Bcl-2 and Bax expression in cartilage and bone cells after high-dose corticosterone treatment in rats

The expression of Bcl-2 and Bax has been evaluated by immunohistochemistry in normal rats, and in rats after treatment with high-dose corticosterone (CORT). Proliferative (PC) and maturative/hypertrophic (MaHC) chondrocytes of the growth plate have been examined, as well as osteoblasts (Obs), osteocytes (Ots) and osteoclasts (Ocs) of the metaphyseal secondary spongiosa. For each cell type, the Bcl-2 and Bax immunopositive cells were expressed as a percentage of the total number of cells. Bcl-2 and Bax expression was considered to be enhanced when the percentage of positive cells rose. Bcl-2 and Bax were expressed in all cell types, and two main kinds of labeling distribution, both suggestive of association with intracellular organelles, were observed in the cytoplasm: scarce and spotty labeling (type 1) or abundant, granular and diffuse labeling (type 2). In some cases, nuclear membranes could also be seen to be positive. Positive PCs and Obs generally showed a labeling of type 1, MaHCs and Ocs of type 2, while Ots varied with labeling of type 1 or type 2. CORT administration induced a fall in the percentage of Bcl-2 immunopositive cells, and a rise in that of Bax immunopositive cells, in PCs and Ots. The same trend was observed in MaHCs, although the Bcl-2 decrease was not significant. The percentage of Bcl-2 and Bax immunopositive Obs rose, and their labeling distribution shifted from type 1- to type 2-labeled cells. Ocs showed the highest immunopositivity for both Bcl-2 and Bax, which did not change after CORT administration. These data suggest that CORT treatment, by lowering Bcl-2, and raising Bax expression, may promote the apoptotic process in PCs, MaHCs and Ots. Obs, however, do not undergo the same variations. This finding, together with the results of a previous study showing that CORT administration raises the frequency of apoptotic Obs, does not support a direct relationship between apoptosis and Bax overexpression, at least in Obs. The CORT effect might be related to cell types and their state of differentiation, so that Bcl-2 and Bax might regulate not only the machinery of cell death, but also cell proliferation and differentiation.