uncoupler by decreasing the synthesis and incorporation of proteins required for bone formation, and inhibiting matrix formation and mineralization in vitro.

Cell cycle controlled histone H1, H3, and H4 genes share unusual arrangements of recognition motifs for HiNF-D supporting a coordinate promoter binding mechanism

Cell cycle and growth control of the DNA binding and transactivation functions of regulatory factors provides a direct mechanism by which cells may coordinate transcription of a multitude of genes in proliferating cells. The promoters of human DNA replication dependent histone H4, H3, and H1 genes interact with at least seven distinct proteins. One of these proteins is a proliferation-specific nuclear factor, HiNF-D, that interacts with a key cis-regulatory element (H4-Site II; 41 bp) present in H4 genes. Here we describe binding sites for HiNF-D in the promoters of H3 and H1 genes using cross-competition, deletion analysis, and methylation interference assays, and we show that HiNF-D recognizes intricate arrangements of at least two sequence elements (CA- and AG-motifs). These recognition motifs are irregularly dispersed and distantly positioned in the proximal promoters (200 bp) of both the H3 and H1 genes. In all cases, these motifs either overlap or are in close proximity to other established transcriptional elements, including ATF and CCAAT sequences. Although HiNF-D can interact with low affinity to a core recognition domain, auxiliary elements in both the distal and proximal portions of each promoter cooperatively enhance HiNF-D binding. Thus, HiNF-D appears to bridge remote regulatory regions, which may juxtapose additional trans-activating proteins interacting within histone gene promoters. Consistent with observations in many cell culture systems, the interactions of HiNF-D with the H4, H3, and H1 promoters are modulated in parallel during the cessation of proliferation in both osteosarcoma cells and normal diploid osteoblasts, and these events occur in conjunction with concerted changes in histone gene expression. Thus, HiNF-D represents a candidate participant in coordinating transcriptional control of several histone gene classes.

Histone gene transcription: a model for responsiveness to an integrated series of regulatory signals mediating cell cycle control and proliferation/differentiation interrelationships

Histone gene expression is restricted to the S-phase of the cell cycle. Control is at multiple levels and is mediated by the integration of regulatory signals in response to cell cycle progression and the onset of differentiation. The H4 gene promoter is organized into a series of independent and overlapping regulatory elements which exhibit selective, phosphorylation-dependent interactions with multiple transactivation factors. The three-dimensional organization of the promoter and, in particular, its chromatin structure, nucleosome organization, and interactions with the nuclear matrix may contribute to interrelationships of activities at multiple promoter elements. Molecular mechanisms are discussed that may participate in the coordinate expression of S-phase-specific core and H1 histone genes, together with other genes functionally coupled with DNA replication.

Expression of bone morphogenetic protein messenger RNA in prolonged cultures of fetal rat calvarial cells

In addition to structural proteins of bone, such as type I collagen, bone cells synthesize a number of growth regulatory peptides that are also stored in the bone matrix, presumably as a consequence of local production by osteoblasts. Among the bone growth regulatory peptides found in the bone matrix are the recently described bone morphogenetic proteins (BMPs). These factors were purified from bone matrix by their capacity to stimulate ectopic bone formation, but it is not known whether they are produced by normal bone cells and influence normal bone formation. To determine whether they are expressed by normal osteoblasts during differentiation, we used the technique of prolonged primary culture of fetal rat calvarial osteoblasts. These cultures have been shown to be an informative model for studying expression of bone-related genes by cultured osteoblasts, since specific genes are expressed as the cells undergo proliferation and differentiation. We found that the bone morphogenetic proteins 1, 2, 4, and 6 are expressed by cultures of fetal rat calvarial osteoblasts before they form mineralized bone nodules and as they express alkaline phosphatase, osteocalcin, and osteopontin. This model can be used for study of regulation of expression of bone morphogenetic proteins by osteoblasts.

In situ hybridization studies suggest a role for the basic region-leucine zipper protein hXBP-1 in exocrine gland and skeletal development during mouse embryogenesis

The spatial and temporal distribution of transcripts for the TRE/CRE-binding basic region-leucine zipper protein hXBP-1 was determined by in situ hybridization. Analysis of embryos from day 10.5 to 18.5 pc revealed high level expression of hXBP-1 RNA in two developing organ systems: 1) in bone and cartilage cells of the developing skeleton and toothbuds, and 2) in exocrine glands including the pancreas and the submandibular and salivary glands. High level expression was also found in whisker follicles and in selected cells in brown adipose tissue. In the developing skeleton, hXBP-1 RNA was expressed starting on day 11.5 pc in osteoblasts of newly formed intramembranous bone. Thereafter, hXBP-1 was expressed in both osteoblasts and preosteoblasts in bone formed directly by intramembranous formation as well as in bone formed during endochondral ossification. The most intense signal was observed in preosteoblasts and osteoblasts of newly forming bone. At day 11.5 pc low level hXBP-1 expression was also observed in matrix secreting chondroblasts of bones which are formed initially of cartilage, at the stage where they consist entirely of cartilage. Signal was also present in matrix producing chondroblasts of the mature zone of the growth region during endochondral ossification although at significantly lower level than in osteoblasts. hXBP-1 is thus the first transcription factor described, to our knowledge, whose level of expression is modulated during the osteoblast developmental sequence in vivo. The pattern of expression of hXBP-1 in the developing skeleton was found to be very similar to that of the genes encoding the tissue inhibitor of metalloproteinase and alkaline phosphatase throughout development. These observations suggest that hXBP-1 may play a role in regulating the expression of tissue specific genes (TIMP, osteonectin, osteopontin, osteocalcin) expressed in osteoblasts. It is intriguing that the promoter regions of several such genes contain potential hXBP-1 binding sites.

A human histone H2B.1 variant gene, located on chromosome 1, utilizes alternative 3' end processing

A variant human H2B histone gene (GL105), previously shown to encode a 2300 nt replication independent mRNA, has been cloned. We demonstrate this gene expresses alternative mRNAs regulated differentially during the HeLa S3 cell cycle. The H2B-Gl105 gene encodes both a 500 nt cell cycle dependent mRNA and a 2300 nt constitutively expressed mRNA. The 3' end of the cell cycle regulated mRNA terminates immediately following the region of hyphenated dyad symmetry typical of most histone mRNAs, whereas the constitutively expressed mRNA has a 1798 nt non-translated trailer that contains the same region of hyphenated dyad symmetry but is polyadenylated. The cap site for the H2B-GL105 mRNAs is located 42 nt upstream of the protein coding region. The H2B-GL105 histone gene was localized to chromosome region 1q21-1q23 by chromosomal in situ hybridization and by analysis of rodent-human somatic cell hybrids using an H2B-GL105 specific probe. The H2B-GL105 gene is paired with a functional H2A histone gene and this H2A/H2B gene pair is separated by a bidirectionally transcribed intergenic promoter region containing consensus TATA and CCAAT boxes and an OTF-1 element. These results demonstrate that cell cycle regulated and constitutively expressed histone mRNAs can be encoded by the same gene, and indicate that alternative 3' end processing may be an important mechanism for regulation of histone mRNA. Such control further increases the versatility by which cells can modulate the synthesis of replication-dependent as well as variant histone proteins during the cell cycle and at the onset of differentiation.

Analysis of hepatocellular proliferation: study of archival liver tissue is facilitated by an endogenous marker of DNA replication

Assessment of liver regeneration with endogenous genes that are expressed during DNA replication is physiological, specific and direct. To determine whether H3 histone messenger RNA expression (which is tightly coupled with DNA synthesis) could be used for this purpose, we initially examined liver regeneration in a mouse model. After partial hepatectomy, RNA transblot studies showed induction of H3 histone messenger RNA expression in regenerating mouse livers. In situ molecular hybridization demonstrated that the overall pattern of H3 histone messenger RNA expression correlated with [3H]thymidine labeling of hepatocytes. After partial hepatectomy, H3 histone messenger RNA expression in hepatocytes peaked at 48 hr (greater than 60 times greater than at 24 hr; p less than 0.001) and then rapidly declined. Although hepatocyte labeling with [3H]thymidine showed similar kinetics of liver regeneration, use of this parameter resulted in overestimation of the proliferative compartment when it was compared with H3 histone messenger RNA expression. Next we determined whether H3 histone messenger RNA expression could be used to study hepatocellular proliferation in archival human material. H3 histone messenger RNA-expressing hepatocytes were identified on in situ hybridization in patients with acute or chronic active hepatitis and active cirrhosis, but not inactive cirrhosis. These studies demonstrate that H3 histone messenger RNA is expressed in a phasic manner during liver regeneration. Use of H3 histone messenger RNA expression to evaluate hepatocellular proliferation should facilitate clinical studies and greatly advance our understanding of the pathophysiology of liver regeneration.

Differential regulation of H4 histone gene expression in 3T3-L1 pre-adipocytes during arrest of proliferation following contact inhibition or differentiation and its modulation by TGF beta 1

The aim of this study was to address whether there is a fundamental difference in regulation of histone gene expression in cells that have become quiescent but retain the ability to proliferate, compared with those cells that have differentiated. We compared multiple levels of regulation of histone gene expression during 3T3-L1 pre-adipocyte differentiation. Confluent cells induced to differentiate by treatment with insulin, dexamethasone, and isobutylmethylxanthine initially exhibited an increased proliferative response compared with cells given serum alone. This initial differentiation response was associated with a twofold increase in both histone gene transcription and cellular histone mRNA levels, as well as with enhanced sequence-specific binding of nuclear factors to the proximal cell-cycle-regulatory element of the H4 histone promoter. Transforming growth factor beta 1, an inhibitor of 3T3-L1 differentiation, increased both the percentage of proliferating cells and the cellular levels of histone mRNA when given in addition to serum stimulation, but no enhancement of these parameters was observed upon addition of TGF beta 1 to the differentiation treatment. Interestingly, although TGF beta 1 enhanced binding of nuclear factors to the proximal cell cycle regulatory element of the histone promoter, these protein/DNA interactions were not associated with an increase in histone transcription. Our results are consistent with the down-regulation of histone gene expression at confluency being controlled primarily at the post-transcriptional level, in contrast to an increased involvement of transcriptional down-regulation at the onset of differentiation.

The proto-oncogene c-myc is involved in cell differentiation as well as cell proliferation: studies on growth plate chondrocytes in situ

A combination of immunocytochemistry and microdensitometry has been used to localize and quantify the expression of the proto-oncogene c-myc within chondrocytes of the proximal growth plates of rat and chick long bones. Although the c-myc protein was localized in all chondrocytes of the growth plate of both species the most intense staining was restricted to the proliferating and differentiating chondrocytes. These were identified by their ability to synthesize DNA (bromodeoxyuridine positive) and the presence of alkaline phosphatase activity, respectively. Species differences did exist with the c-myc concentration of the chick proliferating and differentiating chondrocytes being higher (128% and 240%, respectively) than the respective chondrocytes of the rat. The higher c-myc concentration in the chick proliferating chondrocytes paralleled the differences in the bromodeoxyuridine labelling index between the two species. In the rat, the concentration of c-myc protein present in the differentiating chondrocytes was 74% higher than in the respective proliferating chondrocytes, while in the chick it was 146% higher. The data not only provides further evidence for a role of the c-myc protein in cell proliferation but also suggests involvement of this protein in chondrocyte differentiation and/or hypertrophy.

Transcriptional control of vitamin D-regulated proteins

Vitamin D is a physiological regulator of gene transcription associated with control of a broad spectrum of biological processes that include but is not restricted to growth, differentiation and calcium-mediated homeostatic control. Transcriptional regulation is mediated by sequence-specific interactions of a 1,25(OH)2D3-vitamin D receptor-accessory factor complex with vitamin D responsive elements (VDRE) residing in the promoters of hormone responsive genes. Functioning primarily as a transcription enhancer, activity at the VDRE is controlled by diverse and integrated cellular signalling pathways acting synergistically and/or antagonistically with a series of basal regulatory elements and other hormone regulated sequences that are components of modularly organized vitamin D-responsive gene promoters. Molecular mechanisms that integrate the activities at promoter elements contributing to vitamin D-related transcriptional control include overlapping transcription factor binding domains within regulatory elements and cooperative activities at independent regulatory sequences that determine the level of vitamin D responsiveness.

Regulation of histone gene expression

Histone genes are expressed during the S phase of the cell cycle. Control is at multiple levels and is mediated by the integration of regulatory signals in response to cell-cycle progression and the onset of differentiation. Much work has been carried out on the H4 gene promoter, which appears to be organized into a series of distinct regulatory elements. The three-dimensional organization of the promoter and, in particular, its spatial relationship with the nuclear matrix scaffold, may be important factors of transcription regulation.

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.

Expression of heat shock genes during differentiation of mammalian osteoblasts and promyelocytic leukemia cells

The progressive differentiation of both normal rat osteoblasts and HL-60 promyelocytic leukemia cells involves the sequential expression of specific genes encoding proteins that are characteristic of their respective developing cellular phenotypes. In addition to the selective expression of various phenotype marker genes, several members of the heat shock gene family exhibit differential expression throughout the developmental sequence of these two cell types. As determined by steady state mRNA levels, in both osteoblasts and HL-60 cells expression of hsp27, hsp60, hsp70, hsp89 alpha, and hsp89 beta may be associated with the modifications in gene expression and cellular architecture that occur during differentiation. In both differentiation systems, the expression of hsp27 mRNA shows a 2.5-fold increase with the down-regulation of proliferation while hsp60 mRNA levels are maximal during active proliferation and subsequently decline post-proliferatively. mRNA expression of two members of the hsp90 family decreases with the shutdown of proliferation, with a parallel relationship between hsp89 alpha mRNA levels and proliferation in osteoblasts and a delay in down-regulation of hsp89 alpha mRNA levels in HL-60 cells and of hsp89 beta mRNA in both systems. Hsp70 mRNA rapidly increases, almost twofold, as proliferation decreases in HL-60 cells but during osteoblast growth and differentiation was only minimally detectable and showed no significant changes. Although the presence of the various hsp mRNA species is maintained at some level throughout the developmental sequence of both osteoblasts and HL-60 cells, changes in the extent to which the heat shock genes are expressed occur primarily in association with the decline of proliferative activity. The observed differences in patterns of expression for the various heat shock genes are consistent with involvement in mediating a series of regulatory events functionally related to the control of both cell growth and differentiation.

Regulation of cell cycle and growth control

The potential biological effects of electric and/or magnetic fields on cells and tissues must be addressed systematically within a context of perturbations in cell cycle control. Such studies should not be pursued in an isolated manner but as a component of the fundamental relationship between proliferation and differentiation, the multi-step process by which structural and functional properties of specialized cells, tissues, and organs progressively develop. It is necessary to quantitatively establish the influence of electric and magnetic fields on the integrated signalling mechanisms which transduce regulatory information for 1) control of the proliferative process and 2) down-regulation of proliferation associated with the initiation of gene expression that mediates the development and maintenance of phenotypic properties characteristic of differentiated cells. We will present an overview of our current understanding of regulatory mechanisms that control proliferation and cell specialization in normal diploid cells with emphasis on rate limiting steps that may be the basis for biological perturbations by electric and magnetic fields. Addressing such questions in normal diploid cells is essential since the loss of growth control in transformed and tumor cells is accompanied by an abrogation of developmental regulatory mechanisms that are functionally coupled to proliferation.

Myeloma affects both the growth and function of human osteoblast-like cells

Myeloma behaves differently to other osteolytic tumours which metastasize to bone, in that the latter usually provoke reactive bone formation in the host bone. A previous study showed that a myeloma cell line (GM1500) secreted an osteoblast-inhibiting factor(s). The present study was undertaken to determine whether other myeloma cells also secreted a factor(s) which inhibited both cell proliferation and DNA synthesis of osteoblast-like cells and whether the myeloma also affected the function of osteoblasts. The results showed that a second cell line (Karpas 707) as well as myeloma tissue taken from two patients had a similar effect. The myeloma cells did not affect total collagen or protein synthesis, and did not affect the overall degree of mineralization. A biphasic effect was seen on alkaline phosphatase activity. Thus, although the proliferation of the pre-osteoblast was affected, the synthetic functions of the osteoblasts were not.

Regulation of transcription-factor activity during growth and differentiation: involvement of the nuclear matrix in concentration and localization of promoter binding proteins

Several lines of evidence are presented which support involvement of the nuclear matrix in regulating the transcription of two genes, histone and osteocalcin, that are reciprocally expressed during development of the osteoblast phenotype. In the 5' regulatory region of an H4 histone gene, which is expressed in proliferating osteoblasts early during the developmental/differentiation sequence, a dual role is proposed for the nuclear matrix binding domain designated NMP-1 (-589 to -730 upstream from the transcription start site). In addition to functioning as a nuclear matrix attachment site, the sequences contribute to the upregulation of histone gene transcription, potentially facilitated by concentration and localization of an 84kD ATF DNA binding protein. A homologous nuclear matrix binding domain was identified in the promoter of the osteocalcin gene, which is expressed in mature osteoblasts in an extracellular matrix undergoing mineralization. The NMP binding domain in the osteocalcin gene promoter resides contiguous to the vitamin D responsive element. Together with gene and transcription factor localization, a model is proposed whereby nuclear matrix-associated structural constraints on conformation of the osteocalcin gene promoter facilitates vitamin D responsiveness mediated by cooperativity at multiple regulatory elements.

Influence of dexamethasone on the vitamin D-mediated regulation of osteocalcin gene expression

The influence of dexamethasone on expression of the osteocalcin gene which encodes the most abundant non-collagenous and only reported bone-specific protein was examined in ROS 17/2.8 osteosarcoma cells which express a broad spectrum of genes related to bone formation. Consistent with previous reports, quantitation of cellular osteocalcin mRNA levels by Northern blot analysis, osteocalcin gene transcription by activity of the osteocalcin gene promoter fused to a chloramphenicol acetyl-transferase (CAT) mRNA coding sequence following transfection into ROS 17/2.8 cells, and osteocalcin biosynthesis by radioimmunoassay indicate that dexamethasone in a concentration range of 10(-6) to 10(-9) M only modestly modifies basal levels of osteocalcin gene expression. However, dexamethasone significantly inhibits these parameters of the vitamin D-induced upregulation of osteocalcin gene expression in both proliferating and in confluent ROS 17/2.8 cells. In this study, we observed that the extent to which abrogation of the vitamin D response occurs is dependent on basal levels of osteocalcin gene expression as reflected by a complete inhibition of the vitamin D-induced upregulation in a ROS 17/2.8K subline with low basal expression and only a partial reduction of the vitamin D stimulation in a ROS 17/2.8C subline with eightfold higher levels of basal expression. This effect of glucocorticoid appears to be at the transcriptional and post-transcriptional levels as demonstrated by a parallel decline in the cellular representation of osteocalcin mRNA, osteocalcin gene promoter activity, and osteocalcin biosynthesis. The complexity of the glucocorticoid effect on vitamin D-mediated transcriptional properties of the osteocalcin gene is indicated by persistence of sequence-specific protein-DNA interactions at two principal osteocalcin gene promoter regulatory elements, the osteocalcin (CCAAT) box which modulates basal level of transcription, and the vitamin D responsive element, where vitamin D-mediated enhancement of osteocalcin gene transcription is controlled.

Coordination of protein-DNA interactions in the promoters of human H4, H3, and H1 histone genes during the cell cycle, tumorigenesis, and development

Coordinate transcriptional control of replication-dependent human H4, H3, and H1 histone genes was studied by comparing levels of H3 and H1 histone promoter binding activities with those of H4 histone promoter factor HiNF-D during the cell cycle of both normal diploid and tumor-derived cells, as well as in fetal and adult mammalian tissues. Both H3 and H1 histone promoters interact with binding activities that, as with HiNF-D, are maximal during S-phase but at low levels in the G1-phase of normal diploid cells. However, these analogous DNA binding activities are constitutively maintained at high levels throughout the cell cycle in four different transformed and tumor-derived cells. Downregulation of the H3 and H1 histone promoter factors in conjunction with HiNF-D is observed in vivo at the onset of quiescence and differentiation during hepatic development. Hence, our results indicate a tight temporal coupling of three separate protein-DNA interactions in different histone promoters during the cell cycle, development, and tumorigenesis. This suggests that a key oscillatory, cell-growth-control mechanism modulates three analogous histone gene promoter protein-DNA interactions in concert. The derangement of this mechanism in four distinct tumor cells implies that concerted deregulation of these histone promoter factors is a common event resulting from heterogeneous aberrations in normal cell growth mechanisms during tumorigenesis. We postulate that this mechanism may be involved in the coordinate regulation of the human H4, H3, and H1 histone multigene families.

Involvement of the cell cycle-regulated nuclear factor HiNF-D in cell growth control of a human H4 histone gene during hepatic development in transgenic mice

Regulation of the cell cycle-controlled histone gene promoter factor HiNF-D was examined in vivo. Proliferative activity was measured by DNA replication-dependent histone mRNA levels, and HiNF-D binding activity was found to correlate with cell proliferation in most tissues. Furthermore, HiNF-D is down-regulated during hepatic development, reflecting the onset of differentiation and quiescence. The contribution of transcription to histone gene expression was directly addressed in transgenic mice by using a set of fusion constructs containing a human H4 histone gene promoter linked to three different genes. Transgene expression in both fetal and adult mice paralleled endogenous mouse histone mRNA levels in most tissues, consistent with this promoter conferring developmental, cell growth-related transcriptional regulation. Our results suggest that HiNF-D is stringently regulated in vivo in relation to cell growth and support a primary role for HiNF-D in the proliferation-specific expression of H4 histone genes in the intact animal. Further, the data presented here provide an example in which apparent tissue specificity of gene expression reflects the proliferative state of various tissues and demonstrate that multiple levels of histone gene regulation are operative in vivo.

Phenotype suppression: a postulated molecular mechanism for mediating the relationship of proliferation and differentiation by Fos/Jun interactions at AP-1 sites in steroid responsive promoter elements of tissue-specific genes

There is a generalized reciprocal relationship between cell growth and expression of genes that occurs following completion of proliferation, which supports the progressive development of cell and tissue phenotypes. Molecular mechanisms which couple the shutdown of proliferation with initiation of tissue-specific gene transcription have been addressed experimentally in cultures of primary diploid osteoblasts that undergo a growth and differentiation developmental sequence. Evidence is presented for a model which postulates that genes transcribed post-proliferatively are suppressed during cell growth by binding of the Fos/Jun protein complex to AP-1 promoter sites associated with vitamin D responsive elements of several genes encoding osteoblast phenotype markers (Type I collagen, alkaline phosphatase, osteocalcin).

Coordinate occupancy of AP-1 sites in the vitamin D-responsive and CCAAT box elements by Fos-Jun in the osteocalcin gene: model for phenotype suppression of transcription

Osteocalcin, a bone-specific protein and marker of the mature osteoblast, is expressed only in nonproliferating osteoblasts in a mineralizing extracellular matrix, while type I collagen is expressed in proliferating cells. The nuclear proteins encoded by the c-fos and c-jun protooncogenes are expressed during the proliferation period of osteoblast phenotype development. We present evidence that AP-1 (HeLa cell-activating protein 1) sites residing within two promoter elements of the osteocalcin gene bind the Fos-Jun protein complex: the osteocalcin box (OC box; nucleotides -99 to -76), which contains a CCAAT motif as a central element and influences tissue-specific basal levels of osteocalcin gene transcription, and the vitamin D-responsive element (VDRE; nucleotides -462 to -440), which mediates enhancement of osteocalcin gene transcription. Gel electrophoretic mobility-shift analysis demonstrated high AP-1 binding activity in proliferating osteoblasts and dramatic changes in this activity after the down-regulation of proliferation and the initiation of extracellular-matrix mineralization in primary cultures of normal diploid osteoblasts. Methylation interference analysis established at single nucleotide resolution that purified recombinant Fos and Jun proteins bind in a sequence-specific manner to the AP-1 sites within the VDRE and OC box. Similarly, an AP-1 motif within a putative VDRE of the alkaline phosphatase gene, which is also expressed after the completion of proliferation, binds the Fos-Jun complex. These results support a model in which coordinate occupancy of the AP-1 sites in the VDRE and OC box in proliferating osteoblasts may suppress both basal level and vitamin D-enhanced osteocalcin gene transcription as well as transcription of other genes associated with osteoblast differentiation--a phenomenon we describe as phenotype suppression. This model is further supported by binding of the Fos-Jun complex at an AP-1 site in the type alpha I collagen promoter that is contiguous with, but not overlapping, the VDRE. Such a sequence organization in the collagen VDRE motif is compatible with vitamin D modulation of collagen but not with osteocalcin and alkaline phosphatase expression in proliferating osteoblasts.

Expression and ultrastructural immunolocalization of a major 66 kDa phosphoprotein synthesized by chicken osteoblasts during mineralization in vitro

Embryonic chicken osteoblasts cultured over a 30 day period were used as a model system for studying the expression of bone phosphoproteins during cellular differentiation and the possible role of these proteins in extracellular matrix mineralization. Accumulation of total phosphoprotein in the cultures, as determined by O-phosphoserine (Ser-P) and O-phosphothreonine (Thr-P) amino acid analysis, revealed a greater than 10-fold increase over the 30 day period. Total phosphoprotein synthesis, as assessed by (32P)-, (3H)-Ser-P, and (14C)-Thr-P protein labeling, showed the highest levels concurrent with initial mineral deposition within the matrix. The major phosphoprotein present in chicken bones and synthesized by the cultured osteoblasts had a molecular weight of approximately 66 kDa. This 66 kDa bone phosphoprotein (66 kDa BPP) was purified to homogeneity and was used for antibody production. Application of this antibody in Western blot analysis revealed that 66 kDa BPP was present only in protein extracts of mineralizing cultured osteoblasts and was absent in cultures of non-mineralizing chondrocytes, myoblasts, and tendon fibroblasts. The 66 kDa BPP in vitro accumulated continuously in the extracellular matrix in a manner that paralleled both phosphoprotein synthesis and total phospho-amino acid production. A comparison of the results obtained in vitro to those from developing embryonic tibiae in vivo demonstrated a similar qualitative and temporal expression of phosphoprotein and a continual accumulation of 66 kDa BPP in the matrix with advancing mineralization and developmental age. Ultrastructural immunocytochemistry using the 66 kDa BPP antibody and the protein A-gold technique revealed specific immunolabeling over electron-dense regions of mineralization in the cultures that appeared identical to the distribution of labeling observed in vivo (McKee et al.: Connect. Tissue Res., 21:21-29, 1989; Anat. Rec., 228:77-92, 1990). These results demonstrate that this major 66 kDa BPP was expressed concurrently with other differentiated osteoblast functions and suggests that it may play a role in the initiation or regulation of mineralization.