Specific binding and steroidogenetic effects of atrial natriuretic factor (ANF) in Leydig cells of rats

Calcineurin regulates homologous desensitization of natriuretic peptide receptor-A and inhibits ANP-induced testosterone production in MA-10 cells

Receptor desensitization is a ubiquitous regulatory mechanism that defines the activatable pool of receptors, and thus, the ability of cells to respond to environmental stimuli. In recent years, the molecular mechanisms controlling the desensitization of a variety of receptors have been established. However, little is known about the molecular mechanisms that underlie desensitization of natriuretic peptide receptors, including natriuretic peptide receptor-A (NPR-A). Here we report that calcineurin (protein phosphatase 2B, PP2B, PPP3C) regulates homologous desensitization of NPR-A in murine Leydig tumor (MA-10) cells. We demonstrate that both pharmacological inhibition of calcineurin activity and siRNA-mediated suppression of calcineurin expression potentiate atrial natriuretic peptide (ANP)-induced cGMP synthesis. Treatment of MA-10 cells with inhibitors of other phosphoprotein phosphatases had little or no effect on ANP-induced cGMP accumulation. In addition, overexpression of calcineurin blunts ANP-induced cGMP synthesis. We also present data indicating that the inhibition of calcineurin potentiates ANP-induced testosterone production. To better understand the contribution of calcineurin in the regulation of NPR-A activity, we examined the kinetics of ANP-induced cGMP signals. We observed transient ANP-induced cGMP signals, even in the presence of phosphodiesterase inhibitors. Inhibition of both calcineurin and phosphodiesterase dramatically slowed the decay in the response. These observations are consistent with a model in which calcineurin mediated dephosphorylation and desensitization of NPR-A is associated with significant inhibition of cGMP synthesis. PDE activity hydrolyzes cGMP, thus lowering intracellular cGMP toward the basal level. Taken together, these data suggest that calcineurin plays a previously unrecognized role in the desensitization of NPR-A and, thereby, inhibits ANP-mediated increases in testosterone production.

Biology of natriuretic peptides and their receptors

Increasing evidence suggests that natriuretic peptides (NPs) play diverse roles in mammals, including renal hemodynamics, neuroendocrine, and cardiovascular functions. Collectively, NPs are classified as hypotensive hormones; the main actions of NPs are implicated in eliciting natriuretic, diuretic, steroidogenic, antiproliferative, and vasorelaxant effects, important factors in the control of body fluid volume and blood pressure homeostasis. One of the principal loci involved in the regulatory actions of NPs is their cognate plasma membrane receptor molecules, which are activated by binding with specific NPs. Interaction of NPs with their receptors plays a central role in physiology and pathophysiology of hypertension and cardiovascular disorders. Gaining insight into the intricacies of NPs-specific receptor signaling pathways is of pivotal importance for understanding both hormone-receptor biology and the disease states arising from abnormal hormone receptor interplay. During the last decade there has been a surge in interest in NP receptors; consequently, a wealth of information has emerged concerning molecular structure and function, signaling mechanisms, and use of transgenics and gene-targeted mouse models. The objective of this present review is to summarize and document the previous findings and recent discoveries in the field of the natriuretic peptide hormone family and receptor systems with emphasis on the structure-function relationship, signaling mechanisms, and the physiological and pathophysiological significance in health and disease.

Hypertension associated with decreased testosterone levels in natriuretic peptide receptor-A gene-knockout and gene-duplicated mutant mouse models

Mice lacking the gene (Npr1) encoding the natriuretic peptide receptor A (NPRA) have hypertension with elevated blood pressure and cardiac hypertrophy. In particular, Npr1 gene-deficient male mice exhibit lethal vascular events similar to those seen in untreated human hypertensive patients. Serum testosterone levels tend to be lower in hypertensive male humans than in normal males without hypertension, but the genetic basis for this tendency remains unknown. To determine whether Npr1 gene function affects the testosterone level, we measured serum testosterone in male hypertensive mice lacking a functional Npr1 gene, wild-type animals with two copies, and the gene-duplicated littermates expressing four copies of the gene. In the Npr1 gene-knockout (zero-copy) mice, the serum testosterone level was 62% lower than that in the two-copy control mice (80+/-10 ts. 120+/-14 ng/ml, respectively; P < 0.005). Serum testosterone in the four-copy mice was 144% (P < 0.005) of that in the two-copy wild-type control mice. To investigate the role of NPRA in testicular steroidogenesis, we analyzed atrial natriuretic peptide (ANP)-dependent guanylyl cyclase activation, accumulation of intracellular cGMP, and testosterone production in purified primary Leydig cells from animals with zero, two, or four copies of the Npr1 gene. Leydig cells lacking the Npr1 gene did not show ANP-stimulated guanylyl cyclase activation or cGMP accumulation and had no ANP-dependent testosterone production. ANP stimulation of Leydig cells from the four-copy males elicited a 2-fold greater production of cGMP compared to that in the two-copy wild-type counterparts (260+/-12 vs. 126+/-7 pmol/l x 10(6) cells; P < 0.001). Similarly, ANP-dependent testosterone production in Leydig cells was nearly twice as high in four-copy mice as in two-copy wild-type controls (561+/-18 vs. 325+/-11 ng/l x 10(6) cells; P < 0.001). ANP-dependent guanylyl cyclase activation and production of cGMP in Leydig cells increased progressively with the number of Npr1 gene copies. Our results establish the existence of an alternate mechanism for testicular steroidogenesis that is stimulated by NPRA-dependent cGMP signaling, in addition to that mediated by gonadotropins, via a cAMP pathway. These findings demonstrate the role of Npr1 gene function in the maintenance of serum testosterone levels and testicular steroidogenesis and provide a genetic link between hypertension associated with decreased NPRA and low testosterone levels.

Interaction between cyclic nucleotide second messenger systems in murine Leydig cells

Mouse Leydig cell androgen production can be acutely stimulated by atrial natriuretic factor (ANF) via cyclic guanosine 3',5'-monophosphate (cGMP). This stimulation can approach that seen with high concentrations of luteinizing hormone (LH) acting via cyclic adenosine 3',5'-monophosphate (cAMP). To assess the potential for synergistic interaction between LH/cAMP and ANF/cGMP Leydig cells were co-exposed to ANF and LH or ANF/cGMP and site/type-selective cAMP analogues. Co-exposure to 1 nM ANF and 1 ng/ml LH elicited a synergistic increase in androgen production. Both 500 microM 8-bromo-cGMP and ANF (1.0-2.5 nM) synergized with cAMP analogues selective for either of the two major isoenzymes of protein kinase A. Phosphodiesterase (PDE) inhibition was not involved as inclusion of a PDE inhibitor only augmented the response. It appears that ANF/cGMP may interact cooperatively with LH/cAMP in the stimulatory control of androgen production in the mouse Leydig cell and that the site of synergistic interaction may be the activation of the cAMP-dependent protein kinase.

Effects of acute and chronic treatments with atrial natriuretic factor (ANF) on the Leydig cells of the rat testis

Acute ANF bolus administration (40 micrograms.kg-1) did not affect secretory activity and morphology of rat Leydig cells. Prolonged (7-day) ANF infusion (20 micrograms.kg-1.h-1), on the contrary, elevated both basal and hCG-stimulated testosterone blood concentration, and caused a notable hypertrophy of rat Leydig cells. Leydig-cell hypertrophy was due to increases in the volume of all the organelles involved in cholesterol and testosterone synthesis (i.e. mitochondria, smooth endoplasmic reticulum and peroxisomes). These findings suggest that ANF, when chronically administered, is able to stimulate the growth and steroidogenic capacity of rat Leydig cells.

Physiology of the male reproductive system: endocrine, paracrine and autocrine regulation

This presentation reviews the male reproductive system, concentrating on newer advances in our knowledge of its physiology, biochemistry, and regulation, and introduces the topic of male reproductive toxicology. GnRH is the hypothalamic peptide responsible for the stimulation of LH and FSH release from the pituitary. It is synthesized as a pro-hormone, processed in the hypothalamus and released into the portal system in a pulsatile fashion. The timing of these pulses is critical to the release of LH and FSH into the general circulation. While LH and FSH are the main trophic hormones for the testis, we now realize the importance of not only endocrine control, but also of paracrine and autocrine regulation. Specifically, the local control of Leydig cells, Sertoli cells, and germ cells appears to be modulated by numerous growth factors and local regulators arising from within the testis. This point is emphasized both during a discussion of the interaction of the various cell types in the testis and during a discussion of spermatogenesis, where techniques which show stage-specific secretions are highlighted. Newest advances in the mechanism of action of steroidal and peptide hormones are also emphasized with special reference to the possible interaction between toxicants and endocrine control of the reproductive system. This update of the reproductive system "sets the stage" for an in-depth examination of the site and mechanism of action of reproductive toxicants.

Effects of atrial natriuretic factor (ANF) on rat testicular steroidogenesis in vitro

The aim of this study was to evaluate the effects of rat atrial-peptide type II (rAP-II) on testicular steroidogenesis by isolated adult rat Leydig cells. rAP-II stimulates testosterone secretion. The maximal stimulatory effects of rAP-II on testosterone production occurred at the dose of 10(-11) M. At the same dose this peptide stimulates androstenedione and dehydroepiandrosterone-sulphate production, two important testosterone precursors of delta-4 and delta-5 steroidogenetic pathways, respectively. At higher doses, 10(-9) and 10(-7) M, the stimulatory effect of rAP-II on testosterone secretion is strongly declined, on androstenedione secretion is abolished, whereas on dehydroepiandrosterone-sulphate secretion remains unaffected. These data suggest that rAP-II at low doses exerts a stimulatory effect on the early steroidogenetic step by isolated adult rat Leydig cells. At higher doses this peptide seems to influence the testicular steroidogenesis, probably exerting a limiting reaction step involving the 3-beta-OH-steroid-dehydrogenase activity.