Cholesterol-sensing receptors, liver X receptor alpha and beta, have novel and distinct roles in osteoclast differentiation and activation.
The liver X receptor (alpha,beta) is responsible for regulating cholesterol homeostasis in cells. However, our studies using the LXRalpha-/-, LXRbeta-/-, and LXRalpha-/-beta-/- mice show that both LXRalpha and beta are also important for bone turnover, mainly by regulating osteoclast differentiation/activity.


BACKGROUND
The liver X receptors (alpha,beta) are primarily responsible for regulating cholesterol homeostasis within cells and the whole body. However, as recent studies show that the role for this receptor is expanding, we studied whether the LXRs could be implicated in bone homeostasis and development.


METHODS
pQCT was performed on both male and female LXRalpha-/-, LXRbeta-/-, LXRalpha-/-beta-/-, and WT mice at 4 months and 1 year of age. Four-month-old female mice were additionally analyzed with reference to qPCR, immunohistochemistry, histomorphometry, transmission electron microscopy, and serum bone turnover markers.


RESULTS
At the mRNA level, LXRbeta was more highly expressed than LXRalpha in both whole long bones and differentiating osteoblast-like MC3T3-E1 and osteoclast-like RAW 264.7 cells. Four-month-old female LXRalpha-/- mice had a significant increase in BMD because of an increase in all cortical parameters. No difference was seen regarding trabecular BMD. Quantitative histomorphometry showed that these mice had significantly more endosteal osteoclasts in the cortical bone; however, these cells appeared less active than normal cells as suggested by a significant reduction in serum levels of cross-linked carboxyterminal telopeptides of type I collagen (CTX) and a reduction in bone TRACP activity. Conversely, the female LXRbeta-/- mice exhibited no change in BMD, presumably because a significant decline in the number of the trabecular osteoclasts was compensated for by an increase in the expression of the osteoclast markers cathepsin K and TRACP. These mice also had a significant decrease in serum CTX, suggesting decreased bone resorption; however, in addition presented with an increase in the expression of osteoblast associated genes, bone formation markers, and serum leptin levels.


CONCLUSIONS
Our findings show that both LXRs influence cellular function within the bone, with LXRalpha having an impact on osteoclast activity, primarily in cortical bone, whereas LXRbeta modulates trabecular bone turnover.
Robertson
Kirsten M
KM
Department of Biosciences at Novum, Karolinska Institutet, Huddinge, Sweden.
Norgård
Maria
M
Windahl
Sara H
SH
Hultenby
Kjell
K
Ohlsson
Claes
C
Andersson
Göran
G
Gustafsson
Jan-Ake
JA
D000135:Acid Phosphatase / Q000032:analysis; D000818:Animals; D015415:Biomarkers / Q000097:blood; D015519:Bone Density; D001862:Bone Resorption / Q000097:blood; D001842:Bone and Bones / Q000033:anatomy & histology* / Q000737:chemistry / Q000378:metabolism; D002454:Cell Differentiation / Q000235:genetics; D002784:Cholesterol / Q000378:metabolism*; D024042:Collagen Type I / Q000097:blood; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D005260:Female; D015870:Gene Expression; D006023:Glycoproteins / Q000097:blood; D007527:Isoenzymes / Q000032:analysis; D000071518:Liver X Receptors; D008297:Male; D051379:Mice; D057093:Orphan Nuclear Receptors; D010010:Osteoclasts / Q000378:metabolism / Q000648:ultrastructure*; D053244:Osteoprotegerin; D010455:Peptides / Q000097:blood; D012333:RNA, Messenger / Q000032:analysis / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D018124:Receptors, Tumor Necrosis Factor / Q000097:blood; D000071681:Tartrate-Resistant Acid Phosphatase
D015415:Biomarkers; D024042:Collagen Type I; D004268:DNA-Binding Proteins; D006023:Glycoproteins; D007527:Isoenzymes; D000071518:Liver X Receptors; C000605214:Nr1h3 protein, mouse; D057093:Orphan Nuclear Receptors; D053244:Osteoprotegerin; D010455:Peptides; D012333:RNA, Messenger; D018160:Receptors, Cytoplasmic and Nuclear; D018124:Receptors, Tumor Necrosis Factor; C506031:Tnfrsf11b protein, mouse; C052929:collagen type I trimeric cross-linked peptide; D002784:Cholesterol; D000135:Acid Phosphatase; D000071681:Tartrate-Resistant Acid Phosphatase