Abstract
BackgroundPasteurella multocida toxin (PMT) is a potent inducer of osteoclast formation. Pigs suffering from an infection with toxigenic Pasteurella multocida strains develop atrophic rhinitis characterised by a loss of turbinate bones and conchae. However, on the molecular level the process of bone loss remains largely uncharacterised.ResultsRecently it was found that PMT activates the serine/threonine kinase mammalian target of rapamycin (mTOR) in fibroblasts. Using RAW264.7 macrophages, we investigated the role of the mTOR complex 1 (mTORC1) in PMT-mediated osteoclast formation. PMT induces the differentiation of RAW264.7 macrophages into multinucleated, tartrate resistant acid phosphatase (TRAP) positive osteoclasts that are capable to resorb bone. In the presence of the mTORC1 inhibitor rapamycin, PMT was significantly less able to induce the formation of TRAP-positive osteoclasts. Accordingly, the resulting resorption of bone was strongly reduced. A major target of mTOR is the 70 kDa ribosomal protein S6 kinase 1 (p70 S6K1). Activated p70 S6K1 decreases the expression of programmed cell death protein 4 (PDCD4), a negative transcriptional regulator of osteoclastogenesis, at the protein and gene level. Ultimately this results in the activation of c-Jun, a component of the activator protein 1 (AP-1) complex, which is a major transcription factor for the induction of osteoclast-specific genes. We now demonstrate that c-Jun and its downstream target, the osteoclast-specific bone degrading protease cathepsin K, are upregulated upon PMT treatment in an mTOR-dependent manner.ConclusionsActivation of mTOR signalling plays a central role in the formation of osteoclasts through the bacterial toxin PMT. On the molecular level, PMT-induced activation of mTOR leads to down regulation of PDCD4, a known repressor of AP-1 complex, culminating in the activation of c-Jun, an essential transcription factor for triggering osteoclastogenesis.Electronic supplementary materialThe online version of this article (doi:10.1186/s12964-015-0117-7) contains supplementary material, which is available to authorized users.
Highlights
Pasteurella multocida toxin (PMT) is a potent inducer of osteoclast formation
As PMT is a potent inducer of osteoclast differentiation, we investigated the effect of mammalian target of rapamycin (mTOR) activation in a murine macrophage cell line (RAW264.7 cells) that can be differentiated into osteoclasts [15, 16]
We found that mTOR was activated by treatment with PMT (Fig. 1a)
Summary
Pigs suffering from an infection with toxigenic Pasteurella multocida strains develop atrophic rhinitis characterised by a loss of turbinate bones and conchae. It is well known that PMT is the causative factor of porcine atrophic rhinitis. This disease is characterised by increased osteoclastogenesis and osteoclast activity as well as an inhibition of osteoblast function, eventually leading to the degradation of bone [1]. Interaction partners of mTORC1 are the proteins Raptor, proline-rich Akt substrate (PRAS40), mammalian lethal with SEC13 protein 8 (mLST8) and DEP domain-containing mTOR-interacting protein (Deptor), where PRAS40 and Deptor act as negative regulators for mTOR activity [9]. Phosphorylation of PRAS40 and Deptor through Akt or mTOR releases them from the complex and allows signalling [10, 11]
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