5.8 Glucocorticoid resistance—a defect of the glucocorticoid receptor

The first case of glucocorticoid resistance was reported in 1976 by Vingerhoeds et al. (1). The patient was suffering from hypercortisolism with none of the tissue effects of Cushing’s disease. Further evaluation revealed that the ligand-binding affinity of the glucocorticoid receptor (GR) was diminished. His son and nephew were mildly affected and their GR also showed a reduced hormone affinity, although this was to a lesser extent. Later, the GR gene of the index patient was sequenced and showed a homozygous mutation at position 2054, yielding a valine for aspartic acid substitution at amino acid residue 641 (2). The other two family members appeared to be heterozygous carriers of the same mutation, which can explain their milder clinical picture. Since then, other patients with mutations in the GR gene leading to the syndrome of generalized glucocorticoid resistance have been described (Table 5.8.1) (18).

Familial glucocorticoid resistance is a rare disease, characterized by reduced cortisol action at the tissue level, which is compensated for by elevation of ACTH levels, resulting in an increase of adrenal steroids (glucocorticoids, androgens, mineralocorticoids) (18). It is rather unfamiliar and may confuse clinicians, since the signs and symptoms can be nonspecific. This syndrome has an autosomal recessive or dominant mode of inheritance.

As shown in Fig. 5.8.1a, the production of glucocorticoids is regulated by the hypothalamus. In response to signals from the central nervous system, the secretion of corticotropin-releasing hormone (CRH) and vasopressin is stimulated (19). These hormones stimulate the pituitary to secrete proopiomelanocortin (POMC). After splitting POMC into several proteins, ACTH is released to the circulation, stimulating the adrenal glands to secrete glucocorticoids. To control the activity of the hypothalamic–pituitary–adrenal (HPA) axis, negative feedback action by glucocorticoids is crucial.

If this feedback regulation, mediated by the GR, is disturbed, ACTH levels increase, and the adrenals are stimulated to produce supraphysiological levels of cortisol (Fig. 5.8.1b). However, due to the GR defects, the effects of cortisol on target genes in the nucleus are impaired. The elevated glucocorticoid levels also exert a mineralocorticoid effect, since the capacity of the enzyme 11β-hydroxysteroid dehydrogenase type II, which normally protects the kidneys from an excessive cortisol effect by rapid inactivation, is overridden. In addition, since ACTH levels are increased the adrenal production of androgens (dehydroepiandrosterone (DHEA), DHEA-sulphate, and δ-4-androstenedione) and adrenal corticosteroids with mineralocorticoid activity (corticosterone and deoxycorticosterone) is elevated (3).

The effects of cortisol on target genes are the result of a cascade of events. First, the ligand passively diffuses through the cell membrane and binds to the GR. This receptor is present in the cytoplasm of virtually all human cells. In its unbound form the GR is surrounded by chaperone proteins, which keep it inactivated. When the ligand binds to the receptor, its conformation changes, the heat shock proteins dissociate from the receptor, and the GR translocates to the nucleus (3, 20). Within the nucleus there are two major modes of GR action. The ‘classical’ pathway comprises transcription initiation through binding to positive or negative glucocorticoid responsive elements of the target gene, resulting in, respectively, stimulation or inhibition of transcription. The GR can also act as a transcription factor and indirectly, through interacting with other proteins, stimulate or repress transcription (20).

Several GR gene mutations have been reported as the cause of the syndrome of generalized glucocorticoid resistance. These are predominantly located in the ligand-binding domain, but some have been identified in the DNA-binding domain (3). These mutations lead to a variety of alterations in the GR signalling pathway, e.g. decreased transactivating or transrepressional capacity (11), disturbances in ligand binding (2), decreased GR expression (9), a delay in translocation to the nucleus, changes in interaction with coactivators, alternative splicing, or a combination of these changes in GR function (4, 6–9, 11, 12, 15, 16).

Interestingly, one mutation, close to the boundary of exon 9 in the GR gene, resulted in an increased expression of the GR-β splice variant. In the literature, GR-β is suggested to function as a dominant negative inhibitor of the active GR-α. Therefore increased intracellular presence of the GR-β could lead to either an acquired or an inherited form of glucocorticoid resistance (4).

In some cases, no mutations in the GR gene were found and the mechanism leading to the glucocorticoid resistance is unknown (21). Several mechanisms leading to glucocorticoid resistance, have been suggested in the literature: altered phosphorylation status; hormone-induced conformation changes of the GR and nuclear transformation; thermolability of the GR; and enhanced expression of 90-kDa heat shock protein (hsp90), a chaperone protein (21, 22).

Besides hereditary forms of systemic glucocorticoid resistance acquired forms also occur, in particular in some types of neoplasms. Examples are pituitary tumours (Nelson’s syndrome/Cushing’s disease), ectopic ACTH-producing tumours, as well as haematological malignancies (23). In a wide variety of other diseases, local or systemic, and temporary or chronic forms of glucocorticoid resistance have also been shown, e.g. major depression, AIDS, and several autoimmune diseases (24).

In asthma also, glucocorticoid resistance is a well-known clinical problem (25). Potential mechanisms involved in steroid-resistant asthma are increased expression of the dominant negative GR-β splice variant and local glucocorticoid resistance by diminished binding affinity in inflammatory cells induced by certain cytokines (e.g. interleukin (IL)-2, IL-4, IL-13). Also impaired nuclear localization, and leukaemia inhibitory factor, a cytokine decreasing GR expression in animal studies, may both be contributing factors in the development of glucocorticoid resistance (25).

Within the normal population, variation in glucocorticoid sensitivity has been demonstrated, for which several single-nucleotide polymorphisms in the GR gene seem to be at least partially responsible (26). About 6–9% of the Caucasian population are carriers of the ER22/23EK polymorphism, which is associated with a mild glucocorticoid resistance and results in beneficial effects with respect to insulin sensitivity, lipid profile, body composition, cognition, and longevity (26). Russcher et al. showed that this GR gene variant causes an increased amount of the GR-α translational isoform, which has a lower transcriptional activity compared to the GR-β isoform (27). A polymorphism in the 3′ untranslated region of exon 9β of the GR gene has been associated with mild glucocorticoid resistance with respect to transrepressional effects, which is important for the immune system. In vitro this variant yielded more stable GR-β mRNA, possibly leading to a dominant negative effect on GR-α functioning, and showed diminished transrepressional activity (28, 29). Jiang et al. showed another exon 9 polymorphism in eight out of 39 lupus nephritis patients, resulting in addition of 20 amino acids to the GR protein, potentially affecting GR functioning and thereby increasing the risk of developing autoimmune diseases (30).

In contrast to glucocorticoid resistance, hyperreactivity to endogenous cortisol has also been reported, resulting in clinical features consistent with Cushing’s syndrome despite normal or decreased cortisol levels (22). In the healthy population two poly-morphisms of the GR (Asn363Ser and BclI) have been demonstrated to be associated with mild hypersensitivity to glucocorticoids. Several studies have shown evidence for tissue-specific increased cortisol effects (26).

Glucocorticoid-resistant patients do not suffer from the classical cushingoid effects, such as a moon face, abdominal obesity with red striae, hyperglycaemia, myopathy, etc., despite their elevated cortisol levels. The symptoms in patients with cortisol resistance result from the compensatory increased activation of the HPA axis. Due to these elevated ACTH levels, patients experience symptoms related to an increased production of mineralocorticoids, leading to hypertension, hypokalaemic alkalosis, and fatigue. Female patients also suffer from symptoms of hyperandrogenism, such as hirsutism, male pattern of baldness, and menstrual disturbances, due to increased production of androgens by the adrenals. In male patients, the testicular production of androgens is much higher and outweighs the increased adrenal androgen production. However, some patients with glucocorticoid resistance are asymptomatic or complain only about chronic fatigue. The fatigue could also be attributed to a relative glucocorticoid deficiency in some tissue levels due to insufficient compensatory elevation of cortisol levels (21).

Figure 5.8.2 shows a practical scheme of clinical and biochemical tests for the diagnosis of glucocorticoid resistance. Plasma ACTH and serum cortisol concentrations are increased but, in contrast to Cushing’s syndrome, the diurnal rhythm is maintained, although cortisol levels are on average higher (2, 5). Important for the diagnosis is nonsuppression after a 1 mg overnight dexamethasone suppression test with cortisol levels above 70 nmol/l or even higher—above 140 nmol/l being indicative for glucocorticoid resistance. Urinary cortisol excretion is increased. Serum concentrations of adrenal androgens (DHEA, DHEA-S, and androstenedione) and of ACTH-dependent mineralocorticoids (deoxycorticosterone and corticosterone) are also increased. Imaging may show slightly enlarged adrenal glands.

As shown in Fig. 5.8.2, the standard tests to evaluate hypercortisolism are not sufficient to differentiate between glucocorticoid resistance and Cushing’s syndrome. Some clinical investigations, however, can be used to discriminate between these syndromes. A simple test is measurement of bone mineral density (BMD), which is normal or even increased in patients with glucocorticoid resistance. In contrast, in patients with Cushing’s disease BMD is usually decreased. The increased BMD in glucocorticoid resistance may be explained by a combination of diminished cortisol effects on bone due to the defective GR, and increased adrenal androgen production as a result of elevated ACTH levels. If persistent doubt exists concerning the correct diagnosis, additional endocrine tests can be helpful, e.g. demonstration of a normal response of serum thyroid-stimulating hormone (TSH) to thyrotropin releasing hormone (TRH) administration and/or a normal response of growth hormone to an insulin-induced hypoglycaemia, which would be the case in conditions of glucocorticoid resistance. These responses are invariably reduced in patients with Cushing’s disease. Recently, tests to confirm the diagnosis of glucocorticoid resistance have been developed (Fig. 5.8.2). Disadvantages are that, at present, these tests are labour-intensive and are performed only in research laboratories (4). A fast, alternative way to confirm the diagnosis of hereditary glucocorticoid resistance is to perform a dexamethasone-suppression tests in family members of the patient (22).

Morning ACTH levels can be suppressed by a low dose of dexamethasone taken around midnight. This leads to a reduction in adrenal overproduction of mineralocorticoids and androgens. It is essential that the treatment should be adjusted to the individual signs and symptoms of the patient. Titration to a dose of dexamethasone that normalizes androgens, blood pressure, and serum potassium seems the optimal therapy. After normalization of mineralocorticoids and androgens, the dexamethasone dose to maintain this suppression can be carefully titrated down. To minimize the risks of an effect of dexamethasone in addition to the remaining endogenous cortisol, it is important to slowly decrease the dose of dexamethasone as low as possible. A yearly measurement of BMD is recommended to monitor effects of too high doses of dexamethasone in combination with the endogenous cortisol production. In general, treatment can be started with a dose of about 1 mg dexamethasone at night. This dose can be slowly reduced to 0.5 mg/day or even 0.25 mg/day. To treat hypertension aldosterone antagonists are recommended, since these have additional effects. In particular, their potassium-sparing and antiandrogenic effects are beneficial for glucocorticoid-resistant patients. Thiazide or loop diuretics should not be used to control blood pressure because of their potassium-losing effects.