Sphingomyelinase amplifies BMP-4-induced osteocalcin synthesis in osteoblasts: role of ceramide

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Protein kinase C βII and δ/θ play critical roles in bone morphogenic protein-4-stimulated osteoblastic differentiation of MC3T3-E1 cells

Bone morphogenic protein-4 (BMP-4), one of TGF-β superfamily, is involved in bone and cartilage development, specifically tooth and bone fracture repair. In the present study, the role of protein kinase C (PKC) was investigated in BMP-4-induced differentiation of osteoblast-like MC3T3-E1 cells. PKC inhibitor UCN-01 significantly attenuated the synthesis of osteocalcin, a marker of mature osteoblast phenotype, in a dose-dependent manner as well as blocked osteroblastic differentiation and mineralization in BMP-4-treated MC3T3-E1 cells. Also, UCN-01 suppressed vascular endothelial growth factor (VEGF) production in BMP-4-treated MC3T3-E1 cells. In addition, UCN-01 remarkably suppressed BMP-4-activated PKC βII and PKC δ/θ of PKC family proteins by Western blotting. Consistently, 2-dimensional electrophoresis (2-DE) immunoblotting revealed that UCN-01 inhibited the BMP-4-induced activation of PKC subfamilies in MC3T3-E1 cells. Taken together, our findings suggest that PKC βII and PKC δ/θ mediate BMP-4-induced osteoblastic differentiation.

Upregulation of smpd3 via BMP2 stimulation and Runx2

Deletion of smpd3 induces osteogenesis and dentinogenesis imperfecta in mice. smpd3 is highly elevated in the parietal bones of developing mouse calvaria, but not in sutural mesenchymes. Here, we examine the mechanism of smpd3 regulation, which involves BMP2 stimulation of Runx2. smpd3 mRNA expression increased in response to BMP2 treatment and Runx2 transfection in C2C12 cells. The Runx2-responsive element (RRE) encoded within the -562 to -557 region is important for activation of the smpd3 promoter by Runx2. Electrophoretic mobility shift assays revealed that Runx2 binds strongly to the -355 to -350 RRE and less strongly to the -562 to -557 site. Thus, the smpd3 promoter is activated by BMP2 and is directly regulated by the Runx2 transcription factor. This novel description of smpd3 regulation will aid further studies of bone development and osteogenesis.

Fenretinide prevents the development of osteoporosis in Cftr-KO mice

BACKGROUND

The most recently described phenotype associated with Cystic Fibrosis (CF) is reduced bone mineral density which results in osteopenia and osteoporosis. The etiology of the early onset of osteoporosis in CF patients has remained to be established. It has been suggested that inadequate nutritional absorption of essential fatty acids may play a role in the altered bone metabolism. In this study, we characterized the protective effect of fenretinide [N-(4-hydroxyphenyl) retinamide], a vitamin A derivative, on the early onset of osteoporosis in cystic fibrosis transmembrane conductance regulator knockout (Cftr-KO) mice.

METHODS

Using micro-computed-tomography we examined the effect of fenretinide on the bone composition and architecture in a Cftr-KO mouse model which was then confirmed with histological analyses. Plasma fatty acids were quantified using thin layer chromatography-ELISA method.

RESULTS

Twice-weekly treatments with fenretinide, over four weeks dramatically increased trabecular bone volume compared to controls. This increase in bone volume was also related to an increased concentration of ceramide in the plasma resulting in the down regulation of phospholipid-bound AA in Cftr-KO mice.

CONCLUSIONS

To our knowledge, this is the first time that fenretinide's protective effect against osteoporosis has been demonstrated. The results of this study strongly suggest that fenretinide has potential to be used as a prophylaxis by preventing the early onset of osteoporosis.