Atrial natriuretic peptide (ANP) preconditioning: what is the relevance of the antiapoptotic effects?

Natriuretic peptide receptor A inhibition suppresses gastric cancer development through reactive oxygen species-mediated G2/M cell cycle arrest and cell death

Natriuretic peptide receptor A (NPRA), the major receptor for atrial natriuretic peptide (ANP), has been implicated in tumorigenesis; however, the role of ANP-NPRA signaling in the development of gastric cancer remains unclear. Immunohistochemical analyses indicated that NPRA expression was positively associated with gastric tumor size and cancer stage. NPRA inhibition by shRNA induced G2/M cell cycle arrest, cell death, and autophagy in gastric cancer cells, due to accumulation of reactive oxygen species (ROS). Either genetic or pharmacologic inhibition of autophagy led to caspase-dependent cell death. Therefore, autophagy induced by NPRA silencing may represent a cytoprotective mechanism. ROS accumulation activated c-Jun N-terminal kinase (JNK) and AMP-activated protein kinase (AMPK). ROS-mediated activation of JNK inhibited cell proliferation by disturbing cell cycle and decreased cell viability. In addition, AMPK activation promoted autophagy in NPRA-downregulated cancer cells. Overall, our results indicate that the inhibition of NPRA suppresses gastric cancer development and targeting NPRA may represent a promising strategy for the treatment of gastric cancer.

Atrial natriuretic peptide: an old hormone or a new cytokine?

Atrial natriuretic peptide (ANP) a cardiovascular hormone mainly secreted by heart atria in response to stretching forces induces potent diuretic, natriuretic and vasorelaxant effects and plays a major role in the homeostasis of blood pressure as well as of water and salt balance. The hormone can also act as autocrine/paracrine factor and modulate several immune functions as well as cytoprotective effects. ANP contributes to innate immunity being able to: (i) stimulate the host defense against extracellular microbes by phagocytosis and Reactive Oxygen Species (ROS) release; (ii) inhibit the synthesis and release of proinflammatory markers such as TNF-α, IL-1, MCP-1, nitric oxide (NO), cyclooxygenase-2 (COX-2); (iii) inhibit the expression of adhesion molecules such as ICAM-1 and E-selectin. ANP can also affect the adaptive immunity being able to: (i) reduce the number of CD4(+) CD8(+) lymphocytes as well as to increase the CD4(-) CD8(-) cells; (ii) stimulate the differentiation of naïve CD4(+) cells toward the Th2 and/or Th17 phenotype. The hormone shows protective effects during: (i) ventricular hypertrophy and myocardial injury; (ii) atherosclerosis and hypertension by the induction of antiproliferative effects; (iii) oxidative stress counteracting the dangerous effects of ROS; (iv) growth of tumors cells by the induction of apoptosis or necrosis. Since not much is known about of the role of ANP locally produced and released by non-cardiac cells, this review outlines the contribution of ANP in different aspect of innate as well as adaptive immunity also with respect to the excessive cell growth in physiological and/or pathological conditions.

Post-ischemic infusion of atrial natriuretic peptide attenuates warm ischemia-reperfusion injury in rat lung

BACKGROUND

The serious shortage of organs for transplantation, especially lungs, has drawn increasing attention to donation after cardiac death and protection of organs against warm ischemic injury. Atrial natriuretic peptide (ANP) activates guanylate cyclase receptors and increases cyclic guanosine monophosphate (cGMP) levels, which decrease in the lung during ischemia. In this study we investigated the effect on lung ischemia-reperfusion injury of administering synthetic ANP (carperitide) at the onset of reperfusion after warm ischemia.

METHODS

An isolated rat lung perfusion model was used. The rats were allocated into three groups: the control group; the ANP group; and the sham group. In the control and ANP groups, the heart-lung block was exposed to 60 minutes of ischemia at 37 degrees C, and subsequently reperfused for 60 minutes. At the onset of reperfusion, either saline or ANP was added to the perfusate. In the sham group, lungs were continuously perfused without ischemia and only saline was added to the perfusate.

RESULTS

ANP significantly reduced pulmonary vascular resistance and pulmonary edema, and improved oxygenation. It also significantly increased cGMP levels in reperfused lungs. Histologically, lungs in the ANP group showed significantly fewer signs of injury and fewer cells demonstrated apoptotic changes or single-stranded DNA than lungs in the control group.

CONCLUSIONS

Our results indicate that ANP administered at the onset of reperfusion increases cGMP in lung tissue and attenuates warm ischemia-reperfusion injury in isolated perfused rat lung.

Protein kinase A dependent signalling mediates anti-apoptotic effects of the atrial natriuretic peptide in ischemic livers

BACKGROUND/AIMS

Preconditioning of livers with atrial natriuretic peptide (ANP) attenuates ischemia-reperfusion injury (IRI) via the particulate guanylate cyclase. Recently, we have shown that ANP affects the p38 MAPK signalling cascade in the liver. Thus, aim of the present study was to elucidate the role of cGMP- and p38 MAPK-dependent signalling pathways in ANP-mediated anti-apoptotic effects.

METHODS

Rat livers were perfused with KH-buffer with or without ANP, 8-Br-cGMP (+/-kinase inhibitors) and kept in UW solution (4 degrees C, 24h). Caspase-3-like activity was measured by a fluorometric assay. Expression of cGMP-dependent protein kinases (PKG) in liver tissue was determined by RT-PCR, BAD phosphorylation by Western blot, and cAMP-dependent protein kinase (protein kinase A, PKA) activity by in vitro phosphorylation.

RESULTS

Compared to control organs, ANP-preconditioning reduced post-ischemic caspase-3-like activity. Neither perfusion with a p38 MAPK inhibitor nor with a PKG inhibitor abolished the ANP-mediated anti-apoptotic action. The two PKG isoforms were demonstrated not to be expressed in the liver. In contrast, liver perfusion with a selective PKA inhibitor abrogated the anti-apoptotic effect of ANP. Phosphorylation of pro-apoptotic BAD by ANP-activated PKA might inhibit liver cell apoptosis.

CONCLUSIONS

ANP mediates its anti-apoptotic action during ischemic injury via a crosstalk with the PKA pathway.

ANP-induced decrease of iron regulatory protein activity is independent of HO-1 induction

Atrial natriuretic peptide (ANP)-preconditioned livers are protected from ischemia-reperfusion injury. ANP-treated organs show increased expression of heme oxygenase (HO)-1. Because HO-1 liberates bound iron, the aim of our study was to determine whether ANP affects iron regulatory protein (IRP) activity and, thus, the levels of ferritin. Rat livers were perfused with Krebs-Henseleit buffer [+/-ANP, 8-bromo-cGMP (8-Br-cGMP), and tin protoporphyrin, 20 min], stored in University of Wisconsin solution (4 degrees C, 24 h), and reperfused (120 min). IRP activity was assessed by gel-shift assays, and ferritin, IRP phosphorylation, and PKC localization were assessed by Western blot. Control livers displayed decreased IRP activity at the end of ischemia but no change in ferritin content during ischemia and reperfusion. ANP-pretreated livers showed reduced IRP activity, an effect mimicked by 8-Br-cGMP. Ferritin levels were increased in ANP-pretreated organs. Simultaneous perfusion of livers with ANP and tin protoporphyrin did not reduce ANP-induced action, arguing against a role for HO-1 in changes in IRP activity. ANP and 8-Br-cGMP decreased membrane localization of PKC-alpha and PKC-epsilon, but this modulation of PKC seems unrelated to inhibition of IRP binding. This work shows the cGMP-mediated attenuation of IRP binding activity by ANP, which results in increased hepatic ferritin levels. This change in IRPs is independent of ANP-induced HO-1 and reduced PKC activation.