To address the importance of the farnesoid X-receptor (FXR; NR1H4) for normal cholesterol homeostasis, we evaluated the major pathways of cholesterol metabolism in theFXR-deficient (−/−) mouse model. Compared with wild-type,FXR(−/−) mice have increased plasma high density lipoprotein (HDL) cholesterol and a markedly reduced rate of plasma HDL cholesterol ester clearance. Concomitantly, FXR(−/−) mice exhibit reduced expression of hepatic genes involved in reverse cholesterol transport, most notably, that for scavenger receptor BI.FXR(−/−) mice also have increased: (i) plasma non-HDL cholesterol and triglyceride levels, (ii) apolipoprotein B-containing lipoprotein synthesis, and (iii) intestinal cholesterol absorption. Surprisingly, biliary cholesterol elimination was increased inFXR(−/−) mice, despite decreased expression of hepatic genes thought to be involved in this process. These data demonstrate that FXR is a critical regulator of normal cholesterol metabolism and that genetic changes affecting FXR function have the potential to be pro-atherogenic.

Maintenance of healthy bone mass requires a well-coordinated balance between the ongoing processes of bone formation and bone resorption. Bone-forming osteoblasts derive from resident adult stem cells within bone marrow called bone marrow stromal cells (BMSCs). These BMSCs are multipotent and also can give rise to adipocytes, which do not contribute directly to bone formation but may influence bone remodeling through the release of bioactive signaling molecules. Chemerin is a novel adipocyte-derived signaling molecule that promotes adipocyte differentiation. In this study we examined the role of chemerin and the cognate receptors CMKLR1 and CCRL2 as determinants of osteoblast and adipocyte differentiation of the preosteoblast 7F2 cell line and of primary BMSCs. Expression and secretion of chemerin increased dramatically with adipocyte differentiation of these cells. Functionally, knockdown of chemerin or CMKLR1 expression using RNA interference abrogated adipocyte differentiation, clonal expansion, and basal proliferation of BMSCs. In contrast, knockdown of either gene was associated with increased osteoblast marker gene expression and mineralization in response to osteoblastogenic stimuli. Forced expression of the adipogenic transcription factor peroxisome proliferator-activated receptor γ (PPARγ) induced chemerin expression and partially rescued the loss of adipogenesis associated with chemerin or CMKLR1 knockdown in BMSCs. Taken together, these data support a novel role for chemerin/CMKLR1 signaling in regulating adipogenesis and osteoblastogenesis of bone marrow–derived precursor cells. These data reveal a potential role for this signaling pathway as a modulator of bone mass. © 2010 American Society for Bone and Mineral Research

Adipose tissue secretes a variety of bioactive signaling molecules, termed adipokines, which regulate numerous biological functions including appetite, energy balance, glucose homeostasis, and inflammation. Chemerin is a novel adipokine that regulates adipocyte differentiation and metabolism by binding to and activating the G protein-coupled receptor, chemokine like receptor-1 (CMKLR1). In the present study, we investigated the impact of CMKLR1 deficiency on adipose development, glucose homeostasis, and inflammation in vivo. Herein we report that regardless of diet (low or high fat), CMKLR1^−/− mice had lower food consumption, total body mass, and percent body fat compared with wild-type controls. CMKLR1^−/− mice also exhibited decreased hepatic and white adipose tissue TNFα and IL-6 mRNA levels coincident with decreased hepatic dendritic cell infiltration, decreased adipose CD3+ T cells, and increased adipose natural killer cells. CMKLR1^−/− mice were glucose intolerant compared with wild-type mice, and this was associated with decreased glucose stimulated insulin secretion as well as decreased skeletal muscle and white adipose tissue glucose uptake. Collectively these data provide compelling evidence that CMKLR1 influences adipose tissue development, inflammation, and glucose homeostasis and may contribute to the metabolic derangement characteristic of obesity and obesity-related diseases.

Chemerin is an adipocyte-secreted protein that regulates adipogenesis and the metabolic function of mature adipocytes via activation of chemokine-like receptor 1 (CMKLR1). Herein we report the interaction of peroxisome proliferator-activated receptor γ (PPARγ) and chemerin in the context of adipogenesis. Knockdown of chemerin or CMKLR1 expression or antibody neutralization of secreted chemerin protein arrested adipogenic clonal expansion of bone marrow mesenchymal stem cells (BMSCs) by inducing a loss of G(2)/M cyclins (cyclin A2/B2) but not the G(1)/S cyclin D2. Forced expression of PPARγ in BMSCs did not completely rescue this loss of clonal expansion and adipogenesis following chemerin or CMKLR1 knockdown. However, forced expression and/or activation of PPARγ in BMSCs as well as non-adipogenic cell types such as NIH-3T3 embryonic fibroblasts and MCA38 colon carcinoma cells significantly induced chemerin expression and secretion. Sequence analysis revealed a putative PPARγ response element (PPRE) sequence within the chemerin promoter. This PPRE was able to confer PPARγ responsiveness on a heterologous promoter, and mutation of this sequence abolished activation of the chemerin promoter by PPARγ. Chromatin immunoprecipitation confirmed the direct association of PPARγ with this PPRE. Treatment of mice with rosiglitazone elevated chemerin mRNA levels in adipose tissue and bone marrow coincident with an increase in circulating chemerin levels. Together, these findings support a fundamental role for chemerin/CMKLR1 signaling in clonal expansion during adipocyte differentiation as well as a role for PPARγ in regulating chemerin expression.