Design, synthesis and biological evaluation of new 3,4-dihydroquinoxalin-2(1H)-one derivatives as soluble guanylyl cyclase (sGC) activators.

Herein, we present the structure-based design, synthesis and biological evaluation of novel mono- and di-carboxylic 3,4-dihydroquinoxalin-2(1H)-one derivatives as potential heme-independent activators of soluble guanylate cyclase (sGC). Docking calculations of several known sGC agonists by utilizing both a homology model of human sGC ß1 Η-ΝΟΧ domain and a recent cryo-EM structure of the same domain guided the structural optimization of various designed compounds. Among these, mono- and di-carboxylic 3,4-dihydroquinoxalin-2(1H)-one derivatives arose as promising candidate sGC activators. A series of such compounds was synthesized and assessed for their effect on sGC activity. None of them was able to trigger any detectable activation of native sGC in prostate cancer (LnCaP) or rat aortic smooth muscle (A7r5) cells, even after loss of heme by treatment with the heme oxidant ODQ. Furthermore, selected derivatives did not exhibit any antagonistic effect against the known heme-independent sGC activator BAY 60-2770 nor any additive or synergistic effect with the heme-dependent NO donor sodium nitroprusside (SNP) on heme-associated sGC in A7r5 cells. However, when tested in vitro using purified recombinant sGC enzyme, the dicarboxylic 3,4-dihydroquinoxalin-2(1H)-one derivative 30d was able to increase the enzymatic activity of both the wild-type α1/ß1 sGC dimer (by 4.4-fold, EC50 = 0.77 µΜ) as well as the heme-free α1/ß1 His105Ala mutant sGC (by 4.8-fold, EC50 = 1.8 µΜ). Notably, the activity of compound 30d towards the mutant α1/ß1 Η105A enzyme was comparable with that previously reported by us for the bona fide activator BAY 60-2770, using the functionally equivalent wild-type sGC preparation treated with ODQ. These results indicate that compound 30d can indeed act as a promising sGC activator and may serve as a basic structure in the design of novel, optimized analogues with enhanced sGC agonistic activity and improved efficiency in cell-based and in vivo systems.

Nitric oxide sensitive guanylyl cyclase activity decreases during cerebral postnatal development because of a reduction in heterodimerization.

Soluble guanylyl cyclase (sGC) is the major physiological receptor for nitric oxide (NO) throughout the central nervous system. Three different subunits form the alpha(1)/beta(1) and alpha(2)/beta(1) heterodimeric enzymes that catalyze the reaction of GTP to the second messenger cGMP. Both forms contain a prosthetic heme group which binds NO and mediates activation by NO. A number of studies have shown that NO/cGMP signaling plays a major role in neuronal cell differentiation during development of the central nervous system. In the present work, we studied regulation and expression of sGC in brain of rats during postnatal development using biochemical methods. We consistently observed a surprising decrease in cerebral NO sensitive enzyme activity in adult animals in spite of stable expression of sGC subunits. Total hemoprotein heme content was decreased in cerebrum of adult animals, likely because of an increase in heme oxygenase activity. But the loss of sGC activity was not simply because of heme loss in intact heterodimeric enzymes. This was shown by enzyme activity determinations with cinaciguat which can be used to test heme occupancy in intact heterodimers. A reduction in heterodimerization in cerebrum of adult animals was demonstrated by co-precipitation analysis of sGC subunits. This explained the observed decrease in NO sensitive guanylyl cyclase activity in cerebrum of adult animals. We conclude that differing efficiencies in heterodimer formation may be an important reason for the lack of correlation between sGC protein expression and sGC activity that has been described previously. We suggest that heterodimerization of sGC is a regulated process that changes during cerebral postnatal development because of still unknown signaling mechanisms.

Translocation of heme oxygenase-1 contributes to imatinib resistance in chronic myelogenous leukemia.

Heme oxygenase-1 (HO-1) degrades heme to bilirubin. In addition, it is upregulated in malignant disease and has been described as an important factor for cancer prognosis and therapy. Under physiological conditions HO-1 is anchored to the endoplasmic reticulum (ER). Under stress conditions HO-1 can be cleaved and subsequently translocates to the cytosol and nucleus. In this study we systematically investigated the influence of HO-1's catabolic activity and subcellular localization on resistance against the tyrosine kinase inhibitor imatinib in leukemia cells by confocal laser scanning microscopy, hemoglobin synthesis experiments and cell viability assays. We created two types of monoclonal K562 cell lines stably transfected with GFP-tagged HO-1: cell lines expressing ER anchored HO-1 or anchorless HO-1. Since translocation of HO-1 disrupts the association with cytochrome P450 reductase, heme degrading activity was higher for ER anchored versus anchorless HO-1. Cell viability tests with increasing concentrations of imatinib showed IC50-values for all six cell lines with ER localized HO-1 that were similar to control cells. However, out of the seven cell lines with anchorless HO-1, two showed a statistically significant increase in the imatinib IC50 (19.76 µM and 12.35 µM versus 2.35 - 7.57 µM of sensitive cell lines) corresponding to plasma concentrations outside the therapeutic range. We conclude that the presence of translocated HO-1 in the cytosol and nucleus supports imatinib resistance while it is not sufficient to cause imatinib resistance in every cell line. In contrast, an increase in ER anchored HO-1 with high heme degrading activity does not contribute to imatinib resistance.

Insights into the mechanism of isoenzyme-specific signal peptide peptidase-mediated translocation of heme oxygenase.

It has recently been shown that signal peptide peptidase (SPP) can catalyze the intramembrane cleavage of heme oxygenase-1 (HO-1) that leads to translocation of HO-1 into the cytosol and nucleus. While there is consensus that translocated HO-1 promotes tumor progression and drug resistance, the physiological signals leading to SPP-mediated intramembrane cleavage of HO-1 and the specificity of the process remain unclear. In this study, we used co-immunoprecipitation and confocal laser scanning microscopy to investigate the translocation mechanism of HO-1 and its regulation by SPP. We show that HO-1 and the closely related HO-2 isoenzyme bind to SPP under normoxic conditions. Under hypoxic conditions SPP mediates intramembrane cleavage of HO-1, but not HO-2. In experiments with an inactive HO-1 mutant (H25A) we show that translocation is independent of the catalytic activity of HO-1. Studies with HO-1 / HO-2 chimeras indicate that the membrane anchor, the PEST-domain and the nuclear shuttle sequence of HO-1 are necessary for full cleavage and subsequent translocation under hypoxic conditions. In the presence of co-expressed exogenous SPP, the anchor and the PEST-domain are sufficient for translocation. Taken together, we identified the domains involved in HO-1 translocation and showed that SPP-mediated cleavage is isoform-specific and independent of HO-activity. A closer understanding of the translocation mechanism of HO-1 is of particular importance because nuclear HO-1 seems to lead to tumor progression and drug resistance.

Alterations in soluble guanylate cyclase content and modulation by nitric oxide in liver disease.

Hyperammonemia is the main responsible for the neurological alterations in hepatic encephalopathy in patients with liver failure. We studied the function of the glutamate-nitric oxide (NO)-cGMP pathway in brain in animal models of hyperammonemia and liver failure and in patients died with liver cirrhosis. Activation of glutamate receptors increases intracellular calcium that binds to calmodulin and activates neuronal nitric oxide synthase, increasing nitric oxide, which activates soluble guanylate cyclase (sGC), increasing cGMP. This glutamate-NO-cGMP pathway modulates cerebral processes such as circadian rhythms, the sleep-waking cycle, and some forms of learning and memory. These processes are impaired in patients with hepatic encephalopathy. Activation of sGC by NO is significantly increased in cerebral cortex and significantly reduced in cerebellum from cirrhotic patients died in hepatic coma. Portacaval anastomosis in rats, an animal model of liver failure, reproduces the effects of liver failure on modulation of sGC by NO both in cerebral cortex and cerebellum. In vivo brain microdialisis studies showed that sGC activation by NO is also reduced in vivo in cerebellum in hyperammonemic rats with or without liver failure. The content of alpha but not beta subunits of sGC are increased both in frontal cortex and cerebellum from patients died due to liver disease and from rats with portacaval anastomosis. We assessed whether determination of activation of sGC by NO-generating agent SNAP in lymphocytes could serve as a peripheral marker for the impairment of sGC activation by NO in brain. Chronic hyperammonemia and liver failure also alter sGC activation by NO in lymphocytes from rats or patients. These findings show that the content and modulation by NO of sGC are strongly altered in brain of patients with liver disease. These alterations could be responsible for some of the neurological alterations in hepatic encephalopathy such as sleep disturbances and cognitive impairment.

α1-A680T variant in GUCY1A3 as a candidate conferring protection from pulmonary hypertension among Kyrgyz highlanders.

BACKGROUND: Human variation in susceptibility to hypoxia-induced pulmonary hypertension is well recognized. High-altitude residents who do not develop pulmonary hypertension may host protective gene mutations. METHODS AND RESULTS: Exome sequencing was conducted on 24 unrelated Kyrgyz highlanders living 2400 to 3800 m above sea level, 12 (10 men; mean age, 54 years) with an elevated mean pulmonary artery pressure (mean±SD, 38.7±2.7 mm Hg) and 12 (11 men; mean age, 52 years) with a normal mean pulmonary artery pressure (19.2±0.6 mm Hg) to identify candidate genes that may influence the pulmonary vascular response to hypoxia. A total of 140 789 exomic variants were identified and 26 116 (18.5%) were classified as novel or rare. Thirty-three novel or rare potential pathogenic variants (frameshift, essential splice-site, and nonsynonymous) were found exclusively in either ≥3 subjects with high-altitude pulmonary hypertension or ≥3 highlanders with a normal mean pulmonary artery pressure. A novel missense mutation in GUCY1A3 in 3 subjects with a normal mean pulmonary artery pressure encodes an α1-A680T soluble guanylate cyclase (sGC) variant. Expression of the α1-A680T sGC variant in reporter cells resulted in higher cyclic guanosine monophosphate production compared with the wild-type enzyme and the purified α1-A680T sGC exhibited enhanced sensitivity to nitric oxide in vitro. CONCLUSIONS: The α1-A680T sGC variant may contribute to protection against high-altitude pulmonary hypertension and supports sGC as a pharmacological target for reducing pulmonary artery pressure in humans at altitude.

Heme oxygenase isoforms differ in their subcellular trafficking during hypoxia and are differentially modulated by cytochrome P450 reductase.

Heme oxygenase (HO) degrades heme in concert with NADPH cytochrome P450 reductase (CPR) which donates electrons to the reaction. Earlier studies reveal the importance of the hydrophobic carboxy-terminus of HO-1 for anchorage to the endoplasmic reticulum (ER) which facilitates the interaction with CPR. In addition, HO-1 has been shown to undergo regulated intramembrane proteolysis of the carboxy-terminus during hypoxia and subsequent translocation to the nucleus. Translocated nuclear HO-1 was demonstrated to alter binding of transcription factors and to alter gene expression. Little is known about the homologous membrane anchor of the HO-2 isoform. The current work is the first systematic analysis in a eukaryotic system that demonstrates the crucial role of the membrane anchor of HO-2 for localization at the endoplasmic reticulum, oligomerization and interaction with CPR. We show that although the carboxy-terminal deletion mutant of HO-2 is found in the nucleus, translocation of HO-2 to the nucleus does not occur under conditions of hypoxia. Thus, we demonstrate that proteolytic regulation and nuclear translocation under hypoxic conditions is specific for HO-1. In addition we show for the first time that CPR prevents this translocation and promotes oligomerization of HO-1. Based on these findings, CPR may modulate gene expression via the amount of nuclear HO-1. This is of particular relevance as CPR is a highly polymorphic gene and deficiency syndromes of CPR have been described in humans.

The amino-terminus of nitric oxide sensitive guanylyl cyclase α1 does not affect dimerization but influences subcellular localization.

BACKGROUND: Nitric oxide sensitive guanylyl cyclase (NOsGC) is a heterodimeric enzyme formed by an α- and a ß1-subunit. A splice variant (C-α1) of the α1-subunit, lacking at least the first 236 amino acids has been described by Sharina et al. 2008 and has been shown to be expressed in differentiating human embryonic cells. Wagner et al. 2005 have shown that the amino acids 61-128 of the α1-subunit are mandatory for quantitative heterodimerization implying that the C-α1-splice variant should lose its capacity to dimerize quantitatively. METHODOLOGY/PRINCIPAL FINDINGS: In the current study we demonstrate preserved quantitative dimerization of the C-α1-splice by co-purification with the ß1-subunit. In addition we used fluorescence resonance energy transfer (FRET) based on fluorescence lifetime imaging (FLIM) using fusion proteins of the ß1-subunit and the α1-subunit or the C-α1 variant with ECFP or EYFP. Analysis of the respective combinations in HEK-293 cells showed that the fluorescence lifetime was significantly shorter (≈0.3 ns) for α1/ß1 and C-α1/ß1 than the negative control. In addition we show that lack of the amino-terminus in the α1 splice variant directs it to a more oxidized subcellular compartment. CONCLUSIONS/SIGNIFICANCE: We conclude that the amino-terminus of the α1-subunit is dispensable for dimerization in-vivo and ex-vivo, but influences the subcellular trafficking.

Fluorescent fusion proteins of soluble guanylyl cyclase indicate proximity of the heme nitric oxide domain and catalytic domain.

BACKGROUND: To examine the structural organisation of heterodimeric soluble guanylyl cyclase (sGC) Förster resonance energy transfer (FRET) was measured between fluorescent proteins fused to the amino- and carboxy-terminal ends of the sGC beta1 and alpha subunits. METHODOLOGY/PRINCIPAL FINDINGS: Cyan fluorescent protein (CFP) was used as FRET donor and yellow fluorescent protein (YFP) as FRET acceptor. After generation of recombinant baculovirus, fluorescent-tagged sGC subunits were co-expressed in Sf9 cells. Fluorescent variants of sGC were analyzed in vitro in cytosolic fractions by sensitized emission FRET. Co-expression of the amino-terminally tagged alpha subunits with the carboxy-terminally tagged beta1 subunit resulted in an enzyme complex that showed a FRET efficiency of 10% similar to fluorescent proteins separated by a helix of only 48 amino acids. Because these findings indicated that the amino-terminus of the alpha subunits is close to the carboxy-terminus of the beta1 subunit we constructed fusion proteins where both subunits are connected by a fluorescent protein. The resulting constructs were not only fluorescent, they also showed preserved enzyme activity and regulation by NO. CONCLUSIONS/SIGNIFICANCE: Based on the ability of an amino-terminal fragment of the beta1 subunit to inhibit activity of an heterodimer consisting only of the catalytic domains (alphacatbetacat), Winger and Marletta (Biochemistry 2005, 44:4083-90) have proposed a direct interaction of the amino-terminal region of beta1 with the catalytic domains. In support of such a concept of "trans" regulation of sGC activity by the H-NOX domains our results indicate that the domains within sGC are organized in a way that allows for direct interaction of the amino-terminal regulatory domains with the carboxy-terminal catalytic region. In addition, we constructed "fluorescent-conjoined" sGC's by fusion of the alpha amino-terminus to the beta1 carboxy-terminus leading to a monomeric, fluorescent and functional enzyme complex. To our knowledge this represents the first example where a fluorescent protein links two different subunits of a higher ordered complex to yield a stoichometrically fixed functionally active monomer.

Direct fusion of subunits of heterodimeric nitric oxide sensitive guanylyl cyclase leads to functional enzymes with preserved biochemical properties: evidence for isoform specific activation by ciguates.

Nitric oxide sensitive guanylyl cyclase (NOsGC) is a heterodimeric enzyme consisting of an α and a ß subunit. Two heterodimeric enzymes are known to be important for NO-signalling in humans: α(1)/ß(1) and α(2)/ß(1). No difference had so far been detected with respect to their pharmacological properties, but as we show in the present paper the new drugs cinaciguat and ataciguat activate the α(1)/ß(1) form more effectively. Recent evidence suggests that homodimeric complexes of α and ß subunits exist in vivo and that these non-heterodimerizing subunits have a separate function from cGMP signaling. To isolate the effect of the α(1)/ß(1) or α(2)/ß(1) heterodimeric enzyme in overexpression experiments from potential effects of non-heterodimerizing α(1), ß(1) or α(2) subunits, we cloned constructs that guarantee a 1:1 stochiometry between α and ß subunits and rule out the presence of homodimers. The carboxy-terminus of the ß(1) subunit was directly fused to the amino-terminus of either the α(1) or α(2) subunit. The two different "conjoined" NOsGCs faithfully reproduced the biochemical and pharmacological properties of the α(1)/ß(1) and α(2)/ß(1) heterodimeric enzymes including the differential activation by ciguat-activators. Conjoined NOsGCs can be used for isoform specific overexpression in transgenic animals and therapeutic overexpression may be an application in the future. In both cases possible side effects of homodimeric α or ß subunits are avoided. Crystallization with the goal of structure determination may also be easier for conjoined NOsGCs because enzyme preparations are more homogenous and are free of "contaminating" homodimers.

Soluble guanylate cyclase-dependent relaxation is reduced in the adult rat bronchial smooth muscle.

Cyclic nucleotides are relaxants of the airway smooth muscle, yet most of the available data were obtained in adult animals. The expression and activity of cyclases have been reported to be developmentally regulated in the lung, and little is known about the age-related changes in their bronchial muscle relaxation potential. We evaluated and compared the newborn and adult rat bronchial smooth muscle response to cyclic AMP- and GMP-dependent agonists in isometric mounted bronchial rings. In acetylcholine-precontracted bronchial muscle, the relaxant response to the cAMP agonist forskolin was not age dependent, but the relaxant response to the nitric oxide (NO) donor sodium nitroprusside (SNP) was significantly greater (P<0.01) in the newborn. To further evaluate the cGMP pathway, we stimulated the soluble guanylate cyclase (sGC) with the specific agonists BAY 41-2272 and YC-1. In keeping with the SNP dose-response curves, the sGC agonists significantly relaxed the newborn, but not the adult bronchial muscle. Protein expression of the sGC alpha1- and beta1-subunits were significantly lower (P<0.01) in the adult compared with the newborn bronchial tissue. Consistent with these results, the NO-stimulated sGC activity was significantly greater in the newborn compared with the adult (P<0.01). In conclusion, the bronchial smooth muscle cGMP-, but not cAMP-dependent, relaxant response is developmentally regulated and significantly reduced in the adult rat.