Osteoclasts are known to play a crucial role in both physiologic and pathologic
bone resorption. Moreover, it is generally agreed that
IL-1 has powerful effects on osteoclastic
bone resorption, although the precise cellular sites and mechanisms by which
IL-1 mediates osteoclastic
bone resorption remain unclear. In particular, it is still controversial whether osteoclasts can respond to
IL-1 directly. The expression of
mRNA for type I
IL-1 receptor (IL-1RI) and type II
IL-1 receptor (IL-1RII) in osteoclasts was investigated in normal and inflammatory bone tissues by in situ hybridization to determine whether osteoclasts are the target cells for
IL-1 and to elucidate the mechanism by which
IL-1 induces osteoclastic
bone resorption. For this study, normal tibiae were obtained from newborn, young, and adult mice and rats, and inflammatory bone tissues with bone destruction were obtained from
adjuvant arthritis rat models. The results showed that (a) both
IL-1 receptors (IL-1RI and -II)
mRNA were expressed by osteoclasts in all tissue sections of normal tibiae; (b) at the stage of the
adjuvant arthritis studied, the IL-1RI
mRNA was the most predominant message in osteoclasts present in the area with serious cartilage and bone destruction, whereas the expression level of IL-1RII
mRNA in these osteoclasts was weak; and (c) both IL-1RI and -II
mRNA were expressed by osteoblasts, as well as by osteocytes localized in the osteoid. In addition, these messages were also expressed by chondrocytes, but the signals were not detected in the chondrocytes in the zones of
hypertrophy and provisional calcification. Our present study demonstrates for the first time that mouse and rat osteoclasts express IL-1RI and -II
mRNA, which suggest that a primary effect of
IL-1 on osteoclasts may be one of the mechanisms by which
IL-1 mediates normal bone remodeling and pathologic
bone resorption in chronic inflammatory diseases.