GATA4 negatively regulates osteoblast differentiation by downregulation of Runx2

Osteoblasts are specialized mesenchymal cells that are responsible for bone formation. In this study, we examine the role of GATA4 in osteoblast differentiation. GATA4 was abundantly expressed in preosteoblast cells and gradually down-regulated during osteoblast differentiation. Overexpression of GATA4 in osteoblastic cells inhibited alkaline phosphatase activity and nodule formation in osteogenic conditioned cell culture system. In addition, overexpression of GATA4 attenuated expression of osteogenic marker genes, including Runx2, alkaline phosphatase, bone sialoprotein, and osteocalcin, all of which are important for osteoblast differentiation and function. Overexpression of GATA4 attenuated Runx2 promoter activity, whereas silencing of GATA4 increased Runx2 induction. We found that GATA4 interacted with Dlx5 and subsequently decreased Dlx5 binding activity to Runx2 promoter region. Our data suggest that GATA4 acts as a negative regulator in osteoblast differentiation by downregulation of Runx2. [BMB Reports 2014; 47(8): 463-468]     

Human estrogen receptor alpha gene is a target of Runx2 transcription factor in osteoblasts

Several studies into the mechanisms involved in control of osteoblast-specific gene expression have identified Runx2 and ERalpha (estrogen receptor alpha) as essential regulators of osteoblast differentiation. Recently, interactions between Runx2 and ERalpha have been described. Here, we investigate the role of Runx2 on the regulation of ERalpha expression by determining its interaction with the F promoter, one of the multiple promoters of the human ERalpha gene and the only one active in bone. We found that, in this promoter, three Runx2-like sites are present. By electrophoretic mobility shift assay in combination with supershift and ChIP experiments, we demonstrated that Runx2 preferentially binds one of the Runx2 motifs of the F promoter. To understand whether or not they are involved in influencing F promoter activity, different promoter-reporter deletion and mutation constructs were transiently transfected into human osteoblastic cells. Comparison of luciferase activities allowed the identification of a prevalent negative role of a sequence context, within the -117,877/-117,426 region, which may be under the control of Runx2 (a) site. Finally, silencing and overexpression of endogenous Runx2 provided evidence that Runx2 has a more complex role than initially expected. In fact, Runx2 (a) and Runx2 (b) sites carried out opposite roles which are conditioned by Runx2 levels in bone cells. Therefore, the resulting F promoter activity may be tightly regulated by a dynamic interplay between these two Runx2 sites, with a predominance of negative effect of the Runx2 (a) site.     

Analysis of a conjugate between anti-carcinoembryonic antigen monoclonal antibody and alkaline phosphatase for specific activation of the prodrug etoposide phosphate

Summary The selective targeting of tumours by enzymes conjugated to monoclonal antibodies (mAb) may be an ideal approach to convert relatively nontoxic prodrugs into active agents at the tumour site. We used the anti-carcinoembryonic antigen mAb BW431/26 conjugated to alkaline phosphatase (AP) and phosphorylated etoposide (etoposide-P) as a prodrug to study the feasibility of this concept. Etoposide was phosphorylated with POCl₃. Quantitative hydrolysis of etoposide-P to etoposide occurred within 10 min in the presence of AP. BW431/26 and AP were conjugated using a thioether bond. The AP conjugate retained 93% of its calculated activity.¹²⁵I-labelled AP conjugate did not show a reduction of immunoreactivity as determined by a cell-binding assay. SW1398 colon cancer cells were used to analyse the cytotoxicity of etoposide and etoposide-P. Etoposide (IC₅₀ 22 µM) was 100 times more toxic than etoposide-P (20% growth inhibition at 200 µM). Pretreatment of the cells with BW431/26-AP prior to etoposide-P exposure resulted in a dramatic increase in cytotoxicity (IC₅₀ 70 µM). The pharmacokinetics and tumour-localizing properties of BW431/27 and the AP conjugate were assessed in nude mice bearing SW1398 tumours. BW431/26 showed excellent tumour localization (10% of the injected dose/g tissue retained from 8 h to 120 h), whereas the AP conjugate showed a reduced tumour uptake (3%-0.3% of the injected dose/g tissue at 8–120 h), a faster clearance from the circulation and a high liver uptake. Radiolabelled AP showed a similar pharmacokinetic profile to the AP conjugate. Gel filtration analysis of blood, liver, and tumour samples indicated good stability of the conjugate.     

Nucleotide sequence of the gene for alkaline phosphatase of Thermus caldophilus GK24 and characteristics of the deduced primary structure of the enzyme

The gene encoding Thermus caldophilus GK24 (Tca) alkaline phosphatase was cloned into Escherichia coli. The primary structure of Tca alkaline phosphatase was deduced from its nucleotide sequence. The Tca alkaline phosphatase precursor, including the signal peptide sequence, was comprised of 501 amino acid residues. Its molecular mass was determined to be 54? omitted?760 Da. On the alignment of the amino acid sequence, Tca alkaline phosphatase showed sequence homology with the microbial alkaline phosphatases, 20% identity with E. coli alkaline phosphatase and 22% Bacillus subtilis (Bsu) alkaline phosphatases. High sequence identity was observed in the regions containing the Ser-102 residue of the active site, the zinc and magnesium binding sites of E. coli alkaline phosphatase. Comparison of Tca alkaline phosphatase and E. coli alkaline phosphatase structures suggests that the reduced activity of the Tca alkaline phosphatase, in the presence of zinc, is directly involved in some of the different metal binding sites. Heat-stable Tca alkaline phosphatase activity was detected in E. coli YK537, harboring pJRAP.     

Dissociation between Ca2+-ATPase and alkaline phosphatase activities in plasma membranes of rat duodenum

The presence of Ca²⁺-ATPase activities with high-affinity sites for Ca²⁺ in brush border as well as basolateral plasma membranes of rat duodenal epithelium has been reported previously (Ghijsen, W.E.J.M. and van Os, C.H. (1979) Nature 279, 802–803). Since both plasma membranes contain alkaline phosphatase (EC 3.1.3.1), which also can be stimulated by Ca²⁺, the substrate specificity of Ca²⁺-induced ATP-hydrolysis has been studied to determine whether or not alkaline phosphatase and Ca²⁺-ATPase are two distinct enzymes. In basolateral fragments, the rate of Ca²⁺-dependent ATP-hydrolysis was greater than that of ADP, AMP and $\text{p-}\text{nitrophenylphosphate}$ at Ca²⁺ concentrations below 25 μM. At 0.2 mM Ca²⁺ the rates of ATP, ADP, AMP and $\text{p-}\text{nitrophenylphosphate}$ hydrolysis were not significantly different. In brush border fragments the rates of ATP, ADP and AMP hydrolysis were identical at low Ca²⁺, but at 0.2 mM Ca²⁺, Ca²⁺-induced hydrolysis of ADP and AMP was greater than either ATP or $\text{p-}\text{nitrophenylphosphate}$. Alkaline phosphatase in brush border and basolateral membranes was inhibited by 75% after addition of 2.5 mM theophylline. Ca²⁺-stimulated ATP hydrolysis at 1 μM Ca²⁺ was not sensitive to theophylline in basolateral fragments while the same activity in brush border fragments was totally inhibited. At 0.2 mM Ca²⁺, Ca²⁺-induced ATP hydrolysis in both basolateral and brush border membranes was sensitive to theophylline. Oligomycin and azide had no effect on Ca²⁺-stimulated ATP hydrolysis, either at low or at high Ca²⁺ concentrations. Chlorpromazine fully inhibited Ca²⁺-stimulated ATP hydrolysis in basolateral fragments at 5 μM Ca²⁺, while it had no effect in brush border fragments. From these results we conclude that, (i) Ca²⁺-ATPase and alkaline phosphatase are two distinct enzymes, (ii) high-affinity Ca²⁺-ATPase is exclusively located in basolateral plasma membranes, (iii) alkaline phosphatase activity, present on both sides of duodenal epithelium, is stimulated slightly by low Ca²⁺ concentrations, but this Ca²⁺-induced activity is inhibited by theophylline and shows no specificity with respect to ATP, ADP or AMP.