The human neuroendocrine thyrotropin-releasing hormone receptor promoter is activated by the haematopoietic transcription factor c-Myb

The Thyrotropin-Releasing Hormone-Degrading Ectoenzyme, a Therapeutic Target?

Thyrotropin releasing hormone (TRH: Glp-His-Pro-NH2) is a peptide mainly produced by brain neurons. In mammals, hypophysiotropic TRH neurons of the paraventricular nucleus of the hypothalamus integrate metabolic information and drive the secretion of thyrotropin from the anterior pituitary, and thus the activity of the thyroid axis. Other hypothalamic or extrahypothalamic TRH neurons have less understood functions although pharmacological studies have shown that TRH has multiple central effects, such as promoting arousal, anorexia and anxiolysis, as well as controlling gastric, cardiac and respiratory autonomic functions. Two G-protein-coupled TRH receptors (TRH-R1 and TRH-R2) transduce TRH effects in some mammals although humans lack TRH-R2. TRH effects are of short duration, in part because the peptide is hydrolyzed in blood and extracellular space by a M1 family metallopeptidase, the TRH-degrading ectoenzyme (TRH-DE), also called pyroglutamyl peptidase II. TRH-DE is enriched in various brain regions but is also expressed in peripheral tissues including the anterior pituitary and the liver, which secretes a soluble form into blood. Among the M1 metallopeptidases, TRH-DE is the only member with a very narrow specificity; its best characterized biological substrate is TRH, making it a target for the specific manipulation of TRH activity. Two other substrates of TRH-DE, Glp-Phe-Pro-NH2 and Glp-Tyr-Pro-NH2, are also present in many tissues. Analogs of TRH resistant to hydrolysis by TRH-DE have prolonged central efficiency. Structure-activity studies allowed the identification of residues critical for activity and specificity. Research with specific inhibitors has confirmed that TRH-DE controls TRH actions. TRH-DE expression by β2-tanycytes of the median eminence of the hypothalamus allows the control of TRH flux into the hypothalamus-pituitary portal vessels and may regulate serum thyrotropin secretion. In this review we describe the critical evidences that suggest that modification of TRH-DE activity in tanycytes, and/or in other brain regions, may generate beneficial consequences in some central and metabolic disorders and identify potential drawbacks and missing information needed to test these hypotheses.

The syndrome of central hypothyroidism and macroorchidism: IGSF1 controls TRHR and FSHB expression by differential modulation of pituitary TGFβ and Activin pathways

IGSF1 (Immunoglobulin Superfamily 1) gene defects cause central hypothyroidism and macroorchidism. However, the pathogenic mechanisms of the disease remain unclear. Based on a patient with a full deletion of IGSF1 clinically followed from neonate to adulthood, we investigated a common pituitary origin for hypothyroidism and macroorchidism, and the role of IGSF1 as regulator of pituitary hormone secretion. The patient showed congenital central hypothyroidism with reduced TSH biopotency, over-secretion of FSH at neonatal minipuberty and macroorchidism from 3 years of age. His markedly elevated inhibin B was unable to inhibit FSH secretion, indicating a status of pituitary inhibin B resistance. We show here that IGSF1 is expressed both in thyrotropes and gonadotropes of the pituitary and in Leydig and germ cells in the testes, but at very low levels in Sertoli cells. Furthermore, IGSF1 stimulates transcription of the thyrotropin-releasing hormone receptor (TRHR) by negative modulation of the TGFβ1-Smad signaling pathway, and enhances the synthesis and biopotency of TSH, the hormone secreted by thyrotropes. By contrast, IGSF1 strongly down-regulates the activin-Smad pathway, leading to reduced expression of FSHB, the hormone secreted by gonadotropes. In conclusion, two relevant molecular mechanisms linked to central hypothyroidism and macroorchidism in IGSF1 deficiency are identified, revealing IGSF1 as an important regulator of TGFβ/Activin pathways in the pituitary.

Hormonal control of T-cell development in health and disease

The physiology of the thymus, the primary lymphoid organ in which T cells are generated, is controlled by hormones. Data from animal models indicate that several peptide and nonpeptide hormones act pleiotropically within the thymus to modulate the proliferation, differentiation, migration and death by apoptosis of developing thymocytes. For example, growth hormone and prolactin can enhance thymocyte proliferation and migration, whereas glucocorticoids lead to the apoptosis of these developing cells. The thymus undergoes progressive age-dependent atrophy with a loss of cells being generated and exported, therefore, hormone-based therapies are being developed as an alternative strategy to rejuvenate the organ, as well as to augment thymocyte proliferation and the export of mature T cells to peripheral lymphoid organs. Some hormones (such as growth hormone and progonadoliberin-1) are also being used as therapeutic agents to treat immunodeficiency disorders associated with thymic atrophy, such as HIV infection. In this Review, we discuss the accumulating data that shows the thymus gland is under complex and multifaceted hormonal control that affects the process of T-cell development in health and disease.

Hypothalamic neurohormones and immune responses

The aim of this review is to provide a comprehensive examination of the current literature describing the neural-immune interactions, with emphasis on the most recent findings of the effects of neurohormones on immune system. Particularly, the role of hypothalamic hormones such as Thyrotropin-releasing hormone (TRH), Corticotropin-releasing hormone (CRH) and Gonadotropin-releasing hormone (GnRH). In the past few years, interest has been raised in extrapituitary actions of these neurohormones due to their receptors have been found in many non-pituitary tissues. Also, the receptors are present in immune cells, suggesting an autocrine or paracrine role within the immune system. In general, these neurohormones have been reported to exert immunomodulatory effects on cell proliferation, immune mediators release and cell function. The implications of these findings in understanding the network of hypothalamic neuropeptides and immune system are discussed.

The thyrotropin-releasing hormone (TRH)-immune system homeostatic hypothesis

Decades of research have established that the biological functions of thyrotropin-releasing hormone (TRH) extend far beyond its role as a regulator of the hypothalamic-pituitary-thyroid axis. Gary et al. [Gary, K.A., Sevarino, K.A., Yarbrough, G.G., Prange, A.J. Jr., Winokur, A. (2003). The thyrotropin-releasing hormone (TRH) hypothesis of homeostatic regulation: implications for TRH-based therapeutics. J Pharmacol Exp Ther 305(2):410-416.] and Yarbrough et al. [Yarbrough, G.G., Kamath, J., Winokur, A., Prange, A.J. Jr. (2007). Thyrotropin-releasing hormone (TRH) in the neuroaxis: therapeutic effects reflect physiological functions and molecular actions. Med Hypotheses 69(6):1249-1256.] provided a functional framework, predicated on its global homeostatic influences, to conceptualize the numerous interactions of TRH with the central nervous system (CNS) and endocrine system. Herein, we profer a similar analysis to interactions of TRH with the immune system. Autocrine/paracrine cellular signaling motifs of TRH and TRH receptors are expressed in several tissues and organs of the immune system. Consistent with this functional distribution, in vitro and in vivo evidence suggests a critical role for TRH during the developmental stages of the immune system as well as its numerous interactions with the fully developed immune system. Considerable evidence supports a pivotal role for TRH in the pathophysiology of the inflammatory process with specific relevance to the "cytokine-induced sickness behavior" paradigm. These findings, combined with a number of documented clinical actions of TRH strongly support a potential utility of TRH-based therapeutics in select inflammatory disorders. Similar to its global role in behavioral and energy homeostasis a homeostatic role for TRH in its interactions with the immune system is consonant with the large body of available data. Recent advances in the field of immunology provide a significant opportunity for investigation of the TRH-immune system homeostatic hypothesis. Moreover, this hypothesis may provide a foundation for the development of TRH-based therapeutics for certain medical and psychiatric disorders involving immune dysfunction.

Revisiting a selection of target genes for the hematopoietic transcription factor c-Myb using chromatin immunoprecipitation and c-Myb knockdown

The transcription factor c-Myb is an important regulator of hematopoiesis required for proper development of most blood cell lineages in vertebrates. An increasing number of target genes for c-Myb are being published, although with little or no overlap between the lists of genes reported. This raises the question of which criteria a bona fide c-Myb-target gene should satisfy. In the present paper, we have analyzed a set of previously reported target genes using chromatin immunoprecipitation (ChIP) and siRNA-mediated knockdown. Among the seven well-studied c-Myb target genes that we analyzed by ChIP, only ADA, c-MYC and MAT2A seemed to be occupied by c-Myb under our experimental settings in the Myb-positive cell lines Jurkat and HL60. After siRNA-mediated knockdown of c-Myb expression, the expression levels of two out of three ChIP positive Myb target genes, ADA and c-MYC, were strongly affected. These results clearly demonstrate the importance of combining different methods for target gene validation and suggest that a combination of ChIP and c-Myb knockdown may represent a powerful approach to identify a core collection of c-Myb target genes.

The chromatin remodeling factor Mi-2alpha acts as a novel co-activator for human c-Myb

The c-Myb protein belongs to a group of early hematopoietic transcription factors that are important for progenitor generation and proliferation. These factors have been hypothesized to participate in establishing chromatin patterns specific for hematopoietic genes. In a two-hybrid screening we identified the chromatin remodeling factor Mi-2alpha as an interaction partner for human c-Myb. The main interacting domains were mapped to the N-terminal region of Mi-2alpha and the DNA-binding domain of c-Myb. Surprisingly, functional analysis revealed that Mi-2alpha, previously studied as a subunit in the NuRD co-repressor complex, enhanced c-Myb-dependent reporter activation. Consistently, knock-down of endogenous Mi-2alpha in c-Myb-expressing K562 cells, led to down-regulation of the c-Myb target genes NMU and ADA. When wild-type and helicase-dead Mi-2alpha were compared, the Myb-Mi-2alpha co-activation appeared to be independent of the ATPase/DNA helicase activity of Mi-2alpha. The rationale for the unexpected co-activator function seems to lie in a dual function of Mi-2alpha, by which this factor is able to repress transcription in a helicase-dependent and activate in a helicase-independent fashion, as revealed by Gal4-tethering experiments. Interestingly, desumoylation of c-Myb potentiated the Myb-Mi-2alpha transactivational co-operation, as did co-transfection with p300.

c-Myb associates with PML in nuclear bodies in hematopoietic cells

The c-Myb transcription factor plays a central role in the regulation of cell growth and differentiation of hematopoietic cells. Being the product of a proto-oncogene, one would expect c-Myb function to be modulated by signal transduction pathways, but our knowledge on such regulation of c-Myb is rather limited. Recently, we and others showed that c-Myb is subjected to sumoylation and that this posttranslational modification has considerable effect on c-Myb's activity. Interestingly, many proteins subjected to SUMO-1 conjugation associate with the promyelocytic leukemia (PML) protein and localize to PML nuclear bodies (PML NBs). Although the precise molecular function of PML NBs still remains to be defined, they seem to play a role in regulation of gene expression and are linked to specific cellular signaling. We show here that c-Myb localizes to PML NBs and that c-Myb interacts with PML as judged by immunofluorescence microcopy and co-immunoprecipitation experiments. Enforced expression of PML IV was shown to enhance c-Myb-dependent reporter activation. Our results imply a role for PML and possibly other components of PML NBs in regulating c-Myb's activity. This novel link between c-Myb and PML, two gene products being implicated in leukemic disorders, suggests that previously unknown mechanisms for regulating c-Myb's activity involving PML may exist.