Retinoic acid directly stimulates osteoclastic bone resorption and gene expression of cathepsin K/OC-2

Abstract

Vitamin A metabolites such as all-trans-retinoic acid (all-trans-RA) affect several steps of metabolic processes in vertebrates. In the last few years, several studies have shown the effect of RA on bone formation and metabolism. However, mechanisms of its action still remain unclear, especially with respect to the regulation of bone cells. Therefore, this study was carried out to clarify how RA regulates the activity of osteoclasts. Using a pit assay involving unfractionated bone cells, including osteoclasts obtained from rabbits, we found that RA stimulated an increase in the bone-resorbing activity in a dose- and time-dependent manner. Furthermore, this effect occurred more rapidly than that of treatments with 1 alpha,25-dihydroxyvitamin D3. However, this effect of RA may be partly related to cross-talk between osteoclasts and other types of cells. Therefore we studied the effect of RA on isolated osteoclasts. We found that all-trans-RA regulates the gene expression of cathepsin K/OC-2, a dominant cysteine proteinase, at the transcriptional level in mature osteoclasts isolated from rabbits. Moreover, retinoic acid-receptor alpha mRNA and retinoid X-receptor beta mRNA were expressed in these mature osteoclasts. Our results indicate that osteoclasts are target cells for RA and that RA might regulate a part of bone formation and metabolism through osteoclasts.

Cloning and Function of Rabbit Peroxisome Proliferator-activated Receptor δ/β in Mature Osteoclasts

“…Using these highly enriched mature osteoclasts, we clarified that estrogen directly inhibited bone resorption and cathepsin K gene expression and induced osteoclastic apoptosis through estrogen receptor ␣ (12, 13). Moreover, we demonstrated that retinoids activated both osteoclastic bone resorption and gene expression of cathepsin K, and of retinoic acid receptor ␣ and retinoid X receptor ␤ were expressed in mature osteoclasts (14).…”

“…The total RNA was extracted from the cultured cells by the acid guanidinium-phenol-chloroform method. Total RNA was fractionated on a formaldehyde agarose gel by electrophoresis and then blotted onto a nylon membrane for Northern blot analysis (14). 32 PLabeled cDNA probes were prepared by the random primer labeling procedure.…”

“…Using these highly enriched mature osteoclasts, we clarified that estrogen directly inhibited bone resorption and cathepsin K gene expression and induced osteoclastic apoptosis through estrogen receptor ␣ (12, 13). Moreover, we demonstrated that retinoids activated both osteoclastic bone resorption and gene expression of cathepsin K, and of retinoic acid receptor ␣ and retinoid X receptor ␤ were expressed in mature osteoclasts (14).Receptors for steroid hormones, retinoids, vitamin D, thyroid hormone, and prostanoids comprise a superfamily of regulatory proteins that are structurally and functionally related. Previous studies showed that members of the nuclear hormone receptor superfamily shared high homology in their amino acid sequences, with the highest homology conserved among their DNA binding domains.…”

Emerging Roles for Cysteine Proteases in Human Biology

“…In contrast, cathepsin K is highly expressed in ovaries and osteoclasts (57). Retinoic acid is reported to induce transcription and protein accumulation in osteoclastic cell lines (58). Moreover, cathepsin K appears to be upregulated at sites of inflammation.…”

Immunolocalization of vacuolar‐type H+‐ATPase, cathepsin K, matrix metalloproteinase‐9, and receptor activator of NFkB ligand in odontoclasts during physiological root resorption of human deciduous teeth

“…In the initial phase of osteoclastic bone resorption, hydroxyapatite crystals are decalcified by acidification of the subosteoclastic microenvironment created by vacuolar-type H ϩ -ATPase, which is highly localized in the ruffled border membranes of osteoclasts (Baron et al, 1985;Vaes, 1988;Sundquist et al, 1990;Vaananen et al, 1990;Sasaki et al, 1994;Yokoya et al, 1997). Resorption lacunae are then formed by degradation of bone type-I collagen, mediated mainly by cathepsin K, a lysosomal cysteine proteinase, which is highly and selectively expressed in osteoclasts (Tezuka et al, 1994;Bromme and Okamoto, 1995;Inaoka et al, 1995;Li et al, 1995;Saneshige et al, 1995;Bossard et al, 1996;Bromme et al, 1996;Drake et al, 1996;Inui et al, 1997;LittlewoodEvans et al, 1997;Kamiya et al, 1998;Yazawa et al, 1998). In concert with cysteine proteinase, acid-soluble and -insoluble type I collagens are thought to be further degraded in the subosteoclastic microenvironment by matrix metalloproteinase-9 (MMP-9, 92-kDa gelatinase/type IV collagenase ϭ gelatinase B) produced by osteoclasts (Everts et al, 1992;Reponen et al, 1994;Wucherpfennig et al, 1994;Okada et al, 1995).…”