Interferon-alpha reduces catecholamine secretion from bovine adrenal chromaffin cells stimulated by acetylcholine

Inflammatory Signaling in Hypertension: Regulation of Adrenal Catecholamine Biosynthesis

The immune system is increasingly recognized for its role in the genesis and progression of hypertension. The adrenal gland is a major site that coordinates the stress response via the hypothalamic-pituitary-adrenal axis and the sympathetic-adrenal system. Catecholamines released from the adrenal medulla function in the neuro-hormonal regulation of blood pressure and have a well-established link to hypertension. The immune system has an active role in the progression of hypertension and cytokines are powerful modulators of adrenal cell function. Adrenal medullary cells integrate neural, hormonal, and immune signals. Changes in adrenal cytokines during the progression of hypertension may promote blood pressure elevation by influencing catecholamine biosynthesis. This review highlights the potential interactions of cytokine signaling networks with those of catecholamine biosynthesis within the adrenal, and discusses the role of cytokines in the coordination of blood pressure regulation and the stress response.

Interleukin-6-mediated signaling in adrenal medullary chromaffin cells

The pro-inflammatory cytokines, tumor necrosis factor-α, and interleukin-1β/α modulate catecholamine secretion, and long-term gene regulation, in chromaffin cells of the adrenal medulla. Since interleukin-6 (IL6) also plays a key integrative role during inflammation, we have examined its ability to affect both tyrosine hydroxylase activity and adrenomedullary gene transcription in cultured bovine chromaffin cells. IL6 caused acute tyrosine/threonine phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), and serine/tyrosine phosphorylation of signal transducer and activator of transcription 3 (STAT3). Consistent with ERK1/2 activation, IL6 rapidly increased tyrosine hydroxylase phosphorylation (serine-31) and activity, as well as up-regulated genes, encoding secreted proteins including galanin, vasoactive intestinal peptide, gastrin-releasing peptide, and parathyroid hormone-like hormone. The effects of IL6 on the entire bovine chromaffin cell transcriptome were compared to those generated by G-protein-coupled receptor (GPCR) agonists (histamine and pituitary adenylate cyclase-activating polypeptide) and the cytokine receptor agonists (interferon-α and tumor necrosis factor-α). Of 90 genes up-regulated by IL6, only 16 are known targets of IL6 in the immune system. Those remaining likely represent a combination of novel IL6/STAT3 targets, ERK1/2 targets and, potentially, IL6-dependent genes activated by IL6-induced transcription factors, such as hypoxia-inducible factor 1α. Notably, genes induced by IL6 include both neuroendocrine-specific genes activated by GPCR agonists, and transcripts also activated by the cytokines. These results suggest an integrative role for IL6 in the fine-tuning of the chromaffin cell response to a wide range of physiological and paraphysiological stressors, particularly when immune and endocrine stimuli converge.

Divergent neuroendocrine responses to localized and systemic inflammation

The sympathetic nervous system (SNS) is part of an integrative network that functions to restore homeostasis following injury and infection. The SNS can provide negative feedback control over inflammation through the secretion of catecholamines from postganglionic sympathetic neurons and adrenal chromaffin cells (ACCs). Central autonomic structures receive information regarding the inflammatory status of the body and reflexively modulate SNS activity. However, inflammation and infection can also directly regulate SNS function by peripheral actions on postganglionic cells. The present review discusses how inflammation activates autonomic reflex pathways and compares the effect of localized and systemic inflammation on ACCs and postganglionic sympathetic neurons. Systemic inflammation significantly enhanced catecholamine secretion through an increase in Ca(2+) release from the endoplasmic reticulum. In contrast, acute and chronic GI inflammation reduced voltage-gated Ca(2+) current. Thus it appears that the mechanisms underlying the effects of peripheral and systemic inflammation neuroendocrine function converge on the modulation of intracellular Ca(2+) signaling.

Toll-like receptor 4 activation reduces adrenal chromaffin cell excitability through a nuclear factor-κB-dependent pathway

The adrenal medulla contains fenestrated capillaries that allow catecholamines and neuropeptides secreted by adrenal chromaffin cells (ACCs) to readily access the circulation. These capillaries may also allow bacterial products to enter the adrenal medulla and interact with ACCs during infection. One potential mediator of this interaction is toll-like receptor 4 (TLR-4), a pattern-recognition receptor that detects lipopolysaccharide (LPS) from Gram-negative bacteria. Evidence suggests that excitable cells can express TLR-4 and that LPS can modulate important neuronal and endocrine functions. The present study was therefore performed to test the hypothesis that TLR-4 activation by LPS affects ACC excitability and secretory output. RT-PCR revealed that TLR-4, cluster of differentiation 14, myeloid differentiation protein-2, and myeloid-derived factor 88 are expressed within mouse adrenal medullae. TLR-4 immunoreactivity was observed within all tyrosine hydroxylase immunoreactive ACCs. Incubation of isolated ACCs in LPS dose dependently hyperpolarized the resting membrane potential and enhanced large conductance (BK) Ca(2+)-activated K(+) currents. LPS (10 μg/ml) also increased rheobase, decreased the number of action potentials fired at rheobase, and reduced the percentage of ACCs exhibiting spontaneous and anodal break action potentials. Although catecholamine release was unaltered, LPS significantly reduced high-K(+)-stimulated neuropeptide Y release from isolated ACCs. LPS did not alter the excitability of ACCs from TLR-4(-/-) mice. Inhibition of nuclear factor-κB signaling with SC-514 (20 μm) abolished the effects of LPS on ACC excitability. Our findings suggest that LPS acts at TLR-4 to reduce ACC excitability and neuropeptide Y release through an nuclear factor-κB-dependent pathway.

Immune-neuroendocrine integration at the adrenal gland: cytokine control of the adrenomedullary transcriptome

The bovine chromaffin cell represents an ideal model for the study of cell signaling to gene expression by first messengers. An abundance of GPCR, ionotropic, and growth factor receptors are expressed on these cells, and they can be obtained and studied as an abundant highly enriched cell population; importantly, this is true of no other postmitotic neuroendocrine or neuronal cell type. Chromaffin cells have now been shown to bear receptors for cytokines whose expression in the circulation is highly elevated in inflammation, including tumor necrosis factor, interferon, interleukin-1, and interleukin-6. The use of bovine-specific microarrays, and various biochemical measurements in this highly homogenous cell preparation reveals unique cohorts of distinct genes regulated by cytokines in chromaffin cells, via signaling pathways that are in some cases uniquely neuroendocrine. The transcriptomic signatures of cytokine signaling in chromaffin cells suggest that the adrenal medulla may integrate neuronal, hormonal, and immune signaling during inflammation, through induction of paracrine factors that signal to both adrenal cortex and sensory afferents of the adrenal gland, and autocrine factors, which determine the duration and type of paracrine secretory signaling that occurs in either acute or chronic inflammatory conditions.

Altered adrenal chromaffin cell function during experimental colitis

The sympathetic nervous system regulates visceral function through the release of catecholamines and cotransmitters from postganglionic sympathetic neurons and adrenal chromaffin cells (ACCs). Previous studies have shown that norepinephrine secretion is decreased during experimental colitis due to the inhibition of voltage-gated Ca(2+) current (I(Ca)) in postganglionic sympathetic neurons. The present study examined whether colonic inflammation causes a similar impairment in depolarization-induced Ca(2+) influx in ACCs using the dextran sulfate sodium (DSS) model of acute colitis in mice. Alterations in ACC function during colitis were assessed using fura 2-acetoxymethyl ester Ca(2+) imaging techniques and perforated patch-clamp electrophysiology. In ACCs isolated from mice with DSS-induced acute colitis, the high-K(+)-stimulated increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) was significantly reduced to 74% of the response of ACCs from control mice. Acute colitis caused a 10-mV hyperpolarization of ACC resting membrane potential, without a significant effect on cellular excitability. Delayed-rectifier K(+) and voltage-gated Na(+) current densities were significantly enhanced in ACCs from mice with DSS-induced acute colitis, with peak current densities of 154 and 144% that of controls, respectively. Importantly, acute colitis significantly inhibited I(Ca) in ACCs between -25 and +20 mV. Peak I(Ca) density in ACCs from mice with DSS-induced acute colitis was 61% that of controls. High-K(+)-induced increases in [Ca(2+)](i) were also reduced in ACCs from mice with 2,4,6-trinitrobenzene sulfonic acid-induced acute colitis and DSS-induced chronic colitis to 68 and 78% of the control responses, respectively. Our results suggest that, during colitis, voltage-dependent Ca(2+) influx is impaired in ACCs. Given the importance of Ca(2+) signaling in exocytosis, these alterations may decrease systemic catecholamine levels, which could play an important role in inflammatory bowel disease. This is the first demonstration of aberrant ACC function during experimental colitis.

Cytokine interactions with adrenal medullary chromaffin cells

It is generally accepted that a bi-directional or reciprocal interaction occurs between the immune and neuroendocrine systems, and that this relationship is important for the appropriate physiological functioning of both systems. Similarly, an imbalance in this relationship may contribute to a number of pathologies, most notably those relating to stress. The aim of this article is to consider the interaction of cytokines with the adrenal medulla, a potentially important player in this relationship. The chromaffin cells of the adrenal medulla release catecholamines and a range of biologically active peptides in response to a wide variety of stress-related signals. A growing body of evidence indicates that this stress response is influenced by, and in turn has influence upon, immune signalling. This brief review will focus primarily on the best-described adrenal medullary active cytokines, namely interferon-α, interleukin-6, interleukin-1α/β and tumour necrosis factor-α. In each case, three key issues will be addressed: the physiologically relevant source of the cytokine; the intracellular signalling events arising from activation of its receptor and finally the cellular consequences of such activation in terms of modulation of gene expression and the secretory output of the chromaffin cells.

Interferon-alpha signalling in bovine adrenal chromaffin cells: involvement of signal-transducer and activator of transcription 1 and 2, extracellular signal-regulated protein kinases 1/2 and serine 31 phosphorylation of tyrosine hydroxylase

Adrenal medullary chromaffin cells are an integral part of the neuroendocrine system, playing an important role in the physiological adaptation to stress. In response to a wide variety of stimuli, including acetylcholine released from the splanchnic nerve, hormones such as angiotensin II or paracrine signals such as prostaglandins, chromaffin cells synthesise and secrete catecholamines and a number of biologically active peptides. This adrenal medullary output mediates a complex and diverse stress response. We report that chromaffin cells also respond both acutely and chronically to interferon (IFN)-alpha, thus providing a mechanism of interaction between the immune system and the stress response. Incubation of isolated bovine chromaffin cells maintained in culture, with IFN-alpha resulted in a rapid, transient activation of the extracellular signal-regulated protein kinase (ERK)1/2, which was maximal after 5 min. IFN-alpha mediated activation of ERK1/2 appeared to be responsible for the increased phosphorylation of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine synthesis. This tyrosine hydroxylase phosphorylation was exclusively on serine 31, with no change in the phosphorylation of serine 19 or 40. This increase in the serine 31 phosphorylation of tyrosine hydroxylase was prevented by inhibition of protein kinase C or ERK1/2 activation. Incubation with IFN-alpha also resulted in a time- and concentration-dependent phosphorylation and nuclear translocation of signal transducer and activator of transcription proteins (STAT)1 and 2. This response was maximal after approximately 60 min. Prolonged treatment with IFN-alpha (12-48 h) resulted in increased expression of STAT1 and, to a lesser extent, STAT2. Thus, these findings demonstrate that adrenal medullary chromaffin cells are responsive to IFN-alpha and provide a possible cellular mechanism by which this immune-derived signal can potentially influence and integrate with the stress response.

Chronic intraperitoneal injection of interferon-alpha reduces serotonin levels in various regions of rat brain, but does not change levels of serotonin transporter mRNA, nitrite or nitrate

Interferon-alpha therapy is associated with a high rate of depression, but the pathophysiological mechanisms remain unclear. The purpose of the present study was to investigate the effects of i.p. administered interferon-alpha on monoaminergic neurotransmission in the brain. The levels of monoamines and associated metabolites were measured in various regions of the rat brain using a high-performance liquid chromatography-electrochemical detection system. The serotonin transporter mRNA levels were also measured using in situ hybridization. After 1 day, dopamine turnover was diminished in the cortex. Norepinephrine turnover was decreased in most regions tested after 4 days. However, these changes were transient. After 14 days, serotonin turnover was increased in the frontal cortex and hippocampus in rats given a dose of 20 000 IU/kg; in the frontal cortex, hippocampus, amygdala, thalamus, hypothalamus and brainstem in those on 200 000 IU/kg; and in the thalamus and hypothalamus in those on 2 000 000 IU/kg (all P < 0.05). However, 14-day treatment did not significantly change serotonin transporter mRNA levels. Next, the question of whether interferon-alpha affects monoamine levels via induction of nitric oxide (NO), was investigated. However, there were no changes in either NO2- or NO3-, as markers of NO production, in any brain regions after 14-day treatment. These results suggest that chronic peripheral administration of interferon-alpha induces metabolic changes in the central serotonin system. Further investigation is needed to determine exactly how this cytokine affects the central serotonin system and to assess whether a central serotonin abnormality is involved in interferon-induced depression.