Receptor-mediated effect of thrombin on plasminogen activator inhibitor-1 synthesis in rat osteoblast-like cells

The therapeutic mechanism of Compound Lurong Jiangu Capsule for the treatment of cadmium-induced osteoporosis: network pharmacology and experimental verification

Background

Among bone diseases, osteoporosis-like skeleton, such as trabecular thinning, fracture and so on, is the main pathological change of cadmium-induced osteoporosis(Cd-OP), accompanied by brittle bone and increased fracture rate. However, the mechanism underlying cadmium-induced osteoporosis has remained elusive. Compound Lurong Jiangu Capsule (CLJC) is an experienced formula for the treatment of bone diseases, which has the effect of tonifying kidney and strengthening bones, promoting blood circulation and relieving pain.

Objective

Network pharmacology and molecular docking technology combined with experiments were used to investigate the potential mechanism of CLJC in treating Cd-OP.

Method

The active compounds and corresponding targets of each herb in CLJC were searched in the TCMSP and BATMAN-TCM databases. The DisGeNet, OMIM, and GeneCards databases searched for Cd-OP targets. The relationship between both of them was visualized by establishing an herb-compound-target network using Cytoscape 3.9.1 software. Gene ontology (GO), and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analyses were performed after determining the intersection of the targets from CLJC and Cd-OP. What's more, molecular docking was performed to validate the results. All of them were aim to obtain hud signaling pathways for further study. Finally, BAX, BCL-2, and CASPASE-3 were screened and selected for further experiments, which included bone imaging and reconstruction analysis (Micro-CT), hematoxylin-eosin Staining (HE), and western blot (WB).

Results

106 common targets from CLJC and Cd-OP targets were identified. KEGG pathway analysis suggested that multiple signaling pathways, such as the pathways in cancer, may play roles in treatment. Verification of the molecular docking was successful. Here we showed that Cd-OP displayed Tb.Th and Tb.N significantly reduced and even broke, irregular proliferation of bone cortex, uneven and loose trabecular bone arrangement, changed in apoptosis-related proteins, such as significant upregulation of CASPASE-3, BAX protein and significant downregulation of BCL-2 protein in vivo, while CLJC rescued these phenotypes.

Conclusion

This study revealed that CLJC can reduce the expression of apoptosis-related proteins, and multiple components and multiple targets inhibit Cd-OP through apoptosis signaling pathway.

Thrombin-stimulated growth factor and cytokine expression in osteoblasts is mediated by protease-activated receptor-1 and prostanoids

Thrombin exerts multiple effects upon osteoblasts including stimulating proliferation, and inhibiting osteoblast differentiation and apoptosis. Some of these effects are believed to be mediated by the synthesis and secretion of autocrine factors such as growth factors and cytokines. Many but not all cellular responses to thrombin are mediated by members of the protease-activated receptor (PAR) family of G protein-coupled receptors. The current study was undertaken to investigate the nature of thrombin's induction of autocrine factors by analysing the expression of twelve candidate genes in thrombin-stimulated primary mouse osteoblasts. Analysis by quantitative reverse transcription polymerase chain reaction (qRT-PCR) demonstrated that thrombin induced transforming growth factor beta, cyclooxygenase-2, tenascin C, fibroblast growth factor-1 and -2, connective tissue growth factor and interleukin-6 expression in wild type osteoblasts, but not PAR-1 null mouse osteoblasts. Induction of all the thrombin-responsive genes was blocked by the presence of the non-selective cyclooxygenase inhibitor indomethacin. Further studies were conducted on interleukin-6, which was the gene that showed the greatest increase in expression following stimulation of osteoblast-like cells with thrombin. A PAR-1-specific activating peptide, but neither a PAR-4-activating peptide nor catalytically inactive thrombin induced release of interleukin-6 by osteoblasts. Furthermore, in the presence of the selective cyclooxygenase-1 and -2 inhibitors SC-560 and NS-398 thrombin-induced interleukin-6 release was prevented. Levels of both prostaglandin E(2) and interleukin-6 in medium conditioned by thrombin-stimulated osteoblast-like cells were found to be significantly increased compared to medium conditioned by non-stimulated cells, however release of prostaglandin E(2) was found to precede release of interleukin-6. Treatment of isolated osteoblast-like cells with a number of synthetic prostanoids stimulated secretion of interleukin-6 with differing potencies. These studies suggest that activation of PAR-1 on osteoblasts by thrombin induces cyclooxygenase activity, which in turn results in the increased expression of multiple secreted factors. The induction of these secreted factors may act in an autocrine fashion to alter osteoblast function, allowing these cells to participate in the earliest stages of bone healing by both autocrine and paracrine mechanisms.

Functional responses of bone cells to thrombin

Cells responsible for the formation and maintenance of bone express thrombin-responsive members of the protease-activated receptor family of G protein-coupled receptors. Thrombin has been shown to elicit a number of functional responses in these cells, including proliferation and cytokine production in osteoblasts. Many, but not all, of the effects of thrombin on bone cells are initiated by activation of protease-activated receptor-1. A combination ofin vitroobservations and results ofin vivostudies in protease-activated receptor-1-null mice suggest that thrombin plays multiple roles in the early stages of bone healing.

Inhibition of osteoblast apoptosis by thrombin

The multifunctional serine protease thrombin has been shown to be a specific agonist for a variety of functional responses of cells including osteoblasts. The current study was conducted to determine if thrombin was capable of inhibiting apoptosis in osteoblasts, and if so, to examine the mechanism by which this occurred. Thrombin (20-100 nM) significantly inhibited apoptosis in serum-starved cultures of the human osteoblast-like Saos-2 cell line and cultures of primary osteoblasts isolated from mouse calvariae, as well as dexamethasone-treated primary mouse osteoblasts. Inhibition of serum deprivation-induced apoptosis was shown to require thrombin's specific proteolytic activity. Primary mouse osteoblasts were found to express two functional thrombin receptors, PAR-1 and PAR-4. Thrombin inhibited serum deprivation-induced apoptosis in osteoblasts isolated from PAR-1 null mice to the same degree as in osteoblasts isolated from wild-type mice. Treatment of serum-deprived osteoblasts, isolated from either PAR-1 null or wild-type mice, with a PAR-4-activating peptide failed to significantly inhibit apoptosis compared to the relevant control. Medium conditioned by thrombin-treated osteoblasts, in which thrombin had been inactivated, was able to inhibit serum deprivation-induced osteoblast apoptosis almost as well as thrombin itself. Blocking protein synthesis, by cycloheximide pretreatment of the conditioning cells, prevented this action. The ability of known osteoblast survival factors, such as transforming growth factor beta1, fibroblast growth factor-2, insulin-like growth factor-II, and interleukin-6, to inhibit serum deprivation-induced osteoblast apoptosis was also tested. None of these factors was able to inhibit serum deprivation-induced osteoblast apoptosis to the same extent as thrombin. The results presented here demonstrate that thrombin treatment of osteoblasts inhibits apoptosis induced either by dexamethasone or by serum deprivation. Furthermore, it does so independently of the known thrombin receptors by bringing about the synthesis and/or secretion of an unknown survival factor or factors, which then act in an autocrine fashion to inhibit apoptosis.

Modulation of osteoblast-like cell behavior by activation of protease-activated receptor-1

In addition to playing a central role in thrombosis and hemostasis, the serine protease thrombin is a specific agonist for a variety of functional responses in cells including osteoblast-like cells. Many of the cellular responses to thrombin are mediated by protease-activated receptor-1 (PAR-1). Since osteoblasts express PAR-1 in vivo during development, the effect of PAR-1 activation on proliferation and differentiation in primary rat osteoblast-like cells was investigated. Thrombin or the rat PAR-1-activating peptide SFFLRNPSENTFELVPL (SFFL) stimulated cell proliferation (as assessed by 3H- thymidine incorporation) of primary osteoblast-like cells derived from long bone or calvaria, and treatment with antibodies to PAR-1 abolished the proliferative response to thrombin. Activation of PAR-1 by thrombin or SFFL inhibited endogenous alkaline phosphatase (ALP) activity and caused a transient elevation of intracellular calcium in the osteoblast-like cells. Calcium mobilization was not, however, required for thrombin's effect on proliferation or ALP activity. The ability of a number of growth factors and hormones to regulate expression of PAR-1 in osteoblast-like cells was investigated. Expression of PAR-1 transcript and protein by osteoblast-like cells in vitro was markedly increased by treatment with transforming growth factor-beta (TGF-beta), and the proliferative response to thrombin was enhanced by TGF-beta pretreatment. Platelet-derived growth factor-BB caused a slight but significant down-regulation of PAR-1 mRNA expression. Thrombin caused a transient increase in PAR-1 expression, whereas neither parathyroid hormone-related peptide nor 1, 25-dihydroxyvitamin D3 had any effect. The observations described here suggest that PAR-1 mediates thrombin-induced osteoblast proliferation, which in turn may contribute to responses of osteoblasts to osteogenic growth factors.

Expression of the thrombin receptor in developing bone and associated tissues

Thrombin, a serine protease with a central role in thrombosis and hemostasis, is also a specific agonist for a variety of cellular responses in osteoblasts and stimulates bone resorption in organ culture. Cultured osteoblast-like cells express the proteolytically activated thrombin receptor, but the significance of this finding in vivo remains unknown. Immunohistochemistry was used to investigate the normal tissue distribution of the proteolytically activated thrombin receptor in developing rat bones and associated tissues. In hind limbs, the receptor was first observed on embryonic day 16 and became more abundant within the limb as gestation progressed. Thrombin receptor staining was detected on osteoblasts, macrophages, muscle cells, and endothelial cells, but not osteoclasts. Similarly, osteoblasts in developing calvariae stained positively for the thrombin receptor. The pattern of receptor expression by primary osteoblast cultures and freshly isolated macrophages and osteoclasts corresponded to that observed in vivo. The observed pattern of thrombin receptor expression in bone cells supports the hypothesis that cell-mediated thrombin-induced bone resorption is mediated by osteoblasts.