Glucocorticoid Action: Physiology

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Chapter 2

Glucocorticoid Action

Physiology

Historical Developments and Background

Glucocorticoid physiology at times has occupied the mainstream of endocrinology, at others, a backwater. We begin by describing some of those ebbs and flows up to the dramatic announcement in 1949 that glucocorticoids have powerful antiinflammatory activity, a discovery that for decades isolated glucocorticoid physiology from the major clinical applications of those newly uncovered “miracle drugs.”

The Adrenal Cortex, Survival, and the Role of Glucocorticoids and Mineralocorticoids

Thomas Addison’s discovery in the mid-1800s that the adrenal cortex was essential for survival1,2 preceded by nearly a century the demonstration that this gland produced at least two distinct hormones—eventually called glucocorticoids and mineralocorticoids3—each essential for normal life.4 During that century, many of the actions on glucose metabolism that would characterize glucocorticoids, and on salt and water balance that would characterize mineralocorticoids, were foreshadowed in the symptoms of Addisonian patients2 and adrenalectomized animals, and in the effects of lipid extracts from the adrenal cortex. By 1940, studies with pure glucocorticoids and mineralocorticoids showed that both were essential for survival, mineralocorticoids clearly sustaining life by maintaining electrolyte balance. How glucocorticoids sustained life, however, remained a mystery for decades.

Carbohydrate Metabolism

Glucocorticoids were named for their hyperglycemic effect.3 Low blood glucose in Addisonian patients and adrenalectomized animals and low liver glycogen in the latter were described in the early 1900s. The 1930s saw the use of adrenal extracts to restore normal glucose levels, as well as the striking discovery that adrenalectomy,5 like hypophysectomy,6 ameliorated symptoms of diabetes. A landmark paper by Long, Katzin, and Fry,7 in 1940, demonstrated that glucocorticoids stimulate gluconeogenesis from amino acids derived from protein catabolism, decrease glucose oxidation, and can elicit steroid diabetes. Ingle showed that glucocorticoids decrease glucose utilization8 and cause insulin resistance.9 These papers set the stage for most later work in this area.

Glucocorticoid-Induced Apoptosis of Lymphocytes

Lymphoid tissue as a target for glucocorticoids was perhaps first noted by Addison, who observed “a considerable excess of white corpuscles” in the blood of one of his patients.2 By 1900, thymus enlargement had been described in Addisonian and adrenalectomized rats. Around then, pathologists, unaware that they were dealing with atrophic tissues, used specimens from victims of prolonged illness as standards for normal lymphoid organs. In cases of sudden death in which they judged the thymus and other lymphoid tissues to be enlarged, they pronounced the resounding diagnosis of “status thymico-lymphaticus.”10 Eventually, Selye showed that via the adrenals, any illness or other source of stress can atrophy the thymus.11 These effects on lymphoid tissues were later reproduced with adrenal extracts and pure glucocorticoids, which are now recognized to induce lymphocytolysis by apoptosis.12

Regulatory and Permissive Glucocorticoid Actions in Stress

Intimate connections between stress and adrenocortical hormones were revealed in the 1930s by observations that stress stimulates adrenocortical secretion and adrenal extracts protect against stress.11,13 Protection was traced to glucocorticoids,13 which remain prominent among stress hormones. Selye, who pioneered and popularized the subject of stress, demonstrated that innumerable stimuli activate the adrenal cortex.11 His unified theory of stress introduced such concepts as the “alarm reaction” and the “general adaptation syndrome,” along with the much disputed claim that by raising levels of adrenocortical hormones, stress caused “diseases of adaptation,” which included arthritis and allergy.14 As to how glucocorticoids protect against stress, White and Dougherty proposed that they enhance immune responses through lymphocytolysis by releasing preformed antibodies, and Selye suggested that they satisfy an increased demand for glucose.14 Neither of these ideas gained experimental support.

Ingle15,16 described a protective role for glucocorticoids distinct from the “regulatory” one of high, stress-induced hormone levels. Observing that adrenalectomized animals respond normally to certain forms of stress when administered glucocorticoids at basal levels, he proposed that basal levels exert “permissive” effects that maintain the capacity of some homeostatic functions to respond to moderate stress. He recognized, though, that glucocorticoids are required at stress-induced levels to maintain homeostasis in severe stress.15

Feedback Regulation

Addison2 remarked on “a peculiar change of colour of the skin, occurring in connection with a diseased condition of the supra-renal capsules,” a manifestation of negative feedback linking glucocorticoids to anterior pituitary hormones. This link was explored by Smith, who in 1930 reported that hypophysectomy of rats caused adrenocortical atrophy that was reversed by implanting pituitaries. Pituitaries eventually yielded ACTH, the adrenocorticotropic hormone. Negative feedback control of ACTH by an adrenocortical hormone was described in a remarkable half-page article by Ingle and Kendall,17 which showed that administration of adrenal extracts to rats caused atrophy of the adrenal cortex that was countered by simultaneous administration of a pituitary extract. Stress, furthermore, caused hypertrophy of the adrenal cortex in normal but not hypophysectomized animals, implying that stress stimulated secretion of ACTH.18 Harris’s 1937 proposal that control of secretion of ACTH resides in the hypothalamus,19 followed by evidence that this control is mediated by a hormone via the hypophyseal portal vessels,20 led to identification of CRH, the corticotropin-releasing hormone. Thus arose the concept of the hypothalamo-pituitary-adrenal, or HPA, axis.

Antiinflammatory Actions and Their Repercussions on Glucocorticoid Physiology

By the late 1940s, the main outlines of glucocorticoid physiology appeared to be firmly drawn. Then 1949 brought a watershed event that was to cast a long shadow over this discipline. Hench, Kendall, Slocumb, and Polley21 reported that cortisone in high doses and ACTH exerted powerful anti-inflammatory activity that dramatically improved the condition of patients with rheumatoid arthritis.22 This totally unexpected discovery was celebrated by clinicians and their patients but caused turmoil among glucocorticoid physiologists, who could offer no physiologic explanation for actions wholly inconsistent with their belief that stress-induced levels of glucocorticoids protected against stress by enhancing—not suppressing—defense mechanisms.23 These actions were also at odds with Selye’s idea of diseases of adaptation.13,22 Despite a rare voice to the contrary,23 physiologists concluded that the antiinflammatory and closely linked immunosuppressive actions were pharmacologic rather than physiologic in nature.13

That view persisted for longer than three decades. For example, neither a 1952 review by Ingle15 nor a 1971 review by Hoffman24 on the role of the adrenal cortex in homeostasis even mention the antiinflammatory actions. Consequently, the spectacular rise in therapeutic applications of these hormones, the “miracle drugs” of the 1950s, and the development of synthetic glucocorticoid analogues such as prednisolone, dexamethasone, and countless others proceeded largely without ties to physiology—a situation probably unique in endocrinology.

A central but unrealized goal in the development of those synthetic analogues was to separate antiinflammatory activity from such “side effects” as increased blood glucose and feedback suppression. As we now know, antiinflammatory actions are in fact quintessentially physiologic,25,26 and glucocorticoids administered to produce one physiologic effect generally produce others. Recent progress in circumventing that problem by separating physiologic effects on the basis of their molecular mechanisms is described below.

Background to Modern Glucocorticoid Physiology

Antiinflammatory and immunosuppressive actions today are embedded in the foundations of glucocorticoid physiology, where they belonged all along. Such suppressive actions, along with permissive actions, protect the organism from stress. Glucocorticoid researchers are using cell and molecular biological techniques to unravel the mechanisms of these and many other actions.

The discovery of the glucocorticoid receptor (GR) in the 1960s opened new avenues to physiology. Most cells have GRs, through which they respond in some way to glucocorticoids. Cloning of GRs and other receptors in the 1980s revealed close structural homologies between receptors for steroid hormones, thyroid hormones, and retinoids. All are ligand-dependent regulators of gene transcription, inducing effects that generally take hours or days to appear in the whole organism.

Cortisone, for years considered the prototypical glucocorticoid, hardly binds to GRs and is inactive until converted to cortisol by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1). Quite unexpectedly, cortisol and corticosterone, the natural glucocorticoids, were found to bind to mineralocorticoid receptors (MRs) with much higher affinity than to GRs. Those steroids normally circulate at concentrations that saturate MRs, immediately raising the question of why they do not block binding to MRs in mineralocorticoid target cells by aldosterone, which is present at much lower concentrations. The answer unveiled a fascinating enzyme, 11β-HSD2, which “protects” MRs in target cells by rapidly converting cortisol to cortisone and corticosterone to its inactive 11-keto form.

The GR is a protein that on binding hormone translocates to nuclear sites, where it regulates transcription of certain genes. It accounts so far for most glucocorticoid actions and sometimes is called GRα to distinguish it from an isoform, GRβ, which cannot bind hormone but acts as an antagonist. Several naturally occurring mutants of the human GR can cause generalized glucocorticoid resistance. Some evidence indicates that receptor-like proteins found in cell membranes may initiate rapid glucocorticoid effects through nongenomic mechanisms (see also Chapter 4).

The development of transgenic mice with modified GRs, MRs, 11β-HSDs, and other proteins, sometimes targeted to specific tissues,27 is bringing new insights to glucocorticoid physiology, some of which will be described below.

General Molecular Aspects of Glucocorticoid Physiology

GRs date back hundreds of millions of years, having evolved along with mineralocorticoid receptors from an ancestral estrogen receptor that appeared almost a billion years ago.28 Originally identified in rat thymocytes,29 GRs are found in almost all nucleated cells, where they initiate hormonal activity by regulating transcription of specific genes in a ligand-dependent and cell-specific manner. When unliganded, they are predominantly cytoplasmic. After binding a ligand (cortisol, corticosterone, or powerful synthetic analogues like dexamethasone), they become activated and translocate to the nucleus.30 There they regulate target genes in several ways.31,32

By binding as dimers to glucocorticoid response elements (GREs), which are short palindromic sequences of nucleotides associated with the target gene promoter, liganded GRs activate transcription when appropriate transcription factors and co-activators are present. Similarly, they repress transcription by binding as dimers to negative GREs (nGREs), or to composite GREs consisting of a GRE and a contiguous site for transcription factors such as AP-1 proteins (cJun/cFos).

Liganded GRs also regulate transcription through a mechanism of transcriptional cross-talk that does not require dimerization, DNA binding, or GREs in the regulated gene. Through protein-protein interactions, they bind as monomers to transcription factors like nuclear factor-κB (NF-κB), AP-1, cyclic AMP response element binding protein (CREB), and others, generally repressing transcription of the associated genes.33 This cross-talk probably mediates most anti-inflammatory glucocorticoid actions, which are unimpaired in transgenic mice carrying mutated GRs that cannot dimerize.34 In immortalized fibroblasts from these mice, glucocorticoids suppress the phorbol ester–activated collagenase-3 gene, known to be mediated through AP-1, but barely activate a transfected reporter under control of the mouse mammary tumor virus (MMTV) promoter, which requires GR binding to GREs.35 However, results with dimerization-deficient GRs show no sharp functional separation between these two mechanisms,36 so it may not be possible to separate immunosuppressive from other glucocorticoid effects using “designer” GR ligands that favor gene repression over activation.37,38 Of possible physiologic relevance are observations that, within a cell, translation of GR mRNA can produce several isoforms with different transcriptional specificities.32

Since the discovery that cortisol and corticosterone have much higher affinity for MRs than GRs, it has become clear that under physiologic conditions, some glucocorticoid effects, notably in the hippocampus, are mediated through MRs. Whereas MRs in mineralocorticoid target tissues are protected from high glucocorticoid levels by 11β-HSD2, which oxidizes cortisol to cortisone and corticosterone to 11-dehydrocorticosterone,39-41 that enzyme is absent in the hippocampus.42 In male and female reproductive tracts, 11β-HSD2 may protect GRs from excessive glucocorticoid levels. In pregnancy, it appears to protect the fetus from stress level maternal glucocorticoids, which can cause growth, low birth weight, and permanent postnatal pathologies such as hypertension and impaired HPA function.43,44

In contrast, 11β-HSD1, which is found in many tissues, functions primarily (not exclusively) as a reductase, activating cortisone to cortisol and 11-dehydrocorticosterone to corticosterone: it thereby amplifies local glucocorticoid activity in several tissues45 and may cause what has been called tissue-specific Cushing’s syndrome.46 The importance of 11β-HSD1 for glucocorticoid action has been demonstrated with transgenic mice. Knockout mice that lack 11β-HSD1 have weakened stress-induced glucocorticoid responses. Mice with 11β-HSD1 locally overexpressed in adipose tissue develop the metabolic syndrome.47 Adipose tissue 11β-HSD1 is regulated by both insulin and glucocorticoids.48 Levels of 11β-HSD1 and 11β-HSD2 differ in fetal and adult tissues, and this may have a role in development.49

Because of their higher affinity for MRs than GRs, at low basal levels the natural glucocorticoids occupy mainly unprotected MRs. As levels increase during the circadian cycle, MRs approach saturation and GRs become occupied. With stress, glucocorticoid levels may rise sufficiently to nearly saturate GRs.42

GRβ, an alternative splice isoform of GRα, lacks a hormone-binding domain. Although it cannot bind hormone, it acts as a dominant-negative antagonist to GRα. Its presence at high constitutive levels accounts for the ability of human neutrophils to escape glucocorticoid-induced cell death.50 Increased levels of GRβ may be associated with incidence of rheumatoid arthritis. Levels and functions of both GRα and GRβ are influenced by cytokines.

Glucocorticoid resistance, a significant clinical problem, arises through numerous mechanisms. These include downregulation and altered binding characteristics of GRs in the course of glucocorticoid therapy or disease,51 inactivating mutations and polymorphisms of GRs,52 overexpression of GRβ,53 efflux transporters that remove certain steroids from lymphocytes or the brain,54 and interactions with transcription factors such as AP-1 proteins and NF-κB.55

Membrane receptors for glucocorticoids and rapid actions through nongenomic mechanisms have been reported for many cells and tissues,56,57 but their functional significance is unclear.

Feedback Regulation of Glucocorticoid Production

Although glucocorticoids are essential for the response to stress and for survival, they normally exert major control over few physiologic processes other than their own feedback mechanisms. For example, they influence blood glucose, but the dominant regulators are insulin and glucagon. Reflecting this role and contrasting with hormones like insulin and aldosterone, glucocorticoids control their plasma levels directly by negative feedback via GRs and MRs rather than via a physiologic effect. Such a design is common to hormones with wide-ranging homeostatic functions. This scheme, outlined in Fig. 2-1, emphasizes a central theme of this chapter, namely, the physiologic function of glucocorticoids to protect the organism against stress.58

Normal glucocorticoid levels are regulated in a range and time course that reflect varying physiologic needs as well as the vulnerability of the organism, particularly the brain,59 to harm from excessive exposure. Basal hormone levels follow a circadian rhythm and reach peak values before the period of daily activity.60 Their actions maintain or permissively “prime” homeostatic mechanisms and protect against moderate stress. Stress-induced levels, which can far exceed peak basal levels, appear necessary to cope with severe stress. Peak basal levels cause Cushing’s syndrome if maintained indefinitely, so circadian lowering of glucocorticoid concentrations is physiologically necessary.

Synthesis and secretion of glucocorticoids is controlled by neural and humoral signals that change throughout the day and respond to stress and negative feedback.60 The main components of this system (see Fig. 2-1) are the adrenal cortex, where glucocorticoid secretion is stimulated by ACTH; the anterior pituitary, where ACTH secretion is stimulated by CRH, vasopressin (VP), and other secretagogues, and are inhibited by glucocorticoids; and the central nervous system, where CRH and VP synthesis in the hypothalamus is stimulated by stress and other influences, and is inhibited by glucocorticoids. Paradoxically, chronic actions of glucocorticoids on the brain exerted over days can be excitatory.61

Glucocorticoids exert feedback control on pituitary corticotrophs, the paraventricular nucleus (PVN) of the hypothalamus, and probably the hippocampus.42 Synthetic analogues like dexamethasone and prednisolone are exported from the brain by a multidrug resistance efflux transporter P-glycoprotein in the blood-brain barrier, which acts predominantly on the pituitary.42,54 Glucocorticoid regulation appears to be mediated both by GRs, which are found throughout the brain with high concentrations in the PVN, and by MRs, which are located mainly in the hippocampus and lateral septum. Actions on the brain via MRs can be considered to permissively control sensitivity to rapid CRH responses via CRH-1 receptors, maintaining the capacity of the HPA axis to respond to stress and maintain homeostasis; actions through GRs restrain stress-induced responses and facilitate learning and recovery of homeostasis.58

Studies on transgenic mice with altered GRs extend these conclusions.62,63 Mice with low levels of GRs have increased levels of CRH (not VP), ACTH, and corticosterone, as well as hypertrophy of the adrenal cortex. Overexpressed GRs reverse this picture. Mice with GRs that cannot dimerize have normal CRH and ACTH, showing that feedback via GRs probably is provided through genes controlled by protein-protein cross-talk. However, the gene for pro-opiomelanocortin (POMC), the ACTH precursor, is upregulated, implying control by GR dimers, which probably bind to nGREs, as noted below.

Inactivation of GRs in the nervous system64 leads to higher levels of corticosterone with Cushing’s-like symptoms. CRH is elevated, as is ACTH in pituitary corticotrophs, but circulating ACTH levels are slightly reduced—a divergence between ACTH and corticosterone reminiscent of that seen in clinically depressed patients.

Fig. 2-1 also illustrates one side of the reciprocal relation between the immune and neuroendocrine systems.65 Cytokines like interleukin 1 (IL-1), IL-6, and tumor necrosis factor (TNF)-α, which are produced mainly in the immune system but also by brain cells, stimulate the HPA axis. IL-1, IL-6, and TNF-α are proinflammatory cytokines, so stimulation of glucocorticoid secretion limits their activity throughout the organism. The importance of IL-1, for example, is revealed in knockout mice lacking IL-1 receptors and in mice overexpressing IL-1 antagonist in the brain66: they have reduced stress responses and fail to hypersecrete ACTH after adrenalectomy. Another regulator of the HPA axis may be leptin.67 A remarkable observation is that sucrose ingestion, like glucocorticoid replacement, can restore to normal most of the consequences of adrenalectomy on feeding and metabolism, and on the HPA axis, including ACTH levels, which presumably mimic signals from the metabolic effects of glucocorticoids.68

Each stage of the HPA feedback loop will now be considered.

Adrenal Cortex: Glucocorticoids

Synthesis of glucocorticoids, generally ascribed solely to the adrenal cortex, has been reported to occur in the thymus.69 It also occurs in the intestinal mucosa, where it influences local immune responses.70 In the adrenal cortex, glucocorticoid synthesis is closely tied to plasma levels of ACTH, which exhibit episodic peaks and circadian rhythm similar to plasma levels of glucocorticoids. ACTH stimulates steroidogenesis by binding to membrane receptors on adrenal cells, which activates adenylate cyclase and also causes hypertrophy and hyperplasia of the adrenal cortex. Leptin inhibits ACTH stimulation of cortisol secretion by adrenal cells.67

Pituitary: ACTH

The synthesis and secretion of ACTH in anterior pituitary corticotrophs are stimulated by CRH and VP, modulated by catecholamines, and inhibited by glucocorticoids. CRH binds to receptors on pituitary cell membranes and activates adenylate cyclase; cAMP then stimulates both secretion and synthesis of ACTH. Activity of CRH is strongly potentiated by VP. Whereas CRH increases the amount of ACTH secreted from each responsive corticotroph, VP, probably through the phosphoinositide pathway, increases the number of CRH-responsive corticotrophs. Nonetheless, knockout mice defective in type 1 CRH receptor (CRH-R1) respond to inflammatory stress with pronounced increases in ACTH and corticosterone that do not depend critically on CRH or VP.71

Glucocorticoids inhibit ACTH secretion directly by suppressing POMC expression in pituitary corticotrophs, and indirectly by inhibiting secretion of CRH and VP.60 After adrenalectomy, ACTH secretion rises, retaining its circadian rhythm. CRH and VP levels in the PVN also rise. These and other changes are reversed by glucocorticoids. Annexin 1 (lipocortin 1), a glucocorticoid-induced protein, mediates glucocorticoid inhibition of secretion of ACTH from the pituitary, apparently through a nongenomic mechanism.

Feedback has been classified according to how rapidly it inhibits ACTH secretion60: fast (within 30 minutes of hormone administration), delayed (minutes to hours), and slow (hours to days). The first two are believed to operate after moderate or intermittent stress; the third, in pathologic conditions or therapy with high glucocorticoid levels sustained for days.

Sensitivity to feedback depends on many factors, including the time of day. Basal ACTH release is less sensitive than stimulated release. Furthermore, a stressful stimulus in some way facilitates the ACTH response to a subsequent stress, overcoming the feedback inhibition due to the elevated glucocorticoid levels produced by the first stress. Some feedback can be seen as facilitative or permissive.72

Regulation of basal activity of the HPA axis requires glucocorticoid binding to both MRs and GRs. Inhibition of basal secretion of ACTH by corticosterone in rats at the low point of diurnal HPA activity (the morning) appears to occur through MRs, whereas inhibition at peak activity (evening) occurs through GRs potentiated by MRs.73 Suppression of stimulated ACTH secretion, which prevents overactivity in the stress-induced HPA axis, occurs through binding to GRs in pituitary corticotrophs and hypothalamic CRH neurons.

ACTH is produced as part of the larger precursor protein, POMC, which is also the progenitor of the melanocyte-stimulating hormones α- and β-MSH, β-endorphin, and the lipoproteins. Increased MSH activity associated with increased ACTH appears to be responsible for the changed skin color of Addisonian patients, as originally noted by Addison.2 Synthesis of POMC in pituitary corticotrophs is stimulated by CRH and is inhibited by glucocorticoids, at least partly at the level of transcription of the POMC gene. Direct repression by glucocorticoids occurs through nGREs, which may repress by disrupting interactions that maintain basal transcription. Indirect repression occurs via the hypothalamus.

Corticotrophs are also directly influenced by other hormones, including angiotensin II, paracrine secretions from neighboring pituitary cells, and cytokines such as TNF-α, IL-1, and IL-6.

Hypothalamus: CRH and VP

Secretion of CRH and VP from the paraventricular nuclei, along with other ACTH secretagogues, is subject to both humoral and neural regulation. Secretion increases following adrenalectomy, is stimulated in a stress-specific manner by hemorrhage, injury, hypoglycemia, hypoxia, pain, fear, and other kinds of stress, and generally is inhibited by glucocorticoids74 (see Fig. 2-1). Some inhibition by glucocorticoids may occur via a nongenomic path involving rapid endocannabinoid release in the PVN.75 CRH output can be modulated by catecholamines, leptin, and several cytokines.76 Acute hemorrhage raises levels in hypothalamic neurons of mRNA for CRH but not VP. CRH, via CRH-1 receptors, is thought to orchestrate the immediate behavioral, sympathetic, and HPA axis responses to stress, whereas the CRH-related neuropeptides stresscopin and urocortins, which bind to CRH-2 receptors, may assist slower stress responses.58

In normal rats, stress activates CRH gene expression in the PVN, which is suppressed by glucocorticoids at high levels. In adrenalectomized rats, stress does not activate CRH gene expression unless the animals are first treated with glucocorticoids at low levels.77 The low, facilitative, or permissive levels are thought to act through MRs, and the high, suppressive levels through GRs.77

CRH knockouts homozygous for the defective gene are viable as long as they receive glucocorticoids during the period from a week before birth until 2 weeks after birth. Without glucocorticoids, they die within 12 hours of gestation owing to severe lung abnormalities, including low surfactant mRNA. Glucocorticoids are known to be important for lung development, particularly for synthesis of surfactant.78 Compared with normal mice, the CRH knockouts exhibited a drastically diminished rise in corticosterone levels in response to stress.79

In addition to controlling ACTH secretion, CRH has numerous actions within and outside the brain. When secreted by peripheral nerves, it acts as a proinflammatory agent: mRNAs for CRH-R1 and CRH-R2 are expressed in adipose tissue, whereas CRH downregulates 11β-HSD1.47,80 Stresscopin and urocortins, via CRH-2 receptors, reduce appetite and may participate in delayed stress responses.58

Cytokine Feedback

As first proposed by Besedovsky and Sorkin,81 cytokines communicate between the immune system and the HPA axis. IL-1 has been shown to mediate HPA stimulation by endotoxin. IL-1α, IL-1β, IL-6, and TNF-α administered peripherally increase HPA activity with increased levels of glucocorticoids, ACTH, or POMC mRNA, and CRH or CRH mRNA. IL-1 causes release of both CRH and VP from neurosecretory cells.76 The brain has receptors for IL-1, IL-2, IL-6, and other cytokines, and it produces IL-1.82 (In Fig. 2-1, the question mark indicates uncertainty about which cytokines are most important and how their message is conveyed.)

Transgenic mice reveal the central role of IL-1 in feedback control and stress activation of the HPA axis. Mice lacking IL-1β fail to respond with increased plasma corticosterone to inflammatory stress, whereas mice lacking IL-1α respond normally, suggesting that IL-1β is crucial to the neuro-immuno-endocrine response.83 Mice lacking IL-1 receptor type 1 have diminished corticosterone responses to psychological, metabolic, and restraint stresses. These mice and mice with overexpressed IL-1 receptor antagonist targeted to the brain do not hypersecrete ACTH after adrenalectomy.66

Whether peripherally released cytokines like IL-1 enter the brain in physiologically significant amounts, and if not how their message reaches the hypothalamus, are unsettled issues. Among hypotheses that have been proposed are that cytokine messages are transmitted via the vagus or through specialized brain regions like the organum vascularis laminae terminalis (OVLT), or via mediators like eicosanoids, catecholamines, nitric oxide, or cytokines generated in the brain.

Physiologic Actions of Glucocorticoids

Metabolism

Control of Blood Glucose

Glucocorticoids act in concert with other hormones to maintain or raise blood glucose levels by (1) stimulating hepatic gluconeogenesis, (2) mobilizing gluconeogenic substrates from peripheral tissues, (3) permissively enhancing and prolonging the effects of glucagon and epinephrine on gluconeogenesis and glycogenolysis, (4) inhibiting peripheral glucose utilization, and (5) promoting liver glycogen synthesis to store substrate in preparation for acute responses to glycogenolytic agents such as glucagon and epinephrine.45

From an evolutionary standpoint, glucocorticoids in this way support stress responses that require glucose for rapid and intense exertion, such as an encounter of prey with predator.26 From a physiologic and clinical standpoint, glucocorticoids are counterregulatory hormones that protect the body from insulin-induced hypoglycemia. Both of these roles, in which glucocorticoid effects develop over the course of hours, are shared with the rapidly acting glucagon and epinephrine and to some extent with growth hormone.84

Glucocorticoid actions interact with those of insulin during feeding and fasting in complex ways that not only maintain blood glucose but influence appetite, feeding patterns, disposal of foodstuffs, and body composition.26,85 Antagonism with insulin in both glucose synthesis and utilization accounts at least partly for the diabetogenic actions of excessive glucocorticoids.45,86 Studies with transgenic mice show that expression of hepatic peroxisome-proliferator–activated receptor-α (PPAR-α) may be one mechanism underlying glucocorticoid-induced hypertension and insulin resistance.87

Gluconeogenesis

Hepatic gluconeogenesis is stimulated by glucocorticoids, mainly through the increased activities of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase. These enzymes catalyze the conversion of oxaloacetate to phosphoenolpyruvate and of glucose-6-phosphate to glucose—both rate-limiting steps in gluconeogenesis.45,88 Glucocorticoids also regulate expression of 6-phosphofructo-2-kinase/fructose 2,6-biphosphatase, a bifunctional enzyme that controls the level of fructose-2,6-biphosphate. Fructose-2,6-biphosphate is an allosteric regulator of gluconeogenic and glycolytic enzymes. PEPCK and 6-phosphofructo-2-kinase/fructose 2,6-biphosphatase activities are controlled principally through synthesis of the enzymes.88 On starvation, 11β-HSD1 knockout mice have diminished activation of PEPCK and glucose-6-phosphatase.89

Control of PEPCK gene expression reflects the complexity of regulation of gluconeogenesis in the body, involving glucocorticoids, insulin, glucagon, catecholamines, cyclic adenosine monophosphate (cAMP), and retinoic acid.88,90 In particular, glucocorticoids and insulin, by respectively promoting and indirectly disrupting association of CBP (CREB-binding protein) and RNA polymerase II with the PEPCK promoter, reciprocally regulate PEPCK gene expression. The PEPCK gene has a glucocorticoid response unit (GRU) that spans 110 base pairs. There are two GR-binding sites and four accessory factor elements, all of which are required for glucocorticoid regulation, and within the GRU are insulin-responsive and retinoic acid–responsive sequences.88,91 The 6-phosphofructo-2-kinase/fructose 2,6-biphosphatase gene has a complex glucocorticoid response element that resembles the GRU of the PEPCK gene. Hepatocyte nuclear factor-6 (HNF-6) inhibits glucocorticoid activation of both these genes by binding to DNA and GRs. As would be expected, treatment with glucocorticoids of transgenic mice with dimerization-deficient GRs (i.e., GRs that cannot bind to GREs) failed to induce PEPCK.35

Substrates for gluconeogenesis are generated by glucocorticoids through release of amino acids from muscle and other peripheral tissues and release of glycerol along with lipolysis.

Permissive actions of glucocorticoids on gluconeogenesis by glucagon and epinephrine, possibly due to enhanced responsiveness to cAMP or other intracellular mediators, are evidenced by the impairment of gluconeogenesis caused by adrenalectomy and its normalization by glucocorticoids.

Glucose Utilization

Glucocorticoid inhibition of peripheral glucose utilization can be demonstrated both in intact organisms and with isolated cells.92 It probably accounts for significant insulin antagonism and for the early rise in blood glucose seen after glucocorticoid treatment, and it may play a role in the release of gluconeogenic substrates from peripheral tissues. Glucose uptake is inhibited by direct glucocorticoid actions on normal skin, fibroblast, adipose tissue, adipocytes, lymphoid cells, and polymorphonuclear leukocytes. This inhibition, which requires RNA and protein synthesis, has been postulated to be mediated by a glucocorticoid-induced protein. It results mainly from translocation of glucose transporters from the plasma membrane to intracellular sites.93,94 Glucose uptake by muscle is inhibited in intact organisms treated with glucocorticoids. This action may be indirect.