The heparan sulfate proteoglycan (HSPG) glypican-3 mediates commitment of MC3T3-E1 cells toward osteogenesis

Effects of a sulfated exopolysaccharide produced by Altermonas infernus on bone biology

The growth and differentiation of bone cells is controlled by various factors, which can be modulated by heparan sulfates. Here, we investigated the effects of an oversulfated exopolysaccharide (OS-EPS) on the bone. We compared the effect of this compound with that of a native EPS. Long-term administration of OS-EPS causes cancellous bone loss in mice due, in part, to an increase in the number of osteoclasts lining the trabecular bone surface. No significant difference in cancellous bone volume was found between EPS-treated mice and age-matched control mice, underlying the importance of sulfation in trabecular bone loss. However, the mechanism sustaining this osteoporosis was unclear. To clarify OS-EPS activities, we investigated the effect of OS-EPS on osteogenesis. Our results demonstrated that OS-EPS inhibited osteoclastogenesis in two cell models. Using the surface plasmon resonance technique, we revealed that OS-EPS can form a hetero-molecular complex OS-EPS/receptor activator of NF-κB ligand (RANKL)/RANK and that RANK had a higher affinity for RANKL pre-incubated with OS-EPS than for RANKL alone, which would be in favor of an increase in bone resorption. However, in vitro, OS-EPS inhibited the early steps of osteoclast precursor adhesion and therefore inhibited the cell fusion step. In addition, we showed that OS-EPS reduced proliferation and accelerated osteoblastic differentiation, leading to strong inhibition of mineralized nodule formation, which would be in favor of an increase in bone resorption. Taken together, these data show different levels of bone resorption regulation by EPSs, most of them leading to proresorptive effects.

Glycosaminoglycans modulate RANKL-induced osteoclastogenesis

Skeletal integrity is tightly regulated by the activity of osteoblasts and osteoclasts that are both under the control of extracellular glycosaminoglycans (GAGs) through their interactions with endogenous growth factors and differentiation-promoting ligands. Receptor activator of NF-kappa-B ligand (RANKL), which is a tumor necrosis factor (TNF)-related protein that is critical for osteoclast formation, is produced by osteoblasts and further modulated by certain types of GAGs. Using unfractionated osteoblast-derived GAGs that reflect the complex tissue microenvironment within which osteoclasts reside, we demonstrate that these GAGs block the osteoclastogenic activity of RANKL. Furthermore, RANKL significantly reduces extracellular signal-regulated protein kinase (ERK) activity, a putative suppressor of osteoclastogenesis, but osteoblast-derived GAGs eliminate the inhibitory effects of RANKL on ERK activity. Notably, while imposing an anti-osteoclastic effect, these GAGs also enhanced the proliferation of osteoblasts. Thus, the osteoblast microenvironment is a potent source of GAGs that promote bone anabolic activities. The anti-osteoclastogenic and osteoblast-related mitogenic activities of these GAGs together may provide a key starting point for the development of selective sugar-based therapeutic compounds for the treatment of osteopenic disorders.

Synergism between Wnt3a and heparin enhances osteogenesis via a phosphoinositide 3-kinase/Akt/RUNX2 pathway

A new strategy has emerged to improve healing of bone defects using exogenous glycosaminoglycans by increasing the effectiveness of bone-anabolic growth factors. Wnt ligands play an important role in bone formation. However, their functional interactions with heparan sulfate/heparin have only been investigated in non-osseous tissues. Our study now shows that the osteogenic activity of Wnt3a is cooperatively stimulated through physical interactions with exogenous heparin. N-Sulfation and to a lesser extent O-sulfation of heparin contribute to the physical binding and optimal co-stimulation of Wnt3a. Wnt3a-heparin signaling synergistically increases osteoblast differentiation with minimal effects on cell proliferation. Thus, heparin selectively reduces the effective dose of Wnt3a needed to elicit osteogenic, but not mitogenic responses. Mechanistically, Wnt3a-heparin signaling strongly activates the phosphoinositide 3-kinase/Akt pathway and requires the bone-related transcription factor RUNX2 to stimulate alkaline phosphatase activity, which parallels canonical beta-catenin signaling. Collectively, our findings establish the osteo-inductive potential of a heparin-mediated Wnt3a-phosphoinositide 3-kinase/Akt-RUNX2 signaling network and suggest that heparan sulfate supplementation may selectively reduce the therapeutic doses of peptide factors required to promote bone formation.

Proteoglycans on bone tumor development

Proteoglycans, extracellular matrix components, exert several activities on bone cells and seem crucial for maintaining an appropriate number of osteoblasts and osteoclasts. The overall data strengthen a pro-bone resorptive role for proteoglycans, through the control of osteoprotegerin availability and of receptor activator of NF-kappaB ligand bioactivity. In parallel, proteoglycans participate in the control of tumor development at different levels, including bone tumor development and bone metastases dissemination. This dual role makes them good candidates as regulatory molecules in the vicious cycle between tumor proliferation and bone resorption observed during tumor development in bone site. Knowledge of the biological roles of these molecules in cancer biology, tumor angiogenesis and metastasis has promoted the development of drugs targeting them.