Taurine Inhibits Glucocorticoid-Induced Bone Mitochondrial Injury, Preventing Osteonecrosis in Rabbits and Cultured Osteocytes

Abstract

Mitochondrial injury has recently been implicated in the pathogenesis of glucocorticoid-induced osteonecrosis. Using cultured osteocytes and a rabbit model, we investigated the possibility that taurine (TAU), which is known to play a role in the preservation of mitochondrial function, might also prevent the development of osteonecrosis. To reduplicate the intraosseous environment seen in glucocorticoid-induced osteonecrosis, dexamethasone (Dex) was added to MLO-Y4 cultured in 1% hypoxia (H-D stress environment). An in vitro study was conducted in which changes in mitochondrial transcription factor A (TFAM), a marker of mitochondrial function, and ATP5A produced by mitochondria, induced by the presence/absence of taurine addition were measured. To confirm the effect of taurine in vivo, 15 Japanese White rabbits were administered methylprednisolone (MP) 20 mg/kg as a single injection into the gluteus muscle (MP+/TAU- group), while for 5 consecutive days from the day of MP administration, taurine 100 mg/kg was administered to 15 animals (MP+/TAU+ group). As a control 15 untreated rabbits were also studied. The rabbits in each of the groups were sacrificed on the 14th day after glucocorticoid administration, and the bilateral femora were harvested. Histopathologically, the incidence of osteonecrosis was quantified immunohistochemically by quantifying TFAM and ATP5A expression. In the rabbits exposed to an H-D stress environment and in MP+/TAU- group, TFAM and ATP5A expression markedly decreased. With addition of taurine in the in vitro and in vivo studies, the expression of TFAM and ATP5A was somewhat decreased as compared with Dex-/hypoxia- or MP-/TAU- group, while improvement was noted as compared with Dex+/hypoxia+ or MP+/TAU- group. In rabbits, the incidence of osteonecrosis was 80% in MP+/TAU- group, in contrast to 20% in the taurine administered group (MP+/TAU+), representing a significant decrease. Since taurine was documented to exert a protective effect on mitochondrial function by inhibiting the mitochondrial dysfunction associated with glucocorticoid administration, we speculated that it might also indirectly help to prevent the development of osteonecrosis in this context. Since taurine is already being used clinically, we considered that its clinical application would also likely be smooth.

Keywords: glucocorticoid; mitochondrial function; osteonecrosis; taurine.

Figure 1

Mitochondrial transcription factor A (TFAM) and ATP5A expression in cultured osteocytes in an H-D stress environment. (a) Immunocytochemical study, (b) Western blot. TFAM (29 kDa), ATP5A (50–55 kDa), β-actin (42 kDa). In Dex+/hypoxia+ group as compared with Dex−/hypoxia− group, expression of TFAM and ATP5A levels was decreased. In Dex+/hypoxia+/taurine+ group with addition of taurine, as compared with Dex+/hypoxia+ group, levels of both TFAM and ATP5A expression were increased. Scale bar: 20 µm.

Figure 2

Osteonecrosis inhibition with intravenous administration of taurine in a glucocorticoid-induced rabbit osteonecrosis model. (a) MP−/TAU−, (b) MP+/TAU−, (c,d) MP+/TAU+. Osteonecrosis was found in 12 of 15 rabbits of MP+/TAU− group (b). In 4 of 15 rabbits of the MP+/TAU+ group, some portions showing necrosis of medullary haematopoietic cells or fat cells and empty lacunae or condensed nuclei in osteocytes were found (d). The osteonecrosis development rate was 20%, representing a significant inhibition of osteonecrosis as compared with the MP+/TAU− group (p < 0.05). Scale bar: 200 µm.

Figure 3

TFAM and ATP5A positive cell rate after taurine administration in a glucocorticoid-induced rabbit osteonecrosis model. (a) Immunohistochemical study of TFAM and ATP5A. Scale bar: 100 µm. (b) Graph indicates percentage of positive cells of TFAM and ATP5A in the indicated conditions. Columns and bars indicate means and S.E. As compared with MP−/TAU− group, in MP+/TAU− group the numbers of TFAM and ATP5A positive cells were significantly decreased (^# p < 0.0001 vs. MP−/TAU−, ^## p = 0.001 vs. MP−/TAU−). In MP+/TAU+ group, TFAM and ATP5A levels were maintained as compared with MP−/TAU− group. (* p < 0.0001 vs. MP+/TAU−, ** p < 0.001 vs. MP+/TAU−).