Regulation of expression of plasminogen activator inhibitor-1 in cultured rat osteoblastic cells by osteogenic protein-1 (BMP-7)

A small-molecule PAI-1 inhibitor prevents bone loss by stimulating bone formation in a murine estrogen deficiency-induced osteoporosis model

Osteoporosis is a progressive bone disease caused by an imbalance between bone resorption and formation. Recently, plasminogen activator inhibitor-1 (PAI-1) was shown to play an important role in bone metabolism using PAI-1-deficient mice. In this study, we evaluated the therapeutic benefits of novel, orally available small-molecule PAI-1 inhibitor (iPAI-1) in an estrogen deficiency-induced osteoporosis model. Eight-week-old C57BL/6J female mice were divided into three groups: a sham + vehicle (Sham), ovariectomy + vehicle (OVX + v), and OVX + iPAI-1 (OVX + i) group. iPAI-1 was administered orally each day for 6 weeks starting the day after the operation. Six weeks of iPAI-1 treatment prevented OVX-induced trabecular bone loss in both the femoral bone and lumbar spine. Bone formation activity was significantly higher in the OVX + i group than in the OVX + v and Sham groups. Unexpectedly, OVX-induced osteoclastogenesis was partially, but significantly reduced. Fluorescence-activated cell sorting analyses indicated that the number of bone marrow stromal cells was higher in the OVX + i group than that in the OVX + v group. A colony-forming unit-osteoblast assay indicated enhanced mineralized nodule formation activity in bone marrow cells isolated from iPAI-1-treated animals. Bone marrow ablation analysis indicated that the remodeled trabecular bone volume was significantly higher in the iPAI-1-treated group than that in the control group. In conclusion, our results suggest PAI-1 blockade via a small-molecule inhibitor is a new therapeutic approach for the anabolic treatment of postmenopausal osteoporosis.

Bone morphogenetic protein-induced MSX1 and MSX2 inhibit myocardin-dependent smooth muscle gene transcription

During the onset and progression of atherosclerosis, the vascular smooth muscle cell (VSMC) phenotype changes from differentiated to dedifferentiated, and in some cases, this change is accompanied by osteogenic transition, resulting in vascular calcification. One characteristic of dedifferentiated VSMCs is the down-regulation of smooth muscle cell (SMC) marker gene expression. Bone morphogenetic proteins (BMPs), which are involved in the induction of osteogenic gene expression, are detected in calcified vasculature. In this study, we found that the BMP2-, BMP4-, and BMP6-induced expression of Msx transcription factors (Msx1 and Msx2) preceded the down-regulation of SMC marker expression in cultured differentiated VSMCs. Either Msx1 or Msx2 markedly reduced the myocardin-dependent promoter activities of SMC marker genes (SM22alpha and caldesmon). We further investigated interactions between Msx1 and myocardin/serum response factor (SRF)/CArG-box motif (cis element for SRF) using coimmunoprecipitation, gel-shift, and chromatin immunoprecipitation assays. Our results showed that Msx1 or Msx2 formed a ternary complex with SRF and myocardin and inhibited the binding of SRF or SRF/myocardin to the CArG-box motif, resulting in inhibition of their transcription.

Co-transfection with the osteogenic protein (OP)-1 gene and the insulin-like growth factor (IGF)-I gene enhanced osteoblastic cell differentiation

Previous studies from this laboratory showed that the action of Osteogenic Protein-1 (OP-1, BMP-7) on osteoblastic cell differentiation could be enhanced by other protein factors, such as Insulin-like Growth Factor (IGF)-I. In the present study, we examined the effects of co-transfection with a combination of the OP-1 and the IGF-I gene on osteoblastic cell differentiation. The results first showed that fetal rat calvaria (FRC) cells transfected with the OP-1 gene under the control of the cytomegalovirus (CMV) promoter showed substantial production of the OP-1 protein. Transfected FRC cells also showed a DNA concentration-dependent increase in alkaline phosphatase (AP) activity, an osteoblastic cell differentiation marker. Von Kossa-positive nodules, a hallmark of bone formation in long-term cultures of bone-derived cells, were also observed in the transfected cells after 26 days in culture, whereas none were observed in control cells. Co-transfection of FRC cells with the combination of the OP-1 and the IGF-I gene resulted in a synergistic stimulation of AP activity. The increase was DNA dose-dependent. The current data show that transfection of OP-1 gene into osteoblastic cells stimulates osteoblastic cell differentiation in vitro. The study further demonstrates the feasibility of employing gene transfer of a second gene in combination with an OP-1 vector to synergistically enhance OP-1 activity.

Post-transcriptional regulation of plasminogen activator inhibitor-1 by intracellular iron in cultured human lung fibroblasts--interaction of an 81-kDa nuclear protein with the 3'-UTR

The proteinase inhibitor, type-1 plasminogen activator inhibitor (PAI-1), is a major regulator of the plasminogen activator system involved in plasmin formation and fibrinolysis. The present study explores the effects of intracellular iron on the expression of PAI-1 and associated cell-surface plasmin activity in human lung fibroblasts; and reports the presence of a novel iron-responsive protein. ELISA revealed a dose-dependent increase in PAI-1 antigen levels expressed in the conditioned medium of cells treated with deferoxamine, in the three cell lines studied. A concomitant increase in mRNA levels was also observed by Northern analyses. Presaturation with ferric citrate quenched the effect of deferoxamine. Experiments with transcription and translation inhibitors on TIG 3-20 cells demonstrated that intracellular iron modulated PAI-1 expression at the post-transcriptional level with the requirement of de-novo protein synthesis. Electrophoretic mobility shift assay and UV crosslinking assays revealed the presence of an approximately 81-kDa nuclear protein that interacted with the 3'-UTR of PAI-1 mRNA in an iron-sensitive manner. Finally, we demonstrated that the increased PAI-1 is functional in suppressing cell-surface plasmin activity, a process that can affect wound healing and tissue remodeling.

Iron-mediated stability of PAI-1 mRNA in adenocarcinoma cells—involvement of a mRNA-binding nuclear protein

This study reports the stability of mRNA of type-1 plasminogen activator inhibitor (PAI-1), the major physiologic inhibitor of plasminogen activation, by deferoxamine-aided iron deprivation, in PC3 adenocarcinoma cells. ELISA and Northern analyses studies revealed dose-dependent increase in PAI-1 expression by deferoxamine-treated cells. Co-treatment with ferric citrate quenched the effect of deferoxamine, confirming the role of iron in PAI-1 regulation. DRB-based RNA chase experiments suggested that post-transcriptional mechanism was involved in PAI-1 regulation. De-novo protein synthesis was necessary for this regulation. Electrophoretic mobility shift assay revealed the presence of a nuclear protein, binding to the 3'-UTR of PAI-1 mRNA in an iron-mediated manner. This is the first report of iron-mediated mRNA-protein interaction in PAI-1, involved in mRNA stability.

Bone morphogenetic protein-2 enhances osterix gene expression in chondrocytes

Osterix is a recently identified zinc-finger-containing transcription factor, which is required for skeletogenesis as no bone formation was observed in osterix-deficient mice. Osterix was first cloned as a gene whose expression was enhanced by BMP in C2C12 cells. As BMP induces ectopic bone formation in vivo via a pathway reminiscent to endochondral bone formation, BMP may also regulate osterix gene expression in chondrocytes. However, no information was available regarding the BMP actions on osterix gene expression in chondrocytes. We therefore examined the effects of BMP-2 on osterix gene expression in chondrocytes in culture. RT-PCR analysis indicated that osterix mRNA was expressed in the primary cultures of chondrocytes derived from mouse rib cartilage. The treatment with BMP-2 enhanced the levels of osterix transcripts within 24 h and the enhancement was still observed at 48 h based on RT-PCR analysis. This BMP effect was specific to this cytokine, as TGF-beta did not alter osterix gene expression. BMP effects on the osterix mRNA levels were also confirmed by Northern blot analysis. The enhancing effect of BMP on osterix gene expression was observed in a dose-dependent manner starting at 200 ng/ml. The BMP enhancement of the osterix gene expression in chondrocytes was blocked in the presence of a protein synthesis inhibitor, cycloheximide, while it was still observed in the presence of 5,6-dichloro-1-beta D-ribofuranosylbenzimidazol (DRB) suggesting the involvement of post-transcriptional events, which require new protein synthesis. These results indicated that osterix gene is expressed in the primary cultures of chondrocytes and its expression is under the control of BMP-2.

Plasminogen activator inhibitor-1 and the kidney

Plasminogen activator inhibitor-1 (PAI-1) is a serine protease inhibitor that was isolated 20 years ago. First recognized as an inhibitor of intravascular fibrinolysis, it is now evident that PAI-1 is a multifunctional protein with actions that may be dependent on or independent of its protease inhibitory effects. The latter often involve interactions between PAI-1 and vitronectin or the urokinase receptor. The protease-inhibitory actions of PAI-1 extend beyond fibrinolysis and include extracellular matrix turnover and activation of several proenzymes and latent growth factors. PAI-1 has been implicated in several renal pathogenetic processes, including thrombotic microangiopathies and proliferative and/or crescentic glomerulopathies. Most recently, it has become clear that PAI-1 also plays a pivotal role in progressive renal disease, both glomerulosclerosis and tubulointerstitial fibrosis. An active area of present research interest, untold stories are likely to be uncovered soon.

Post-transcriptional regulation of gene expression in the plasminogen activation system

The urokinase-mediated plasminogen activation (PA) system has been shown to play a key role in cell migration and tissue invasion by regulating both cell-associated proteolysis and cell-cell and cell-matrix interactions. The expression and activity of the components of this complex system are strictly regulated. The control of the expression occurs both at transcriptional and post-transcriptional levels. This review is focused on the post-transcriptional regulation of gene expression of all components of the PA system.