Atrial natriuretic peptide: a magic bullet for cancer therapy targeting Wnt signaling and cellular pH regulators

Abstract

Atrial natriuretic peptide (ANP) is a cardiac hormone playing a crucial role in cardiovascular homeostasis mainly through blood volume and pressure regulation. In the last years, the new property ascribed to ANP of inhibiting tumor growth both in vitro and in vivo has made this peptide an attractive candidate for anticancer therapy. The molecular mechanism underlying the anti-proliferative effect of ANP has been mainly related to its interaction with the specific receptors NPRs, through which this natriuretic hormone inhibits some metabolic targets critical for cancer development, including the Ras-MEK1⁄2-ERK1⁄2 kinase cascade, functioning as a multikinase inhibitor. In this review we summarize the current knowledge on this topic, focusing on our recent data demonstrating that the antitumor activity of this natriuretic hormone is also mediated by a concomitant effect on the Wnt/β-catenin pathway and on the pH regulation ability of cancer cells, through a Frizzled-related mechanism. This peculiarity of simultaneously targeting two processes crucial for neoplastic transformation and solid tumor survival reinforces the utility of ANP for the development of both preventive and therapeutic strategies.

Fig. (1)

Processing of pro-ANP (A), Corin structure and proANP/Corin interaction (B, C). A. ANP is originally synthesized as a 151-amino acids prepropeptide; after removal of signal peptide (Sp) by signal peptidase in the endoplasmic reticulum, pro-ANP is store in atrial cardiomyocyte granules. On stimulation, the pro-hormone is proteolytically cleaved at the cell membrane by the convertase Corin, that gives rise to the mature ANP (COOH-terminal) and an NH₂-terminal propeptide from which the other three natriuretic hormones LANP, VSDL and KP derive. [Adapted from 12] B. The membrane-associated protease Corin has a transmembrane domain (TM), two Frizzled-like cysteine-rich domains (Fz1 and Fz2), eight LDL receptor repeats (LDLR), a scavenger receptor domain (SR) and a protease catalytic domain with the three amino acids His (H), Asp (D), and Ser (S) in the active site. C. As proposed by Knappe et al. [13], upon release from the dense granules of cardiomyocytes, pro-ANP (solid black line) binds to Fz1 and LDLR repeats 1-4 and is cleaved by the catalytic domain, releasing the mature and biologically active ANP [Adapted from 13].

Fig. (2)

Schematic representation of the Wnt/β-catenin signaling activation (top) and β-catenin and E-cadherin distribution in normal and transformed cells (bottom). In normal epithelial cells such as colonocytes, Wnt pathway is not activated and β-catenin (β-cat) and E-cadherin (E-cad) are mainly located at cell-cell junction; activation of Wnt signaling in tumor cells, and specifically in colon carcinoma cells, leads to β-catenin translocation into the nucleus and loss of E-cadherin at the cell contacts. Bottom panels: confocal microscopy images from our laboratory showing the different intracellular distribution of β-catenin and E-cadherin (green hue) in colonocytes and in colon adenocarcinoma cells (DHD/K12/Trb cell line). Red hue: nuclei were stained with propidium iodide. APC, adenomatous polyposis coli; AKT, Serine/Threonine Protein Kinase; β-cat, β-catenin; CBP, CREB-binding protein; CK, casein kinase; DKK, Dickkopf; DSH, Dishevelled; GBP, GSK3-binding protein; GSK, glycogen synthase kinase; LRP, LDL receptor-related protein; P, phosphorylation; sFRP, secreted Frizzled-related protein; TCF, T-cell factor.

Fig. (3)

A. Generation of extracellular acidosis during tumor growth and strategy adopted by cancer cells to survive to extracellular acidic stress. Tumor cells are characterized by an alkaline cytosolic pH and an acidic extracellular pH: this pH gradient, whose preservation is crucial for tumor cell survival, is maintained by cancer cells through various pH regulators over-expressed on tumor cell surface, including Proton Pumps (PP) and Sodium/Proton Exchangers (NHE). B. Schematic representation of the effect of PP and NHE inhibitors on tumor microenvironment and cancer cell proliferation and survival; PP or NHE inhibitors, by decreasing the activity of the specific pH regulators, induce intracellular acidification that ultimately results in inhibition of tumor growth and metastasis, chemosensitivity and apoptosis.

Fig. (4)

Proposed model for the mechanism of ANP action involving a cross-talk between the NHE-1-regulated intracellular pH and the Wnt signaling. APC, adenomatous polyposis coli; AKT, Serine/Threonine Protein Kinase; β-cat, β-catenin; CBP, CREB-binding protein; CK, casein kinase; DKK, Dickkopf; DSH, Dishevelled; GBP, GSK3-binding protein; GSK, glycogen synthase kinase; LRP, LDL receptor-related protein; NHE, Sodium/Proton Exchanger; P, phosphorylation; TCF, T-cell factor. [Adapted from 30].

Fig. (5)

Receptors and cellular signaling pathways implicated in the antitumor effect of ANP. NHE-1, Na⁺/H⁺ exchanger isoform 1; NPRs, Natriuretic peptide receptors; TKR, Tyrosine kinase receptor; VEGF, vascular endothelial growth factor; VEGFR, VEGF receptor. Details and main function/s of the different molecular components, and references supporting the role of each molecular component and pathway in ANP antitumor action are reported in (Table 1).