PTX3 Effects on Osteogenic Differentiation in Osteoporosis: An In Vitro Study

Genetic Deficiency of the Long Pentraxin 3 Affects Osteogenesis and Osteoclastogenesis in Homeostatic and Inflammatory Conditions

The long pentraxin 3 (PTX3) is a soluble glycoprotein made by immune and nonimmune cells endowed with pleiotropic functions in innate immunity, inflammation, and tissue remodeling. PTX3 has recently emerged as a mediator of bone turnover in both physiological and pathological conditions, with direct and indirect effects on osteoblasts and osteoclasts. This notwithstanding, its role in bone biology, with major regard to the osteogenic potential of osteoblasts and their interplay with osteoclasts, is at present unclear. Here, we investigated the contribution of this pentraxin to bone deposition in the osteogenic lineage by assessing collagen production, mineralization capacity, osteoblast maturation, extracellular matrix gene expression, and inflammatory mediators' production in primary osteoblasts from the calvaria of wild-type (WT) and Ptx3-deficient (Ptx3-/-) mice. Also, we evaluated the effect of PTX3 on osteoclastogenesis in cocultures of primary osteoblasts and bone marrow-derived osteoclasts. Our investigations were carried out both in physiological and inflammatory conditions to recapitulate in vitro aspects of inflammatory diseases of the bone. We found that primary osteoblasts from WT animals constitutively expressed low levels of the protein in osteogenic noninflammatory conditions, and genetic ablation of PTX3 in these cells had no major impact on collagen and hydroxyapatite deposition. However, Ptx3-/- osteoblasts had an increased RANKL/OPG ratio and CD44 expression, which resulted in in enhanced osteoclastogenesis when cocultured with bone marrow monocytes. Inflammation (modelled through administration of tumor necrosis factor-α, TNF-α) boosted the expression and accumulation of PTX3 and inflammatory mediators in WT osteoblasts. In these conditions, Ptx3 genetic depletion was associated with reduced collagen deposition and immune modulators' production. Our study shed light on the role of PTX3 in osteoblast and osteoclast biology and identified a major effect of inflammation on the bone-related properties of this pentraxin, which might be relevant for therapeutic and/or diagnostic purposes in musculoskeletal pathology.

Exposure to Random Positioning Machine Alters the Mineralization Process and PTX3 Expression in the SAOS-2 Cell Line

Bone loss is among the most frequent changes seen in astronauts during space missions. Although weightlessness is known to cause high bone resorption and a rapid decrease in bone minerals and calcium, the underlying mechanisms are not yet fully understood. In our work, we investigated the influence of random positioning machine (RPM) exposure on the mineralization process in the SAOS-2 cell line, in osteogenic and non-osteogenic conditions, by examining changes in their mineralizing capacity and in the expression of PTX3, a positive regulator of bone mineralization. We analyzed cell viability by MTS assay and the mineralization process after staining with Toluidine Blue and Alizarin Red, while PTX3 expression was investigated by immunocytochemistry and western blotting analysis. Our results showed that RPM exposure increased cells’ viability and improved their mineralizing competence when not treated with osteogenic cocktail. In contrast, in osteogenic conditions, cells exposed to RPM showed a reduction in the presence of calcification-like structures, mineral deposits and PTX3 expression, suggesting that the effects of RPM exposure on mineralizing matrix deposition depend on the presence of osteogenic factors in the culture medium. Further studies will be needed to clarify the role of potential mineralization markers in the cellular response to the simulated biological effects of microgravity, paving the way for a new approach to treating osteoporosis in astronauts exposed to spaceflight.