The many faces of pheochromocytoma.

OBJECTIVE: To recognize and manage pheochromocytomas in unusual settings. METHODS: Three case reports are presented with clinical, biochemical, imaging, and operative findings. The pitfalls in diagnosis of pheochromocytomas and management are addressed. RESULTS: We begin with a 27-yr-old gravida 2, para 1 Caucasian woman with unexplained tachycardia and hypertension during a routine pre-natal visit at 30 weeks estimated gestational age. Urinary studies revealed elevated catecholamines. Magnetic resonance imaging localized a 6.6-cm right adrenal mass with features consistent with a pheochromocytoma. She was medically managed with phenoxybenzamine and propranolol until 35 weeks, after which she underwent a combined Cesarean section, and open right adrenalectomy. Another patient, a 36-yr-old African-American woman presented to a hospital in cardiac arrest, with elevated serum troponins, and underwent cardiac catheterization, which revealed normal coronary arteries. A computed tomography (CT) scan revealed a left adrenal mass and CT-guided biopsy was consistent with a pheochromocytoma, although prior studies were negative. Finally, we present a 49-yr-old Caucasian woman who had a right adrenalectomy 10 yr prior and presented to the clinic with fluctuating blood pressures, headaches, and palpitations. Further testing revealed she had a recurrent metastatic pheochromocytoma. The challenges behind treating these patients are further explored. CONCLUSION: Antenatal diagnosis of pheochromocytoma, though challenging, is associated with lower maternal and fetal morbidity and mortality. The differential diagnosis for cardiac arrest in the presence of normal coronary arteries should include a pheochromocytoma. Finally, treatment with iodinated metaiodobenzylguanidine may be a therapeutic option for those patients with metastatic pheochromocytomas.

Scavenger receptor, class B, type I-dependent stimulation of cholesterol esterification by high density lipoproteins, low density lipoproteins, and nonlipoprotein cholesterol.

Scavenger receptor, class B, type I (SR-BI) is a cell surface glycoprotein that mediates selective uptake and efflux of sterols from high density lipoproteins (HDL) to cells. A Chinese hamster ovary cell line that is deficient in functional LDL receptors, but has high expression levels of recombinant SR-BI (ldlA7-SR-BI), was used to examine the effect of SR-BI on the trafficking of sterols between lipoproteins and cells. To monitor the fate of sterols transported by SR-BI into cells, we measured the incorporation of [14C]oleate into cholesterol esters by acyl-CoA:cholesteryl acyltransferase in the endoplasmic reticulum. We show that incubation of ldlA7-SRBI cells with either LDL or HDL resulted in an equally dramatic increase in the formation of [14C]oleate-labeled cholesterol esters. The lipoprotein-stimulated, SR-BI-dependent increase in cholesterol esterification was inhibited by chloroquine. The uptake of sterols and their incorporation into cholesterol esters by SR-BI from LDL was largely a selective process. The addition of free cholesterol to ldlA7-SRBI cells also stimulated cholesterol ester formation in a chloroquine-sensitive fashion. We also show that SR-BI mediates the efflux of endogenously synthesized sterols from the cell membrane. From these studies we conclude that, in the absence of the LDL receptor, overexpression of SR-BI can mediate significant transport of sterols between lipoproteins and the endoplasmic reticulum of cells.

Transport of maternal LDL and HDL to the fetal membranes and placenta of the Golden Syrian hamster is mediated by receptor-dependent and receptor-independent processes.

Maternal lipoproteins provide nutrients to the fetus via the placenta, yolk sac, and uterine membrane plus decidua. To determine the transport processes that are responsible for the removal of lipoproteins from the maternal circulation, we measured the clearance rates of maternal LDL and HDL in vivo, as well as the tissue distribution of expression of the LDL receptor, glycoprotein 330 (gp330) and the newly described HDL receptor, SR-BI, in the placenta, yolk sac, and uterine membrane plus decidua at mid- and late-gestation of the hamster. In mid-gestation (day 10.5), LDL clearance rates of the placenta and yolk sac were similar to those in the liver (approximately 100 microl/h per g) and higher than those in the decidua (18 +/- 3 microl/h per g). Clearance rates for HDL-apoA-I and HDL-cholesteryl ether were similar to those of LDL in the placenta and decidua whereas rates in the yolk sac were dramatically higher (>1700 microl/h per g). Additionally, albumin was cleared in the placenta and decidua at approximately 16 microl/h per g whereas the yolk sac cleared the protein at much higher rates (196 +/- 22 microl/h per g). Low levels of LDL receptor were detected by immunoblot analysis in the placenta with trace amounts in the yolk sac. Gp330 and SR-BI were both barely detectable in the placenta but were expressed at high levels in the yolk sac. As gestation progressed to day 14.5, LDL and HDL clearance rates decreased in all three tissues; immunodetectable LDL receptor decreased in the placenta whereas the expression of gp330 and SR-BI in the placenta and yolk sac remained relatively constant. These data suggest that the clearance of maternal lipoproteins by the placenta, yolk sac, and decidua are mediated by receptor-mediated as well as receptor-independent processes.

Structure and localization of the human gene encoding SR-BI/CLA-1. Evidence for transcriptional control by steroidogenic factor 1.

The scavenger receptor, class B, type 1 receptor (SR-BI) mediates the selective transport of lipids from high density lipoprotein to cells. We describe the structure and subchromosomal location of human SR-BI and provide evidence that it is regulated by the transcription factor, steroidogenic factor 1 (SF-1). SR-BI resides on chromosome 12q24.2-qter, spans approximately 75 kilobase pairs, and contains 13 exons. RNA blot analysis of human tissues reveals an expression pattern similar to that described previously for rodents with the highest levels of mRNA in the adrenal gland, ovary, and liver. Unlike rodents, human SR-BI was expressed at high levels in the placenta. The transcription start site for SR-BI was mapped, and DNA sequence analysis revealed a binding site for SF-1 in the proximal 5'-flanking sequence. SF-1, an orphan member of the nuclear hormone receptor gene family, plays a key role in the regulation of steroidogenesis and is expressed at high levels in steroidogenic tissues. SF-1 binds to the SR-BI promoter in a sequence-specific manner, and efficient transcription from this promoter in adrenocortical Y1 cells is dependent on an intact SF-1 site. These data extend our understanding of SF-1 function within steroidogenic tissues and suggest that SR-BI, which serves to supply selected tissues with lipoprotein-derived lipids, is part of the repertoire of SF-1-responsive genes involved in steroidogenesis.

Expression of the VLDL receptor in endothelial cells.

In this article we describe the cellular distribution of the very low density lipoprotein receptor (VLDLR), a transmembrane protein that is expressed at high concentrations in skeletal muscle, heart, adipose tissue, and brain but in only trace amounts in the liver. Indirect immunofluorescence localization studies were performed in murine and bovine tissues using a rabbit polyclonal anti-human VLDLR antibody. Immunoreactive VLDLR protein was detected in the endothelium of capillaries and small arterioles but not in veins or venules of bovine skeletal muscle, heart, ovary, and brain. In the liver, there was intense staining of the capillaries and arterioles that supply the capsule and hepatic vessels but no staining of the sinusoidal surfaces. We failed to detect any signal from nonendothelial cells in the liver or peripheral organs. The VLDLR was also expressed at high levels on the endothelial surface of bovine coronary arteries; in contrast, little or no staining was seen in aortic endothelium. Antibody staining of cultured bovine coronary artery endothelial cells demonstrated punctate cell-surface staining, as well as staining of large and small cytoplasmic vesicles. This tissue and cell pattern of expression suggests that the VLDLR plays a role in the transport of VLDL or another plasma constituent from the vascular compartment to adjacent tissues.

Regulation of the very low density lipoprotein receptor by thyroid hormone in rat skeletal muscle.

A new member of the low density lipoprotein receptor gene family that binds and internalizes very low density lipoprotein (VLDL) particles was previously cloned and characterized from the rabbit and human. The physiological role of this putative VLDL receptor is not known, but its tissue distribution and ligand specificity suggest a possible role in the delivery of triglycerides to peripheral tissue. To learn more about the potential function of this receptor, we measured the changes in VLDL receptor mRNA and protein in various tissues following dietary or hormonal manipulation of rats. No significant changes in the VLDL receptor mRNA or protein were seen after a 48-h fast and subsequent to refeeding. A striking change in receptor mRNA and protein was observed in skeletal muscle of hypothyroid and hyperthyroid rats. In hypothyroid rats, the amount of immunodetectable VLDL receptor was reduced by 80%, while in the hyperthyroid animals it was increased by 300%. These maneuvers did not affect VLDL receptor mRNA or protein levels in adipose tissue or heart. The changes in VLDL receptor mRNA in muscle were opposite to those observed with lipoprotein lipase. These studies suggest that the VLDL receptor plays a role in a metabolic process in muscle that is regulated by thyroid hormone.

Rat granulosa cell apolipoprotein E secretion. Regulation by cell cholesterol.

We have demonstrated previously that cultured rat ovarian granulosa cells synthesize and secrete apoE, and this production of apoE is increased by agents that stimulate protein kinase A (cyclic AMP-dependent enzyme) (for example, cholera toxin) and protein kinase C (Ca2+/phospholipid-dependent enzyme) (for example, 12-O-tetradecanoylphorbol-13-acetate, a phorbol ester). In the studies presented in this report, we have examined the effect of changes in cell cholesterol synthesis on the production of apoE by rat ovarian granulosa cells. Mevinolin, an inhibitor of hydroxymethylglutaryl (HMG)-CoA reductase (the rate-limiting enzyme in cholesterol synthesis), and 4,4,10 beta-trimethyl-trans-decal-3 beta-ol, an inhibitor of squalene cyclization, both attenuate the cholera toxin or 12-O-tetradecanoylphorbol-13-acetate stimulation of granulosa cell apoE secretion and apoE mRNA content in a dose-responsive manner. The inhibitory effect of mevinolin is reversed by the concomitant administration of mevalolactone, which provides the cells with the product of the reaction catalyzed by HMG-CoA reductase. Steroidogenesis per se has no effect on apoE production. Aminoglutethimide, which blocks the rate-limiting step in steroidogenesis, has no effect on apoE or apoE mRNA. The data indicate that products of HMG-CoA reductase (isoprenes, cholesterol and/or cholesterol metabolites) are required along with stimulators of protein kinases A and C, to regulate ovarian granulosa cell apoE production.

Regulation of apolipoprotein E biosynthesis by cAMP and phorbol ester in rat ovarian granulosa cells.

Apolipoprotein E (apoE) is synthesized by the liver and many peripheral cells. Rat ovarian granulosa cells synthesize and secrete apoE, and this apoE production is increased by agents that increase cellular cAMP. In these studies of granulosa cell apoE synthesis we have examined the effect of agents that stimulate various cell kinases, including protein kinases A, G, and C. The cell content of apoE mRNA was measured simultaneously. Cholera toxin (1.25 micrograms/ml), dibutyryl-cAMP (5 mg/ml), and forskolin (10(-4) M), all of which increase cellular cAMP, stimulate apoE accumulation in the medium 7-10-fold. On the other hand, dibutyryl-cGMP (20 mg/ml) has no effect on apoE synthesis or secretion. The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (100 ng/ml), a protein kinase C stimulator, increases apoE accumulation in the medium 8-10-fold, while 4 alpha-phorbol 12,13-didecanoate, the inactive phorbol congener, has no such effect. The cAMP effect on apoE synthesis by granulosa cells is maximal at 48 h, while the phorbol ester effect is maximal at 72-96 h in culture. The data indicate that agents whose effects are mediated by activation of protein kinases A and C, but not G, stimulate granulosa cell apoE production. These effects on the amount of secreted apoE are temporally preceded by increases in the granulosa cell content of apoE messenger RNA. Together, these data suggest that the regulation of apoE production in the rat ovarian granulosa cell could involve transcriptional and post-transcriptional mechanisms.