AT1R-CB1R heteromerization reveals a new mechanism for the pathogenic properties of angiotensin II.

The mechanism of G protein-coupled receptor (GPCR) signal integration is controversial. While GPCR assembly into hetero-oligomers facilitates signal integration of different receptor types, cross-talk between Gαi- and Gαq-coupled receptors is often thought to be oligomerization independent. In this study, we examined the mechanism of signal integration between the Gαi-coupled type I cannabinoid receptor (CB(1)R) and the Gαq-coupled AT1R. We find that these two receptors functionally interact, resulting in the potentiation of AT1R signalling and coupling of AT1R to multiple G proteins. Importantly, using several methods, that is, co-immunoprecipitation and resonance energy transfer assays, as well as receptor- and heteromer-selective antibodies, we show that AT1R and CB(1)R form receptor heteromers. We examined the physiological relevance of this interaction in hepatic stellate cells from ethanol-administered rats in which CB(1)R is upregulated. We found a significant upregulation of AT1R-CB(1)R heteromers and enhancement of angiotensin II-mediated signalling, as compared with cells from control animals. Moreover, blocking CB(1)R activity prevented angiotensin II-mediated mitogenic signalling and profibrogenic gene expression. These results provide a molecular basis for the pivotal role of heteromer-dependent signal integration in pathology.

XTX301, a Tumor-Activated Interleukin-12 Has the Potential to Widen the Therapeutic Index of IL12 Treatment for Solid Tumors as Evidenced by Preclinical Studies.

IL12 is a proinflammatory cytokine, that has shown promising antitumor activity in humans by promoting the recruitment and activation of immune cells in tumors. However, the systemic administration of IL12 has been accompanied by considerable toxicity, prompting interest in researching alternatives to drive preferential IL12 bioactivity in the tumor. Here, we have generated XTX301, a tumor-activated IL12 linked to the human Fc protein via a protease cleavable linker that is pharmacologically inactivated by an IL12 receptor subunit beta 2 masking domain. In vitro characterization demonstrates multiple matrix metalloproteases, as well as human primary tumors cultured as cell suspensions, can effectively activate XTX301. Intravenous administration of a mouse surrogate mXTX301 demonstrated significant tumor growth inhibition (TGI) in inflamed and non-inflamed mouse models without causing systemic toxicities. The superiority of mXTX301 in mediating TGI compared with non-activatable control molecules and the greater percentage of active mXTX301 in tumors versus other organs further confirms activation by the tumor microenvironment-associated proteases in vivo. Pharmacodynamic characterization shows tumor selective increases in inflammation and upregulation of immune-related genes involved in IFNγ cell signaling, antigen processing, presentation, and adaptive immune response. XTX301 was tolerated following four repeat doses up to 2.0 mg/kg in a nonhuman primate study; XTX301 exposures were substantially higher than those at the minimally efficacious dose in mice. Thus, XTX301 has the potential to achieve potent antitumor activity while widening the therapeutic index of IL12 treatment and is currently being evaluated in a phase I clinical trial.

Type I cannabinoid receptor trafficking: all roads lead to lysosome.

The majority of G-protein-coupled receptors (GPCRs) function at the cell surface, where they are activated by their ligands present in the extracellular milieu. Interestingly, type I cannabinoid receptor (CB(1) R), one of the most abundant GPCRs in the central nervous system, is predominantly intracellular. The important physiological roles of CB(1) R have sparked interest in the elucidation of the molecular mechanisms underlying the trafficking of this receptor and the role of intracellular CB(1) Rs. Thus far, results from different groups have been, at least in part, contradictory and the basis of CB(1) R intracellular localization remains controversial. In this commentary, by comparing the studies examining CB(1) R trafficking and localization, we identify technical or experimental ground responsible for the conflicting results. Finally, we propose a possible mechanism of CB(1) R trafficking that may reconcile the different models.

Endoplasmic reticulum stress induces fibrogenic activity in hepatic stellate cells through autophagy.

BACKGROUND & AIMS: Metabolic stress during liver injury enhances autophagy and provokes stellate cell activation, with secretion of scar matrix. Conditions that augment protein synthesis increase demands on the endoplasmic reticulum (ER) folding capacity and trigger the unfolded protein response (UPR) to cope with resulting ER stress. Generation of reactive oxygen species (ROS) is a common feature of hepatic fibrogenesis, and crosstalk between oxidant stress and ER stress has been proposed. The aim of our study was to determine the impact of oxidant and ER stress on stellate cell activation. METHODS: Oxidant stress was induced in hepatic stellate cells using H2O2 in culture or by ethanol feeding in vivo, and the UPR was analyzed. Because the branch of the UPR mainly affected was IREα, we blocked this pathway in stellate cells and analyzed the fibrogenic response, together with autophagy and downstream MAPK signaling. The Nrf2 antioxidant response was also evaluated in stellate cells under oxidant stress conditions. RESULTS: H2O2 treatment in culture or ethanol feeding in vivo increased the UPR based on splicing of XBP1 mRNA, which triggered autophagy. The Nrf2-mediated antioxidant response, as measured by qRT-PCR of its target genes was also induced under ER stress conditions. Conversely, blockade of the IRE1α pathway in stellate cells significantly decreased both their activation and autophagic activity in a p38 MAPK-dependent manner, leading to a reduced fibrogenic response. CONCLUSIONS: These data implicate mechanisms underlying protein folding quality control in regulating the fibrogenic response in hepatic stellate cells.

Autophagy releases lipid that promotes fibrogenesis by activated hepatic stellate cells in mice and in human tissues.

BACKGROUND & AIMS: The pathogenesis of liver fibrosis involves activation of hepatic stellate cells, which is associated with depletion of intracellular lipid droplets. When hepatocytes undergo autophagy, intracellular lipids are degraded in lysosomes. We investigated whether autophagy also promotes loss of lipids in hepatic stellate cells to provide energy for their activation and extended these findings to other fibrogenic cells. METHODS: We analyzed hepatic stellate cells from C57BL/6 wild-type, Atg7(F/F), and Atg7(F/F)-GFAP-Cre mice, as well as the mouse stellate cell line JS1. Fibrosis was induced in mice using CCl(4) or thioacetamide (TAA); liver tissues and stellate cells were analyzed. Autophagy was blocked in fibrogenic cells from liver and other tissues using small interfering RNAs against Atg5 or Atg7 and chemical antagonists. Human pulmonary fibroblasts were isolated from samples of lung tissue from patients with idiopathic pulmonary fibrosis or from healthy donors. RESULTS: In mice, induction of liver injury with CCl(4) or TAA increased levels of autophagy. We also observed features of autophagy in activated stellate cells within injured human liver tissue. Loss of autophagic function in cultured mouse stellate cells and in mice following injury reduced fibrogenesis and matrix accumulation; this effect was partially overcome by providing oleic acid as an energy substrate. Autophagy also regulated expression of fibrogenic genes in embryonic, lung, and renal fibroblasts. CONCLUSIONS: Autophagy of activated stellate cells is required for hepatic fibrogenesis in mice. Selective reduction of autophagic activity in fibrogenic cells in liver and other tissues might be used to treat patients with fibrotic diseases.

Exploring a role for heteromerization in GPCR signalling specificity.

The critical involvement of GPCRs (G-protein-coupled receptors) in nearly all physiological processes, and the presence of these receptors at the interface between the extracellular and the intracellular milieu, has positioned these receptors as pivotal therapeutic targets. Although a large number of drugs targeting GPCRs are currently available, significant efforts have been directed towards understanding receptor properties, with the goal of identifying and designing improved receptor ligands. Recent advances in GPCR pharmacology have demonstrated that different ligands binding to the same receptor can activate discrete sets of downstream effectors, a phenomenon known as 'ligand-directed signal specificity', which is currently being explored for drug development due to its potential therapeutic advantage. Emerging studies suggest that GPCR responses can also be modulated by contextual factors, such as interactions with other GPCRs. Association between different GPCR types leads to the formation of complexes, or GPCR heteromers, with distinct and unique signalling properties. Some of these heteromers activate discrete sets of signalling effectors upon activation by the same ligand, a phenomenon termed 'heteromer-directed signalling specificity'. This has been shown to be involved in the physiological role of receptors and, in some cases, in disease-specific dysregulation of a receptor effect. Hence targeting GPCR heteromers constitutes an emerging strategy to select receptor-specific responses and is likely to be useful in achieving specific beneficial therapeutic effects.

Cell surface targeting of mu-delta opioid receptor heterodimers by RTP4.

Mu opioid receptors are G protein-coupled receptors that mediate the pain-relieving effects of clinically used analgesics, such as morphine. Accumulating evidence shows that mu-delta opioid heterodimers have a pharmacologic profile distinct from those of the mu or delta homodimers. Because the heterodimers exhibit distinct signaling properties, the protein and mechanism regulating their levels have significant effects on morphine-mediated physiology. We report the characterization of RTP4, a Golgi chaperone, as a regulator of the levels of heterodimers at the cell surface. We show that the association with RTP4 protects mu-delta receptors from ubiquitination and degradation. This leads to increases in surface heterodimer levels, thereby affecting signaling. Thus, the oligomeric organization of opioid receptors is controlled by RTP4, and this governs their membrane targeting and functional activity. This work is the first report of the identification of a chaperone involved in the regulation of the biogenesis of a family A GPCR heterodimer. The identification of such factors as RTP4 controlling dimerization will provide insight into the regulation of heterodimers in vivo. This has implications in the modulation of pharmacology of their endogenous ligands, and in the development of drugs with specific therapeutic effects.

Regulation of CB1 cannabinoid receptor trafficking by the adaptor protein AP-3.

Cannabinoid receptor 1 (CB(1)) is an abundant G protein-coupled receptor, involved in a number of physiological processes. This receptor is localized at the plasma membrane, as well as in intracellular vesicles. The trafficking events leading to this intracellular localization remain controversial. In this study, we examine the differential trafficking of CB(1) receptors and its implication on signaling. We find that the transfected tagged receptors are predominantly at the plasma membrane, whereas endogenous receptors exhibit an intracellular localization. We also find that intracellular endogenous CB(1) receptors do not have an endocytic origin. Instead, these receptors associate with the adaptor protein AP-3 and traffic to the lysosomes. siRNA-mediated AP-3delta knockdown leads to enhanced cell surface localization of CB(1) receptors. Finally, we show that CB(1) receptors in the late endosomal/lysosomal compartment are associated with heterotrimeric G proteins and mediate signal transduction. These results suggest that intracellular CB(1) receptors are functional and that their spatial segregation is likely to significantly affect receptor function.

Asp218 participates with Asp213 to bind a Ca2+ atom into the S1 subsite of aminopeptidase A: a key element for substrate specificity.

APA (aminopeptidase A; EC 3.4.11.7) is a membrane-bound zinc metallopeptidase, also activated by Ca(2+), involved in the formation of brain angiotensin III, which exerts a tonic stimulatory action on the central control of blood pressure in hypertensive animals. In the present study, in the three-dimensional model of the ectodomain of mouse APA, we docked the specific APA inhibitor glutamate phosphonate, in the presence of Ca(2+). The model showed the presence of one Ca(2+) atom in an hydrophilic pocket corresponding to the S1 subsite in which the lateral chain of the inhibitor is pointing. In this pocket, the Ca(2+) atom was hexaco-ordinated with the acidic side chains of Asp(213) and Asp(218), the carbonyl group of Glu(215) and three water molecules, one of them being engaged in a hydrogen bond with the negatively charged carboxylate side chain of the inhibitor. Mutagenic replacement of Asp(213) and Asp(218) with a conservative residue maintained the ability of mutated APAs to be activated by Ca(2+). However, the replacement by a non-conservative residue abolished this property, demonstrating the crucial role of these residues in Ca(2+) binding. We also showed the involvement of these residues in the strict specificity of APA in the presence of Ca(2+) for N-terminal acidic residues from substrates or inhibitors, since mutagenic replacement of Asp(213) and Asp(218) induced a decrease of the inhibitory potencies of inhibitors homologous with acidic residues. Finally, this led to the rational design of a new potent APA inhibitor, NI926 (K(i)=70 nM), which allowed us to precisely localize Asp(213) at the entrance and Asp(218) at the bottom of the S1 subsite. Taken together, these data provide new insight into the organization and functional role of the APA S1 subsite and will allow the design of pharmacophore of the inhibitor, helpful for the development of a new generation of APA inhibitors as central-acting antihypertensive agents.

Homology modeling and site-directed mutagenesis to identify selective inhibitors of endothelin-converting enzyme-2.

Endothelin-converting enzyme-2 (ECE-2), a member of M13 family of zinc metallopeptidases, has previously been shown to process a number of neuropeptides including those derived from prodynorphin, proenkephalin, proSAAS, and amyloid precursor protein. ECE-2, unlike ECE-1, exhibits restricted neuroendocrine distribution and acidic pH optimum; it is consistent with a role in the regulation of neuropeptide levels in vivo. Here, we report the generation of a three-dimensional (3D) molecular model of ECE-2 using the crystal structure of neprilysin (EC 3.4.24.11) as a template. On the basis of the predictions made from the molecular model, we mutated and tested two residues, Trp 148 and Tyr 563, in the catalytic site. The mutation of Tyr 563 was found to significantly affect the catalytic activity and inhibitor binding. The molecular model was used to virtually screen a small molecule library of 13 000 compounds. Among the top-scoring compounds three were found to inhibit ECE-2 with high affinity and exhibited specificity for ECE-2 compared to neprilysin. Thus, the model provides a new useful tool to probe the active site of ECE-2 and design additional selective inhibitors of this enzyme.

Receptor heterodimerization leads to a switch in signaling: beta-arrestin2-mediated ERK activation by mu-delta opioid receptor heterodimers.

Opiates are analgesics of choice in the treatment of chronic pain, but their long-term use leads to the development of physiological tolerance. Thus, understanding the mechanisms modulating the response of their receptor, the mu opioid receptor (muOR), is of great clinical relevance. Here we show that heterodimerization of muOR with delta opioid receptors (deltaOR) leads to a constitutive recruitment of beta-arrestin2 to the receptor complex resulting in changes in the spatio-temporal regulation of ERK1/2 signaling. The involvement of beta-arrestin2 is further supported by studies using beta-arrestin2 siRNA in cells endogenously expressing the heterodimers. The association of beta-arrestin2 with the heterodimer can be altered by treatment with a combination of muOR agonist (DAMGO) and deltaOR antagonist (Tipp(psi)), and this leads to a shift in the pattern of ERK1/2 phosphorylation to the pattern observed with muOR alone. These data indicate that, in the naive state, muOR-deltaOR heterodimers are in a conformation conducive to beta-arrestin-mediated signaling. Destabilization of this conformation by cotreatment with muOR and deltaOR ligands leads to a switch to a non-beta-arrestin-mediated signaling. Taken together, these results show for the first time that muOR-deltaOR heterodimers, by differentially recruiting beta-arrestin, modulate the spatio-temporal dynamics of opioid receptor signaling.

An emerging role for the delta opioid receptor in the regulation of mu opioid receptor function.

Morphine and related opiates are commonly used in the clinical management of various types of pain. However, the antinociceptive properties of morphine are often overshadowed by the development of tolerance and dependence following its chronic use. The mechanisms underlying opiate tolerance are not fully understood, but appear to involve numerous and complex physiological adaptations. Recently, a role for the heterodimerization of mu and delta opioid receptors in the development of morphine tolerance has been proposed. This novel mechanism could help us to understand several observations, such as the critical role of delta opioid receptor regulation, the impact of delta opioid receptor binding site occupancy, and the participation of beta-arrestin2, in the development of morphine tolerance.

Cannabidiol protects liver from binge alcohol-induced steatosis by mechanisms including inhibition of oxidative stress and increase in autophagy.

Acute alcohol drinking induces steatosis, and effective prevention of steatosis can protect liver from progressive damage caused by alcohol. Increased oxidative stress has been reported as one mechanism underlying alcohol-induced steatosis. We evaluated whether cannabidiol, which has been reported to function as an antioxidant, can protect the liver from alcohol-generated oxidative stress-induced steatosis. Cannabidiol can prevent acute alcohol-induced liver steatosis in mice, possibly by preventing the increase in oxidative stress and the activation of the JNK MAPK pathway. Cannabidiol per se can increase autophagy both in CYP2E1-expressing HepG2 cells and in mouse liver. Importantly, cannabidiol can prevent the decrease in autophagy induced by alcohol. In conclusion, these results show that cannabidiol protects mouse liver from acute alcohol-induced steatosis through multiple mechanisms including attenuation of alcohol-mediated oxidative stress, prevention of JNK MAPK activation, and increasing autophagy.

Receptor heteromerization expands the repertoire of cannabinoid signaling in rodent neurons.

A fundamental question in G protein coupled receptor biology is how a single ligand acting at a specific receptor is able to induce a range of signaling that results in a variety of physiological responses. We focused on Type 1 cannabinoid receptor (CB1R) as a model GPCR involved in a variety of processes spanning from analgesia and euphoria to neuronal development, survival and differentiation. We examined receptor dimerization as a possible mechanism underlying expanded signaling responses by a single ligand and focused on interactions between CB1R and delta opioid receptor (DOR). Using co-immunoprecipitation assays as well as analysis of changes in receptor subcellular localization upon co-expression, we show that CB1R and DOR form receptor heteromers. We find that heteromerization affects receptor signaling since the potency of the CB1R ligand to stimulate G-protein activity is increased in the absence of DOR, suggesting that the decrease in CB1R activity in the presence of DOR could, at least in part, be due to heteromerization. We also find that the decrease in activity is associated with enhanced PLC-dependent recruitment of arrestin3 to the CB1R-DOR complex, suggesting that interaction with DOR enhances arrestin-mediated CB1R desensitization. Additionally, presence of DOR facilitates signaling via a new CB1R-mediated anti-apoptotic pathway leading to enhanced neuronal survival. Taken together, these results support a role for CB1R-DOR heteromerization in diversification of endocannabinoid signaling and highlight the importance of heteromer-directed signal trafficking in enhancing the repertoire of GPCR signaling.

Receptor heteromerization and drug discovery.

G-protein-coupled receptors (GPCRs) are membrane proteins that convert extracellular information into intracellular signals. They are involved in many biological processes and therefore represent powerful targets to modulate physiological and pathological states. Recent studies have demonstrated that GPCR activity is regulated by several mechanisms. Among these, protein-protein interactions (and in particular interactions with other receptors leading to heteromerization) has been shown to have an important role in modulating GPCR function. This has expanded their repertoire of signaling and added a new level of regulation to their physiological roles. Emerging studies provide evidence for tissue-specific and disease-specific receptor heteromerization. This suggests that heteromers represent novel drug targets for the identification of selective compounds with potentially fewer side-effects.

Cannabinoid-opioid interactions during neuropathic pain and analgesia.

Opiates and exogenous cannabinoids, both potent analgesics used for the treatment of patients with neuropathic pain, bind to and activate class A G-protein-coupled receptors (GPCRs). Several lines of evidence have recently suggested that opioid and cannabinoid receptors can functionally interact in the central nervous system (CNS). These interactions may be direct, such as through receptor heteromerization, or indirect, such as through signaling cross-talk that includes agonist-mediated release and/or synthesis of endogenous ligands that can activate downstream receptors. Interactions between opioid and cannabinoid receptors may mediate many of the behavioral phenomena associated with the use of these drugs, including the production of acute antinociception and the development of tolerance and cross-tolerance to the antinociceptive effects of opioid and cannabinoid-specific ligands. This review summarizes behavioral, anatomical, and molecular data characterizing these interactions during the development of neuropathic pain and during antinociceptive treatment with these drugs alone or in combination. These studies are critical for understanding how the receptor systems involved in pain relief are altered during acute or chronic pain, and for designing better antinociceptive drug therapies, such as the combined use of opioid and cannabinoid receptor agonists or selective activation of receptor heteromers, that directly target the altered neurophysiology of patients experiencing pain.

Increased abundance of opioid receptor heteromers after chronic morphine administration.

The mu and delta types of opioid receptors form heteromers that exhibit pharmacological and functional properties distinct from those of homomeric receptors. To characterize these complexes in the brain, we generated antibodies that selectively recognize the mu-delta heteromer and blocked its in vitro signaling. With these antibodies, we showed that chronic, but not acute, morphine treatment caused an increase in the abundance of mu-delta heteromers in key areas of the central nervous system that are implicated in pain processing. Because of its distinct signaling properties, the mu-delta heteromer could be a therapeutic target in the treatment of chronic pain and addiction.

Modulation of opioid receptor function by protein-protein interactions.

Opioid receptors, MORP, DORP and KORP, belong to the family A of G protein coupled receptors (GPCR), and have been found to modulate a large number of physiological functions, including mood, stress, appetite, nociception and immune responses. Exogenously applied opioid alkaloids produce analgesia, hedonia and addiction. Addiction is linked to alterations in function and responsiveness of all three opioid receptors in the brain. Over the last few years, a large number of studies identified protein-protein interactions that play an essential role in opioid receptor function and responsiveness. Here, we summarize interactions shown to affect receptor biogenesis and trafficking, as well as those affecting signal transduction events following receptor activation. This article also examines protein interactions modulating the rate of receptor endocytosis and degradation, events that play a major role in opiate analgesia. Like several other GPCRs, opioid receptors may form homo or heterodimers. The last part of this review summarizes recent knowledge on proteins known to affect opioid receptor dimerization.

Identification of threonine 348 as a residue involved in aminopeptidase A substrate specificity.

Aminopeptidase A (APA; EC 3.4.11.7) is a membrane-bound zinc metalloprotease cleaving in the brain the N-terminal aspartyl residue of angiotensin II to generate angiotensin III, which exerts a tonic stimulatory effect on the central control of blood pressure in hypertensive animals. We docked the specific APA inhibitor, glutamate phosphonate, in the three-dimensional model of the mouse APA ectodomain in the presence of Ca(2+). In the S1 subsite of this model, the Ca(2+) atom was coordinated with Asp-213, Asp-218,y and Glu-215 and three water molecules, one of which formed a hydrogen bond with the carboxylate side chain of the inhibitor. We report here that the carboxylate side chain of glutamate phosphonate also formed a hydrogen bond with the alcohol side chain of Thr-348. Mutagenic replacement of Thr-348 with an aspartate, tyrosine, or serine residue led to a modification of the hydrolysis velocity, with no change in the affinity of the recombinant enzymes for the substrate GluNA, either in the absence or presence of Ca(2+). In the absence of Ca(2+), the mutations modified the substrate specificity of APA, which was nevertheless restored by the addition of Ca(2+). An analysis of three-dimensional models of the corresponding Thr-348 mutants revealed that the interaction between this residue and the inhibitor was abolished or disturbed, leading to a change in the position of the inhibitor in the active site. These findings demonstrate a key role of Thr-348 in substrate specificity of APA for N-terminal acidic amino acids by insuring the optimal positioning of the substrate during catalysis.

Post-activation-mediated changes in opioid receptors detected by N-terminal antibodies.

The majority of studies examining activity-induced conformational changes in G protein-coupled receptors have focused on transmembrane helices or intracellular regions. Relatively few studies have examined the involvement of the extracellular region in general and the N-terminal region in particular in this process. To begin to address this, we generated a series of antibodies to the N-terminal region of opioid receptors. Characterization of these antibodies revealed that they differentially recognize activated receptors. Recently, we generated monoclonal antibodies that recognize regions proximal to glycosylation sites in the receptor N terminus. Characterization of these antibodies revealed that agonist treatment leads to a decrease in epitope recognition by the antibody presumably because of a movement of the region of the N terminus proximal to glycosylation sites. The time course of the decrease in antibody recognition suggested that it could be due to a post-activation-mediated event. Examination of the involvement of receptor residues in the C-tail and beta-arrestin binding using site-directed mutagenesis and cells or tissues lacking beta-arrestin 2 suggests a role for these desensitization-related mechanisms in governing antibody binding to the receptor. Thus, these N-terminally directed antibodies can differentially recognize post-activation-mediated changes in the C-terminal (intracellular) region of the receptor. Therefore, these conformation-sensitive antibodies represent powerful reagents to probe receptor activation states and provide a potential tool for identifying and characterizing new compounds of therapeutic interest.