Internalization of endogenous and exogenous atrial natriuretic peptide by target tissues

Serum atrial natriuretic peptide: a suspected biomarker of breast cancer

Aim of the study

To assess serum levels of ANP in breast cancer female patients and its relationship to metastasis and some clinical parameters among those patients.

Material and methods

One hundred breast cancer patients with and without metastasis along with 20 healthy closely matched controls, were enrolled in the present cross sectional study. Background: To assess the serum levels of atrial natriuretic peptide in breast cancer Serum levels of ANP were assessed using ELISA.

Results

Mean serum levels of ANP breast cancer patients (13.9 ±10.1 ng/ml) were significantly elevated compared to healthy control group (2.2 ±1.3 ng/ml) (p < 0.001). The metastatic breast cancer patients showed significant elevated ANP levels (17.1 ±8.9 ng/ml) compared to non-metastatic group (6.4 ±8.8 ng/ml) p < 0.001. Within the metastatic group significant difference was detected between de novo metastatic, under follow-up, under hormonal control and locally advanced group (p = 0.007).

Conclusions

This study showed significant elevated levels of ANP in the serum of metastatic breast cancer patients compared to non-metastatic patients. Within the metastatic group the lowest levels were detected in metastatic breast Cancer under hormonal treatment either tamoxifen or aromatase inhibitor.

Immunocytochemical Localization of Atrial Natriuretic Peptide, Vessel Dilator, Long-acting Natriuretic Peptide, and Kaliuretic Peptide in Human Pancreatic Adenocarcinomas

We recently found that four peptide hormones synthesized by the same gene completely inhibit the growth of human pancreatic adenocarcinomas in athymic mice. The present immunocytochemical investigation was designed to determine where in the adenocarcinomas these peptide hormones localize. Atrial natriuretic peptide, vessel dilator, long-acting natriuretic peptide, and kaliuretic peptide localized to the cytoplasm and nucleus of the human pancreatic adenocarcinomas, which is consistent with their ability to decrease DNA synthesis in the nucleus of this cancer. In this first investigation of where these peptide hormones with anticancer effects localize in any cancer, these peptide hormones also localized to the endothelium of capillaries and fibroblasts within these cancers. This is the first demonstration of growth-inhibiting peptide hormones localizing to the nucleus, where they inhibit DNA synthesis and may interact with growth-promoting hormones that localize there as the etiology of their ability to inhibit the growth of adenocarcinomas both in vitro and in vivo.

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.

Lysosomal delivery of ANP receptors following internalization in PC12 cell

Internalization and intracellular processing of ANP-B and C receptors play an important role in regulating cell responsiveness to atrial natriuretic peptides (ANP). Receptor internalization was indirectly monitored with 125I labelled ligand. When 125I-ANP(99-126) was internalized by the cells at 37 degrees C, 55% of the internalized radioactivity was localized in the lysosomal fraction. When receptors were affinity-labelled with 125I-ANP(99-126) and allowed to internalize for varying time periods, two radiolabelled proteins in the m.wt range of 56 and 52 KDa were detected in the cytosolic extract. These proteins appear to be the hydrolytic products of the ANP-C receptor expressed on the plasma membrane. In addition to lysosomal delivery, shedding of the ANP-C receptor from the cell surface was detected following incubation of cells with 125I-ANP(99-126). The dual processes may function to clear exogenous ANP from the extracellular compartments.

Immunocytological localization of endogenous calcitonin-like molecules in the crustacean Orchestia

Immunocytological mapping of calcitonin-like molecules (human form) performed in the terrestrial crustacean Orchestia, using PAP procedure on paraffin sections and immunogold method on ultrathin cryosections, reveals two reactive organs: central nervous system and posterior caeca of the midgut. Immunoreactivity within the nervous system is mainly located throughout perikarya and nerve fibers from both dueto- and tritocerebron and ventral nervous chain. Immunolabeling in the posterior caeca is detected on both cell components of the epithelium, with significant quantitative differences between molt and intermolt periods. The role of calcitonin-like substances in these organs is then discussed: at the nervous system level, a neuro transmitter function is suggested; the direct participation of these peptides in the regulation of calcium shifts through the caecal epithelium is hypothesized.

Regulatory peptides produced in the anterior pituitary

Several peptide families have been detected in the anterior pituitary of several species, including man, and for many of them evidence for local synthesis has been found. Although a paracrine action seems evident for a few, the precise function of most of these peptides remains unknown.

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.

Immunocytochemical localization of ACTH-like immunoreactivity in rat adrenal glomerulosa and fasciculata cells

The role of ACTH in the synthesis of the adrenocortical hormones has been largely described. In order to investigate the localization of this peptide at the subcellular level of the adrenal glomerulosa and fasciculata cells, an immunocytological method was used. Rat adrenals were fixed and frozen. Ultrathin sections obtained by cryoultramicrotomy, were incubated with anti-beta (1-24) ACTH or anti-alpha (17-39) ACTH sera. The antigen-antibody reaction was detected by PAP complexes (revealed by 4-chloro-1-naphthol) or with protein A-colloidal gold or IgG-colloidal gold. The results obtained were the same whatever the antisera of the technique employed. All the cells of the adrenal zona glomerulosa and zona fasciculata were labelled. ACTH-like immunoreactivity in zona glomerulosa and zona fasciculata cells was observed at the plasma membrane level, in cytoplasmic matrix, mitochondria and nucleus (in the euchromatin close to the heterochromatin aggregations and, occasionally, associated with the nucleolus). No immunoreactivity was observed when non-immune serum or anti-ACTH serum preincubated with ACTH were used, nor there was any modification of the immunocytochemical reaction when anti-ACTH serum incubated with heterologous antigens was employed. These data, demonstrate the presence of endogenous ACTH in both adrenal glomerulosa and fasciculata cells, and suggest that the peptide is internalized after binding to the plasma membrane.