Hormone-sensitive lipase deficiency in mice causes lipid storage in the adrenal cortex and impaired corticosterone response to corticotropin stimulation

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.

An Overview of Hormone-Sensitive Lipase (HSL)

Hormone-sensitive lipase (HSL) is a pivotal enzyme that mediates triglyceride hydrolysis to provide free fatty acids and glycerol in adipocytes in a hormonally controlled lipolysis process. Elevated plasma-free fatty acids were accompanied by insulin resistance, type-2 diabetes, and obesity. Inhibition of lipolysis through HSL inhibition may provide a mechanism to prevent the accumulation of free fatty acids and to improve the affectability of insulin and blood glucose handling in type II diabetes. The published studies that examine the structure, regulation, and function of HSL and major inhibitors were reviewed in this paper.

Cloning and Molecular Characterization of HSL and Its Expression Pattern in HPG Axis and Testis during Different Stages in Bactrian Camel

Hormone-sensitive lipase (HSL) is a key enzyme in animal fat metabolism and is involved in the rate-limiting step of catalyzing the decomposition of fat and cholesterol. It also plays an important regulatory role in maintaining seminiferous epithelial structure, androgen synthesis and primordial germ cell differentiation. We previously reported that HSL is involved the synthesis of steroids in Bactrian camels, although it is unclear what role it plays in testicular development. The present study was conducted to characterize the biological function and expression pattern of the HSL gene in the hypothalamic pituitary gonadal (HPG) axis and the development of testis in Bactrian camels. We analyzed cloning of the cDNA sequence of the HSL gene of Bactrian camels by RT-PCR, as well as the structural features of HSL proteins, using bioinformatics software, such as ProtParam, TMHMM, Signal P 4.1, SOPMA and MEGA 7.0. We used qRT-PCR, Western blotting and immunofluorescence staining to clarify the expression pattern of HSL in the HPG axis and testis of two-week-old (2W), two-year-old (2Y), four-year-old (4Y) and six-year-old (6Y) Bactrian camels. According to sequence analysis, the coding sequence (CDS) region of the HSL gene is 648 bp in length and encodes 204 amino acids. According to bioinformatics analysis, the nucleotide and amino acid sequence of Bactrian camel HSL are most similar to those of Camelus pacos and Camelusdromedarius, with the lowest sequence similarity with Mus musculus. In adult Bactrian camel HPG axis tissues, both HSL mRNA and protein expression were significantly higher in the testis than in other tissues (hypothalamus, pituitary and pineal tissues) (p < 0.05). The expression of mRNA in the testis increased with age and was the highest in six-year-old testis (p < 0.01). The protein expression levels of HSL in 2Y and 6Y testis were clearly higher than in 2W and 4Y testis tissues (p < 0.01). Immunofluorescence results indicate that the HSL protein was mainly localized in the germ cells, Sertoli cells and Leydig cells from Bactrian camel testis, and strong positive signals were detected in epididymal epithelial cells, basal cells, spermatocytes and smooth muscle cells, with partially expression in hypothalamic glial cells, pituitary suspensory cells and pineal cells. According to the results of gene ontology (GO) analysis enrichment, HSL indirectly regulates the anabolism of steroid hormones through interactions with various targets. Therefore, we conclude that the HSL gene may be associated with the development and reproduction of Bactrian camels in different stages of maturity, and these results will contribute to further understanding of the regulatory mechanisms of HSL in Bactrian camel reproduction.

The Association of Cholesterol Uptake and Synthesis with Histology and Genotype in Cortisol-Producing Adenoma (CPA)

Cortisol-producing adenoma (CPA) is composed of clear and compact cells. Clear cells are lipid abundant, and compact ones lipid poor but associated with higher production of steroid hormones. PRKACA mutation (PRKACA mt) in CPA patients was reported to be associated with more pronounced clinical manifestation of Cushing's syndrome. In this study, we examined the association of histological features and genotypes with cholesterol uptake receptors and synthetic enzymes in 40 CPA cases, and with the quantitative results obtained by gas chromatography-mass spectrometry (GC-MS) analysis in 33 cases to explore their biological and clinical significance. Both cholesterol uptake receptors and synthetic enzymes were more abundant in compact cells. GC-MS analysis demonstrated that the percentage of compact cells was inversely correlated with the concentrations of cholesterol and cholesterol esters, and positively with the activity of cholesterol biosynthesis from cholesterol esters. In addition, hormone-sensitive lipase (HSL), which catalyzes cholesterol biosynthesis from cholesterol esters, tended to be more abundant in compact cells of PRKACA mt CPAs. These results demonstrated that both cholesterol uptake and biosynthesis were more pronounced in compact cells in CPA. In addition, more pronounced HSL expression in compact cells of PRKACA mt CPA could contribute to their more pronounced clinical manifestation.

Loss of Hepatic Surf4 Depletes Lipid Droplets in the Adrenal Cortex but Does Not Impair Adrenal Hormone Production

The adrenal gland produces steroid hormones to play essential roles in regulating various physiological processes. Our previous studies showed that knockout of hepatic Surf4 (Surf4^LKO) markedly reduced fasting plasma total cholesterol levels in adult mice, including low-density lipoprotein and high-density lipoprotein cholesterol. Here, we found that plasma cholesterol levels were also dramatically reduced in 4-week-old young mice and non-fasted adult mice. Circulating lipoprotein cholesterol is an important source of the substrate for the production of adrenal steroid hormones. Therefore, we investigated whether adrenal steroid hormone production was affected in Surf4^LKO mice. We observed that lacking hepatic Surf4 essentially eliminated lipid droplets and significantly reduced cholesterol levels in the adrenal gland; however, plasma levels of aldosterone and corticosterone were comparable in Surf4^LKO and the control mice under basal and stress conditions. Further analysis revealed that mRNA levels of genes encoding enzymes important for hormone synthesis were not altered, whereas the expression of scavenger receptor class B type I (SR-BI), low-density lipoprotein receptor (LDLR) and 3-hydroxy-3-methyl-glutaryl-CoA reductase was significantly increased in the adrenal gland of Surf4^LKO mice, indicating increased de novo cholesterol biosynthesis and enhanced LDLR and SR-BI-mediated lipoprotein cholesterol uptake. We also observed that the nuclear form of SREBP2 was increased in the adrenal gland of Surf4 ^LKO mice. Taken together, these findings indicate that the very low levels of circulating lipoprotein cholesterol in Surf4^LKO mice cause a significant reduction in adrenal cholesterol levels but do not significantly affect adrenal steroid hormone production. Reduced adrenal cholesterol levels activate SREBP2 and thus increase the expression of genes involved in cholesterol biosynthesis, which increases de novo cholesterol synthesis to compensate for the loss of circulating lipoprotein-derived cholesterol in the adrenal gland of Surf4^LKO mice.

Carboxylesterase 1d (Ces1d) does not contribute to cholesteryl ester hydrolysis in the liver

The liver is the central organ regulating cholesterol synthesis, storage, transport, and elimination. Mouse carboxylesterase 1d (Ces1d) and its human ortholog CES1 have been described to possess lipase activity and play roles in hepatic triacylglycerol metabolism and VLDL assembly. It has been proposed that Ces1d/CES1 might also catalyze cholesteryl ester (CE) hydrolysis in the liver and thus be responsible for the hydrolysis of HDL-derived CE; this could contribute to the final step in the reverse cholesterol transport (RCT) pathway, wherein cholesterol is secreted from the liver into bile and feces, either directly or after conversion to water-soluble bile salts. However, the proposed function of Ces1d/CES1 as a CE hydrolase is controversial. In this study, we interrogated the role hepatic Ces1d plays in cholesterol homeostasis using liver-specific Ces1d-deficient mice. We rationalized that if Ces1d is a major hepatic CE hydrolase, its absence would (1) reduce in vivo RCT flux and (2) provoke liver CE accumulation after a high-cholesterol diet challenge. We found that liver-specific Ces1d-deficient mice did not show any difference in the flux of in vivo HDL-to-feces RCT nor did it cause additional liver CE accumulation after high-fat, high-cholesterol Western-type diet feeding. These findings challenge the importance of Ces1d as a major hepatic CE hydrolase.

Plin2 deletion increases cholesteryl ester lipid droplet content and disturbs cholesterol balance in adrenal cortex

Cholesteryl esters (CEs) are the water-insoluble transport and storage form of cholesterol. Steroidogenic cells primarily store CEs in cytoplasmic lipid droplet (LD) organelles, as contrasted to the majority of mammalian cell types that predominantly store triacylglycerol (TAG) in LDs. The LD-binding Plin2 binds to both CE- and TAG-rich LDs, and although Plin2 is known to regulate degradation of TAG-rich LDs, its role for regulation of CE-rich LDs is unclear. To investigate the role of Plin2 in the regulation of CE-rich LDs, we performed histological and molecular characterization of adrenal glands from Plin2^+/+ and Plin2^−/− mice. Adrenal glands of Plin2^−/− mice had significantly enlarged organ size, increased size and numbers of CE-rich LDs in cortical cells, elevated cellular unesterified cholesterol levels, and increased expression of macrophage markers and genes facilitating reverse cholesterol transport. Despite altered LD storage, mobilization of adrenal LDs and secretion of corticosterone induced by adrenocorticotropic hormone stimulation or starvation were similar in Plin2^+/+ and Plin2^−/− mice. Plin2^−/− adrenals accumulated ceroid-like structures rich in multilamellar bodies in the adrenal cortex-medulla boundary, which increased with age, particularly in females. Finally, Plin2^−/− mice displayed unexpectedly high levels of phosphatidylglycerols, which directly paralleled the accumulation of these ceroid-like structures. Our findings demonstrate an important role of Plin2 for regulation of CE-rich LDs and cellular cholesterol balance in the adrenal cortex.

Hormone-sensitive lipase: sixty years later

Hormone-sensitive lipase (HSL) was initially characterized as the hormonally regulated neutral lipase activity responsible for the breakdown of triacylglycerols into fatty acids in adipose tissue. This review aims at providing up-to-date information on structural properties, regulation of expression, activity and function as well as therapeutic potential. The lipase is expressed as different isoforms produced from tissue-specific alternative promoters. All isoforms are composed of an N-terminal domain and a C-terminal catalytic domain within which a regulatory domain containing the phosphorylation sites is embedded. Some isoforms possess additional N-terminal regions. The catalytic domain shares similarities with bacteria, fungus and vascular plant proteins but not with other mammalian lipases. HSL singularity is provided by regulatory and N-terminal domains sharing no homology with other proteins. HSL has a broad substrate specificity compared to other neutral lipases. It hydrolyzes acylglycerols, cholesteryl and retinyl esters among other substrates. A novel role of HSL, independent of its enzymatic function, has recently been described in adipocytes. Clinical studies revealed dysregulations of HSL expression and activity in disorders, such as lipodystrophy, obesity, type 2 diabetes and cancer-associated cachexia. Development of specific inhibitors positions HSL as a pharmacological target for the treatment of metabolic complications.

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.

Cholesterol accumulation, lipid droplet formation, and steroid production in Leydig cells: Role of translocator protein (18-kDa)

BACKGROUND

Cholesterol import into the mitochondria of steroid-producing cells is the rate-determining step in steroidogenesis. Numerous studies have provided evidence that the cholesterol-binding translocator protein (18 kDa TSPO) plays an important role in cholesterol translocation into mitochondria and that it also might act on cholesterol homeostasis. Several TSPO-specific ligands have been shown to increase steroid production in vitro and in vivo.

OBJECTIVES

The present study assessed the effects of the TSPO drug ligand FGIN-1-27 on cholesterol accumulation and lipid droplet formation in relationship to steroid formation.

MATERIALS AND METHODS

Using MA-10 and primary Leydig cells, immunocytochemical and molecular methods were used to examine cholesterol accumulation, the formation of lipid droplets, and steroid formation in response to LH and FGIN-1-27. Additionally, we determined the effects of Tspo knockout by CRISPR/Cas9, and of siRNA knockdowns of Tspo and Plin2 (Perilipin 2; also known as adipose differentiation-related protein, ADFP) on LH- and FGIN-1-27-induced steroidogenesis.

RESULTS

In response to LH and FGIN-1-27, cultured MA-10 cells and primary Leydig cells increased steroid formation, cholesterol accumulation, and lipid droplet formation. Cholesterol accumulation in the lipid droplets also was increased in Tspo knockout cells. Knockout of Tspo or its knockdown in MA-10 cells resulted in reduced progesterone formation in response to both LH and FGIN-1-27, as did knockdown of Plin2. Steroid production also was inhibited by the cholesteryl ester hydrolase inhibitor diethylumbelliferyl phosphate.

DISCUSSION AND CONCLUSION

These results support the conclusion that FGIN-1-27 stimulates steroid formation by increasing TSPO-mediated cholesterol translocation into the inner mitochondria for steroidogenesis, as well as into the cytosol for lipid droplet formation. FGIN-1-27 also increased steroid formation at least in part by inducing the conversion of cholesteryl ester located in lipid droplets to cholesterol, thus making available more substrate for steroid formation.

Uptake of HDL-cholesterol contributes to lipid accumulation in clear cell renal cell carcinoma

Clear cell renal cell carcinoma (ccRCC), which accounts for the majority of kidney cancer, is known to accumulate excess cholesterol. However, the mechanism and functional significance of the lipid accumulation for development of the cancer remains obscure. In this study, we analyzed 42 primary ccRCC samples, and determined that cholesterol levels of ~ 70% of the tumors were at least two-fold higher than that of benign kidney tissues. Compared to tumors without cholesterol accumulation, those containing excess cholesterol expressed higher levels of scavenger receptor BI (SR-B1), a receptor for uptake of HDL-associated cholesterol, but not genes involved in cholesterol synthesis and uptake of LDL-associated cholesterol. To further determine the roles of sterol accumulation for cancer development, we implanted ccRCC from patients into mouse kidneys using a mouse ccRCC xenograft model. Feeding mice with probucol, a compound lowing HDL-cholesterol, markedly reduced levels of cholesterol in tumors containing excess cholesterol. This treatment, however, did not affect growth of these tumors. Our study suggests that cholesterol overaccumulation in ccRCC is the consequence of increased uptake of HDL-cholesterol as a result of SR-B1 overexpression, but the lipid accumulation by itself may not play a significant role in progression of the cancer.

Toxicity of Flame Retardant Isopropylated Triphenyl Phosphate: Liver, Adrenal, and Metabolic Effects

The use of organophosphates phosphate flame retardants, particularly isopropylated triphenyl phosphate (IPTPP), has increased in recent years as replacements for polybrominated diphenyl ethers. This is despite limited understanding of the hazards of IPTPP. To examine the general and endocrine toxicity of IPTPP, adult Wistar rats were fed for 90 days on diets containing IPTPP estimated to deliver daily doses of 5 to 140 mg/kg/d. Exposure to IPTPP caused a dose-related increase in liver and adrenal gland weight in both sexes. Cells in the zona fasciculate (ZF) of the adrenal cortex were observed to be filled with droplets that stained with Nile red, suggesting they contained neutral lipid. Despite marked structural changes, there was no change in basal or stress-induced serum levels of their major secreted ZF product corticosterone (B), suggesting cell function was not altered. There were no effects on responses to glucose or insulin challenge, but serum levels of fructosamine were elevated by IPTPP exposure, suggesting a slight tendency of exposed animals to be hyperglycemic. Serum levels of total cholesterol and high-density lipoprotein cholesterol were significantly elevated in both sexes at the 2 highest doses. This study demonstrates that IPTPP exposure causes hypertrophy and neutral lipid accumulation in adrenal cortex ZF cells but does not result in impaired B production.

TSPO mutations in rats and a human polymorphism impair the rate of steroid synthesis

The 18 kDa translocator protein (TSPO) is a ubiquitous conserved outer mitochondrial membrane protein implicated in numerous cell and tissue functions, including steroid hormone biosynthesis, respiration, cell proliferation, and apoptosis. TSPO binds with high affinity to cholesterol and numerous compounds, is expressed at high levels in steroid-synthesizing tissues, and mediates cholesterol import into mitochondria, which is the rate-limiting step in steroid formation. In humans, the rs6971 polymorphism on the TSPO gene leads to an amino acid substitution in the fifth transmembrane loop of the protein, which is where the cholesterol-binding domain of TSPO is located, and this polymorphism has been associated with anxiety-related disorders. However, recent knockout mouse models have provided inconsistent conclusions of whether TSPO is directly involved in steroid synthesis. In this report, we show that TSPO deletion mutations in rat and its corresponding rs6971 polymorphism in humans alter adrenocorticotropic hormone-induced plasma corticosteroid concentrations. Rat tissues examined show increased cholesteryl ester accumulation, and neurosteroid formation was undetectable in homozygous rats. These results also support a role for TSPO ligands in diseases with steroid-dependent stress and anxiety elements.

Usefulness of Simultaneous Measurement of Plasma Steroids, Including Precursors, for the Evaluation of Drug Effects on Adrenal Steroidogenesis in Rats

The aim of this study was to evaluate the usefulness of simultaneous measurement of plasma steroids, including precursors, for the evaluation of drug effects on adrenal steroidogenesis in vivo. Plasma concentrations of corticosterone and its precursors were examined in rats dosed with compounds that affect adrenal steroidogenesis via different modes of action as well as the relationships of the changes with blood chemistry and adrenal histopathology. Male rats were dosed with tricresyl phosphate, aminoglutethimide, trilostane (TRL), metyrapone (MET), ketoconazole (KET), or mifepristone for 7 days. In the TRL, MET, and KET groups, precursor levels were markedly increased, while there were no significant changes in the corticosterone level, suggesting that the precursors are more sensitive biomarkers to detect the effect on adrenal steroidogenesis. Also, the precursors with increased levels were those that are normally metabolized by the inhibited enzymes, reflecting the modes of action of the compounds. In addition, different patterns of changes were observed in blood chemistry and histopathology, supporting the mechanism suggested by the steroid changes. These results show that simultaneous measurement of plasma steroids, including precursors, can be a valuable method to sensitively evaluate drug effects on adrenal steroidogenesis and to investigate the underlying mechanisms.

Monitoring of Dual CRISPR/Cas9-Mediated Steroidogenic Acute Regulatory Protein Gene Deletion and Cholesterol Accumulation Using High-Resolution Fluorescence In Situ Hybridization in a Single Cell

Recent advances in fluorescence microscopy, coupled with CRISPR/Cas9 gene editing technology, provide opportunities for understanding gene regulation at the single-cell level. The application of direct imaging shown here provides an in situ side-by-side comparison of CRISPR/Cas9-edited cells and adjacent unedited cells. We apply this methodology to the steroidogenic acute regulatory protein (StAR) gene in Y-1 adrenal cells and MA-10 testis cells. StAR is a gatekeeper protein that controls the access of cholesterol from the cytoplasm to the inner mitochondria. The loss of this mitochondrial cholesterol transfer mediator rapidly increases lipid droplets in cells, as seen in StAR^-/- mice. Here, we describe a dual CRISPR/Cas9 strategy marked by GFP/mCherry expression that deletes StAR activity within 12 h. We used single-molecule fluorescence in situ hybridization (sm-FISH) imaging to directly monitor the time course of gene editing in single cells. We achieved StAR gene deletion at high efficiency dual gRNA targeting to the proximal promoter and exon 2. Seventy percent of transfected cells showed a slow DNA deletion as measured by PCR, and loss of Br-cAMP stimulated transcription. This DNA deletion was seen by sm-FISH in both loci of individual cells relative to non-target Cyp11a1 and StAR exon 7. sm-FISH also distinguishes two effects on stimulated StAR expression without this deletion. Br-cAMP stimulation of primary and spliced StAR RNA at the gene loci were removed within 4 h in this dual CRISPR/Cas9 strategy before any effect on cytoplasmic mRNA and protein occurred. StAR mRNA disappeared between 12 and 24 h in parallel with this deletion, while cholesterol ester droplets increased fourfold. These alternative changes match distinct StAR expression processes. This dual gRNA and sm-FISH approach to CRISPR/Cas9 editing facilitates rapid testing of editing strategies and immediate assessment of single-cell adaptation responses without the perturbation of clonal expansion procedures.

Role of the steroidogenic acute regulatory protein in health and disease

Steroid hormones are an important class of regulatory molecules that are synthesized in steroidogenic cells of the adrenal, ovary, testis, placenta, brain, and skin, and influence a spectrum of developmental and physiological processes. The steroidogenic acute regulatory protein (STAR) predominantly mediates the rate-limiting step in steroid biosynthesis, i.e., the transport of the substrate of all steroid hormones, cholesterol, from the outer to the inner mitochondrial membrane. At the inner membrane, cytochrome P450 cholesterol side chain cleavage enzyme cleaves the cholesterol side chain to form the first steroid, pregnenolone, which is converted by a series of enzymes to various steroid hormones in specific tissues. Both basic and clinical evidence have demonstrated the crucial involvement of the STAR protein in the regulation of steroid biosynthesis. Multiple levels of regulation impinge on STAR action. Recent findings demonstrate that hormone-sensitive lipase, through its action on the hydrolysis of cholesteryl esters, plays an important role in regulating STAR expression and steroidogenesis which involve the liver X receptor pathway. Activation of the latter influences macrophage cholesterol efflux that is a key process in the prevention of atherosclerotic cardiovascular disease. Appropriate regulation of steroid hormones is vital for proper functioning of many important biological activities, which are also paramount for geriatric populations to live longer and healthier. This review summarizes the current level of understanding on tissue-specific and hormone-induced regulation of STAR expression and steroidogenesis, and provides insights into a number of cholesterol and/or steroid coupled physiological and pathophysiological consequences.

Osteopontin is associated with inflammation and mortality in a mouse model of polymicrobial sepsis

BACKGROUND

Osteopontin (OPN) is a multifunctional glycoprotein with pro-inflammatory properties. In severe sepsis, levels of plasma OPN are significantly higher in non-survivors than in survivors. We hypothesized that OPN results in greater inflammation and worse outcome through modulation of endogenous glucocorticoid production in sepsis.

METHODS AND RESULTS

Sepsis was induced by cecal ligation and puncture (CLP) in wild type (WT) and OPN gene knockout (OPN(-/-) ) mice. In response to sepsis, the OPN(-/-) mice had lower levels of plasma cytokines and chemokines than the WT mice. The levels of corticosterone in plasma were similar between WT and OPN(-/-) sham animals but they increased 24 h after CLP induction in the WT mice, but not in the OPN(-/-) mice. The mortality rate was lower in the OPN(-/-) mice than in the WT mice.

CONCLUSION

OPN is associated with greater inflammatory response and increased mortality, despite the higher corticosterone levels in plasma. Corticosterone production is not impaired in the absence of OPN.

Inborn errors of cytoplasmic triglyceride metabolism

Triglyceride (TG) synthesis, storage, and degradation together constitute cytoplasmic TG metabolism (CTGM). CTGM is mostly studied in adipocytes, where starting from glycerol-3-phosphate and fatty acyl (FA)-coenzyme A (CoA), TGs are synthesized then stored in cytoplasmic lipid droplets. TG hydrolysis proceeds sequentially, producing FAs and glycerol. Several reactions of CTGM can be catalyzed by more than one enzyme, creating great potential for complex tissue-specific physiology. In adipose tissue, CTGM provides FA as a systemic energy source during fasting and is related to obesity. Inborn errors and mouse models have demonstrated the importance of CTGM for non-adipose tissues, including skeletal muscle, myocardium and liver, because steatosis and dysfunction can occur. We discuss known inborn errors of CTGM, including deficiencies of: AGPAT2 (a form of generalized lipodystrophy), LPIN1 (childhood rhabdomyolysis), LPIN2 (an inflammatory condition, Majeed syndrome, described elsewhere in this issue), DGAT1 (protein loosing enteropathy), perilipin 1 (partial lipodystrophy), CGI-58 (gene ABHD5, neutral lipid storage disease (NLSD) with ichthyosis and "Jordan's anomaly" of vacuolated polymorphonuclear leukocytes), adipose triglyceride lipase (ATGL, gene PNPLA2, NLSD with myopathy, cardiomyopathy and Jordan's anomaly), hormone-sensitive lipase (HSL, gene LIPE, hypertriglyceridemia, and insulin resistance). Two inborn errors of glycerol metabolism are known: glycerol kinase (GK, causing pseudohypertriglyceridemia) and glycerol-3-phosphate dehydrogenase (GPD1, childhood hepatic steatosis). Mouse models often resemble human phenotypes but may diverge markedly. Inborn errors have been described for less than one-third of CTGM enzymes, and new phenotypes may yet be identified.

Mechanisms mediating environmental chemical-induced endocrine disruption in the adrenal gland

Humans are continuously exposed to hundreds of man-made chemicals that pollute the environment in addition to multiple therapeutic drug treatments administered throughout life. Some of these chemicals, known as endocrine disruptors (EDs), mimic endogenous signals, thereby altering gene expression, influencing development, and promoting disease. Although EDs are eventually removed from the market or replaced with safer alternatives, new evidence suggests that early-life exposure leaves a fingerprint on the epigenome, which may increase the risk of disease later in life. Epigenetic changes occurring in early life in response to environmental toxicants have been shown to affect behavior, increase cancer risk, and modify the physiology of the cardiovascular system. Thus, exposure to an ED or combination of EDs may represent a first hit to the epigenome. Only limited information is available regarding the effect of ED exposure on adrenal function. The adrenal gland controls the stress response, blood pressure, and electrolyte homeostasis. This endocrine organ therefore has an important role in physiology and is a sensitive target of EDs. We review herein the effect of ED exposure on the adrenal gland with particular focus on in utero exposure to the plasticizer di(2-ethylehyl) phthalate. We discuss the challenges associated with identifying the mechanism mediating the epigenetic origins of disease and availability of biomarkers that may identify individual or population risks.

The catalytic function of hormone-sensitive lipase is essential for fertility in male mice

In male mice, deficiency of hormone sensitive lipase (HSL, Lipe gene, E.C.3.1.1.3) causes deficient spermatogenesis, azoospermia, and infertility. Postmeiotic germ cells express a specific HSL isoform that includes a 313 amino acid N-terminus encoded by a testis-specific exon (exon T1). The remainder of testicular HSL is identical to adipocyte HSL. The amino acid sequence of the testis-specific exon is poorly conserved, showing only a 46% amino acid identity with orthologous human and rat sequences, compared with 87% over the remainder of the HSL coding sequence, providing no evidence in favor of a vital functional role for the testis-specific N-terminus of HSL. However, exon T1 is important for Lipe transcription; in mouse testicular mRNA, we identified 3 major Lipe transcription start sites, finding numerous testicular transcription factor binding motifs upstream of the transcription start site. We directly explored two possible mechanisms for the infertility of HSL-deficient mice, using mice that expressed mutant HSL transgenes only in postmeiotic germ cells on a HSL-deficient background. One transgene expressed human HSL lacking enzyme activity but containing the testis-specific N-terminus (HSL-/-muttg mice). The other transgene expressed catalytically inactive HSL with the testis-specific N-terminal peptide (HSL-/-atg mice). HSL-/-muttg mice were infertile, with abnormal histology of the seminiferous epithelium and absence of spermatozoa in the epididymal lumen. In contrast, HSL-/-atg mice had normal fertility and normal testicular morphology. In conclusion, whereas the catalytic function of HSL is necessary for spermatogenesis in mice, the presence of the N-terminal testis-specific fragment is not essential.

Multidimensional profiling platforms reveal metabolic dysregulation caused by organophosphorus pesticides

We are environmentally exposed to countless synthetic chemicals on a daily basis, with an increasing number of these chemical exposures linked to adverse health effects. However, our understanding of the (patho)physiological effects of these chemicals remains poorly understood, due in part to a general lack of effort to systematically and comprehensively identify the direct interactions of environmental chemicals with biological macromolecules in mammalian systems in vivo. Here, we have used functional chemoproteomic and metabolomic platforms to broadly identify direct enzyme targets that are inhibited by widely used organophosphorus (OP) pesticides in vivo in mice and to determine metabolic alterations that are caused by these chemicals. We find that these pesticides directly inhibit over 20 serine hydrolases in vivo leading to widespread disruptions in lipid metabolism. Through identifying direct biological targets of OP pesticides, we show heretofore unrecognized modes of toxicity that may be associated with these agents and underscore the utility of using multidimensional profiling approaches to obtain a more complete understanding of toxicities associated with environmental chemicals.

Expression of selected genes involved in steroidogenesis in the course of enucleation-induced rat adrenal regeneration

The enucleation-induced (EI) rapid proliferation of adrenocortical cells is followed by their differentiation, the degree of which may be characterized by the expression of genes directly and indirectly involved in steroid hormone biosynthesis. In this study, out of 30,000 transcripts of genes identified by means of Affymetrix Rat Gene 1.1 ST Array, we aimed to select genes (either up- or downregulated) involved in steroidogenesis in the course of enucleation-induced adrenal regeneration. On day 1, we found 32 genes with altered expression levels, 15 were upregulated and 17 were downregulated [i.e., 3β-hydroxysteroid dehydrogenase (Hsd3β), nuclear receptor subfamily 0, group B, member 1 (Nr0b1), cytochrome P450 aldosterone synthase (Cyp11b2) and sterol O-acyltransferase 1 (Soat1)]. On day 15, the expression of only 2 genes was increased and that of 3 was decreased. The investigated genes were clustered according to an hierarchical clustering algorithm and 4 clusters were obtained. Quantitative PCR (qPCR) confirmed the much lower mRNA expression levels of steroidogenic acute regulatory protein (StAR) during the regeneration process compared to the control, while the cholesterol side-chain cleavage enzyme (cholesterol desmolase; Cyp11a1) and Hsd3β genes presented similar expression profiles throughout the entire regeneration process. Cyp11b2 mRNA levels remained very low during the whole regeneration period. Fatty acid binding protein 6 (Fabp6) was markedly upregulated, whereas hormone-sensitive lipase (Lipe) was downregulated. The expression of Soat1 was lowest on regeneration day 1 and, subsequently, its expression increased from there on, reaching levels higher than the control. Dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1 (Dax-1) mRNA levels were lowest on day 1 of the experiment; however, throughout the entire experimental period, there were no statistically significant differences observed. After the initial decrease in steroidogenic factor 1 (Sf-1) mRNA levels observed on the 1st day of the experiment, a marked upregulation in its expression was observed from there on. Data from the current study strongly suggest the role of Fabp6, Lipe and Soat1 in supplying substrates of regenerating adrenocortical cells for steroid synthesis. Our results indicate that during the first days of adrenal regeneration, intense synthesis of cholesterol may occur, which is then followed by its conversion into cholesteryl esters. Moreover, our data demonstrated that in enucleation-induced regeneration, the restoration of genes involved in glucocorticoid synthesis is notably shorter than that of those involved in aldosterone synthesis.

Liver-specific cholesteryl ester hydrolase deficiency attenuates sterol elimination in the feces and increases atherosclerosis in ldlr-/- mice

OBJECTIVE

Liver is the major organ responsible for the final elimination of cholesterol from the body either as biliary cholesterol or as bile acids. Intracellular hydrolysis of lipoprotein-derived cholesteryl esters (CEs) is essential to generate the free cholesterol required for this process. Earlier, we demonstrated that overexpression of human CE hydrolase (Gene symbol CES1) increased bile acid synthesis in human hepatocytes and enhanced reverse cholesterol transport in mice. The objective of the present study was to demonstrate that liver-specific deletion of its murine ortholog, Ces3, would decrease cholesterol elimination from the body and increase atherosclerosis.

APPROACH AND RESULTS

Liver-specific Ces3 knockout mice (Ces3-LKO) were generated, and Ces3 deficiency did not affect the expression of genes involved in cholesterol homeostasis and free cholesterol or bile acid transport. The effects of Ces3 deficiency on the development of Western diet-induced atherosclerosis were examined in low density lipoprotein receptor knock out(-/-) mice. Despite similar plasma lipoprotein profiles, there was increased lesion development in low density lipoprotein receptor knock out(-/-)Ces3-LKO mice along with a significant decrease in the bile acid content of bile. Ces3 deficiency significantly reduced the flux of cholesterol from [(3)H]-CE-labeled high-density lipoproteins to feces (as free cholesterol and bile acids) and decreased total fecal sterol elimination.

CONCLUSIONS

Our results demonstrate that hepatic Ces3 modulates the hydrolysis of lipoprotein-delivered CEs and thereby regulates free cholesterol and bile acid secretion into the feces. Therefore, its deficiency results in reduced cholesterol elimination from the body, leading to significant increase in atherosclerosis. Collectively, these data establish the antiatherogenic role of hepatic CE hydrolysis.

Mechanisms of action of hormone-sensitive lipase in mouse Leydig cells: its role in the regulation of the steroidogenic acute regulatory protein

Hormone-sensitive lipase (HSL) catalyzes the hydrolysis of cholesteryl esters in steroidogenic tissues and, thus, facilitates cholesterol availability for steroidogenesis. The steroidogenic acute regulatory protein (StAR) controls the rate-limiting step in steroid biosynthesis. However, the modes of action of HSL in the regulation of StAR expression remain obscure. We demonstrate in MA-10 mouse Leydig cells that activation of the protein kinase A (PKA) pathway, by a cAMP analog Bt2cAMP, enhanced expression of HSL and its phosphorylation (P) at Ser-660 and Ser-563, but not at Ser-565, concomitant with increased HSL activity. Phosphorylation and activation of HSL coincided with increases in StAR, P-StAR (Ser-194), and progesterone levels. Inhibition of HSL activity by CAY10499 effectively suppressed Bt2cAMP-induced StAR expression and progesterone synthesis. Targeted silencing of endogenous HSL, with siRNAs, resulted in increased cholesteryl ester levels and decreased cholesterol content in MA-10 cells. Depletion of HSL affected lipoprotein-derived cellular cholesterol influx, diminished the supply of cholesterol to the mitochondria, and resulted in the repression of StAR and P-StAR levels. Cells overexpressing HSL increased the efficacy of liver X receptor (LXR) ligands on StAR expression and steroid synthesis, suggesting HSL-mediated steroidogenesis entails enhanced oxysterol production. Conversely, cells deficient in LXRs exhibited decreased HSL responsiveness. Furthermore, an increase in HSL was correlated with the LXR target genes, steroid receptor element-binding protein 1c and ATP binding cassette transporter A1, demonstrating HSL-dependent regulation of steroidogenesis predominantly involves LXR signaling. LXRs interact/cooperate with RXRs and result in the activation of StAR gene transcription. These findings provide novel insight and demonstrate the molecular events by which HSL acts to drive cAMP/PKA-mediated regulation of StAR expression and steroidogenesis in mouse Leydig cells.

LCAT deficiency in mice is associated with a diminished adrenal glucocorticoid function

In vitro studies have suggested that HDL and apoB-containing lipoproteins can provide cholesterol for synthesis of glucocorticoids. Here we assessed adrenal glucocorticoid function in LCAT knockout (KO) mice to determine the specific contribution of HDL-cholesteryl esters to adrenal glucocorticoid output in vivo. LCAT KO mice exhibit an 8-fold higher plasma free cholesterol-to-cholesteryl ester ratio (P < 0.001) and complete HDL-cholesteryl ester deficiency. ApoB-containing lipoprotein and associated triglyceride levels are increased in LCAT KO mice as compared with C57BL/6 control mice (44%; P < 0.05). Glucocorticoid-producing adrenocortical cells within the zona fasciculata in LCAT KO mice are devoid of neutral lipids. However, adrenal weights and basal corticosterone levels are not significantly changed in LCAT KO mice. In contrast, adrenals of LCAT KO mice show compensatory up-regulation of genes involved in cholesterol synthesis (HMG-CoA reductase; 516%; P < 0.001) and acquisition (LDL receptor; 385%; P < 0.001) and a marked 40-50% lower glucocorticoid response to adrenocorticotropic hormone exposure, endotoxemia, or fasting (P < 0.001 for all). In conclusion, our studies show that HDL-cholesteryl ester deficiency in LCAT KO mice is associated with a 40-50% lower adrenal glucocorticoid output. These findings further highlight the important novel role for HDL as cholesterol donor for the synthesis of glucocorticoids by the adrenals.

FXR agonist GW4064 increases plasma glucocorticoid levels in C57BL/6 mice

Since high expression of farnesoid X receptor (FXR) has been detected in glucocorticoid-producing adrenocortical cells, we evaluated the potential role of FXR in adrenal glucocorticoid production. FXR agonist GW4064 increased fasting plasma corticosterone levels (+45%; P<0.01) in C57BL/6 mice, indicative of enhanced adrenal steroidogenesis. GW4064 treatment did not affect plasma ACTH levels, adrenal weight, or adrenal expression of steroidogenic genes. Scavenger receptor BI (SR-BI) mRNA and protein expression, respectively, increased 1.9-fold (P<0.01) and 1.5-fold, which suggests a stimulated lipoprotein-associated cholesterol uptake into the adrenals upon GW4064 treatment. In line with an enhanced flux of cellular cholesterol into the steroidogenic pathway, adrenal unesterified and esterified cholesterol stores were 21-41% decreased (P<0.01) upon GW4064 treatment. In conclusion, we have shown that the FXR agonist GW4064 stimulates plasma corticosterone levels in C57BL/6 mice. Our findings suggest a novel role for FXR in the modulation of adrenal cholesterol metabolism and glucocorticoid synthesis in mice.

Abrogation of neutral cholesterol ester hydrolytic activity causes adrenal enlargement

We have previously demonstrated that neutral cholesterol ester hydrolase 1 (Nceh1) regulates foam cell formation and atherogenesis through the catalytic activity of cholesterol ester hydrolysis, and that Nceh1 and hormone-sensitive lipase (Lipe) are responsible for the majority of neutral cholesterol ester hydrolase activity in macrophages. There are several cholesterol ester-metabolizing tissues and cells other than macrophages, among which adrenocortical cells are also known to utilize the intracellular cholesterol for steroidogenesis. It has been believed that the mobilization of intracellular cholesterol ester in adrenal glands was facilitated solely by Lipe. We herein demonstrate that Nceh1 is also involved in cholesterol ester hydrolysis in adrenal glands. While Lipe deficiency remarkably reduced the neutral cholesterol ester hydrolase activity in adrenal glands as previously reported, additional inactivation of Nceh1 gene completely abrogated the activity. Adrenal glands were enlarged in proportion to the degree of reduced neutral cholesterol ester hydrolase activity, and the enlargement of adrenal glands and the accumulation of cholesterol esters were most pronounced in the Nceh1/Lipe double-deficient mice. Thus Nceh1 is involved in the adrenal cholesterol metabolism, and the cholesterol ester hydrolytic activity in adrenal glands is associated with the organ enlargement.

Cellular cholesterol delivery, intracellular processing and utilization for biosynthesis of steroid hormones

Steroid hormones regulate diverse physiological functions such as reproduction, blood salt balance, maintenance of secondary sexual characteristics, response to stress, neuronal function and various metabolic processes. They are synthesized from cholesterol mainly in the adrenal gland and gonads in response to tissue-specific tropic hormones. These steroidogenic tissues are unique in that they require cholesterol not only for membrane biogenesis, maintenance of membrane fluidity and cell signaling, but also as the starting material for the biosynthesis of steroid hormones. It is not surprising, then, that cells of steroidogenic tissues have evolved with multiple pathways to assure the constant supply of cholesterol needed to maintain optimum steroid synthesis. The cholesterol utilized for steroidogenesis is derived from a combination of sources: 1) de novo synthesis in the endoplasmic reticulum (ER); 2) the mobilization of cholesteryl esters (CEs) stored in lipid droplets through cholesteryl ester hydrolase; 3) plasma lipoprotein-derived CEs obtained by either LDL receptor-mediated endocytic and/or SR-BI-mediated selective uptake; and 4) in some cultured cell systems from plasma membrane-associated free cholesterol. Here, we focus on recent insights into the molecules and cellular processes that mediate the uptake of plasma lipoprotein-derived cholesterol, events connected with the intracellular cholesterol processing and the role of crucial proteins that mediate cholesterol transport to mitochondria for its utilization for steroid hormone production. In particular, we discuss the structure and function of SR-BI, the importance of the selective cholesterol transport pathway in providing cholesterol substrate for steroid biosynthesis and the role of two key proteins, StAR and PBR/TSO in facilitating cholesterol delivery to inner mitochondrial membrane sites, where P450scc (CYP11A) is localized and where the conversion of cholesterol to pregnenolone (the common steroid precursor) takes place.

Effects of hormone-sensitive lipase disruption on cardiac energy metabolism in response to fasting and refeeding

Increased fatty acid (FA) flux and intracellular lipid accumulation (steatosis) give rise to cardiac lipotoxicity in both pathological and physiological conditions. Since hormone-sensitive lipase (HSL) contributes to intracellular lipolysis in adipose tissue and heart, we investigated the impact of HSL disruption on cardiac energy metabolism in response to fasting and refeeding. HSL-knockout (KO) mice and wild-type (WT) littermates were fasted for 24 h, followed by ∼6 h of refeeding. Plasma FA concentration in WT mice was elevated twofold with fasting, whereas KO mice lacked this elevation, resulting in twofold lower cardiac FA uptake compared with WT mice. Echocardiography showed that fractional shortening was 15% decreased during fasting in WT mice and was associated with steatosis, whereas both of these changes were absent in KO mice. Compared with Langendorff-perfused hearts isolated from fasted WT mice, the isolated KO hearts also displayed higher contractile function and a blunted response to FA. Although cardiac glucose uptake in KO mice was comparable with WT mice under all conditions tested, cardiac VLDL uptake and lipoprotein lipase (LPL) activity were twofold higher in KO mice during fasting. The KO hearts showed undetectable activity of neutral cholesteryl esterase and 40% lower non-LPL triglyceride lipase activity compared with WT hearts in refed conditions accompanied by overt steatosis, normal cardiac function, and increased mRNA expression of adipose differentiation-related protein. Thus, the dissociation between cardiac steatosis and functional sequelae observed in HSL-KO mice suggests that excess FA influx, rather than steatosis per se, appears to play an important role in the pathogenesis of cardiac lipotoxicity.

Digital fluorescence analysis of trafficking of single endosomes containing low-density lipoprotein in adrenocortical cells: facilitation of centripetal motion by adrenocorticotropic hormone

Imaging of trafficking of endosomes containing low-density lipoprotein (LDL) is useful to analyze cholesterol transport in adrenocortical cells. At 60 min after the application of fluorescently labeled LDL to adrenocortical cells, individual endosomes containing LDL were demonstrated to undergo frequent switching between forward and reverse movement and immobility. The population of moving endosomes (>or=0.065 microm/s) was approximately 75% in control cells. The remaining endosomes were either slowly moving or temporarily immobile. At 3h after the LDL addition, endosomes were concentrated around the circumference of the cell nuclei. The endosome movement was inhibited by nocodazole, implying that endosomes undergo movement along microtubule networks. Anti-dynein antibodies inhibited the motion of endosomes towards the nucleus, and anti-kinesin antibodies inhibited peripherally directed motion. These results imply that both dynein-like and kinesin-like motor proteins bind to the same endosome, resulting in saltatory movements with centripetal or peripherally directed direction, depending on which motor binds to microtubules. Though the dynein and kinesin motors drive the endosomes very rapidly (microm/s), frequent saltatory motions of single endosomes may induce the very slow net centripetal motion (microm/h).The application of adrenocorticotropic hormone (ACTH) resulted in a facilitation of the centripetal motion of endosomes, resulting in the establishment of the concentration of endosomes around cell nuclei within 1 h.

Regulation of the steroidogenic acute regulatory protein gene expression: present and future perspectives

Steroid hormones are synthesized in the adrenal gland, gonads, placenta and brain and are critical for normal reproductive function and bodily homeostasis. The steroidogenic acute regulatory (StAR) protein regulates the rate-limiting step in steroid biosynthesis, i.e. the delivery of cholesterol from the outer to the inner mitochondrial membrane. The expression of the StAR protein is predominantly regulated by cAMP-dependent mechanisms in the adrenal and gonads. Whereas StAR plays an indispensable role in the regulation of steroid biosynthesis, a complete understanding of the regulation of its expression and function in steroidogenesis is not available. It has become clear that the regulation of StAR gene expression is a complex process that involves the interaction of a diversity of hormones and multiple signaling pathways that coordinate the cooperation and interaction of transcriptional machinery, as well as a number of post-transcriptional mechanisms that govern mRNA and protein expression. However, information is lacking on how the StAR gene is regulated in vivo such that it is expressed at appropriate times during development and is confined to the steroidogenic cells. Thus, it is not surprising that the precise mechanism involved in the regulation of StAR gene has not yet been established, which is the key to understanding the regulation of steroidogenesis in the context of both male and female development and function.

Lys-gamma3-MSH: a global regulator of hormone sensitive lipase activity?

Gamma-melanocyte stimulating hormone (gamma-MSH) is a peptide derived from the ACTH precursor, pro-opiomelanocortin (POMC), and belongs to a family of peptides called the melanocortins that also comprises alpha- and beta-MSH. Although conserved in tetrapods, the biological role of gamma-MSH remains largely undefined. It has been demonstrated previously that gamma-MSH is involved in the regulating the activity of hormone sensitive lipase (HSL) activity in the adrenal and more recently, in the adipocyte. It has been shown also to have effects on the cardiovascular and renal systems. This short review will provide a brief overview of the role of gamma-MSH in the adrenal and the more recent report that it can also regulate HSL function in the adipocyte. We also present some preliminary data purporting a direct role for Lys-gamma(3)-MSH in the regulation of HSL phosphorylation in the heart. Taken together these data suggest that gamma-MSH peptides might play a more widespread role in lipid and cholesterol utilization.

Multiple roles for lipids in the Hedgehog signalling pathway

The identification of endogenous sterol derivatives that modulate the Hedgehog (Hh) signalling pathway has begun to suggest testable hypotheses for the cellular biological functions of Patched, and for the lipoprotein association of Hh. Progress in the field of intracellular sterol trafficking has emphasized how tightly the distribution of intracellular sterol is controlled, and suggests that the synthesis of sterol derivatives can be influenced by specific sterol-delivery pathways. The combination of this field with Hh studies will rapidly give us a more sophisticated understanding of both the Hh signal-transduction pathway and the cell biology of sterol metabolism.

Hormone-sensitive lipase is involved in hepatic cholesteryl ester hydrolysis

Hormone-sensitive lipase (HSL) regulates the hydrolysis of acylglycerol and cholesteryl ester (CE) in various organs, including adipose tissues. However, the hepatic expression level of HSL has been reported to be almost negligible. In the present study, we found that mice lacking both leptin and HSL (Lep(ob/ob)/HSL(-/-)) showed massive accumulation of CE in the liver compared with Lep(ob/ob)/HSL(+/+) mice, while triacylglycerol (TG) accumulation was modest. Similarly, feeding with a high-cholesterol diet induced hepatic CE accumulation in HSL(-/-) mice. Supporting these observations, we detected significant expression of protein as well as mRNA of HSL in the liver. HSL(-/-) mice showed reduced activity of CE hydrolase, but not of TG lipase, in the liver compared with wild-type mice. Furthermore, we confirmed the expression of HSL in viable parenchymal cells isolated from wild-type mice. The hepatocytes from HSL(-/-) mice showed reduced activity of CE hydrolase and contained more CE than those from HSL(+/+) mice even without the incubation with lipoproteins. Incubation with LDL further augmented the accumulation of CE in the HSL-deficient hepatocytes. From these results, we conclude that HSL is involved in the hydrolysis of CE in hepatocyes.

Alterations in the testis of hormone sensitive lipase-deficient mice is associated with decreased sperm counts, sperm motility, and fertility

Hormone-sensitive lipase (HSL, Lipe, E.C.3.1.1.3) functions as a triglyceride and cholesteryl esterase, supplying fatty acids, and cholesterol to cells. Gene-targeted HSL-deficient (HSL(-/-)) mice reveal abnormal spermatids and are infertile at 24 weeks after birth. The purpose of this study was to follow the evolution of spermatid abnormalities as HSL(-/-) mice age, characterize sperm motility in older HSL(-/-) mice, and determine if mice expressing a human testicular HSL transgene (HSL(-/-)ttg) produce normal motile sperm. In situ hybridization indicated that HSL is expressed exclusively in steps 5-16 spermatids, but not in Sertoli cells. In HSL(-/-) mice, abnormalities were evident in step 16 spermatids at 5 weeks after birth, with defects progressively increasing in spermatids with age. The defects included multinucleation of spermatids, abnormal shapes and a reduction of elongating spermatids. In older HSL(-/-) mice, sperm counts appeared reduced by 42%, but this value was lower because samples were compromised by the presence of small degenerating germ cells in addition to sperm, both of which appeared of similar size and density. Sperm motility was dramatically reduced with only 11% classified as motile in HSL(-/-) mice compared to 76-78% of sperm in wild-type and HSL(-/-)ttg mice. Sperm morphology, counts, and motility were normal in HSL(-/-)ttg mice, as was their fertility. Collectively, the data indicate that HSL deficiency results in abnormal spermatid development with defects arising at 5 weeks of age and progressively increasing at later ages. HSL(-/-) mice also show a dramatic reduction in sperm counts and motility and are infertile.

Transcriptional regulation of adrenocortical steroidogenic gene expression

By serving as ligands for nuclear and plasma membrane receptors, steroid hormones are key regulators of a diverse array of physiological processes. These hormones are synthesized from cholesterol in tissues such as the adrenal cortex, ovaries, testes, and placenta. Because steroid hormones control the expression of numerous genes, steroidogenic cells utilize multiple mechanisms that ensure tight control of the synthesis of these molecules. This review will give an overview of the molecular mechanisms by which the expression of steroidogenic genes is regulated in the human adrenal cortex.

Adrenal cholesterol utilization

There are multiple systems for cellular cholesterol delivery for steroidogenesis, including uptake of lipoprotein-derived cholesterol via LDL receptor mediated endocytic pathways and SR-BI mediated "selective" pathways, as well as from endogenous cholesterol synthesis and the mobilization of stored cholesteryl esters. The vast majority of lipoprotein-derived cholesterol utilized for murine adrenal steroidogenesis is obtained via SR-BI mediated "selective" uptake of cholesteryl esters. Hormone-sensitive lipase (HSL) is responsible for neutral cholesteryl ester hydrolase activity in the adrenal and is critical for hydrolyzing stored cholesteryl esters, as well as cholesteryl esters that are selectively delivered from lipoproteins via SR-BI. Marked defects in steroid production are observed in adrenal cells from HSL knockout mice, due to an inability to process and utilize cholesteryl esters selectively derived from lipoproteins. Although the LDL receptor is responsible for receptor-mediated endocytic delivery of cholesteryl esters, adrenal steroid hormone production is normal in mice lacking LDL receptors.

Role of gamma-MSH peptides in the regulation of adrenal steroidogenesis

Alpha-, beta- and gamma-melanocyte stimulating hormones (MSHs) are peptides derived from the ACTH precursor, pro-opiomelanocortin. All three peptides have been highly conserved throughout evolution but their exact biological function in mammals is still largely obscure. In recent years, there has been a surge of interest in alpha-MSH and its role in the regulation of feeding. Gamma-MSH by contrast has been shown to be involved in the regulation of adrenal steroidogenesis and also has effects on the cardiovascular and renal systems. This review will provide an overview of the role that gamma-MSH peptides play in the regulation of adrenal steroidogenesis.

Specific inhibition of hormone-sensitive lipase improves lipid profile while reducing plasma glucose

Elevation of plasma free fatty acids has been linked with insulin resistance and diabetes. Inhibition of lipolysis may provide a mechanism to decrease plasma fatty acids, thereby improving insulin sensitivity. Hormone-sensitive lipase (HSL) is a critical enzyme involved in the hormonally regulated release of fatty acids and glycerol from adipocyte lipid stores, and its inhibition may thus improve insulin sensitivity and blood glucose handling in type 2 diabetes. In rat adipocytes, forskolin-activated lipolysis was blocked by in vitro addition of a potent and selective HSL inhibitor or by prior treatment of the animals themselves. Antilipolytic effects also were demonstrated in overnight-fasted mice, rats, and dogs with species-dependent effects on plasma free fatty acid levels but with similar reductions in plasma glycerol being observed in all species. Inhibition of HSL also reduced hyperglycemia in streptozotocin-induced diabetic rats. The data support a connection between adipose tissue lipolysis and plasma glucose levels.

Human hormone-sensitive lipase (HSL): expression in white fat corrects the white adipose phenotype of HSL-deficient mice

In white adipose tissue (WAT), hormone-sensitive lipase (HSL) can mediate lipolysis, a central pathway in obesity and diabetes. Gene-targeted HSL-deficient (HSL-/-) mice with no detectable HSL peptide or activity (measured as cholesteryl esterase) have WAT abnormalities, including low mass, marked heterogeneity of cell diameter, increased diacylglycerol content, and low beta-adrenergic stimulation of adipocyte lipolysis. Three transgenic mouse strains preferentially expressing human HSL in WAT were bred to a HSL-/- background. One, HSL-/- N, expresses normal human HSL (41.3 +/- 9.1% of normal activity); two express a serine-to-alanine mutant (S554A) initially hypothesized to be constitutively active: HSL-/- ML, 50.3 +/- 12.3% of normal, and HSL-/- MH, 69.8 +/- 15.8% of normal. In WAT, HSL-/- N mice resembled HSL+/+ controls in WAT mass, histology, diacylglyceride content, and lipolytic response to beta-adrenergic agents. In contrast, HSL-/- ML and HSL-/- MH mice resembled nontransgenic HSL-/- mice, except that diacylglycerol content and perirenal and inguinal WAT masses approached normal in HSL-/- MH mice. Therefore, 1) WAT expression of normal human HSL markedly improves HSL-/- WAT biochemically, physiologically, and morphologically; 2) similar levels of S554A HSL have a low physiological effect despite being active in vitro; and 3) diacylglycerol accumulation is not essential for the development of the characteristic WAT pathology of HSL-/- mice.

Novel signaling stimulated by arsenite increases cholesterol metabolism through increases in unphosphorylated steroidogenic acute regulatory (StAR) protein

Cholesterol metabolism to pregnenolone is dependent on the steroidogenic acute regulatory protein (StAR), which activates mitochondrial transfer of cholesterol to cytochrome CYP450scc. In mouse Y-1 adrenal cells and testis MA10 cells stimulation by 8-Bromo-cAMP (Br-cAMP) is augmented by a novel signaling initiated by low concentrations of arsenite (3-20 microM) and anisomycin (0.2 microM), a more selective stress agent. Each elevated StAR mRNA (three-fold after 6 h treatment) even with simultaneous stimulation by Br-cAMP. Arsenite produced parallel increases in StAR protein expression and cholesterol metabolism, but not for P450scc-mediated metabolism of 20alpha-hydroxycholesterol. Although arsenite and anisomycin each stimulated the phosphorylation of p38, the p38 inhibitor SB203580 (SB) produced additive increases in StAR expression. Cholesterol metabolism increased in parallel but without the increased StAR protein phosphorylation produced by Br-cAMP. Arsenite and anisomycin each elevated StAR mRNA but preferentially increased the 3.5 kb form relative to the 1.6 kb form. Arsenite and anisomycin each enhanced the stability of the more labile 3.5 kb mRNA which contains AU-rich elements that control mRNA stability. Although there were increases in both forms of StAR mRNA, arsenite did not stimulate a StAR promoter-reporter that exhibited a typical three-fold response to Br-cAMP. Arsenite and anisomycin may therefore activate a novel SB-independent MAP kinase which in part increases StAR expression through stabilizing the 3.5 kb mRNA but which may also activate a mechanism that by-passes transcription factors detected by the reporter. SB stimulation, which was completely blocked by a MEK inhibitor, was also selective towards the 3.5 kb StAR mRNA suggesting a second pathway for mRNA stabilization. These activations contrast with inhibition of StAR expression by arsenite at higher concentrations or longer incubation times.

Expression of human hormone-sensitive lipase (HSL) in postmeiotic germ cells confers normal fertility to HSL-deficient mice

Hormone-sensitive lipase (HSL, Lipe, E.C.3.1.1.3) is a multifunctional fatty acyl esterase that is essential for male fertility and spermatogenesis and that also plays important roles in the function of adipocytes, pancreatic beta-cells, and adrenal cortical cells. Gene-targeted HSL-deficient (HSL-/-) male mice are infertile, have a 2-fold reduction in testicular mass, a 2-fold elevation of the ratio of esterified to free cholesterol in testis, and unique morphological abnormalities in round and elongating spermatids. Postmeiotic germ cells in the testis express a specific HSL isoform. We created transgenic mice expressing a normal human testicular HSL cDNA from the mouse protamine-1 promoter, which mediates expression specifically in postmeiotic germ cells. Testicular cholesteryl esterase activity was undetectable in HSL-/- mice, but in HSL-/- males expressing the testicular transgene, activity was 2-fold greater than normal. HSL transgene mRNA became detectable in testes between 19 and 25 days of age, coinciding with the first wave of postmeiotic transcription in round spermatids. In contrast to nontransgenic HSL-/- mice, HSL-/- males expressing the testicular transgene were normal with respect to fertility, testicular mass, testicular esterified/free cholesterol ratio, and testicular histology. Their cauda epididymides contained abundant, normal-appearing spermatozoa. We conclude that human testicular HSL is functional in mouse testis and that the mechanism of infertility in HSL-deficient males is cell autonomous and resides in postmeiotic germ cells, because HSL expression in these cells is in itself sufficient to restore normal fertility.

Hormone-sensitive lipase-independent adipocyte lipolysis during beta-adrenergic stimulation, fasting, and dietary fat loading

In white adipose tissue, lipolysis can occur by hormone-sensitive lipase (HSL)-dependent or HSL-independent pathways. To study HSL-independent lipolysis, we placed HSL-deficient mice in conditions of increased fatty acid flux: beta-adrenergic stimulation, fasting, and dietary fat loading. Intraperitoneal administration of the beta(3)-adrenergic agonist CL-316243 caused a greater increase in nonesterified fatty acid level in controls (0.33 +/- 0.05 mmol/l) than in HSL(-/-) mice (0.12 +/- 0.01 mmol/l, P < 0.01). Similarly, in isolated adipocytes, lipolytic response to CL-316243 was greatly reduced in HSL(-/-) mice compared with controls. Fasting for <or=48 h produced normal mobilization and oxidation of fatty acids in HSL(-/-) mice, as judged by similar values of respiratory quotient and oxygen consumption as in HSL(+/+) controls. In isolated adipocytes, lipolysis in the absence of beta-adrenergic stimulation was 1.9-fold greater in HSL(-/-) than in HSL(+/+) cells (P < 0.05), increasing to 6.5-fold after fasting (P < 0.01). After 6 wk of a fat-rich diet containing 31.5% of energy as lipid, weight gain of HSL(-/-) mice was 4.4-fold less than in HSL(+/+) mice (P < 0.01), and total abdominal fat mass was 5.2-fold lower in HSL(-/-) than in HSL(+/+) mice (P < 0.01). In white adipose tissue, HSL is essential for normal acute beta-adrenergic-stimulated lipolysis and permits normal triglyceride storage capacity in response to dietary fat loading. However, HSL-independent lipolysis can markedly increase during fasting, both in isolated adipocytes and in intact mice, and can mediate a normal flux of fatty acids during fasting.

In vitro SAR of (5-(2H)-isoxazolonyl) ureas, potent inhibitors of hormone-sensitive lipase

A series of (5-(2H)-isoxazolonyl) ureas were developed as nanomolar inhibitors of hormone-sensitive lipase, an enzyme of potential importance in the treatment of diabetes.

Carboxyl Ester Lipase Cofractionates with Scavenger Receptor BI in Hepatocyte Lipid Rafts and Enhances Selective Uptake and Hydrolysis of Cholesteryl Esters from HDL3

Cholesteryl esters are selectively removed from high density lipoproteins by hepatocytes and steroidogenic cells through a process mediated by scavenger receptor BI. In the liver this cholesterol is secreted into bile, primarily as free cholesterol. Previous work showed that carboxyl ester lipase enhanced selective uptake of cholesteryl ether from high density lipoprotein by an unknown mechanism. Experiments were performed to determine whether carboxyl ester lipase plays a role in scavenger receptor BI-mediated selective uptake. When added to cultures of HepG2 cells, carboxyl ester lipase cofractionated with scavenger receptor BI and [(3)H]cholesteryl ether-labeled high density lipoprotein in lipid raft fractions of cell homogenates. Confocal microscopy of immunostained carboxyl ester lipase and scavenger receptor BI showed a close association of these proteins in HepG2 cells. The enzyme and receptor also cofractionated from homogenates of mouse liver using two different fractionation methods. Antibodies that block scavenger receptor BI function prevented carboxyl ester lipase stimulation of selective uptake in primary hepatocytes from carboxyl ester lipase knockout mice. Heparin blockage of cell-surface proteoglycans also prevented carboxyl ester lipase stimulation of cholesteryl ester uptake by HepG2 cells. Inhibition of carboxyl ester lipase activity in HepG2 cells reduced hydrolysis of high density lipoprotein-cholesteryl esters approximately 40%. In vivo, hydrolysis was similarly reduced in lipid rafts from the livers of carboxyl ester lipase-null mice compared with control animals. Primary hepatocytes from these mice yielded similar results. The data suggest that carboxyl ester lipase plays a physiological role in hepatic selective uptake and metabolism of high density lipoprotein cholesteryl esters by direct and indirect interactions with the scavenger receptor BI pathway.

Metabolomic analysis of adrenal lipids during hypoxia in the neonatal rat: implications in steroidogenesis

The nursing rat pup exposed to hypoxia from birth exhibits ACTH-independent increases in corticosterone and renin/ANG II-independent increases in aldosterone. These increases are accompanied by significant elevation of plasma lipid concentrations in the hypoxic neonates. The purpose of the present study was to compare changes in the concentrations of specific fatty acid metabolites and lipid classes in serum and adrenal tissue from normoxic and hypoxic rat pups. We hypothesized that lipid alterations resulting from hypoxia may partly explain increases in steroidogenesis. Rats were exposed to normoxia or hypoxia from birth, and pooled serum and adrenal tissue from 7-day-old pups were subjected to metabolomic analyses. Hypoxia resulted in specific and significant changes in a number of fatty acid metabolites in both serum and the adrenal. Hypoxia increased the concentrations of oleic (18:1 n-9), eicosapentaenoic (EPA; 20:5 n-3), and arachidonic (20:4 n-6) acids in the triacylglyceride fraction of serum and decreased oleic and EPA concentrations in the cholesterol ester fraction. In the adrenal, hypoxia caused an increase in several n-6 fatty acids in the triacylglyceride fraction, including linoleic (18:2 n-6) and arachidonic acid. There was also an increase in the concentration of alpha-linolenic acid (18:3 n-3) in the triacylglyceride fraction of the hypoxic adrenal, along with an increase in linoleic acid concentration in the diacylglyceride fraction. We propose that specific changes in lipid metabolism in the adrenal, as a result of hypoxia, may partly explain the increased steroidogenesis previously observed. The mechanism responsible may involve alterations in cellular signaling and/or mitochondrial function. These cellular changes may be a mechanism by which the neonate can increase circulating adrenal steroids necessary for survival, therefore bypassing a relative insensitivity to normal stimuli.

Absence of hormone-sensitive lipase inhibits obesity and adipogenesis in Lep ob/ob mice

Hormone-sensitive lipase (HSL) plays a crucial role in the hydrolysis of triacylglycerol and cholesteryl ester in various tissues including adipose tissues. To explore the role of HSL in the metabolism of fat and carbohydrate, we have generated mice lacking both leptin and HSL (Lep(ob/ob)/HSL(-/-)) by cross-breeding HSL(-/-) mice with genetically obese Lep(ob/ob) mice. Unexpectedly, Lep(ob/ob)/HSL(-/-) mice ate less food, gained less weight, and had lower adiposity than Lep(ob/ob)/HSL(+/+) mice. Lep(ob/ob)/HSL(-/-) mice had massive accumulation of preadipocytes in white adipose tissues with increased expression of preadipocyte-specific genes (CAAT/enhancer-binding protein beta and adipose differentiation-related protein) and decreased expression of genes characteristic of mature adipocytes (CCAAT/enhancer-binding protein alpha, peroxisome proliferator activator receptor gamma, and adipocyte determination and differentiation factor 1/sterol regulatory element-binding protein-1). Consistent with the reduced food intake, hypothalamic expression of neuropeptide Y and agouti-related peptide was decreased. Since HSL is expressed in hypothalamus, we speculate that defective generation of free fatty acids in the hypothalamus due to the absence of HSL mediates the altered expression of these orexigenic neuropeptides. Thus, deficiency of both leptin and HSL has unmasked novel roles of HSL in adipogenesis as well as in feeding behavior.