Neurological disorders after severe pneumonia are associated with translocation of endogenous bacteria from the lung to the brain

Neurological disorders are a common feature in patients who recover from severe acute pneumonia. However, the underlying mechanisms remain poorly understood. Here, we show that the neurological syndromes after severe acute pneumonia are partly attributed to the translocation of endogenous bacteria from the lung to the brain during pneumonia. Using principal components analysis, similarities were found between the brain's flora species and those of the lungs, indicating that the bacteria detected in the brain may originate from the lungs. We also observed impairment of both the lung-blood and brain-blood barriers, allowing endogenous lung bacteria to invade the brain during pneumonia. An elevated microglia and astrocyte activation signature via bacterial infection-related pathways was observed, indicating a bacterial-induced disruption of brain homeostasis. Collectively, we identify endogenous lung bacteria that play a role in altering brain homeostasis, which provides insight into the mechanism of neurological syndromes after severe pneumonia.

Inhibition of pro-inflammatory signaling in human primary macrophages by enhancing arginase-2 via target site blockers

The modulation of macrophage phenotype from a pro-inflammatory to an anti-inflammatory state holds therapeutic potential in the treatment of inflammatory disease. We have previously shown that arginase-2 (Arg2), a mitochondrial enzyme, is a key regulator of the macrophage anti-inflammatory response. Here, we investigate the therapeutic potential of Arg2 enhancement via target site blockers (TSBs) in human macrophages. TSBs are locked nucleic acid antisense oligonucleotides that were specifically designed to protect specific microRNA recognition elements (MREs) in human ARG2 3' UTR mRNA. TSBs targeting miR-155 (TSB-155) and miR-3202 (TSB-3202) MREs increased ARG2 expression in human monocyte-derived macrophages. This resulted in decreased gene expression and cytokine production of TNF-α and CCL2 and, for TSB-3202, in an increase in the anti-inflammatory macrophage marker, CD206. Proteomic analysis demonstrated that a network of pro-inflammatory responsive proteins was modulated by TSBs. In silico bioinformatic analysis predicted that TSB-3202 suppressed upstream pro-inflammatory regulators including STAT-1 while enhancing anti-inflammatory associated proteins. Proteomic data were validated by confirming increased levels of sequestosome-1 and decreased levels of phosphorylated STAT-1 and STAT-1 upon TSB treatment. In conclusion, upregulation of Arg2 by TSBs inhibits pro-inflammatory signaling and is a promising novel therapeutic strategy to modulate inflammatory signaling in human macrophages.

NOD1 mediates interleukin-18 processing in epithelial cells responding to Helicobacter pylori infection in mice

The interleukin-1 family members, IL-1β and IL-18, are processed into their biologically active forms by multi-protein complexes, known as inflammasomes. Although the inflammasome pathways that mediate IL-1β processing in myeloid cells have been defined, those involved in IL-18 processing, particularly in non-myeloid cells, are still not well understood. Here we report that the host defence molecule NOD1 regulates IL-18 processing in mouse epithelial cells in response to the mucosal pathogen, Helicobacter pylori. Specifically, NOD1 in epithelial cells mediates IL-18 processing and maturation via interactions with caspase-1, instead of the canonical inflammasome pathway involving RIPK2, NF-κB, NLRP3 and ASC. NOD1 activation and IL-18 then help maintain epithelial homoeostasis to mediate protection against pre-neoplastic changes induced by gastric H. pylori infection in vivo. Our findings thus demonstrate a function for NOD1 in epithelial cell production of bioactive IL-18 and protection against H. pylori-induced pathology.

Enhancing arginase 2 expression using target site blockers as a strategy to modulate macrophage phenotype

Macrophages are plastic cells playing a crucial role in innate immunity. While fundamental in responding to infections, when persistently maintained in a pro-inflammatory state they can initiate and sustain inflammatory diseases. Therefore, a strategy that reprograms pro-inflammatory macrophages toward an anti-inflammatory phenotype could hold therapeutic potential in that context. We have recently shown that arginase 2 (Arg2), a mitochondrial enzyme involved in arginine metabolism, promotes the resolution of inflammation in macrophages and it is targeted by miR-155. Here, we designed and tested a target site blocker (TSB) that specifically interferes and blocks the interaction between miR-155 and Arg2 mRNA, leading to Arg2 increased expression and activity. In bone marrow-derived macrophages transfected with Arg2 TSB (in the presence or absence of the pro-inflammatory stimulus LPS), we observed an overall shift of the polarization status of macrophages toward an anti-inflammatory phenotype, as shown by significant changes in surface markers (CD80 and CD71), metabolic parameters (mitochondrial oxidative phosphorylation) and cytokines secretion (IL-1β, IL-6, and TNF). Moreover, in an in vivo model of LPS-induced acute inflammation, intraperitoneal administration of Arg2 TSB led to an overall decrease in systemic levels of pro-inflammatory cytokines. Overall, this proof-of-concept strategy represent a promising approach to modulating macrophage phenotype.

Biomaterial and Therapeutic Approaches for the Manipulation of Macrophage Phenotype in Peripheral and Central Nerve Repair

Injury to the peripheral or central nervous systems often results in extensive loss of motor and sensory function that can greatly diminish quality of life. In both cases, macrophage infiltration into the injury site plays an integral role in the host tissue inflammatory response. In particular, the temporally related transition of macrophage phenotype between the M1/M2 inflammatory/repair states is critical for successful tissue repair. In recent years, biomaterial implants have emerged as a novel approach to bridge lesion sites and provide a growth-inductive environment for regenerating axons. This has more recently seen these two areas of research increasingly intersecting in the creation of 'immune-modulatory' biomaterials. These synthetic or naturally derived materials are fabricated to drive macrophages towards a pro-repair phenotype. This review considers the macrophage-mediated inflammatory events that occur following nervous tissue injury and outlines the latest developments in biomaterial-based strategies to influence macrophage phenotype and enhance repair.

Mitochondrial arginase-2 is essential for IL-10 metabolic reprogramming of inflammatory macrophages

Mitochondria are important regulators of macrophage polarisation. Here, we show that arginase-2 (Arg2) is a microRNA-155 (miR-155) and interleukin-10 (IL-10) regulated protein localized at the mitochondria in inflammatory macrophages, and is critical for IL-10-induced modulation of mitochondrial dynamics and oxidative respiration. Mechanistically, the catalytic activity and presence of Arg2 at the mitochondria is crucial for oxidative phosphorylation. We further show that Arg2 mediates this process by increasing the activity of complex II (succinate dehydrogenase). Moreover, Arg2 is essential for IL-10-mediated downregulation of the inflammatory mediators succinate, hypoxia inducible factor 1α (HIF-1α) and IL-1β in vitro. Accordingly, HIF-1α and IL-1β are highly expressed in an LPS-induced in vivo model of acute inflammation using Arg2^-/- mice. These findings shed light on a new arm of IL-10-mediated metabolic regulation, working to resolve the inflammatory status of the cell.

Novel mineralocorticoid receptor mechanisms regulate cardiac tissue inflammation in male mice

MR activation in macrophages is critical for the development of cardiac inflammation and fibrosis. We previously showed that MR activation modifies macrophage pro-inflammatory signalling, changing the cardiac tissue response to injury via both direct gene transcription and JNK/AP-1 second messenger pathways. In contrast, MR-mediated renal electrolyte homeostasis is critically determined by DNA-binding-dependent processes. Hence, ascertaining the relative contribution of MR actions via DNA binding or alternative pathways on macrophage behaviour and cardiac inflammation may provide therapeutic opportunities which separate the cardioprotective effects of MR antagonists from their undesirable renal potassium-conserving effects. We developed new macrophage cell lines either lacking MR or harbouring a mutant MR incapable of DNA binding. Western blot analysis demonstrated that MR DNA binding is required for lipopolysaccharide (LPS), but not phorbol 12-myristate-13-acetate (PMA), induction of the MAPK/pJNK pathway in macrophages. Quantitative RTPCR for pro-inflammatory and pro-fibrotic targets revealed subsets of LPS- and PMA-induced genes that were either enhanced or repressed by the MR via actions that do not always require direct MR-DNA binding. Analysis of the MR target gene and profibrotic factor MMP12 identified promoter elements that are regulated by combined MR/MAPK/JNK signalling. Evaluation of cardiac tissue responses to an 8-day DOC/salt challenge in mice selectively lacking MR DNA-binding in macrophages demonstrated levels of inflammatory markers equivalent to WT, indicating non-DNA binding-dependent MR signalling in macrophages is sufficient for DOC/salt-induced tissue inflammation. Our data demonstrate that the MR regulates a macrophage pro-inflammatory phenotype and cardiac tissue inflammation, partially via pathways that do not require DNA binding.

Parsing the IL-37-Mediated Suppression of Inflammasome Function

Interleukin (IL)-37 is a member of the IL-1 family of cytokines. Although its broad anti-inflammatory properties are well described, the effects of IL-37 on inflammasome function remain poorly understood. Performing gene expression analyses, ASC oligomerization/speck assays and caspase-1 assays in bone marrow-derived macrophages (BMDM), and employing an in vivo endotoxemia model, we studied how IL-37 affects the expression and maturation of IL-1β and IL-18, inflammasome activation, and pyroptosis in detail. IL-37 inhibited IL-1β production by NLRP3 and AIM2 inflammasomes, and IL-18 production by the NLRP3 inflammasome. This inhibition was partially attributable to effects on gene expression: whereas IL-37 did not affect lipopolysaccharide (LPS)-induced mRNA expression of Il18 or inflammasome components, IL-37-transgenic BMDM displayed an up to 83% inhibition of baseline and LPS-stimulated Il1b compared to their wild-type counterparts. Importantly, we observed that IL-37 suppresses nigericin- and silica-induced ASC oligomerization/speck formation (a step in inflammasome activation and subsequent caspase-1 activation), and pyroptosis (-50%). In mice subjected to endotoxemia, IL-37 inhibited plasma IL-1β (-78% compared to wild-type animals) and IL-18 (-61%). Thus, our study adds suppression of inflammasome activity to the portfolio of anti-inflammatory pathways employed by IL-37, highlighting this cytokine as a potential tool for treating inflammasome-driven diseases.

The Single Nucleotide Polymorphism Mal-D96N Mice Provide New Insights into Functionality of Mal in TLR Immune Responses

MyD88 adaptor-like (Mal) protein is the most polymorphic of the four key adaptor proteins involved in TLR signaling. TLRs play a critical role in the recognition and immune response to pathogens through activation of the prototypic inflammatory transcription factor NF-κB. The study of single nucleotide polymorphisms in TLRs, adaptors, and signaling mediators has provided key insights into the function of the corresponding genes but also into the susceptibility to infectious diseases in humans. In this study, we have analyzed the immune response of mice carrying the human Mal-D96N genetic variation that has previously been proposed to confer protection against septic shock. We have found that Mal-D96N macrophages display reduced cytokine expression in response to TLR4 and TLR2 ligand challenge. Mal-D96N macrophages also display reduced MAPK activation, NF-κB transactivation, and delayed NF-κB nuclear translocation, presumably via delayed kinetics of Mal interaction with MyD88 following LPS stimulation. Importantly, Mal-D96N genetic variation confers a physiological protective phenotype to in vivo models of LPS-, Escherichia coli-, and influenza A virus-induced hyperinflammatory disease in a gene dosage-dependent manner. Together, these results highlight the critical role Mal plays in regulating optimal TLR-induced inflammatory signaling pathways and suggest the potential therapeutic advantages of targeting the Mal D96 signaling nexus.

Membrane vesicles from Pseudomonas aeruginosa activate the noncanonical inflammasome through caspase-5 in human monocytes

Outer membrane vesicles (OMVs) are constitutively produced by Gram-negative bacteria both in vivo and in vitro. These lipid-bound structures carry a range of immunogenic components derived from the parent cell, which are transported into host target cells and activate the innate immune system. Recent advances in the field have shed light on some of the multifaceted roles of OMVs in host-pathogen interactions. In this study, we investigated the ability of OMVs from two clinically important pathogens, Pseudomonas aeruginosa and Helicobacter pylori, to activate canonical and noncanonical inflammasomes. P. aeruginosa OMVs induced inflammasome activation in mouse macrophages, as evidenced by "speck" formation, as well as the cleavage and secretion of interleukin-1β and caspase-1. These responses were independent of AIM2 and NLRC4 canonical inflammasomes, but dependent on the noncanonical caspase-11 pathway. Moreover, P. aeruginosa OMVs alone were able to activate the inflammasome in a TLR-dependent manner, without requiring an exogenous priming signal. In contrast, H. pylori OMVs were not able to induce inflammasome activation in macrophages. Using CRISPR/Cas9 knockout THP-1 cells lacking the human caspase-11 homologs, caspase-4 and -5,we demonstrated that caspase-5 but not caspase-4 is required for inflammasome activation by P. aeruginosa OMVs in human monocytes. In contrast, free P. aeruginosa lipopolysaccharide (LPS) transfected into cells induced inflammasome responses via caspase-4. This suggests that caspase-4 and caspase-5 differentially recognize LPS depending on its physical form or route of delivery into the cell. These findings have relevance to Gram-negative infections in humans and the use of OMVs as novel vaccines.

PB1-F2 Peptide Derived from Avian Influenza A Virus H7N9 Induces Inflammation via Activation of the NLRP3 Inflammasome

The emergence of avian H7N9 influenza A virus in humans with associated high mortality has highlighted the threat of a potential pandemic. Fatal H7N9 infections are characterized by hyperinflammation and increased cellular infiltrates in the lung. Currently there are limited therapies to address the pathologies associated with H7N9 infection and the virulence factors that contribute to these pathologies. We have found that PB1-F2 derived from H7N9 activates the NLRP3 inflammasome and induces lung inflammation and cellular recruitment that is NLRP3-dependent. We have also shown that H7N9 and A/Puerto Rico/H1N1 (PR8)PB1-F2 peptide treatment induces significant mitochondrial reactive oxygen production, which contributes to NLRP3 activation. Importantly, treatment of cells or mice with the specific NLRP3 inhibitor MCC950 significantly reduces IL-1β maturation, lung cellular recruitment, and cytokine production. Together, these results suggest that PB1-F2 from H7N9 avian influenza A virus may be a major contributory factor to disease pathophysiology and excessive inflammation characteristic of clinical infections and that targeting the NLRP3 inflammasome may be an effective means to reduce the inflammatory burden associated with H7N9 infections.

Toll-like receptors: the swiss army knife of immunity and vaccine development

Innate immune cells have a critical role in defense against infection and disease. Central to this is the broad specificity with which they can detect pathogen-associated patterns and danger-associated patterns via the pattern recognition receptors (PRRs) they express. Several families of PRRs have been identified including: Toll-like receptors (TLRs), C-type lectin-like receptors, retinoic acid-inducible gene-like receptors and nucleotide-binding oligomerization domain-like receptors. TLRs are one of the most largely studied families of PRRs. The binding of ligands to TLRs on antigen presenting cells (APCs), mainly dendritic cells, leads to APC maturation, induction of inflammatory cytokines and the priming of naive T cells to drive acquired immunity. Therefore, activation of TLRs promotes both innate inflammatory responses and the induction of adaptive immunity. Consequently, in the last two decades mounting evidence has inextricably linked TLR activation with the pathogenesis of immune diseases and cancer. It has become advantageous to harness these aspects of TLR signaling therapeutically to accelerate and enhance the induction of vaccine-specific responses and also target TLRs with the use of biologics and small molecule inhibitors for the treatment of disease. In these respects, TLRs may be considered a 'Swiss Army' knife of the immune system, ready to respond in a multitude of infectious and disease states. Here we describe the latest advances in TLR-targeted therapeutics and the use of TLR ligands as vaccine adjuvants.

Toll-Like Receptors: Ligands, Cell-Based Models, and Readouts for Receptor Action

This chapter details Toll-like receptors (TLRs) and the tools available to study their biology in vitro. Key parameters to consider before exploring TLR action such as receptor localization, signaling pathways, nature of ligands and cellular expression are introduced. Cellular models (i.e., host cells and readouts) based on the use of cell lines, primary cells, or whole blood are presented. The use of modified TLRs to circumvent some technical problems is also discussed.

Inflammasome activity is essential for one kidney/deoxycorticosterone acetate/salt-induced hypertension in mice

BACKGROUND AND PURPOSE

Inflammasomes are multimeric complexes that facilitate caspase-1-mediated processing of the pro-inflammatory cytokines IL-1β and IL-18. Clinical hypertension is associated with renal inflammation and elevated circulating levels of IL-1β and IL-18. Therefore, we investigated whether hypertension in mice is associated with increased expression and/or activation of the inflammasome in the kidney, and if inhibition of inflammasome activity reduces BP, markers of renal inflammation and fibrosis.

EXPERIMENTAL APPROACH

Wild-type and inflammasome-deficient ASC(-/-) mice were uninephrectomized and received deoxycorticosterone acetate and saline to drink (1K/DOCA/salt). Control mice were uninephrectomized but received a placebo pellet and water. BP was measured by tail cuff; renal expression of inflammasome subunits and inflammatory markers was measured by real-time PCR and immunoblotting; macrophage and collagen accumulation was assessed by immunohistochemistry.

KEY RESULTS

1K/DOCA/salt-induced hypertension in mice was associated with increased renal mRNA expression of inflammasome subunits NLRP3, ASC and pro-caspase-1, and the cytokine, pro-IL-1β, as well as protein levels of active caspase-1 and mature IL-1β. Following treatment with 1K/DOCA/salt, ASC(-/-) mice displayed blunted pressor responses and were also protected from increases in renal expression of IL-6, IL-17A, CCL2, ICAM-1 and VCAM-1, and accumulation of macrophages and collagen. Finally, treatment with a novel inflammasome inhibitor, MCC950, reversed hypertension in 1K/DOCA/salt-treated mice.

CONCLUSIONS AND IMPLICATIONS

Renal inflammation, fibrosis and elevated BP induced by 1K/DOCA/salt treatment are dependent on inflammasome activity, highlighting the inflammasome/IL-1β pathway as a potential therapeutic target in hypertension.

Promyelocytic leukemia protein interacts with the apoptosis-associated speck-like protein to limit inflammasome activation

The apoptosis-associated speck-like protein containing a caspase-activating recruitment domain (ASC) is an essential component of several inflammasomes, multiprotein complexes that regulate caspase-1 activation and inflammation. We report here an interaction between promyelocytic leukemia protein (PML) and ASC. We observed enhanced formation of ASC dimers in PML-deficient macrophages. These macrophages also display enhanced levels of ASC in the cytosol. Furthermore, IL-1β production was markedly enhanced in these macrophages in response to both NLRP3 and AIM2 inflammasome activation and following bone marrow-derived macrophage infection with herpes simplex virus-1 (HSV-1) and Salmonella typhimurium. Collectively, our data indicate that PML limits ASC function, retaining ASC in the nucleus.

Activation of liver X receptor suppresses the production of the IL-12 family of cytokines by blocking nuclear translocation of NF-κBp50

There is now convincing evidence that liver X receptor (LXR) is an important modulator of the inflammatory response; however, its mechanism of action remains unclear. This study aimed to examine the effect of LXR on the IL-12 family of cytokines and examined the mechanism by which LXR exerted this effect. We first demonstrated that activation of murine-derived dendritic cells (DC) with a specific agonist to LXR enhanced expression of LXR following activation with LPS, suggesting a role in inflammation. Furthermore, we showed LXR expression to be increased in vivo in dextrane sulphate sodium-induced colitis. LXR activation also suppressed production of IL-12p40, IL-12p70, IL-27 and IL-23 in murine-derived DC following stimulation with LPS, and specifically targeted the p35, p40 and EBI3 subunits of the IL-12 cytokine family, which are under the control of the NF-κB subunit p50 (NF-κBp50). Finally, we demonstrated that LXR can associate with NF-κBp50 in DC and that LXR activation prevents translocation of the p50 subunit into the nucleus. In summary, our study indicates that LXR can specifically suppress the IL-12 family of cytokines though its association with NF-κBp50 and highlights its potential as a therapeutic target for chronic inflammatory diseases.

Biochemical regulation of the inflammasome

The extensively studied cytokine IL-1β is an important mediator of the inflammatory response. However, dysregulated release of IL-1β can be detrimental and is attributed to the progression and pathogenesis of multiple inflammatory diseases including, rhuematoid arthritis (RA), atherosclerosis, type 2 diabetes (T2D), Alzheimers disease and gout. IL-1β is encoded as a pro-protein. A multi-protein molecular scaffold termed the "Inflammasome" is responsible for the tightly controlled and coordinated processing of pro-IL-1β. The activation of several NLR (nucleotide-binding oligomerization domain (NOD)-like receptor) family members and PYHIN (pyrin and HIN domain) proteins can drive the formation of inflammasomes. However, the exact biochemical mechanisms governing their activation have been the subject of much research. Different inflammasomes have been demonstrated to respond to the same pathogen inducing a cooperative immune response accountable for the clearance of infection. Here, we review current knowledge surrounding the biochemical regulation of the NLRP1, NLRP3, NLRC4, AIM2 and IFI16 inflammasomes.

Mycobacterial PIMs inhibit host inflammatory responses through CD14-dependent and CD14-independent mechanisms

Mycobacteria develop strategies to evade the host immune system. Among them, mycobacterial LAM or PIMs inhibit the expression of pro-inflammatory cytokines by activated macrophages. Here, using synthetic PIM analogues, we analyzed the mode of action of PIM anti-inflammatory effects. Synthetic PIM(1) isomer and PIM(2) mimetic potently inhibit TNF and IL-12 p40 expression induced by TLR2 or TLR4 pathways, but not by TLR9, in murine macrophages. We show inhibition of LPS binding to TLR4/MD2/CD14 expressing HEK cells by PIM(1) and PIM(2) analogues. More specifically, the binding of LPS to CD14 was inhibited by PIM(1) and PIM(2) analogues. CD14 was dispensable for PIM(1) and PIM(2) analogues functional inhibition of TLR2 agonists induced TNF, as shown in CD14-deficient macrophages. The use of rough-LPS, that stimulates TLR4 pathway independently of CD14, allowed to discriminate between CD14-dependent and CD14-independent anti-inflammatory effects of PIMs on LPS-induced macrophage responses. PIM(1) and PIM(2) analogues inhibited LPS-induced TNF release by a CD14-dependent pathway, while IL-12 p40 inhibition was CD14-independent, suggesting that PIMs have multifold inhibitory effects on the TLR4 signalling pathway.

A role for TLR4 in Clostridium difficile infection and the recognition of surface layer proteins

Clostridium difficile is the etiological agent of antibiotic-associated diarrhoea (AAD) and pseudomembranous colitis in humans. The role of the surface layer proteins (SLPs) in this disease has not yet been fully explored. The aim of this study was to investigate a role for SLPs in the recognition of C. difficile and the subsequent activation of the immune system. Bone marrow derived dendritic cells (DCs) exposed to SLPs were assessed for production of inflammatory cytokines, expression of cell surface markers and their ability to generate T helper (Th) cell responses. DCs isolated from C3H/HeN and C3H/HeJ mice were used in order to examine whether SLPs are recognised by TLR4. The role of TLR4 in infection was examined in TLR4-deficient mice. SLPs induced maturation of DCs characterised by production of IL-12, TNFα and IL-10 and expression of MHC class II, CD40, CD80 and CD86. Furthermore, SLP-activated DCs generated Th cells producing IFNγ and IL-17. SLPs were unable to activate DCs isolated from TLR4-mutant C3H/HeJ mice and failed to induce a subsequent Th cell response. TLR4⁻/⁻ and Myd88⁻/⁻, but not TRIF⁻/⁻ mice were more susceptible than wild-type mice to C. difficile infection. Furthermore, SLPs activated NFκB, but not IRF3, downstream of TLR4. Our results indicate that SLPs isolated from C. difficile can activate innate and adaptive immunity and that these effects are mediated by TLR4, with TLR4 having a functional role in experimental C. difficile infection. This suggests an important role for SLPs in the recognition of C. difficile by the immune system.

Regulation of Thromboxane Receptor Trafficking Through the Prostacyclin Receptor in Vascular Smooth Muscle Cells

BACKGROUND

Prostacyclin (PGI2) and thromboxane (TxA2) effect disparate outcomes for atherogenesis and the response to vascular injury; PGI2, a vasodilator and inhibitor of platelet aggregation, limits the deleterious actions of TxA2, a vasoconstrictor and platelet activator. Dimerization of their G protein-coupled receptors, IP and TP, evokes a modified cellular response through which IP/TP counter-balance may be effected. We examined the consequence of IP/TP interaction for the regulatory pathways of both receptors.

METHODS AND RESULTS

TPalpha overexpressed in HEK293 cells or expressed endogenously in aortic smooth muscle cells (ASMCs) was internalized after selective activation of either TP or IP. Homologous trafficking of TP was unaltered by coexpression of IP. Heterologous sequestration of TPalpha required the physical presence of activated IP, in transfected and native cells, but was independent of IP signaling to adenylyl cyclase. Reciprocal heterologous regulation of IP, via activated TP, was evident in both HEK293 cells and ASMCs. Homologous TP internalization led to receptor retention and degradation. In contrast, when internalization was IP-induced, TPalpha was recycled to the cell surface in coexpressing HEK293 cells, but not in ASMCs, in accord with the postendocytotic pathway of IP.

CONCLUSIONS

IP/TPalpha interaction permits reciprocal regulation of receptor endocytosis via the trafficking pathway determined by the activated dimeric partner.