Contribution of multidrug resistance protein MRP5 in control of cyclic guanosine 5'-monophosphate intracellular signaling in anterior pituitary cells

Inhibition of ABCC5-mediated cGMP transport by progesterone, testosterone and their analogues

The biodynamics and biokinetics of sex hormones are complex. In addition to the classical steroid receptors (nuclear receptors), these hormones act through several non-genomic mechanisms. Modulation of ABC-transporters by progesterone represents a non-genomic mechanism. In the present study, we employed inside out vesicles from human erythrocytes to characterize high affinity cGMP transport by ABCC5 (member 5 of the ATP-Binding Cassette subfamily C). Progesterone and testosterone inhibited the transport with respective K_i of 1.2 ± 0.3 and 2.0 ± 0.6 μmol/L. We used virtual ligand screening (VLS) to identify analogues to progesterone and testosterone. A large number of substances were screened in silico and the 19 most promising candidates were screened in vitro. Each substance was tested for a concentration of 10 μmol/L. The range of cGMP transport reduction was 21.5% to 86.2% for progesterone analogues and 8.6% to 93.8 % for testosterone analogues. Three of the most potent test compounds (TC) of each analogue class, in addition to progesterone and testosterone, were characterized for concentrations from 1 nanomol/L to 1 mmol/L. The progesterone analogues showed following K_i-values (μmol/L): TC-08: 0.61, TC-16: 0.66 and TC-15: 9.3. The K_i-values (μmol/L) for the testosterone analogues were: TC-18: 0.10, TC-07: 0.67 andTC-05: 2.0. The present study shows that VLS may be a versatile tool in the development of membrane transport modulating agents (MTMAs).

Chaperones, somatotroph tumors and the cyclic AMP (cAMP)-dependent protein kinase (PKA) pathway

The cAMP-PKA pathway plays an essential role in the pituitary gland, governing cell differentiation and survival, and maintenance of endocrine function. Somatotroph growth hormone transcription and release as well as cell proliferation are regulated by the cAMP-PKA pathway; cAMP-PKA pathway abnormalities are frequently detected in sporadic as well as in hereditary somatotroph tumors and more rarely in other pituitary tumors. Inactivating variants of the aryl hydrocarbon receptor-interacting protein (AIP)-coding gene are the genetic cause of a subset of familial isolated pituitary adenomas (FIPA). Multiple functional links between the co-chaperone AIP and the cAMP-PKA pathway have been described. This review explores the role of chaperones including AIP in normal pituitary function as well as in somatotroph tumors, and their interaction with the cAMP-PKA pathway.

ABC Family Transporters

The transport of specific molecules across lipid membranes is an essential function of all living organisms. The processes are usually mediated by specific transporters. One of the largest transporter families is the ATP-binding cassette (ABC) family. More than 40 ABC transporters have been identified in human, which are divided into 7 subfamilies (ABCA to ABCG) based on their gene structure, amino acid sequence, domain organization, and phylogenetic analysis. Of them, at least 11 ABC transporters including P-glycoprotein (P-GP/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2) are involved in multidrug resistance (MDR) development. These ABC transporters are expressed in various tissues such as the liver, intestine, kidney, and brain, playing important roles in absorption, distribution, and excretion of drugs. Some ABC transporters are also involved in diverse cellular processes such as maintenance of osmotic homeostasis, antigen processing, cell division, immunity, cholesterol, and lipid trafficking. Several human diseases such as cystic fibrosis, sitosterolemia, Tangier disease, intrahepatic cholestasis, and retinal degeneration are associated with mutations in corresponding transporters. This chapter will describe function and expression of several ABC transporters (such as P-GP, BCRP, and MRPs), their substrates and inhibitors, as well as their clinical significance.

Gestational Zearalenone Exposure Causes Reproductive and Developmental Toxicity in Pregnant Rats and Female Offspring

Zearalenone (ZEN) is an oestrogenic mycotoxin commonly found in food and feed products and can affect reproduction and development in both humans and animals. This study aimed to determine the toxic effects of ZEN on maternal SD rats and the F1 female offspring. Sixty-four pregnant rats were divided into 4 groups and exposed to feed contaminated with ZEN (0, 5, 10, and 20 mg/kg feed) on gestational days (GDs) 0–21. Compared with the controls, the groups exposed to 10 and 20 mg/kg ZEN showed significantly decreased feed intake and body weight of pregnant rats and/or female offspring. Meanwhile, 20 mg/kg ZEN significantly decreased the birth weight and viability of F1 newborn rats. Moreover, 10 and 20 mg/kg ZEN diets increased follicle-stimulating hormone concentrations but decreased oestradiol in both maternal and F1 adult rats. In the F1 generation, ZEN caused no pathological changes in ovaries and uterus in weaned rats, but significant follicular atresia and a thinning uterine layer were found in F1 female adult rats in the 20 mg/kg ZEN group. These impairments concurred with the inhibited mRNA and protein levels of oestrogen receptor-alpha (Esr1) and 3β-hydroxysteroid dehydrogenase (HSD) in the adult uterus and/or ovaries. Furthermore, 10 and/or 20 mg/kg ZEN exposure significantly reduced Esr1, gonadotropin-releasing hormone receptor (GnRHr), and ATP binding cassette transporters b1 and c1 (ABCb1 and ABCc1) in the placenta and foetal and weaned F1 brains, and also produced a dose-dependent increase in 3β-HSD in the placenta. Additionally, 20 mg/kg ZEN significantly upregulated ABCc5 expression in the placenta and ovaries of weaned rats. These results suggested that prenatal ZEN exposure in rats affected maternal and foetal development and may lead to long-term reproductive impairment in F1 adult females.

Impaired platelet activation and cAMP homeostasis in MRP4-deficient mice

Molecules that reduce the level of cyclic adenosine 5'-monophosphate (cAMP) in the platelet cytosol, such as adenosine 5'-diphosphate (ADP) secreted from dense granules, trigger platelet activation. Therefore, any change in the distribution and/or availability of cyclic nucleotides or ADP may interfere with platelet reactivity. In this study, we evaluated the role of multidrug resistance protein 4 (MRP4, or ABCC4), a nucleotide transporter, in platelet functions in vivo and in vitro by investigating MRP4-deficient mice. MRP4 deletion resulted in a slight increase in platelet count but had no impact on platelet ultrastructure. In MRP4-deficient mice, the arterial occlusion was delayed and the tail bleeding time was prolonged. In a model of platelet depletion and transfusion mimicking a platelet-specific knockout, mice injected with MRP4(-/-) platelets also showed a significant increase in blood loss compared with mice injected with wild-type platelets. Defective thrombus formation and platelet activation were confirmed in vitro by studying platelet adhesion to collagen in flow conditions, integrin αIIbβ3 activation, washed platelet secretion, and aggregation induced by low concentrations of proteinase-activated receptor 4-activating peptide, U46619, or ADP. We found no role of MRP4 in ADP dense-granule storage, but MRP4 redistributed cAMP from the cytosol to dense granules, as confirmed by increased vasodilator-stimulated phosphoprotein phosphorylation in MRP4-deficient platelets. These data suggest that MRP4 promotes platelet aggregation by modulating the cAMP-protein kinase A signaling pathway, suggesting that MRP4 might serve as a target for novel antiplatelet agents.

Purinergic signaling pathways in endocrine system

Adenosine-5'-triphosphate is released by neuroendocrine, endocrine, and other cell types and acts as an extracellular agonist for ligand-gated P2X cationic channels and G protein-coupled P2Y receptors in numerous organs and tissues, including the endocrine system. The breakdown of ATP by ectonucleotidases not only terminates its extracellular messenger functions, but also provides a pathway for the generation of two additional agonists: adenosine 5'-diphosphate, acting via some P2Y receptors, and adenosine, a native agonist for G protein-coupled adenosine receptors, also expressed in the endocrine system. This article provides a review of purinergic signaling pathways in the hypothalamic magnocellular neurosecretory cells and neurohypophysis, hypothalamic parvocellular neuroendocrine system, adenohypophysis, and effector glands organized in five axes: hypothalamic-pituitary-gonadal, hypothalamic-pituitary-thyroid, hypothalamic-pituitary-adrenal, hypothalamic-pituitary-growth hormone, and hypothalamic-pituitary-prolactin. We attempted to summarize current knowledge of purinergic receptor subtypes expressed in the endocrine system, including their roles in intracellular signaling, hormone secretion, and other cell functions. We also briefly review the release mechanism for adenosine-5'-triphosphate by neuroendocrine, endocrine and surrounding cells, the enzymes involved in adenosine-5'-triphosphate hydrolysis to adenosine-5'-diphosphate and adenosine, and the relevance of this pathway for sequential activation of receptors and termination of signaling.

The role of cyclic nucleotides in pituitary lactotroph functions

Lactotrophs are one of the five secretory anterior pituitary cell types specialized to synthesize and release prolactin. In vitro, these cells fire action potentials (APs) spontaneously and the accompanied Ca(2+) transients are of sufficient amplitude to keep the exocytotic pathway, the transcription of prolactin gene, and de novo hormone synthesis continuously active. Basal cyclic nucleotide production is also substantial in cultured cells but not critical for the APs secretion/transcription coupling in lactotrophs. However, elevated intracellular cAMP levels enhance the excitability of lactotrophs by stimulating the depolarizing non-selective cationic hyperpolarization-activated cyclic nucleotide-regulated and background channels, whereas cGMP inhibits it by activating Ca(2+)-controlled K(+) channels. Elevated cAMP also modulates prolactin release downstream of Ca(2+) influx by changing the kinetic of secretory pores: stimulate at low and inhibit at high concentrations. Induction of prolactin gene and lactotroph proliferation is also stimulated by elevated cAMP through protein kinase A. Together, these observations suggest that in lactotrophs cAMP exhibits complex regulatory effects on voltage-gated Ca(2+) influx and Ca(2+)-dependent cellular processes.

Differential expression of multidrug resistance protein 5 and phosphodiesterase 5 and regulation of cGMP levels in phasic and tonic smooth muscle

Previous studies have identified differences in the expression of proteins that regulate myosin light chain phosphorylation and contraction in tonic and phasic smooth muscle. cGMP plays a critical role in smooth muscle relaxation and is important for optimal function of phasic and tonic smooth muscle. The intracellular cGMP levels are regulated by its hydrolysis via phosphodiesterase 5 (PDE5) and efflux via novel multidrug resistance protein 5 (MRP5). In the present study we tested the hypothesis that the differences in the phasic and tonic behavior of smooth muscles may be related to differences in mechanisms that terminate cGMP signaling. Expression of PDE5 and MRP5 was significantly (more than 2-fold) higher in fundus compared with antrum. The NO donor S-nitrosoglutathione (GSNO) caused an increase in PDE5 activity and intra- and extracellular cGMP levels in both fundus and antrum. Stimulation of PDE5 activity and increase in extracellular cGMP were significantly higher in fundus, whereas increase in intracellular cGMP was significantly higher in antrum. GSNO-induced increase in extracellular cGMP was blocked in dispersed cells by the cyclic nucleotide export blocker probenecid and in cultured muscle cells by depletion of ATP or suppression of MRP5 by siRNA, providing evidence that cGMP efflux was mediated by ATP-dependent export via MRP5. Consistent with the higher expression and activity levels of PDE5 and MRP5, GSNO-induced PKG activity and muscle relaxation were significantly lower in muscle cells from fundus compared with antrum. Thus higher expression of PDE5 and MRP5 in muscle cells from fundus correlates with tonic phenotype of muscle.

Dependence of the excitability of pituitary cells on cyclic nucleotides

Cyclic 3',5'-adenosine monophosphate and cyclic 3',5'-guanosine monophosphate are intracellular (second) messengers that are produced from the nucleotide triphosphates by a family of enzymes consisting of adenylyl and guanylyl cyclases. These enzymes are involved in a broad array of signal transduction pathways mediated by the cyclic nucleotide monophosphates and their kinases, which control multiple aspects of cell function through the phosphorylation of protein substrates. We review the findings and working hypotheses on the role of the cyclic nucleotides and their kinases in the control of electrical activity of the endocrine pituitary cells and the plasma membrane channels involved in this process.

Sickling cells, cyclic nucleotides, and protein kinases: the pathophysiology of urogenital disorders in sickle cell anemia

Sickle cell anemia is one of the best studied inherited diseases, and despite being caused by a single point mutation in the HBB gene, multiple pleiotropic effects of the abnormal hemoglobin S production range from vaso-occlusive crisis, stroke, and pulmonary hypertension to osteonecrosis and leg ulcers. Urogenital function is not spared, and although priapism is most frequently remembered, other related clinical manifestations have been described, such as nocturia, enuresis, increased frequence of lower urinary tract infections, urinary incontinence, hypogonadism, and testicular infarction. Studies on sickle cell vaso-occlusion and priapism using both in vitro and in vivo models have shed light on the pathogenesis of some of these events. The authors review what is known about the deleterious effects of sickling on the genitourinary tract and how the role of cyclic nucleotides signaling and protein kinases may help understand the pathophysiology underlying these manifestations and develop novel therapies in the setting of urogenital disorders in sickle cell disease.

Cyclic nucleotide phosphodiesterase 1A: a key regulator of cardiac fibroblast activation and extracellular matrix remodeling in the heart

Cardiac fibroblasts become activated and differentiate to smooth muscle-like myofibroblasts in response to hypertension and myocardial infarction (MI), resulting in extracellular matrix (ECM) remodeling, scar formation and impaired cardiac function. cAMP and cGMP-dependent signaling have been implicated in cardiac fibroblast activation and ECM synthesis. Dysregulation of cyclic nucleotide phosphodiesterase (PDE) activity/expression is also associated with various diseases and several PDE inhibitors are currently available or in development for treating these pathological conditions. The objective of this study is to define and characterize the specific PDE isoform that is altered during cardiac fibroblast activation and functionally important for regulating myofibroblast activation and ECM synthesis. We have found that Ca(2+)/calmodulin-stimulated PDE1A isoform is specifically induced in activated cardiac myofibroblasts stimulated by Ang II and TGF-β in vitro as well as in vivo within fibrotic regions of mouse, rat, and human diseased hearts. Inhibition of PDE1A function via PDE1-selective inhibitor or PDE1A shRNA significantly reduced Ang II or TGF-β-induced myofibroblast activation, ECM synthesis, and pro-fibrotic gene expression in rat cardiac fibroblasts. Moreover, the PDE1 inhibitor attenuated isoproterenol-induced interstitial fibrosis in mice. Mechanistic studies revealed that PDE1A modulates unique pools of cAMP and cGMP, predominantly in perinuclear and nuclear regions of cardiac fibroblasts. Further, both cAMP-Epac-Rap1 and cGMP-PKG signaling was involved in PDE1A-mediated regulation of collagen synthesis. These results suggest that induction of PDE1A plays a critical role in cardiac fibroblast activation and cardiac fibrosis, and targeting PDE1A may lead to regression of the adverse cardiac remodeling associated with various cardiac diseases.

Expression and differential localization of xenobiotic transporters in the rat olfactory neuro-epithelium

Transporters, such as multidrug resistance P-glycoproteins (MDR), multidrug resistance-related proteins (MRP) and organic anion transporters (OATs), are involved in xenobiotic metabolism, particularly the cellular uptake or efflux of xenobiotics (and endobiotics) or their metabolites. The olfactory epithelium is exposed to both inhaled xenobiotics and those coming from systemic circulation. This tissue has been described as a pathway for xenobiotics to the brain via olfactory perineural space. Thereby, olfactory transporters and xenobiotic metabolizing enzymes, dedicated to the inactivation and the elimination of xenobiotics, have been involved in the toxicological protection of the brain, the olfactory epithelium itself and the whole body. These proteins could also have a role in the preservation of the olfactory sensitivity by inactivation and clearance of the excess of odorant molecules from the perireceptor space. The goal of the present study was to increase our understanding of the expression and the localization of transporters in this tissue. For most of the studied transporters, we observed an opposite mRNA expression pattern (RT-PCR) in the olfactory epithelium compared to the liver, which is considered to be the main metabolic organ. Olfactory epithelium mainly expressed efflux transporters (MRP, MDR). However, a similar pattern was observed between the olfactory epithelium and the olfactory bulb. We also demonstrate distinct cellular immunolocalization of the transporters in the olfactory epithelium. As previously reported, Mrp1 was mainly found in the supranuclear portions of supporting cells. In addition, Mrp3 and Mrp5 proteins, which were detected for the first time in olfactory epithelium, were localized to the olfactory neuron layer, while Mdr1 was localized to the capillary endothelium of lymphatic vessels in the subepithelial region. The pattern of expression and the distinct localization of the olfactory transporters showed in this work may highlight on their specific function in the whole olfactory epithelium.

Multidrug resistance proteins (MRPs/ABCCs) in cancer chemotherapy and genetic diseases

The ATP-binding cassette (ABC) transporters are a superfamily of membrane proteins that are best known for their ability to transport a wide variety of exogenous and endogenous substances across membranes against a concentration gradient via ATP hydrolysis. There are seven subfamilies of human ABC transporters, one of the largest being the 'C' subfamily (gene symbol ABCC). Nine ABCC subfamily members, the so-called multidrug resistance proteins (MRPs) 1-9, have been implicated in mediating multidrug resistance in tumor cells to varying degrees as the efflux extrude chemotherapeutic compounds (or their metabolites) from malignant cells. Some of the MRPs are also known to either influence drug disposition in normal tissues or modulate the elimination of drugs (or their metabolites) via hepatobiliary or renal excretory pathways. In addition, the cellular efflux of physiologically important organic anions such as leukotriene C(4) and cAMP is mediated by one or more of the MRPs. Finally, mutations in several MRPs are associated with human genetic disorders. In this minireview, the current biochemical and physiological knowledge of MRP1-MRP9 in cancer chemotherapy and human genetic disease is summarized. The mutations in MRP2/ABCC2 leading to conjugated hyperbilirubinemia (Dubin-Johnson syndrome) and in MRP6/ABCC6 leading to the connective tissue disorder Pseudoxanthoma elasticum are also discussed.

Inactivation of multidrug resistance proteins disrupts both cellular extrusion and intracellular degradation of cAMP

In addition to xenobiotics and several other endogenous metabolites, multidrug-resistance proteins (MRPs) extrude the second-messenger cAMP from various cells. Pharmacological and/or genetic inactivation of MRPs has been shown to augment intracellular cAMP signaling, an effect assumed to be a direct consequence of the blockade of cAMP extrusion. Here we provide evidence that the augmented intracellular cAMP levels are not due exclusively to the prevention of cAMP efflux because MRP inactivation is also associated with reduced cAMP degradation by phosphodiesterases (PDEs). Several prototypical MRP inhibitors block PDE activity at concentrations widely used to inhibit MRPs. Their dose-dependent effects in several paradigms of cAMP signaling are more consistent with their potency in inhibiting PDEs than MRPs. Moreover, genetic manipulation of MRP expression results in concomitant changes in PDE activity and protein levels, thus affecting cAMP degradation in parallel with cAMP efflux. These findings suggest that the effects of MRP inactivation on intracellular cAMP levels reported previously may be due in part to reduced degradation by PDEs and identify MRP-dependent transport mechanisms as novel regulators of cellular PDE expression levels. Mathematical simulations of cAMP signaling predict that selective ablation of MRP-dependent cAMP efflux per se does not affect bulk cytosolic cAMP levels, but may control cAMP levels in restricted submembrane compartments that are defined by small volume, high MRP activity, limited PDE activity, and limited exchange of cAMP with the bulk-cytosolic cAMP pool. Whether this regulation occurs in cells remains to be confirmed experimentally under conditions that do not affect PDE activity.

Multidrug resistance protein 4 (MRP4/ABCC4) regulates cAMP cellular levels and controls human leukemia cell proliferation and differentiation

Increased intracellular cAMP concentration plays a well established role in leukemic cell maturation. We previously reported that U937 cells stimulated by H2 receptor agonists, despite a robust increase in cAMP, fail to mature because of rapid H2 receptor desensitization and phosphodiesterase (PDE) activation. Here we show that intracellular cAMP levels not only in U937 cells but also in other acute myeloid leukemia cell lines are also regulated by multidrug resistance-associated proteins (MRPs), particularly MRP4. U937, HL-60, and KG-1a cells, exposed to amthamine (H2-receptor agonist), augmented intracellular cAMP concentration with a concomitant increase in the efflux. Extrusion of cAMP was ATP-dependent and probenecid-sensitive, supporting that the transport was MRP-mediated. Cells exposed to amthamine and the PDE4 inhibitor showed enhanced cAMP extrusion, but this response was inhibited by MRP blockade. Amthamine stimulation, combined with PDE4 and MRP inhibition, induced maximal cell arrest proliferation. Knockdown strategy by shRNA revealed that this process was mediated by MRP4. Furthermore, blockade by probenecid or MRP4 knockdown showed that increased intracellular cAMP levels induce maturation in U937 cells. These findings confirm the key role of intracellular cAMP levels in leukemic cell maturation and provide the first evidence that MRP4 may represent a new potential target for leukemia differentiation therapy.

Differential gene expression in liver and small intestine from lactating rats compared to age-matched virgin controls detects increased mRNA of cholesterol biosynthetic genes

Background

Lactation increases energy demands four- to five-fold, leading to a two- to three-fold increase in food consumption, requiring a proportional adjustment in the ability of the lactating dam to absorb nutrients and to synthesize critical biomolecules, such as cholesterol, to meet the dietary needs of both the offspring and the dam. The size and hydrophobicity of the bile acid pool increases during lactation, implying an increased absorption and disposition of lipids, sterols, nutrients, and xenobiotics. In order to investigate changes at the transcriptomics level, we utilized an exon array and calculated expression levels to investigate changes in gene expression in the liver, duodenum, jejunum, and ileum of lactating dams when compared against age-matched virgin controls.

Results

A two-way mixed models ANOVA was applied to detect differentially expressed genes. Significance calls were defined as a p < 0.05 for the overall physiologic state effect (lactation vs. control), and a within tissue pairwise comparison of p < 0.01. The proportion of false positives, an estimate of the ratio of false positives in the list of differentially expressed genes, was calculated for each tissue. The number of differentially expressed genes was 420 in the liver, 337 in the duodenum, 402 in the jejunum, and 523 in the ileum. The list of differentially expressed genes was in turn analyzed by Ingenuity Pathways Analysis (IPA) to detect biological pathways that were overrepresented. In all tissues, sterol regulatory element binding protein (Srebp)-regulated genes involved in cholesterol synthesis showed increased mRNA expression, with the fewest changes detected in the jejunum. We detected increased Scap mRNA in the liver only, suggesting an explanation for the difference in response to lactation between the liver and small intestine. Expression of Cyp7a1, which catalyzes the rate limiting step in the bile acid biosynthetic pathway, was also significantly increased in liver. In addition, decreased levels of mRNA associated with T-cell signaling were found in the jejunum and ileum. Several members of the Solute Carrier (SLC) and Adenosine Triphosphate Binding Cassette (ABC) superfamilies of membrane transporters were found to be differentially expressed; these genes may play a role in differences in nutrient and xenobiotic absorption and disposition. mRNA expression of SLC39a4_predicted, a zinc transporter, was increased in all tissues, suggesting that it is involved in increased zinc uptake during lactation. Microarray data are available through GEO under GSE19175.

Conclusions

We detected differential expression of mRNA from several pathways in lactating dams, including upregulation of the cholesterol biosynthetic pathway in liver and intestine, consistent with Srebp activation. Differential T-Cell signaling in the two most distal regions of the small intestine (ileum and jejunum) was also noted, as well as differential expression of transporters that likely play a key role in nutrient uptake.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Upregulation of peripubertal rat Leydig cell steroidogenesis following 24 h in vitro and in vivo exposure to atrazine

Atrazine is currently one of the most widely used herbicides in the United States and elsewhere. Here we examined 24 h in vitro and in vivo effects of atrazine on androgen production and on expression and activity of steroidogenic enzymes and regulatory proteins involved in cyclic adenosine monophosphate (cAMP)-signaling pathway in peripubertal rat Leydig cells. When in vitro added, 1-50 μM atrazine increased basal and human chorion gonadotropin-stimulated testosterone production and accumulation of cAMP in the medium of treated cells. The stimulatory action of atrazine on androgen production but not on cAMP accumulation was abolished in cells with inhibited protein kinase A. Atrazine also stimulated the expression of mRNA transcripts for steroidogenic factor-1, steroidogenic acute regulatory protein, cytochrome P450 (CYP)17A1, and 17β-hydroxysteroid dehydrogenase (HSD), as well as the activity of CYP17A1 and 17βHSD. The stimulatory effects of atrazine on cAMP accumulation and androgen production were also observed during the first 3 days of in vivo treatment (200 mg/kg body weight, by gavage) followed by a decline during further treatment. These results indicate that atrazine has a transient stimulatory action on cAMP signaling pathway in Leydig cells, leading to facilitated androgenesis.

Testosterone-induced modulation of nitric oxide-cGMP signaling pathway and androgenesis in the rat Leydig cells

Testosterone, acting as a systemic and local factor, is one of the major regulatory molecules that initiate and maintain testicular function. In the present study, different experimental approaches were used to evaluate the role of testosterone in regulation of the nitric oxide (NO)-cGMP pathway in Leydig cells derived from normal and hypogonadotropic male rats treated with testosterone for 24 h and 2 wk. Real-time quantitative PCR and Western blot analysis revealed increased inducible NO synthase (NOS2) expression followed by increased NO secretion from Leydig cells ex vivo after continuous treatment with testosterone for 2 wk in vivo. The cGMP-specific phosphodiesterases Pde5, Pde6, and Pde9 were up-regulated, whereas PRKG1 protein was decreased after a 2-wk testosterone treatment. Induction of Nos2 and Pde5 in Leydig cells was blocked by androgen receptor antagonist. In experimental hypogonadotropic hypogonadism, expression of NOS2 was significantly reduced, and treatment with testosterone increased NOS2 expression above control levels. PDE5 protein level was unchanged in hypogonadal rats, whereas treatment of hypogonadal rats with testosterone significantly increased it. In contrast, hypogonadism and testosterone replacement reduced PRKG1 protein in Leydig cells. In vitro treatment with testosterone caused gradually increased Nos2 gene expression followed by increased nitrite and cGMP production by purified Leydig cells. In summary, testosterone up-regulated NO signaling via increased NOS2 expression and contributed to down-regulation of cGMP signaling in Leydig cells. Thus, testosterone-induced modulation of NO-cGMP signaling may serve as a potent autocrine regulator of testicular steroidogenesis.

Characterization of purinergic P2X4 receptor channels expressed in anterior pituitary cells

Anterior pituitary cells express cation-conducting P2X receptor channels (P2XRs), but their molecular identity, electrophysiological properties, cell-specific expression pattern, and physiological roles have been only partially characterized. In this study, we show by quantitative RT-PCR that mRNA transcripts for the P2X(4) subunit are the most abundant in rat anterior pituitary tissue and confirm the P2X(4)R protein expression by Western blot analysis. Single-cell patch-clamp recordings show that extracellular ATP induced an inward depolarizing current in a majority of thyrotropin-releasing hormone-responsive pituitary cells, which resembled the current profile generated by recombinant P2X(4)R. The channels were activated and desensitized in a dose-dependent manner and deactivated rapidly. Activation of these channels led to stimulation of electrical activity and promotion of voltage-gated and voltage-insensitive Ca(2+) influx. In the presence of ivermectin, a specific allosteric modulator of P2X(4)Rs, there was an approximately fourfold increase in the maximum amplitude of the ATP-induced inward current, accompanied by an increase in the sensitivity of receptors for ATP, slowed deactivation of receptors, and enhanced ATP-induced prolactin release. These results indicate that thyrotropin-releasing hormone-responsive cells, including lactotrophs, express homomeric and/or heteromeric P2X(4)Rs, which facilitate Ca(2+) influx and hormone secretion.

Signaling by purinergic receptors and channels in the pituitary gland

Adenosine 5'-triphosphate is frequently released by cells and acts as an agonist for G protein-coupled P2Y receptors and ligand-gated P2X cationic channels in numerous tissues. The breakdown of ATP by ectonucleotidases not only terminates its extracellular messenger functions, but also provides a pathway for the generation of two additional agonists: adenosine 5'-diphosphate, acting via some P2Y receptors, and adenosine, a native agonist for G protein-coupled adenosine receptors. In the pituitary gland, adenosine 5'-triphosphate is released from the endings of magnocellular hypothalamic neurons and by anterior pituitary cells through pathway(s) that are still not well characterized. This gland also expresses several members of each family of purinergic receptors. P2X and adenosine receptors are co-expressed in the somata and nerve terminals of vasopressin-releasing neurons as well as in some secretory pituitary cells. P2X receptors stimulate electrical activity and modulate InsP(3)-dependent calcium release from intracellular stores, whereas adenosine receptors terminate electrical activity. Calcium-mobilizing P2Y receptors are expressed in pituicytes, folliculo-stellate cells and some secretory cells of the anterior pituitary.

PACAP stimulation of maturational gonadotropin secretion in goldfish involves extracellular signal-regulated kinase, but not nitric oxide or guanylate cyclase, signaling

In goldfish, nitric oxide synthase (NOS) immunoreactivity is present in gonadotropes and extracellular signal-regulated protein kinase (ERK) mediates GnRH stimulation of gonadotropin release and synthesis. In this study, we tested the possible involvement of nitric oxide (NO) and ERK in mediating PACAP-stimulated maturational gonadotropin (GTH-II) release from primary cultures of dispersed goldfish pituitary cells. In static incubation experiments, PACAP-induced GTH-II release was unaffected by two inhibitors of NOS synthase, AGH and 1400W; whereas addition of a NO donor, SNAP, elevated GTH-II secretion. In perifusion experiments, neither NOS inhibitors (AGH, 1400W and 7-Ni) nor NO scavengers (PTIO and rutin hydrate) attenuated the GTH-II response to pulse applications of PACAP. In addition, the GTH-II responses to PACAP and the NO donor SNP were additive while PTIO blocked SNP action. Although dibutyryl cGMP increased GTH-II secretion in static incubation, inhibition of guanylate cyclase (GC), a known down-stream target for NO signaling, did not reduce the GTH-II response to pulse application of PACAP. On the other hand, GTH-II responses to PACAP in perifusion were attenuated in the presence of two inhibitors of ERK kinase (MEK), U 0126 and PD 98059. These results suggest that although increased availability of NO and cGMP can lead to increased GTH-II secretion, MEK/ERK signaling, rather than NOS/NO/GC activation, mediates PACAP action on GTH-II release in goldfish.

Dependence of multidrug resistance protein-mediated cyclic nucleotide efflux on the background sodium conductance

Anterior pituitary cells fire action potentials and release cyclic nucleotides both spontaneously and in response to agonist stimulation, but the relationship between electrical activity and cyclic nucleotide efflux has not been studied. In these cells, a tetrodotoxin-resistant background N(+) conductance is critical for firing of action potentials, and multidrug resistance proteins (MRPs) MRP4 and MRP5 contribute to cyclic nucleotide efflux. Here, we show that abolition of the background Na(+) conductance in rat pituitary cells by complete or partial replacement of extracellular Na(+) with organic cations or sucrose induced a rapid and reversible hyperpolarization of cell membranes and inhibition of action potential firing, accompanied by a rapid inhibition of cyclic nucleotide efflux. Valinomycin-induced hyperpolarization of plasma membranes also inhibited cyclic nucleotide efflux, whereas depolarization of cell membranes induced by the inhibition of Ca(2+) influx or stimulation of Na(+) influx by gramicidin was accompanied by a facilitation of cyclic nucleotide efflux. In contrast, inhibition of cyclic nucleotide efflux by probenecid did not affect the background Na(+) conductance. In human embryonic kidney 293 cells stably transfected with human MRP4 or MRP5, replacement of bath Na(+) with organic cations also hyperpolarized the cell membranes and inhibited cyclic nucleotide efflux. In these cells, the Na(+)/H(+) antiporter monensin did not affect the membrane potential and was practically ineffective in altering cyclic nucleotide efflux. In both pituitary and MRP4- and MRP5-expressing cells, 3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid (MK571) inhibited cyclic nucleotide efflux. These results indicate that the MRP4/5-mediated cyclic nucleotide efflux can be rapidly modulated by membrane potential determined by the background Na(+) conductance.

Signaling by G-protein-coupled receptor (GPCR): studies on the GnRH receptor

Gonadotropin-releasing hormone (GnRH) is the first key hormone of reproduction. GnRH analogs are extensively used in in vitro fertilization, and treatment of sex hormone-dependent cancers, due to their ability to bring about 'chemical castration'. The interaction of GnRH with its cognate type I receptor (GnRHR) in pituitary gonadotropes results in the activation of Gq/G(11), phospholipase Cbeta (PLCbetaI), PLA(2), and PLD. Sequential activation of the phospholipases generates the second messengers inositol 1, 4, 5-trisphosphate (IP(3)), diacylglycerol (DAG), and arachidonic acid (AA), which are required for Ca(2+) mobilization, the activation of various protein kinase C isoforms (PKCs), and the production of prostaglandin (PG) and other metabolites of AA, respectively. PKC isoforms are the major mediators of the downstream activation of a number of mitogen-activated protein kinase (MAPK) cascades by GnRH, namely: extracellular signal-regulated kinase (ERK), jun-N-terminal kinase (JNK), and p38MAPK. The activated MAPKs phosphorylate both cytosolic and nuclear proteins to initiate the transcriptional activation of the gonadotropin subunit genes and the GnRHR. While Ca(2+) mobilization has been found to initiate rapid gonadotropin secretion, Ca(2+), together with various PKC isoforms, MAPKs and AA metabolites also serve as key nodes, in the GnRH-stimulated signaling network that enables the gonadotropes to decode GnRH pulse frequencies and translating that into differential gonadotropin synthesis and release. Even though pulsatility of GnRH is recognized as a major determinant for differential gonadotropin subunit gene expression and gonadotropin secretion very little is yet known about the signaling circuits governing GnRH action at the 'Systems Biology' level. Direct apoptotic and metastatic effects of GnRH analogs in gonadal steroid-dependent cancers expressing the GnRHR also seem to be mediated by the activation of the PKC/MAPK pathways. However, the mechanisms dictating life (pituitary) vs. death (cancer) decisions made by the same GnRHR remain elusive. Understanding these molecular mechanisms triggered by the GnRHR through biochemical and 'Systems Biology' approaches would provide the basis for the construction of the dynamic connectivity maps, which operate in the various cell types (endocrine, cancer, and immune system) targeted by GnRH. The connectivity maps will open a new vista for exploring the direct effects of GnRH analogs in tumors and the design of novel combined therapies for fertility control, reproductive disorders and cancers.

Nitric oxide stimulates embryonic somatotroph differentiation and growth hormone mRNA and protein expression through a cyclic guanosine monophosphate-independent mechanism

In the pituitary gland, NO is locally synthesized by gonadotroph and folliculo-stellate cells. Many reports have shown that NO can modulate the growth hormone (GH) secretion. However, its role on mice embryo GH regulation remains unclear. In addition, it is unknown whether the regulation is associated with the proliferation of pituitary cells. In this study, we have investigated the regulatory effects of NO on somatotroph differentiation, proliferation and GH mRNA and protein expression using primary cell cultures of mice fetal pituitaries (embryonic days 16.5, ED 16.5). Our results show that incubation of pituitary cells in the presence of sodium nitroprusside (SNP; 1mM), a NO donor, for 4.5h resulted in a significant increase in GH mRNA and protein expression (P<0.05) and the stimulation of SNP can be inhibited by hemoglobin, a NO scavenger. But the addition of cyclic guanosine monophosphate (cGMP; 3.0mM), the second messenger of multiple NO actions cannot influence GH mRNA and protein expression. The cyclic nucleotide cellular efflux pumps existed in the pituitary cells can transport the majority of de novo-produced cGMP and effectively block cGMP accumulation. For maintaining intracellular concentration of cGMP, probenecid (0.5mM), a blocker of cGMP efflux pump, combined with cGMP (3.0mM) was used to treat the pituitary cells. This also cannot influence GH mRNA and protein expression. In addition, the ratio of GH-positive cells is increased significantly after the stimulation of SNP (P<0.05). However, SNP cannot modulate the pituitary cell proliferation. From these results we conclude that NO can increase GH mRNA and protein expression in fetal pituitary cells and cGMP is not involved in this hormonal regulation. Stimulation of NO on the somatotroph differentiation does not occur due to pituitary cell proliferation.

Organic anion transporter 2 (SLC22A7) is a facilitative transporter of cGMP

The second messenger, cGMP, mediates a host of cellular responses to various stimuli, resulting in the regulation of many critical physiologic functions. The existence of specific cGMP transporters on the plasma membrane that participate in the regulation of cGMP levels has been suggested in a large number of studies. In this study, we identified a novel plasma membrane transporter for cGMP. In particular, we showed that hOAT2 (SLC22A7), a member of the solute carrier (SLC) superfamily, was a facilitative transporter for cGMP and other guanine nucleotides. hOAT2, which is ubiquitously expressed at high levels in many cell types, was previously thought to primarily transport organic anions. Among purine and pyrimidine nucleobases, nucleosides, and nucleotides, hOAT2 showed the greatest preference for cGMP, which transported cGMP with a K(m) value of 88 +/- 11 muM and exhibited between 50- and 100-fold enhanced uptake over control cells. Our data revealed that hOAT2 is a bidirectional facilitative transporter that can control both intracellular and extracellular levels of cGMP. In addition, we observed that a common alternatively spliced variant of hOAT2 demonstrated a complete loss of transport function as a result of a low expression level on the plasma membrane. We conclude that hOAT2 is a highly efficient, facilitative transporter of cGMP and may be involved in cGMP signaling in many tissues. Our study suggests that hOAT2 represents a potential new drug target for regulating cGMP levels.

Cyclic AMP in rat ileum: evidence for the presence of an extracellular cyclic AMP-adenosine pathway

BACKGROUND & AIMS

Extracellular adenosine plays a relevant role in regulating intestinal motility and preventing inflammatory processes. Adenosine 3',5'-cyclic monophosphate (cAMP) extruded from cells may be converted to adenosine monophosphate and then to adenosine by ecto-phosphodiesterase and CD73/ecto-5'nucleotidase, respectively, thus representing a source of adenosine. Our purpose was to assess the existence of a functional extracellular cAMP-adenosine pathway in intestinal tissue, obtaining evidence for CD73 expression and evaluating the effect of cAMP on ileum motility.

METHODS

The formation of cAMP metabolites in rat ileum strips incubated with exogenous cAMP or [(3)H]cAMP was monitored by high-performance liquid chromatography. CD73 was detected by immunoprecipitation and immunofluorescence. The functional activity of exogenous cAMP on ileum strips was recorded by measuring tension changes.

RESULTS

In ileum strips, the generation of cAMP-derived adenosine monophosphate, adenosine, and inosine was time and concentration dependent and was blocked by phosphodiesterase or CD73 inhibitors in a manner consistent with exogenous cAMP being processed through the extracellular cAMP-adenosine pathway. Accordingly, [(3)H]cAMP uptake in ileum strips was negligible. Immunofluorescence revealed CD73 surface expression on intestinal smooth muscle cells and intact smooth muscle. Exogenous cAMP concentration-dependently increased ileum muscle tension partially inhibited by adenosine inactivation or receptor blockade. Forskolin-stimulated endogenous cAMP induced concentration-dependent ileum relaxations.

CONCLUSIONS

A functioning extracellular cAMP-adenosine pathway featuring CD73 expression is present in rat ileum and affects intestinal motility. Extracellular cAMP may therefore act on intestinal muscle both directly by binding to specific smooth muscle cell membrane sites and indirectly through its degradation products.

Inhibition of hyaluronan export reduces collagen degradation in interleukin-1 treated cartilage

Background

Osteoarthrosis is characterized by cartilage erosion, proteolysis of aggrecan and collagen, and disturbed rates of synthesis of aggrecan and hyaluronan by chondrocytes, with hyaluronan over-production being an early reaction. We considered that inhibition of hyaluronan export might prevent subsequent proteoglycan loss and collagen degradation.

Methods

To test this hypothesis, we studied a tissue culture model using bovine cartilages explants activated with IL-1α to induce osteoarthritic reactions using the phosphodiesterase-5 inhibitors tadalafil, zaprinast and vardenafil.

Results

These drugs inhibited hyaluronan export, but they did not inhibit hyaluronan synthase activity. Simultaneously, they inhibited proteoglycan loss and collagen degradation, but not their synthesis. They also reduced the release of gelatinases into the culture media and diffusion of the indicator protein horseradish peroxidase through the cartilage explants. The mechanism of action of these compounds may be through inhibition of hyaluronan exporter multidrug resistance-associated protein 5 (MRP5), because the effective drug concentrations were much higher than required for phosphodiesterase-5 inhibition and intracellular cGMP accumulation.

Conclusion

Inhibition of hyaluronan over-production may be an appropriate target to attenuate IL-1-induced reactions in osteoarthritic cartilage.

Extracellular calcium and cAMP: second messengers as "third messengers"?

Calcium and cyclic AMP are familiar second messengers that typically become elevated inside cells on activation of cell surface receptors. This article will explore emerging evidence that transport of these signaling molecules across the plasma membrane allows them to be recycled as "third messengers," extending their ability to convey information in a domain outside the cell.

Evaluation of Two Cationic Delivery Systems for siRNA

Small interference RNA (siRNA) is an important research tool, and also has the potential for clinical application. RNA interference (RNAi) approaches allow degradation of selective mRNA coding for pathogenic or disease-related proteins. RNAi pathway can be taken advantage of by the delivery of chemically synthesized siRNA. To fully attain its potential a sufficient siRNA must be delivered to the cell's cytoplasm. Cellular delivery of polyanions such as siRNA, while a challenging problem, may be addressed by the use of cationic macromolecules, the two major classes being lipids and polymers. In this study we compared two model cationic vectors liposomes (lipoplexes) and polyethelyenimine (PEI) (polyplexes). Complexes of the cationic macromolecules and siRNA did not differ in terms of their cellular uptake as determined by flow cytometry. However, it was demonstrated that the lipoplexes decomplexed more easily than the polyplexes. Differences in the biological activity of the siRNA were observed using commercially available siTOX siRNA. Lipoplexes resulted in dose-dependent siRNA activity; to 76.4 +/- 5.9% cell death was seen 48 hours posttransfection using 80 nmol siTOX. In summary, the selection of delivery vector can have a profound impact on biological activity of siRNA molecules. siRNA decomplexation from the cationic vector might be an important factor in the future development of new vectors.

Hyaluronan export by the ABC transporter MRP5 and its modulation by intracellular cGMP

Hyaluronan must be exported from its site of synthesis, the inner side of plasma membrane, to the extracellular matrix. Here, we identified the multidrug-associated protein MRP5 as the principle hyaluronan exporter from fibroblasts. The expression of the MRP5 (ABC-C5) transporter was silenced in fibroblasts using RNA interference, and a dose-dependent inhibition of hyaluronan export was observed. Hyaluronan oligosaccharides introduced into the cytosol competed with the export of endogenously labeled hyaluronan and the MRP5 substrate fluorescein. Because cGMP is a physiological substrate of MRP5, the intracellular concentrations of cGMP were modulated by the drugs 3-isobutyl-1-methylxanthin, propentofyllin, L-NAME, zaprinast, and bromo-cGMP, and the effects on hyaluronan export were analyzed. Increasing the cGMP levels inhibited hyaluronan export and decreasing it afforded higher concentrations of zaprinast to inhibit the export. Thus, cGMP may be a physiological regulator of hyaluronan export at the level of the export MRP5.

Dependence of electrical activity and calcium influx-controlled prolactin release on adenylyl cyclase signaling pathway in pituitary lactotrophs

Pituitary lactotrophs in vitro fire extracellular Ca2+-dependent action potentials spontaneously through still unidentified pacemaking channels, and the associated voltage-gated Ca2+influx (VGCI) is sufficient to maintain basal prolactin (PRL) secretion high and steady. Numerous plasma membrane channels have been characterized in these cells, but the mechanism underlying their pacemaking activity is still not known. Here we studied the relevance of cyclic nucleotide signaling pathways in control of pacemaking, VGCI, and PRL release. In mixed anterior pituitary cells, both VGCI-inhibitable and -insensitive adenylyl cyclase (AC) subtypes contributed to the basal cAMP production, and soluble guanylyl cyclase was exclusively responsible for basal cGMP production. Inhibition of basal AC activity, but not soluble guanylyl cyclase activity, reduced PRL release. In contrast, forskolin stimulated cAMP and cGMP production as well as pacemaking, VGCI, and PRL secretion. Elevation in cAMP and cGMP levels by inhibition of phosphodiesterase activity was also accompanied with increased PRL release. The AC inhibitors attenuated forskolin-stimulated cyclic nucleotide production, VGCI, and PRL release. The cell-permeable 8-bromo-cAMP stimulated firing of action potentials and PRL release and rescued hormone secretion in cells with inhibited ACs in an extracellular Ca2+-dependent manner, whereas 8-bromo-cGMP and 8-(4-chlorophenylthio)-2'-O-methyl-cAMP were ineffective. Protein kinase A inhibitors did not stop spontaneous and forskolin-stimulated pacemaking, VGCI, and PRL release. These results indicate that cAMP facilitates pacemaking, VGCI, and PRL release in lactotrophs predominantly in a protein kinase A- and Epac cAMP receptor-independent manner.