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

Abstract

Osteoclasts modulate bone resorption under physiological and pathological conditions. Previously, we showed that both estrogens and retinoids regulated osteoclastic bone resorption and postulated that such regulation was directly mediated through their cognate receptors expressed in mature osteoclasts. In this study, we searched for expression of other members of the nuclear hormone receptor superfamily in osteoclasts. Using the low stringency homologous hybridization method, we isolated the peroxisome proliferator-activated receptor ␦/␤ (PPAR␦/␤) cDNA from mature rabbit osteoclasts. Northern blot analysis showed that PPAR␦/␤ mRNA was highly expressed in highly enriched rabbit osteoclasts. Carbaprostacyclin, a prostacyclin analogue known to be a ligand for PPAR␦/␤, significantly induced both bone-resorbing activities of isolated mature rabbit osteoclasts and mRNA expression of the cathepsin K, carbonic anhydrase type II, and tartrate-resistant acid phosphatase genes in these cells. Moreover, the carbaprostacyclin-induced bone resorption was completely blocked by an antisense phosphothiorate oligodeoxynucleotide of PPAR␦/␤ but not by the sense phosphothiorate oligodeoxynucleotide of the same DNA sequence. Our results suggest that PPAR␦/␤ may be involved in direct modulation of osteoclastic bone resorption.Mature osteoclasts are differentiated from hemopoietic stem cells and fuse each other into multinucleate giant cells. These cells are characterized by a combination of some unique properties. They possess tartrate-resistant acid phosphatase (TRAP) 1 activity, an abundance of cathepsin K, and highly developed ion transport systems (1, 2). Mature osteoclasts are involved in bone metabolism, especially they resorb mineralized bone surfaces under both physiological and pathological conditions (3).Much evidence obtained from in vitro studies has demonstrated that the differentiation of osteoclasts is controlled by various factors, such as vitamin D 3 , osteoclast differentiation factor, osteoclastogenesis inhibitory factor, macrophage colonystimulating factor, and parathyroid hormone (4 -7). The osteoclast is also active in postmenopausal osteoporosis (bone loss), a major health problem (2). Estrogen deficiency has long been recognized as a cause of postmenopausal osteoporosis, and estrogen replacement therapy is an effective treatment for the prevention of bone loss. Research has shown that estrogen loss promotes interleukin-6 secretion in nonosteoclastic cells and that interleukin-6 up-regulates osteoclast differentiation and formation (8,9). However, the function and metabolism of fully mature osteoclasts remain unclear, especially at the molecular level, because of the difficulty in obtaining large quantities of highly enriched osteoclasts for use in such in vitro studies.For assessment of the bone-resorbing activity of mature osteoclasts, it would be desirable to have a cell suspension consisting of highly enriched mature osteoclasts that could be cultured on a mineralized substratum such as a bone or dentine ...

The Peroxisome Proliferator-activated Receptor δ Promotes Lipid Accumulation in Human Macrophages

“…PPAR␦, also known as PPAR␤, NUC1, and FAAR, has been shown to be expressed in a wide range of tissues, but progress in understanding the function of this protein has been hampered by the lack of selective ligands. PPAR␦ has recently been implicated in a wide range of physiological and pathophysiological processes such as embryonic implantation, wound healing, inflammation, cancer, and osteoporosis (23)(24)(25)(26)(27)(28)(29). In this report, we show that PPAR␦ mRNA and protein levels are dramatically elevated during macrophage differentiation and provide both genetic and pharmacological evidence that PPAR␦ is a positive effector of lipid accumulation in human macrophage cultures.…”

The Free Fatty Acid Receptor G Protein-coupled Receptor 40 (GPR40) Protects from Bone Loss through Inhibition of Osteoclast Differentiation*

“…The effects of PPARs on osteoclastogenesis remain controversial and depend on the receptor subtype. PPAR␥ activation was found either to inhibit (32,33) or stimulate (34) osteoclast differentiation, whereas antisense strategies for PPAR␦/␤ revealed a pro-resorbing impact of these two isoforms (35). Interestingly, PPAR␥ agonists such as thiazolidinediones and rosiglitazone also stimulate GPR40 signaling pathways, therefore inhibition of osteoclastogenesis by PPAR␥ activators may in fact result from an uninvestigated GPR40 activation (25,36,37).…”

Differentiation of Trophoblast Giant Cells and Their Metabolic Functions Are Dependent on Peroxisome Proliferator-Activated Receptor β/δ

“…While PPAR␣ and PPAR␥ have clearly defined roles in controlling lipid and glucose homeostasis (64), various physiological roles of PPAR␤ are still being investigated. PPAR␤ has been linked to embryo implantation (41,42), myelination in the brain (51), osteoclastic bone resorption (44), and skin wound healing (46,58). Its role in lipid metabolism comprises diverse facets associated with the preadipocyte clonal expansion (26,29), fatty acid oxidation in muscle (63), or lipoprotein homeostasis (50).…”