Identification of a β-arrestin-biased negative allosteric modulator for the β2-adrenergic receptor

Catecholamine-stimulated β2-adrenergic receptor (β2AR) signaling via the canonical Gs-adenylyl cyclase-cAMP-PKA pathway regulates numerous physiological functions, including the therapeutic effects of exogenous β-agonists in the treatment of airway disease. β2AR signaling is tightly regulated by GRKs and β-arrestins, which together promote β2AR desensitization and internalization as well as downstream signaling, often antithetical to the canonical pathway. Thus, the ability to bias β2AR signaling toward the Gs pathway while avoiding β-arrestin-mediated effects may provide a strategy to improve the functional consequences of β2AR activation. Since attempts to develop Gs-biased agonists and allosteric modulators for the β2AR have been largely unsuccessful, here we screened small molecule libraries for allosteric modulators that selectively inhibit β-arrestin recruitment to the receptor. This screen identified several compounds that met this profile, and, of these, a difluorophenyl quinazoline (DFPQ) derivative was found to be a selective negative allosteric modulator of β-arrestin recruitment to the β2AR while having no effect on β2AR coupling to Gs. DFPQ effectively inhibits agonist-promoted phosphorylation and internalization of the β2AR and protects against the functional desensitization of β-agonist mediated regulation in cell and tissue models. The effects of DFPQ were also specific to the β2AR with minimal effects on the β1AR. Modeling, mutagenesis, and medicinal chemistry studies support DFPQ derivatives binding to an intracellular membrane-facing region of the β2AR, including residues within transmembrane domains 3 and 4 and intracellular loop 2. DFPQ thus represents a class of biased allosteric modulators that targets an allosteric site of the β2AR.

β2 -Adrenoceptor agonist profiling reveals biased signalling phenotypes for the β2 -adrenoceptor with possible implications for the treatment of asthma

BACKGROUND AND PURPOSE

β-Adrenoceptor agonists relieve airflow obstruction by activating β2 -adrenoceptors, which are G protein-coupled receptors (GPCRs) expressed on human airway smooth muscle (HASM) cells. The currently available β-adrenoceptor agonists are balanced agonists, however, and signal through both the stimulatory G protein (Gs )- and β-arrestin-mediated pathways. While Gs signalling is beneficial and promotes HASM relaxation, β-arrestin activation is associated with reduced Gs efficacy. In this context, biased ligands that selectively promote β2 -adrenoceptor coupling to Gs signalling represent a promising strategy to treat asthma. Here, we examined several β-adrenoceptor agonists to identify Gs -biased ligands devoid of β-arrestin-mediated effects.

EXPERIMENTAL APPROACH

Gs -biased ligands for the β2 -adrenoceptor were identified by high-throughput screening and then evaluated for Gs interaction, Gi interaction, cAMP production, β-arrestin interaction, GPCR kinase (GRK) phosphorylation of the receptor, receptor trafficking, ERK activation, and functional desensitization of the β2 -adrenoceptor.

KEY RESULTS

We identified ractopamine, dobutamine, and higenamine as Gs -biased agonists that activate the Gs /cAMP pathway upon β2 -adrenoceptor stimulation while showing minimal Gi or β-arrestin interaction. Furthermore, these compounds did not induce any receptor trafficking and had reduced GRK5-mediated phosphorylation of the β2 -adrenoceptor. Finally, we observed minimal physiological desensitization of the β2 -adrenoceptor in primary HASM cells upon treatment with biased agonists.

CONCLUSION AND IMPLICATIONS

Our work demonstrates that Gs -biased signalling through the β2 -adrenoceptor may prove to be an effective strategy to promote HASM relaxation in the treatment of asthma. Such biased compounds may also be useful in identifying the molecular mechanisms that determine biased signalling and in design of safer drugs.

Phenylalanine 193 in Extracellular Loop 2 of the β 2-Adrenergic Receptor Coordinates β-Arrestin Interaction

G protein-coupled receptors (GPCRs) transduce a diverse variety of extracellular stimuli into intracellular signaling. These receptors are the most clinically productive drug targets at present. Despite decades of research on the signaling consequences of molecule-receptor interactions, conformational components of receptor-effector interactions remain incompletely described. The β ₂-adrenergic receptor (β ₂AR) is a prototypical and extensively studied GPCR that can provide insight into this aspect of GPCR signaling thanks to robust structural data and rich pharmacopeia. Using bioluminescence resonance energy transfer -based biosensors, second messenger assays, and biochemical techniques, we characterize the properties of β ₂AR-F193A. This single point mutation in extracellular loop 2 of the β ₂AR is sufficient to intrinsically bias the β ₂AR away from β-arrestin interaction and demonstrates altered regulatory outcomes downstream of this functional selectivity. This study highlights the importance of extracellular control of intracellular response to stimuli and suggests a previously undescribed role for the extracellular loops of the receptor and the extracellular pocket formed by transmembrane domains 2, 3, and 7 in GPCR regulation that may contribute to biased signaling at GPCRs. SIGNIFICANCE STATEMENT: The role of extracellular G protein-coupled receptor (GPCR) domains in mediating intracellular interactions is poorly understood. We characterized the effects of extracellular loop mutations on agonist-promoted interactions of GPCRs with G protein and β-arrestin. Our studies reveal that F193 in extracellular loop 2 in the β₂-adrenergic receptor mediates interactions with G protein and β-arrestin with a biased loss of β-arrestin binding. These results provide new insights on the role of the extracellular domain in differentially modulating intracellular interactions with GPCRs.

Characterization of a new WHIM syndrome mutant reveals mechanistic differences in regulation of the chemokine receptor CXCR4

WHIM syndrome is a rare immunodeficiency disorder that is characterized by warts, hypogammaglobulinemia, infections, and myelokathexis. While several gain-of-function mutations that lead to C-terminal truncations, frame shifts and point mutations in the chemokine receptor CXCR4 have been identified in WHIM syndrome patients, the functional effect of these mutations are not fully understood. Here, we report on a new WHIM syndrome mutation that results in a frame shift within the codon for Ser339 (S339fs5) and compare the properties of S339fs5 with wild-type CXCR4 and a previously identified WHIM syndrome mutant, R334X. The S339fs5 and R334X mutants exhibited significantly increased signaling compared to wild-type CXCR4 including agonist-promoted calcium flux and extracellular-signal-regulated kinase activation. This increase is at least partially due to a significant decrease in agonist-promoted phosphorylation, β-arrestin binding, and endocytosis of S339fs5 and R334X compared with wild-type CXCR4. Interestingly, there were also significant differences in receptor degradation, with S339fs5 having a very high basal level of degradation compared with that of R334X and wild-type CXCR4. In contrast to wild-type CXCR4, both R334X and S339fs5 were largely insensitive to CXCL12-promoted degradation. Moreover, while basal and agonist-promoted degradation of wild-type CXCR4 was effectively inhibited by the CXCR4 antagonist TE-14016, this had no effect on the degradation of the WHIM mutants. Taken together, these studies identify a new WHIM syndrome mutant, CXCR4-S339fs5, which promotes enhanced signaling, reduced phosphorylation, β-arrestin binding and endocytosis, and a very high basal rate of degradation that is not protected by antagonist treatment.

Direct impact of gonadotropins on glucose uptake and storage in preovulatory granulosa cells: Implications in the pathogenesis of polycystic ovary syndrome

BACKGROUND

Polycystic ovary syndrome (PCOS) is often associated with higher levels of LH, and arrested ovarian follicular growth. The direct impact of high LH on FSH mediated metabolic responses in PCOS patients is not clearly understood.

METHOD

In order to investigate the impact of FSH and LH on glucose metabolism in preovulatory granulosa cells (GCs), we used [U¹⁴C]-2 deoxyglucose, D-[U¹⁴C]-glucose or 2-NBD glucose to analyse glucose uptake and its incorporation into glycogen. To reproduce the high androgenic potential in PCOS patients, we administered hCG both in vitro and in vivo. The role of IRS-2/PI3K/Akt2 pathway was studied after knockdown with specific siRNA. Immunoprecipitation and specific assays were used for the assessment of IRS-2, glycogen synthase and protein phosphatase 1. Furthermore, we examined the in vivo effects of hCG on FSH mediated glycogen increase in normal and PCOS rat model. HEK293 cells co-expressing FSHR and LHR were used to demonstrate glucose uptake and BRET change by FSH and hCG.

RESULTS

In normal human and rat granulosa cells, FSH is more potent than hCG in stimulating glucose uptake, however glycogen synthesis was significantly upregulated only by FSH through increase in activity of glycogen synthase via IRS-2/PI3K/Akt2 pathway. On the contrary, an impaired FSH-stimulated glucose uptake and glycogen synthesis in granulosa cells of PCOS-patients indicated a selective defect in FSHR activation. Further, in normal human granulosa cells, and in immature rat model, the impact of hCG on FSH responses was such that it inhibited the FSH-mediated glucose uptake as well as glycogen synthesis through inhibition of FSH-stimulated IRS-2 expression. These findings were further validated in HEK293 cells overexpressing Flag-LHR and HA-FSHR, where high hCG inhibited the FSH-stimulated glucose uptake. Notably, an increased BRET change was observed in HEK293 cells expressing FSHR-Rluc8 and LHR-Venus possibly suggesting increased heteromerization of LHR and FSHR in the presence of both hCG and FSH in comparison to FSH or hCG alone.

CONCLUSION

Our findings confirm a selective attenuation of metabolic responses to FSH such as glucose uptake and glycogen synthesis by high activation level of LHR leading to the inhibition of IRS-2 pathway, resulting in depleted glycogen stores and follicular growth arrest in PCOS patients.

Pharmacological Programming of Endosomal Signaling Activated by Small Molecule Ligands of the Follicle Stimulating Hormone Receptor

Follicle-stimulating hormone receptor (FSHR) is a G protein-coupled receptor (GPCR) with pivotal roles in reproduction. One key mechanism dictating the signal activity of GPCRs is membrane trafficking. After binding its hormone FSH, FSHR undergoes internalization to very early endosomes (VEEs) for its acute signaling and sorting to a rapid recycling pathway. The VEE is a heterogeneous compartment containing the Adaptor Protein Phosphotyrosine Interacting with Pleckstrin homology Domain and Leucine Zipper 1 (APPL1) with distinct functions in regulating endosomal Gαs/cAMP signaling and rapid recycling. Low molecular weight (LMW) allosteric FSHR ligands were developed for use in assisted reproductive technology yet could also provide novel pharmacological tools to study FSHR. Given the critical nature of receptor internalization and endosomal signaling for FSHR activity, we assessed whether these compounds exhibit differential abilities to alter receptor endosomal trafficking and signaling within the VEE. Two chemically distinct LMW agonists (benzamide, termed B3 and thiazolidinone, termed T1) were employed. T1 was able to induce a greater level of cAMP than FSH and B3. As cAMP signaling drives gonadotrophin hormone receptor recycling, rapid exocytic events were evaluated at single event resolution. Strikingly, T1 was able to induce a 3-fold increase in recycling events compared to FSH and two-fold more compared to B3. As T1-induced internalization was only marginally greater, the dramatic increase in recycling and cAMP signaling may be due to additional mechanisms. All compounds exhibited a similar requirement for receptor internalization to increase cAMP and proportion of FSHR endosomes with active Gαs, suggesting regulation of cAMP signaling induced by T1 may be altered. APPL1 plays a central role for GPCRs targeted to the VEE, and indeed, loss of APPL1 inhibited FSH-induced recycling and increased endosomal cAMP signaling. While T1-induced FSHR recycling was APPL1-dependent, its elevated cAMP signaling was only partially increased following APPL1 knockdown. Unexpectedly, B3 altered the dependence of FSHR to APPL1 in an opposing manner, whereby its endosomal signaling was negatively regulated by APPL1, while B3-induced FSHR recycling was APPL1-independent. Overall, FSHR allosteric compounds have the potential to re-program FSHR activity via altering engagement with VEE machinery and also suggests that these two distinct functions of APPL1 can potentially be selected pharmacologically.

Membrane Estrogen Receptor (GPER) and Follicle-Stimulating Hormone Receptor (FSHR) Heteromeric Complexes Promote Human Ovarian Follicle Survival

Classically, follicle-stimulating hormone receptor (FSHR)-driven cAMP-mediated signaling boosts human ovarian follicle growth and oocyte maturation. However, contradicting in vitro data suggest a different view on physiological significance of FSHR-mediated cAMP signaling. We found that the G-protein-coupled estrogen receptor (GPER) heteromerizes with FSHR, reprogramming cAMP/death signals into proliferative stimuli fundamental for sustaining oocyte survival. In human granulosa cells, survival signals are missing at high FSHR:GPER ratio, which negatively impacts follicle maturation and strongly correlates with preferential Gαs protein/cAMP-pathway coupling and FSH responsiveness of patients undergoing controlled ovarian stimulation. In contrast, FSHR/GPER heteromers triggered anti-apoptotic/proliferative FSH signaling delivered via the Gβγ dimer, whereas impairment of heteromer formation or GPER knockdown enhanced the FSH-dependent cell death and steroidogenesis. Therefore, our findings indicate how oocyte maturation depends on the capability of GPER to shape FSHR selective signals, indicating hormone receptor heteromers may be a marker of cell proliferation.

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G-protein-coupled estrogen receptor (GPER) interacts with FSH receptor (FSHR)

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FSHR/GPER heteromers reprogram FSH-induced death signals to proliferative stimuli

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Anti-apoptotic signaling of heteromers is via a GPER-Gαs inhibitory complex and Gβγ

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Heteromer formation impacts follicle maturation and FSH responses of IVF patients

Glycosylation Pattern and in vitro Bioactivity of Reference Follitropin alfa and Biosimilars

Recombinant follicle-stimulating hormone (FSH) (follitropin alfa) and biosimilar preparations are available for clinical use. They have specific FSH activity and a unique glycosylation profile dependent on source cells. The aim of the study is to compare the originator (reference) follitropin alfa (Gonal-f®)- with biosimilar preparations (Bemfola® and Ovaleap®)-induced cellular responses in vitro. Gonadotropin N-glycosylation profiles were analyzed by ELISA lectin assay, revealing preparation specific-patterns of glycan species (Kruskal-Wallis test; p < 0.05, n = 6) and by glycotope mapping. Increasing concentrations of Gonal-f® or biosimilar (1 × 10^-3-1 × 10³ ng/ml) were used for treating human primary granulosa lutein cells (hGLC) and FSH receptor (FSHR)-transfected HEK293 cells in vitro. Intracellular cAMP production, Ca²⁺ increase and β-arrestin 2 recruitment were evaluated by BRET, CREB, and ERK1/2 phosphorylation by Western blotting. 12-h gene expression, and 8- and 24-h progesterone and estradiol synthesis were measured by real-time PCR and immunoassay, respectively. We found preparation-specific glycosylation patterns by lectin assay (Kruskal-Wallis test; p < 0.001; n = 6), and similar cAMP production and β-arrestin 2 recruitment in FSHR-transfected HEK293 cells (cAMP EC₅₀ range = 12 ± 0.9-24 ± 1.7 ng/ml; β-arrestin 2 EC₅₀ range = 140 ± 14.1-313 ± 18.7 ng/ml; Kruskal-Wallis test; p ≥ 0.05; n = 4). Kinetics analysis revealed that intracellular Ca²⁺ increased upon cell treatment by 4 μg/ml Gonal-f®, while equal concentrations of biosimilars failed to induced a response (Kruskal-Wallis test; p < 0.05; n = 3). All preparations induced both 8 and 24 h-progesterone and estradiol synthesis in hGLC, while no different EC₅₀s were demonstrated (Kruskal-Wallis test; p > 0.05; n = 5). Apart from preparation-specific intracellular Ca²⁺ increases achieved at supra-physiological hormone doses, all compounds induced similar intracellular responses and steroidogenesis, reflecting similar bioactivity, and overall structural homogeneity.

Biased Signaling and Allosteric Modulation at the FSHR

Knowledge on G protein-coupled receptor (GPCRs) structure and mechanism of activation has profoundly evolved over the past years. The way drugs targeting this family of receptors are discovered and used has also changed. Ligands appear to bind a growing number of GPCRs in a competitive or allosteric manner to elicit balanced signaling or biased signaling (i.e., differential efficacy in activating or inhibiting selective signaling pathway(s) compared to the reference ligand). These novel concepts and developments transform our understanding of the follicle-stimulating hormone (FSH) receptor (FSHR) biology and the way it could be pharmacologically modulated in the future. The FSHR is expressed in somatic cells of the gonads and plays a major role in reproduction. When compared to classical GPCRs, the FSHR exhibits intrinsic peculiarities, such as a very large NH2-terminal extracellular domain that binds a naturally heterogeneous, large heterodimeric glycoprotein, namely FSH. Once activated, the FSHR couples to Gαs and, in some instances, to other Gα subunits. G protein-coupled receptor kinases and β-arrestins are also recruited to this receptor and account for its desensitization, trafficking, and intracellular signaling. Different classes of pharmacological tools capable of biasing FSHR signaling have been reported and open promising prospects both in basic research and for therapeutic applications. Here we provide an updated review of the most salient peculiarities of FSHR signaling and its selective modulation.

Workflow Description to Dynamically Model β-Arrestin Signaling Networks

Dynamic models of signaling networks allow the formulation of hypotheses on the topology and kinetic rate laws characterizing a given molecular network, in-depth exploration, and confrontation with kinetic biological data. Despite its standardization, dynamic modeling of signaling networks still requires successive technical steps that need to be carefully performed. Here, we detail these steps by going through the mathematical and statistical framework. We explain how it can be applied to the understanding of β-arrestin-dependent signaling networks. We illustrate our methodology through the modeling of β-arrestin recruitment kinetics at the follicle-stimulating hormone (FSH) receptor supported by in-house bioluminescence resonance energy transfer (BRET) data.

Inferring biallelism of two FSH receptor mutations associated with spontaneous ovarian hyperstimulation syndrome by evaluating FSH, LH and HCG cross-activity

RESEARCH QUESTION

What is the cumulative effect of two follicle-stimulating hormone receptor (FSHR) mutations in spontaneous ovarian hyperstimulation syndrome (sOHSS) pathogenesis? Are these mutations in the mono- or biallelic state?

DESIGN

Two FSHR mutations were found in a pregnant patient affected by sOHSS with no predisposing conditions. While the p.Asn106His mutation is novel, the p.Ser128Tyr mutation has been associated with sOHSS previously. The patient's FSHR gene was analysed by Sanger sequencing, and FSHR cDNAs carrying a single or both point mutations were created by mutagenesis in vitro. cAMP activation by recombinant FSH, luteinizing hormone (LH), human chorionic gonadotropin (HCG) and thyroid-stimulating hormone (TSH) was evaluated in transfected HEK293 cells by bioluminescence resonance energy transfer.

RESULTS

All mutations decreased the 50% effective concentration of FSH calculated for cAMP (P < 0.05, n = 6), resulting in two- to 10-fold lower ligand potency. TSH failed to induce an FSHR-mediated increase in intracellular cAMP, while LH was approximately four-fold more potent than HCG in p.Ser128Tyr FSHR-expressing HEK293 cells despite lower cAMP plateau levels (P < 0.05, n = 5). The p.Ser128Tyr FSHR mutation was found to be responsible for an LH-/HCG-induced increase in cAMP when it was in the biallelic heterozygous state with p.Asn106His, but no increase in cAMP was induced in the monoallelic state.

CONCLUSION

In-vitro data support that, in pregnant patients with sOHSS, the two FSHR mutations have an opposing effect on the pathogenesis of sOHSS and are in the biallelic heterozygous form, allowing HCG to induce a p.Ser128Tyr FSHR-mediated increase in cAMP.

β-arrestins and biased signaling in gonadotropin receptors

Gonadotropin receptors include the follicle stimulating hormone receptor (FSHR) and the luteinizing hormone/choriogonadotropin receptor (LHCGR), both belong to the G protein-coupled receptor (GPCR) superfamily and are essential to reproduction. FSHR is activated by follicle stimulating hormone (FSH) while LHCGR is activated by either luteinizing hormone (LH) or choriogonadotropin (CG). Upon ligand binding, gonadotropin receptors undergo conformational changes that lead to the activation of the heterotrimeric G protein, resulting in the production of different second messengers. Gonadotropin receptors can also recruit and bind β-arrestins. This particular class of scaffold proteins were initially identified to mediate GPCRs desensitization and recycling, but it is now well established that β-arrestins can also initiate Gs-independent signaling by assembling signaling modules. Furthermore, new advances in structural biology and biophysical techniques have revealed novel activation mechanisms allowing β-arrestins and G proteins to control signaling in time and space. The ability of different ligands to preferentially elicit G- or β-arrestin-mediated signaling is known as functional selectivity or biased signaling. This new concept has switched the view of pharmacology efficacy from monodimensional to multidimensional. Biased signaling offers the possibility to separate therapeutic benefits of a drug from its adverse effects. The proof of concept that gonadotropin receptors can be subjected to biased signaling is now established. The challenge will now be the design of molecules that can specifically activate beneficial signaling pathway at gonadotropin receptors while reducing or abolishing those leading to side effects. Such strategy could for instance lead to improved treatments for infertility.

β-arrestin signalling and bias in hormone-responsive GPCRs

G protein-coupled receptors (GPCRs) play crucial roles in the ability of target organs to respond to hormonal cues. GPCRs' activation mechanisms have long been considered as a two-state process connecting the agonist-bound receptor to heterotrimeric G proteins. This view is now challenged as mounting evidence point to GPCRs being connected to large arrays of transduction mechanisms involving heterotrimeric G proteins as well as other players. Amongst the G protein-independent transduction mechanisms, those elicited by β-arrestins upon their recruitment to the active receptors are by far the best characterized and apply to most GPCRs. These concepts, in conjunction with remarkable advances made in the field of GPCR structural biology and biophysics, have supported the notion of ligand-selective signalling also known as pharmacological bias. Interestingly, recent reports have opened intriguing prospects to the way β-arrestins control GPCR-mediated signalling in space and time within the cells. In the present paper, we review the existing evidence linking endocrine-related GPCRs to β-arrestin recruitement, signalling, pathophysiological implications and selective activation by biased ligands and/or receptor modifications. Emerging concepts surrounding β-arrestin-mediated transduction are discussed in the light of the peculiarities of endocrine systems.

Human LH and hCG stimulate differently the early signalling pathways but result in equal testosterone synthesis in mouse Leydig cells in vitro

BACKGROUND

Human luteinizing hormone (LH) and chorionic gonadotropin (hCG) are glycoprotein hormones regulating development and reproductive functions by acting on the same receptor (LHCGR). We compared the LH and hCG activity in gonadal cells from male mouse in vitro, i.e. primary Leydig cells, which is a common tool used for gonadotropin bioassay. Murine Leydig cells are naturally expressing the murine LH receptor (mLhr), which binds human LH/hCG.

METHODS

Cultured Leydig cells were treated by increasing doses of recombinant LH and hCG, and cell signaling, gene expression and steroid synthesis were evaluated.

RESULTS

We found that hCG is about 10-fold more potent than LH in cAMP recruitment, and slightly but significantly more potent on cAMP-dependent Erk1/2 phosphorylation. However, no significant differences occur between LH and hCG treatments, measured as activation of downstream signals, such as Creb phosphorylation, Stard1 gene expression and testosterone synthesis.

CONCLUSIONS

These data demonstrate that the responses to human LH/hCG are only quantitatively and not qualitatively different in murine cells, at least in terms of cAMP and Erk1/2 activation, and equal in activating downstream steroidogenic events. This is at odds with what we previously described in human primary granulosa cells, where LHCGR mediates a different pattern of signaling cascades, depending on the natural ligand. This finding is relevant for gonadotropin quantification used in the official pharmacopoeia, which are based on murine, in vivo bioassay and rely on the evaluation of long-term, testosterone-dependent effects mediated by rodent receptor.

Follicle-stimulating hormone potentiates the steroidogenic activity of chorionic gonadotropin and the anti-apoptotic activity of luteinizing hormone in human granulosa-lutein cells in vitro

Luteinizing hormone (LH) and choriogonadotropin (hCG) are glycoprotein hormones regulating ovarian function and pregnancy, respectively. Since these molecules act on the same receptor (LHCGR), they were traditionally assumed as equivalent in assisted reproduction techniques (ART), although differences between LH and hCG were demonstrated at molecular and physiological level. In this study, we demonstrated for the first time that co-treatment with a follicle-stimulating hormone (FSH) dose in the ART therapeutic range potentiates different LH- and hCG-dependent responses in vitro, measured in terms of cAMP, phospho-CREB, -ERK1/2 and -AKT activation, gene expression, progesterone and estradiol production in human granulosa-lutein cells (hGLC). We show that in the presence of FSH, hCG biopotency is about 5-fold increased, in the presence of FSH, in terms of cAMP activation. Accordingly, CREB phosphorylation and steroid production is increased under hCG and FSH co-treatment. LH effects, evaluated as steroidogenic cAMP/PKA pathway activation, do not change in the presence of FSH, which, however, increases LH-dependent ERK1/2 and AKT, but not CREB phosphorylation, resulting in anti-apoptotic effects. The different modulatory activity of FSH on LH and hCG action in vitro corresponds to their different physiological functions, reflecting proliferative effects exerted by LH during the follicular phase and before trophoblast development, and the high steroidogenic potential of hCG requested to sustain pregnancy from the luteal phase onwards.

Depletion of NADP(H) due to CD38 activation triggers endothelial dysfunction in the postischemic heart

In the postischemic heart, coronary vasodilation is impaired due to loss of endothelial nitric oxide synthase (eNOS) function. Although the eNOS cofactor tetrahydrobiopterin (BH4) is depleted, its repletion only partially restores eNOS-mediated coronary vasodilation, indicating that other critical factors trigger endothelial dysfunction. Therefore, studies were performed to characterize the unidentified factor(s) that trigger endothelial dysfunction in the postischemic heart. We observed that depletion of the eNOS substrate NADPH occurs in the postischemic heart with near total depletion from the endothelium, triggering impaired eNOS function and limiting BH4 rescue through NADPH-dependent salvage pathways. In isolated rat hearts subjected to 30 min of ischemia and reperfusion (I/R), depletion of the NADP(H) pool occurred and was most marked in the endothelium, with >85% depletion. Repletion of NADPH after I/R increased NOS-dependent coronary flow well above that with BH4 alone. With combined NADPH and BH4 repletion, full restoration of NOS-dependent coronary flow occurred. Profound endothelial NADPH depletion was identified to be due to marked activation of the NAD(P)ase-activity of CD38 and could be prevented by inhibition or specific knockdown of this protein. Depletion of the NADPH precursor, NADP(+), coincided with formation of 2'-phospho-ADP ribose, a CD38-derived signaling molecule. Inhibition of CD38 prevented NADP(H) depletion and preserved endothelium-dependent relaxation and NO generation with increased recovery of contractile function and decreased infarction in the postischemic heart. Thus, CD38 activation is an important cause of postischemic endothelial dysfunction and presents a novel therapeutic target for prevention of this dysfunction in unstable coronary syndromes.

Reversal of SIN-1-induced eNOS dysfunction by the spin trap, DMPO, in bovine aortic endothelial cells via eNOS phosphorylation

BACKGROUND AND PURPOSE

Nitric oxide (NO) derived from eNOS is mostly responsible for the maintenance of vascular homeostasis and its decreased bioavailability is characteristic of reactive oxygen species (ROS)-induced endothelial dysfunction (ED). Because 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), a commonly used spin trap, can control intracellular nitroso-redox balance by scavenging ROS and donating NO, it was employed as a cardioprotective agent against ED but the mechanism of its protection is still not clear. This study elucidated the mechanism of protection by DMPO against SIN-1-induced oxidative injury to bovine aortic endothelial cells (BAEC).

EXPERIMENTAL APPROACH

BAEC were treated with SIN-1, as a source of peroxynitrite anion (ONOO⁻), and then incubated with DMPO. Cytotoxicity following SIN-1 alone and cytoprotection by adding DMPO was assessed by MTT assay. Levels of ROS and NO generation from HEK293 cells transfected with wild-type and mutant eNOS cDNAs, tetrahydrobiopterin bioavailability, eNOS activity, eNOS and Akt kinase phosphorylation were measured.

KEY RESULTS

Post-treatment of cells with DMPO attenuated SIN-1-mediated cytotoxicity and ROS generation, restoration of NO levels via increased in eNOS activity and phospho-eNOS levels. Treatment with DMPO alone significantly increased NO levels and induced phosphorylation of eNOS Ser¹¹⁷⁹ via Akt kinase. Transfection studies with wild-type and mutant human eNOS confirmed the dual role of eNOS as a producer of superoxide anion (O₂⁻) with SIN-1 treatment, and a producer of NO in the presence of DMPO.

CONCLUSION AND IMPLICATIONS

Post-treatment with DMPO of oxidatively challenged cells reversed eNOS dysfunction and could have pharmacological implications in the treatment of cardiovascular diseases.

Hypoxia and reoxygenation induce endothelial nitric oxide synthase uncoupling in endothelial cells through tetrahydrobiopterin depletion and S-glutathionylation

Ischemia-reperfusion injury is accompanied by endothelial hypoxia and reoxygenation that trigger oxidative stress with enhanced superoxide generation and diminished nitric oxide (NO) production leading to endothelial dysfunction. Oxidative depletion of the endothelial NO synthase (eNOS) cofactor tetrahydrobiopterin can trigger eNOS uncoupling, in which the enzyme generates superoxide rather than NO. Recently, it has also been shown that oxidative stress can induce eNOS S-glutathionylation at critical cysteine residues of the reductase site that serves as a redox switch to control eNOS coupling. While superoxide can deplete tetrahydrobiopterin and induce eNOS S-glutathionylation, the extent of and interaction between these processes in the pathogenesis of eNOS dysfunction in endothelial cells following hypoxia and reoxygenation remain unknown. Therefore, studies were performed on endothelial cells subjected to hypoxia and reoxygenation to determine the severity of eNOS uncoupling and the role of cofactor depletion and S-glutathionylation in this process. Hypoxia and reoxygenation of aortic endothelial cells triggered xanthine oxidase-mediated superoxide generation, causing both tetrahydrobiopterin depletion and S-glutathionylation with resultant eNOS uncoupling. Replenishing cells with tetrahydrobiopterin along with increasing intracellular levels of glutathione greatly preserved eNOS activity after hypoxia and reoxygenation, while targeting either mechanism alone only partially ameliorated the decrease in NO. Endothelial oxidative stress, secondary to hypoxia and reoxygenation, uncoupled eNOS with an altered ratio of oxidized to reduced glutathione inducing eNOS S-glutathionylation. These mechanisms triggered by oxidative stress combine to cause eNOS dysfunction with shift of the enzyme from NO to superoxide production. Thus, in endothelial reoxygenation injury, normalization of both tetrahydrobiopterin levels and the glutathione pool are needed for maximal restoration of eNOS function and NO generation.

Redox modulation of endothelial nitric oxide synthase by glutaredoxin-1 through reversible oxidative post-translational modification

S-Glutathionylation is a redox-regulated modification that uncouples endothelial nitric oxide synthase (eNOS), switching its function from nitric oxide (NO) synthesis to (•)O2(-) generation, and serves to regulate vascular function. While in vitro or in vivo eNOS S-glutathionylation with modification of Cys689 and Cys908 of its reductase domain is triggered by high levels of glutathione disulfide (GSSG) or oxidative thiyl radical formation, it remains unclear how this process may be reversed. Glutaredoxin-1 (Grx1), a cytosolic and glutathione-dependent enzyme, can reverse protein S-glutathionylation; however, its role in regulating eNOS S-glutathionylation remains unknown. We demonstrate that Grx1 in the presence of glutathione (GSH) (1 mM) reverses GSSG-mediated eNOS S-glutathionylation with restoration of NO synthase activity. Because Grx1 also catalyzes protein S-glutathionylation with an increased [GSSG]/[GSH] ratio, we measured its effect on eNOS S-glutathionylation when the [GSSG]/[GSH] ratio was >0.2, which can occur in cells and tissues under oxidative stress, and observed an increased level of eNOS S-glutathionylation with a marked decrease in eNOS activity without uncoupling. This eNOS S-glutathionylation was reversed with a decrease in the [GSSG]/[GSH] ratio to <0.1. Liquid chromatography and tandem mass spectrometry identified a new site of eNOS S-glutathionylation by Grx1 at Cys382, on the surface of the oxygenase domain, without modification of Cys689 or Cys908, each of which is buried within the reductase. Furthermore, Grx1 was demonstrated to be a protein partner of eNOS in vitro and in normal endothelial cells, supporting its role in eNOS redox regulation. In endothelial cells, Grx1 inhibition or gene silencing increased the level of eNOS S-glutathionylation and decreased the level of cellular NO generation. Thus, Grx1 can exert an important role in the redox regulation of eNOS in cells.

Tetrathiatriarylmethyl radical with a single aromatic hydrogen as a highly sensitive and specific superoxide probe

Superoxide (O(2)(•-)) plays crucial roles in normal physiology and disease; however, its measurement remains challenging because of the limited sensitivity and/or specificity of prior detection methods. We demonstrate that a tetrathiatriarylmethyl (TAM) radical with a single aromatic hydrogen (CT02-H) can serve as a highly sensitive and specific O(2)(•-) probe. CT02-H is an analogue of the fully substituted TAM radical CT-03 (Finland trityl) with an electron paramagnetic resonance (EPR) doublet signal due to its aromatic hydrogen. Owing to the neutral nature and negligible steric hindrance of the hydrogen, O(2)(•-) preferentially reacts with CT02-H at this site with production of the diamagnetic quinone methide via oxidative dehydrogenation. Upon reaction with O(2)(•-), CT02-H loses its EPR signal and this EPR signal decay can be used to quantitatively measure O(2)(•-). This is accompanied by a change in color from green to purple, with the quinone methide product exhibiting a unique UV-Vis absorbance (ε=15,900 M(-1) cm(-1)) at 540 nm, providing an additional O(2)(•-) detection method. More than five-fold higher reactivity of CT02-H for O(2)(•-) relative to CT-03 was demonstrated, with a second-order rate constant of 1.7×10(4) M(-1) s(-1) compared to 3.1×10(3) M(-1) s(-1) for CT-03. CT02-H exhibited high specificity for O(2)(•-) as evidenced by its inertness to other oxidoreductants. The O(2)(•-) generation rates detected by CT02-H from xanthine/xanthine oxidase were consistent with those measured by cytochrome c reduction but detection sensitivity was 10- to 100-fold higher. EPR detection of CT02-H enabled measurement of very low O(2)(•-) flux with a detection limit of 0.34 nM/min over 120 min. HPLC in tandem with electrochemical detection was used to quantitatively detect the stable quinone methide product and is a highly sensitive and specific method for measurement of O(2)(•-), with a sensitivity limit of ~2×10(-13) mol (10 nM with 20-μl injection volume). Based on the O(2)-dependent linewidth broadening of its EPR spectrum, CT02-H also enables simultaneous measurement of O(2) concentration and O(2)(•-) generation and was shown to provide sensitive detection of extracellular O(2)(•-) generation in endothelial cells stimulated either by menadione or with anoxia/reoxygenation. Thus, CT02-H is a unique probe that provides very high sensitivity and specificity for measurement of O(2)(•-) by either EPR or HPLC methods.

HPLC analysis of tetrahydrobiopterin and its pteridine derivatives using sequential electrochemical and fluorimetric detection: application to tetrahydrobiopterin autoxidation and chemical oxidation

Tetrahydrobiopterin (BH(4)) is an essential cofactor of endothelial nitric oxide (NO) synthase and when depleted, endothelial dysfunction results with decreased production of NO. BH(4) is also an anti-oxidant being a good "scavenger" of oxidative species. NADPH oxidase, xanthine oxidase, and mitochondrial enzymes producing reactive oxygen species (ROS) can induce elevated oxidant stress and cause BH(4) oxidation and subsequent decrease in NO production and bioavailability. In order to define the process of ROS-mediated BH(4) degradation, a sensitive method for monitoring pteridine redox-state changes is required. Considering that the conventional fluorescence method is an indirect method requiring conversion of all pteridines to oxidized forms, it would be beneficial to use a rapid quantitative assay for the individual detection of BH(4) and its related pteridine metabolites. To study, in detail, the BH(4) oxidative pathways, a rapid direct sensitive HPLC assay of BH(4) and its pteridine derivatives was adapted using sequential electrochemical and fluorimetric detection. We examined BH(4) autoxidation, hydrogen peroxide- and superoxide-driven oxidation, and Fenton reaction hydroxyl radical-driven BH(4) transformation. We demonstrate that the formation of the primary two-electron oxidation product, dihydrobiopterin (BH(2)), predominates with oxygen-induced BH(4) autoxidation and superoxide-catalyzed oxidation, while the irreversible metabolites, pterin and dihydroxanthopterin (XH(2)), are largely produced during hydroxyl radical-driven BH(4) oxidation.

Development of a pharmacologically improved peptide agonist of the leptin receptor

Leptin, a hormone produced by adipose tissue, regulates energy balance in the hypothalamus and is involved in fertility, immune response and carcinogenesis. The existence of disorders related to leptin deficit and leptin overabundance calls for the development of drugs activating or inhibiting the leptin receptor (ObR). We synthesized four proposed receptor-binding leptin fragments (sites I, IIa and IIb, III), their reportedly antagonist analogs, and a peptide chimera composed of the two discontinuous site II arms. To assess the pharmacological utility of leptin fragments, we studied the peptides' ability to stimulate the growth of ObR-positive and ObR-negative cells. The combined site II construct and site III derivatives selectively reversed leptin-induced growth of ObR-positive cells at mid-nanomolar concentrations. However, these peptides appeared to be partial agonists/antagonists as they activated cell growth in the absence of exogenous leptin. A designer site III analog, featuring non-natural amino acids at terminal positions to decrease proteolysis and a blood-brain barrier (BBB) penetration-enhancing carbohydrate moiety, proved to be full agonist to ObR, i.e., stimulated proliferation of different ObR-positive but not ObR-negative cells in the presence or absence of leptin. This glycopeptide bound to isolated ObR on solid-phase assays and activated ERK-1/2 signaling in ObR-positive MCF-7 cells at 100-500 nM concentrations. The glycopeptide was stable in mouse serum, readily crossed endothelial/astrocyte cell layers in a cellular BBB model, and was distributed into the brain of Balb/c mice after intraperitoneal administration. These characteristics suggest a potential pharmaceutical utility of the designer site III glycopeptide in leptin-deficient diseases.

Interplay between the retinoblastoma related pRb2/p130 and E2F-4 and -5 in relation to JCV-TAg

Human polyomaviruses, which include JC virus (JCV) and BK virus (BKV), as well as the simian virus 40 (SV40), have been associated with human tumors and have been shown to be highly tumorigenic in experimental animal models. Although the mechanism by which JCV induces tumorigenesis is not entirely clear, earlier studies point to the involvement of the viral early protein T-antigen which has the ability to bind and inactivate tumor suppressors and cell cycle regulatory proteins, such as the retinoblastoma family proteins and p53. We investigated if the distribution between nucleus and cytoplasm of the transcription factors E2F4 and E2F5 is mediated by pRb2/p130 and if the presence of JCV T-antigen may impair this shuttling by sequestering pRb2/p130. The results showed that E2F4 was prevalently localized in the nucleus of both T-antigen positive and -negative R503 cells independently of the cell cycle phase. E2F5 instead was prevalently localized in the cytoplasmic fraction in G(0)/G(1), S-phase synchronized, and asynchronous R503 and R503 T-Ag positive cells. The presence of T-antigen did not influence the subcellular localization of these transcription factors E2F4 and E2F5, at least in this murine cellular model. Moreover, Small interference RNA experiments directed toward silencing the Rb2/p130 gene demonstrated that pRb2/p130 does not play a predominant role in the nuclear transportation of E2F4 and E2F5.