Synthesis and internalization of atrial natriuretic factor in anterior pituitary cells

Non-lysosomal cycling pathway for atrial natriuretic peptide activated by protein kinase C in human NPE cells

Atrial natriuretic peptide (ANP) stimulates aqueous humor formation in primates, but the membrane-bound receptors which mediate this effect have not been well studied in the eye. Endocytosis of [125I]ANP bound to natriuretic peptide C receptors was characterized in fetal human nonpigmented ciliary epithelial (NPE) cells. [125I]ANP which bound to cells at 4 degreesC was detected in the cell interior after a temperature shift to 37 degreesC. Appearance of ligand within the cell peaked at 5 min, and then declined towards zero over 20 min. The endocytosis inhibitor phenylarsine oxide blocked the appearance of internalized ligand, whereas the lysosomotropic drug chloroquine had no effect on internalization but blocked subsequent loss of internalized ligand. Chloroquine also blocked the accumulation of degraded ligand in the extracellular medium. Treatment with phorbol 12-myristate, 13-acetate accelerated the loss of internalized ligand from cells and increased the accumulation of ligand in the extracellular medium. Ligand in the medium was also increased by dioctanoylglycerol but not by 4alpha phorbol didecanoate, an isomer which does not activate protein kinase C. The protein kinase inhibitors staurosporine and bisindolylymaleimide blocked the increase in ligand. Phorbol ester-stimulated loss of internalized ligand occurred in the presence of chloroquine. TCA precipitation of ligand in the extracellular medium showed that both degraded and undegraded [125I]ANP were present. However, in the presence of chloroquine only, undegraded ANP was detected in the medium, and phorbol esters stimulated its rate of appearance by approximately 2 fold. A similar stimulation occurred when cells containing internalized ligand, but stripped of membrane-bound ligand, were exposed to phorbol esters. The data suggest that ANP bound to natriuretic peptide C receptors on NPE cells is endocytosed, and that protein kinase C activates a non-lysosomal pathway for ANP retroendocytosis in these cells.

Localization of mRNA coding for the three subtypes of atrial natriuretic factor (ANF) receptors in rat anterior pituitary gland cells

Atrial Natriuretic Factor (ANF) action is mediated by highly selective and specific receptors. Three subtypes have been characterized and cloned: ANF receptor-A, -B and -C. These subtypes are all expressed in the anterior pituitary of the rat. In the present study, the mRNA for each subtype was detected by in situ hybridization. The amounts of ANFR-A and -B mRNA were found to be similar, and to be twice that of ANFR-C mRNA. At the ultrastructural level, the three types of ANFR mRNA were expressed in three anterior pituitary cell types, namely lactotrophs, corticotrophs, and gonadotrophs, identified by their hormonal content. No signal was revealed in somatotrophs or thyrotrophs. The different forms of mRNA were similar in terms of subcellular localization: in the cytoplasmic matrix and the nuclear euchromatin. These data indicate that the anterior pituitary is an important target tissue for ANF action.

Internalization and nuclear localization of peptide hormones

The presence of neuropeptide receptors on the plasma membrane is well accepted, as is its internalization and down-regulation. The analysis of the fate of these peptides within their target-cells is difficult. Endogenous peptides or administered native peptides are visualized in these cells using immunocytology after cryoultramicrotomy. Labelled peptides can be injected and their internalization kinetics studied using ultrastructural autoradiography. The pituitary gland is a suitable model for the study of the neuropeptide mechanism, with the lactotroph function being taken as an example in the present case. Prolactin (PRL) release depends on two main neuropeptides: thyrotropin-releasing hormone (TRH) and somatostatin (SS). The TRH immunoreactivity obtained from endogenous as well as injected material was restricted to the plasma membrane, secretory granules, cytoplasmic matrix and nucleus. The internalization kinetics of exogenous native TRH showed an increase of immunoreactive material in all compartments including the nucleus. The endogenous SSs (SS14 and SS28) were detected in the same subcellular lactotroph compartments. Injection of 125I-SS showed a rapid binding of SS at the plasma membrane level before internalization. For 60 min of in vivo uptake, 125I-SS28, the large SS molecule, was detected in the cytoplasm only, while 125I-SS14 was found in the nuclear matrix. In vitro 125I-SS28 was restricted to the nuclear membrane. Under physiological conditions the endogenous neuropeptides were visualized in the nucleus, but after injection of labelled peptides only small molecules were found in the nucleus. The significance of the presence of these neuropeptides is discussed.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

Autoradiographic visualization of the natriuretic peptide receptor-B in rat tissues

Natriuretic peptide receptor-B (NPRB) was visualized in rat tissues by in vitro autoradiography, using its putative physiological agonist C-type natriuretic peptide (CNP). In initial studies, we determined that atrial natriuretic peptide (ANP) is not a suitable ligand for labeling the NPRB: in tissues reported to contain NPRB transcripts, CNP did not inhibit [125I]ANP binding except to NPRC sites. Therefore, to visualize the NPRB we used 125I[Tyr(o)]-CNP as a radioligand with an excess of NPRC-blocking peptide: C-ANP. With this approach we detected the highest number of NPRB-like sites in the pars intermedia of the pituitary gland. A large number of these sites were present in pituitary neural and anterior lobes, area postrema, adrenal medulla and cortex. A moderate NPRB population was observed in the subfornical organ, plexiform layer of the olfactory bulb and kidney. Low concentrations of NPRB were noted in the cerebellum and cerebrum but not in the choroid plexus and pia-arachnoid. Saturation experiments performed on cerebellum sections revealed a very low concentration (Bmax 4.8 fmol/mg protein) of high affinity (Kd 1.2 nM) NPRB-like sites. This study is the first demonstration of 125I[Tyr(o)]-CNP binding sites with characteristics of the NPRB in intact tissues.

Thyroidal ANF: a possible mediator of autocrine regulation in the porcine thyroid gland

Atrial natriuretic factor immunoreactivity (ir-ANF) was examined in thyroid tissue sections and cultured thyroid cells using immunohistochemical staining with a monoclonal anti-ANF antibody. Localization of ir-ANF in perinuclear granules in cultured cells and in the basal region of follicular cells in sectioned tissue suggests that ir-ANF is a basally secreted product. Thyroidal ir-ANF was also characterized using reversed phase high performance liquid chromatography (RP-HPLC) of acidic thyroid extracts. An ir-ANF peak coeluting with synthetic rat ANF(99-126) suggests that thyroidal ir-ANF may be identical in form to circulating atrial ANF. However, the detection of ir-ANF in cultured thyroid cells confirms that the immunoreactivity is locally produced. Saturation analysis revealed high affinity ANF receptors (Kd = 0.1 nM; MBC = 17.2 fmol/mg protein) on these cultured cells, and a competition study demonstrated the ability of extracted thyroidal ir-ANF to inhibit 125I rat ANF binding to the membrane receptors. The evidence presented here suggests that ir-ANF in the thyroid may be secreted locally to exert an autocrine effect on neighboring follicular cells.

Differential internalization and processing of atrial-natriuretic-factor B and C receptor in PC12 cells

PC12 cells express two atrial-natriuretic-factor-(ANF)-receptor subtypes with molecular masses of 130,000 (B receptor) and 70,000 (C receptor). The B-receptor subtype constitutes 65% of the cell-surface receptor population, and the remaining 35% are C receptors as determined by saturation binding studies in the presence of C-ANF, a C-receptor-selective analogue. ANF-(99-126)-peptide [ANF(99-126)], which can bind to both B- and C-receptor subtypes, was rapidly internalized into the cells after incubation at 37 degrees C. Internalization of 125I-ANF(99-126) was used as an index of the receptor-mediated endocytosis and to quantify receptor internalization. In the presence of a saturating concentration of C-ANF, receptor-mediated internalization of 125I-ANF(99-126) was reduced by 24%, indicating B receptor mediate 76% of ligand internalization. Incubation of cells with 10 microM-ANF at 37 degrees C down-regulated both receptor subtypes as reflected by decreased surface binding. Time-dependent studies suggest that B- and C-receptor subtypes undergo differential down-regulation. Incubation of down-regulated cells for 120 min in ANF-free medium produced a recovery of 35% of the original cell-surface binding. Affinity cross-linking of 125I-ANF to the receptors on the plasma membrane in re-incubated (up-regulated) cells demonstrated expression of predominantly the B-receptor subtype. Monensin blocked 72% of receptor up-regulation, whereas cycloheximide inhibited 43%, suggesting an active recycling mechanism involved in mediating up-regulation of the B receptors. The present study demonstrates a rapid internalization and intracellular recycling mechanism for B receptors in PC12 cells. C receptors also undergo internalization and down-regulation, but recycling of this receptor subtype into the plasma membrane occurs at a lower rate and to a lesser extent than is the case for the B receptor.

Binding and intracellular degradation of atrial natriuretic factor by cultured vascular smooth muscle cells

Binding studies were performed on vascular smooth muscle cells (VSMC) from the rat aorta, using 125I-atrial natriuretic factor (Ser99-Tyr126) (ANF (Ser99-Tyr126] as the ligand. Kinetic studies at 37 degrees C indicated a rapid onset of binding with a maximum total binding of 25% being reached by 60 min. Crosslinking experiments demonstrated that ANF bound to a 120 kDa and a 60 kDa protein with the former dissociating into the 60 kDa species in presence of beta-mercaptoethanol. Of the total radioactivity bound, 15% represented internalized material. Analysis of the medium after different incubation periods revealed a 42% degradation of 125I-ANF by 120 min. At 4 degrees C, no internalization of 125I-ANF was observed. However, surface binding occurred, albeit at a much slower rate, and not reaching a maximum even at the end of 3 h. No degraded material was detected in the extracellular medium even after a 2-h incubation. Chloroquine (100 microM) and monensin (10 microM) significantly increased the cell-associated radioactivity, causing a 2- to 3-fold elevation of internalized material and a 1.5- to 2-fold rise in the surface-bound ligand. Both lysosomotropic agents also inhibited ANF degradation by 70-80%. Kinetic of the intracellular labeled material was analyzed: within 5-10 min it reaches a maximum level and it decreases rapidly. In presence of monensin the intracellular signal was amplified and the decay was minimized. The intracellular material was found to be mostly bound to a 60 kDa protein. These studies suggest an intracellular degradation of ANF, probably in the lysosomal compartment, following receptor-mediated endocytosis.

Direct binding of atrial natriuretic factor to adrenocortical mitochondria

Atrial natriuretic factor (ANF) is a known antagonist of adrenocortical aldosterone synthesis and secretion. Immunocytochemical and ultrastructural autoradiographic evidence suggests that ANF may bind to mitochondria of a number of target tissues including adrenal cortex. Consequently, the ability of [125I]ANF to bind directly to isolated bovine adrenocortical mitochondria was assessed. Mitochondrial-enriched subfractions of adrenocortical homogenates were prepared by differential and sucrose gradient centrifugation. Mitochondrial membranes specifically bound [125I]ANF. At 20 degrees C equilibrium was achieved between 90 and 120 min. [125I]ANF binding was inhibited in a concentration-dependent manner by unlabelled ANF (IC50 about 5 x 10(-10) M); other substances with biological actions on glomerulosa cells (arginine vasopressin, angiotensin II) did not alter [125I]ANF binding. Similarly shorter ANF fragments including ANF-(103-125), ANF-(99-109) and ANF-(111-126) had no significant competitive effect on binding of the labelled ligand. While Ca2+ and Mg2+ had little effect on ANF binding, the divalent cation Ni2+ inhibited binding of radiolabelled ANF by 90% (IC50 about 8.3 x 10(-5) M). Scatchard analysis revealed both high and low affinity binding sites for [125I]ANF with respective KDs of 4.7 +/- 7 pM and 0.3 +/- 0.02 nM and receptor densities of 1.1 +/- 0.2 and 8.6 +/- 0.1 pmol/mg protein. At 0.2 nM, Ni2+ caused a 5-fold and 100-fold decrease in high and low affinity [125I]ANF binding, respectively. The data demonstrate that ANF binds directly to mitochondria and perhaps it is at this site that the atrial peptide negatively modulates agonist-induced aldosterone biosynthesis.

Receptor sites for atrial natriuretic factors in brain and associated structures: an overview

1. Recent data have clearly shown the existence of specific receptor binding sites for atrial natriuretic factors (ANF) or polypeptides in mammalian brain tissues. 2. Ligand selectivity pattern and coupling to cGMP production suggest that brain ANF sites are similar to high-affinity/low-capacity sites found in various peripheral tissues (kidney, adrenal gland, blood vessels). These brain ANF sites possibly are of the B-ANP subtype. 3. High densities of ANF binding sites are found especially in areas of the central nervous system associated with the control of various cardiovascular parameters (such as the subfornical organ and area postrema). However, high densities of sites are also present in other regions such as the hippocampus, cerebellum, and thalamus in the brain of certain mammalian species, suggesting that brain ANF could act as a neuromodulator of noncardiovascular functions. 4. The density of brain ANF binding sites is modified in certain animal models of cardiovascular disorders and during postnatal ontogeny, demonstrating the plasticity of these sites in the central nervous system (CNS). 5. Specific ANF binding sites are also found in various other CNS-associated tissues such as the eye, pituitary gland, and adrenal medulla. In these tissues ANF appears to act as a modulator of fluid production and hormone release. 6. Thus, ANF-like peptides and ANF receptor sites are present in brain and various peripheral tissues, demonstrating the existence of a family of brain/heart peptides.

Immunocytochemical localization, binding, and effects of atrial natriuretic peptide in rat adipocytes

The metabolic effects of atrial natriuretic peptide (ANP) have not been widely investigated. Since adipocyte cells represent a model system extensively used to examine the metabolic actions of many peptide hormones, we sought to establish whether ANP could bind to adipocyte membranes, alter cyclic nucleotide metabolism, and affect spontaneous or hormone-stimulated lipolysis. Using in vitro autoradiographic techniques, radiolabelled ANP was found to bind specifically to mammary gland fat cells. Additionally, endogenous ANP-like immunoreactivity could be localized in the plasma membrane compartment and cytoplasmic matrix of fat cells, but not in fat vacuoles. [125I]ANP bound to single high affinity sites (Kd = 0.72 nM) in fat cell membranes. The binding was rapid (equilibrium within 1 min at 25 degrees C) and specific. The atrial peptide was capable of stimulating a time- and concentration-dependent increase in cGMP accumulation in isolated adipocytes, but had no effect on spontaneous or stimulated [-)-isoproterenol, ACTH, forskolin) cAMP formation. ANP did not alter the increase in glycerol production stimulated by l-epinephrine in isolated fat cells. While i.v. infusion of ANP stimulated a marked increase in circulating levels of cGMP, the atrial peptide did not alter plasma triglyceride levels. These data demonstrate the presence of specific ANP binding sites on adipocyte membranes and internalization of ANP-associated immunoreactivity. These receptors are biochemically functional given the ability of ANP to augment cGMP formation. The peptide, however, does not exert an action on adipocyte lipolysis. Adipocytes, therefore, represent an ANP target tissue in which the physiological action of the peptide is yet to be defined.

Indirect relationship between vasopressin-induced secretion of ACTH and cyclic nucleotides in cultured anterior pituitary cells

The present study examines whether a correlation exists between cyclic nucleotides and the mechanism of action of arginine vasopressin (AVP) on adrenocorticotropin hormone (ACTH) secretion from pituitary corticotrophs. Incubation of cultured anterior pituitary cells with 3-isobutyl-1-methylxanthine (IBMX) or Rolipram elevated the basal intracellular content of both adenosine 3':5'-cyclic monophosphate (cAMP) and guanosine 3':5'-cyclic monophosphate (cGMP) or cAMP alone, respectively. Both IBMX and Rolipram enhanced the AVP-stimulated secretion of ACTH in cultured anterior pituitary cells, but not in AtT-20 corticotrophs which lack functional AVP receptors. Rolipram was less potent than IBMX in this regard, which suggests a possible involvement of cGMP. In contrast, both drugs showed similar potency to stimulate CRF-induced ACTH secretion. Incubation of pituitary cells with atrial natriuretic factor elevated tissue cGMP levels and increased the ACTH response to AVP. The results of this study show that, although AVP fails to directly affect the levels of cAMP and cGMP in anterior pituitary cells, the stimulatory effect of AVP on ACTH secretion was modulated by the cellular cAMP content.

Atrial natriuretic peptide receptors are present in brown adipose tissue

Binding activities and metabolic effects of atrial natriuretic peptide (ANP) were examined in rat brown fat. ANP binding sites were identified in thin sections of brown adipose. Binding of [125I]-ANP to brown fat membranes was specific, saturable and time-dependent. A single class of high affinity sites (Kd, 1.7 nM; Bmax, 226 fmol/mg protein) was present. ANP increased cGMP synthesis in isolated cells (EC50, 2.5 x 10(-9) M), but did not alter epinephrine-stimulated glycerol production. The results demonstrate the presence of specific ANP receptors in brown fat, activation of which increases cGMP formation; however, since neither ANP nor cGMP affect lipolysis, the biological importance of these receptors remains unknown.

Binding and effect of atrial natriuretic factor on cyclic GMP formation and alpha-MSH secretion of intermediate pituitary cells

The present study shows for the first time that in proopiomelanocortin cells of the rat intermediate pituitary gland ANF binds to two receptor forms, with apparent molecular weights of 150K and 70K. Scatchard plots revealed specific and high affinity non-interacting sites, with a KD value of about 3 nM and a density of 7,000 sites/cell. The presence of these binding sites was further confirmed by autoradiographic studies. Activation of these receptors led to an increase in cellular content of cGMP, with half-maximal effect being elicited with about 5 nM ANF, while cAMP formation was unaltered. Alpha-MSH secretion of intermediate pituitary cells was unaffected by ANF, whether the cells were incubated in the absence or presence of corticotropin-releasing factor or bromocryptine. These data thus indicate the presence of multiple ANF receptor sites in the intermediate pituitary which are coupled to cell production of cGMP, but independent of alpha-MSH secretion.