COBALT IONS INDUCE A MESENCHYMAL PHENOTYPE IN PRIMARY HUMAN OSTEOBLASTS

Abstract

Introduction: Despite a resurgence in cobalt-chromium metal-on-metal arthroplasty and hip resurfacing, the potential toxicity of cobalt ions in the periprosthetic area remains a cause for concern. Cytotoxic effects have been demonstrated in macrophages with cobalt ions inducing apoptosis and TNF-α secretion. A similar cytotoxic effect has been demonstrated in osteoblast-like cells. However, these studies assessed the acute cellular response to cobalt ions over 48 hours. To date, the effect on osteoblasts of chronic exposure to cobalt ions is unknown.

Aim: In this study we investigated the effect on osteoblasts of chronic exposure to cobalt ions. Specifically we investigated the chemokine response and effect on osteoblast function. We also investigated for a change in osteoblast phenotype to a less differentiated mesenchymal cell type.

Methods. Primary human osteoblasts were cultured and treated with cobalt (10ppm) over 21 days. Secreted chemokines (IL-8, MCP-1, TNF-α) were assayed using enzyme-linked immunosorbent assays (ELISA). Osteoblast function was assessed via alkaline phosphatase activity and calcium deposition. For a change in osteoblast phenotype, osteoblast gene expression was assessed using real time PCR. Immunoflourescent cell staining of actin filaments was used to examine for a change in osteoblast morphology.

Results: Chemokine (IL-8) secretion by osteoblasts was significantly increased after 7 days of stimulation with cobalt ions. In parallel with this, osteoblast function was also significantly inhibited as demonstrated by reduced alkaline phosphatase activity and calcium deposition. Regarding osteoblast phenotype, FSP-1, CTGF and TGF-β gene expression were upregulated after 7 days exposure indicating a transition in osteoblast phenotype to a less differentiated mesenchymal cell type. Immunoflourescent staining of actin filaments also showed a change in osteoblast morphology. Taken together, these data demonstrate cobalt ions induce a change in the osteoblast phenotype to that of a mesenchymal cell type. This is the first study to investigate osteoblast plasticity in the context of periprosthetic osteolysis.

Conclusion: After prolonged exposure to cobalt ions, IL-8 chemokine secretion is increased which attracts neutrophils to the periprosthetic area. Furthermore, osteoblasts no longer function as osteogenic cells as demonstrated by a decrease in osteoblast alkaline phosphatase activity and calcium deposition. Instead, they undergo transition to a mesenchymal cell type as demonstrated by an increase in the expression of genes associated with a mesenchymal cell lineage. Instead of secreting osteoid matrix the new cell type secretes unmineralized collagen. Cobalt ions are not benign and may play an important role in periprosthetic osteolysis by inducing osteoblasts to undergo transition to a less differentiated mesenchymal cell type.