Cholesterol-sensing receptors, liver X receptor alpha and beta, have novel and distinct roles in osteoclast differentiation and activation.

UNLABELLED: 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. INTRODUCTION: 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. MATERIALS AND 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.

The liver X receptor-{beta} is essential for maintaining cholesterol homeostasis in the testis.

The liver X receptor (LXR)alpha and -beta has been found to play a central role in maintaining cellular cholesterol homeostasis. In this study we comprehensively investigated the effect of deleting LXRalpha and -beta on testicular morphology and function. In the absence of LXRbeta, excessive cholesterol accumulated in the Sertoli cells from 2.5 months, resulting in severe cellular disruption and dysregulation of spermatogenesis by 10 months of age. This correlated with gene expression analyses that clearly indicated that LXRbeta was the dominant transcript in the testis Although the LXRalpha(-/-) testis was normal, the LXRalpha(-/-)beta(-/-) testis presented with a more severe phenotype than the LXRbeta(-/-) mice, and males were infertile by 4 months of age, indicating LXRalpha may partially rescue the testicular phenotype. Although Leydig cells did not accumulate excessive cholesterol, declining serum and intratesticular androgen levels with age suggested that these cells were in fact less functional. Treatment of a Sertoli cell line with the LXR agonist T0901317 led to increased expression of known LXR target genes like ATP binding cassette-G1 and sterol regulatory binding protein-1c; similar results were observed in wild-type testis after in vivo administration, suggesting the LXR is functioning in the same way as in other tissues. Ordinarily increased levels of cholesterol activate intracellular sensors to decrease these levels; however, the increasing amount of cholesterol in the Sertoli cells indicates improper control of cholesterol metabolism when LXRbeta is absent. Although the precise molecular mechanism at this time remains unclear, our study highlights the crucial role for LXRbeta in retaining cholesterol homeostasis in Sertoli cells.

The phenotype of the aromatase knockout mouse reveals dietary phytoestrogens impact significantly on testis function.

Estrogen is synthesized in the testis, both in Leydig cells and seminiferous epithelium, and its importance in spermatogenesis is highlighted by the phenotype of the aromatase knockout (ArKO) mouse. These mice are unable to synthesize endogenous estrogens. The males develop postmeiotic defects by 18 wk of age. We hypothesized that maintenance of spermatogenesis in younger animals may be mediated by exogenous estrogenic substances. Dietary soy meal, contained in almost all commercial rodent diets, provides a source of estrogenic isoflavones. We thus investigated spermatogenesis in wild-type and ArKO mice raised on a diet containing soy, compared with a soy-free diet, to elucidate the biological action of phytoestrogens on the testis. In ArKO mice, dietary phytoestrogens could partially prevent disruptions to spermatogenesis, in that they prevented the decline in germ cell numbers. They also seemed to maintain Sertoli cell function, and they blocked elevations in FSH. The impairment of spermatogenesis seen in soy-free ArKOs occurred in the absence of a decreased gonadotropic stimulus, suggesting that the effects of dietary phytoestrogens are independent of changes to the pituitary-gonadal axis. Our study highlights the importance of estrogen in spermatogenesis and shows that relatively low levels of dietary phytoestrogens have a biological effect in the testis.