Osteoblasts Respond to Mild-Heat Stress By Change in OPG/RANKl Ratio

Objectives: Molecular chaperones, also known as heat shock proteins (Hsps) and cell stress proteins, have a potent effect on bone remodeling(1;2). Hsps have evolved as intracellular protein-folding molecules essential in enabling cells to cope with environmental and physiological stress(3). Mild physiological stress such as change in pH or oxygen tension also has a potent effect on bone remodeling(4;5). We have investigated the effect of mild thermal stress on osteoblasts with reference to bone remodeling. Methods: Osteoblastic cell line; MG63 cells were exposed to 33^oC, 37^oC and 42^oC for 90 minutes, then incubated overnight at 37^oC. We assayed for OPG and sRANKL and tumour necrosis factor alpha (TNFa) protein and mRNA and heat shock proteins 27, 60, 70 and 90. Results: There was no expression of hsp60 and 70, HSP27 was slightly more up regulated at 33^oC compared to 42^oC, hsp90 was expressed at all temperatures studied. There was no change in TNFa levels. The expression of RANKL was significantly reduced at 42^oC whereas there was no difference in the OPG levels. At 33^oC OPG levels were significantly raised, and the RANKL levels were not affected. Conclusions: These results show osteoblasts respond differently to hyper-hypothermia, mild thermal stress suppresses RANKL synthesis at 42^oC, with no effect on OPG in contrast at 33^oC, OPG synthesis is suppressed with no effect on RANKL synthesis. In both cases altering the OPG/RANKL ratio, the balance between RANKL and OPG is critical for modulation of bone remodeling and in both cases would lead to inhibition of bone resorption (1)Nair et al. Calcif Tissue Int 1999 Mar;64(3):214-8. (2)Meghji et al. Bone 2003 33(3):419-25. (3)Gething M-J. Guidebook to molecular chaperones and proteins folding catalysts. Oxford University Press; 1997. (4)Arnett TR, et al. J Cell Physiol 2003 ;196(1):2-8. (5)Meghji et al.. Am J Physiol Endocrinol Metab 2001;280(1):E112-E119.