Abstract
Commonly used alterations of prosthetic surfaces include grit-blasting (GB), plasma-sprayed titanium (Ti) or hydroxyapatite (HA) coating. Systemic concentrations of cobalt (Co) and chromium (Cr) are elevated in patients with metal-on-metal hip replacement, but can occur for all modular hip replacements. Here, we use whole genome microarrays to assess differential gene expression in primary human osteoblasts grown in vitro and on these prosthesis surfaces following exposure to clinically relevant concentrations of Co and Cr.
Mesenchymal cells obtained from bone-fragments of 3 patients undergoing joint replacement surgery were differentiated into osteoblasts. Subsequently, cells were cultured in vitro on tissue-culture plates (TCP), or on GB, Ti and HA surfaces (JRI Orthopaedics Ltd, Sheffield, UK). Following 24hr exposure to a combination of clinically equivalent concentrations of Co2+:Cr3+, RNA was extracted and hybridized to SurePrint-G3 Gene Expression Microarray. Probe signals were normalised using ‘Limma’ package on R-Bioconductor and differential gene expression assessed with empirical Bayes approach (Log2FC>1.00, P<0.001 for differentially expressed genes).
For cells grown on TCP, 11 genes were upregulated with 500μg/L Co2+:Cr3+. Of these, 4 were associated to HIF-1 signalling based on KEGG pathway analysis (P=5.4e-5). Exposure to 1000μg/L Co2+:Cr3+ altered expression at 164 loci for HA surfaces, and a separate 50 loci for Ti surfaces compared to GB surfaces. Genes for osteoblast differentiation (BMP2 and RGS2) were downregulated on HA surfaces compared to GB, whilst genes for cell-adhesion (ESAM), vesicular trafficking (RAB37) and protection against oxidative damage (NRF2) were upregulated. Ti surfaces caused an upregulation in ERBB3 and CNTF, which are associated with inhibition of osteoblast differentiation and mineralisation, when compared to GB surfaces.
This study confirms the role of HIF-1 signalling in response to prosthesis generated metal ions, and is the first to provide a comprehensive genome-wide insight into transcriptional response of osteoblasts at prosthesis surface to clinically equivalent metal exposure.
