Synthetic peroxisome proliferator-activated receptor γ agonists rosiglitazone and troglitazone suppress transcription by promoter 3 of the human thromboxane A2 receptor gene in human erythroleukemia cells

The Role and Regulation of Thromboxane A2 Signaling in Cancer-Trojan Horses and Misdirection

Over the last two decades, there has been an increasing awareness of the role of eicosanoids in the development and progression of several types of cancer, including breast, prostate, lung, and colorectal cancers. Several processes involved in cancer development, such as cell growth, migration, and angiogenesis, are regulated by the arachidonic acid derivative thromboxane A₂ (TXA₂). Higher levels of circulating TXA₂ are observed in patients with multiple cancers, and this is accompanied by overexpression of TXA₂ synthase (TBXAS1, TXA₂S) and/or TXA₂ receptors (TBXA2R, TP). Overexpression of TXA₂S or TP in tumor cells is generally associated with poor prognosis, reduced survival, and metastatic disease. However, the role of TXA₂ signaling in the stroma during oncogenesis has been underappreciated. TXA₂ signaling regulates the tumor microenvironment by modulating angiogenic potential, tumor ECM stiffness, and host immune response. Moreover, the by-products of TXA₂S are highly mutagenic and oncogenic, adding to the overall phenotype where TXA₂ synthesis promotes tumor formation at various levels. The stability of synthetic enzymes and receptors in this pathway in most cancers (with few mutations reported) suggests that TXA₂ signaling is a viable target for adjunct therapy in various tumors to reduce immune evasion, primary tumor growth, and metastasis.

Bile Salt Homeostasis in Normal and Bsep Gene Knockout Rats with Single and Repeated Doses of Troglitazone

The interference of bile acid secretion through bile salt export pump (BSEP) inhibition is one of the mechanisms for troglitazone (TGZ)-induced hepatotoxicity. Here, we investigated the impact of single or repeated oral doses of TGZ (200 mg/kg/day, 7 days) on bile acid homoeostasis in wild-type (WT) and Bsep knockout (KO) rats. Following oral doses, plasma exposures of TGZ were not different between WT and KO rats, and were similar on day 1 and day 7. However, plasma exposures of the major metabolite, troglitazone sulfate (TS), in KO rats were 7.6- and 9.3-fold lower than in WT on day 1 and day 7, respectively, due to increased TS biliary excretion. With Bsep KO, the mRNA levels of multidrug resistance-associated protein 2 (Mrp2), Mrp3, Mrp4, Mdr1, breast cancer resistance protein (Bcrp), sodium taurocholate cotransporting polypeptide, small heterodimer partner, and Sult2A1 were significantly altered in KO rats. Following seven daily TGZ treatments, Cyp7A1 was significantly increased in both WT and KO rats. In the vehicle groups, plasma exposures of individual bile acids demonstrated variable changes in KO rats as compared with WT. WT rats dosed with TGZ showed an increase of many bile acid species in plasma on day 1, suggesting the inhibition of Bsep. Conversely, these changes returned to base levels on day 7. In KO rats, alterations of most bile acids were observed after seven doses of TGZ. Collectively, bile acid homeostasis in rats was regulated through bile acid synthesis and transport in response to Bsep deficiency and TGZ inhibition. Additionally, our study is the first to demonstrate that repeated TGZ doses can upregulate Cyp7A1 in rats.

Protein kinase C-related kinase 1 and 2 play an essential role in thromboxane-mediated neoplastic responses in prostate cancer

The prostanoid thromboxane (TX) A₂ is increasingly implicated in neoplastic progression, including prostate cancer (PCa). Mechanistically, we recently identified protein kinase C-related kinase (PRK) 1 as a functional interactant of both the TPα and TPβ isoforms of the human T prostanoid receptor (TP). The interaction with PRK1 was not only essential for TPα/TPβ-induced PCa cell migration but also enabled the TXA₂-TP axis to induce phosphorylation of histone H3 at Thr11 (H3Thr11), an epigenetic marker both essential for and previously exclusively associated with androgen-induced chromatin remodelling and transcriptional activation. PRK1 is a member of a subfamily of three structurally related kinases comprising PRK1/PKNα, PRK2/PKNγ and PRK3/PKNβ that are widely yet differentially implicated in various cancers. Hence, focusing on the setting of prostate cancer, this study investigated whether TPα and/or TPβ might also complex with PRK2 and PRK3 to regulate their activity and neoplastic responses. While TPα and TPβ were found in immune complexes with PRK1, PRK2 and PRK3 to regulate their activation and signalling, they do so differentially and in a TP agonist-regulated manner dependent on the T-loop activation status of the PRKs but independent of their kinase activity. Furthermore, TXA₂-mediated neoplastic responses in prostate adenocarcinoma PC-3 cells, including histone H3Thr11 phosphorylation, was found to occur through a PRK1- and PRK2-, but not PRK3-, dependent mechanism. Collectively, these data suggest that TXA₂ acts as both a neoplastic and epigenetic regulator and provides a mechanistic explanation, at least in part, for the prophylactic benefits of Aspirin in reducing the risk of certain cancers.

Transcriptional regulation of the human thromboxane A2 receptor gene by Wilms' tumor (WT)1 and hypermethylated in cancer (HIC) 1 in prostate and breast cancers

The prostanoid thromboxane (TX) A(2) plays a central role in hemostasis and is increasingly implicated in neoplastic disease, including prostate and breast cancers. In humans, TXA(2) signals through the TPα and TPβ isoforms of the T prostanoid receptor, two structurally related receptors transcriptionally regulated by distinct promoters, Prm1 and Prm3, respectively, within the TP gene. Focusing on TPα, the current study investigated its expression and transcriptional regulation through Prm1 in prostate and breast cancers. Expression of TPα correlated with increasing prostate and breast tissue tumor grade while the TXA(2) mimetic U46619 promoted both proliferation and migration of the respective prostate (PC3) and breast (MCF-7 and MDA-MD-231) derived-carcinoma cell lines. Through 5' deletional and genetic reporter analyses, several functional upstream repressor regions (URRs) were identified within Prm1 in PC3, MCF-7 and MDA-MB-231 cells while site-directed mutagenesis identified the tumor suppressors Wilms' tumor (WT)1 and hypermethylated in cancer (HIC) 1 as the trans-acting factors regulating those repressor regions. Chromatin immunoprecipitation (ChIP) studies confirmed that WT1 binds in vivo to multiple GC-enriched WT1 cis-elements within the URRs of Prm1 in PC3, MCF-7 and MDA-MB-231 cells. Furthermore, ChIP analyses established that HIC1 binds in vivo to the HIC1((b))cis-element within Prm1 in PC3 and MCF-7 cells but not in the MDA-MB-231 carcinoma line. Collectively, these data establish that WT1 and HIC1, both tumor suppressors implicated in prostate and breast cancers, transcriptionally repress TPα expression and thereby provide a strong genetic basis for understanding the role of TXA2 in the progression of certain human cancers.

Sensitizing primary acute lymphoblastic leukemia to natural killer cell recognition by induction of NKG2D ligands

Natural killer (NK) cell immunosurveillance may be impaired by malignant disease, resulting in tumor escape and disease progression. Therapies that enhance NK cytotoxicity may therefore prove valuable in remission-induction and maintenance treatment regimens. Acute lymphoblastic leukemia (ALL) has previously been considered resistant to NK cell lysis and not tractable to this approach. Our study demonstrates that bortezomib, valproate and troglitazone can up-regulate NK activating ligands on a B-ALL cell line and on a proportion but not all adult primary B-ALL samples. Drug-treated ALL cells trigger higher levels of NK degranulation, as measured by CD107a expression, and this effect is dependent on signaling through the NK activating receptor NKG2D. These results suggest that bortezomib, valproate and troglitazone may have clinical utility in sensitizing ALL to NK mediated lysis in vivo.

Peroxisome proliferator-activated receptor-gamma agonists suppress tissue factor overexpression in rat balloon injury model with paclitaxel infusion

The role and underlying mechanisms of rosiglitazone, a peroxisome proliferator-activated receptor-gamma (PPAR-γ) agonist, on myocardial infarction are poorly understood. We investigated the effects of this PPAR-γ agonist on the expression of tissue factor (TF), a primary molecule for thrombosis, and elucidated its underlying mechanisms. The PPAR-γ agonist inhibited TF expression in response to TNF-α in human umbilical vein endothelial cells, human monocytic leukemia cell line, and human umbilical arterial smooth muscle cells. The overexpression of TF was mediated by increased phosphorylation of mitogen-activated protein kinase (MAPK), which was blocked by the PPAR-γ agonist. The effective MAPK differed depending on each cell type. Luciferase and ChIP assays showed that transcription factor, activator protein-1 (AP-1), was a pivotal target of the PPAR-γ agonist to lower TF transcription. Intriguingly, two main drugs for drug-eluting stent, paclitaxel or rapamycin, significantly exaggerated thrombin-induced TF expression, which was also effectively blocked by the PPAR-γ agonist in all cell types. This PPAR-γ agonist did not impair TF pathway inhibitor (TFPI) in three cell types. In rat balloon injury model (Sprague-Dawley rats, n = 10/group) with continuous paclitaxel infusion, the PPAR-γ agonist attenuated TF expression by 70±5% (n = 4; P<0.0001) in injured vasculature. Taken together, rosiglitazone reduced TF expression in three critical cell types involved in vascular thrombus formation via MAPK and AP-1 inhibitions. Also, this PPAR-γ agonist reversed the paclitaxel-induced aggravation of TF expression, which suggests a possibility that the benefits might outweigh its risks in a group of patients with paclitaxel-eluting stent implanted.

The Role of PPARgamma in the Cyclooxygenase Pathway in Lung Cancer

Decreased expression of peroxisome proliferator activated receptor-γ (PPARγ) and high levels of the proinflammatory enzyme cyclooxygenase-2 (COX-2) have been observed in many tumor types. Both reduced (PPARγ) expression and elevated COX-2 within the tumor are associated with poor prognosis in lung cancer patients, and recent work has indicated that these signaling pathways may be interrelated. Synthetic (PPARγ) agonists such as the thiazolidinedione (TZD) class of anti-diabetic drugs can decrease COX-2 levels, inhibit growth of non-small-cell lung cancer (NSCLC) cells in vitro, and block tumor progression in xenograft models. TZDs alter the expression of COX-2 and consequent production of the protumorigenic inflammatory molecule prostaglandin E2 (PGE2) through both (PPARγ) dependent and independent mechanisms. Certain TZDs also reduce expression of PGE2 receptors or upregulate the PGE2 catabolic enzyme 15-prostaglandin dehydrogenase. As several COX-2 enzymatic products have antitumor properties and specific COX-2 inhibition has been associated with increased risk of adverse cardiac events, directly reducing the effects or concentration of PGE2 may provide a more safe and effective strategy for lung cancer treatment. Understanding the mechanisms underlying these effects may be helpful for designing anticancer therapies. This article summarizes recent research on the relationship between (PPARγ), TZDs, and the COX-2/PGE2 pathways in lung cancer.

Identification of an interaction between the TPalpha and TPbeta isoforms of the human thromboxane A2 receptor with protein kinase C-related kinase (PRK) 1: implications for prostate cancer

In humans, thromboxane (TX) A(2) signals through the TPα and TPβ isoforms of the TXA(2) receptor or TP. Here, the RhoA effector protein kinase C-related kinase (PRK) 1 was identified as an interactant of both TPα and ΤPβ involving common and unique sequences within their respective C-terminal (C)-tail domains and the kinase domain of PRK1 (PRK1(640-942)). Although the interaction with PRK1 is constitutive, agonist activation of TPα/TPβ did not regulate the complex per se but enhanced PRK1 activation leading to phosphorylation of its general substrate histone H1 in vitro. Altered PRK1 and TP expression and signaling are increasingly implicated in certain neoplasms, particularly in androgen-associated prostate carcinomas. Agonist activation of TPα/TPβ led to phosphorylation of histone H3 at Thr(11) (H3 Thr(11)), a previously recognized specific marker of androgen-induced chromatin remodeling, in the prostate LNCaP and PC-3 cell lines but not in primary vascular smooth muscle or endothelial cells. Moreover, this effect was augmented by dihydrotestosterone in androgen-responsive LNCaP but not in nonresponsive PC-3 cells. Furthermore, PRK1 was confirmed to constitutively interact with TPα/TPβ in both LNCaP and PC-3 cells, and targeted disruption of PRK1 impaired TPα/TPβ-mediated H3 Thr(11) phosphorylation in, and cell migration of, both prostate cell types. Collectively, considering the role of TXA(2) as a potent mediator of RhoA signaling, the identification of PRK1 as a bona fide interactant of TPα/TPβ, and leading to H3 Thr(11) phosphorylation to regulate cell migration, has broad functional significance such as within the vasculature and in neoplasms in which both PRK1 and the TPs are increasingly implicated.

PPARgamma as a potential therapeutic target in pulmonary hypertension

Pulmonary hypertension (PH) is a progressive disorder of the pulmonary circulation associated with significant morbidity and mortality. The pathobiology of PH involves a complex series of derangements causing endothelial dysfunction, vasoconstriction and abnormal proliferation of pulmonary vascular wall cells that lead to increases in pulmonary vascular resistance and pressure. Recent evidence indicates that the ligand-activated transcription factor, peroxisome proliferator-activated receptor gamma (PPARgamma) can have a favorable impact on a variety of pathways involved in the pathogenesis of PH. This review summarizes PPARgamma biology and the emerging evidence that therapies designed to activate this receptor may provide novel approaches to the treatment of PH. Mediators of PH that are regulated by PPARgamma are reviewed to provide insights into potential mechanisms underlying therapeutic effects of PPARgamma ligands in PH.

The Wilms' tumour suppressor protein WT1 acts as a key transcriptional repressor of the human thromboxane A2 receptor gene in megakaryocytes

In humans, the TPalpha and TPbeta isoforms of the thromboxane A2 receptor are transcriptionally regulated by distinct promoters, designated Prm1 and Prm3. Previous investigations identified two upstream repressor regions (URR) 1 and URR2 within Prm1. Herein, it was sought to characterize Prm1, identifying the factor(s) regulating URR1 and URR2 in human erythroleukaemia (HEL) 92.1.7 cells. Genetic reporter assays and 5' deletions confirmed the presence of URR1 and URR2 but also identified a third repressor, designated RR3, within the proximal 'core' promoter. Bioinformatic analysis revealed several GC elements representing putative sites for Egr1/Sp1/Wilms tumour (WT)1 within URR1, URR2 and RR3. While mutation of three GC elements within URR1 and of an adjacent GC element suggested that repressor binding occurs through a cooperative mechanism, repressors binding to the single GC elements within URR2 and RR3 act independently to regulate Prm1. While electrophoretic mobility shift assays and supershift assays demonstrated that each of the GC elements can bind Egr1 and WT1 in vitro, chromatin immunoprecipitations established that WT1 is the factor predominantly bound to each of the repressor regions in vivo. Additionally, ectopic expression of -KTS isoforms of WT1 decreased Prm1-directed gene expression and TPalpha mRNA expression. Collectively, these data establish WT1 as a critical repressor of Prm1, suppressing TPalpha expression in the platelet progenitor megakaryoblastic HEL cells.

Intermolecular cross-talk between the prostaglandin E2 receptor (EP)3 of subtype and thromboxane A(2) receptor signalling in human erythroleukaemic cells

BACKGROUND AND PURPOSE

In previous studies investigating cross-talk of signalling between prostaglandin (PG)E(2) receptor (EP) and the TPalpha and TPbeta isoforms of the human thromboxane (TX)A(2) receptor (TP), 17-phenyl trinor PGE(2)-induced desensitization of TP receptor signalling through activation of the AH6809 and SC19220-sensitive EP(1) subtype of the EP receptor family, in a cell-specific manner. Here, we sought to further investigate that cross-talk in human erythroleukaemic (HEL) 92.1.7 cells.

EXPERIMENTAL APPROACH

Specificity of 17-phenyl trinor PGE(2) signalling and its possible cross-talk with signalling by TPalpha/TPbeta receptors endogenously expressed in HEL cells was examined through assessment of agonist-induced inositol 1,4,5-trisphosphate (IP)(3) generation and intracellular calcium ([Ca(2+)](i)) mobilization.

KEY RESULTS

While 17-Phenyl trinor PGE(2) led to activation of phospholipase (PL)Cbeta to yield increases in IP(3) generation and [Ca(2+)](i), it did not desensitize but rather augmented that signalling in response to subsequent stimulation with the TXA(2) mimetic U46619. Furthermore, the augmentation was reciprocal. Signalling by 17-phenyl trinor PGE(2) was found to occur through AH6809- and SC19920-insensitive, Pertussis toxin-sensitive, G(i)/G(betagamma)-dependent activation of PLCbeta. Further pharmacological investigation using selective EP receptor subtype agonists and antagonists confirmed that 17-phenyl trinor PGE(2)-mediated signalling and reciprocal cross-talk with the TP receptors occurred through the EP(3), rather than the EP(1), EP(2) or EP(4) receptor subtype in HEL cells.

CONCLUSIONS AND IMPLICATIONS

The EP(1) and EP(3) subtypes of the EP receptor family mediated intermolecular cross-talk to differentially regulate TP receptor-mediated signalling whereby activation of EP(1) receptors impaired or desensitized, while that of EP(3) receptors augmented signalling through TPalpha/TPbeta receptors, in a cell type-specific manner.

Peroxisome proliferator-activated receptor gamma down-regulates follistatin in intestinal epithelial cells through SP1

Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) down-regulates the expression of follistatin mRNA in intestinal epithelial cells in vivo. The mechanism of PPARgamma-mediated down-regulation of follistatin was investigated using non-transformed, rat intestinal epithelial cells (RIE-1). RIE cells expressed activin A, the activin receptors ActRI and ActRII, and the follistatin-315 mRNA. RIE-1 cells responded to endogenous activin A, and this response was antagonized by follistatin, as evidenced by changes in cell growth and regulation of an activin-responsive reporter. Using RIE-1 cells, we show that activation of PPARgamma by rosiglitazone reduced follistatin mRNA levels in a dose- and concentration-dependent manner. Down-regulation of follistatin by rosiglitazone required the DNA binding domain of PPARgamma and was dependent upon dimerization with the retinoid X receptor. Inhibition of follistatin expression by rosiglitazone was not associated with decreased follistatin mRNA stability, suggesting that regulation may be at the promoter level. Analysis of the follistatin promoter revealed consensus binding sites for AP-1, AP-2, and Sp1. Targeting the AP-1 pathway with SP600125, an inhibitor of JNK, and TAM67, a dominant negative c-Jun, had no effect on PPARgamma-mediated down-regulation of follistatin. However, the follistatin promoter was dramatically regulated by Sp1, and this regulation was inhibited by PPARgamma expression. Knockdown of Sp1 expression relieved repression of follistatin levels by rosiglitazone. Moreover, PPARgamma was found to interact with Sp1 and repress its transcriptional activation function. Collectively, our data indicate that repression of Sp1 transcriptional activity by PPARgamma is the underlying mechanism responsible for PPARgamma-mediated regulation of follistatin expression.

Regulation of the human thromboxane A2 receptor gene by Sp1, Egr1, NF-E2, GATA-1, and Ets-1 in megakaryocytes

The alpha and beta isoforms of the human thromboxane A(2) (TXA(2)) receptor (TP) are encoded by a single gene but are transcriptionally regulated by distinct promoters, termed promoter 1 (Prm1) and Prm3, respectively. Herein, it was sought to identify factors regulating Prm1 within the megakaryocytic human erythroleukemia 92.1.7 cell line. Through gene deletion and reporter assays, the core Prm1 was localized to between nucleotides -6,320 and -5,895, proximal to the transcription initiation site. Furthermore, two upstream repressor and two upstream activator regions were identified. Site-directed mutagenesis of four overlapping Sp1/Egr1 elements and an NF-E2/AP1 element within the proximal region substantially reduced Prm1 activity. Deletion/mutation of GATA and Ets elements disrupted the upstream activator sequence located between -7,962 and -7,717, significantly impairing Prm1 activity. Electrophoretic mobility shift assays and chromatin immunoprecipitations confirmed that Sp1, Egr1, and NF-E2 bind to elements within the core promoter, whereas GATA-1 and Ets-1 factors bind to the upstream activator sequence (between -7,962 and -7,717). Collectively, these data establish that Sp1, Egr1, and NF-E2 regulate core Prm1 activity in the megakaryocytic-platelet progenitor cells, whereas GATA-1 and Ets-1 act as critical upstream activators, hence providing the first genetic basis for the expression of the human TXA(2) receptor (TP) within the vasculature.

Ciglitazone, a Peroxisome Proliferator-Activated Receptor γ Inducer, Ameliorates Renal Preglomerular Production and Activity of Angiotensin II and Thromboxane A2 in Glycerol-Induced Acute Renal Failure

Peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear transcription factor, modulates vascular responses to angiotensin II (AII) or thromboxane A(2) (TxA(2)) via regulation of their gene/receptor. Increased vasoconstriction and deteriorating renal function in glycerol-induced acute renal failure (ARF) may be attributed to down-regulation of PPARgamma. In this study, we investigated the effect of ciglitazone (CG), a PPARgamma inducer, on AII and TxA(2) production and activity in glycerol-induced ARF. Vascular responses to AII or 9,11-dideoxy-11alpha,9alpha-epoxymethano prostaglandin F(2alpha) (U46619), a TxA(2) mimetic, were determined in preglomerular vessels following induction of ARF with glycerol. Renal damage and function were assessed in CG-treated (9 nmol/kg for 21 days) rats. PPARgamma protein expression and activity, which were significantly lower in ARF rats, were enhanced by CG (26 and 30%). CG also increased PPARgamma mRNA by 67 +/- 6%, which was reduced in ARF. In ARF, there was significant tubular necrosis and apoptosis, a 5-fold increase in proteinuria and a 2-fold enhancement in vasoconstriction to AII and U46619. CG reduced proteinuria (49 +/- 3%), enhanced Na(+) (124 +/- 35%) and creatinine excretion (92 +/- 25%), markedly diminished tubular necrosis, and reduced ARF-induced increase in AII (40 +/- 3%) and TxA(2) (39 +/- 2%) production, the attending increase in vasoconstriction to AII (36 +/- 2%) and U46619 (50 +/- 11%), and the increase in angiotensin receptor-1 (AT(1)) (23 +/- 3%) or thromboxane prostaglandin (TP) receptor (13 +/- 1%). CG reduced free radical generation by 55 +/- 14% while elevating nitrite excretion (65 +/- 13%). Our results suggest that enhanced activity of AII and TxA(2), increased AT(1) or TP receptor expression, and renal injury in glycerol-induced ARF are consequent to down-regulation of PPARgamma gene. CG ameliorated glycerol-induced effects through maintaining PPARgamma gene.

Homologous desensitization of signalling by the alpha (alpha) isoform of the human thromboxane A2 receptor: a specific role for nitric oxide signalling

Thromboxane (TX) A₂ plays a central role in hemostasis, regulating platelet activation status and vascular tone. We have recently established that the TPβ isoform of the human TXA₂ receptor (TP) undergoes rapid, agonist-induced homologous desensitization of signalling largely through a G protein-coupled receptor kinase (GRK) 2/3-dependent mechanism with a lesser role for protein kinase (PK) C. Herein, we investigated the mechanism of desensitization of signalling by the TPα isoform. TPα undergoes profound agonist-induced desensitization of signalling (intracellular calcium mobilization and inositol 1,4,5 trisphosphate generation) in response to the TXA₂ mimetic U46619 but, unlike that of TPβ, this is independent of GRKs. Similar to TPβ, TPα undergoes partial agonist-induced desensitization that occurs through a GF 109203X-sensitive, PKC mechanism where Ser¹⁴⁵ within intracellular domain (IC)₂ represents the key phospho-target. TPα also undergoes more profound sustained PKC- and PKG-dependent desensitization where Thr³³⁷ and Ser³³¹, respectively, within its unique C-tail domain were identified as the phospho-targets. Desensitization was impaired by the nitric oxide synthase (NOS), soluble guanylyl cyclase (sGC) and PKG inhibitors l-NAME, LY 83583 and KT5823, respectively, indicating that homologous desensitization of TPα involves nitric oxide generation and signalling. Consistent with this, U46619 led to rapid phosphorylation/activation of endogenous eNOS. Collectively, data herein suggest a mechanism whereby agonist-induced PKC phosphorylation of Ser¹⁴⁵ partially and transiently impairs TPα signalling while PKG- and PKC-phosphorylation at both Ser³³¹ and Thr³³⁷, respectively, within its C-tail domain profoundly desensitizes TPα, effectively terminating its signalling. Hence, in addition to the agonist-mediated PKC feedback mechanism, U46619-activation of the NOS/sGC/PKG pathway plays a significant role in inducing homologous desensitization of TPα.

Palmitoylation of the TPbeta isoform of the human thromboxane A2 receptor. Modulation of G protein: effector coupling and modes of receptor internalization

Palmitoylation is a prevalent feature amongst G protein-coupled receptors. In this study we sought to establish whether the TPα and TPβ isoforms of the human prostanoid thromboxane (TX) A₂ receptor (TP) are palmitoylated and to assess the functional consequences thereof. Consistent with the presence of three cysteines within its unique carboxyl-terminal domain, metabolic labelling and site-directed mutagenesis confirmed that TPβ is palmitoylated at Cys³⁴⁷ and, to a lesser extent, at Cys^373,377 whereas TPα is not palmitoylated. Impairment of palmitoylation did not affect TPβ expression or its ligand affinity. Conversely, agonist-induced [Ca²⁺]_i mobilization by TPβ^C347S and the non-palmitoylated TPβ^{C347,373,377S}, but not by TPβ^C373S or TPβ^C373,377S, was significantly reduced relative to the wild type TPβ suggesting that palmitoylation at Cys³⁴⁷ is specifically required for efficient Gq/phospholipase Cβ effector coupling. Furthermore, palmitoylation at Cys^373,377 is critical for TPβ internalization with TPβ^C373S, TPβ^C373,377S and TPβ^{C347,373,377S} failing to undergo either agonist-induced or temperature-dependent tonic internalization. On the other hand, whilst TPβ^C347S underwent reduced agonist-induced internalization, it underwent tonic internalization to a similar extent as TPβ. The deficiency in agonist-induced internalization by TPβ^C347S, but not by TPβ^C373,377 nor TPβ^{C347,373,377S}, was overcome by over-expression of either β-arrestin1 or β-arrestin2. Taken together, data herein suggest that whilst palmitoylation of TPβ at Cys^373,377 is critical for both agonist- and tonic-induced internalization, palmitoylation at Cys³⁴⁷ has a role in determining which pathway is followed, be it by the β-arrestin-dependent agonist-induced pathway or by the β-arrestin-independent tonic internalization pathway.

Downregulation of cyclooxygenase-2 expression and activation of caspase-3 are involved in peroxisome proliferator-activated receptor-gamma agonists induced apoptosis in human monocyte leukemia cells in vitro

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a transcription factor important in fat metabolism and PPAR-gamma agonists were recently demonstrated to affect proliferation, differentiation, and apoptosis of different cell types. In the present study, two PPAR-gamma agonists, 15-deoxy-delta (12,14)-prostaglandin J2 (15d-PGJ2) and a synthetic PPAR-gamma agonist troglitazone (TGZ), were used to investigate activated PPAR-gamma-induced apoptosis on human monocyte leukemia U937 and Mono Mac 6 cells in vitro. The results showed that both U937 and Mono Mac 6 cells demonstrated constitutive activation of COX-2 expression; treatment by 15d-PGJ2 and TGZ could induce apoptosis remarkably in human monocyte leukemia cells by disruption of mitochondrial membrane potential, activation of caspase-3, and causing cleavage of the caspase substrate poly (ADP-ribose) polymerase (PARP). Further studies revealed that treatment by both 15d-PGJ2 and TGZ remarkably downregulated COX-2 expression in these two kind of monocyte leukemia cells as measured by reverse transcriptase PCR (RT-PCR) and Western blot. Furthermore, the expression of Bcl-2 and Bcl-Xl and Mcl-1 was downregulated while Bax expression was upregulated concurrently after the cells were treated by these two agonists, and no variations were found in other Bcl-2 family members such as Bak, Bid, and Bad. Taken together, our results demonstrate for the first time that downregulation of cyclooxygenase-2 expression, disruption of mitochondrial membrane potential, activation of caspase-3, downregulation of Bcl-2, Bcl-Xl, and Mcl-1, and upregulation of Bax are involved in PPAR-gamma agonists-induced apoptosis in these two human monocyte leukemia cells.

Homologous desensitization of signalling by the beta (β) isoform of the human thromboxane A2 receptor

Thromboxane (TX) A(2) is a potent stimulator of platelet activation/aggregation and smooth muscle contraction and contributes to a variety of pathologies within the vasculature. In this study, we investigated the mechanism whereby the cellular responses to TXA(2) mediated through the TPbeta isoform of the human TXA(2) receptor (TP) are dynamically regulated by examining the mechanism of agonist-induced desensitization of intracellular signalling and second messenger generation by TPbeta. It was established that TPbeta is subject to profound agonist-induced homologous desensitization of signalling (intracellular calcium mobilization and inositol 1,3,5 trisphosphate generation) in response to stimulation with the TXA(2) mimetic U46619 and this occurs through two key mechanisms: TPbeta undergoes partial agonist-induced desensitization that occurs through a GF 109203X-sensitive, protein kinase (PK)C mechanism whereby Ser(145) within intracellular domain (IC)(2) has been identified as the key phospho-target. In addition, TPbeta also undergoes more profound and sustained agonist-induced desensitization involving G protein-coupled receptor kinase (GRK)2/3-phosphorylation of both Ser(239) and Ser(357) within its IC(3) and carboxyl-terminal C-tail domains, respectively. Inhibition of phosphorylation of either Ser(239) or Ser(357), through site directed mutagenesis, impaired desensitization while mutation of both Ser(239) and Ser(357) almost completely abolished desensitization of signalling, GRK phosphorylation and beta-arrestin association, thereby blocking TPbeta internalization. These data suggest a model whereby agonist-induced PKC phosphorylation of Ser(145) partially impairs. TPbeta signalling while GRK2/3 phosphorylation at both Ser(239) and Ser(357) within its IC(3) and C-tail domains, respectively, sterically inhibits G-protein coupling, profoundly desensitizing signalling, and promotes beta-arrestin association and, in turn, facilitates TPbeta internalization. Thromboxane (TX) A(2) is a potent stimulator of platelet aggregation and smooth muscle contraction and contributes to a variety of vascular pathologies. Herein the mechanism whereby the cellular responses to TXA(2) mediated through the TPbeta isoform of the human TXA(2) receptor (TP) are dynamically regulated was investigated by examining the mechanism of its agonist-induced desensitization of intracellular signalling and second messenger generation. TPbeta is subject to profound agonist-induced homologous desensitization of signalling (intracellular calcium mobilization and inositol 1,3,5 trisphosphate generation) in response to stimulation with the TXA(2) mimetic U46619 and this occurs through two key mechanisms: TPbeta undergoes partial agonist-induced desensitization that occurs through a GF 109203X-sensitive, protein kinase (PK)C mechanism whereby Ser(145) within intracellular domain (IC)(2) was identified as the key phospho-target. In addition, TPbeta also undergoes more profound and sustained agonist-induced desensitization involving G protein-coupled receptor kinase (GRK)2/3-phosphorylation of both Ser(239) and Ser(357) within its IC(3) and carboxyl-terminal C-tail domains, respectively. Inhibition of phosphorylation of either Ser(239) or Ser(357), through site directed mutagenesis, impaired desensitization while mutation of both Ser(239) and Ser(357) almost completely abolished desensitization of signalling, GRK phosphorylation and beta-arrestin association, thereby blocking TPbeta internalization. These data suggest a model whereby agonist-induced PKC phosphorylation of Ser(145) partially impairs TPbeta signalling while GRK2/3 phosphorylation at both Ser(239) and Ser(357) within its IC(3) and C-tail domains, respectively, sterically inhibits G-protein coupling, profoundly desensitizing signalling, and promotes beta-arrestin association and, in turn, facilitates TPbeta internalization.