New aspects of the interactions between the cardiovascular nitric oxide system and natriuretic peptides

Arterial blood pressure is regulated by a variety of endocrine, autocrine and neuronal systems. Natriuretic peptides and nitric oxide are important factors that exert synergistic vascular and cardiac actions and their activities are closely linked. The existence of a novel signal transduction mechanism involved in activation of nitric oxide synthase via natriuretic peptides is currently being explored. Since several cardiovascular disorders are associated with dysfunction of natriuretic peptides activity, selective modulation of the natriuretic peptides pathway represents an important therapeutic target. This review article highlights the current findings on cross-talk between natriuretic peptides and the nitric oxide system.     

Glycosylation and processing of pro-B-type natriuretic peptide in cardiomyocytes

B-type natriuretic peptide (BNP) and its related peptides are biomarkers for the diagnosis of heart failure. Recent studies identified several O-glycosylation sites, including Thr-71, on human pro-BNP but the functional significance was unclear. In this study, we analyzed glycosylation and proteolytic processing of pro-BNP in cardiomyocytes. Human pro-BNP wild-type (WT) and mutants were expressed in HEK 293 cells and murine HL-1 cardiomyocytes. Pro-BNP and BNP were analyzed by immunoprecipitation and Western blotting. Glycosidases and glycosylation inhibitors were used to examine carbohydrates on pro-BNP. The effects of furin and corin expression on pro-BNP processing in cells also were examined. We found that in HEK 293 cells, recombinant pro-BNP contained significant amounts of O-glycans with terminal oligosialic acids. Mutation at Thr-71 reduced O-glycans on pro-BNP and increased pro-BNP processing. In HL-1 cardiomyocytes, residue Thr-71 contained little O-glycans, and pro-BNP WT and T71A mutant were processed similarly. In HEK 293 cells, pro-BNP was processed by furin. Mutations at Arg-73 and Arg-76, but not Lys-79, prevented pro-BNP processing. In HL-1 cardiomyocytes, which express furin and corin, single or double mutations at Arg-73, Arg-76 and Lys-79 did not prevent pro-BNP processing. Only when all these three residues were mutated, was pro-BNP processing completely blocked. Our data indicate that pro-BNP glycosylation in cardiomyocytes differed significantly from that in HEK 293 cells. In HEK 293 cells, furin cleaved pro-BNP at Arg-76 whereas in cardiomyocytes corin cleaved pro-BNP at multiple residues including Arg-73, Arg-76 and Lys-79.     

Inhibition of hepatic damage and liver fibrosis by brain natriuretic peptide

Anti-fibrotic and organ protective effects of brain natriuretic peptide (BNP) have been reported. In this study, effects of BNP on liver fibrosis were examined in the carbon tetrachloride (CCl₄)-induced liver fibrosis model using BNP-transgenic (Tg) and wild-type (WT) mice. Twice-a-week intraperitoneal injections of CCl₄ for 8 weeks resulted in massive liver fibrosis, augmented transforming growth factor (TGF)-β₁ and type I procollagen α₁ chain (Col1a1) mRNA expression, and the hepatic stellate cell (HSC) activation in WT mice, all of which were significantly suppressed in Tg mice. These observations indicate that BNP inhibits liver fibrosis by attenuating the activation of HSCs.     

Glycogen synthase kinase 3beta together with 14-3-3 protein regulates diabetic cardiomyopathy: effect of losartan and tempol

Glycogen synthase kinase (GSK) 3beta is a multifunctional protein that positively regulates myocardial apoptosis and negatively regulates hypertrophy. However, the role of GSK3beta in the diabetic myocardium is largely unknown. We found that GSK3beta became more active (less phosphorylated at serine 9) via decreased Akt phosphorylation, in parallel to c-Jun NH2 terminal kinase activation, which correlated with increased activated caspase 3 and myocardial apoptosis 3 days after streptozotocin (STZ) injection in mice. However, 28 days after STZ injection, GSK3beta became inactive, which correlated with the enhanced protein kinase C beta2 and p38 mitogen activated protein kinase expression, nuclear translocation of nuclear factor of activated T cells c3, cardiac hypertrophy and fibrosis. All of the above parameters were exacerbated in dominant-negative 14-3-3 transgenic mice. Our results suggest that GSK3beta together with 14-3-3 protein plays essential roles in the signaling of diabetic cardiomyopathy, and treatment with either losartan or tempol prevents these changes.     

Activation of catalase by apelin prevents oxidative stress-linked cardiac hypertrophy

Adipose tissue secretes a variety of bioactive factors, which can regulate cardiomyocyte hypertrophy via reactive oxygen species (ROS). In the present study we investigated whether apelin affects ROS-dependent cardiac hypertrophy. In cardiomyocytes apelin inhibited the hypertrophic response to 5-HT and oxidative stress induced by 5-HT- or H₂O₂ in a dose-dependent manner. These effects were concomitant to the increase in mRNA expression and activity of catalase. Chronic treatment of mice with apelin attenuated pressure-overload-induced left ventricular hypertrophy. The prevention of hypertrophy by apelin was associated with increased myocardial catalase activity and decreased plasma lipid hydroperoxide, as an index of oxidative stress. These results show that apelin behaves as a catalase activator and prevents cardiac ROS-dependent hypertrophy.     

Differential ubiquitination of Smad1 mediated by CHIP: implications in the regulation of the bone morphogenetic protein signaling pathway

Smad1, a downstream regulator of the bone morphogenetic protein (BMP) receptors, is tightly regulated by the ubiquitin-proteasomal degradation system. To dissect the mechanisms that underlie the regulation of Smad1, it is important to investigate the specific ubiquitination site(s) in Smad1. Here we report that the α-NH₂ group of the N terminus and the ε-NH₂ groups of internal lysine residues 116, 118 and 269 (K116, K118 and K269) of Smad1 are ubiquitin acceptor sites mediated by the carboxyl terminus of Hsc70-interacting protein (CHIP). The in vitro degradation assay indicates that ubiquitination at the N terminus partially contributes to the degradation of Smad1. Furthermore, we demonstrate that the ubiquitination level of pseudo-phosphorylated Smad1 by CHIP is stronger than that of wild-type Smad1 and can be strongly inhibited by a phosphorylated tail of Smad1, PIS(pS)V(pS). Third, our results indicate that Hsp70 facilitates CHIP-mediated poly-ubiquitination of Smad1 whereas it attenuates CHIP-meditated mono-ubiquitination of Smad1. Finally, consistent with the in vitro observation, we show that CHIP preferentially mediates the degradation of phospho-Smad1/5 in vivo. Taken together, these results provide us a hint that CHIP might preferentially regulate phosphorylated Smad1 and thus the BMP signaling.     

Mutations in the hydrophobic core of ubiquitin differentially affect its recognition by receptor proteins

Ubiquitin (Ub) is one of the most highly conserved signaling proteins in eukaryotes. In carrying out its myriad functions, Ub conjugated to substrate proteins interacts with dozens of receptor proteins that link the Ub signal to various biological outcomes. Here we report mutations in conserved residues of Ub's hydrophobic core that have surprisingly potent and specific effects on molecular recognition. Mutant Ubs bind tightly to the Ub-associated domain of the receptor proteins Rad23 and hHR23A but fail to bind the Ub-interacting motif present in the receptors Rpn10 and S5a. Moreover, chains assembled on target substrates with mutant Ubs are unable to support substrate degradation by the proteasome in vitro or sustain viability of yeast cells. The mutations have relatively little effect on Ub's overall structure but reduce its rigidity and cause a slight displacement of the C-terminal β-sheet, thereby compromising association with Ub-interacting motif but not with Ub-associated domains. These studies emphasize an unexpected role for Ub's core in molecular recognition and suggest that the diversity of protein-protein interactions in which Ub engages placed enormous constraints on its evolvability.