Adrenocorticotropin (ACTH)- and Non-ACTH-Mediated Regulation of the Adrenal Cortex: Neural and Immune Inputs

The maintenance of life depends on the capacity of the body to sustain its steady state or homeostasis; hence, the survival of the organism relies on its ability to react and adapt to the constant bombardment by physical and emotional threats or stressors (1, 2). To accomplish this, all living beings have developed an efficient but complex adaptive response system that allows the integration of the necessary defense mechanisms directed against both external and internal stressors. This coordinated response, the stress response, involves the nervous, endocrine, and immune systems. A qualitatively and quantitatively appropriate and time-limited stress response is a prerequisite for a healthy life. Inappropriate or inappropriately excessive and chronic hyperactivation of the stress system may lead to disease and, eventually, premature death. The two major peripheral limbs of the stress system are the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system (1, 2). Their central components are, respectively, located in the hypothalamus and the brain stem. Stress-induced activation of the HPA axis is associated with release of hypothalamic CRH and vasopressin (AVP), the principal regulators of anterior pituitary corticotropin (ACTH) secretion, into the hypophyseal portal system. These hormones synergistically stimulate systemic ACTH secretion, which, in turn, stimulates the adrenal cortexes to secrete glucocorticoids. Central activation of the sympathetic neurons leads to activation of both the systemic sympathetic nervous system and, through the splanchnic nerves, the adrenal medullae (3). As the proper functioning of both the HPA axis and the sympathetic nervous system is crucial for survival and maintenance of health, it is not surprising that their regulation is developmentally plastic and complex, multilevel, and redundant. The regulation and central interaction of the HPA axis and the sympathetic nervous system and the immune system have been extensively studied and summarized in recent review articles (1, 3–5). The purpose of this brief review is to outline and discuss recent advances in the interactions and regulation of the adjacent peripheral limbs of the stress system, particularly the adrenal cortex and medulla, and to point out their implications for clinical endocrinology (Fig. 1). The adrenal gland has an astonishing capacity to adapt to various forms of acute and chronic stress (6–8). After central activation of the HPA axis, ACTH triggers a physiologic, molecular, and morphological response of the adrenal cortex. This leads not only to glucocorticoid release, but also to up-regulation of steroidogenic cytochrome P450 messenger ribonucleic acids (9, 10) and to conspicuous structural changes in the adrenal gland characterized by both hypervascularization and cellular hypertrophy and hyperplasia (11, 12). These morphological changes are mirrored on the ultrastructural level; adrenocortical cells increase the number of their mitochondria, whereas their inner membranes form a dense vesicular pattern (13, 14). In addition, there is an increase in smooth endoplasmic reticulum and filopodia and a decrease in liposomes known to store cholesterol, the substrate for glucocorticoid biosynthesis (13–15). The adrenal gland, as the end organ of the human stress system, reacts with the above changes in many clinical situations that involve severe or chronic stress. Subacute or chronic stress, for example major surgery, or lingering affective disorders, chronic infections, and chronic autoimmune diseases are frequently associated with adrenal alterations (16). Only recently it became evident that in these states, dissociation between central activation of the HPA axis and the adrenal cortex may occur (7, 17). Thus, frequently, ACTH levels do not correspond to the chronically elevated concentrations of glucocorticoids and the hypertrophy/hyperplasia of the adrenal gland. This dissociation cannot be explained by the different half-lives of the pituitary and adrenal hormones and suggests a reset of the HPA axis and/or the presence of extrapituitary mechanisms of adrenal regulation. Mounting evidence in recent years suggests that the interaction of the two ontogenetically different parts of the adrenal gland, the cortex and the medulla, is not a one-way street but, rather, a bidirectional phenomenon that also receives input from the nervous and immune systems (Fig. 1) (17, 18). The influence of the adrenal cortex on the adrenal medulla and on the expression of catecholamine biosynthetic enzymes and synthesis has been well characterized in vitro (19) and in vivo (20). On the other hand, the influence of the sympatho-adrenomedullary system on adrenocortical functions includes the diurnal variation of adrenal steroidogenesis, which depends on the integrity of sympathetic innervation (for review, see Refs. 21, 22). Also, neural inputs seem to mediate compensatory growth in the remaining adrenal after unilateral adrenalectomy (23). The sympatho-adrenomedullary system produces these effects on the adrenal cortex in part by increasing sensitivity to ACTH. Indeed, splanchnic nerve stimulation enhanced the production of glucocorticoids in response to ACTH, whereas sectioning of both splanchnic nerves in calves decreased adrenocortical sensitivity to ACTH (24). In recent experiments in isolated perfused pig adrenal glands with intact splanchnic innervation, steroidogenesis was stimulated independently of ACTH through electrical activation of the sphlanic nerves and, hence, the sympathoadrenomedullary system (25–28). Therefore, this ganglion turned gland in the middle of the steroid-producing endocrine gland appears to be intimately involved in the regulation of adrenocortical function in mammals. ACTH- and non-ACTH-mediated regulation of the adrenal cortex: neural and immune inputs. Neuroendocrine regulation of the adrenal zona fasciculata may depend on neurotransmitters released from nerve endings that originate from two different sources but terminate in the adrenal cortex (22, 29). Some of these nerves have their cell bodies outside the adrenal gland and reach the cortex with the blood vessels; these are quite independent from the splanchnic nerves. Other neurons originate in cell bodies from within the adrenal medulla and may be regulated by splanchnic nerve activity. These adrenal nerves are mainly catecholaminergic and peptidergic, storing the catecholamines dopamine, epinephrine, and norepinephrine and a wide variety of neuropeptides, including opioid peptides, calcitonin gene-related peptide, neuropeptide Y, vasoactive intestinal polypeptide (VIP), CRH, and substance P (6, 17). These neurotransmitters and neuropeptides have been shown to modulate adrenocortical function (Table 1). Non-ACTH factors involved in regulating adrenocortical function VIP, Vasoactive intestinal peptide; NPY, neuropeptide Y; PACAP, pituitary adenylate cyclase-activating peptide; ANP, atrial natriuretic peptide; IL, interleukin; TNF, tumor necrosis factor; MIF, migration inhibitory factor; TGF, transforming growth factor; IGF, insulin-like growth factor; EGF, epidermal growth factor. Non-ACTH factors involved in regulating adrenocortical function VIP, Vasoactive intestinal peptide; NPY, neuropeptide Y; PACAP, pituitary adenylate cyclase-activating peptide; ANP, atrial natriuretic peptide; IL, interleukin; TNF, tumor necrosis factor; MIF, migration inhibitory factor; TGF, transforming growth factor; IGF, insulin-like growth factor; EGF, epidermal growth factor. The main adrenomedullary secretory products, epinephrine and norepinephrine, stimulate mammalian adrenocortical function in situ in perfused adrenals (25, 27) and in vitro in adrenocortical cells in primary culture (for review, see Ref. 17) by enhancing the transcriptional activity of several steroidogenic factors and enzymes (10, 30). Thus, it appears that catecholamines and costored neuropeptides released from adrenomedullary chromaffin cells in large amounts stimulate adrenocortical steroidogenesis in a direct fashion and are probably responsible for most of the observed activation of the mammalian adrenal cortex by the splanchnic nerve. As mentioned above, adrenomedullary chromaffin cells also produce, store, and secrete a whole host of neuropeptides, including CRH, enkephalins, calcitonin gene-related peptide, neuropeptide Y, neurotensin, galanin, substance P, AVP, oxytocin, VIP, somatostatin, PACAP, and POMC-derived peptides (for review, see Ref. 17). In the normal human adrenal, catecholamines have no major effect on adrenal steroidogenesis, whereas several neuropeptides produced in the adrenal medulla, such as VIP, PACAP, ANP, and vasopressin regulate adrenocortical steroid production. Finally, the adrenal medulla and intraadrenal immune cells are a source of extrahypothalamic CRH and extrapituitary ACTH (17). ACTH immunoreactivity was demonstrated in extracts of human adrenals and in the adrenal venous effluent of hypophysectomized calves in response to splanchnic stimulation (31). Therefore, local ACTH, possibly in response to local CRH, may stimulate adrenal cortisol production in the absence of pituitary ACTH. Nevertheless, local ACTH plays only a minor role in the up-regulation of basal cortisol release in bovine cortico-chromaffin cell cocultures (32). Coculture systems of bovine adrenomedullary chromaffin with adrenocortical cells have now supplied direct evidence for the paracrine influence of chromaffin cells on adrenocortical cells. In these systems, medullary and adrenocortical cells are separated by semipermeable membranes. We demonstrated that secretory products released from chromaffin cells under basal conditions were potent stimulators of adrenocortical steroidogenesis; this stimulatory effect was independent of a direct cell-cell contact (32). The accepted textbook view holds that the two different components of the adrenal gland are clearly separated in an outer steroid-producing cortex and a central medulla with blood circulation directed centripetally (33). However, this concept is not supported by the morphological characteristics of this organ. In fact, the adrenal medulla and cortex are significantly interwoven in many mammals, including humans. Indeed, adrenomedullary chromaffin cells can be found in all zones of the adult adrenal cortex. Some form ray-like structures stretching from the adrenal medulla through the entire cortex (14, 34, 35). Others form small islets or are simply dispersed in the zona fasciculata and reticularis, fully surrounded by steroid-producing cells. In the zona glomerulosa, chromaffin cells spread frequently into the capsular region, forming subcapsular nests of cells (34–36). The presence of adrenomedullary chromaffin cells in the adrenal cortex may be explained on the basis of the embryologic development of the adrenal gland. In humans, chromaffin precursor cells start to invade the adrenal primordium from the outside at the sixth week of embryonic life (37); thus, chromaffin cells located in the cortex probably discontinued their migration toward the future medulla before they reached the center of the gland. On the other hand, some adrenocortical cells are located within the adrenal medulla, suggesting that they extended their migration from the subcapsular region inward. Besides a small number of adrenocortical cells surrounding the greater vessels, isolated accumulations of such cells are found within the adrenomedullary chromaffin tissue, whereas other accumulations are connected to the adrenal cortex; in some cases, the human adrenal medulla appears to be peppered with cortical cells (38); interestingly, in the human adrenal, these adrenocortical islets are composed of cells from all three cortical zones (39). Ultrastuctural analyses of the contact areas in all three zones of the adrenal cortex revealed that adrenocortical and adrenomedullary chromaffin cells were posed next to each other without separation by connective tissue or an interstitial membrane (36, 38, 40, 41); this intimate intermingling of the two cell types allows extensive contact for paracrine and juxtacrine interactions (18). In addition to a direct paracrine action, some adrenomedullary secretory products may reach the adrenal cortex via interstitial fluid and lymphatics, as has been shown in the cat adrenal. Small molecules, such as catecholamines, appear to enter the blood vessels directly and therefore can only influence adrenocortical cells that are in direct contact with the producing chromaffin cell. Larger molecules, such as neuropeptides and proteins, may cross into and from the lymph, reaching adrenocortical cells (42). Immune cells, including macrophages, monocytes, dendritic cells, mast cells, and lymphocytes, are located within the adrenal cortex of rodents and humans. Resident macrophages are mostly located in the inner adrenocortical zone and express tumor necrosis factor-α (TNFα) (43), interleukin-1 (IL-1) (44), IL-6 (45), and transforming growth factor-β (46) when activated. These cytokines may differently influence adrenal function by exerting stimulatory and/or inhibitory effects (47, 48). Similarly, circulating leukocytes and lipopolysaccharide-stimulated macrophages respectively stimulate or inhibit glucocorticoid biosynthesis by human or rabbit adrenocortical cells and may participate in a local immune-adrenal regulation. Lymphocytes may also be found in the inner cortical layers in a focal manner; these foci are observed in childhood (49), and their number increases with age (50). They mostly belong to the compartment of CD4-positive cells and express IL-2 receptors. As cells of this phenotype produce a variety of cytokines, it is likely that these cells are also an important source of cytokines in the adrenal gland. Finally, lymphocytic infiltration of adrenal tissue has been noted in histological sections of the adrenals of some patients with adrenal Cushing’s syndrome (51, 52). Adrenocortical cells themselves are able to synthesize several cytokines. Similarly to macrophages within the adrenal, they contain TNFα (43), IL-1 (44), and IL-6 messenger ribonucleic acid (53). The distribution of this expression varies in a species-specific manner; in rats, high amounts of cytokines have been detected in the zona glomerulosa (47), whereas in humans, the main site of cytokine production is the inner zona reticularis (43, 44, 53). Studies in mice and rats indicated that potent activators of hormone synthesis in the adrenal cortex, such as angiotensin II (47) and ACTH (47), induce IL-6 secretion in the adrenal cortex, whereas TNFα release is inhibited by ACTH (47). The recent discovery of migration inhibitory factor expression in the rat adrenal may provide an explanation for the ability of adrenal lymphocytes and steroid-secreting cells to synthesize and secrete these cytokines in the presence of high local production of glucocorticoids inhibitory a hormone for glucocorticoid action, the effects of on cytokine production and cellular of the cytokines shown to be produced in the adrenal cortex are able to direct effects on adrenocortical cells (47, (Table 1). These effects on growth and of the adrenocortical cells and changes in adrenocortical the cytokines and IL-6 all seem to a role in local immune-adrenal regulation. IL-1 biosynthesis in vivo independently from ACTH and glucocorticoid secretion in hypophysectomized rats perfused rat adrenals and dispersed human adrenal cells IL-6 stimulated release from rat adrenocortical cells and in with ACTH, an effect probably and by human IL-6 also the secretion of cortisol and adrenal in both via ACTH and of the IL-6 on steroid-producing cells was demonstrated TNFα inhibited the secretion of from rat adrenal cells whereas in human adrenal cells, it decreased both basal and cortisol production. In the cell system, it a toward synthesis and both inhibited the expression of insulin-like growth factor a factor that in human adrenals the regulation between the systemic sympathetic and adrenomedullary components of the sympathetic nervous system has been well the regulation of the adrenal cortex by ACTH- or non-ACTH-mediated mechanisms has to be The have been this 1) As leads to adrenal and ACTH to of adrenal glucocorticoid secretion, non-ACTH-mediated regulation of the adrenal cortex have non-ACTH-mediated regulation occur in or is it only a in severe or chronic or non-ACTH-mediated regulation an or of adrenal Finally, can between the different components of this regulation of the HPA axis by and such be in the and of human ACTH is the primary of glucocorticoid secretion by and adult adrenal However, there is evidence for factors a role in adrenal to in and life In in the the elevated cortisol concentrations were independent from ACTH and on the splanchnic innervation of the gland Similarly, nerve the in cortisol in response to acute without the ACTH response of the in changes in cortisol secretion, which were not in changes in ACTH secretion pituitary cells were by in circulating ACTH the of there was no in adrenocortical between the mice that no pituitary cells and the These that this the and to normal adrenal function were independent from pituitary ACTH. In fact, the in life is most likely an The cellular mechanisms involved in the neural to ACTH regulation that in life are at this these have to humans, the to the is in of a for non-ACTH-mediated regulation of the adrenal cortex, under both and be to the role of ACTH the of life in human and in healthy may a dissociation of ACTH and cortisol secretion In many clinical there is a dissociation between ACTH concentrations and cortisol secretion in that cannot be explained by the different of these hormones (Table 2). To of ACTH secretion with not a in cortisol in response to stress, whereas it in the stress of (17, in patients and in patients with and in the chronic severe of human elevated cortisol levels were by or normal ACTH (17, The phenomenon was observed in patients to of human IL-6 Similarly, in patients with adrenals and cortisol the ACTH levels are and their response to CRH In of ACTH and cortisol revealed a dissociation of the hormone that within A dissociation also in patients with chronic in patients was by a ACTH response to of life and clinical suggesting of non-ACTH-mediated regulation of adrenocortical function of life and clinical suggesting of non-ACTH-mediated regulation of adrenocortical function have supported a between the HPA axis and adrenomedullary stress (3). Thus, levels and of cortisol and epinephrine, with only small changes in norepinephrine levels Also, in a of ACTH levels with of epinephrine, but not norepinephrine (3). In rats, venous concentrations of ACTH, and epinephrine, whereas concentrations of norepinephrine large epinephrine and but small norepinephrine In this it is that various forms of stress that the adrenomedullary system, such as chronic or stress, and affective are also in non-ACTH-mediated increases in adrenocortical In such as and ACTH not a crucial role in adrenocortical function and local paracrine mechanisms can fully ACTH regulation of adrenocortical In mechanisms can adrenal function in and life to a However, in adult mammalian of ACTH clearly leads to adrenal and the presence of ACTH is for maintenance of normal adrenocortical regulation of adrenocortical may be responsible for a basal small of cortisol production in as frequently observed in hypophysectomized Also, on a body of in various the innervation of the adrenal is important for a normal adrenal of secretion, and compensatory adrenal growth after unilateral adrenalectomy (17, The is particularly to is ACTH adrenocortical cell and cell when to isolated adrenal cells it was recently that secretion a the of ACTH by stress As the sympathetic nervous system the HPA axis to it is that the of the response that of ACTH is explained by an splanchnic nerve stimulation of the adrenal cortex. In acute stress the body all three levels of the HPA axis, and an acute increase of ACTH is and by an increase in In severe forms of stress, such as major extensive surgery, acute or the human body can increase its adrenal cortisol production to This increase is crucial for with these severe forms of stress, that such situations are for or patients with adrenocortical the extrapituitary regulation of adrenocortical function into major in chronic forms of stress. Thus, cortisol production to be a ACTH levels to the normal or normal whereas cortisol levels elevated This suggests that extrapituitary mechanisms may in the maintenance of high cortisol levels in chronic forms of stress. This of the adrenal cortex to chronic stress is by an increase in adrenal and of the necessary for steroidogenesis, the and the smooth endoplasmic reticulum The increase in steroidogenesis after ACTH stimulation is by the of in liposomes within the adrenocortical cells, by of from the and by synthesis of ACTH stimulates adrenal it is well known that ACTH is to a normal cortisol to In fact, there are many and conditions in which there is a dissociation of adrenal and cortisol production. This has been the and between adrenal and cortisol has been in Cushing’s disease in patients steroid in in and in stress In of chronic or severe steroid synthesis may be from adrenal to glucocorticoids to maintenance of high glucocorticoid which are crucial for with the The mechanisms involved in this dissociation of adrenal and cortisol production have not been and both and intraadrenal factors have been such as other pituitary POMC-derived peptides, and as well as growth factors and cytokines produced within the adrenal can regulate adrenal production. A regulation of which the biosynthesis of through a factor not been demonstrated as On the other hand, adrenal cells express major complex II and that in to or or in autoimmune lymphocytes may a major complex cell in the inner zone of the adrenal cortex, which to the observed of adrenal secretion (for review, see Ref. or produced IL-6 has the capacity to increase adrenal steroid production chronically but not This cytokine is elevated in situations characterized by chronic stress, such as in patients in severe stress of for example the maintenance of high glucocorticoid production is and chronic activation of the stress system may have to chronically elevated glucocorticoid Also, chronic of the adrenal cortex in with of for neuropeptides neurotransmitters or cytokines may lead to adrenal tumor and possibly non-ACTH-mediated Cushing’s syndrome in humans. Thus, it appears that the stress response was not by to be an but, rather, to be an that in the chronic regulation of the adrenal gland. To the role of the stress response in clinical be This is not an an in vivo of these systems be to be complex (18). for of both ACTH and cortisol levels may dissociation of the two a phenomenon that observed in this the A of be this be or Thus, the of the with may be to the role of the regulation of adrenocortical is to pituitary ACTH release by CRH and AVP, whereas activation of the adrenal medulla most likely not be as A or of the sympathetic nervous system on adrenocortical cortisol secretion by several medullary products be studied such a In this it is of that in CRH the ACTH release after was whereas there was a increase in adrenal secretion be to a of CRH and These have a greater inhibitory effect on the ACTH- on the non-ACTH-mediated of adrenocortical regulation. of the and from isolated cell systems of the stress axis has that there is a and important interaction all major components of the stress system, including the endocrine, nervous, and immune systems. adrenocortical cell of its tissue of input from the nervous system, and of its with and immune cells its normal capacity to produce glucocorticoids and to to the of stress (18). the of the extrapituitary mechanisms of adrenocortical regulation may be a point for a of the human stress system in is also a to at the systems in not only in endocrine disorders, but in all diseases to stress This to and efficient and for such