Adrenal Function and Metabolism

Steroids and catecholamines released from the adrenal gland both play a crucial role in the regulation of blood pressure, energy homeostasis, fat cell mass, insulin release and bone metabolism.

Although we owe the first description of the adrenals to Eustachius in 1563, the Bible already referred to the fat pad in which the adrenals are embedded in Leviticus 3 : 4, “and the two kidneys and the fat that is on them which is by the flanks …”

As early as in the 17^th century, Riolan postulated that the adrenal glands are involved in „generating fat”. Finally, the close relation between the adrenals and metabolism is not only reflected in anatomical descriptions of the adrenal but also in the names of its hormones such as glucocorticoids.

Recent years have seen much research on the close functional involvement of the adrenals in metabolism. This revival of an old concept mainly developed from three novel lines of research.

With the discovery of several new hormones and adipocytokines such as leptin produced in fat cells, which express receptors in adrenal tissue and act on both adrenal steroidogenesis and catecholamine biosynthesis, a novel direct link between fat cell metabolism and adrenal function has been established 1 2 3. In addition to the well-known activity of glucocorticoids and catecholamines in the metabolic system, it has become clear that local intracrine modulation, particularly of adrenocortical steroid hormones in the adipocyte, may be a key mechanism to fat cell maturation and obesity 4 5. Selective inhibition of intracellular steroid metabolism in adipose tissue or kidneys (e. g. reduction of cortisone to cortisol by 11β-hydroxysteroid-dehydrogenases) may hold promise for new therapeutic concepts. Finally, it has been suggested by modern imaging technique that the incidentally discovered adrenal masses frequently found may account for some form of subclinical hypercortisolism that could contribute to the metabolic syndrome X 6 7.

Altogether, these exciting new findings suggest that focussing the main program in the 6^th German Adrenal Conference on this topic may provide a useful contribution to our knowledge in the area. The conference was held in Kloster Drübeck, Germany, on November 21-23, 2003.

In his presentation on pregnancy specific corticosteroid-binding globulin (CBG), David Torpy from the University of Adelaide pointed out that pregnancy induces progressive hypercortisolism. CBG levels also increased at least threefold. Moreover, variant pregnancy-specific glycoforms of CBG (pregCBG) appear. PregCBG displays enhanced placental binding [8], and may contribute to the placental CRH-cortisol loop, which appears to be important in determining human gestation time.
Characterization of pregCBG glycoforms and determining their relationship to gestational age are important steps towards understanding the physiological role of pregCBG and its role in obesity developing later in life.

Horst Fehm from the University of Lübeck suggested a new and comprehensive model of central regulation of body weight in humans.
Specifically, he stressed the pivotal role of the POMC gene underlying the influence of both the HPA axis and the arcuate nucleus. Intranasal alphaMSH analoga and insulin application can also influence the human melanocortin system in the arcuate nucleus. He showed that a loss of weight can be achieved in patients with normal weight by intranasal application of alphaMSH-40 without any dieting. AlphaMSH failed, however, to reduce weight in obese subjects.

Jeremy W. Tomlinson from the University of Birmingham presented data on a provocative concept of obesity as Cushing’s Syndrome of adipose tissue. Clinical studies, however, indicate global inhibition of 11β-HSD1 reductase activity in obesity amidst continued debate over adipose tissue-specific expression levels. Studies have been unable to demonstrate regulation through BMI within whole adipose tissue. Indeed, 11β-reductase activity and enzyme mRNA expression inversely correlated with obesity in cultured omental preadipocytes. Furthermore, they observed a significant increase in 11β-HSD1 reductase activity and mRNA expression in subcutaneous adipose tissue biopsies following weight loss.
Therefore, global inhibition of 11β-HSD1 reductase activity in human obesity and the changes that occur with weight loss may represent a compensatory mechanism that decreases tissue-specific cortisol concentrations and improves insulin sensitivity. Therapeutic inhibition of 11β-HSD1 in rodents improves insulin sensitivity, remaining a novel and attractive therapeutic target in patients with type 2 diabetes mellitus and metabolic syndrome.

Zeev Hochberg from Meyer Children’s Hospital in Haifa provided new ideas and data on the role of adipose tissue steroidogenesis in hypothalamic obesity, pointing out that communication from the hypothalamus to adipocyte 11β-HSD-1 involves hormones, the sympathetic nervous system and cytokines. CRH and ACTH downregulate 11β-HSD-1 activity and cause a reduction in lipolysis [9]. Indeed, CRH-receptors are expressed in human adipose tissue [10]. Tumour necrosis factor-α (TNFα) and interleukin-1β upregulate 11β-HSD-1 expression and activity while enhancing lipolysis. The sympathetic nervous system acts through β-adrenergic upregulation and α-adrenergic downregulation of 11β-HSD-1 activity. Inhibition of 11β-HSD-1 suppresses preadipocyte differentiation into mature adipocytes, and may provide a novel therapeutic tool.

While leptin has a direct inhibiting effect on adrenal cortisol production, Olaf Jöhren from the University of Lübeck pointed out that orexins, which are known to have an orexigenic effect, stimulate adrenal cortisol production through their specific receptors in the adrenal [11].

Monika Ehrhart-Bornstein from the German Diabetes Research Institute in Düsseldorf presented new information on adipoadrenocortical co-cultures, testing the hypothesis that adipocyte secretory products directly stimulate adrenocortical aldosterone secretion. Secretory products from isolated human adipocytes strongly stimulated steroidogenesis in human adrenocortical cells (NCI-H295R) with a predominant effect on mineralocorticoid secretion. Aldosterone secretion increased sevenfold over 24 h of incubation. This stimulation was comparable to maximal stimulation of these cells with forskolin (2 × 10 ^-5 M). At molecular level, there was a tenfold increase in the expression of steroid acute regulatory peptide (StAR) mRNA. She concluded that human adipocytes secrete potent mineralocorticoid releasing factors suggesting a novel direct link between obesity and hypertension [12] [13].

Fig. 1 Adipoadrenal interactions mediated by adipocytokines and intracrine modulation of steroid metabolism (modified from [12]).

M. Quinkler from the University of Birmingham and Berlin pointed out that progesterone metabolism in the kidney influences salt and water regulation during pregnancy and the menstrual cycle. Locally formed androgens in the kidney could also have an influence on blood pressure that may be transmitted over a regulation of the vascular antagonism of the renin-angiotensin-system or the expression of sodium channels.

In a fascinating and captivating after-dinner lecture, Günther Schütz from the German Cancer Institute in Heidelberg elucidated novel mechanisms of glucocorticoid action based on genetic dissection of glucocorticoid receptor function in gene inactivation and transgenic strategies. He showed that germline and somatic gene targeting as well as increased dosage of the glucocorticoid receptor (GR) allows characterization of various functions and molecular modes of action of these receptors. Most of the effects of the glucocorticoid receptor are mediated via activation and repression of gene expression.
Overexpression of GR in mice demonstrates that tight regulation of GR expression is crucial for proper control of neuroendocrine and immunological processes [14]. The GR gene was inactivated in parenchymal cells of the liver, in thymocytes, monocytes/macrophages, skin and brain, respectively [15]. The absence of GR in hepatocytes, for example, leads to a dramatic reduction in body size. He demonstrated that growth hormone (GH) signaling mediated by STAT5 in hepatocytes is impaired in these mice, leading to lower synthesis of IGF-I and other STAT5-dependent mRNAs. We exspect deeper insights into the role of glucocorticoid receptor function in the mouse and in diseases caused by these cell type-specific mutants [16].

Antje Böttner from the University of Leipzig provided a comprehensive overview on what we can learn about adrenal gland function from gene-deletion and overexpression. Models with gene-deletion or overexpression provide an opportunity to analyze effects of single genetic alterations in a living organism including the entire stress system in vivo [17] [18].

Stefan Bornstein from the University of Düsseldorf demonstrated that deletion of the vitamin C transporter leads to severe impairment of adrenal function.
His findings establish a crucial role for ascorbic acid in adrenal chromaffin cell function, but do not support any important role that this may play in the lethal SVCT2 null mouse phenotype. He further concluded that at least three mechanisms contribute to impaired adrenal chromaffin cell function in SVCT2 null mice: 1) impaired activity of dopamine β-hydroxylase; 2) reduced HPA axis activity; and 3) induction of programmed cell death [19].

Felix Beuschlein from the University of Freiburg discussed the role of inhibin (INH) and activin in adrenal tumorigenesis. Taken together, the findings support the concept that in contrast to LH, which exerts a growth promoting effect on adrenocortical tumors in INH-/- mice, activin prevents adrenal tumor growth by inducing x-zone regression [20].
Further clinical studies are needed to evaluate the role of the activin/inhibin system in patients with benign and malignant adrenal tumors.

Matthias M. Weber from the University of Mainz elucidated the role of the insulin-like growth factor system in adrenocortical growth control and carcinogenesis. In addition to IGF-II overexpression, increased IGF-I receptor and IGFBP-2 levels have been found in advanced human adrenocortical carcinomas, suggesting an important role of the IGF system in adrenocortical carcinogenesis. However, since transgenic mice with overexpression of IGF-II in the adrenal gland do not show an increased adrenal tumor frequency, IGF-II overproduction by itself might not be sufficient for malignant transformation, and additional factors could play an essential role in adrenocortical tumorigenesis.

Oliver Zwermann from the University of Freiburg explained the potential role that the ACTH receptor might play in adrenal tumorigenesis. The ACTH receptor exerts an anti-proliferative effect in his model that can be reversed by a higher dose of ACTH. In this complex and dual role of ACTH on tumor growth, other melanocortin receptors may be involved.

Matthias Schott from the University of Düsseldorf reported on dendritic cell vaccination. His approach may represent the basis for the development of a new therapeutic strategy in adrenal cancer. A DC-based vaccination trial in 2 patients with adrenocortical cancer (follow-up: œ and 2 months) has been initiated. Further follow-up of these and other patients enrolled in the study will establish whether this strategy may represent an option for the treatment of metastatic adrenal cancer.

Martin Reincke from the University of Munich considered other ways of immune therapy of adrenal carcinoma in animal models. The result showed that the T-cell tolerance toward steroidogenic acute regulatory protein (StAR) can be broken, reducing anti-tumor immunity. Thus, StAR represents a candidate target antigen for immunotherapeutic strategies against adrenocortical cancer.

Hartmut P. H. Neumann from the University of Freiburg revealed new information about germline mutations in non-syndromic pheochromocytoma. Almost 25 % of mutation carriers with pheochromocytoma can present as apparently sporadic cases; routine mutation analysis for RET, VHL, SDHD and SDHB should be carried out to identify pheochromocytoma-associated syndromes that would otherwise be missed.

Christian A. Koch gave a comprehensive overview on novel tumor markers for pheochromocytoma. Future studies searching for potential markers that can discriminate reliably between benign and malignant pheochromocytomas will have to be developed.

Jacques W. M. Lenders from the St. Radboud University Medical Center in Nijmegen suggested plasma metanephrines as the optimal diagnostic tool for pheochromocytoma. Plasma-free metanephrine or urinary fractionated metanephrine measurements are equally effective in excluding pheochromocytoma. However, due to the higher specificity of plasma than urinary fractionated metanephrines, plasma free metanephrines should be the test of first choice in the diagnostic work-up of a patient with suspected pheochromocytoma [21].

Klaus-Martin Schulte from the University of Düsseldorf explained the endoscopic treatment of pheochromocytoma, specifying technical features, pitfalls and possibilities of laparoscopic surgery and comparing them to our experience with open access surgery in this entity. Long-term results after subtotal adrenal resection will be presented.

Stefan A. Wudy from the University of Giessen discussed mass spectrometry as a tool in the differential diagnosis of disorders of steroid metabolism. With the analysis of intact liquid chromatography mass spectrometry (LC-MS) on steroids or steroids with high molecular masses, the use of (LC-MS) with ionization techniques is the technique of choice. Techniques such as tandem-mass spectrometry (LC-MS-MS) allow rapid and efficient analysis but do not have the potential of the high specific multi-component analysis such as gas chromatography MS.

Sven Diederich from the Freie Universität Berlin described the pharmacodynamics and -kinetics of synthetic mineralocorticoids and glucocorticoids. His in vitro data allow the following conclusions: 9α-fluorcortisol, the substance most frequently used in mineralocorticoid substitution, seems to be the best choice of available steroids for this approach. Whereas glucocorticoid (GC) selectivity can be improved by hydrophobic substituents in position 16 (e. g. prednylidene, deflazacort) and the ▵1-dehydro-configuration, maximal GC activity needs additional fluorination in position 6α and/or 9α (e. g. dexamethasone). The widely used prednisolone does not seem to be the optimal recommendation for GC therapy directed to HSD2-expressing organs such as the kidney.

L. Seiler from the University of Freiburg presented new data on the diagnosis of the normokalemic Conn-syndrome by using the plasma aldosterone-renin ratio. A simple confirmatory test is the salt loading test. Alternatively, the fludrocortisone suppression test or determination of urinary aldosterone-18-glucuronide and tetrahydroaldosterone glucuronide may be used. Since many patients with primary aldosteronism can be cured by operation, and missing the diagnosis often leads to significant end-organ damage, it is important to evaluate hypertensive patients with therapy-resistant hypertension for primary aldosteronism.

Wolfgang Oelkers from the Freie Universität Berlin provided new ideas and information on the clinical picture of severe hyponatremia in patients with hypopituitarism including secondary adrenal insufficiency. Hypopituitarism including secondary adrenal insufficiency seems to be a frequently overlooked cause of severe hyponatremia. A high level of suspicion is the best way of recognizing the underlying disorder. Treatment with hydrocortisone is very effective.

Bruno Allolio from the University of Würzburg introduced the German adrenal cancer register. The prospective register will only accept newly diagnosed patients with adrenal cancer. Concepts for international treatment studies are currently under development so that patients from the register can participate in the studies (www.nebennierenkarzinom.de).

M. K. Walz from Essen gave us information about minimal invasive adrenal gland surgery in different cases of chromaffin tumors. His experience shows that large chromaffin tumors, bilateral, locally recurring and extraadrenally located, can be safely excised using the minimally invasive approach.

Georg Mansmann from the University of Düsseldorf presented an update in diagnosis and management of the clinically inapparent adrenal mass. In preparation for an NIH State-of-Science Conference on this topic, an extensive literature review was performed, including Medline, BIOSIS, Embase between 1966 and March 2001, references of published metaanalyses and selected review articles, identifying 5,386 citations as well as more than 600 articles that were retrieved for further examination, allowing a comprehensive update of the diagnostic and therapeutic approaches, focusing on endocrine and radiological features as well as surgical options. Based on the statements at the conference, the available literature and ongoing studies, he attempted to provide practical management recommendations and to highlight areas for future research.

M. W. A. Angstwurm from the University of Munich suggested that patients with septic shock show an impaired adrenal hormone synthesis. Patients with septic or cardiogenic shock had higher plasma progesterone and 17-OH progesterone but only slightly elevated cortisol levels compared to control. Septic patients showed diminished response to cosyntropin stimulation regarding cortisol or aldosterone levels despite a normal response of the cortisol precursors progesterone and 17-OH progesterone. This impairment of steroid hormone synthesis at the level of the enzymes 21-hydroxylase or 11-hydroxylase might have clinical implications in patients with septic shock.

Altogether, the meeting highlighted the great interest in adrenal research and its relevance and potential for many areas of clinical medicine outside endocrinology.

Based on fundamental mechanisms derived from our understanding of adrenal steroid and catecholamine metabolism, exploration of new avenues for treating metabolic disease and other disorders will continue.

We would like to thank the German Endocrine Society and Merck KGaA, Darmstadt, in particular Dr. Erkens and Dr. Benker, for their generous support of the 6^th German Adrenal Conference.

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