MMQ cells: a model for evaluating the role of G proteins in the modulation of prolactin release

Gene therapy for pituitary tumors

Pituitary tumors are the most common primary intracranial neoplasms. Although most pituitary tumors are considered typically benign, others can cause severe and progressive disease. The principal aims of pituitary tumor treatment are the elimination or reduction of the tumor mass, normalization of hormone secretion and preservation of remaining pituitary function. In spite of major advances in the therapy of pituitary tumors, for some of the most difficult tumors, current therapies that include medical, surgical and radiotherapeutic methods are often unsatisfactory and there is a need to develop new treatment strategies. Gene therapy, which uses nucleic acids as drugs, has emerged as an attractive therapeutic option for the treatment of pituitary tumors that do not respond to classical treatment strategies if the patients become intolerant to the therapy. The development of animal models for pituitary tumors and hormone hypersecretion has proven to be critical for the implementation of novel treatment strategies and gene therapy approaches. Preclinical trials using several gene therapy approaches for the treatment of anterior pituitary diseases have been successfully implemented. Several issues need to be addressed before clinical implementation becomes a reality, including the development of more effective and safer viral vectors, uncovering novel therapeutic targets and development of targeted expression of therapeutic transgenes. With the development of efficient gene delivery vectors allowing long-term transgene expression with minimal toxicity, gene therapy will become one of the most promising approaches for treating pituitary adenomas.

Pituitary cell lines and their endocrine applications

The pituitary gland is an important component of the endocrine system, and together with the hypothalamus, exerts considerable influence over the functions of other endocrine glands. The hypothalamus either positively or negatively regulates hormonal productions in the pituitary through its release of various trophic hormones which act on specific cell types in the pituitary to secrete a variety of pituitary hormones that are important for growth and development, metabolism, reproductive and nervous system functions. The pituitary is divided into three sections-the anterior lobe which constitute the majority of the pituitary mass and is composed primarily of five hormone-producing cell types (thyrotropes, lactotropes, corticotropes, somatotropes and gonadotropes) each secreting thyrotropin, prolactin, ACTH, growth hormone and gonadotropins (FSH and LH) respectively. There is also a sixth cell type in the anterior lobe-the non-endocrine, agranular, folliculostellate cells. The intermediate lobe produces melanocyte-stimulating hormone and endorphins, whereas the posterior lobe secretes anti-diuretic hormone (vasopressin) and oxytocin. Representative cell lines of all the six cell types of the anterior pituitary have been established and have provided valuable information on genealogy of the various cell lineages, endocrine feedback control of hormone synthesis and secretions, intrapituitary interactions between the various cell types, as well as the role of specific transcription factors that determine each differentiated cell phenotype. In this review, we will discuss the morphology and function of the cell types that make up the anterior pituitary, and the characteristics of the various functional anterior pituitary cell systems that have been established to be representative of each anterior pituitary cell lineage.

Influence of VIP on prolactinemia in turkey anterior pituitary cells: role of cAMP second messenger in VIP-induced prolactin gene expression

Vasoactive intestinal peptide (VIP) is the avian prolactin (PRL)-releasing factor. In the turkey, hypothalamic VIP immunoreactivity and mRNA content, as well as VIP levels in hypophyseal portal blood, are closely related to the state of prolactinemia and the reproductive stage. The present study investigated the role of VIP on prolactinemia in turkey anterior pituitary (AP) cells through PRL gene expression and the role of a cAMP second messenger system on VIP-induced PRL expression. In primary AP cells harvested from hens in different prolactinemic states, steady state promoter activities were positively correlated with secreted PRL levels. VIP increased PRL promoter activities in AP cells from hens with intermediate PRL levels (laying), but not in AP cells from hypoprolactinemic hens (nonphotostimulated reproductively quiescent). However, in AP cells from hyperprolactinemic hens (incubating), PRL promoter activity was down-regulated by VIP. PRL mRNA steady state levels were significantly decreased by the cAMP analogue, 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), and PRL secretion was down-regulated by the phosphodiesterase blocker, 3-isobutyl-1-methylxanthine (IBMX) in a dose-dependent manner, suggesting that the cAMP second messenger system might be involved in the inhibitory action of dopamine upon VIP-stimulated PRL secretion and gene expression at the pituitary level. In a study of VIP immediate and long-term effects on c-fos expression in relation to PRL expression, VIP dramatically induced c-fos mRNA expression within 5 min, suggesting that VIP-induced c-fos expression might be involved in VIP-stimulated PRL secretion and gene expression. These results provide additional evidence of the functional significance of VIP in PRL gene expression and suggest that changes in PRL promoter activity by VIP may be one of the important inductive mechanisms leading to prolactinemia.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

The intrinsic activity of (-)-3-PPP vis-à-vis prolactin-suppressing dopamine D2 receptors in transfected GH4C1 cells is dependent on which secretagogue that is used to provoke prolactin release

The abilities of dopamine (DA) and the partial DA D2 receptor agonist (-)-(3-hydroxyphenyl)-N-n-propylpiperidine, (-)-3-PPP, to suppress prolactin (PRL) release induced by any of five different PRL secretagogues in GH4C1 cells transfected with the human D2 receptor (short isoform) were investigated. Whereas DA reduced the response to all five secretagogues. (-)-3-PPP reduced the response to vasoactive intestinal peptide (VIP) and thyrotropin-releasing hormone (TRH), but not to high medium potassium (K+) or to the potassium channel antagonist tetraethylammonium (TEA). (-)-3-PPP tended to reduce the PRL release induced by the Ca2+ channel agonist BAY K-8644 (BAY); however, this effect of the partial agonist was modest and not significant. Whereas the effects of both DA and (-)-3-PPP on the PRL response to VIP and TRH were counteracted by co-incubation with the D2 antagonist raclopride, the effects of DA on the PRL response to K+, BAY, and TEA were antagonized by co-incubation with either raclopride or (-)-3-PPP. The results show that, at a given receptor density, the intrinsic activity of a partial D2 agonist with respect to D2-mediated suppression of PRL release may vary from agonism to antagonism depending on which intracellular transduction systems that are being concomitantly activated.

Fatty acids and brain peptides

The role of fatty acids (FA) as a mediator and modulator of central nervous system activity in general, and peptides in particular, is only recently becoming understood. This paper reviews numerous findings concerned with the activity of fatty acids, particularly with their interaction with diverse neurochemical systems and their consequences for better understanding neurotransmitters, hormones and peptides. The effects include FA as precursors in the manufacture of neurochemical elements, including enzymes, neurotransmitters, and hormones. Of particular interest is the important changes in neuronal membrane composition that have been attributed to FA. Such changes may account for the changes in thermoregulation, learning, and other functions that accompany dietary manipulation of FA intake. While the total level of FA has been the object of many investigations, this report addresses the need to focus on the ratio of FA, especially alpha-linolenic/linoleic acid, which has been shown to be a critical factor in a number of research studies.

Regulation of dopaminergic pathways by retinoids: activation of the D2 receptor promoter by members of the retinoic acid receptor-retinoid X receptor family

Dopamine is a neuromodulator involved in the control of key physiological functions. Dopamine-dependent signal transduction is activated through the interaction with membrane receptors of the seven-transmembrane domain G protein-coupled family. Among them, dopamine D2 receptor is highly expressed in the striatum and the pituitary gland as well as by mesencephalic dopaminergic neurons. Lack of D2 receptors in mice leads to a locomotor parkinsonian-like phenotype and to pituitary tumors. The D2 receptor promoter has characteristics of a housekeeping gene. However, the restricted expression of this gene to particular neurons and cells points to a strict regulation of its expression by cell-specific transcription factors. We demonstrate here that the D2 receptor promoter contains a functional retinoic acid response element. Furthermore, analysis of retinoic acid receptor-null mice supports our finding and shows that in these animals D2 receptor expression is reduced. This finding assigns to retinoids an important role in the control of gene expression in the central nervous system.