Forskolin derivatives with increased selectivity for cardiac adenylyl cyclase

Adenylyl cyclase isoforms 5 and 6 in the cardiovascular system: complex regulation and divergent roles

Adenylyl cyclases (ACs) are crucial effector enzymes that transduce divergent signals from upstream receptor pathways and are responsible for catalyzing the conversion of ATP to cAMP. The ten AC isoforms are categorized into four main groups; the class III or calcium-inhibited family of ACs comprises AC5 and AC6. These enzymes are very closely related in structure and have a paucity of selective activators or inhibitors, making it difficult to distinguish them experimentally. AC5 and AC6 are highly expressed in the heart and vasculature, as well as the spinal cord and brain; AC6 is also abundant in the lungs, kidney, and liver. However, while AC5 and AC6 have similar expression patterns with some redundant functions, they have distinct physiological roles due to differing regulation and cAMP signaling compartmentation. AC5 is critical in cardiac and vascular function; AC6 is a key effector of vasodilatory pathways in vascular myocytes and is enriched in fetal/neonatal tissues. Expression of both AC5 and AC6 decreases in heart failure; however, AC5 disruption is cardio-protective, while overexpression of AC6 rescues cardiac function in cardiac injury. This is a comprehensive review of the complex regulation of AC5 and AC6 in the cardiovascular system, highlighting overexpression and knockout studies as well as transgenic models illuminating each enzyme and focusing on post-translational modifications that regulate their cellular localization and biological functions. We also describe pharmacological challenges in the design of isoform-selective activators or inhibitors for AC5 and AC6, which may be relevant to developing new therapeutic approaches for several cardiovascular diseases.

A bioluminescent and homogeneous assay for monitoring GPCR-mediated cAMP modulation and PDE activity

3',5'-Cyclic adenosine monophosphate (cAMP), the first identified second messenger, is implicated in diverse cellular processes involving cellular metabolism, cell proliferation and differentiation, apoptosis, and gene expression. cAMP is synthesized by adenylyl cyclase (AC), which converts ATP to cAMP upon activation of Gαs-protein coupled receptors (GPCRs) in most cases and hydrolyzed by cyclic nucleotide phosphodiesterases (PDEs) to 5'-AMP. Dysregulation of cAMP signaling is implicated in a wide range of pathophysiological conditions such as cardiovascular diseases, neurodegenerative and behavioral disorders, cancers, diabetes, obesity, cataracts, and others. Therefore, cAMP targeted therapies have been and are still undergoing intense investigation for the treatment of these and other diseases. This highlights the need for developing assays to detect and monitor cAMP levels. In this study, we show cAMP Lumit assay as a highly specific homogeneous bioluminescent assay suitable for high throughput screenings with a large assay window and a wide dynamic range for cAMP detection. We believe that this assay will aid and simplify drug discovery screening efforts for cAMP signaling targeted therapies.

Host cell cAMP-Epac-Rap1b pathway inhibition by hawthorn extract as a potential target against Trypanosoma cruzi infection

Although the two drugs currently available for the treatment of Chagas disease, Benznidazole and Nifurtimox, have proven to be effective in the acute phase of the disease, the 60-90-day treatment leads to high toxicity and unwanted side effects, presenting, in addition, a low efficacy in the chronic phase of the disease. For this reason, new therapies that are more effective are needed. In this regard, we have recently shown that the inhibition of the Epac-Rap1b pathway suppressed the cAMP-mediated host cell invasion by Trypanosoma cruzi. Interestingly, it has been described that vitexin, a natural flavone that protects against ischemia-reperfusion damage, acts by inhibiting the expression of Epac and Rap1 proteins. Vitexin can be found in plants of the genus Crataegus spp., traditionally known as hawthorn, which are of great interest considering their highly documented use as cardio-protectors. Pre-treating cells with an extract of Crataegus oxyacantha produced levels of T. cruzi invasion comparable to the ones observed for the commercially available Epac1-specific inhibitor, ESI-09. In addition, extract-treated cells exhibited a decrease in the activation of Rap1b, suggesting that the effects of the extract would be mediated by the inhibition of the cAMP-Epac-Rap1 signaling pathway. Using HPLC-HRMS2, we could confirm the presence of vitexin, and other flavones that could act as inhibitors of Epac/Rap1b, in the extracts of C. oxyacantha. Most significantly, when cells were treated with the extract of C. oxyacantha in conjunction with Nifurtimox, an increased modulation of invasion was observed.

Natural products as modulators of the cyclic-AMP pathway: evaluation and synthesis of lead compounds

It is now well recognized that the normal cellular response in mammalian cells is critically regulated by the cyclic-AMP (cAMP) pathway through the appropriate balance of adenylyl cyclase (AC) and phosphodiesterase-4 (PDE4) activities. Dysfunctions in the cAMP pathway have major implications in various diseases like CNS disorders, inflammation and cardiac syndromes and, hence, the modulation of cAMP signalling through appropriate intervention of AC/PDE4 activities has emerged as a promising new drug discovery strategy of current interest. In this context, synthetic small molecules have had limited success so far and therefore parallel efforts on natural product leads have been actively pursued. The early promise of using the diterpene forskolin and its semi-synthetic analogs as AC activators has given way to new leads in the last decade from novel natural products like the marine sesterterpenoids alotaketals and ansellones and the 9,9'-diarylfluorenone cored selaginpulvilins, etc. and their synthesis has drawn much attention. This review captures these contemporary developments, particularly total synthesis campaigns and structure-guided analog design in the context of AC and PDE-4 modulating attributes and the scope for future possibilities.

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

Total biosynthesis of the cyclic AMP booster forskolin from Coleus forskohlii

Forskolin is a unique structurally complex labdane-type diterpenoid used in the treatment of glaucoma and heart failure based on its activity as a cyclic AMP booster. Commercial production of forskolin relies exclusively on extraction from its only known natural source, the plant Coleus forskohlii, in which forskolin accumulates in the root cork. Here, we report the discovery of five cytochrome P450s and two acetyltransferases which catalyze a cascade of reactions converting the forskolin precursor 13R-manoyl oxide into forskolin and a diverse array of additional labdane-type diterpenoids. A minimal set of three P450s in combination with a single acetyl transferase was identified that catalyzes the conversion of 13R-manoyl oxide into forskolin as demonstrated by transient expression in Nicotiana benthamiana. The entire pathway for forskolin production from glucose encompassing expression of nine genes was stably integrated into Saccharomyces cerevisiae and afforded forskolin titers of 40 mg/L.

The cAMP Pathway as Therapeutic Target in Autoimmune and Inflammatory Diseases

Nucleotide signaling molecules contribute to the regulation of cellular pathways. In the immune system, cyclic adenosine monophosphate (cAMP) is well established as a potent regulator of innate and adaptive immune cell functions. Therapeutic strategies to interrupt or enhance cAMP generation or effects have immunoregulatory potential in autoimmune and inflammatory disorders. Here, we provide an overview of the cyclic AMP axis and its role as a regulator of immune functions and discuss the clinical and translational relevance of interventions with these processes.

Calcium influx enhances neuropeptide activation of ecdysteroid hormone production by mosquito ovaries

A critical step in mosquito reproduction is the ingestion of a blood meal from a vertebrate host. In mosquitoes like Aedes aegypti, blood feeding stimulates the release of ovary ecdysteroidogenic hormone (OEH) and insulin-like peptide 3 (ILP3). This induces the ovaries to produce ecdysteroid hormone (ECD), which then drives egg maturation. In many immature insects, prothoracicotropic hormone (PTTH) stimulates the prothoracic glands to produce ECD that directs molting and metamorphosis. The receptors for OEH, ILP3 and PTTH are different receptor tyrosine kinases with OEH and ILP3 signaling converging downstream in the insulin pathway and PTTH activating the mitogen-activated protein kinase pathway. Calcium (Ca(2+)) flux and cAMP have also been implicated in PTTH signaling, but the role of Ca(2+) in OEH, ILP3, and cAMP signaling in ovaries is unknown. Here, we assessed whether Ca(2+) flux affects OEH, ILP3, and cAMP activity in A. aegypti ovaries and also asked whether PTTH stimulated ovaries to produce ECD. Results indicated that Ca(2+) flux enhanced but was not essential for OEH or ILP3 activity, whereas cAMP signaling was dependent on Ca(2+) flux. Recombinant PTTH from Bombyx mori fully activated ECD production by B. mori PTGs, but exhibited no activity toward A. aegypti ovaries. Recombinant PTTH from A. aegypti also failed to stimulate either B. mori PTGs or A. aegypti ovaries to produce ECD. We discuss the implications of these results in the context of mosquito reproduction and ECD biosynthesis by insects generally.

Coupling of β1-adrenergic receptor to type 5 adenylyl cyclase and its physiological relevance in cardiac myocytes

Myocardial β-adrenergic receptor (β-AR) β1- and β2-subtypes are highly homologous, but play opposite roles in cardiac apoptosis and heart failure, as do cardiac adenylyl cyclase (AC) subtypes 5 (AC5) and 6 (AC6): β1-AR and AC5 promote cardiac remodeling, while β2-AR and AC6 activate cell survival pathways. However, the mechanisms involved remain poorly understood. We hypothesized that AC5 is coupled preferentially to β1-AR rather than β2-AR, and we examined this idea by means of pharmacological and genetic approaches. We found that selective inhibition of AC5 with 2'5'-dideoxyadenosine significantly suppressed cAMP accumulation and cardiac apoptosis induced by selective β1-AR stimulation, but had no effect on cAMP accumulation and cardiac apoptosis in response to selective β2-AR stimulation. The results of selective stimulation of β1-AR and β2-AR in neonatal cardiac myocytes prepared from wild-type and AC5-knockout mice were also consistent with the idea that β1-AR selectively couples with AC5. We believe these results are helpful for understanding the mechanisms underlying the different roles of AR subtypes in healthy and diseased hearts.

Influence of acidosis on cardiotonic effects of colforsin and epinephrine: a dose-response study

OBJECTIVE

Acidosis produces a negative inotropic effect on cardiac muscle against which catecholamines and phosphodiesterase III inhibitors have limited therapeutic effects. This study evaluated the effects of colforsin, which directly activates adenylate cyclase without β-adrenergic receptor activation, in isolated Langendorff rat hearts in a pH- and concentration-dependent manner.

DESIGN

Experimental animal study.

SETTING

A university laboratory.

PARTICIPANTS

Sprague-Dawley rats.

INTERVENTIONS

Hearts were isolated and perfused with 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid/Tyrode solution (pH 7.4) in the Langendorff preparation. The hearts were assigned randomly to the control (pH 7.4), mild acidosis (pH 7.0), or severe acidosis (pH 6.6) group (n = 8 per group) and were perfused continuously with colforsin 10(-7), 10(-6), and 10(-5) mol/L.

MEASUREMENTS AND MAIN RESULTS

Maximum dP/dt was determined, and the concentration-response relation was evaluated at each pH. Colforsin at 10(-6) mol/L increased the maximum dP/dt from 2,592 ± 557 to 5,189 ± 721 mmHg/s (p < 0.001) and from 1,942 ± 325 to 3,399 ± 608 mmHg/s (p < 0.001) in the control and mild acidosis groups, respectively; whereas colforsin, 10(-5) mol/L, significantly increased the maximum dP/dt even in the severe acidosis group. No significant difference was seen in maximum dP/dt among the 3 groups after infusion with colforsin 10(-5) mol/L.

CONCLUSIONS

In contrast to catecholamines and other inodilators, colforsin at a high concentration restores decreased cardiac contractility against severe acidosis to an extent similar to physiologic pH.

Cannabinoid-induced enhanced interaction and protein levels of serotonin 5-HT(2A) and dopamine D₂ receptors in rat prefrontal cortex

Recent evidence suggests that non-selective cannabinoid receptor agonists may regulate serotonin 2A (5-HT(2A)) receptor neurotransmission in brain. The molecular mechanisms of this regulation are unknown, but could involve cannabinoid-induced enhanced interaction between 5-HT(2A) and dopamine D2 (D₂) receptors. Here, we present experimental evidence that Sprague-Dawley rats treated with a non-selective cannabinoid receptor agonist (CP55,940, 50 µg/kg, 7 days, i.p.) showed enhanced co-immunoprecipitation of 5-HT(2A) and D₂ receptors and enhanced membrane-associated expression of D₂ and 5-HT(2A) receptors in prefrontal cortex (PFCx). Furthermore, 5-HT(2A) receptor mRNA levels were increased in PFCx, suggesting a cannabinoid-induced upregulation of 5-HT(2A) receptors. To date, two cannabinoids receptors have been found in brain, CB1 and CB2 receptors. We used selective cannabinoid agonists in a neuronal cell line to study mechanisms that could mediate this 5-HT(2A) receptor upregulation. We found that selective CB2 receptor agonists upregulate 5-HT(2A) receptors by a mechanism that seems to involve activation of Gα(i) G-proteins, ERK1/2, and AP-1 transcription factor. We hypothesize that the enhanced cannabinoid-induced interaction between 5-HT(2A) and D₂ receptors and in 5-HT(2A) and D₂ receptors protein levels in the PFCx might provide a molecular mechanism by which activation of cannabinoid receptors might be contribute to the pathophysiology of some cognitive and mood disorders.

Pharmacological stimulation of type 5 adenylyl cyclase stabilizes heart rate under both microgravity and hypergravity induced by parabolic flight

We previously demonstrated that type 5 adenylyl cyclase (AC5) functions in autonomic regulation in the heart. Based on that work, we hypothesized that pharmacological modulation of AC5 activity could regulate the autonomic control of the heart rate under micro- and hypergravity. To test this hypothesis, we selected the approach of activating AC5 activity in mice with a selective AC5 activator (NKH477) or inhibitor (vidarabine) and examining heart rate variability during parabolic flight. The standard deviation of normal R-R intervals, a marker of total autonomic variability, was significantly greater under micro- and hypergravity in the vidarabine group, while there were no significant changes in the NKH477 group, suggesting that autonomic regulation was unstable in the vidarabine group. The ratio of low frequency and high frequency (HF) in heart rate variability analysis, a marker of sympathetic activity, became significantly decreased under micro- and hypergravity in the NKH477 group, while there was no such decrease in the vidarabine group. Normalized HF, a marker of parasympathetic activity, became significantly greater under micro- and hypergravity in the NKH477 group. In contrast, there was no such increase in the vidarabine group. This study is the first to indicate that pharmacological modulation of AC5 activity under micro- and hypergravity could be useful to regulate the autonomic control of the heart rate.

Impact of divalent metal ions on regulation of adenylyl cyclase isoforms by forskolin analogs

Mammalian membranous adenylyl cyclases (mACs) play an important role in transmembrane signalling events in almost every cell and represent an interesting drug target. Forskolin (FS) is an invaluable research tool, activating AC isoforms 1-8. However, there is a paucity of AC isoform-selective FS analogs. Therefore, we examined the effects of FS and six FS derivatives on recombinant ACs 1, 2 and 5, representing members of different mAC families. Correlations of the pharmacological properties of the different AC isoforms revealed pronounced differences between ACs 1, 2 and 5. Additionally, potencies and efficacies of FS derivatives changed for any given AC isoform, depending on the metal ion, Mg(2+) or Mn(2+). The most striking effects of Mg(2+) and Mn(2+) on the diterpene profile were observed for AC2 where the large inhibitory effect of BODIPY-FS in the presence of Mg(2+) was considerably reduced in the presence of Mn(2+). Sequence alignment and docking experiments confirmed an exceptional position of AC2 compared to ACs 1 and 5 with respect to the structural environment of the catalytic core and cation-dependent diterpene effects. In conclusion, mAC isoforms 1, 2 and 5 exhibit a distinct pharmacological diterpene profile, depending on the divalent cation present. mAC crystal structures and modelling/docking studies provided an explanation for the pharmacological differences between the AC isoforms. Our study constitutes an important step towards the development of isoform-specific diterpenes exhibiting stimulatory or inhibitory effects.

Forskolin compared with beclomethasone for prevention of asthma attacks: a single-blind clinical trial

This single-blind study compared the efficacy of oral forskolin versus inhaled beclomethasone for mild or moderately persistent adult asthma. Patients were randomly assigned to receive forskolin (one 10-mg capsule orally per day; n = 30) or beclomethasone (two 50 microg inhalations every 12 h; n = 30) for 2 months. No statistically significant improvement occurred in any lung function parameter in the forskolin-treated patients. Subjects in the beclomethasone-treated group presented a slight but statistically significant improvement in percentage forced expiratory volume in 1 s (FEV(1)), percentage forced expiratory flow in the middle (25 - 75%) expiratory phase (FEF(25 - 75%)) and percentage forced vital capacity (FVC) after 2 months of treatment, though the improvement in absolute values for FEV(1), FEF(25 - 75%), FVC and FEV(1):FVC did not reach statistical significance. There was no statistically significant difference between the forskolin and beclomethasone treatment groups for any lung function parameter at baseline or after treatment. None of the beclomethasone-treated patients had an asthma attack and one forskolin-treated patient had a mild asthma attack during the 2-month study period. More studies are needed in adult asthma patients to confirm whether forskolin may be a useful preventive treatment for mild or moderately persistent adult asthma.

Adenylyl cyclases as innovative therapeutic goals

Pharmacological modulation of intracellular cyclic AMP (cAMP) signalling could provide new therapeutic and experimental tools. Although drugs interfering with this pathway have traditionally targeted membrane receptors, the effector enzyme adenylyl cyclase (AC), which functions as a signalling catalyst, also presents an interesting target. Thus, development of isoform-selective stimulator and/or inhibitor compounds for AC could lead to organ-specific pharmacotherapeutics for treating heart failure, cancer and neurodegenerative diseases. In this review, the potential of AC as the object of drug therapy is discussed.

Capturing adenylyl cyclases as potential drug targets

Cyclic AMP (cAMP) is an important intracellular signalling mediator. It is generated in mammals by nine membrane-bound and one soluble adenylyl cyclases (ACs), each with distinct regulation and expression patterns. Although many drugs inhibit or stimulate AC activity through the respective upstream G-protein coupled receptors (for example, opioid or beta-adrenergic receptors), ACs themselves have not been major drug targets. Over the past decade studies on the physiological functions of the different mammalian AC isoforms as well as advances in the development of isoform-selective AC inhibitors and activators suggest that ACs could be useful drug targets. Here we discuss the therapeutic potential of isoform-selective compounds in various clinical settings, including neuropathic pain, neurodegenerative disorders, congestive heart failure, asthma and male contraception.

Functional changes in adenylyl cyclases and associated decreases in relaxation responses in mesenteric arteries from diabetic rats

To assess the functional change in adenylyl cyclases (AC) associated with the diabetic state, we investigated AC-mediated relaxations and cAMP production in mesenteric arteries from rats with streptozotocin (STZ)-induced diabetes. The relaxations induced by the water-soluble forskolin (FSK) analog NKH477, which is a putative AC5 activator, but not by the beta-adrenoceptor agonist isoproterenol (Iso) and the AC activator FSK, were reduced in intact diabetic mesenteric artery. In diabetic rats, however, Iso-, FSK-, and NKH477-induced relaxations were attenuated in the presence of inhibitors of nitric oxide synthase and cyclooxygenase. To exclude the influence of phosphodiesterase (PDE), we also examined the relaxations induced by several AC activators in the presence of 3-isobutyl-1-methylxanthine (IBMX; a PDE inhibitor). Under these conditions, the relaxation induced by Iso was greatly impaired in STZ-diabetic rats. This Iso-induced relaxation was significantly attenuated by pretreatment with SQ-22536, an AC inhibitor, in mesenteric rings from age-matched controls but not in those from STZ-diabetic rats. Under the same conditions, the relaxations induced by FSK or NKH477 were impaired in STZ-diabetic rats. Neither FSK- nor A-23187 (a Ca2+ ionophore)-induced cAMP production was significantly different between diabetics and controls. However, cAMP production induced by Iso or NKH477 was significantly impaired in diabetic mesenteric arteries. Expression of mRNAs and proteins for AC5/6 was lower in diabetic mesenteric arteries than in controls. These results suggest that AC-mediated relaxation is impaired in the STZ-diabetic rat mesenteric artery, perhaps reflecting a reduction in AC5/6 activity.

Antiproliferative effects of NKH477, a forskolin derivative, on cytokine profile in rat lung allografts

OBJECTIVE

NKH477 was recently identified as a water-soluble forskolin derivative and was reported to prolong survival of murine cardiac allografts. However, the mechanism of the efficacy is not clear in vivo. The aim of this study was to investigate the immunosuppressive effects of NKH477 on acute lung allograft rejection in the rat model and its mechanism of action in vivo.

METHODS

Left lungs were transplanted orthotopically from Brown-Norway donors to Lewis recipients. Recipient rats were untreated or treated daily with different doses of NKH477. Grafts were excised on Day 3 or Day 5 to determine histopathological rejection and expressions of interleukin (IL)-2, IL-4, IL-10, and interferon (IFN)-gamma by enzyme-linked immunosorbent assay. The cytokine expression at Day 3 or Day 5 was also evaluated in recipient spleens by immunohistochemistry. Furthermore, mesenteric lymph node cells from recipients at Day 5 were cultured alone or stimulated with donor antigens for 72 hours to determine cell proliferation by means of thymidine incorporation.

RESULTS

NKH477 significantly extended allograft survival time in a dose-dependent manner and reduced histopathological rejection. Treatment with NKH477 inhibited IFN-gamma and IL-10 expression, whereas expression of these cytokines were markedly upregulated in the untreated allografts. Expression of IL-2 and IL-10 also increased in the spleen of untreated allorecipients. NKH477 suppressed expression of both cytokines in the spleen. In addition, lymphocyte proliferation was inhibited in NKH477-treated recipients as compared with untreated recipients.

CONCLUSION

These results suggest that NKH477 exerts an antiproliferative effect on lymphocytes in vivo with an altered cytokine profile in rat recipients of lung allografts.

Direct inhibition of type 5 adenylyl cyclase prevents myocardial apoptosis without functional deterioration

Adenylyl cyclase, a major target enzyme of beta-adrenergic receptor signals, is potently and directly inhibited by P-site inhibitors, classic inhibitors of this enzyme, when the enzyme catalytic activity is high. Unlike beta-adrenergic receptor antagonists, this is a non- or uncompetitive inhibition with respect to ATP. We have examined whether we can utilize this enzymatic property to regulate the effects of beta-adrenergic receptor stimulation differentially. After screening multiple new and classic compounds, we found that some compounds, including 1R,4R-3-(6-aminopurin-9-yl)-cyclopentanecarboxylic acid hydroxyamide, potently inhibited type 5 adenylyl cyclase, the major cardiac isoform, but not other isoforms. In normal mouse cardiac myocytes, contraction induced by low beta-adrenergic receptor stimulation was poorly inhibited with this compound, but the induction of cardiac myocyte apoptosis by high beta-adrenergic receptor stimulation was effectively prevented by type 5 adenylyl cyclase inhibitors. In contrast, when cardiac myocytes from type 5 adenylyl cyclase knock-out mice were examined, beta-adrenergic stimulation poorly induced apoptosis. Our data suggest that the inhibition of beta-adrenergic signaling at the level of the type 5 adenylyl cyclase isoform by P-site inhibitors may serve as an effective method to prevent cardiac myocyte apoptosis induced by excessive beta-adrenergic stimulation without deleterious effect on cardiac myocyte contraction.

Mechanism of cicaprost-induced desensitization in rat pulmonary artery smooth muscle cells involves a PKA-mediated inhibition of adenylyl cyclase

Long-term infusion of prostacyclin, or its analogs, is an effective treatment for severe pulmonary arterial hypertension. However, dose escalation is often required to maintain efficacy. The aim of this study was to investigate the mechanisms of prostacyclin receptor desensitization using the prostacyclin analog cicaprost in rat pulmonary artery smooth muscle cells (PASMCs). Desensitization of the cAMP response occurred in 63 nM cicaprost after a 6-h preincubation with agonist. This desensitization was reversed 12 h after agonist removal, and resensitization was inhibited by 10 microg/ml of cycloheximide. Desensitization was heterologous since desensitization to other G(s)alpha-adenylyl cyclase (AC)-coupled agonists, isoproterenol (1 microM), adrenomedullin (100 nM), or bradykinin (1 microM), was also reduced by preincubation with cicaprost. The reduced cAMP response to prolonged cicaprost exposure appeared to be due to inhibition of AC activity since the responses to the directly acting AC agonist forskolin (3 microM) and the selective AC5 activator NKH-477 were similarly reduced. Expression of AC2 and AC5/6 protein levels transiently decreased after 1 h of cicaprost exposure. The PKA inhibitor H-89 (1 microM) added 1 h before cicaprost preincubation (6 h, 63 nM) completely reversed cicaprost-induced desensitization, whereas the PKC inhibitor bisindolylmaleimide (100 nM) was only partly effective. Desensitization was not prevented by the G(i) inhibitor pertussis toxin. In conclusion, chronic treatment of PASMCs with cicaprost induced heterologous, reversible desensitization by inhibition of AC activity. Our data suggest that heterologous G(s)alpha desensitization by cicaprost is mediated predominantly by a PKA-inhibitable isoform of AC, most likely AC5/6.

A forskolin derivative, colforsin daropate hydrochloride, inhibits rat mesangial cell mitogenesis via the cyclic AMP pathway

A forskolin derivative, colforsin daropate hydrochloride (CDH), has been introduced as an inotropic agent that acts directly on adenylate cyclase to increase intracellular cyclic AMP (cAMP) levels and ventricular contractility, resulting in positive inotropic activity. We investigated the effects of CDH on rat mesangial cell (MC) proliferation. CDH (10(-7)-10(-5) mol/l) inhibited [(3)H]thymidine incorporation into cultured rat MCs in a concentration-dependent manner. CDH (10(-7)-10(-5) mol/l) also decreased cell numbers in a similar manner, and stimulated cAMP accumulation in MCs in a concentration-dependent manner. A protein kinase A inhibitor, H-89, abolished the inhibitory effects of CDH on MC mitogenesis. These findings suggest that CDH would inhibit the proliferation of rat MCs via the cAMP pathway.

Ischemic preconditioning prevents ischemia-induced beta-adrenergic receptor sequestration

Preconditioning enables endogenous protection to repeated myocardial ischemia. However, the effect of preconditioning on beta1 adrenergic receptor (AR) signal remains controversial. We have recently developed receptor assay system using whole cells, in which overexpressed cell surface beta ARs can be readily quantitated without disrupting the cell. Using this technique, we examined the effects of chemical/metabolic ischemia on the beta1 AR sequestration and adenylyl cyclase activity. Isoproterenol treatment, but not forskolin treatment, of HEK293T cells overexpressing beta1 ARs led to a rapid decrease (within 2 hours) in the number of the cell surface receptor, which was negated in the presence of concanavalin A. Similarly, treatment of cells with potassium cyanide and 2-deoxy-D-glucose (chemical/metabolic ischemia) induced similar receptor sequestration. When isoproterenol was superimposed on chemical/metabolic ischemia, the degree of sequestration became greater. However, when cells were pre-exposed to potassium cyanide on the preceding day (chemical preconditioning), the sequestration induced by either isoproterenol or chemical/metabolic ischemia was attenuated. Adenylyl cyclase catalytic activity as assessed by stimulation with forskolin was decreased by chemical/metabolic ischemia but fully recovered after 24 hours, suggesting that chemical/metabolic ischemia treatment did not alter cell viability. Putting together, chemical/metabolic ischemia induced beta1 AR sequestration in a similar manner to isoproterenol. In addition, preconditioning prevented the beta1 AR sequestration induced by both isoproterenol and chemical/metabolic ischemia. Pre-conditioning may play a role in preserving the cell surface beta ARs by inhibiting the sequestration that is usually induced by an ischemic event or beta adrenergic stimulation.

Isoform-specific regulation of adenylyl cyclase: a potential target in future pharmacotherapy

Adenylyl cyclase (AC) is a target enzyme of multiple G-protein-coupled receptors (GPCRs). In the past decade, the cloning, structure and biochemical properties of nine AC isoforms were reported, and each isoform of AC shows distinct patterns of tissue distribution and biochemical/pharmacological properties. In addition to the conventional regulators of this enzyme, such as calmodulin (CaM) or PKC, novel regulators, for example, caveolin, have been identified. Most importantly, these regulators work on AC in an isoform dependent manner. Recent studies have demonstrated that certain classic AC inhibitors, i.e., P-site inhibitors, show an isoform-dependent inhibition of AC. The side chain modifications of forskolin, a diterpene extract from Coleus forskolii, markedly enhance its isoform selectivity. When taken together, these findings suggest that it is feasible to develop new pharmacotherapeutic agents that target AC isoforms to regulate various neurohormonal signals in a highly tissue-/organ-specific manner.

Metal coordination-based inhibitors of adenylyl cyclase: novel potent P-site antagonists

The adenylyl cyclases (ACs) are a family of intracellular enzymes associated with signal transduction by virtue of their ability to convert ATP to cAMP. The catalytic mechanism of this transformation proceeds through initial binding of ATP to the so-called purine binding site (P-site) of the enzyme followed by metal-mediated cyclization with loss of pyrophosphate. Crystallographic analysis of ACs with known inhibitors reveals the presence of two metals in the active site. Presently, nine isoforms of adenylyl cyclase are known, and unique isoform combinations are expressed in a tissue-specific manner. The development of isoform-specific inhibitors of adenylyl cyclase may prove to be a useful strategy toward the design of unique signal transduction inhibitors. To develop novel AC inhibitors, we have chosen an approach to inhibitor design utilizing an adenine ring system joined to a metal-coordinating hydroxamic acid via various linkers. Previous work in our group has validated this approach and identified novel inhibitors that possess an adenine ring joined to a metal-coordinating hydroxamic acid through flexible acyclic linkers (Levy, D. E., et al. Bioorg. Med. Chem. Lett. 2002, 12, 3085-3088). Subsequent studies have focused on the introduction of conformational restrictions into the tether of the inhibitors with the goal of increasing potency (Levy, D. E., et al. Bioorg. Med. Chem. Lett. 2002, 12, 3089-3092). Building upon the favorable spatial positioning of the adenine and hydroxamate groups coupled with potentially favorable entropic factors, the unit joining the carbocycle to the hydroxamate was explored further and a stereochemical-based SAR was elucidated, leading to a new series of highly potent AC inhibitors.

The therapeutic potential of novel cardiotonic agents

During the course of treatment of heart failure patients, cardiotonic agents are inevitable for improvement of myocardial dysfunction. Clinically available agents, such as beta-adrenoceptor agonists and selective phosphodiesterase 3 inhibitors, act mainly via cyclic AMP/protein kinase A-mediated facilitation of Ca(2+) mobilisation (upstream mechanism). These agents are associated with the risk of Ca(2+) overload leading to arrhythmias, myocardial cell injury and premature cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and metabolic effects. Cardiac glycosides act also via an upstream mechanism and readily elicit Ca(2+) overload with a narrow safety margin. No currently available agents act primarily via an increase in the myofilament sensitivity to Ca(2+) ions (central and/or downstream mechanisms). Novel Ca(2+) sensitisers under basic research may deserve clinical trials to examine the therapeutic potential to replace currently employed agents in acute and chronic heart failure patients. Molecular mechanisms of action of Ca(2+) sensitisers are divergent. In addition, they show a wide range of discrete pharmacological profiles due to additional actions associated with individual compounds. Therefore, the outcome of clinical trials has to be explained carefully based on these mechanisms of actions.

Cardiovascular and adenylate cyclase stimulating effects of colforsin daropate, a water-soluble forskolin derivative, compared with those of isoproterenol, dopamine and dobutamine

Colforsin daropate is a recently developed water-soluble derivative of forskolin that directly stimulates adenylate cyclase, unlike the catecholamines. The chronotropic, inotropic and coronary vasodilator actions of colforsin daropate were compared with those of isoproterenol, dopamine and dobutamine, using canine isolated, blood-perfused heart preparations. The stimulating effect of each drug on adenylate cyclase activity was also assessed. Colforsin daropate, as well as each of the catecholamines, exerted positive chronotropic, inotropic and coronary vasodilator actions. The order of selectivity for the cardiovascular variables of colforsin daropate was coronary vasodilation >> positive inotropy > positive chronotropy; whereas that of isoproterenol, dopamine and dobutamine was positive inotropy >> coronary vasodilation > positive chronotropy. Thus, a marked characteristic of colforsin daropate is its potent coronary vasodilator action. On the other hand, each drug significantly increased the adenylate cyclase activity in a dose-related manner: colforsin daropate >> isoproterenol > dopamine = dobutamine. These results suggest that colforsin daropate may be preferable in the treatment of severe heart failure where the coronary blood flow is reduced and beta-adrenoceptor-dependent signal transduction pathway is down-regulated.

Hydroxamate based inhibitors of adenylyl cyclase. Part 1: the effect of acyclic linkers on P-site binding

The adenylyl cyclases (ACs) are a family of enzymes that are key elements of signal transduction by virtue of their ability to convert ATP to cAMP. The catalytic mechanism of this transformation proceeds through initial binding of ATP to the purine binding site (P-site) followed by metal mediated cyclization with loss of pyrophosphate. Crystallographic analysis of ACs with known inhibitors reveals the presence of two metals in the active site. Presently, nine isoforms of adenylyl cyclase are known and unique isoform combinations are expressed in a tissue specific manner. The development of isoform specific inhibitors of adenylyl cyclase may prove to be a useful strategy toward the design of novel therapeutic agents. In order to develop novel AC inhibitors, we have chosen a design approach utilizing molecules with the adenine ring system joined to a metal-coordinating hydroxamic acid via flexible acyclic linkers. The designed inhibitors were assayed against type V AC with the size and heteroatom content of the linkers varied to probe the interaction of the nucleotide and metal binding sites within the enzyme.

Hydroxamate based inhibitors of adenylyl cyclase. Part 2: the effect of cyclic linkers on P-site binding

The adenylyl cyclases (ACs) are a family of enzymes that are key elements of signal transduction by virtue of their ability to convert ATP to cAMP. The catalytic mechanism of this transformation proceeds through initial binding of ATP to the purine binding site (P-site) followed by metal mediated cyclization with loss of pyrophosphate. Previous work in our group identified novel inhibitors which possess an adenine ring joined to a metal-coordinating hydroxamic acid through flexible linkers. Considering the spatial positioning of the metals with respect to the adenine binding site coupled with potentially favorable entropic factors, conformational restriction of the tether through a stereochemistry based SAR employing a rigid cyclic scaffold was explored.

Mechanisms of action of novel cardiotonic agents

Regulation of myocardial contractility by cardiotonic agents is achieved by an increase in intracellular Ca2+ mobilization (upstream mechanism), an increase in Ca2+ binding affinity to troponin C (central mechanism), or facilitation of the process subsequent to Ca2+ binding to troponin C (downstream mechanism). cAMP mediates the regulation induced by Ca2+ mobilizers such as beta-adrenoceptor agonists and selective phosphodiesterase III inhibitors acting through the upstream mechanism. These agents act likewise on the central mechanism to decrease Ca2+ sensitivity of troponin C in association with the cAMP-mediated phosphorylation of troponin I. In addition to such a well-known action of cAMP, recent experimental findings have revealed that Ca2+ sensitizers, such as levosimendan, OR-1896, and UD-CG 212 Cl, require the cAMP-mediated signaling for induction of Ca2+ sensitizing effect. These agents shift the [Ca2+] -force relationship to the left, but their positive inotropic effect (PIE) is inhibited by carbachol, which suppresses selectively the cAMP-mediated PIE. These findings imply that cAMP may play a crucial role in increasing the myofilament Ca2+ sensitivity by cross-talk with the action of individual cardiotonic agents. No clinically available cardiotonic agents act primarily via Ca2+ sensitization, but the PIE of pimobendan and levosimendan is partly mediated by an increase in myofilament Ca2+ sensitivity. Evidence is accumulating that cardiotonic agents with Ca2+ sensitizing action are more effective than agents that act purely via the upstream mechanism in clinical settings. Further clinical trials are required to establish the effectiveness of Ca2+ sensitizers in long-term therapy for congestive heart failure patients.

Type-specific regulation of adenylyl cyclase. Selective pharmacological stimulation and inhibition of adenylyl cyclase isoforms

Crystallographic studies have elucidated the binding mechanism of forskolin and P-site inhibitors to adenylyl cyclase. Accordingly, computer-assisted drug design has enabled us to identify isoform-selective regulators of adenylyl cyclase. After examining more than 200 newly synthesized derivatives of forskolin, we found that the modification at the positions of C6 and C7, in general, enhances isoform selectivity. The 6-(3-dimethylaminopropionyl) modification led to an enhanced selectivity for type V, whereas 6-[N-(2-isothiocyanatoethyl) aminocarbonyl] and 6-(4-acrylbutyryl) modification led to an enhanced selectivity for type II. In contrast, 2'-deoxyadenosine 3'-monophosphate, a classical and 3'-phosphate-substituted P-site inhibitor, demonstrated a 27-fold selectivity for inhibiting type V relative to type II, whereas 9-(tetrahydro-2-furyl) adenine, a ribose-substituted P-site ligand, showed a markedly increased, 130-fold selectivity for inhibiting type V. Consequently, on the basis of the pharmacophore analysis of 9-(tetrahydro-2-furyl) adenine and adenylyl cyclase, a novel non-nucleoside inhibitor, 2-amino-7-(2-furanyl)-7,8-dihydro-5(6H)-quinazolinone (NKY80), was identified after virtual screening of more than 850,000 compounds. NKY80 demonstrated a 210-fold selectivity for inhibiting type V relative to type II. More importantly, the combination of a type III-selective forskolin derivative and 9-(tetrahydro-2-furyl) adenine or NKY80 demonstrated a further enhanced selectivity for type III stimulation over other isoforms. Our data suggest the feasibility of adenylyl cyclase isoform-targeted regulation of cyclic AMP signaling by pharmacological reagents, either alone or in combination.

Characterization of adenylyl cyclase isoforms in rat peripheral pulmonary arteries

Activation of adenylyl cyclase (AC), of which there are 10 diversely regulated isoforms, is important in regulating pulmonary vascular tone and remodeling. Immunohistochemistry in rat lungs demonstrated that AC2, AC3, and AC5/6 predominated in vascular and bronchial smooth muscle. Isoforms 1, 4, 7, and 8 localized to the bronchial epithelium. Exposure of animals to hypoxia did not change the pattern of isoform expression. RT-PCR confirmed mRNA expression of AC2, AC3, AC5, and AC6 and demonstrated AC7 and AC8 transcripts in smooth muscle. Western blotting confirmed the presence of AC2, AC3, and AC5/6 proteins. Functional studies provided evidence of cAMP regulation by Ca(2+) and protein kinase C-activated but not G(i)-inhibited pathways, supporting a role for AC2 and a Ca(2+)-stimulated isoform, AC8. However, NKH-477, an AC5-selective activator, was more potent than forskolin in elevating cAMP and inhibiting serum-stimulated [(3)H]thymidine incorporation, supporting the presence of AC5. These studies demonstrate differential expression of AC isoforms in rat lungs and provide evidence that AC2, AC5, and AC8 are functionally important in cAMP regulation and growth pathways in pulmonary artery myocytes.

Effects of NKH477 on renal nerve stimulation-induced responses in anesthetized dogs

We evaluated the effects of an adenylate cyclase activator, N, N-dimetyl-beta-alanine[3R-(3alpha,4alphabeta,5beta+ ++,6beta,6aalpha, 10alpha,10abeta,10balpha)]-5(acetyloxy)-3-eth enyldodecahydro-10, 10b-dihydroxy-3,4a,7,7,10a-pentamethyl-1-oxo-1H-naphtho[2, 1-b]pyran-6-yl ester hydrochloride (NKH477), on neural control of renal functions in anesthetized dogs. Renal nerve stimulation (2 Hz) increased renal norepinephrine efflux and reduced renal blood flow, glomerular filtration rate, urine flow rate, urinary Na(+) excretion and fractional Na(+) excretion. Intrarenal arterial infusion of NKH477 (300 ng/kg/min) suppressed the stimulation-induced reductions in renal blood flow and glomerular filtration rate and attenuated the reductions in urine flow rate and urinary Na(+) excretion but not the changes in renal norepinephrine efflux and fractional Na(+) excretion. Infusion of NKH477 did not affect the urinary responses induced by renal nerve stimulation at a lower frequency (0.5-1 Hz) which had little influence on renal blood flow and glomerular filtration rate. The present results demonstrate that NKH477 inhibits renal vasoconstriction and hypofiltration but not the enhanced tubular Na(+) reabsorption during activation of the renal sympathetic nervous system.

Effects of NKH477 on renal functions and cyclic AMP production in anesthetized dogs

The present study was undertaken to evaluate the effects of an adenylate cyclase activator N,N-dimethyl-beta-alanine[3R-(3alpha, 4alphabeta, 5beta, 6beta, 6aalpha, 10alpha, 10abeta, 10balpha)]-5(acetyloxy)-3-ethenyldodecahydro-10, 10b-dihydroxy-3, 4a, 7, 7, 10a-pentamethyl-1-oxo-1H-naphtho [2,1-b] pyran-6-yl ester hydrochloride (NKH477) on renal functions and cyclic AMP production in the dog kidney. The intrarenal arterial infusion of NKH477 (30, 100 and 300 ng kg(-1) min(-1)) increased renal blood flow, glomerular filtration rate, urine flow rate, urinary Na+ and cyclic AMP excretion, fractional Na+ excretion and arterial and renal venous plasma cyclic AMP concentrations in a dose-dependent manner. The intrarenal arterial infusion of rolipram (0.3 microg kg(-1) min(-1)), a cyclic AMP-specific phosphodiesterase inhibitor, also caused the same renal responses as NKH477. The increasing effects of NKH477 on renal blood flow, fractional Na+ excretion and renal venous plasma cyclic AMP concentration were facilitated in the presence of rolipram. NKH477 reduced glomerular filtration rate and filtration fraction in the presence of rolipram. The increasing effects of NKH477 on urine flow rate and urinary Na+ excretion were not affected by rolipram. The present results suggest that NKH477 increases glomerular filtration and suppresses tubular sodium reabsorption through activation of cyclic AMP production, and thereby induces natriuresis. The results also demonstrate that renal cyclic AMP level during the activation of adenylate cyclase is regulated by phosphodiesterase IV in both the vascular and tubular sites.

Characterization of positive inotropic effect of colforsin dapropate hydrochloride, a water-soluble forskolin derivative, in isolated adult rat cardiomyocytes

The present study was undertaken to characterize the positive inotropic action of colforsin dapropate hydrochloride (NKH477), a novel water-soluble forskolin derivative, on isolated cardiomyocytes of adult rats. Simultaneous measurements of cellular contraction and intracellular calcium concentration ([Ca2+]i) were carried out. The effects of isoprenaline and ouabain on these parameters were also determined for comparison. The contraction and maximum [Ca2+]i of NKH477-, isoprenaline-, or ouabain-treated cells were increased concentration dependently. Peak shortening of NKH477-treated cells was positively correlated with the shortening velocity and inversely with the time to peak shortening. Maximum, but not minimum, [Ca2+]i in NKH477-treated cells was correlated with the rate of increase in [Ca2+]i and inversely with the time to maximum [Ca2+]i. Similar results were obtained with isoprenaline. In contrast, ouabain increased both maximum and minimum [Ca2+]i. Treatment with either NKH477 or isoprenaline increased cellular cAMP content, but treatment with ouabain did not. These results suggest that the positive inotropic action of NKH477 is associated with an increase in [Ca2+]i and acceleration of its kinetics.Key words: adenylate cyclase, calcium transient, colforsin dapropate, isoprenaline, ouabain.

G protein regulation of adenylate cyclase

Adenylate cyclase integrates positive and negative signals that act through G protein-coupled cell-surface receptors with other extracellular stimuli to finely regulate levels of cAMP within the cell. Recently, the structures of the cyclase catalytic core complexed with the plant diterpene forskolin, and a cyclase-forskolin complex bound to an activated form of the stimulatory G protein subunit Gs alpha have been solved by X-ray crystallography. These structures provide a wealth of detail about how different signals could converge at the core cyclase domains to regulate catalysis. In this article, William Simonds reviews recent advances in the molecular and structural biology of this key regulatory enzyme, which provide new insight into its ability to integrate multiple signals in diverse cellular contexts.