Role for BLT1 in regulating inflammation within adipose tissue immune cells of aged mice.

BACKGROUND: Aging is a complex biological process characterized by obesity and immunosenescence throughout the organism. Immunosenescence involves a decline in immune function and the increase in chronic-low grade inflammation, called inflammaging. Adipose tissue expansion, particularly that of visceral adipose tissue (VAT), is associated with an increase in pro-inflammatory macrophages that play an important role in modulating immune responses and producing inflammatory cytokines. The leukotriene B4 receptor 1 (BLT1) is a regulator of obesity-induced inflammation. Its ligand, LTB4, acts as a chemoattractant for immune cells and induces inflammation. Studies have shown that BLT1 is crucial for cytokine production during lipopolysaccharide (LPS) endotoxemia challenge in younger organisms. However, the expression patterns and function of BLT1 in older organisms remains unknown. RESULTS: In this study, we investigated BLT1 expression in immune cell subsets within the VAT of aged male and female mice. Moreover, we examined how antagonizing BLT1 signaling could alter the inflammatory response to LPS in aged mice. Our results demonstrate that aged mice exhibit increased adiposity and inflammation, characterized by elevated frequencies of B and T cells, along with pro-inflammatory macrophages in VAT. BLT1 expression is the highest in VAT macrophages. LPS and LTB4 treatment result in increased BLT1 in young and aged bone marrow-derived macrophages (BMDMs). However, LTB4 treatment resulted in amplified Il6 from aged, but not young BMDMs. Treatment of aged mice with the BLT1 antagonist, U75302, followed by LPS-induced endotoxemia resulted in an increase in anti-inflammatory macrophages, reduced phosphorylated NFκB and reduced Il6. CONCLUSIONS: This study provides valuable insights into the age- and sex- specific changes in BLT1 expression on immune cell subsets within VAT. This study offers support for the potential of BLT1 in modulating inflammation in aging.

Guanylyl cyclase-A phosphorylation decreases cardiac hypertrophy and improves systolic function in male, but not female, mice.

Atrial natriuretic peptide (NP) and BNP increase cGMP, which reduces blood pressure and cardiac hypertrophy by activating guanylyl cyclase (GC)-A, also known as NPR-A or Npr1. Although GC-A is highly phosphorylated, and dephosphorylation inactivates the enzyme, the significance of GC-A phosphorylation to heart structure and function remains unknown. To identify in vivo processes that are regulated by GC-A phosphorylation, we substituted glutamates for known phosphorylation sites to make GC-A8E/8E mice that express an enzyme that cannot be inactivated by dephosphorylation. GC-A activity, but not protein, was increased in heart and kidney membranes from GC-A8E/8E mice. Activities were threefold higher in female compared to male cardiac ventricles. Plasma cGMP and testosterone were elevated in male and female GC-A8E/8E mice, but aldosterone was only increased in mutant male mice. Plasma and urinary creatinine concentrations were decreased and increased, respectively, but blood pressure and heart rate were unchanged in male GC-A8E/8E mice. Heart weight to body weight ratios for GC-A8E/8E male, but not female, mice were 12% lower with a 14% reduction in cardiomyocyte cross-sectional area. Subcutaneous injection of fsANP, a long-lived ANP analog, increased plasma cGMP and decreased aldosterone in male GC-AWT/WT and GC-A8E/8E mice at 15 min, but only GC-A8E/8E mice had elevated levels of plasma cGMP and aldosterone at 60 min. fsANP reduced ventricular ERK1/2 phosphorylation to a greater extent and for a longer time in the male mutant compared to WT mice. Finally, ejection fractions were increased in male but not female hearts from GC-A8E/8E mice. We conclude that increased phosphorylation-dependent GC-A activity decreases cardiac ERK activity, which results in smaller male hearts with improved systolic function.

Regulation of the Natriuretic Peptide Receptor 2 (Npr2) by Phosphorylation of Juxtamembrane Serine and Threonine Residues Is Essential for Bifurcation of Sensory Axons.

cGMP signaling elicited by activation of the transmembrane receptor guanylyl cyclase Npr2 (also known as guanylyl cyclase B) by the ligand CNP controls sensory axon bifurcation of DRG and cranial sensory ganglion (CSG) neurons entering the spinal cord or hindbrain, respectively. Previous studies have shown that Npr2 is phosphorylated on serine and threonine residues in its kinase homology domain (KHD). However, it is unknown whether phosphorylation of Npr2 is essential for axon bifurcation. Here, we generated a knock-in mouse line in which the seven regulatory serine and threonine residues in the KHD of Npr2 were substituted by alanine (Npr2-7A), resulting in a nonphosphorylatable enzyme. Real-time imaging of cGMP in DRG neurons with a genetically encoded fluorescent cGMP sensor or biochemical analysis of guanylyl cyclase activity in brain or lung tissue revealed the absence of CNP-induced cGMP generation in the Npr27A/7A mutant. Consequently, bifurcation of axons, but not collateral formation, from DRG or CSG in this mouse mutant was perturbed at embryonic and mature stages. In contrast, axon branching was normal in a mouse mutant in which constitutive phosphorylation of Npr2 is mimicked by a replacement of all of the seven serine and threonine sites by glutamic acid (Npr2-7E). Furthermore, we demonstrate that the Npr27A/7A mutation causes dwarfism as described for global Npr2 mutants. In conclusion, our in vivo studies provide strong evidence that phosphorylation of the seven serine and threonine residues in the KHD of Npr2 is an important regulatory element of Npr2-mediated cGMP signaling which affects physiological processes, such as axon bifurcation and bone growth.SIGNIFICANCE STATEMENT The branching of axons is a morphological hallmark of virtually all neurons. It allows an individual neuron to innervate different targets and to communicate with neurons located in different regions of the nervous system. The natriuretic peptide receptor 2 (Npr2), a transmembrane guanylyl cyclase, is essential for the initiation of bifurcation of sensory axons when entering the spinal cord or the hindbrain. By using two genetically engineered mouse lines, we show that phosphorylation of specific serine and threonine residues in juxtamembrane regions of Npr2 are required for its enzymatic activity and for axon bifurcation. These investigations might help to understand the regulation of Npr2 and its integration in intracellular signaling systems.

Multiomic Profiling of Tyrosine Kinase Inhibitor-Resistant K562 Cells Suggests Metabolic Reprogramming To Promote Cell Survival.

Resistance to chemotherapy can occur through a wide variety of mechanisms. Resistance to tyrosine kinase inhibitors (TKIs) often arises from kinase mutations-however, "off-target" resistance occurs but is poorly understood. Previously, we established cell line resistance models for three TKIs used in chronic myeloid leukemia treatment, and found that resistance was not attributed entirely to failure of kinase inhibition. Here, we performed global, integrated proteomic and transcriptomic profiling of these cell lines to describe mechanisms of resistance at the protein and gene expression level. We used whole transcriptome sequencing and SWATH-based data-independent acquisition mass spectrometry (DIA-MS), which does not require isotopic labels and provides quantitative measurements of proteins in a comprehensive, unbiased fashion. The proteomic and transcriptional data were correlated to generate an integrated understanding of the gene expression and protein alterations associated with TKI resistance. We defined mechanisms of resistance and two novel markers, CA1 and alpha-synuclein, that were common to all TKIs tested. Resistance to all of the TKIs was associated with oxidative stress responses, hypoxia signatures, and apparent metabolic reprogramming of the cells. Metabolite profiling and glucose-dependence experiments showed that resistant cells had routed their metabolism through glycolysis (particularly through the pentose phosphate pathway) and exhibited disruptions in mitochondrial metabolism. These experiments are the first to report a global, integrated proteomic, transcriptomic, and metabolic analysis of TKI resistance. These data suggest that although the mechanisms are complex, targeting metabolic pathways along with TKI treatment may overcome pan-TKI resistance.

Skeletal overgrowth-causing mutations mimic an allosterically activated conformation of guanylyl cyclase-B that is inhibited by 2,4,6,-trinitrophenyl ATP.

Activating mutations in the receptor for C-type natriuretic peptide (CNP), guanylyl cyclase B (GC-B, also known as Npr2 or NPR-B), increase cellular cGMP and cause skeletal overgrowth, but how these mutations affect GTP catalysis is poorly understood. The A488P and R655C mutations were compared with the known mutation V883M. Neither mutation affected GC-B concentrations. The A488P mutation decreased the EC50 5-fold, increased Vmax 2.6-fold, and decreased the Km 13-fold, whereas the R655C mutation decreased the EC50 5-fold, increased the Vmax 2.1-fold, and decreased the Km 4.7-fold. Neither mutation affected maximum activity at saturating CNP concentrations. Activation by R655C did not require disulfide bond formation. Surprisingly, the A488P mutant only activated the receptor when it was phosphorylated. In contrast, the R655C mutation converted GC-B-7A from CNP-unresponsive to CNP-responsive. Interestingly, neither mutant was activated by ATP, and the Km and Hill coefficient of each mutant assayed in the absence of ATP were similar to those of wild-type GC-B assayed in the presence of ATP. Finally, 1 mm 2,4,6,-trinitrophenyl ATP inhibited all three mutants by as much as 80% but failed to inhibit WT-GC-B. We conclude that 1) the A488P and R655C missense mutations result in a GC-B conformation that mimics the allosterically activated conformation, 2) GC-B phosphorylation is required for CNP-dependent activation by the A488P mutation, 3) the R655C mutation abrogates the need for phosphorylation in receptor activation, and 4) an ATP analog selectively inhibits the GC-B mutants, indicating that a pharmacologic approach could reduce GC-B dependent human skeletal overgrowth.

A Glutamate-Substituted Mutant Mimics the Phosphorylated and Active Form of Guanylyl Cyclase-A.

Multisite phosphorylation is required for activation of guanylyl cyclase (GC)-A, also known as NPR-A or NPR1, by cardiac natriuretic peptides (NPs). Seven chemically identified sites (Ser-487, Ser-497, Thr-500, Ser-502, Ser-506, Ser-510, and Thr-513) and one functionally identified putative site (Ser-473) were reported. Single alanine substitutions for Ser-497, Thr-500, Ser-502, Ser-506, and Ser-510 reduced maximal velocity (Vmax), whereas glutamate substitutions had no effect or increased Vmax Ala but not Glu substitution for Ser-497 increased the Michaelis constant (Km) approximately 400%. A GC-A mutant containing Glu substitutions for all seven chemically identified sites (GC-A-7E) had a Km approximately 10-fold higher than phosphorylated wild-type (WT) GC-A, but one additional substitution for Ser-473 to make GC-A-8E resulted in the same Vmax, Km, and EC50 as the phosphorylated WT enzyme. Adding more glutamates to make GC-A-9E or GC-A-10E had little effect on activity, and sequential deletion of individual glutamates in GC-A-8E progressively increased the Km Double Ala substitutions for Ser-497 and either Thr-500, Ser-510 or Thr-513 in WT-GC-A increased the Km 23- to 70-fold but the same mutations in GC-A-8E only increased the Km 8-fold, consistent with one site affecting the phosphorylation of other sites. Phosphate measurements confirmed that single-site Ala substitutions reduced receptor phosphate levels more than expected for the loss of a single site. We conclude that a concentrated region of negative charge, not steric properties, resulting from multiple interdependent phosphorylation sites is required for a GC-A conformation capable of transmitting the hormone binding signal to the catalytic domain.

Dephosphorylation of juxtamembrane serines and threonines of the NPR2 guanylyl cyclase is required for rapid resumption of oocyte meiosis in response to luteinizing hormone.

The meiotic cell cycle of mammalian oocytes starts during embryogenesis and then pauses until luteinizing hormone (LH) acts on the granulosa cells of the follicle surrounding the oocyte to restart the cell cycle. An essential event in this process is a decrease in cyclic GMP in the granulosa cells, and part of the cGMP decrease results from dephosphorylation and inactivation of the natriuretic peptide receptor 2 (NPR2) guanylyl cyclase, also known as guanylyl cyclase B. However, it is unknown whether NPR2 dephosphorylation is essential for LH-induced meiotic resumption. Here, we prevented NPR2 dephosphorylation by generating a mouse line in which the seven regulatory serines and threonines of NPR2 were changed to the phosphomimetic amino acid glutamate (Npr2-7E). Npr2-7E/7E follicles failed to show a decrease in enzyme activity in response to LH, and the cGMP decrease was attenuated; correspondingly, LH-induced meiotic resumption was delayed. Meiotic resumption in response to EGF receptor activation was likewise delayed, indicating that NPR2 dephosphorylation is a component of the pathway by which EGF receptor activation mediates LH signaling. We also found that most of the NPR2 protein in the follicle was present in the mural granulosa cells. These findings indicate that NPR2 dephosphorylation in the mural granulosa cells is essential for the normal progression of meiosis in response to LH and EGF receptor activation. In addition, these studies provide the first demonstration that a change in phosphorylation of a transmembrane guanylyl cyclase regulates a physiological process, a mechanism that may also control other developmental events.

Catalytically Active Guanylyl Cyclase B Requires Endoplasmic Reticulum-mediated Glycosylation, and Mutations That Inhibit This Process Cause Dwarfism.

C-type natriuretic peptide activation of guanylyl cyclase B (GC-B), also known as natriuretic peptide receptor B or NPR2, stimulates long bone growth, and missense mutations in GC-B cause dwarfism. Four such mutants (L658F, Y708C, R776W, and G959A) bound (125)I-C-type natriuretic peptide on the surface of cells but failed to synthesize cGMP in membrane GC assays. Immunofluorescence microscopy also indicated that the mutant receptors were on the cell surface. All mutant proteins were dephosphorylated and incompletely glycosylated, but dephosphorylation did not explain the inactivation because the mutations inactivated a "constitutively phosphorylated" enzyme. Tunicamycin inhibition of glycosylation in the endoplasmic reticulum or mutation of the Asn-24 glycosylation site decreased GC activity, but neither inhibition of glycosylation in the Golgi by N-acetylglucosaminyltransferase I gene inactivation nor PNGase F deglycosylation of fully processed GC-B reduced GC activity. We conclude that endoplasmic reticulum-mediated glycosylation is required for the formation of an active catalytic, but not ligand-binding domain, and that mutations that inhibit this process cause dwarfism.

Lipid peroxidation products induce carbonyl stress, mitochondrial dysfunction, and cellular senescence in human and murine cells.

Lipid enals are electrophilic products of lipid peroxidation that induce genotoxic and proteotoxic stress by covalent modification of DNA and proteins, respectively. As lipid enals accumulate to substantial amounts in visceral adipose during obesity and aging, we hypothesized that biogenic lipid enals may represent an endogenously generated, and therefore physiologically relevant, senescence inducers. To that end, we identified that 4-hydroxynonenal (4-HNE), 4-hydroxyhexenal (4-HHE) or 4-oxo-2-nonenal (4-ONE) initiate the cellular senescence program of IMR90 fibroblasts and murine adipose stem cells. In such cells, lipid enals induced accumulation of γH2AX foci, increased p53 signaling, enhanced expression of p21Cip1, and upregulated the expression and secretion of numerous cytokines, chemokines, and regulatory factors independently from NF-κB activation. Concomitantly, lipid enal treatment resulted in covalent modification of mitochondrial proteins, reduced mitochondrial spare respiratory capacity, altered nucleotide pools, and increased the phosphorylation of AMP kinase. Lipid-induced senescent cells upregulated BCL2L1 (Bcl-xL) and BCL2L2 (Bcl-w). and were resistant to apoptosis while pharmacologic inhibition of BAX/BAK macropores attenuated lipid-induced senescence. In situ, the 4-HNE scavenger L-carnosine ameliorated the development of the cellular senescence, while in visceral fat of obese C57BL/6J mice, L-carnosine reduced the abundance of 4-HNE-modified proteins and blunted the expression of senescence biomarkers CDKN1A (p21Cip1), PLAUR, BCL2L1, and BCL2L2. Taken together, the results suggest that lipid enals are endogenous regulators of cellular senescence and that biogenic lipid-induced senescence (BLIS) may represent a mechanistic link between oxidative stress and age-dependent pathologies.

Novel enhancers of guanylyl cyclase-A activity acting via allosteric modulation.

BACKGROUND AND PURPOSE: Guanylyl cyclase-A (GC-A), activated by endogenous atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), plays an important role in the regulation of cardiovascular and renal homeostasis and is an attractive drug target. Even though small molecule modulators allow oral administration and longer half-life, drug targeting of GC-A has so far been limited to peptides. Thus, in this study we aimed to develop small molecular activators of GC-A. EXPERIMENTAL APPROACH: Hits were identified through high-throughput screening and optimized by in silico design. Cyclic GMP was measured in QBIHEK293A cells expressing GC-A, GC-B or chimerae of the two receptors using AlphaScreen technology. Binding assays were performed in membrane preparations or whole cells using 125 I-ANP. Vasorelaxation was measured in aortic rings isolated from Wistar rats. KEY RESULTS: We have identified small molecular allosteric enhancers of GC-A, which enhanced ANP or BNP effects in cellular systems and ANP-induced vasorelaxation in rat aortic rings. The mechanism of action appears novel and not mediated through previously described allosteric binding sites. In addition, the selectivity and activity depend on a single amino acid residue that differs between the two similar receptors GC-A and GC-B. CONCLUSION AND IMPLICATIONS: We describe a novel allosteric binding site on GC-A, which can be targeted by small molecules to enhance ANP and BNP effects. These compounds will be valuable tools in further development and proof-of-concept of GC-A enhancement for the potential use in cardiovascular therapy.

Guanylyl cyclase (GC)-A and GC-B activities in ventricles and cardiomyocytes from failed and non-failed human hearts: GC-A is inactive in the failed cardiomyocyte.

Cardiomyocytes release atrial natriuretic peptide (ANP) and B-type natriuretic peptide to stimulate processes that compensate for the failing heart by activating guanylyl cyclase (GC)-A. C-type natriuretic peptide is also elevated in the failing heart and inhibits cardiac remodeling by activating the homologous receptor, GC-B. We previously reported that GC-A is the most active membrane GC in normal mouse ventricles while GC-B is the most active membrane GC in failing ventricles due to increased GC-B and decreased GC-A activities. Here, we examined ANP and CNP-specific GC activity in membranes obtained from non-failing and failing human left ventricles and in membranes from matched cardiomyocyte-enriched pellet preparations. Similar to our findings in the murine study, we found that CNP-dependent GC activity was about half of the ANP-dependent GC activity in the non-failing ventricular and was increased in the failing ventricle. ANP and CNP increased GC activity 9- and 5-fold in non-failing ventricles, respectively. In contrast to the mouse study, in failing human ventricles, ANP-dependent activity was unchanged compared to non-failing values whereas CNP-dependent activity increased 35% (p=0.005). Compared with ventricular membranes, basal GC activity was reduced an order of magnitude in membranes derived from myocyte-enriched pellets from non-failing ventricles. ANP increased GC activity 2.4-fold but CNP only increased GC activity 1.3-fold. In contrast, neither ANP nor CNP increased GC activity in equivalent preparations from failing ventricles. We conclude that: 1) GC-B activity is increased in non-myocytes from failing human ventricles, possibly as a result of increased fibrosis, 2) human ventricular cardiomyocytes express low levels of GC-A and much lower levels or possibly no GC-B, and 3) GC-A in cardiomyocytes from failing human hearts is refractory to ANP stimulation.

Insulin-degrading enzyme modulates the natriuretic peptide-mediated signaling response.

Natriuretic peptides (NPs) are cyclic vasoactive peptide hormones with high therapeutic potential. Three distinct NPs (ANP, BNP, and CNP) can selectively activate natriuretic peptide receptors, NPR-A and NPR-B, raising the cyclic GMP (cGMP) levels. Insulin-degrading enzyme (IDE) was found to rapidly cleave ANP, but the functional consequences of such cleavages in the cellular environment and the molecular mechanism of recognition and cleavage remain unknown. Here, we show that reducing expression levels of IDE profoundly alters the response of NPR-A and NPR-B to the stimulation of ANP, BNP, and CNP in cultured cells. IDE rapidly cleaves ANP and CNP, thus inactivating their ability to raise intracellular cGMP. Conversely, reduced IDE expression enhances the stimulation of NPR-A and NPR-B by ANP and CNP, respectively. Instead of proteolytic inactivation, IDE cleavage can lead to hyperactivation of BNP toward NPR-A. Conversely, decreasing IDE expression reduces BNP-mediated signaling. Additionally, the cleavages of ANP and BNP by IDE render them active with NPR-B and a reduction of IDE expression diminishes the ability of ANP and BNP to stimulate NPR-B. Our kinetic and crystallographic analyses offer the molecular basis for the selective degradation of NPs and their variants by IDE. Furthermore, our studies reveal how IDE utilizes its catalytic chamber and exosite to engulf and bind up to two NPs leading to biased stochastic, non-sequential cleavages and the ability of IDE to switch its substrate selectivity. Thus, the evolutionarily conserved IDE may play a key role in modulating and reshaping the strength and duration of NP-mediated signaling.

Biodesign of a renal-protective peptide based on alternative splicing of B-type natriuretic peptide.

Alternative RNA splicing may provide unique opportunities to identify drug targets and therapeutics. We identified an alternative spliced transcript for B-type natriuretic peptide (BNP) resulting from intronic retention. This transcript is present in failing human hearts and is reduced following mechanical unloading. The intron-retained transcript would generate a unique 34 amino acid (aa) carboxyl terminus while maintaining the remaining structure of native BNP. We generated antisera to this carboxyl terminus and identified immunoreactivity in failing human heart tissue. The alternatively spliced peptide (ASBNP) was synthesized and unlike BNP, failed to stimulate cGMP in vascular cells or vasorelax preconstricted arterial rings. This suggests that ASBNP may lack the dose-limiting effects of recombinant BNP. Given structural considerations, a carboxyl-terminal truncated form of ASBNP was generated (ASBNP.1) and was determined to retain the ability of BNP to stimulate cGMP in canine glomerular isolates and cultured human mesangial cells but lacked similar effects in vascular cells. In a canine-pacing model of heart failure, systemic infusion of ASBNP.1 did not alter mean arterial pressure but increased the glomerular filtration rate (GFR), suppressed plasma renin and angiotensin, while inducing natriuresis and diuresis. Consistent with its distinct in vivo effects, the activity of ASBNP.1 may not be explained through binding and activation of NPR-A or NPR-B. Thus, the biodesigner peptide ASBNP.1 enhances GFR associated with heart failure while lacking the vasoactive properties of BNP. These findings demonstrate that peptides with unique properties may be designed based on products of alternatively splicing.

Inflammasome Activation and Pyroptosis via a Lipid-regulated SIRT1-p53-ASC Axis in Macrophages From Male Mice and Humans.

Obesity-linked diabetes is associated with accumulation of proinflammatory macrophages into adipose tissue leading to inflammasome activation and pyroptotic secretion of interleukin (IL)-1ß and IL-18. Targeting fatty acid binding protein 4 (FABP4) uncouples obesity from inflammation, attenuates characteristics of type 2 diabetes and is mechanistically linked to the cellular accumulation of monounsaturated fatty acids in macrophages. Herein we show that pharmacologic inhibition or genetic deletion of FABP4 activates silent mating type information regulation 2 homolog 1 (SIRT1) and deacetylates its downstream targets p53 and signal transducer and activator of transcription 3 (STAT3). Pharmacologic inhibition of fatty acid synthase or stearoyl-coenzyme A desaturase inhibits, whereas exogenous addition of C16:1 or C18:1 but not their saturated acyl chain counterparts, activates SIRT1 and p53/STAT3 signaling and IL-1ß/IL-18 release. Expression of the p53 target gene ASC [apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (CARD)] required for assembly of the NLR family pyrin domain containing 3 (NLRP3) inflammasome is downregulated in FABP4 null mice and macrophage cell lines leading to loss of procaspase 1 activation and pyroptosis. Concomitant with loss of ASC expression in FABP4-/- macrophages, inflammasome activation, gasdermin D processing, and functional activation of pyroptosis are all diminished in FABP4 null macrophages but can be rescued by silencing SIRT1 or exogenous expression of ASC. Taken together, these results reveal a novel lipid-regulated pathway linking to SIRT1-p53-ASC signaling and activation of inflammasome action and pyroptosis.

Fatty acid binding protein 4 regulates pancreatic cancer cell proliferation via activation of nuclear factor E2-related factor 2.

BACKGROUND: Obesity and diabetes are associated with an increased incidence of pancreatic cancer. Fatty acid binding protein 4 (FABP4), noted to be higher in patients with severe obesity, is linked to the development and progression of several cancers, and its level in the circulation decreases after bariatric surgery. OBJECTIVE: In this paper, we evaluate the role of FABP4 in pancreatic cancer progression. SETTING: University Hospital and Laboratories, United States. METHODS AND RESULTS: When Panc-1 (human) and Pan02 (mouse) pancreatic cancer cells were treated with FABP4 or the-single-point mutant FABP4 (R126Q, fatty acid binding site mutant), only FABP4 stimulated cellular proliferation. The transcriptional activity of nuclear factor E2-related factor 2 (NRF2) was increased in response to FABP4 but not the R126Q. FABP4 treatment also led to downregulation of reactive oxygen species (ROS) activity. Consistent with induced cell propagation by FABP4, the growth of Pan02 tumor was decreased in FABP4-null animals compared with C57BL/6J controls. CONCLUSION: These results suggest that FABP4 increases pancreatic cancer proliferation via activation of NRF2 and downregulation of ROS activity.

Dendroaspis natriuretic peptide and the designer natriuretic peptide, CD-NP, are resistant to proteolytic inactivation.

Designer natriuretic peptides (NPs) represent an active area of drug development. In canine and human studies, the designer natriuretic peptide CD-NP demonstrated more desirable therapeutic potential than recombinant B-type NP (BNP), which is known as nesiritide and is approved for treatment of acute decompensated heart failure. However, why CD-NP is more effective than BNP is not known. We previously reported that CD-NP is a poorer activator of human guanylyl cyclase-A (GC-A) and a better activator of human guanylyl cyclase-B than BNP. Here, guanylyl cyclase bioassays were used to compare the susceptibility of CD-NP verses ANP, BNP, CNP and DNP to inactivation by human kidney membranes. The half time (t(1/2)) for CD-NP inactivation was increased by factors of 13, 3 and 4 compared to ANP, BNP and CNP, respectively, when measured in the same assay. Surprisingly, DNP failed to undergo complete inactivation and was the most degradation resistant of the peptides tested. The neutral endopeptidase (NEP) inhibitor, phosphoramidon, blocked inactivation of CNP and CD-NP, but not BNP or DNP. In contrast, the general serine and cysteine protease inhibitor, leupeptin, completely blocked the degradation of BNP and CD-NP, but did not block CNP inactivation unless phosphoramidon was included in the assay. Thus, NPs with shorter carboxyl tails (ANP and CNP) are degraded by phosphoramidon-sensitive proteases and NPs with extended carboxyl tails (BNP, DNP and CD-NP) are resistant to NEP degradation and degraded by leupeptin-sensitive proteases. We conclude that DNP and CD-NP are highly resistant to proteolysis and that proteolytic resistance contributes to the beneficial cardiovascular properties of CD-NP. We suggest that this property may be exploited to increase the half-life of NP-based drugs.

Antibody tracking demonstrates cell type-specific and ligand-independent internalization of guanylyl cyclase a and natriuretic peptide receptor C.

Atrial natriuretic peptide (ANP) binds guanylyl cyclase-A (GC-A) and natriuretic peptide receptor-C (NPR-C). Internalization of GC-A and NPR-C is poorly understood, in part, because previous studies used (125)I-ANP binding to track these receptors, which are expressed in the same cell. Here, we evaluated GC-A and NPR-C internalization using traditional and novel approaches. Although HeLa cells endogenously express GC-A, (125)I-ANP binding and cross-linking studies only detected NPR-C, raising the possibility that past studies ascribed NPR-C-mediated processes to GC-A. To specifically measure internalization of a single receptor, we developed an (125)I-IgG-binding assay that tracks extracellular FLAG-tagged versions of GC-A and NPR-C independently of each other and ligand for the first time. FLAG-GC-A bound ANP identically with wild-type GC-A and was internalized slowly (0.5%/min), whereas FLAG-NPR-C was internalized rapidly (2.5%/min) in HeLa cells. In 293 cells, (125)I-ANP and (125)I-IgG uptake curves were superimposable because these cells only express a single ANP receptor. Basal internalization of both receptors was 8-fold higher in 293 compared with HeLa cells and ANP did not increase internalization of FLAG-GC-A. For FLAG-NPR-C, neither ANP, BNP, nor CNP increased its internalization in either cell line. Prolonged ANP exposure concomitantly reduced surface and total GC-A levels, consistent with rapid exchange of extracellular and intracellular receptor pools. We conclude that ligand binding does not stimulate natriuretic peptide receptor internalization and that cellular environment determines the rate of this process. We further deduce that NPR-C is internalized faster than GC-A and that increased internalization is not required for GC-A down-regulation.

ProBNP(1-108) is resistant to degradation and activates guanylyl cyclase-A with reduced potency.

BACKGROUND: B-type natriuretic peptide (BNP) compensates for the failing heart and is synthesized as a 108-residue prohormone that is cleaved to a 32-residue C-terminal maximally active peptide. During heart failure, serum concentrations of proBNP(1-108) exceed concentrations of BNP(1-32). The aim of this study was to determine why the proBNP(1-108)/BNP(1-32) ratio increases and whether proBNP(1-108) is bioactive. METHODS: Using cGMP elevation and (125)I-ANP binding assays, we measured binding and activation of individual human natriuretic peptide receptor populations by recombinant human proBNP(1-108) and human synthetic BNP(1-32). Using receptor bioassays, we measured degradation of recombinant proBNP(1-108) and BNP(1-32) by human kidney membranes. RESULTS: ProBNP(1-108) stimulated guanylyl cyclase-A (GC-A) to near-maximum activities but was 13-fold less potent than BNP(1-32). ProBNP(1-108) bound human GC-A 35-fold less tightly than BNP(1-32). Neither proBNP(1-108) nor BNP(1-32) activated GC-B. The natriuretic peptide clearance receptor bound proBNP(1-108) 3-fold less tightly than BNP(1-32). The half time for degradation of proBNP(1-108) by human kidney membranes was 2.7-fold longer than for BNP(1-32), and the time required for complete degradation was 6-fold longer. BNP(1-32) and proBNP(1-108) were best fitted by first- and second-order exponential decay models, respectively. CONCLUSIONS: ProBNP(1-108) activates GC-A with reduced potency and is resistant to degradation. Reduced degradation of proBNP(1-108) may contribute to the increased ratio of serum proBNP(1-108) to BNP(1-32) observed in patients with congestive heart failure.

Arg13 of B-type natriuretic Peptide reciprocally modulates binding to guanylyl cyclase but not clearance receptors.

B-type natriuretic peptide (BNP) decreases cardiac preload and hypertrophy. As such, synthetic BNP, nesiritide, was approved for the treatment of acutely decompensated heart failure. However, two problems limit its therapeutic potential. First, ensuing hypertension decreases urine output, and second, guanylyl cyclase-A (GC-A), the primary signaling receptor for BNP, is down-regulated in heart failure. Thus, alternative or chimeric natriuretic peptides maintaining the renal but lacking the vasorelaxation properties of BNP provide an alternative approach. Here, we examined the ability of single amino acid substitutions in the conserved 17-amino acid disulfide ring structure of human BNP to activate GC-A and guanylyl cyclase-B (GC-B), which is not reduced in heart failure. We hypothesized that substitution of highly conserved residues in BNP with highly conserved residues from a GC-B-specific peptide would yield BNP variants with increased and decreased potency for human GC-B and GC-A, respectively. Substitution of Leu for Arg13 (l-bnp) yielded a 5-fold more potent activator of GC-B and 7-fold less potent activator of GC-A compared with wild type. l-bnp also bound GC-A 4.5-fold less tightly than wild type. In contrast, substitution of Met for Ser21 (M-BNP) had no effect. A peptide containing both the Leu and Met substitutions behaved similarly to l-bnp. Meanwhile, wild-type and l-bnp bound the natriuretic peptide clearance receptor with similar affinities. These data indicate that Arg13 of BNP is a critical discriminator of binding to guanylyl cyclase-linked but not clearance natriuretic peptide receptors, supporting designer natriuretic peptides as an alternative to wild-type BNP for the treatment of heart failure.

Natriuretic peptides, their receptors, and cyclic guanosine monophosphate-dependent signaling functions.

Natriuretic peptides are a family of structurally related but genetically distinct hormones/paracrine factors that regulate blood volume, blood pressure, ventricular hypertrophy, pulmonary hypertension, fat metabolism, and long bone growth. The mammalian members are atrial natriuretic peptide, B-type natriuretic peptide, C-type natriuretic peptide, and possibly osteocrin/musclin. Three single membrane-spanning natriuretic peptide receptors (NPRs) have been identified. Two, NPR-A/GC-A/NPR1 and NPR-B/GC-B/NPR2, are transmembrane guanylyl cyclases, enzymes that catalyze the synthesis of cGMP. One, NPR-C/NPR3, lacks intrinsic enzymatic activity and controls the local concentrations of natriuretic peptides through constitutive receptor-mediated internalization and degradation. Single allele-inactivating mutations in the promoter of human NPR-A are associated with hypertension and heart failure, whereas homozygous inactivating mutations in human NPR-B cause a form of short-limbed dwarfism known as acromesomelic dysplasia type Maroteaux. The physiological effects of natriuretic peptides are elicited through three classes of cGMP binding proteins: cGMP-dependent protein kinases, cGMP-regulated phosphodiesterases, and cyclic nucleotide-gated ion channels. In this comprehensive review, the structure, function, regulation, and biological consequences of natriuretic peptides and their associated signaling proteins are described.