HSP72: A NEW MEDIATOR IN WEAR PARTICLES-INDUCED OSTEOLYSIS

Abstract

The biological response to implant-derived wear particles is recognized as one of the main factors involved in the development of periprosthetic osteolysis. Wear particles induce a foreign-body inflammatory response that results in the formation of a periprosthetic membrane and progresses over time to aseptic loosening and implant failure. Upon exposure to particles, macrophages and other cell types release inflammatory cytokines to the periprosthetic milieu such as inter-leukin-1 beta (IL-1 beta, tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) which contribute to bone resorption. Heat shock proteins (HSP) are intra-cellular proteins involved in the maintenance of cellular homeostasis. The stress inducible form of the Hsp70 family protein, Hsp72, has been detected in circulation, acting as a factor capable of regulating pro-inflammatory cytokines secretion and it has been demonstrated that induces the production of pro-inflammatory cytokines via the CD14 and Toll-like receptor-mediated signal transduction pathway.

We hypothesized that Hsp72 could be involved in the inflammatory response to wear particles. To this aim, we investigated Hsp72 and its receptor, CD14, in interfacial membrane specimens obtained from patients undergoing revision surgery for aseptic loosening of uncemented acetabular cups (n=7). Distribution of both proteins was assessed by immunofluorescence and examined by confocal laser scanning microscopy. Hsp72 was detected in the periprostehetic membranes, colocalizing with CD14. Explants of membranes were cultured in vitro and levels of Hsp72 and IL-6 were determined by ELISA after 24, 48 and 72 h (n=9). Cultured membranes released IL-6 to culture medium in a time-dependent manner (p< 0.05), while Hsp72 levels decreased during same observation period (p< 0.05). These data suggest that, rather than being produced by the periprosthetic tissue, Hsp72 might be recruited by CD14+ cells from extracellular fluids. In this regard, preliminary data indicated that soluble Hsp72 levels in sera from patients undergoing revision surgery due to aseptic loosening were significantly lower than those from age-matched control subjects (n=6; p< 0.001). To investigate the involvement of Hsp72 in the inflammatory response to wear particles, we used a cell culture model of THP-1 cells driven to the monocyte/macrophage differentiation pathway. These cells were exposed to titanium particles of phagocytosable sizes, either in the presence or absence of exogenously added Hsp72. results obtained to date indicate that Hsp72 is able to modulate the titanium-induced TNF-alpha, IL-1 beta and IL-6 secretion (p< 0.05). Altogether, our data suggest that Hsp72 could be a novel mediator involved in wear particles-induced osteolysis and prosthetic failure.