No abstract

IL-4 is a pleiotropic immune cytokine secreted by activated TH2 cells that inhibits bone resorption both in vitro and in vivo. The cellular targets of IL-4 action as well as its intracellular mechanism of action remain to be determined. We show here that IL-4 inhibits receptor activator of NF-B ligand-induced osteoclast differentiation through an action on osteoclast precursors that is independent of stromal cells. Interestingly, this inhibitory effect can be mimicked by both natural as well as synthetic peroxisome proliferatoractivated receptor ␥1 (PPAR␥1) ligands and can be blocked by the irreversible PPAR␥ antagonist GW 9662. These findings suggest that the actions of IL-4 on osteoclast differentiation are mediated by PPAR␥1, an interpretation strengthened by the observation that IL-4 can activate a PPAR␥1-sensitive luciferase reporter gene in RAW264.7 cells. We also show that inhibitors of enzymes such as 12͞15-lipoxygenase and the cyclooxygenases that produce known PPAR␥1 ligands do not abrogate the IL-4 effect. These findings, together with the observation that bone marrow cells from 12͞ 15-lipoxygenase-deficient mice retain sensitivity to IL-4, suggest that the cytokine may induce novel PPAR␥1 ligands. Our results reveal that PPAR␥1 plays an important role in the suppression of osteoclast formation by IL-4 and may explain the beneficial effects of the thiazolidinedione class of PPAR␥1 ligands on bone loss in diabetic patients.

Androgen deficiency in males leads to an increase in osteoclastic bone resorption and a progressive decrease in bone mineral density. In the current studies, we examined the ability of 5 alpha-dihydrotestosterone to suppress osteoclast formation induced by receptor activator of NF-kB ligand (RANKL) and macrophage-colony stimulating factor in vitro. 5 alpha-Dihydrotestosterone suppressed the differentiation of bone marrow monocytes into osteoclasts from both sham-operated and orchidectomized mice. Androgen deficiency also led to an increase in the number of hematopoietic precursors capable of forming osteoclasts and increased the relative responsiveness of these cells to androgens in vitro. Interestingly, E2 was as effective as 5 alpha-dihydrotestosterone in suppressing osteoclast formation in bone marrow monocytes from both sham and orchidectomized mice. As with bone marrow monocytes, 5 alpha-dihydrotestosterone also suppressed RANKL-induced osteoclast formation in the monocyte-macrophagic cell line RAW264.7. In RAW264.7 cells, androgens appear to block RANKL-induced osteoclast formation through selective regulation of c-JUN: Accordingly, 5 alpha-dihydrotestosterone suppressed RANKL-induced c-Jun N-terminal kinase activation and reduced c-Jun expression levels. These effects resulted in a reduction in RANKL-induced activator protein-1 DNA binding activity and a corresponding suppression in activator protein-1-mediated transcriptional activation. These studies indicate that both E and androgens can suppress osteoclast formation via a direct, stromal cell-independent action on osteoclast precursors to block key transcription factors such as c-Jun essential for osteoclast differentiation.