Experimental study on the role of osteoclasts and free radicals in the mandibular invasion of VX2 carcinoma in Japanese white rabbits

Oxygen-derived free radicals are stimulators of bone resorption in vitro and in vivo. We hypothesized that oxygen-derived free radicals or reactive oxygen species (ROS) generated around bone matrix invaded by cancer cells might be associated with the activation or formation of osteoclasts in bone metastasis, and thus the administration of an ROS scavenger or a free-radical scavenger might control osteoclastic bone destruction in tumor metastasis. We examined the ability of VX2 carcinoma cells to generate superoxide anion (O2−) as well as the effects of the O2− scavenger superoxide dismutase (SOD) and the hydrogen peroxide (H2O2) scavenger catalase (CAT) on the activation and formation of osteoclasts in VX2 carcinoma tissue of rabbits. VX2 carcinoma cells generated more O2− than tartrate-resistant acid phosphatase-positive (TRAP+) cells. The TRAP activity from TRAP+ cells in the presence of VX2 carcinoma cells was higher than that from TRAP+ cells in the absence of the carcinoma cells. The addition of SOD but not of CAT into the incubation medium inhibited the TRAP activity from TRAP+ cells in the presence or absence of VX2 carcinoma cells. Similar inhibitory effects were also observed with SOD plus CAT on TRAP+ cells in the presence or absence of carcinoma cells. Intravenous administration of SOD and SOD plus CAT, but not of CAT, caused a decrease in the number of osteoclasts in mandibles implanted with VX2 carcinoma cells, and showed a predominant occupation by osteoclasts with poorly developed ruffled borders in lesions of bone resorption. In contrast, the administration of CAT, but not of SOD and SOD plus CAT, decreased the more mature forms of osteoclasts in implanted mandibles. From the results obtained, it is suggested that O2− may stimulate bone resorption by increasing the activity and number of osteoclasts, and H2O2 may stimulate resorption by enhancing the formation of mature osteoclasts in tumor metastasis. The administration of some ROS or free-radical scavengers to patients with cancer may provide a defense against bone destruction in bone metastatic lesions.