Ribonucleic acid expression of the CLA-1 gene, a human homolog to mouse high density lipoprotein receptor SR-BI, in human adrenal tumors and cultured adrenal cells

Scavenger receptor B type 1: expression, molecular regulation, and cholesterol transport function

Cholesterol is required for maintenance of plasma membrane fluidity and integrity and for many cellular functions. Cellular cholesterol can be obtained from lipoproteins in a selective pathway of HDL-cholesteryl ester (CE) uptake without parallel apolipoprotein uptake. Scavenger receptor B type 1 (SR-B1) is a cell surface HDL receptor that mediates HDL-CE uptake. It is most abundantly expressed in liver, where it provides cholesterol for bile acid synthesis, and in steroidogenic tissues, where it delivers cholesterol needed for storage or steroidogenesis in rodents. SR-B1 transcription is regulated by trophic hormones in the adrenal gland, ovary, and testis; in the liver and elsewhere, SR-B1 is subject to posttranscriptional and posttranslational regulation. SR-B1 operates in several metabolic processes and contributes to pathogenesis of atherosclerosis, inflammation, hepatitis C virus infection, and other conditions. Here, we summarize characteristics of the selective uptake pathway and involvement of microvillar channels as facilitators of selective HDL-CE uptake. We also present the potential mechanisms of SR-B1-mediated selective cholesterol transport; the transcriptional, posttranscriptional, and posttranslational regulation of SR-B1; and the impact of gene variants on expression and function of human SR-B1. A better understanding of this unique pathway and SR-B1's role may yield improved therapies for a wide variety of conditions.

SR-B1: A Unique Multifunctional Receptor for Cholesterol Influx and Efflux

The scavenger receptor, class B type 1 (SR-B1), is a multiligand membrane receptor protein that functions as a physiologically relevant high-density lipoprotein (HDL) receptor whose primary role is to mediate selective uptake or influx of HDL-derived cholesteryl esters into cells and tissues. SR-B1 also facilitates the efflux of cholesterol from peripheral tissues, including macrophages, back to liver. As a regulator of plasma membrane cholesterol content, SR-B1 promotes the uptake of lipid soluble vitamins as well as viral entry into host cells. These collective functions of SR-B1 ultimately affect programmed cell death, female fertility, platelet function, vasculature inflammation, and diet-induced atherosclerosis and myocardial infarction. SR-B1 has also been identified as a potential marker for cancer diagnosis and prognosis. Finally, the SR-B1-linked selective HDL-cholesteryl ester uptake pathway is now being evaluated as a gateway for the delivery of therapeutic and diagnostic agents. In this review, we focus on the regulation and functional significance of SR-B1 in mediating cholesterol movement into and out of cells.

SR-BI: Linking Cholesterol and Lipoprotein Metabolism with Breast and Prostate Cancer

Studies have demonstrated the significant role of cholesterol and lipoprotein metabolism in the progression of cancer. The SCARB1 gene encodes the scavenger receptor class B type I (SR-BI), which is an 82-kDa glycoprotein with two transmembrane domains separated by a large extracellular loop. SR-BI plays an important role in the regulation of cholesterol exchange between cells and high-density lipoproteins. Accordingly, hepatic SR-BI has been shown to play an essential role in the regulation of the reverse cholesterol transport pathway, which promotes the removal and excretion of excess body cholesterol. In the context of atherosclerosis, SR-BI has been implicated in the regulation of intracellular signaling, lipid accumulation, foam cell formation, and cellular apoptosis. Furthermore, since lipid metabolism is a relevant target for cancer treatment, recent studies have focused on examining the role of SR-BI in this pathology. While signaling pathways have initially been explored in non-tumoral cells, studies with cancer cells have now demonstrated SR-BI's function in tumor progression. In this review, we will discuss the role of SR-BI during tumor development and malignant progression. In addition, we will provide insights into the transcriptional and post-transcriptional regulation of the SCARB1 gene. Overall, studying the role of SR-BI in tumor development and progression should allow us to gain useful information for the development of new therapeutic strategies.

ACTH Regulation of Adrenal SR-B1

The adrenal gland is one of the prominent sites for steroid hormone synthesis. Lipoprotein-derived cholesterol esters (CEs) delivered via SR-B1 constitute the dominant source of cholesterol for steroidogenesis, particularly in rodents. Adrenocorticotropic hormone (ACTH) stimulates steroidogenesis through downstream actions on multiple components involved in steroidogenesis. Both acute and chronic ACTH treatments can modulate SR-B1 function, including its transcription, posttranscriptional stability, phosphorylation and dimerization status, as well as the interaction with other protein partners, all of which result in changes in the ability of SR-B1 to mediate HDL-CE uptake and the supply of cholesterol for conversion to steroids. Here, we provide a review of the recent findings on the regulation of adrenal SR-B1 function by ACTH.

Aldosterone production in human adrenocortical cells is stimulated by high-density lipoprotein 2 (HDL2) through increased expression of aldosterone synthase (CYP11B2)

Adrenal aldosterone production is regulated by physiological agonists at the level of early and late rate-limiting steps. Numerous studies have focused on the role of lipoproteins including high-density lipoprotein (HDL) as cholesterol providers in this process; however, recent research suggests that HDL can also act as a signaling molecule. Herein, we used the human H295R adrenocortical cell model to study the effects of HDL on adrenal aldosterone production and CYP11B2 expression. HDL, especially HDL2, stimulated aldosterone synthesis by increasing expression of CYP11B2. HDL treatment increased CYP11B2 mRNA in both a concentration- and time-dependent manner, with a maximal 19-fold increase (24 h, 250 μg/ml of HDL). Effects of HDL on CYP11B2 were not additive with natural agonists including angiotensin II or K(+). HDL effects were likely mediated by a calcium signaling cascade, because a calcium channel blocker and a calmodulin kinase inhibitor abolished the CYP11B2-stimulating effects. Of the two subfractions of HDL, HDL2 was more potent than HDL3 in stimulating aldosterone and CYP11B2. Further studies are needed to identify the active components of HDL, which regulate aldosterone production.

Adrenal-exhaustion syndrome in patients with liver disease

OBJECTIVE

Adrenal failure is emerging as an important cause of excess morbidity and mortality in critically ill patients. We have recently reported the high incidence of adrenal failure in patients with liver disease, the so called "hepato-adrenal syndrome." It has been noted that patients who on initial testing may have "normal" adrenal function subsequently progress to overt adrenal failure, the adrenal-exhaustion syndrome. The goal of this study was to further characterize this syndrome.

DESIGN

Retrospective review of the "Hepatic Cortisol Research and Adrenal Pathophysiology Study" database.

SETTING

Liver transplant ICU of a tertiary care university hospital.

PATIENTS AND METHODS

Patients who on initial testing had "normal" adrenal function were followed, and adrenal function testing was repeated in those who failed to improve. The patients were grouped as follows: (1) patients who developed adrenal failure on follow-up testing; and (2) patients who had normal adrenal function during their ICU stay. The incidence and risk factors for the development of adrenal-exhaustion syndrome were determined.

MEASUREMENTS AND RESULTS

The study cohort consisted of 221 patients, of whom 120 (54%) were diagnosed with adrenal insufficiency on initial diagnostic testing and were excluded from further analysis. The remaining 101 patients comprised those who made up the group of interest. On follow-up, 16 (16%) of these developed adrenal failure a mean of 3 days after initial testing. The only factor that predicted the development of adrenal-exhaustion syndrome was a low HDL level (p<0.001).

CONCLUSION

This study demonstrates that adrenal failure is a dynamic process and that repeat adrenal function testing is indicated in patients who remain hemodynamically unstable or fail to improve with aggressive supportive treatment. Low HDL levels may be pathogenetically linked to the development of adrenal failure.

The hepatoadrenal syndrome: a common yet unrecognized clinical condition

OBJECTIVE

Adrenal failure is common in critically ill patients, particularly those with sepsis. As liver failure and sepsis are both associated with increased circulating levels of endotoxin and proinflammatory mediators and reduced levels of apoprotein-1/high-density lipoprotein, we postulated that adrenal failure may be common in patients with liver disease.

DESIGN

Clinical study.

SETTING

Liver transplant intensive care unit.

PATIENTS

The study cohort included 340 patients with liver disease.

INTERVENTIONS

Based on preliminary observational data, all patients admitted to our 28-bed liver transplant intensive care unit (LTICU) undergo adrenal function testing. An honest broker system was used to extract clinical, hemodynamic, medication, and laboratory data on patients admitted to the LTICU from March 2002 to March 2004. A random (stress) cortisol level <20 microg/dL in a highly stressed patient (respiratory failure, hypotension) was used to diagnose adrenal insufficiency. In all other patients, a random cortisol level <15 microg/dL or a 30-min level <20 microg/dL post-low-dose (1 microg) cosyntropin was considered diagnostic of adrenal insufficiency. Patients were grouped as follows: a) chronic liver failure; b) fulminant hepatic failure; c) patients immediately status post-orthotopic liver transplantation receiving a steroid-free protocol of immunosuppression; and d) patients status post-remote liver transplant (>/=6 months). The decision to treat patients with stress doses of hydrocortisone was at the discretion of the treating intensivist and transplant surgeon.

MEASUREMENTS AND MAIN RESULTS

Two-hundred and forty-five (72%) patients met our criteria for adrenal insufficiency (the hepatoadrenal syndrome). Eight (33%) patients with fulminant hepatic failure, 97 (66%) patients with chronic liver disease, 31(61%) patients with a remote history of liver transplantation, and 109 (92%) patients who had undergone liver transplantation under steroid-free immunosuppression were diagnosed with adrenal insufficiency. The high-density lipoprotein level at the time of adrenal testing was the only variable predictive of adrenal insufficiency (p < .0001). In vasopressor-dependent patients with adrenal insufficiency, treatment with hydrocortisone was associated with a significant reduction (p = .02) in the dose of norepinephrine at 24 hrs, whereas the dose of norepinephrine was significantly higher (p = .04) in those patients with adrenal failure not treated with hydrocortisone. In vasopressor-dependent patients without adrenal insufficiency, treatment with hydrocortisone did not affect vasopressor dose at 24 hrs. One hundred and forty-one patients (26.4%) died during their hospitalization. The baseline serum cortisol was 18.8 +/- 16.2 microg/dL in the nonsurvivors compared with 13.0 +/- 11.8 microg/dL in the survivors (p < .001). Of those patients with adrenal failure who were treated with glucocorticoids, the mortality rate was 26% compared with 46% (p = .002) in those who were not treated. In those patients receiving vasopressor agents at the time of adrenal testing, the baseline cortisol was 10.0 +/- 4.8 microg/dL in those with adrenal insufficiency compared with 35.6 +/- 21.2 microg/dL in those with normal adrenal function. Vasopressor-dependent patients who did not have adrenal failure had a mortality rate of 75%.

CONCLUSIONS

Patients with liver failure and patients post-liver transplantation have an exceedingly high incidence of adrenal failure, which may be pathophysiologically related to low levels of high-density lipoprotein. Treatment of patients with adrenal failure may improve outcome. High baseline serum cortisol levels may be a maker of disease severity and portend a poor prognosis.

Influence of the high density lipoprotein receptor SR-BI on reproductive and cardiovascular pathophysiology

The high density lipoprotein (HDL) receptor SR-BI (scavenger receptor class B type I) mediates the selective uptake of plasma HDL cholesterol by the liver and steroidogenic tissues. As a consequence, SR-BI can influence plasma HDL cholesterol levels, HDL structure, biliary cholesterol concentrations, and the uptake, storage, and utilization of cholesterol by steroid hormone-producing cells. Here we used homozygous null SR-BI knockout mice to show that SR-BI is required for maintaining normal biliary cholesterol levels, oocyte development, and female fertility. We also used SR-BI/apolipoprotein E double homozygous knockout mice to show that SR-BI can protect against early-onset atherosclerosis. Although the mechanisms underlying the effects of SR-BI loss on reproduction and atherosclerosis have not been established, potential causes include changes in (i) plasma lipoprotein levels and/or structure, (ii) cholesterol flux into or out of peripheral tissues (ovary, aortic wall), and (iii) reverse cholesterol transport, as indicated by the significant reduction of gallbladder bile cholesterol levels in SR-BI and SR-BI/apolipoprotein E double knockout mice relative to controls. If SR-BI has similar activities in humans, it may become an attractive target for therapeutic intervention in a variety of diseases.