Osteoclast formation, survival and morphology are highly dependent on exogenous cholesterol/lipoproteins

Abstract

Osteoporosis is associated with both atherosclerosis and vascular calcification. No mechanism yet explains the parallel progression of these diseases. Here, we demonstrate that osteoclasts (OCL) depend on lipoproteins to modulate cellular cholesterol levels and that this controls OCL formation and survival. Removal of cholesterol in OCL via high-density lipoprotein or cyclodextrin treatment dose-dependently induced apoptosis, with actin disruption, nuclear condensation and caspase-3 activation. One mechanism linked to the induction of OCL apoptosis was the cell-type-specific failure to induce HMG-CoA reductase mRNA expression, suggesting an absence of feedback regulation of de novo cholesterol biosynthesis. Furthermore, cyclodextrin treatment substantially suppressed essential M-CSF and RANKL-induced survival signaling pathways via Akt, mTOR and S6K. Consistent with these findings, cholesterol delivery via lowdensity lipoprotein (LDL) significantly increased OCL viability. Interestingly, OCLs from the LDL receptor (LDLR)À/À mouse exhibited reduced size and lifespan in vitro. Remarkably, LDLR þ / þ OCL in lipoprotein-deficient medium phenocopied LDLRÀ/À OCL, while fusion and spreading of LDLRÀ/À OCL was rescued when cholesterol was chemically delivered during differentiation. With hyperlipidemia being associated with disease of the vascular system and bone, these findings provide novel insights into the selective lipoprotein and cholesterol dependency of the bone resorbing cell.

Low-density Lipoprotein Receptor Deficiency Causes Impaired Osteoclastogenesis and Increased Bone Mass in Mice because of Defect in Osteoclastic Cell-Cell Fusion

“…More recently, we found that during osteoclastogenesis, RANKL induces the expression of caveolin-1 (Cav-1) (40), a principal scaffolding protein of lipid rafts and caveolae. Furthermore, depletion of exogenous LDL causes impaired NFATc1 activation and consequently reduces osteoclast formation (40), consistent with other studies (29,30). These results suggest a tight correlation between osteoclast differentiation and cholesterol.…”

“…Furthermore, an in vivo study has shown that a mouse strain with susceptibility to atherosclerosis (C57BL/6) loses a greater amount of bone mass in response to a high-fat diet than atherosclerosis-resistant strains (26). In addition, several in vitro studies have demonstrated that oxidized LDL (ox-LDL) inhibits osteoblast differentiation (27,28), whereas an absence of native LDL suppresses osteoclast formation in cocultures of spleen cells or bone marrow cells as osteoclast progenitors and osteoclast differentiation-supporting stromal or osteoblastic cells (29,30). Such coculture systems, however, could not provide a evidence for direct action of LDL on osteoclast precursors.…”

Systems level analysis of osteoclastogenesis reveals intrinsic and extrinsic regulatory interactions

“…In vitro and in vivo studies in rodents (Mundy et al, 1999;Grasser et al, 2003;Luegmayr et al, 2004) and epi-demiological studies (Bauer, 2003) have found that statins, inhibitors of HMG-CoA reductase, increase bone density especially in postmenopausal women (Edwards et al, 2000). While these studies are difficult to interpret in detail because of complexity and confounding factors inherent in epidemiological studies, our analysis suggests that cholesterol levels regulate osteoclastogenesis at multiple levels.…”

The vital osteoclast: how is it regulated?

“…Studies of this pathway have clarified how diverse physiological and pathophysiological signals exert their effects on osteoclast formation, function and survival, and ultimately control skeletal remodelling and bone mass. 2 Two papers 3,4 in the current issue of Cell Death and Differentiation further extend our understanding of osteoclast formation and survival. The paper by Yang et al 3 proposes a new role for decoy receptor 3 (DcR3) in regulating osteoclastogenic differentiation of haematopoietic precursor cells, whereas the paper by Luegmayr et al raises the possibility that osteoclast function and survival are regulated by exogenous cholesterol/lipoprotein levels.…”

“…The paper by Luegmayr et al 4 provides evidence to suggest that osteoclast function and survival is highly dependent on exogenous sources of cholesterol. They show that cholesterol acceptors, including high-density lipoprotein, induce osteoclast apoptosis; that osteoclasts deficient in the low-density lipoprotein receptor (LDLR), in the presence of M-CSF and RANKL, undergo apoptotic cell death at an increased rate compared with wild-type osteoclasts; and that osteoclasts are highly sensitive to forced cholesterol removal because of the absence of an endogenous cholesterol biosynthesis feedback response.…”