Strategies and limitations associated with in vitro characterization of vitamin D receptor activators

In vitro cell-based assays are common screening tools used for the identification of new VDR ligands. For 25-hydroxyvitamin D₃ [25(OH)D₃] and 1α-hydroxyvitamin D₃ [1α(OH)D₃], protein expressions of CYP2R1 and CYP27B1, respectively, that form the active 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] ligand were detected in human embryonic kidney (HEK293) cells expressing the GAL4-hVDR, the human brain microvessel endothelial (hCMEC/D3) and adenocarcinoma colonic (Caco-2) cells. The impact of bioactivation enzymes was shown upon the addition of ketoconazole (10 μM KTZ), a pan-CYP inhibitor, which reduced the apparent potency of 25(OH)D₃ and increased the EC₅₀ from 272 to 608 nM in HEK293 cells. EIA assays verified that 1,25(OH)₂D₃ was formed and contributed to VDR activity independently of its precursors. In hCMEC/D3 cells where enzyme protein levels were lowest, changes in MDR1/P-gp expression with KTZ were minimal. In Caco-2 cells, the induction of TRPV6 (calcium channel), CYP24A1, CYP3A4, OATP1A2 and MDR1 mRNA expression was 1,25(OH)₂D₃ > 1α(OH)D₃ > 25(OH)D₃, with the magnitude of change being blunted by KTZ. Upon inclusion of KTZ in the cell-based assays, high transcriptional activities were observed for synthetic VDR activators from Teijin Pharma. Cyclopentanone derivatives: TPD-003, TPD-005, TPD-006, TPD-008 and TPD-009 (EC₅₀s 0.06 to 67 nM, unchanged with KTZ) were found more potent over straight chain and lactone derivatives (antagonists). Most TPD compounds activated OATP1A2, CYP24A1, CYP3A4, and MDR1 (28-67%) and TRPV6 transcriptionally in Caco-2 cells. The results identified that cell-based assays with added KTZ could accurately identify new VDR activators, although these may be hypercalcemic with strong TRPV6 inducing properties.

Loss of the Liver X Receptors Disrupts the Balance of Hematopoietic Populations, With Detrimental Effects on Endothelial Progenitor Cells

Background

The liver X receptors (LXRs; α/β) are nuclear receptors known to regulate cholesterol homeostasis and the production of select hematopoietic populations. The objective of this study was to determine the importance of LXRs and a high‐fat high‐cholesterol diet on global hematopoiesis, with special emphasis on endothelial progenitor cells (EPCs), a vasoreparative cell type that is derived from bone marrow hematopoietic stem cells.

Methods and Results

Wild‐type and LXR double‐knockout (Lxrαβ^−/−) mice were fed a Western diet (WD) to increase plasma cholesterol levels. In WD‐fed Lxrαβ^−/− mice, flow cytometry and complete blood cell counts revealed that hematopoietic stem cells, a myeloid progenitor, and mature circulating myeloid cells were increased; EPC numbers were significantly decreased. Hematopoietic stem cells from WD‐fed Lxrαβ^−/− mice showed increased cholesterol content, along with increased myeloid colony formation compared with chow‐fed mice. In contrast, EPCs from WD‐fed Lxrαβ^−/− mice also demonstrated increased cellular cholesterol content that was associated with greater expression of the endothelial lineage markers Cd144 and Vegfr2, suggesting accelerated differentiation of the EPCs. Treatment of human umbilical vein endothelial cells with conditioned medium collected from these EPCs increased THP‐1 monocyte adhesion. Increased monocyte adhesion to conditioned medium–treated endothelial cells was recapitulated with conditioned medium from Lxrαβ^−/−EPCs treated with cholesterol ex vivo, suggesting cholesterol is the main component of the WD inducing EPC dysfunction.

Conclusions

LXRs are crucial for maintaining the balance of hematopoietic cells in a hypercholesterolemic environment and for mitigating the negative effects of cholesterol on EPC differentiation/secretome changes that promote monocyte‐endothelial adhesion.

Abstract 623: Activating Liver X Receptors Alters Endothelial Progenitor Cell Differentiation and Enhances the Release of Factors that Reduce Atherosclerosis

The liver x receptors (LXRα/β) are nuclear receptors that are highly expressed in macrophages and endothelial progenitor cells (EPCs). EPCs are bone marrow (BM) derived cells that facilitate endothelial repair through the secretion of factors. The importance of LXRs in preventing atherosclerosis has been attributed to promoting cholesterol efflux from macrophages. However, a role for EPCs in the LXR response has not been explored. We hypothesized that LXR activation in EPCs contributes to their anti-atherogenic effects by preventing endothelial activation during atherogenesis. In the Lxr -/- mice, circulating EPCs were decreased by 27% compared to WT, while myeloid populations were increased. Isolated hematopoietic stem cells treated with LXR agonist (1μM GW3965) ex vivo showed a 36% increase in the number of EPC colonies formed, while myeloid colonies were decreased by 25%. GW3965 treatment during EPC differentiation resulted in a 76% decrease in endothelial lineage marker ( Cd144 and Vegfr2 ) expression. To investigate endothelial cell activation, activated human umbilical vein endothelial cells were incubated with the conditioned media (CM) from GW3965-treated WT EPCs, which reduced THP-1 monocyte adhesion by 36% compared to the CM from vehicle treatment. To determine the effects of the EPC-derived CM in vivo , Ldlr -/- mice were injected with concentrated CM bi-weekly over an 8-week period, which decreased aortic sinus lesion area by 52% compared to the media control injections. Taken together, the loss of LXRs perturbed hematopoietic balance in vivo , while the activation of LXRs were associated with enhanced ex vivo EPC formation. Furthermore, LXR activation altered EPC differentiation and secretome resulting in decreased in vitro endothelial-monocyte adhesion. When administered to atherosclerotic-prone mice, these secreted factors reduced lesion formation in the aortic sinus. These data suggest that LXRs reduce the development of atherosclerosis in part through the release of EPC-derived factors that can reduce endothelial activation.

Multilayered Control of Alternative Splicing Regulatory Networks by Transcription Factors

Networks of coordinated alternative splicing (AS) events play critical roles in development and disease. However, a comprehensive knowledge of the factors that regulate these networks is lacking. We describe a high-throughput system for systematically linking trans-acting factors to endogenous RNA regulatory events. Using this system, we identify hundreds of factors associated with diverse regulatory layers that positively or negatively control AS events linked to cell fate. Remarkably, more than one-third of the regulators are transcription factors. Further analyses of the zinc finger protein Zfp871 and BTB/POZ domain transcription factor Nacc1, which regulate neural and stem cell AS programs, respectively, reveal roles in controlling the expression of specific splicing regulators. Surprisingly, these proteins also appear to regulate target AS programs via binding RNA. Our results thus uncover a large "missing cache" of splicing regulators among annotated transcription factors, some of which dually regulate AS through direct and indirect mechanisms.

Oxidized Low-Density Lipoprotein Loading of Macrophages Downregulates TLR-Induced Proinflammatory Responses in a Gene-Specific and Temporal Manner through Transcriptional Control

Hypercholesterolemia is a key risk factor for atherosclerosis and leads to the uptake of native and oxidized low-density lipoprotein (oxLDL) by macrophages (Mϕs) and foam cell formation. Inflammatory processes accompany Mϕ foam cell formation in the artery wall, yet the relationship between Mϕ lipid loading and their response to inflammatory stimuli remains elusive. We investigated proinflammatory gene expression in thioglycollate-elicited peritoneal Mϕs, bone marrow-derived Mϕs and dendritic cells, and RAW264.7 cells. Loading with oxLDL did not induce peritoneal Mϕ apoptosis or modulate basal-level expression of proinflammatory genes. Upon stimulation of TLR4, the rapid induction of IFN-β was inhibited in cells loaded with oxLDL, whereas the induction of other proinflammatory genes by TLR4 (LPS), TLR3 (polyriboinosinic-polyribocytidylic acid), TLR2 (Pam₃CSK₄), and TLR9 (CpG) remained comparable within the first 2 h. Subsequently, the expression of a subset of proinflammatory genes (e.g., IL-1β, IL-6, CCL5) was reduced in oxLDL-loaded cells at the level of transcription. This phenomenon was partially dependent on NF erythroid 2-related factor 2 (NRF2) but not on nuclear liver X receptors α and β (LXRα,β), peroxisome proliferator-activated receptor-γ (PPARγ), and activating transcription factor 3 (ATF3). LPS-induced NF-κB reporter activity and intracellular signaling by NF-κB and MAPK pathways were comparable in oxLDL-loaded Mϕs, yet the binding of p65/RelA (the prototypic NF-κB family member) was reduced at IL-6 and CCL5 promoters. This study revealed that oxLDL loading of Mϕs negatively regulates transcription at late stages of TLR-induced proinflammatory gene expression and implicates epigenetic mechanisms such as histone deacetylase activity.

The CRH-Transgenic Cushingoid Mouse Is a Model of Glucocorticoid-Induced Osteoporosis

Glucocorticoids (GCs) have unparalleled anti‐inflammatory and immunosuppressive properties, which accounts for their widespread prescription and use. Unfortunately, a limitation to GC therapy is a wide range of negative side effects including Cushing's syndrome, a disease characterized by metabolic abnormalities including muscle wasting and osteoporosis. GC‐induced osteoporosis occurs in 30% to 50% of patients on GC therapy and thus, represents an important area of study. Herein, we characterize the molecular and physiologic effects of GC‐induced osteoporosis using the Cushing's mouse model, the corticotropin releasing hormone (CRH) transgenic mouse (CRH‐Tg). The humeri, femurs, and tibias from wild‐type (WT) and CRH‐Tg male mice, aged 13 to 14 weeks old were subjected to multiple bone tests including, micro–computed tomography (μCT), static and dynamic histomorphometry, strength testing, and gene expression analyses. The CRH‐Tg mice had a 38% decrease in cortical bone area, a 35% decrease in cortical thickness, a 16% decrease in trabecular thickness, a sixfold increase in bone adiposity, a 27% reduction in osteoid width, a 75% increase in bone‐resorbing osteoclast number/bone surface, a 34% decrease in bone formation rate, and a 40% decrease in bone strength compared to WT mice. At the gene expression level, CRH‐Tg bone showed significantly increased osteoclast markers and decreased osteoblast markers, whereas CRH‐Tg muscle had increased muscle atrophy gene markers compared to WT mice. Overall, the CRH‐Tg mouse model aged to 14 weeks recapitulated many features of osteoporosis in Cushing's syndrome and thus, represents a useful model to study GC‐induced osteoporosis and interventions that target the effects of GCs on the skeleton. © 2017 The Authors. JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Separating the Anti-Inflammatory and Diabetogenic Effects of Glucocorticoids Through LXRβ Antagonism

Synthetic glucocorticoids (GCs), including dexamethasone (DEX), are powerful anti-inflammatory drugs. Long-term use of GCs, however, can result in metabolic side effects such as hyperglycemia, hepatosteatosis, and insulin resistance. The GC receptor (GR) and liver X receptors (LXRα and LXRβ) regulate overlapping genes involved in gluconeogenesis and inflammation. We have previously shown that Lxrβ-/- mice are resistant to the diabetogenic effects of DEX but still sensitive to its immunosuppressive actions. To determine whether this finding could be exploited for therapeutic intervention, we treated mice with GSK2033, a pan-LXR antagonist, alone or combined with DEX. GSK2033 suppressed GC-induced gluconeogenic gene expression without affecting immune-responsive GR target genes. The suppressive effect of GSK2033 on DEX-induced gluconeogenic genes was specific to LXRβ, was liver cell autonomous, and occurred in a target gene-specific manner. Compared with DEX treatment alone, the coadministration of GSK2033 with DEX decreased the recruitment of GR and its accessory factors MED1 and C/EBPβ to the phosphoenolpyruvate carboxykinase promoter. However, GSK2033 had no effect on DEX-mediated suppression of inflammatory genes expressed in the liver or in mouse primary macrophages stimulated with lipopolysaccharides. In conclusion, our study provides evidence that the gluconeogenic and immunosuppressive actions of GR activation can be mechanistically dissociated by pharmacological antagonism of LXRβ. Treatment with an LXRβ antagonist could allow the safer use of existing GC drugs in patients requiring chronic dosing of anti-inflammatory agents for the treatment of diseases such as rheumatoid arthritis and inflammatory bowel disease.

Diet Polyphenol Curcumin Stimulates Hepatic Fgf21 Production and Restores Its Sensitivity in High-Fat-Diet-Fed Male Mice

We found previously that short-term curcumin gavage stimulated mouse hepatic fibroblast growth factor 21 (Fgf21) expression. Here we conducted mechanistic exploration and investigated the potential pathophysiological relevance on this regulation. Fgf21 stimulation was observed at messenger RNA and protein levels in mice with daily curcumin gavage for 4 or 8 days and in primary hepatocytes with curcumin treatment. Using peroxisome proliferator-activated receptor α (PPARα) agonist and antagonist, along with luciferase reporter and chromatin immune-precipitation approaches, we determined that curcumin stimulates Fgf21 transcription in a mechanism involving PPARα activation. High-fat diet (HFD) feeding also increased mouse hepatic and serum Fgf21 levels, whereas dietary curcumin intervention attenuated these increases. We found that HFD feeding reduced hepatic expression levels of genes that encode FGFR1 and βKlotho, PGC1α, and the targets of the PPARα-PGC1α axis, whereas concomitant curcumin intervention restored or partially restored their expression levels. Importantly, hepatocytes from HFD-fed mice showed a loss of response to FGF21 treatment on Erk phosphorylation and the expression of Egr1 and cFos; this response was restored in hepatocytes from HFD-fed mice with curcumin intervention. This investigation expanded our mechanistic understanding of the metabolic beneficial effects of dietary curcumin intervention involving the regulation of Fgf21 production and the attenuation of HFD-induced Fgf21 resistance.

Glucocorticoids and Metabolic Control

In response to stress, the central nervous system initiates a signaling cascade, which leads to the production of glucocorticoids (GCs). GCs act through the glucocorticoid receptor (GR) to coordinate the appropriate cellular response with the primary goal of mobilizing the storage forms of carbon precursors to generate a continuous glucose supply for the brain. Although GCs are critical for maintaining energy homeostasis, excessive GC stimulation leads to a number of undesirable side effects, including hyperglycemia, insulin resistance, fatty liver, obesity, and muscle wasting leading to severe metabolic dysfunction. Summarized below are the diverse metabolic roles of glucocorticoids in energy homeostasis and dysregulation, focusing specifically on glucose, lipid, and protein metabolism.

Loss of the Mono-ADP-ribosyltransferase, Tiparp, Increases Sensitivity to Dioxin-induced Steatohepatitis and Lethality

The aryl hydrocarbon receptor (AHR) mediates the toxic effects of the environmental contaminant dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin; TCDD). Dioxin causes a range of toxic responses, including hepatic damage, steatohepatitis, and a lethal wasting syndrome; however, the mechanisms are still unknown. Here, we show that the loss of TCDD-inducible poly(ADP-ribose) polymerase (Tiparp), an ADP-ribosyltransferase and AHR repressor, increases sensitivity to dioxin-induced toxicity, steatohepatitis, and lethality. Tiparp(-/-) mice given a single injection of 100 μg/kg dioxin did not survive beyond day 5; all Tiparp(+/+) mice survived the 30-day treatment. Dioxin-treated Tiparp(-/-) mice exhibited increased liver steatosis and hepatotoxicity. Tiparp ADP-ribosylated AHR but not its dimerization partner, the AHR nuclear translocator, and the repressive effects of TIPARP on AHR were reversed by the macrodomain containing mono-ADP-ribosylase MACROD1 but not MACROD2. These results reveal previously unidentified roles for Tiparp, MacroD1, and ADP-ribosylation in AHR-mediated steatohepatitis and lethality in response to dioxin.

Abstract 327: Liver X Receptors (LXR) Modulate the Negative Effects on Endothelial Progenitors Cells after Feeding a High Cholesterol Diet

The liver X receptors (LXRα/β) belong to the nuclear receptor superfamily of ligand activated transcription factors. LXRs are endogenously activated by oxysterols; and one of their primary functions is to facilitate cholesterol efflux out of cells, such as macrophages, via the ABCA1 & ABCG1 transporters. Bone marrow (BM) transplant studies have shown that LXRs are important for preventing the formation of atherosclerotic plaques. Loss of LXRs in the monocyte/macrophage population of the BM was thought to be the primary mediator of the increased atherosclerosis in these studies. However, the BM is comprised of a variety of cell types that are differentiated from hematopoietic stem cells (HSCs). Like numerous other vascular complications, endothelial defects are central to the pathogenesis of the early stages of atherosclerosis. Endothelial progenitor cells (EPCs), derived from HSCs in the BM, contribute to vascular health primarily through the secretion of factors that maintain and repair the endothelium of the damaged vasculature. We hypothesized that LXRs also modulate the negative effects of cholesterol on EPCs, in addition to its effects in macrophages. To study the effects of dietary cholesterol on EPCs and the roles of LXRs in modulating these effects, WT and LXRα/β-/- mice were fed a high-fat/high-cholesterol (HF/HC) diet starting at 8-9 weeks of age for a total of 12 weeks. Complete blood counts showed that in the LXRα/β-/- but not the WT mice fed the HF/HC diet, there was a significant expansion of select myeloid populations (neutrophils, monocytes, platelets). Furthermore, EPCs derived from the HF/HC fed LXRα/β-/- mice had a higher expression of the endothelial markers CD144 and VEGFR2 compared to WT mice and LXRα/β-/- mice on chow diet, suggesting enhanced differentiation to a more endothelial-like cell in the absence of LXRs. Notably, the paracrine reparative actions of EPCs have been shown in populations of cells with low expression of endothelial markers. Taken together, these results suggest that LXRs modulate the effects of HF/HC diet on the differentiation of EPCs to endothelial cells and thus, may provide another cell type within the BM that would benefit from LXR activation in modulating vascular disease.[[Unable to Display Character:  ]]

SIRT1 activation ameliorates hyperglycaemia by inducing a torpor-like state in an obese mouse model of type 2 diabetes

AIMS/HYPOTHESIS

Nutrient overabundance and diminished physical activity underlie the epidemic of obesity and its consequences of insulin resistance and type 2 diabetes. These same phenomena, obesity and insulin resistance, are also observed in mammals as they ready themselves for the nutrient deprivation of winter, yet their plasma glucose does not rise. Given the role of silent information regulator 2 (Sir2) and its mammalian orthologue, Sirt1, in survival and life extension during energy deprivation, we hypothesised that enhancing its activity may reduce the insensible energy loss engendered by hyperglycaemia and glycosuria.

METHODS

At 8 weeks of age, db/db and db/m mice were randomised to receive the SIRT1 activator SRT3025 milled in chow (3.18 g/kg) or regular chow and followed for a further 12 weeks.

RESULTS

When compared with vehicle, SIRT1 activation greatly improved glycaemic control, augmented plasma insulin concentrations, increased pancreatic islet beta cell mass and elevated hepatic expression of the beta cell growth factor, betatrophin in db/db mice. Despite the dramatic reduction in hyperglycaemia, db/db mice displayed worsening insulin resistance, diminished physical activity and further weight gain. These findings along with reduced food intake and reduction in body temperature resembled torpor and hibernation. By contrast, SIRT1 activation conferred only minimal changes in non-diabetic db/m mice.

CONCLUSIONS/INTERPRETATION

While reducing hyperglycaemia and promoting beta cell expansion, enhancing the activity of SIRT1 facilitates a phenotypic change in a db/db mouse model of diabetes to one that more closely resembles the physiological state of torpor or hibernation.

Glucocorticoids regulate the metabolic hormone FGF21 in a feed-forward loop

Hormones such as fibroblast growth factor 21 (FGF21) and glucocorticoids (GCs) play crucial roles in coordinating the adaptive starvation response. Here we examine the interplay between these hormones. It was previously shown that FGF21 induces corticosterone levels in mice by acting on the brain. We now show that this induces the expression of genes required for GC synthesis in the adrenal gland. FGF21 also increases corticosterone secretion from the adrenal in response to ACTH. We further show that the relationship between FGF21 and GCs is bidirectional. GCs induce Fgf21 expression in the liver by acting on the GC receptor (GR). The GR binds in a ligand-dependent manner to a noncanonical GR response element located approximately 4.4 kb upstream of the Fgf21 transcription start site. The GR cooperates with the nuclear fatty acid receptor, peroxisome proliferator-activated receptor-α, to stimulate Fgf21 transcription. GR and peroxisome proliferator-activated receptor-α ligands have additive effects on Fgf21 expression both in vivo and in primary cultures of mouse hepatocytes. We conclude that FGF21 and GCs regulate each other's production in a feed-forward loop and suggest that this provides a mechanism for bypassing negative feedback on the hypothalamic-pituitary-adrenal axis to allow sustained gluconeogenesis during starvation.

Mapping the cellular response to small molecules using chemogenomic fitness signatures

Genome-wide characterization of the in vivo cellular response to perturbation is fundamental to understanding how cells survive stress. Identifying the proteins and pathways perturbed by small molecules affects biology and medicine by revealing the mechanisms of drug action. We used a yeast chemogenomics platform that quantifies the requirement for each gene for resistance to a compound in vivo to profile 3250 small molecules in a systematic and unbiased manner. We identified 317 compounds that specifically perturb the function of 121 genes and characterized the mechanism of specific compounds. Global analysis revealed that the cellular response to small molecules is limited and described by a network of 45 major chemogenomic signatures. Our results provide a resource for the discovery of functional interactions among genes, chemicals, and biological processes.

Minireview: new molecular mediators of glucocorticoid receptor activity in metabolic tissues

The glucocorticoid receptor (GR) was one of the first nuclear hormone receptors cloned and represents one of the most effective drug targets available today for the treatment of severe inflammation. The physiologic consequences of endogenous or exogenous glucocorticoid excess are well established and include hyperglycemia, insulin resistance, fatty liver, obesity, and muscle wasting. However, at the molecular and tissue-specific level, there are still many unknown protein mediators of glucocorticoid response and thus, much remains to be uncovered that will help determine whether activation of the GR can be tailored to improve therapeutic efficacy while minimizing unwanted side effects. This review summarizes recent discoveries of tissue-selective modulators of glucocorticoid signaling that are important in mediating the unwanted side effects of therapeutic glucocorticoid use, emphasizing the downstream molecular effects of GR activation in the liver, adipose tissue, muscle, and pancreas.

Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol

BACKGROUND & AIMS

Little is known about the effects of the vitamin D receptor (VDR) on hepatic activity of human cholesterol 7α-hydroxylase (CYP7A1) and cholesterol metabolism. We studied these processes in mice in vivo and mouse and human hepatocytes.

METHODS

Farnesoid X receptor (Fxr)(-/-), small heterodimer partner (Shp)(-/-), and C57BL/6 (wild-type control) mice were fed normal or Western diets for 3 weeks and were then given intraperitoneal injections of vehicle (corn oil) or 1α,25-dihydroxyvitamin D3 (1,25[OH]2D3; 4 doses, 2.5 μg/kg, every other day). Plasma and tissue samples were collected and levels of Vdr, Shp, Cyp7a1, Cyp24a1, and rodent fibroblast growth factor (Fgf) 15 expression, as well as levels of cholesterol, were measured. We studied the regulation of Shp by Vdr using reporter and mobility shift assays in transfected human embryonic kidney 293 cells, quantitative polymerase chain reaction with mouse tissues and mouse and human hepatocytes, and chromatin immunoprecipitation assays with mouse liver.

RESULTS

We first confirmed the presence of Vdr mRNA and protein expression in livers of mice. In mice fed normal diets and given injections of 1,25(OH)2D3, liver and plasma concentrations of 1,25(OH)2D3 increased and decreased in unison. Changes in hepatic Cyp7a1 messenger RNA (mRNA) correlated with those of Cyp24a1 (a Vdr target gene) and inversely with Shp mRNA, but not ileal Fgf15 mRNA. Similarly, incubation with 1,25(OH)2D3 increased levels of Cyp24a1/CYP24A1 and Cyp7a1/CYP7A1 mRNA in mouse and human hepatocytes, and reduced levels of Shp mRNA in mouse hepatocytes. In Fxr(-/-) and wild-type mice with hypercholesterolemia, injection of 1,25(OH)2D3 consistently reduced levels of plasma and liver cholesterol and Shp mRNA, and increased hepatic Cyp7a1 mRNA and protein; these changes were not observed in Shp(-/-) mice given 1,25(OH)2D3 and fed Western diets. Truncation of the human small heterodimer partner (SHP) promoter and deletion analyses revealed VDR-dependent inhibition of SHP, and mobility shift assays showed direct binding of VDR to enhancer regions of SHP. In addition, chromatin immunoprecipitation analysis of livers from mice showed that injection of 1,25(OH)2D3 increased recruitment of Vdr and rodent retinoid X receptor to the Shp promoter.

CONCLUSIONS

Activation of the VDR represses hepatic SHP to increase levels of mouse and human CYP7A1 and reduce cholesterol.

Liver X receptors preserve renal glomerular integrity under normoglycaemia and in diabetes in mice

AIMS/HYPOTHESIS

Liver X receptors (LXRs) α and β are nuclear hormone receptors that are widely expressed in the kidney. They promote cholesterol efflux from cells and inhibit inflammatory responses by regulating gene transcription. Here, we hypothesised (1) that LXR deficiency would promote renal decline in a mouse model of diabetes by accelerating intraglomerular cholesterol accumulation and, conversely, (2) that LXR agonism would attenuate renal decline in diabetes.

METHODS

Diabetes was induced with streptozotocin (STZ) and maintained for 14 weeks in Lxrα/β (+/+) (Lxrα, also known as Nr1h3; Lxrβ, also known as Nr1h2) and Lxrα/β (-/-) mice. In addition, STZ-injected DBA/2J mice were treated with vehicle or the LXR agonist N,N-dimethyl-hydroxycholenamide (DMHCA) (80 mg/kg daily) for 10 weeks. To determine the role of cholesterol in diabetic nephropathy (DN), mice were placed on a Western diet after hyperglycaemia developed.

RESULTS

Even in the absence of diabetes, Lxrα/β (-/-) mice exhibited a tenfold increase in the albumin:creatinine ratio and a 40-fold increase in glomerular lipid accumulation compared with Lxrα/β (+/+) mice. When challenged with diabetes, Lxrα/β (-/-) mice showed accelerated mesangial matrix expansion and glomerular lipid accumulation, with upregulation of inflammatory and oxidative stress markers. In the DN-sensitive STZ DBA/2J mouse model, DMHCA treatment significantly decreased albumin and nephrin excretion (by 50% each), glomerular lipids and plasma triacylglycerol (by 70%) and cholesterol (by 48%); it also decreased kidney inflammatory and oxidative stress markers compared with vehicle-treated mice.

CONCLUSIONS/INTERPRETATION

These data support the idea that LXR plays an important role in the normal and diabetic kidney, while showing that LXR, through its inhibitory effect on inflammation and cholesterol accumulation in glomeruli, could also be a novel therapeutic target for DN.

Liver X receptor modulates diabetic retinopathy outcome in a mouse model of streptozotocin-induced diabetes

Endothelial progenitor cells (EPCs), critical for mediating vascular repair, are dysfunctional in a hyperglycemic and/or hypercholesterolemic environment. Their dysfunction contributes to the progression of diabetic macro- and microvascular complications. Activation of "cholesterol-sensing" nuclear receptors, the liver X receptors (LXRα/LXRβ), protects against atherosclerosis by transcriptional regulation of genes important in promoting cholesterol efflux and inhibiting inflammation. We hypothesized that LXR activation with a synthetic ligand would correct diabetes-induced EPC dysfunction and improve diabetic retinopathy. Studies were performed in streptozotocin (STZ)-injected DBA/2J mice fed a high-fat Western diet (DBA/STZ/WD) and treated with the LXR agonist GW3965 and in LXRα(-/-), LXRβ(-/-), and LXRα/β(-/-) mice. Retinas were evaluated for number of acellular capillaries and glial fibrillary acidic protein (GFAP) immunoreactivity. Bone marrow EPCs were analyzed for migratory function and gene expression. Compared with vehicle-treated DBA/STZ/WD mice, GW3965 treated mice showed fewer acellular capillaries and reduced GFAP expression. These mice also exhibited enhanced EPC migration and restoration of inflammatory and oxidative stress genes toward nondiabetic levels. LXRα(-/-), LXRβ(-/-), and LXRα/β(-/-) mice developed acellular capillaries and EPC dysfunction similar to the DBA/STZ/WD mice. These studies support a key role for LXR in retinal and bone marrow progenitor dysfunction associated with type 1 diabetes. LXR agonists may represent promising pharmacologic targets for correcting retinopathy and EPC dysfunction.

Lecithin:cholesterol acyltransferase deficiency protects against cholesterol-induced hepatic endoplasmic reticulum stress in mice

We recently reported that lecithin:cholesterol acyltransferase (LCAT) knock-out mice, particularly in the LDL receptor knock-out background, are hypersensitive to insulin and resistant to high fat diet-induced insulin resistance (IR) and obesity. We demonstrated that chow-fed Ldlr-/-xLcat+/+ mice have elevated hepatic endoplasmic reticulum (ER) stress, which promotes IR, compared with wild-type controls, and this effect is normalized in Ldlr-/-xLcat-/- mice. In the present study, we tested the hypothesis that hepatic ER cholesterol metabolism differentially regulates ER stress using these models. We observed that the Ldlr-/-xLcat+/+ mice accumulate excess hepatic total and ER cholesterol primarily attributed to increased reuptake of biliary cholesterol as we observed reduced biliary cholesterol in conjunction with decreased hepatic Abcg5/g8 mRNA, increased Npc1l1 mRNA, and decreased Hmgr mRNA and nuclear SREBP2 protein. Intestinal NPC1L1 protein was induced. Expression of these genes was reversed in the Ldlr-/-xLcat-/- mice, accounting for the normalization of total and ER cholesterol and ER stress. Upon feeding a 2% high cholesterol diet (HCD), Ldlr-/-xLcat-/- mice accumulated a similar amount of total hepatic cholesterol compared with the Ldlr-/-xLcat+/+ mice, but the hepatic ER cholesterol levels remained low in conjunction with being protected from HCD-induced ER stress and IR. Hepatic ER stress correlates strongly with hepatic ER free cholesterol but poorly with hepatic tissue free cholesterol. The unexpectedly low ER cholesterol seen in HCD-fed Ldlr-/-xLcat-/- mice was attributable to a coordinated marked up-regulation of ACAT2 and suppressed SREBP2 processing. Thus, factors influencing the accumulation of ER cholesterol may be important for the development of hepatic insulin resistance.

A novel 3-hydroxysteroid dehydrogenase that regulates reproductive development and longevity

Endogenous small molecule metabolites that regulate animal longevity are emerging as a novel means to influence health and life span. In C. elegans, bile acid-like steroids called the dafachronic acids (DAs) regulate developmental timing and longevity through the conserved nuclear hormone receptor DAF-12, a homolog of mammalian sterol-regulated receptors LXR and FXR. Using metabolic genetics, mass spectrometry, and biochemical approaches, we identify new activities in DA biosynthesis and characterize an evolutionarily conserved short chain dehydrogenase, DHS-16, as a novel 3-hydroxysteroid dehydrogenase. Through regulation of DA production, DHS-16 controls DAF-12 activity governing longevity in response to signals from the gonad. Our elucidation of C. elegans bile acid biosynthetic pathways reveals the possibility of novel ligands as well as striking biochemical conservation to other animals, which could illuminate new targets for manipulating longevity in metazoans.

Compound prioritization methods increase rates of chemical probe discovery in model organisms

Preselection of compounds that are more likely to induce a phenotype can increase the efficiency and reduce the costs for model organism screening. To identify such molecules, we screened ~81,000 compounds in Saccharomyces cerevisiae and identified ~7500 that inhibit cell growth. Screening these growth-inhibitory molecules across a diverse panel of model organisms resulted in an increased phenotypic hit-rate. These data were used to build a model to predict compounds that inhibit yeast growth. Empirical and in silico application of the model enriched the discovery of bioactive compounds in diverse model organisms. To demonstrate the potential of these molecules as lead chemical probes, we used chemogenomic profiling in yeast and identified specific inhibitors of lanosterol synthase and of stearoyl-CoA 9-desaturase. As community resources, the ~7500 growth-inhibitory molecules have been made commercially available and the computational model and filter used are provided.