Altered adrenal gland cholesterol metabolism in the apoE-deficient mouse

Meta-analysis of gonadal transcriptome provides novel insights into sex change mechanism across protogynous fishes

Protogyny, being capable of changing from female to male during their lifetime, is prevalent in 20 families of teleosts but is believed to have evolved within specific evolutionary lineages. Therefore, shared regulatory factors governing the sex change process are expected to be conserved across protogynous fishes. However, a comprehensive understanding of this mechanism remains elusive. To identify these factors, we conducted a meta-analysis using gonadal transcriptome data from seven species. We curated data pairs of ovarian tissue and transitional gonad, and employed ratios of expression level as a unified criterion for differential expression, enabling a meta-analysis across species. Our approach revealed that classical sex change-related genes exhibited differential expression levels between the ovary and transitional gonads, consistent with previous reports. These results validate our methodology's robustness. Additionally, we identified novel genes not previously linked to gonadal sex change in fish. Notably, changes in the expression levels of acetoacetyl-CoA synthetase and apolipoprotein Eb, which are involved in cholesterol synthesis and transport, respectively, suggest that the levels of cholesterol, a precursor of steroid hormones crucial for sex change, are decreased upon sex change onset in the gonads. This implies a potential universal influence of cholesterol dynamics on gonadal transformation in protogyny.

Regulation of lipid droplets and cholesterol metabolism in adrenal cortical cells

The adrenal gland is composed of two distinctly different endocrine moieties. The interior medulla consists of neuroendocrine chromaffin cells that secrete catecholamines like adrenaline and noradrenaline, while the exterior cortex consists of steroidogenic cortical cells that produce steroid hormones, such as mineralocorticoids (aldosterone), glucocorticoids (cortisone and cortisol) and androgens. Synthesis of steroid hormones in cortical cells requires substantial amounts of cholesterol, which is the common precursor for steroidogenesis. Cortical cells may acquire cholesterol from de novo synthesis and uptake from circulating low- and high-density lipoprotein particles (LDL and HDL). As cholesterol is part of the plasma membrane in all mammalian cells and an important regulator of membrane fluidity, cellular levels of free cholesterol are tightly regulated. To ensure a robust supply of cholesterol for steroidogenesis and to avoid cholesterol toxicity, cortical cells store large amounts of cholesterol as cholesteryl esters in intracellular lipid droplets. Cortical steroidogenesis relies on both mobilization of cholesterol from lipid droplets and constant uptake of circulating cholesterol to replenish lipid droplet stores. This chapter will describe mechanisms involved in cholesterol uptake, cholesteryl ester synthesis, lipid droplet formation, hydrolysis of stored cholesteryl esters, as well as their impact on steroidogenesis. Additionally, animal models and human diseases characterized by altered cortical cholesteryl ester storage, with or without abnormal steroidogenesis, will be discussed.

Early transcriptomic response of mouse adrenal gland and Y-1 cells to dexamethasone

Glucocorticoids have short- and long-term effects on adrenal gland function and development. RNA sequencing (RNA-seq) was performed to identify early transcriptomic responses to the synthetic glucocorticoid, dexamethasone (Dex), in vitro and in vivo. In total, 1711 genes were differentially expressed in the adrenal glands of the 1-h Dex-treated mice. Among them, only 113 were also considered differentially expressed genes (DEGs) in murine adrenocortical Y-1 cells treated with Dex for 1 h. Gene ontology analysis showed that the upregulated DEGs in the adrenal gland of the 1-h Dex-treated mice were highly associated with the development of neuronal cells, suggesting the adrenal medulla had a rapid response to Dex. Interestingly, only 4.3% of Dex-responsive genes in the Y-1 cell line under Dex treatment for 1 h were differentially expressed under Dex treatment for 24 h. The heatmaps revealed that most early responsive DEGs in Y-1 cells during 1 h of treatment exhibited a transient response. The expression of these genes under treatment for 24 h returned to basal levels similar to that during control treatment. In summary, this research compared the rapid transcriptomic effects of Dex stimulation in vivo and in vitro. Notably, adrenocortical Y-1 cells had a transient early response to Dex treatment. Furthermore, the DEGs had a minimal overlap in the 1-h Dex-treated group in vivo and in vitro.

A human cell atlas of fetal gene expression

The gene expression program underlying the specification of human cell types is of fundamental interest. We generated human cell atlases of gene expression and chromatin accessibility in fetal tissues. For gene expression, we applied three-level combinatorial indexing to >110 samples representing 15 organs, ultimately profiling ~4 million single cells. We leveraged the literature and other atlases to identify and annotate hundreds of cell types and subtypes, both within and across tissues. Our analyses focused on organ-specific specializations of broadly distributed cell types (such as blood, endothelial, and epithelial), sites of fetal erythropoiesis (which notably included the adrenal gland), and integration with mouse developmental atlases (such as conserved specification of blood cells). These data represent a rich resource for the exploration of in vivo human gene expression in diverse tissues and cell types.

VLDL/LDL serves as the primary source of cholesterol in the adrenal glucocorticoid response to food deprivation

The contribution of individual lipoprotein species to the generation of the adrenal cholesterol pool used for the synthesis of anti-inflammatory glucocorticoid species remains unknown. Here we examined the impact of specific lowering of very low-density lipoprotein (VLDL) and low-density (LDL) levels on adrenal cholesterol and glucocorticoid homeostasis. Hereto, lethally-irradiated hypercholesterolemic apolipoprotein E (APOE) knockout mice received APOE-containing bone marrow from wild-type mice (n = 6) or APOE knockout control bone marrow (n = 10) and were subsequently fed a regular chow diet. Transplantation with wild-type bone marrow was associated with a 10-fold decrease in VLDL/LDL-cholesterol levels. No changes were observed in adrenal weights, adrenal cholesterol content, or basal plasma corticosterone levels. However, food deprivation-induced corticosterone secretion was 64% lower (P < 0.05) in wild-type bone marrow recipients as compared to APOE knockout bone marrow recipients, in the context of similar plasma adrenocorticotropic hormone (ACTH) levels. A parallel 19-29% decrease in adrenal relative mRNA expression levels of ACTH-responsive genes SR-BI (P < 0.01), STAR (P < 0.05), and CYP11A1 (P < 0.05) was detected. In support of relative glucocorticoid insufficiency, blood lymphocyte and eosinophil concentrations were respectively 2.4-fold (P < 0.01) and 8-fold (P < 0.001) higher in wild-type bone marrow recipients under food deprivation stress conditions. In conclusion, we have shown that a selective lowering of VLDL/LDL levels in APOE knockout mice through a transplantation with APOE-containing wild-type bone marrow is associated with a decreased maximal adrenal glucocorticoid output. Our studies provide experimental support for the hypothesis that, in vivo, VLDL/LDL serves as the primary source of cholesterol used for glucocorticoid synthesis during food deprivation stress.

Cholestasis-associated glucocorticoid overexposure does not increase atherogenesis

Chronic glucocorticoid overexposure predisposes to the development of atherosclerotic cardiovascular disease in humans. Cholestatic liver disease is associated with increased plasma glucocorticoid levels. Here, we determined - in a preclinical setting - whether the chronic presence of cholestatic liver disease also induces a concomitant negative impact on atherosclerosis susceptibility. Hereto, regular chow diet-fed atherosclerosis-susceptible hypercholesterolemic apolipoprotein E (APOE)-knockout mice were treated with the bile duct toxicant alpha-naphthylisothiocyanate (ANIT) for 8 weeks. ANIT exposure induced the development of fibrotic cholestatic liver disease as evident from collagen deposits and compensatory bile duct hyperproliferation within the liver and the rise in plasma levels of bilirubin (+60%; P < 0.01) and bile acids (10-fold higher; P < 0.01). Adrenal weights (+22%; P < 0.01) and plasma corticosterone levels (+72%; P < 0.01) were increased in ANIT-treated mice. In contrast, atherosclerosis susceptibility was not increased in response to ANIT feeding, despite the concomitant increase in plasma free cholesterol (+30%; P < 0.01) and cholesteryl ester (+42%; P < 0.001) levels. The ANIT-induced hypercorticosteronemia coincided with marked immunosuppression as judged from the 50% reduction (P < 0.001) in circulating lymphocyte numbers. However, hepatic glucocorticoid signaling was not enhanced after ANIT treatment. It thus appears that the immunosuppressive effect of glucocorticoids is uncoupled from their metabolic effect under cholestatic disease conditions. In conclusion, we have shown that cholestatic liver disease-associated endogenous glucocorticoid overexposure does not increase atherosclerosis susceptibility in APOE-knockout mice. Our studies provide novel preclinical evidence for the observations that the hypercholesterolemia seen in cholestatic human subjects does not translate into a higher risk for atherosclerotic cardiovascular disease.

Elimination of adrenocortical apolipoprotein E production does not impact glucocorticoid output in wild-type mice

Apolipoprotein E (APOE) deficient mice exhibit unexplained hypercorticosteronemia. Given that APOE is also produced locally within the adrenals, we evaluated the effect of adrenal-specific APOE deficiency on the glucocorticoid function. Hereto, one adrenal containing or lacking APOE was transplanted into adrenalectomized wild-type mice. Adrenal APOE deficiency did not impact adrenal total cholesterol levels. Importantly, the ability of the two adrenal types to produce glucocorticoids was also not different as judged from the similar plasma corticosterone levels. Adrenal mRNA expression levels of HMG-CoA reductase and the LDL receptor were decreased by respectively 72% (p < 0.01) and 65% (p = 0.07), suggesting that cholesterol acquisition pathways were inhibited to possibly compensate the lack of APOE. In support, a parallel increase in the expression level of the cholesterol accumulation-associated ER stress marker CHOP was detected (+117%; p < 0.05). In conclusion, our studies show that elimination of adrenocortical APOE production does not impact glucocorticoid output in wild-type mice.

ATP-binding cassette transporter G1 deficiency is associated with mild glucocorticoid insufficiency in mice

OBJECTIVE

Since cholesterol is the sole precursor for glucocorticoid synthesis, it is hypothesized that genetic defects in proteins that impact the cellular cholesterol pool may underlie glucocorticoid insufficiency in humans. In the current study, we specifically focused on the cholesterol efflux mediator ATP-binding cassette transporter G1 (ABCG1) as gene candidate.

METHODS

The adrenal transcriptional response to fasting stress was measured in wild-type mice to identify putative novel gene candidates. Subsequently, the adrenal glucocorticoid function was compared between ABCG1 knockout mice and wild-type controls.

RESULTS

Overnight food deprivation induced a change in relative mRNA expression levels of cholesterol metabolism-related proteins previously linked to steroidogenesis, i.e. scavenger receptor class B type I (+149%; P < 0.001), LDL receptor (-70%; P < 0.001) and apolipoprotein E (-41%; P < 0.01). Strikingly, ABCG1 transcript levels were also markedly decreased (-61%; P < 0.05). In contrast to our hypothesis that decreasing cholesterol efflux would increase the adrenal cholesterol pool and enhance glucocorticoid output, ABCG1 knockout mice as compared to wild-type mice exhibited a reduced ability to secrete corticosterone in response to an ACTH challenge (two-way ANOVA: P < 0.001 for genotype) or fasting stress. As a result, glucocorticoid target gene expression levels in liver and hypothalamus were reduced and blood lymphocyte concentrations and spleen weights increased in ABCG1 knockout mice under fasting stress conditions. This was paralleled by a 48% reduction in adrenal cholesteryl ester stores and stimulation of adrenal NPC intracellular cholesterol transporter 2 (+37%; P < 0.05) and apolipoprotein E (+59%; P < 0.01) mRNA expression.

CONCLUSION

ABCG1 deficiency is associated with mild glucocorticoid insufficiency in mice.

Transcription controls growth, cell kinetics and cholesterol supply to sustain ACTH responses

Chronic ACTH exposure is associated with adrenal hypertrophy and steroidogenesis. The underlying molecular processes in mice have been analysed by microarray, histological and immunohistochemical techniques. Synacthen infused for 2 weeks markedly increased adrenal mass and plasma corticosterone levels. Microarray analysis found greater than 2-fold changes in expression of 928 genes (P < 0.001; 397 up, 531 down). These clustered in pathways involved in signalling, sterol/lipid metabolism, cell proliferation/hypertrophy and apoptosis. Signalling genes included some implicated in adrenal adenomas but also upregulated genes associated with cyclic AMP and downregulated genes associated with aldosterone synthesis. Sterol metabolism genes were those promoting cholesterol supply (Scarb1, Sqle, Apoa1) and disposal (Cyp27a1, Cyp7b1). Oil red O staining showed lipid depletion consistent with reduced expression of genes involved in lipid synthesis. Genes involved in steroidogenesis (Star, Cyp11a1, Cyp11b1) were modestly affected (P < 0.05; <1.3-fold). Increased Ki67, Ccna2, Ccnb2 and Tk1 expression complemented immunohistochemical evidence of a 3-fold change in cell proliferation. Growth arrest genes, Cdkn1a and Cdkn1c, which are known to be active in hypertrophied cells, were increased >4-fold and cross-sectional area of fasciculata cells was 2-fold greater. In contrast, genes associated with apoptosis (eg Casp12, Clu,) were downregulated and apoptotic cells (Tunel staining) were fewer (P < 0.001) and more widely distributed throughout the cortex. In summary, long-term steroidogenesis with ACTH excess is sustained by genes controlling cholesterol supply and adrenal mass. ACTH effects on adrenal morphology and genes controlling cell hypertrophy, proliferation and apoptosis suggest the involvement of different cell types and separate molecular pathways.

ApoE2 Exaggerates PTSD-Related Behavioral, Cognitive, and Neuroendocrine Alterations

Apolipoprotein E (apoE) is an essential component of lipoprotein particles in both the brain and periphery, and exists in three isoforms in the human population: E2, E3, and E4. ApoE has numerous, well-established roles in neurobiology. Most notably, E4 is associated with earlier onset and increased risk of Alzheimer's disease (AD). Although possession of E2 is protective in the context of AD, E2 appears to confer an increased incidence and severity of posttraumatic stress disorder (PTSD). However, the biological processes underlying this link remain unclear. In this study, we began to elucidate these associations by examining the effects of apoE on PTSD severity in combat veterans, and on PTSD-like behavior in mice with human apoE. In a group of 92 veterans with PTSD, we observed significantly higher Clinician-Administered PTSD Scale and PTSD Checklist scores in E2+ individuals, as well as alterations in salivary cortisol levels. Furthermore, we measured behavioral and biological outcomes in mice expressing human apoE after a single stressful event as well as following a period of chronic variable stress, a model of combat-related trauma. Mice with E2 showed impairments in fear extinction, and behavioral, cognitive, and neuroendocrine alterations following trauma. To the best of our knowledge, these data constitute the first translational demonstration of PTSD severity in men and PTSD-like symptoms in mice with E2, and point to apoE as a novel biomarker of susceptibility, and potential therapeutic target, for PTSD.

Adrenocortical LDL receptor function negatively influences glucocorticoid output

Over 50% of the cholesterol needed by adrenocortical cells for the production of glucocorticoids is derived from lipoproteins. However, the overall contribution of the different lipoproteins and associated uptake pathways to steroidogenesis remains to be determined. Here we aimed to show the importance of LDL receptor (LDLR)-mediated cholesterol acquisition for adrenal steroidogenesis in vivo. Female total body LDLR knockout mice with a human-like lipoprotein profile were bilaterally adrenalectomized and subsequently provided with one adrenal either expressing or genetically lacking the LDLR under their renal capsule to solely modulate adrenocortical LDLR function. Plasma total cholesterol levels and basal plasma corticosterone levels were identical in the two types of adrenal transplanted mice. Strikingly, restoration of adrenal LDLR function significantly reduced the ACTH-mediated stimulation of adrenal steroidogenesis (P<0.001), with plasma corticosterone levels that were respectively 44-59% lower (P<0.01) as compared to adrenal LDLR negative controls. In addition, LDLR positive adrenal transplanted mice exhibited a significant decrease (-39%; P<0.001) in their plasma corticosterone level under fasting stress conditions. Biochemical analysis did not show changes in the expression of genes involved in cholesterol mobilization. However, LDLR expressing adrenal transplants displayed a marked 62% reduction (P<0.05) in the transcript level of the key steroidogenic enzyme HSD3B2. In conclusion, our studies in a mouse model with a human-like lipoprotein profile provide the first in vivo evidence for a novel inhibitory role of the LDLR in the control of adrenal glucocorticoid production.

Proteomic Analysis of Plasma from California Sea Lions (Zalophus californianus) Reveals Apolipoprotein E as a Candidate Biomarker of Chronic Domoic Acid Toxicosis

Domoic acid toxicosis (DAT) in California sea lions (Zalophus californianus) is caused by exposure to the marine biotoxin domoic acid and has been linked to massive stranding events and mortality. Diagnosis is based on clinical signs in addition to the presence of domoic acid in body fluids. Chronic DAT further is characterized by reoccurring seizures progressing to status epilepticus. Diagnosis of chronic DAT is often slow and problematic, and minimally invasive tests for DAT have been the focus of numerous recent biomarker studies. The goal of this study was to retrospectively profile plasma proteins in a population of sea lions with chronic DAT and those without DAT using two dimensional gel electrophoresis to discover whether individual, multiple, or combinations of protein and clinical data could be utilized to identify sea lions with DAT. Using a training set of 32 sea lion sera, 20 proteins and their isoforms were identified that were significantly different between the two groups (p<0.05). Interestingly, 11 apolipoprotein E (ApoE) charge forms were decreased in DAT samples, indicating that ApoE charge form distributions may be important in the progression of DAT. In order to develop a classifier of chronic DAT, an independent blinded test set of 20 sea lions, seven with chronic DAT, was used to validate models utilizing ApoE charge forms and eosinophil counts. The resulting support vector machine had high sensitivity (85.7% with 92.3% negative predictive value) and high specificity (92.3% with 85.7% positive predictive value). These results suggest that ApoE and eosinophil counts along with machine learning can perform as a robust and accurate tool to diagnose chronic DAT. Although this analysis is specifically focused on blood biomarkers and routine clinical data, the results demonstrate promise for future studies combining additional variables in multidimensional space to create robust classifiers.

Oxidative stress regulates adipocyte apolipoprotein e and suppresses its expression in obesity

OBJECTIVE

Endogenous expression of apolipoprotein E (apoE) has a significant impact on adipocyte lipid metabolism and is markedly suppressed in obesity. Adipose tissue oxidant stress is emerging as an important mediator of adipocyte dysfunction. These studies were undertaken to evaluate the role of oxidant stress for regulation of adipocyte apoE.

RESEARCH DESIGN AND METHODS

ApoE gene and protein expression in 3T3-L1 adipocytes or mature adipocytes and adipose tissue from C57/BL6 mice was evaluated after induction of oxidant stress. The response of adipose tissue and adipocytes from obese compared with lean mice to antioxidants was also assessed.

RESULTS

Oxidant stress in 3T3-L1 cells or adipocytes and adipose tissue from lean mice significantly reduced apoE mRNA and protein level. Inclusion of an antioxidant eliminated this reduction. Oxidant stress was accompanied by activation of the nuclear factor-kappaB (NF-kappaB) transcription complex, and its effect on apoE was eliminated by an NF-kappaB activation inhibitor. Treatment of freshly isolated adipose tissue or mature adipocytes from obese mice with antioxidant increased apoE expression but had no effect on cells or tissue from lean mice. Incubation of freshly isolated adipocytes from lean mice with stromovascular cells from obese mice significantly suppressed adipocyte apoE compared with incubation with stromovascular cells from lean mice, but this suppression was reversed by inclusion of antioxidant or a neutralizing antibody to tumor necrosis factor-alpha.

CONCLUSIONS

Oxidant stress significantly modulates adipose tissue and adipocyte apoE expression. Furthermore, oxidant stress contributes to suppression of adipocyte apoE in obesity. This suppression depends on interaction between adipose tissue stromovascular cells and adipocytes.

Altered gene expression in early atherosclerosis is blocked by low level apolipoprotein E

BACKGROUND

Mice deficient in apolipoprotein E (apoE(-/-)) develop atherosclerosis. The possible linkage between expression of adhesion molecules/cofactors and atherosclerosis was probed at the level of mRNA and protein expression. The hypothesis of a linkage between changes of adhesion molecules/cofactors and atherosclerosis was tested further by suppression of aortic lesion formation in apoE(-/-) mice by expression of very low levels of transgenic apolipoprotein E.

METHODOLOGY/PRINCIPAL FINDINGS

We show that at 8.5 months of age, the apoE(-/-) mice display elevated expression of mRNA for LFA-1, MAC-1, VCAM-1, ICAM-1, and for CD44, as well as MCP-1, cathepsin B, and COX-2 (but not that for eNOS) in atherosclerotic aortic arches. At earlier age, (10-13 week old) apoE(-/-) mice already display elevated expression of mRNA of CD44, LFA-1, MAC-1, VCAM-1, ICAM-1, cathepsin, and of COX-2 in lesioned aortic arches. Expressing very low levels of transgenic apolipoprotein E suppresses both aortic lesions and the expression of mRNA of LFA-1, VCAM-1, MCP-1, cathepsin B, and of ICAM-1 in ApoE(-/-) mice. We tested at the level of protein, the observations obtained for mRNA expression. CD11a (a component of LFA-1), VCAM-1 and cathepsin B expression was found to be elevated in apoE(-/-) aortas at 8-9 months; low level expression of transgenic apolipoprotein E rectifies these changes.

CONCLUSIONS/SIGNIFICANCE

Atherosclerotic lesions in apoE(-/-) mice are detected as early as 4 weeks of age. Expression of low levels of apoE is shown to be both atheroprotective and to suppress these changes in key adhesion and inflammatory molecules observed in early atherosclerotic lesions.

Liver X receptors and cholesterol homoeostasis: spotlight on the adrenal gland

The LXRs (liver X receptors) (LXRalpha and LXRbeta) are nuclear hormone receptors that are activated by oxysterols, endogenous oxidative metabolites of cholesterol. These receptors regulate an integrated network of genes that control whole body cholesterol and lipid homoeostasis. A brief overview of the mechanism of this regulation by LXRs in the liver, macrophage and intestine will be outlined, followed by data from our recent work demonstrating that LXRalpha is crucial in maintaining adrenal cholesterol homoeostasis. In the adrenal gland, oxysterols are formed as intermediates in the conversion of cholesterol into steroid hormones and can act as endogenous activators of LXR. We have found using both gain- and loss-of-function models that LXR acts to maintain free cholesterol below toxic levels in the adrenal gland, through the co-ordinated regulation of genes involved in cholesterol efflux [ABCA1 (ATP-binding-cassette transporter A1)], storage (sterol-regulatory-element-binding protein-1c and apolipoprotein E) and metabolism to steroid hormones (steroidogenic acute regulatory protein). Furthermore, we show that under chronic dietary stress, the adrenal glands of LXR-null mice (and not wild-type mice) accumulate free cholesterol. These results support the role of LXR as a global regulator of cholesterol homoeostasis, where LXR provides a safety valve to limit free cholesterol in tissues experiencing high cholesterol flux.

Role of the nuclear receptors for oxysterols LXRs in steroidogenic tissues: beyond the "foie gras", the steroids and sex?

Various physiological functions have been ascribed to the liver X receptors (LXRs). Recently, we have identified new functions of these nuclear receptors in steroidogenic tissues. In adrenal, LXRalpha prevents accumulation of free cholesterol in mouse by controlling expression of genes involved in all aspects of cholesterol utilization. Under chronic dietary stress, adrenals from LXR-deficient mice accumulate free cholesterol while wild-type animals maintain cholesterol homeostasis through basal regulation of cholesterol efflux and storage. Hence, LXRalpha provides a safety valve to limit free cholesterol levels as a basal protective mechanism in the adrenal. Beside, mice lacking LXRalpha show lower levels of testicular testosterone while wild-type mice treated with the specific LXR agonist present an increase of testosterone production. Altogether, these data identify new roles for LXRs, in the regulation of cholesterol homeostasis in steroidogenic tissues and hormone synthesis.

Liver X receptors regulate adrenal cholesterol balance

Cholesterol is the obligate precursor to adrenal steroids but is cytotoxic at high concentrations. Here, we show the role of the liver X receptors (LXRalpha and LXRbeta) in preventing accumulation of free cholesterol in mouse adrenal glands by controlling expression of genes involved in all aspects of cholesterol utilization, including the steroidogenic acute regulatory protein, StAR, a novel LXR target. Under chronic dietary stress, adrenal glands from Lxralphabeta-/- mice accumulated free cholesterol. In contrast, wild-type animals maintained cholesterol homeostasis through basal expression of genes involved in cholesterol efflux and storage (ABC transporter A1 [ABCA1], apoE, SREBP-1c) while preventing steroidogenic gene (StAR) expression. Upon treatment with an LXR agonist that mimics activation by oxysterols, expression of these target genes was increased. Basally, Lxralphabeta-/- mice exhibited a marked decrease in ABCA1 and a derepression of StAR expression, causing a net decrease in cholesterol efflux and an increase in steroidogenesis. These changes occurred under conditions that prevented the acute stress response and resulted in a phenotype more specific to the loss of LXRalpha, including hypercorticosteronemia, cholesterol ester accumulation, and adrenomegaly. These results imply LXRalpha provides a safety valve to limit free cholesterol levels as a basal protective mechanism in the adrenal gland, where cholesterol is under constant flux.

Attenuated corticosterone response to chronic ACTH stimulation in hepatic lipase-deficient mice: evidence for a role for hepatic lipase in adrenal physiology

Hepatic lipase (HL), a liver-expressed lipolytic enzyme, hydrolyzes triglycerides and phospholipids in lipoproteins and promotes cholesterol delivery through receptor-mediated whole particle and selective cholesterol uptake. HL activity also occurs in the adrenal glands, which utilize lipoprotein cholesterol to synthesize glucocorticoids in response to pituitary ACTH. It is likely that the role of adrenal HL is to facilitate delivery of exogenous cholesterol for glucocorticoid synthesis. On this basis, we hypothesized that HL deficiency would blunt the glucocorticoid response to ACTH. Furthermore, because exogenous cholesterol also is derived from the LDL receptor (LDLR) pathway, we hypothesized that LDLR deficiency would blunt the response to ACTH. To test these hypotheses, we compared the corticosterone response to eight daily ACTH injections in HL-deficient (hl-/-), LDLR-deficient (Ldlr-/-), and HL- and LDLR-doubly deficient (Ldlr-/- hl-/-) mice with that in wild-type (WT) mice. Plasma corticosterone levels were measured on days 2, 5, and 8. Differences in plasma corticosterone levels between genotypes were analyzed by Kruskal-Wallis one-way ANOVA on ranks and pairwise multiple comparisons by Dunn's test. Our results demonstrate a trend toward reductions in plasma corticosterone levels on day 2 and significant reductions on day 5 and day 8 in the knockout models. Thus, on day 5, plasma corticosterone levels were reduced by 57, 70, and 73% (all P < 0.05) and on day 8 by 76, 59, and 63% (all P < 0.05) in hl-/-, Ldlr-/-, and Ldlr-/- hl-/- mice, respectively. These results demonstrate that HL deficiency, like LDLR deficiency, blunts the adrenal response to chronic ACTH stimulation and suggest a novel role for HL in adrenal physiology.