Strontium regulates the mineralization of osteocompetent cells at the molecular level and increases the bioactivity of PMMA cement

The bone tissue provides mechanical support and protection to soft tissues and regulates the availability and storage of calcium. Conditions like estrogen deficiency, long-term immobilization, and aging are commonly associated with the loss of bone mass and predisposition to fractures. The use of bioactive nanoparticles (NPs) that mimic the structure of bone apatite is a compelling approach to bone healing. Strontium ions (Sr2+) have the ability to control osteoblast activity and affect extracellular matrix (ECM) mineralization. However, how strontium acts on mineralization-competent cells are unclear. In the present work, we hypothesized that Sr2+ affects mineralization in two ways: (1) at a molecular level by regulating indirectly osteogenic genes, and (2) by regulating secretion and molecular composition, hence, the function of matrix vesicle (MV). These vesicles are thought to play a major role in the initiation of ECM mineralization. Here, we used alizarin red staining, TNAP activity, and qRT-PCR to check the NPs´ potential in inducing mineralization of the pre-osteoblast MC3T3-E1 cell line. Subsequently, we compared by computerized micrography, histology, qRT-PCR, and mechanical test rabbits´ femur implanted with polymethylmethacrylate cement loaded and unloaded with the NPs. Using an odontoblastic cell line (17IIA11), we analyzed the Sr2+ effect upon MVs secretion (by nanoparticle tracking system) and protein composition (by Western blot). Our results demonstrate that the NPs in which 90 % of Ca2+ ions were substituted by Sr2+ (NanoSr 90%) upregulate TNAP activity and increase ECM mineralization. Moreover, NanoSr 90% increase the mRNA levels of markers of osteoblast differentiation: Runx2, Ocn, and Sp7. In vivo, studies showed that NanoSr 90% increases BMP2 and OCN gene expression. The mechanical test (push-out) revealed that porous cement loaded with the NPs increases the maximum strength of the interface bone/cement de-bonding. On the molecular level, Sr2+ stimulates Erk1/2 and CREB signaling and represses MVs secretion. However, MVs secreted in the presence of Sr2+ are enriched in proteins engaged in calcium and phosphate regulation. Finally, our data suggest that Sr2+ .modulates mineralization at the gene expression and MVs secretion level, hence its use as a potential bioactive agent could be applied in the bone regeneration field.