Ron Kooijman

Protein hormones and immunity

A number of observations and discoveries over the past 20 years support the concept of important physiological interactions between the endocrine and immune systems. The best known pathway for transmission of information from the immune system to the neuroendocrine system is humoral in the form of cytokines, although neural transmission via the afferent vagus is well documented also. In the other direction, efferent signals from the nervous system to the immune system are conveyed by both the neuroendocrine and autonomic nervous systems. Communication is possible because the nervous and immune systems share a common biochemical language involving shared ligands and receptors, including neurotransmitters, neuropeptides, growth factors, neuroendocrine hormones and cytokines. This means that the brain functions as an immune-regulating organ participating in immune responses. A great deal of evidence has accumulated and confirmed that hormones secreted by the neuroendocrine system play an important role in communication and regulation of the cells of the immune system. Among protein hormones, this has been most clearly documented for prolactin (PRL), growth hormone (GH), and insulin-like growth factor-1 (IGF-I), but significant influences on immunity by thyroid-stimulating hormone (TSH) have also been demonstrated. Here we review evidence obtained during the past 20 years to clearly demonstrate that neuroendocrine protein hormones influence immunity and that immune processes affect the neuroendocrine system. New findings highlight a previously undiscovered route of communication between the immune and endocrine systems that is now known to occur at the cellular level. This communication system is activated when inflammatory processes induced by proinflammatory cytokines antagonize the function of a variety of hormones, which then causes endocrine resistance in both the periphery and brain. Homeostasis during inflammation is achieved by a balance between cytokines and endocrine hormones.

The dual role of the neuroinflammatory response after ischemic stroke: modulatory effects of hypothermia

Neuroinflammation is a key element in the ischemic cascade after cerebral ischemia that results in cell damage and death in the subacute phase. However, anti-inflammatory drugs do not improve outcome in clinical settings suggesting that the neuroinflammatory response after an ischemic stroke is not entirely detrimental. This review describes the different key players in neuroinflammation and their possible detrimental and protective effects in stroke. Because of its inhibitory influence on several pathways of the ischemic cascade, hypothermia has been introduced as a promising neuroprotective strategy. This review also discusses the influence of hypothermia on the neuroinflammatory response. We conclude that hypothermia exerts both stimulating and inhibiting effects on different aspects of neuroinflammation and hypothesize that these effects are key to neuroprotection.

Insulin-like growth factor I: a potential neuroprotective compound for the treatment of acute ischemic stroke?

Background and Purpose— Insulin-like growth factor I (IGF-I) exerts neuroprotective effects in both white and gray matter under different detrimental conditions. The purpose of this review is to collect the evidence whether IGF-I is a candidate neuroprotective drug in patients with acute ischemic stroke. Results— IGF-I was found to be neuroprotective in animal models of focal brain ischemia when given ≥2 hours after the insult. Different routes of administration (eg, cerebroventricular, intravenous, and intranasal) were found to be effective. In addition to inhibition of apoptosis and reduction of the infarct volume, IGF-I also improved neurological outcome. Furthermore, there are strong indications that IGF-I can also stimulate the regeneration of neural tissue. Conclusions— Additional studies are required to reveal the neuroprotective mechanisms of IGF-I in detail and to elucidate the role of IGF-binding proteins. Preclinical studies in relevant animal models for studying stroke (ie, hypertensive, diabetic, or aged animals) should be done testing different doses and routes of IGF-I administration and different combinations of IGF-I and IGF-binding proteins.

Prolactin in man: a tale of two promoters

The pituitary hormone prolactin (PRL) is best known for its role in the regulation of lactation. Recent evidence furthermore indicates PRL is required for normal reproduction in rodents. Here, we report on the insertion of two transposon-like DNA sequences in the human prolactin gene, which together function as an alternative promoter directing extrapituitary PRL expression. Indeed, the transposable elements contain transcription factor binding sites that have been shown to mediate PRL transcription in human uterine decidualised endometrial cells and lymphocytes. We hypothesize that the transposon insertion event has resulted in divergent (pituitary versus extrapituitary) expression of prolactin in primates, and in differential actions of pituitary versus extrapituitary prolactin in lactation versus pregnancy respectively. Importantly, the TE insertion might provide a context for some of the conflicting results obtained in studies of PRL function in mice and man. BioEssays 28: 1051–1055, 2006.

Cyclic AMP: a selective modulator of NF-κB action

It has been known for several decades that cyclic AMP (cAMP), a prototypical second messenger, transducing the action of a variety of G-protein-coupled receptor ligands, has potent immunosuppressive and anti-inflammatory actions. These actions have been attributed in part to the ability of cAMP-induced signals to interfere with the function of the proinflammatory transcription factor Nuclear Factor-kappaB (NF-κB). NF-κB plays a crucial role in switching on the gene expression of a plethora of inflammatory and immune mediators, and as such is one of the master regulators of the immune response and a key target for anti-inflammatory drug design. A number of fundamental molecular mechanisms, contributing to the overall inhibitory actions of cAMP on NF-κB function, are well established. Paradoxically, recent reports indicate that cAMP, via its main effector, the protein kinase A (PKA), also promotes NF-κB activity. Indeed, cAMP actions appear to be highly cell type- and context-dependent. Importantly, several novel players in the cAMP/NF-κB connection, which selectively direct cAMP action, have been recently identified. These findings not only open up exciting new research avenues but also reveal novel opportunities for the design of more selective, NF-κB-targeting, anti-inflammatory drugs.

Insulin-like growth factor-I receptor signal transduction and the Janus Kinase/Signal Transducer and Activator of Transcription (JAK-STAT) pathway.

The insulin‐like growth factor IGF‐I is an important fetal and postnatal growth factor, which is also involved in tissue homeostasis via regulation of proliferation, differentiation, and cell survival. To understand the role of IGF‐I in the pathophysiology of a variety of disorders, including growth disorders, cancer, and neurodegenerative diseases, a detailed knowledge of IGF‐I signal transduction is required. This knowledge may also contribute to the development of new therapies directed at the IGF‐I receptor or other signaling molecules. In this review, we will address IGF‐I receptor signaling through the JAK/STAT pathway in IGF‐I signaling and the role of cytokine‐induced inhibitors of signaling (CIS) and suppressors of cytokine signaling (SOCS). It appears that, in addition to the canonical IGF‐I signaling pathways through extracellular‐regulated kinase (ERK) and phosphatidylinositol‐3 kinase (PI3K)‐Akt, IGF‐I also signals through the JAK/STAT pathway. Activation of this pathway may lead to induction of SOCS molecules, well‐known feedback inhibitors of the JAK/STAT pathway, which also suppress of IGF‐I‐induced JAK/STAT signaling. Furthermore, other IGF‐I‐induced signaling pathways may also be modulated by SOCS. It is conceivable that the effect of these classical inhibitors of cytokine signaling directly affect IGF‐I receptor signaling, because they are able to associate to the intracellular part of the IGF‐I receptor. These observations indicate that CIS and SOCS molecules are key to cross‐talk between IGF‐I receptor signaling and signaling through receptors belonging to the hematopoietic/cytokine receptor superfamily. Theoretically, dysregulation of CIS or SOCS may affect IGF‐I‐mediated effects on body growth, cell differentiation, proliferation, and cell survival.

The role of growth hormone and insulin-like growth factors in the immune system

Growth hormone (GH) and insulin-like growth factor I (IGF-I) can modulate the development and function of the immune system. In this chapter, we present data on the expression of receptors for GH and IGFs and the in vitro and in vivo effects of these proteins. We show that expression of GH and IGFs in the immune system opens up the possibility that these proteins are not only involved in endocrine control of the immune system but can also play a role as local growth and differentiation factors (cytokines). Endocrine control of GH could be direct or mediated via endocrine or autocrine/paracrine IGF-I. In addition, GH can act as an autocrine or paracrine factor itself. Furthermore, IGF-I in the immune system has been shown to be regulated by cytokines, such as interleukin-1 and interferon-γ, alluding to a cytokine-like function of IGF-I. In addition to data on the function of GH and IGF-I in the immune system, we present new findings which imply a possible function of IGF-II and IGF-binding proteins.

Insulin-like growth factor-I stimulates IL-10 production in human T cells

There is vast body of evidence that the insulin‐like growth factor (IGF)‐I exerts immunomodulatory effects in vitro and in vivo. In vitro studies indicate that stimulatory effects of IGF‐I may be exerted through augmentation of inflammatory cytokine production. To further explore the immunomodulatory effects of IGF‐I through regulation of cytokine production, we tested the in vitro effects of IGF‐I on the secretion of inflammatory T helper cell type 1 (Th1) and Th2 cytokines by human peripheral blood mononuclear cells (PBMC). To this end, PBMC were stimulated with the T cell mitogen phytohemagglutinin (PHA), and cytokines in the culture media were assessed after 18, 42, 66, and 80 h of culture. We found that IGF‐I stimulated the secretion of the Th2 cytokine interleukin (IL)‐10 by 40–70% in PHA‐stimulated PBMC. In addition, we observed a small stimulatory effect (15%) on the secretion of another Th2 cytokine IL‐4. The secretion of IL‐2, IL‐5, IL‐6, interferon‐γ, and the inflammatory cytokines IL‐1β, IL‐8, and tumor necrosis factor α was not or was hardly affected. IL‐10 secretion was also stimulated in purified T cells, and we established that IGF‐I also stimulated IL‐10 mRNA expression by 100–150%. The monocyte‐activating bacterial cell‐wall product lipopolysaccharide induced IL‐10 production in PBMC, but this was not affected by IGF‐I. As IL‐10 predominantly exerts anti‐inflammatory actions and suppresses Th1‐dependent immune responses, our results indicate that IGF‐I may exert inhibitory actions on inflammatory and Th1‐mediated cellular immune responses through stimulation of IL‐10 production in T cells.

Cooperation of NFκB and CREB to induce synergistic IL-6 expression in astrocytes

Astrocytes are critical players in the innate immune response of the central nervous system. Upon encountering proinflammatory stimuli, astrocytes produce a plethora of inflammatory mediators. Here, we have investigated how beta(2)-adrenergic receptor activation modulates proinflammatory gene expression in astrocytes. We have observed that treatment of human 1321N1 astrocytes with the beta-adrenergic agonist isoproterenol synergistically enhanced TNF-alpha-induced expression of the cytokine IL-6. The effect of isoproterenol was cAMP-dependent and mediated by the beta(2)-adrenergic subtype. Using pharmacological inhibitors and siRNA we showed that protein kinase A (PKA) is an indispensable mediator of the synergy. Simultaneous induction with isoproterenol and TNF-alpha was moreover associated with combined recruitment of CREB and p65 to the native IL-6 promoter. The role of CREB and NFkappaB in promoting the synergy was corroborated using IL-6 promoter point mutants, as well as via siRNA-mediated silencing of CREB and NFkappaB. Interestingly, whereas CREB and NFkappaB usually compete for the limiting cofactor CREB binding protein (CBP), we detected enhanced recruitment of CBP at the IL-6 promoter in our system. The transcriptional synergy seems to be a gene specific process, occurring at the IL-6 and COX-2 gene, but not at other typical NFkappaB-dependent genes such as IL-8, ICAM-1 or VCAM-1. As astrocytic IL-6 overexpression has been associated with neuroinflammatory and neurodegenerative processes, our findings might have important physiological consequences.

Mechanism of prostaglandin (PG)E2-induced prolactin expression in human T cells : cooperation of two PGE2 receptor subtypes, E-prostanoid (EP) 3 and EP4, via calcium- and cyclic adenosine 5'-monophosphate-mediated signaling pathways

We previously reported that prolactin gene expression in the T-leukemic cell line Jurkat is stimulated by PGE2 and that cAMP acts synergistically with Ca2+ or protein kinase C on the activation of the upstream prolactin promoter. Using the transcription inhibitor actinomycin D, we now show that PGE2-induced prolactin expression requires de novo prolactin mRNA synthesis and that PGE2 does not influence prolactin mRNA stability. Furthermore, PGE2-induced prolactin expression was inhibited by protein kinase inhibitor fragment 14–22 and BAPTA-AM, which respectively, inhibit protein kinase A- and Ca2+-mediated signaling cascades. Using specific PGE2 receptor agonists and antagonists, we show that PGE2 induces prolactin expression through engagement of E-prostanoid (EP) 3 and EP4 receptors. We also found that PGE2 induces an increase in intracellular cAMP concentration as well as intracellular calcium concentration via EP4 and EP3 receptors, respectively. In transient transfections, 3000 bp flanking the leukocyte prolactin promoter conferred a weak induction of the luciferase reporter gene by PGE2 and cAMP, whereas cAMP in synergy with ionomycin strongly activated the promoter. Mutation of a C/EBP responsive element at −214 partially abolished the response of the leukocyte prolactin promoter to PGE2, cAMP, and ionomycin plus cAMP.