Direct and indirect induction of apoptosis in human mesenchymal stem cells in response to titanium particles

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The most frequent complication of total joint arthroplasty is periprosthetic osteolysis initiated by an inflammatory response to orthopaedic wear debris, which if left untreated, can result in implant instability and failure, eventually requiring revision surgery. We have previously reported that osteogenic differentiation of human marrow stroma-derived mesenchymal stem cells (hMSCs) is suppressed upon exposure to titanium particles, accompanied by reduced bone sialoprotein (BSP) gene expression, diminished production of collagen type I and BSP, decreased cellular viability and proliferation, and inhibition of extracellular matrix mineralization. In this study, we have further investigated hMSC cytotoxicity upon exposure to submicron particles of commercially pure titanium (cpTi) and zirconium oxide (ZrO 2). Our results showed that direct exposure to cpTi and ZrO 2 particles compromises cell viability through the induction of apoptosis, eliciting increased levels of the tumor suppressor proteins p53 and p73, in a manner dependent on material composition, particle dosage, and time. Additionally, conditioned medium collected from hMSCs exposed to cpTi particles, but not to ZrO 2 particles, is cytotoxic to hMSCs, inducing apoptosis in the absence of particles. These findings demonstrate that exposure to orthopaedically derived wear particles can compromise hMSC viability through the direct and indirect induction of apoptosis. Thus, prolonged in vivo exposure of marrow-derived hMSCs to implant-derived wear debris is likely to reduce the population of viable osteoprogenitor cells, and may contribute to poor periprosthetic bone quality and implant loosening.