EXPRESSION OF BCL-2 PROTEIN IN THE EPIPHYSEAL PLATE CARTILAGE AND TRABECULAR BONE OF GROWING RATS

The protooncogene protein, Bcl-2, protects cells from apoptosis and en sures their survival in vitro by inhibiting the action of the apoptosi s-inducer, Bar. Its expression in proliferative and long-lived cells i n vivo also indicates that it protects against cell death. The chondro cytes of the epiphyseal plate cartilage undergo a series of maturation steps and deposit mineral in the cartilage matrix before dying. The p ossibility that Bcl-2 helps protect chondrocytes until mineral deposit ion is completed was investigated by determining the distribution of B cl-2 immunoreactivity in the epiphyseal plate cartilage of growing rat s and its subcellular localization, using a specific antibody. The inv olvement of Bar in the triggering of chondrocyte death was checked by immunocytochemistry. Bcl-2 expression in the osteoblasts and the final result of their evolution, the osteocytes, was also examined in trabe cular bone. Bcl-2 immunoreactivity was non-uniformly distributed throu ghout the epiphyseal cartilage. It was maximal in proliferative chondr ocytes, decreased in mature chondrocytes, and low in hypertrophic chon drocytes, whereas there was Bar immunoreactivity in all chondrocytes e xamined. immunolabeling was intense in osteoblasts but considerably lo wer in fully differentiated osteocytes. Bcl-2 immunoreactivity was mai nly in the cytoplasm of chondrocytes, osteoblasts, and early osteocyte s; the nuclei appeared clear. The subcellular distribution of Bcl-2 im munolabeling in chondrocytes, revealed by gold particles in the electr on microscope, showed that gold particles were frequently concentrated in the mitochondria in all the cartilage zones and lay mainly within the organelles, not at their periphery. The endoplasmic reticulum cont ained moderate immunoreactivity and there were few gold particles in t he cytoplasm and nuclei. The number of gold particles decreased in all the subcellular compartments from proliferative to hypertrophic chond rocytes. In contrast, Bar immunoreactivity changed little during chond rocyte terminal evolution, and its subcellular distribution mirrored t hat of Bcl-2. These immunocytochemical data indicate that Bcl-2 helps maintain chondrocytes and osteoblasts until their terminal maturation.