Retinoic acid prevents experimental Cushing syndrome

Cushing syndrome is caused by an excess of adrenocorticotropic hormone (ACTH) production by neuroendocrine tumors, which subsequently results in chronic glucocorticoid excess. We found that retinoic acid inhibits the transcriptional activity of AP-1 and the orphan receptors Nur77 and Nurr1 in ACTH-secreting tumor cells. Retinoic acid treatment resulted in reduced pro-opiomelanocortin transcription and ACTH production. ACTH inhibition was also observed in human pituitary ACTH-secreting tumor cells and a small-cell lung cancer cell line, but not in normal cells. This correlated with the expression of the orphan receptor COUP-TFI, which was found in normal corticotrophs but not in pituitary Cushing tumors. COUP-TFI expression in ACTH-secreting tumor cells blocked retinoic acid action. Retinoic acid also inhibited cell proliferation and, after prolonged treatment, increased caspase-3 activity and induced cell death in ACTH-secreting cells. In adrenal cortex cells, retinoic acid inhibited corticosterone production and cell proliferation. The antiproliferative action and the inhibition of ACTH and corticosterone produced by retinoic acid were confirmed in vivo in experimental ACTH-secreting tumors in nude mice. Thus, we conclude that the effects of retinoic acid combine in vivo to reverse the endocrine alterations and symptoms observed in experimental Cushing syndrome.

Functional cross-talk among cytokines, T-cell receptor, and glucocorticoid receptor transcriptional activity and action

The main communicators between the neuroendocrine and immune systems are cytokines and hormones. We studied the molecular interaction between immune activators (cytokines and T-cell receptors [TCRs]) and the glucocorticoid receptor (GR) in cells in which glucocorticoids play a key regulatory function: (1) cellular targets of TNF-induced cytotoxicity; (2) the pituitary gland; and (3) thymic cells. Cytokines (TNF-alpha and IL-1) increase glucocorticoid-induced transcriptional activity of the GR via the DNA-glucocorticoid response elements (GREs) in cells transfected with a glucocorticoid-inducible reporter plasmid. As a functional physiological correlate, priming of fibroblastic cells with a low dose of TNF significantly increases the sensitivity to glucocorticoid inhibition of TNF-induced apoptosis (without involving NF-kappa B). Priming of AtT-20 mouse corticotrophs and Cushing pituitary cells with IL-1 increases the sensitivity to glucocorticoid inhibition of CRH-induced ACTH/POMC expression. In thymocytes, activation of the T-cell receptor counteracts the glucocorticoid-induced thymic apoptosis by downregulating the glucocorticoid action on GRE-driven apoptotic genes. Thus, cytokines and immune mediators prevent their own deleterious effects not only by stimulating glucocorticoid production, but also by modifying the sensitivity of the target cells for the glucocorticoid counter-regulatory action. The functional cross-talk at the molecular level between immune signals and glucocorticoids is essential to determine the biological response to both mediators and constitutes the ultimate level of interaction between the immune and neuroendocrine mediators.

Interleukin-6 is inhibited by glucocorticoids and stimulates ACTH secretion and POMC expression in human corticotroph pituitary adenomas

Interleukins and their receptors are expressed intrinsically in the anterior pituitary and regulate hormone production and cell proliferation. It has previously been shown that interleukin-6 (IL-6) regulates hormone secretion in normal pituitary cells and cell lines. Here we examined the effects of IL-6 on propiomelanocortin (POMC) expression and ACTH production in corticotroph adenoma cells in vitro. We found that IL-6 stimulates both ACTH secretion and POMC gene expression in corticotroph adenoma cell cultures. This first demonstration of the stimulatory action of IL-6 on human corticotroph adenoma cell function provides further evidence for a direct action of IL-6 on corticotroph pituitary cells. We have confirmed previous reports of IL-6 production by corticotroph adenoma cells and in addition, demonstrated for the first time that the synthetic glucocorticoid dexamethasone is a potent suppressor of intratumoral IL-6 production. This intratumoral produced IL-6 may be in part responsible, in an autocrine manner, for the stimulation of ACTH synthesis and secretion. Our results suggest that IL-6 might play a role in the pathogenesis of Cushing's disease. However, elevated glucocorticoid levels in patients with Cushing's disease may prevent excessive action of IL-6 on ACTH production and tumor progression of corticotroph adenomas in vivo.

Molecular mechanisms and Th1/Th2 pathways in corticosteroid regulation of cytokine production

We focus on how the IL-1 system, T-helper1 (Th1) or Th2 cytokines and glucocorticoids, converge to give a unified physiological response. Glucocorticoids inhibit IL-1 and IL-1 receptor antagonist (IL-1ra) expression, Th1 cytokines stimulate both and Th2 cytokines stimulate IL-1ra and inhibit IL-1. Thus, during the Th1 response there is a window for IL-1 inflammatory activity, absent during the Th2 response. We also study the interactions among glucocorticoid and cytokine transcriptional activity. Glucocorticoids inhibit cytokine-induced transcription factors (NFkB, AP1) and cytokines enhance glucocorticoid receptor (GR) transcriptional activity, thus reciprocally fine tuning immunological control mechanisms.

Differential regulation of interleukin-1 receptor antagonist by proopiomelanocortin peptides adrenocorticotropic hormone and beta-endorphin

We have previously described the regulation of interleukin-1 receptor antagonist (IL-1ra) protein secretion and expression by IL-1, glucocorticoids and corticotropin-releasing hormone in monocytes in culture. In the present work, we analyze the direct effect of adrenocorticotropic hormone (ACTH) and beta-endorphin on the expression and secretion of IL-1ra by human monocytes in culture. ACTH exerted a dose-dependent inhibitory effect on lipopolysaccharide (LPS)-induced IL-1ra production and mRNA expression. Basal IL-1ra levels were not affected by treatment with any ACTH dose. In contrast, on human monocytes, beta-endorphin at concentrations as low as 10 pg/ml produced an increase of basal IL-1ra protein secretion and mRNA expression, this effect being reverted by pretreatment with naloxone. No effect of beta-endorphin was observed either in IL-1ra mRNA expression or protein secretion when cells were treated with LPS. The different effects of ACTH and beta-endorphin could account for their differential contribution to the inflammatory response: while ACTH contributes to the glucocorticoid overall control of the inflammatory response, beta-endorphin exerts an inhibitory tone on the resting IL-1 system. Because IL-1ra is essential in setting the level of monocyte and inflammatory response its differential regulation by the HPA axis hormones contributes to regulating the IL-1/inflammatory temporal response.

The Th1 and Th2 cytokines IFN-gamma and IL-4 antagonize the inhibition of monocyte IL-1 receptor antagonist by glucocorticoids: involvement of IL-1

Monocytes express IL-1 and IL-1 receptor antagonist (IL-1Ra) in response to lipopolysaccharide (LPS). IL-1 self-induction contributes to the increase in IL-1 following LPS stimulation. LPS-stimulated IL-1 and IL-1Ra production are inhibited by glucocorticoids. In the present work we examined the regulation of IL-1Ra by Th1 cytokine IFN-gamma, Th2 cytokine IL-4, glucocorticoids and IL-1 in human monocytes. We demonstrate that IL-1 contributes to LPS-induced IL-1 Ra expression as shown by IL-1 blockade in LPS-stimulated monocytes using a specific anti-IL-1beta antibody or recombinant IL-1Ra. Glucocorticoids inhibited IL-1beta-stimulated IL-1Ra mRNA expression and protein production. Glucocorticoids inhibited both IL-1-mediated and non-mediated LPS stimulation of IL-1Ra expression. Both IFN-gamma and IL-4 reversed the inhibitory effect of glucocorticoids on IL-1Ra expression and secretion. The effect of IFN-gamma was blocked by pretreatment of monocytes with an anti-IL-1beta blocking antibody, whereas the effect of IL-4 could not be blocked, demonstrating that IFN-gamma acts through a mechanism dependent on endogenous IL-1 production, whereas IL-4 acts through an IL-1-independent one. Consistent with this finding, IFN-gamma (but not IL-4) failed to reverse the inhibitory effect of glucocorticoids when stimulated by IL-1, and only IL-4 combined with IL-1 showed synergism resulting in an increase in IL-1 Ra production. The differential regulation and involvement of IL-1 in the expression of IL-1Ra by IFN-gamma, IL-4 and glucocorticoids sets the level of monocyte responsiveness during the Th1 or Th2 responses.

[Mechanisms of glucocorticoid sensitivity modulation by cytokines]

We have previously shown that TNF-alpha and IL-1 may enhance the glucocorticoid (GC)-induced transcriptional activity of glucocorticoid receptor (GR) in different cell lines transfected with a reporter plasmid carrying GC response elements (GRE). In TNF-alpha and GC target cell lines, it was found that: 1) TNF-alpha enhanced GR number in L-929 cells, and 2) by transfection of these cells with a reporter plasmid carrying the GR promoter, that TNF-alpha-induced increase in GR is at the transcriptional level, 3) by electrophoretic mobility shift assay, using nuclear extracts of TNF-alpha (0.02 ng/ml) or TNF-alpha plus DEX (10 nM) stimulated L-929 cells, that cytokines can increase the binding of GR to GRE (45 min, 1.8 x), while the TNF-alpha-induced NFkB factor expression was not affected by GC. 4) As a biological correlate of this mechanism, priming of L-929 cells with TNF-alpha significantly increased (p < 0.001) the sensitivity to GC inhibition of TNF-alpha-induced apoptosis. The organism protects itself from an immune overreaction, not only via the HPA axis induction and an increase in GC by cytokines, but also enhancing the sensitivity to GC: by an increase in GR number, the binding to GRE and the transcription of GC target genes (e.g. TNF-alpha-induced apoptosis inhibitory genes). These mechanisms contribute to enhance the immunosuppressive and antiinflammatory GC activity, in order to maintain homeostasis.