A novel link between the proteasome pathway and the signal transduction pathway of the Bone Morphogenetic Proteins (BMPs)

BACKGROUND: The intracellular signaling events of the Bone Morphogenetic Proteins (BMPs) involve the R-Smad family members Smad1, Smad5, Smad8 and the Co-Smad, Smad4. Smads are currently considered to be DNA-binding transcriptional modulators and shown to recruit the master transcriptional co-activator CBP/p300 for transcriptional activation. SNIP1 is a recently discovered novel repressor of CBP/p300. Currently, the detailed molecular mechanisms that allow R-Smads and Co-Smad to co-operatively modulate transcription events are not fully understood. RESULTS: Here we report a novel physical and functional link between Smad1 and the 26S proteasome that contributes to Smad1- and Smad4-mediated transcriptional regulation. Smad1 forms a complex with a proteasome β subunit HsN3 and the ornithine decarboxylase antizyme (Az). The interaction is enhanced upon BMP type I receptor activation and occur prior to the incorporation of HsN3 into the mature 20S proteasome. Furthermore, BMPs trigger the translocation of Smad1, HsN3 and Az into the nucleus, where the novel CBP/p300 repressor protein SNIP1 is further recruited to Smad1/HsN3/Az complex and degraded in a Smad1-, Smad4- and Az-dependent fashion. The degradation of the CBP/p300 repressor SNIP1 is likely an essential step for Smad1-, Smad4-mediated transcriptional activation, since increased SNIP1 expression inhibits BMP-induced gene responses. CONCLUSIONS: Our studies thus add two additional important functional partners of Smad1 into the signaling web of BMPs and also suggest a novel mechanism for Smad1 and Smad4 to co-modulate transcription via regulating proteasomal degradation of CBP/p300 repressor SNIP1

Constitutively elevated blood serotonin is associated with bone loss and type 2 diabetes in rats

Reduced peripheral serotonin (5HT) in mice lacking tryptophan hydroxylase (TPH1), the rate limiting enzyme for 5HT synthesis, was reported to be anabolic to the skeleton. However, in other studies TPH1 deletion either had no bone effect or an age dependent inhibition of osteoclastic bone resorption. The role of 5HT in bone therefore remains poorly understood. To address this issue, we used selective breeding to create rat sublines with constitutively high (high-5HT) and low (low-5HT) platelet 5HT level (PSL) and platelet 5HT uptake (PSU). High-5HT rats had decreased bone volume due to increased bone turnover characterized by increased bone formation and mineral apposition rate, increased osteoclast number and serum C-telopeptide level. Daily oral administration of the TPH1 inhibitor (LX1032) for 6 weeks reduced PSL and increased the trabecular bone volume and trabecular number of the spine and femur in high-5HT rats. High-5HT animals also developed a type 2 diabetes (T2D) phenotype with increased: plasma insulin, glucose, hemoglobin A1c, body weight, visceral fat, β-cell pancreatic islets size, serum cholesterol, and decreased muscle strength. Serum calcium accretion mediated by parathyroid hormone slightly increased, whereas treatment with 1,25(OH)2D3 decreased PSL. Insulin reduction was paralleled by a drop in PSL in high-5HT rats. In vitro, insulin and 5HT synergistically up-regulated osteoblast differentiation isolated from high-5HT rats, whereas TPH1 inhibition decreased the number of bone marrow-derived osteoclasts. These results suggest that constitutively elevated PSL is associated with bone loss and T2D via a homeostatic interplay between the peripheral 5HT, bone and insulin

Recombinant human bone morphogenetic protein 2B stimulates PC12 cell differentiation: potentiation and binding to type IV collagen

Abstract. Bone morphogenetic protein 2B (BMP 2B, also known as BMP 4) induces cartilage and bone morphogenesis in ectopic extraskeletal sites. BMP 2B is one of several bone morphogenetic proteins which along with activins and inhibins are members of the transforming growth factor-/3 (TGF-B) family. Both BMP 2B and activin A, but not TGF-B~, induce rat pheochromocytoma PC12 neuronal cell differentiation and expression of VGF, a nervous system-specific mRNA. PC12 cells exhibited •2,500 receptors per cell for BMP 2B with an apparent dissociation constant of 19 pM. Extracellular matrix components, in-ROWTH and differentiation factors in bone enable demineralized bone matrix to induce endochondral bone formation at ectopic sites in rat (Reddi an

Circulating Bone Morphogenetic Protein 1–3 Isoform Increases Renal Fibrosis

Bone morphogenetic proteins (BMPs) participate in organ regeneration through autocrine and paracrine actions, but the existence and effects of these proteins in the systemic circulation is unknown. Using liquid chromatography–mass spectrometry, we identified BMP6, GDF15, and the BMP1–3 isoform of the Bmp1 gene in plasma samples from healthy volunteers and patients with CKD. We isolated the endogenous BMP1–3 protein and demonstrated that it circulates as an active enzyme, evidenced by its ability to cleave dentin matrix protein-1 in vitro. In rats with CKD, administration of recombinant BMP1–3 increased renal fibrosis and reduced survival. In contrast, administration of a BMP1–3-neutralizing antibody reduced renal fibrosis, preserved renal function, and increased survival. In addition, treating with the neutralizing antibody was associated with low plasma levels of TGFβ1 and connective tissue growth factor. In HEK293 cells and remnant kidneys, BMP1–3 increased the transcription of collagen type I, TGFβ1, β-catenin, and BMP7 via a BMP- and Wnt-independent mechanism that involved signaling through an integrin β1 subunit. The profibrotic effect of BMP1–3 may, in part, be a result of the accompanied decrease in decorin (DCN) expression. Taken together, inhibition of circulating BMP1–3 reduces renal fibrosis, suggesting that this pathway may be a therapeutic target for CKD

Bone biomarkers and hormones in high-5HT and low-5HT rats.

<p>Depicted are relative differences (%): for the overall difference between the high-5HT (H) and low-5HT (L) sublines, P<0.05 is considered significant; for contrasts between sublines at a given age and between different ages within the same subline, P<0.025 is considered significant (n = 5–10 rats/group). H-L indicates a difference between high-5HT and low-5HT animals. 12–2 indicates a difference between 12 months and 2 months old animals.</p