Distinct Concentration-Dependent Molecular Pathways Regulate Bone Cell Responses to Cobalt and Chromium Exposure from Joint Replacement Prostheses

Abstract

Systemic cobalt (Co) and chromium (Cr) concentrations may be elevated in patients with metal joint replacement prostheses. Several studies have highlighted the detrimental effects of this exposure on bone cells in vitro, but the underlying mechanisms remain unclear. In this study, we use whole-genome microarrays to comprehensively assess gene expression in primary human osteoblasts, osteoclast precursors and mature resorbing osteoclasts following exposure to clinically relevant circulating versus local periprosthetic tissue concentrations of Co²⁺ and Cr³⁺ ions and CoCr nanoparticles. We also describe the gene expression response in osteoblasts on routinely used prosthesis surfaces in the presence of metal exposure. Our results suggest that systemic levels of metal exposure have no effect on osteoblasts, and primarily inhibit osteoclast differentiation and function via altering the focal adhesion and extracellular matrix interaction pathways. In contrast, periprosthetic levels of metal exposure inhibit both osteoblast and osteoclast activity by altering HIF-1α signaling and endocytic/cytoskeletal genes respectively, as well as increasing inflammatory signaling with mechanistic implications for adverse reactions to metal debris. Furthermore, we identify gene clusters and KEGG pathways for which the expression correlates with increasing Co²⁺:Cr³⁺ concentrations, and has the potential to serve as early markers of metal toxicity. Finally, our study provides a molecular basis for the improved clinical outcomes for hydroxyapatite-coated prostheses that elicit a pro-survival osteogenic gene signature compared to grit-blasted and plasma-sprayed titanium-coated surfaces in the presence of metal exposure.

Keywords: chromium; cobalt; gene expression; hip replacement; microarray; osteoblasts; osteoclasts; prosthesis surface.

Figure 1

Expression profiles for osteoblast gene clusters with statistically significant expression changes with increasing concentrations of Co²⁺:Cr³⁺ (0, 5 and 500 µg/L). Corresponding heat map for each plot is illustrated underneath. Gene list for each cluster is available in Supplementary Table S1.

Figure 2

Functional classification of genes down-regulated in osteoclast precursors following exposure to 5 µg/L Co²⁺:Cr³⁺. KEGG pathways that were enriched in the differentially expressed genes, with the number of genes in each pathway and the corresponding P value are illustrated here.

Figure 3

Expression profiles for osteoclast precursor gene clusters with statistically significant expression changes with increasing concentrations of Co²⁺:Cr³⁺ (0, 5 and 500 µg/L). Corresponding heat map for each plot is illustrated underneath. (A) Cluster 1 with genes that are overrepresented in calcium signaling and Ras signaling pathway, and associate negatively to metal concentrations, and (B) Cluster 4 have genes overrepresented in cytokine-cytokine receptor interaction pathway and have positive association with increasing concentrations of metal ions. Gene list for all eight clusters can be found in Supplementary Table S4.