Study on the Mechanism of Bone-mineral Destruction by Human Monocytes

To clarify the mechanism of bone resorption by osteoclasts or macrophages, the bone resorption assay using 45Ca-labeled rat calvaria or killed bone is usually used. However, it is difficult to determine the effector cell that resorbs the bone because of the variety of bone cell types (osteocytes, etc.), cytokines and collagen present in the assay system. So we introduced a simple bone resorption assay system which uses hydroxyapatite (HA), the main mineral component of bone and purified human monocytes. And we have reported that human monocytes destroyed 45Ca-labeled synthetic hydroxyapatite (45Ca-HA) and this activity was enhanced by addition of the cultured supernate of PHA stimulated human peripheral leukocytes (PHA-sup). In this paper, the mechanism of bone-mineral destruction (45Ca release from 45Ca-HA) by human monocytes was investigated. The results were as follows: 1. Monocytes adhering to HA were observed by lightmicroscopy and electronmicroscopy in 48 hr culture. 2. By addition of PHA-sup, monocytes-adhering activity to HA was enhanced significantly and the quantity of released 45Ca from 45Ca-HA by monocytes was increased. 3. By addition of cytochalasin B (1μg/ml-10μg/ ml), the quantity of released 45Ca from 45Ca-HA was decreased dependently to the concentration. 4. Released 45Ca in media from 45Ca-HA was detected in 24 hr culture, but cell-intral 45Ca was detected in 96 hr culture. These results suggest that bone-mineral destruction by monocytes is related to the concentration of citric acid in the micro-environment of the adherence site.