Release of ouabain-like compound (OLC) from the intact perfused rat adrenal gland

Role of endogenous ouabain in the etiology of bipolar disorder

Background

Bipolar disorder is a severe psychiatric illness with poor prognosis and problematic and suboptimal treatments. Understanding the pathoetiologic mechanisms may improve treatment and outcomes.

Discussion

Dysregulation of cationic homeostasis is the most reproducible aspect of bipolar pathophysiology. Correction of ionic balance is the universal mechanism of action of all mood stabilizing medications. Recent discoveries of the role of endogenous sodium pump modulators (which include ‘endogenous ouabain’) in regulation of sodium and potassium distribution, inflammation, and activation of key cellular second messenger systems that are important in cell survival, and the demonstration that these stress-responsive chemicals may be dysregulated in bipolar patients, suggest that these compounds may be candidates for the coupling of environmental stressors and illness onset. Specifically, individuals with bipolar disorder appear to be unable to upregulate endogenous ouabain under conditions that require it, and therefore may experience a relative deficiency of this important regulatory hormone. In the absence of elevated endogenous ouabain, neurons are unable to maintain their normal resting potential, become relatively depolarized, and are then susceptible to inappropriate activation. Furthermore, sodium pump activity appears to be necessary to prevent inflammatory signals within the central nervous system. Nearly all available data currently support this model, but additional studies are required to solidify the role of this system.

Conclusion

Endogenous ouabain dysregulation appears to be a reasonable candidate for understanding the pathophysiology of bipolar disorder.

Determination of Endogenous Bufalin in Serum of Patients With Hepatocellular Carcinoma Based on HPLC-MS/MS

Bufalin is a cardiotonic steroid and a key active ingredient of the Chinese medicine ChanSu. It has significant anti-tumor activity against many malignancies, including hepatocellular carcinoma (HCC). Previous studies have shown that human bodies contain an endogenous bufalin-like substance. This study aimed to confirm whether the endogenous bufalin-like substances is bufalin and further detect the differences between HCC and control groups of endogenous bufalin concentration by the high-performance liquid chromatography coupled tandem mass spectrometry (HPLC-MS/MS). The results confirmed the endogenous bufalin-like substance is bufalin. Totally, 227 serum samples were collected: 54 from HCC patients and 173 from healthy volunteers constituting a control group. Both the test group and the control group contained bufalin in serum, revealing that bufalin is indeed an endogenous substance. The bufalin concentration was 1.3 nM in HCC patients and 5.7 nM in normal people (P < 0.0001). These results indicate that human bodies contain endogenous bufalin, and it may be negatively correlated with the incidence of HCC.

Ouabain, endogenous ouabain and ouabain-like factors: The Na+ pump/ouabain receptor, its linkage to NCX, and its myriad functions

In this brief review we discuss some aspects of the Na⁺ pump and its roles in mediating the effects of ouabain and endogenous ouabain (EO): i) in regulating the cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) via Na/Ca exchange (NCX), and ii) in activating a number of protein kinase (PK) signaling cascades that control a myriad of cell functions. Importantly, [Ca²⁺]_CYT and the other signaling pathways intersect at numerous points because of the influence of Ca²⁺ and calmodulin in modulating some steps in those other pathways. While both mechanisms operate in virtually all cells and tissues, this article focuses primarily on their functions in the cardiovascular system, the central nervous system (CNS) and the kidneys.

Ouabain Decreases Regulatory T Cell Number in Mice by Reducing IL-2 Secretion

BACKGROUND

Ouabain (OUA) is a cardiotonic glycoside originally extracted from African plants. It has also been described as an endogenous component in mammals, being released in stress situations mainly by the adrenal gland. OUA has been reported to be capable of inhibiting mitogen-induced lymphocyte proliferation and also affects B and T lymphocytes.

OBJECTIVES

The aim of this work is to show the effects of OUA in peripheral T lymphocytes.

METHODS

In the in vivo experiments, mice were injected intraperitoneally for 3 consecutive days with RPMI medium (control group) or 0.56 mg/kg of OUA diluted in RPMI medium (OUA group). On the fourth day, spleen or mesenteric lymph nodes were removed.

RESULTS

OUA significantly reduced the number of CD4+ T lymphocytes in the spleen, especially regulatory T cells (Tregs). In vitro OUA did not inhibit the proliferation of CD4+T lymphocytes stimulated with anti-CD3 neither was able to induce the apoptosis of CD4+ nor Tregs. There was no increase in the number or percentage of T lymphocytes in the mesenteric lymph nodes, suggesting that there was no preferential accumulation of these cells in this organ. Secretion of IL-2 by activated T lymphocytes was decreased by the OUA, explaining at least in part the reduction of Tregs, since this cytokine is involved in the peripheral conversion and maintenance of Tregs.

CONCLUSION

The impact of this reduction in autoimmune diseases, allergy and cancer as well as the potential use of OUA as a therapeutic approach in tumor treatment still needs more investigation.

Dynamics of murine B lymphocytes is modulated by in vivo treatment with steroid ouabain

Ouabain (OUA) is a steroid hormone capable of inhibiting the protein Na+K+ATPase present in the plasma membrane of cells. Ouabain was initially extracted from the roots of African trees such as Acocanthera ouabaio and Strophantus gratus seeds and later described as an endogenous component found in higher mammals. The adrenal gland is the main site of synthesis of ouabain and it is released in stressful situations, conditions similar to those where there is secretion of corticosteroids. Immunological functions have been shown to be regulated by ouabain. In order to understand the effects of ouabain on B lymphocyte populations in different lymphoid organs, mice received intraperitoneal injections of ouabain for 3 consecutive days. Twenty-four hours after the last injection, cells were analyzed by flow cytometry. In the spleen, ouabain modulated especially follicular B cells, inducing a significant decrease in the percentage and absolute numbers of those cells. Ouabain also reduced the absolute number of marginal zone B lymphocytes. No difference in the percentage or absolute number of B lymphocytes in the spleen forty-eight hours after the last injection was observed. An increase in the number of B cells was seen in mesenteric lymph nodes and this retention appears to be directly related to increased expression of CXCR5 chemokine receptor and reduction of CD62L, which also explains the observed reduction of B cells in the spleen. Our results indicate that ouabain regulates the dynamics of B lymphocytes in peripheral organs but production of total IgM and IgG in the serum of animals treated in vivo with ouabain was not affected.

Ouabain inhibits monocyte activation in vitro: prevention of the proinflammatory mCD14(+)/CD16(+) subset appearance and cell-size progression

Classically described as a potent inhibitor of the sodium-potassium adenosine triphosphatase enzyme, ouabain has been further shown to act as an effective immunomodulator in mammals. Recently, our group showed that this hormone downregulates membrane CD14 (mCD14) in human monocytes, though it is not known whether monocyte activation status could modify ouabain influence. Hence, we aimed to investigate ouabain effect during monocyte activation in vitro, analyzing mCD14, CD16 and CD69 expression in total monocytes after two periods of adhesion (2 hours and 24 hours) or in small and large monocyte subpopulations separately. Ouabain (100 nM) inhibited monocyte-size increase, characteristic of activation, only when added to cells immediately after 2 hours’ adhesion. Moreover, downregulation of both mCD14 and CD16 expression by ouabain was more effective in small monocytes and in cells after 2 hours’ adhesion. Since monocytes after 24 hours’ adhesion showed no lack of ouabain binding and no CD69 upregulation, it seems that ouabain is somehow incapable of triggering an appropriate cell-signaling induction once monocytes become activated. Furthermore, though p38 MAPK activation was crucial for the impairment in cell-size progression induced by ouabain, its inhibition did not alter ouabain-induced CD69 upregulation, suggesting that other molecules may participate in the response to this hormone by monocytes. Our data suggest that ouabain inhibits monocyte activation in vitro, preventing both cell-size increase and the appearance of the proinflammatory mCD14⁺/CD16⁺ subpopulation. Thus, the findings suggest that individuals suffering from disorders commonly associated with high ouabain plasma levels, like hypertension, may present defective monocyte activation under inflammation or infection.

"The Lower Threshold" phenomenon in tumor cells toward endogenous digitalis-like compounds: Responsible for tumorigenesis?

Since their first discovery as potential anti-cancer drugs decades ago, there is increasing evidence that digitalis-like compounds (DLC) have anti-tumor effects. Less is known about endogenous DLC (EDLC) metabolism and regulation. As stress hormones synthesized in and secreted from the adrenal gland, they likely take part in the hypothalamo-pituitary-adrenal (HPA) axis. In a previous study, we revealed reduced EDLC concentrations in plasma and organs from immune-compromised animals and proposed that a similar situation of a deregulated HPA axis with "adrenal EDLF exhaustion" may contribute to tumorigenesis in chronic stress situations. Here, we put forward the hypothesis that a lowered EDLC response threshold of tumor cells as compared with normal cells increases the risk of tumorigenesis, especially in those individuals with reduced EDLC plasma concentrations after chronic stress exposure. We will evaluate this hypothesis by (a) summarizing the effects of different DLC concentrations on tumor as compared with normal cells and (b) reviewing some essential differences in the Na/K-ATPase of tumor as compared with normal cells (isoform pattern, pump activity, mutations of other signalosome receptors). We will conclude that (1) tumor cells, indeed, seem to have their individual "physiologic" EDLC response range that already starts at pmolar levels and (2) that individuals with markedly reduced (pmolar) EDLC plasma levels are predisposed to cancer because these EDLC concentrations will predominantly stimulate the proliferation of tumor cells. Finally, we will summarize preliminary results from our department supporting this hypothesis.

Influence of N-methyl-D-aspartate receptors on ouabain activation of nuclear factor-κB in the rat hippocampus

It has been shown that ouabain (OUA) can activate the Na,K-ATPase complex and mediate intracellular signaling in the central nervous system (CNS). Inflammatory stimulus increases glutamatergic transmission, especially at N-methyl-D-aspartate (NMDA) receptors, which are usually coupled to the activation of nitric oxide synthase (NOS). Nuclear factor-κB (NF-κB) activation modulates the expression of genes involved in development, plasticity, and inflammation. The present work investigated the effects of OUA on NF-κB binding activity in rat hippocampus and the influence of this OUA-Na,K-ATPase signaling cascade in NMDA-mediated NF-κB activation. The findings presented here are the first report indicating that intrahippocampal administration of OUA, in a concentration that did not alter Na,K-ATPase or NOS activity, induced an activation of NF-κB, leading to increases in brain-derived neurotrophic factor (Bdnf), inducible NOS (iNos), tumor necrosis factor-α (Tnf-α), and B-cell leukemia/lymphoma 2 (Bcl2) mRNA levels. This response was not linked to any significant signs of neurodegeneration as showed via Fluoro-Jade B and Nissl stain. Intrahippocampal administration of NMDA induced NF-κB activation and increased NOS and α(2/3) -Na,K-ATPase activities. NMDA treatment further increased OUA-induced NF-κB activation, which was partially blocked by MK-801, an antagonist of NMDA receptor. These results suggest that OUA-induced NF-κB activation is at least in part dependent on Na,K-ATPase modulatory action of NMDA receptor in hippocampus. The interaction of these signaling pathways could be associated with biological mechanisms that may underlie the basal homeostatic state linked to the inflammatory signaling cascade in the brain.

Modulation of the immune system by ouabain

Ouabain, a known inhibitor of the Na,K-ATPase, has been shown to regulate a number of lymphocyte functions in vitro and in vivo. Lymphocyte proliferation, apoptosis, cytokine production, and monocyte function are all affected by ouabain. The ouabain-binding site occurs at the alpha subunit of the enzyme. The alpha subunit plays a critical role in the transport process, and four different alpha-subunit isoforms have been described with different sensitivities to ouabain. Analysis by RT-PCR indicates that alpha1, alpha2, and alpha3 isoforms are all present in murine lymphoid cells obtained from thymus, lymph nodes, and spleen. In these cells ouabain exerts an effect at concentrations that do not induce plasma membrane depolarization, suggesting a mechanism independent of the classical inhibition of the pump. In other systems, the Na,K-ATPase acts as a signal transducer in addition to being an ion pump, and ouabain is capable of inducing the activation of various signal transduction cascades. Neither resting nor concanavalin A (Con A)-activated thymocytes had their levels of phosphorylated-extracellular signal-regulated kinase (P-ERK) modified by ouabain. However, ouabain decreased p38 phosphorylation induced by Con A in these cells. The pathway induced by ouabain in lymphoid cells is still unclear but might vary with the type and state of activation of the cell.

Activation of brain renin-angiotensin-aldosterone system by central sodium in Wistar rats

Functional studies indicate that the sympathoexcitatory and pressor responses to an increase in cerebrospinal fluid (CSF) [Na+] by central infusion of Na+-rich artificial cerebrospinal fluid (aCSF) in Wistar rats are mediated in the brain by mineralocorticoid receptor (MR) activation, ouabain-like compounds (OLC), and AT1-receptor stimulation. In the present study, we examined whether increasing CSF [Na+] by intracerebroventricular infusion of Na+-rich aCSF activates MR and thereby increases OLC and components of the renin-angiotensin system in the brain. Male Wistar rats received via osmotic minipump an intracerebroventricular infusion of aCSF or Na+-rich aCSF, in some groups combined with intracerebroventricular infusion of spironolactone (100 ng/h), antibody Fab fragments (to bind OLC), or as control gamma-globulins. After 2 wk of infusion, resting blood pressure and heart rate were recorded, OLC and aldosterone content in the hypothalamus were assessed by a specific ELISA or radioimmunoassay, and angiotensin-converting enzyme (ACE) and AT1-receptor binding densities in various brain nuclei were measured by autoradiography using 125I-labeled 351 A and 125I-labeled ANG II. When compared with intracerebroventricular aCSF, intracerebroventricular Na+-rich aCSF increased CSF [Na+] by approximately 5 mmol/l, mean arterial pressure by approximately 20 mmHg, heart rate by approximately 65 beats/min, and hypothalamic content of OLC by 50% and of aldosterone by 33%. Intracerebroventricular spironolactone did not affect CSF [Na+] but blocked the Na+-rich aCSF-induced increases in blood pressure and heart rate and OLC content. Intracerebroventricular Na+-rich aCSF increased ACE and AT1-receptor-binding densities in several brain nuclei, and Fab fragments blocked these increases. These data indicate that in Wistar rats, a chronic increase in CSF [Na+] may increase hypothalamic aldosterone and activate CNS pathways involving MR, and OLC, leading to increases in AT1-receptor and ACE densities in brain areas involved in cardiovascular regulation and hypertension.

Ouabain induces an increase of retinal ganglion cell survival in vitro: the involvement of protein kinase C

Ouabain is a steroid derivative that can regulate many cellular events such as growth and proliferation. It modulates Na+,K+-ATPase activity leading to the activation of different intracellular pathways through protein-protein interactions that have been characterized during the last few years. The aim of this work was to study the role of ouabain in rat retinal ganglion cell survival after 48 h in culture. Our results demonstrated that ouabain significantly induced an increase in retinal ganglion cell survival. The effect was dose-dependent and was maximal with 3.0 nM. The blockade of protein kinase C activity by 1.25 microM chelerythrine chloride abolished the ouabain effect, indicating an involvement of this intracellular pathway. None of the protein kinase inhibitors usually employed in the study of ouabain-driven intracellular pathways (PD98059, Ly294002, herbimycin, and genistein) was able to influence neuronal survival induced by ouabain. The data presented suggest that ouabain may be the trigger of an intracellular pathway responsible for neuronal survival.

The highly conserved cardiac glycoside binding site of Na,K-ATPase plays a role in blood pressure regulation

The Na,K-ATPase contains a binding site for cardiac glycosides, such as ouabain, digoxin, and digitoxin, which is highly conserved among species ranging from Drosophila to humans. Although advantage has been taken of this site to treat congestive heart failure with drugs such as digoxin, it is unknown whether this site has a natural function in vivo. Here we show that this site plays an important role in the regulation of blood pressure, and it specifically mediates adrenocorticotropic hormone (ACTH)-induced hypertension in mice. We used genetically engineered mice in which the Na,K-ATPase alpha2 isoform, which is normally sensitive to cardiac glycosides, was made resistant to these compounds. Chronic administration of ACTH caused hypertension in WT mice but not in mice with an ouabain-resistant alpha2 isoform of Na,K-ATPase. This finding demonstrates that the cardiac glycoside binding site of the Na,K-ATPase plays an important role in blood pressure regulation, most likely by responding to a naturally occurring ligand. Because the alpha1 isoform is sensitive to cardiac glycosides in humans, we developed mice in which the naturally occurring ouabain-resistant alpha1 isoform was made ouabain-sensitive. Mice with the ouabain-sensitive "human-like" alpha1 isoform and an ouabain-resistant alpha2 isoform developed ACTH-induced hypertension to greater extent than WT animals. This result indicates that the cardiac glycoside binding site of the alpha1 isoform can also mediate ACTH-induced hypertension. Taken together these results demonstrate that the cardiac glycoside binding site of the alpha isoforms of the Na,K-ATPase have a physiological function and supports the hypothesis for a role of the endogenous cardiac glycosides.

De novo biosynthesis and radiolabeling of mammalian digitalis-like factors

BACKGROUND

Digoxin-like immunoreactive factors (DLIFs) are endogenous mammalian cardenolides with structural features similar to those of the plant-derived digitalis compounds. DLIFs and their structurally related forms (Dh-DLIFs) may serve as effectors of ion-transport activity mediated by their interaction with Na,K-ATPase and thus play a role as a new hormonal axis. Although some evidence implicates the adrenal gland as a tissue source for the DLIFs, little is known about the biosynthetic pathway producing these compounds. We now demonstrate de novo biosynthesis of DLIF by incorporation of radioactive carbon ((14)C) into the structures of both DLIF and Dh-DLIF.

METHODS

We used a combination of reversed-phase HPLC techniques to separate the radioactive DLIF components after incorporation of (14)C into their structure by use of either [1,2-(14)C]acetic acid or [4-(14)C]cholesterol as precursors and a Y-1 mouse adrenocortical tumor cell line. We also stimulated and suppressed production of steroidogenesis by use of cAMP analogs and Mevastatin, respectively, to demonstrate the dependence of DLIF production on the cholesterol-dependent biosynthetic pathway. A combination of chromatographic mobility, immunoassays specific for digoxin and dihydrodigoxin, and deglycosylation using 5-sulfosalicylic acid were used to identify the DLIF and Dh-DLIF components.

RESULTS

With cholesterol as precursor, the cells produced DLIF (7.5 mCi/mmol) with a labeling efficiency of 10%, whereas with acetate the cells produced DLIF (72.2 mCi/mmol) with a labeling efficiency of 0.08% of the total DLIF produced. The radiolabeled DLIF and Dh-DLIF molecules had identical chromatographic mobilities and stoichiometric removal of sugars as the previously characterized DLIFs isolated from different mammalian species and tissues. With radioactive cholesterol as precursor, the (14)C was incorporated into the DLIF-genin portion of the compounds and not the sugars. Interestingly, treatment of Y-1 cells with 8-bromoadenosine 3':5'-cAMP to stimulate steroidogenesis did not increase production of DLIF or Dh-DLIF but did increase production of progesterone. Mevastatin (5 micromol), an inhibitor of the enzyme hydroxymethylglutaryl-CoA reductase and thus of cholesterol biosynthesis, gave an 85% decrease in the production of (14)C-DLIF and progesterone, but only a modest 15% decrease in (14)C-Dh-DLIF production.

CONCLUSIONS

These data demonstrate that the adrenal cell has the cellular machinery necessary for de novo biosynthesis of DLIF and Dh-DLIF starting from a simple carbon pool and also support the concept that cholesterol is a major precursor of the DLIF compounds. This cell culture model provides a source of radiolabeled DLIF compounds for future experimental work.

11-hydroxylation in the biosynthesis of endogenous ouabain: multiple implications

Accumulating evidence indicates that mammals use steroidal glycosides with "digitalis-like" activity. An endogenous ouabain (EO) has been described and is linked with long-term changes in sodium balance and cardiovascular structure and function. In the adrenal gland, the biosynthesis of EO and similar compounds appears to involve cholesterol side-chain cleavage with sequential metabolism of pregnenolone and progesterone. The more distal events in the biosynthesis have not been elucidated. Preliminary work using primary cell cultures from the bovine adrenal cortex suggests that the biosynthesis of EO is affected by inhibitors of 11beta-hydroxylase. Direct participation of 11-hydoxylase in EO synthesis would lead to an 11beta isomer of ouabain in mammals and, in vivo, an 11beta-oriented hydroxyl group would spontaneously form a mixture of two 11-19 hemiketal isomers. The latter isomers would likely be converted back to a single 11beta isomer of ouabain during isolation. The existence of an additional ring in the hemiketals, along with reduced flexion of the steroidal A, B, and C rings, raises the possibility that their in vivo physiological targets and actions differ from the isolated form of EO.

Human Adrenal Cells in Culture Produce Both Ouabain-like and Dihydroouabain-like Factors

Background: Ouabain-like factor (OLF) and its newly discovered reduced species, dihydroouabain-like factor (Dh-OLF), are mammalian cardenolides whose structural and functional characteristics are similar to the plant-derived compounds ouabain and dihydroouabain. These endogenous compounds are believed to be produced by the adrenals and to constitute part of an hormonal axis that may regulate the catalytic activity of the α-subunit of Na+,K+-ATPase. We developed antibodies sufficiently specific to distinguish between OLF and Dh-OLF, and in this study demonstrate the selective secretion of OLF and Dh-OLF from human H295R-1 adrenocortical cells in culture.

Methods: We used reversed-phase HPLC, inhibition of Na+,K+-ATPase catalytic activity, and two enzyme immunoassays developed with antibodies specific to ouabain and dihydroouabain to purify and characterize the secretion of these two compounds by human adrenal cells in culture. Purified antisera had high titers (1 × 106 for ouabain and 8 ×105 for dihydroouabain) and were specific to their corresponding antigens.

Results: Human H295R-1 cells grown in serum-free medium secreted 0.18 ± 0.03 pmol of OLF and 0.39 ± 0.04 pmol of Dh-OLF per 106 cells in 24 h. Both OLF and Dh-OLF inhibited the ouabain-sensitive catalytic activity of the sodium pump (0.03 μmol/L OLF inhibited 29% of the catalytic activity; 0.07 μmol/L Dh-OLF inhibited 17%). Stimulation of the cell culture by dibutryl cAMP increased the secretion of Dh-OLF 50% over control (unstimulated), whereas the secretion of OLF did not increase significantly.

Conclusions: OLF and Dh-OLF are secreted by human adrenal cells, and antibodies specific to these two compounds can be developed, using the plant-derived counterparts as antigens. The secretion of Dh-OLF is responsive to a cAMP-dependent stimulation mechanism, whereas OLF is not. Our data suggest that either the secretory or biosynthetic pathways for production of these two compounds by human adrenal cells may have different control mechanisms or that they may be linked via a precursor–product relationship.

Cardiotonic steroids: potential endogenous sodium pump ligands with diverse function

The highly conserved cardiotonic steroid (CS) binding site present on the ubiquitous membrane sodium pump, sodium, potassium-ATPase, appears to have been conserved by no force other than its capacity to bind CS: a family that includes plant-derived cardiac glycosides and putative endogenous vertebrate counterparts. Binding of ligand is inhibited by increased extracellular potassium. This implies functional coordination because inhibition of the sodium pump would be counterproductive when extracellular potassium is elevated. The interesting biology of the CS binding site continues to stimulate investigations into the identity of endogenous ligands, their role as pump regulators at the cellular level, and as mediators of body fluid balance and blood pressure regulation. In addition to inhibition of sodium and potassium transport, there is considerable recent evidence suggesting that the sodium pump may act as a cell signaling receptor activated by CS binding and responding by coordination of intracellular signaling pathways that can be dependent on and also independent of the reduction in transmembrane ion flux resulting directly from pump inhibition. This signaling may influence cell survival, growth, and differentiation. Recent insight into the biology of pump regulation by CS is reviewed.