Elevated levels of plasma inactive stromal cell derived factor-1α predict poor long-term outcomes in diabetic patients following percutaneous coronary intervention.

BACKGROUND: Since the complication of diabetes mellitus (DM) is a risk for adverse cardiovascular outcomes in patients with coronary artery disease (CAD), appropriate risk estimation is needed in diabetic patients following percutaneous coronary intervention (PCI). However, there is no useful biomarker to predict outcomes in this population. Although stromal cell derived factor-1α (SDF-1α), a circulating chemokine, was shown to have cardioprotective roles, the prognostic impact of SDF-1α in diabetic patients with CAD is yet to be fully elucidated. Moreover, roles of SDF-1α isoforms in outcome prediction remain unclear. Therefore, this study aimed to assess the prognostic implication of three forms of SDF-1α including total, active, and inactive forms of SDF-1α in patients with DM and after PCI. METHODS: This single-center retrospective analysis involved consecutive patients with diabetes who underwent PCI for the first time between 2008 and 2018 (n = 849). Primary and secondary outcome measures were all-cause death and the composite of cardiovascular death, non-fatal myocardial infarction, and ischemic stroke (3P-MACE), respectively. For determining plasma levels of SDF-1α, we measured not only total, but also the active type of SDF-1α by ELISA. Inactive isoform of the SDF-1α was calculated by subtracting the active isoform from total SDF-1α. RESULTS: Unadjusted Kaplan-Meier analyses revealed increased risk of both all-cause death and 3P-MACE in patients with elevated levels of inactive SDF-1α. However, plasma levels of total and active SDF-1α were not associated with cumulative incidences of outcome measures. Multivariate Cox hazard analyses repeatedly indicated the 1 higher log-transformed inactive SDF-1α was significantly associated with increased risk of all-cause death (hazard ratio (HR): 2.64, 95% confidence interval (CI): 1.28-5.34, p = 0.008) and 3P-MACE (HR: 2.51, 95% CI: 1.12-5.46, p = 0.02). Moreover, the predictive performance of inactive SDF-1α was higher than that of total SDF-1α (C-statistics of inactive and total SDF-1α for all-cause death: 0.631 vs 0.554, for 3P-MACE: 0.623 vs 0.524, respectively). CONCLUSION: The study results indicate that elevated levels of plasma inactive SDF-1α might be a useful indicator of poor long-term outcomes in diabetic patients following PCI. TRIAL REGISTRATION: This study describes a retrospective analysis of a prospective registry database of patients who underwent PCI at Juntendo University Hospital, Tokyo, Japan (Juntendo Physicians' Alliance for Clinical Trials, J-PACT), which is publicly registered (University Medical Information Network Japan-Clinical Trials Registry, UMIN-CTR 000035587).

Dipeptidyl peptidase-4 inhibitors reduced long-term cardiovascular risk in diabetic patients after percutaneous coronary intervention via insulin-like growth factor-1 axis.

Dipeptidyl-peptidase-4 inhibitors (DPP4i) have been the most used antidiabetic medications worldwide due to their good safety profiles and tolerability with a low risk of hypoglycemia, however, large cardiovascular outcome trials (CVOTs) have not shown any significant the prognostic superiority. On the contrary, since observational studies have suggested the effects of DPP4i are enhanced some populations, such as Asians and those who without overweight, their prognostic benefit is still under debate. The aim of this study was thus to assess the prognostic impact of DPP4i in patients with both diabetes and coronary artery disease (CAD) who underwent percutaneous coronary intervention (PCI) through the insulin-like growth factor-1 (IGF-1) axis, a substrate of DPP4. This single-center analysis involved consecutive Japanese diabetic patients who underwent PCI for the first time between 2008 and 2018 (n = 885). Primary and secondary endpoints were set as cardiovascular (CV) death and the composite of CV death, non-fatal myocardial infarction and ischemic stroke (3P-MACE). Serum levels of IGF-1 and its main binding protein (insulin-like growth factor binding protein-3: IGFBP-3) were measured. In consequences, unadjusted Kaplan-Meier analyses revealed reduced incidences of CV-death and 3P-MACE by DPP4i, which was particularly enhanced in patients who were not overweight (BMI ≤ 25). Multivariate Cox hazard analyses consistently indicated reduced risks of CV death by DPP4i at PCI (hazard ratio (HR) 0.39, 95% confidence interval (CI) 0.16-0.82, p = 0.01) and 3P-MACE (HR 0.47, 95% CI 0.25-0.84, p = 0.01), respectively. Moreover, elevated IGF-1 activity indicated by the IGF-1/IGFBP-3 ratio was associated with decreased risks of both endpoints and it was significantly higher in patients with DPP4i (p < 0.0001). In conclusion, the findings of the present study indicate beneficial effects of DPP4i to improve outcomes in Japanese diabetic patients following PCI, which might be mediated by DPP4-IGF-1 axis.

The association between impairment of HDL cholesterol efflux capacity and atrial remodeling in atrial fibrillation.

This cross-sectional study enrolled 202 patients with atrial fibrillation (AF) who had undergone catheter ablation and evaluated the association between high-density lipoprotein (HDL) functionality, cholesterol efflux capacity (CEC) of HDL, and the pathophysiology of left atrial structural remodeling. Participants were divided into two groups, based on their left atrial volume index (LAVI) (< 34 mL/m2, n = 60 vs. LAVI ≥ 34 mL/m2, n = 142). We quantified three types of HDL CECs by the presence or absence of cyclic-AMP, as entire, and CEC dependent or not dependent on ATP binding cassette transporter A1 (ABCA1) and termed them Global CEC, ABCA1 CEC, and Non-ABCA1 CEC, respectively. Consequently, Global and Non-ABCA1 CECs were significantly impaired in patients with an enlarged LA (Global CEC: p = 0.039, Non-ABCA1 CEC: p = 0.022). Logistic regression analyses demonstrated that Non-ABCA1 CEC was significantly associated with an enlarged LA after adjusting for the conventional risk factors of AF. Furthermore, the association of higher Non-ABCA1 CEC with an enlarged LA was independent of serum levels of HDL cholesterol and serum myeloperoxidase (Odds ratio of 1 standard deviation higher: 0.64, 95% confidence interval: 0.43-0.95, p = 0.027). The findings of this study indicate the potential contribution of reduced Non-ABCA1 CEC in HDL to the pathophysiology in left atrial structural remodeling of patients with AF.

Enhanced monocyte migratory activity in the pathogenesis of structural remodeling in atrial fibrillation.

BACKGROUND AND AIMS: Pathophysiological roles of monocytes in atrial fibrillation (AF), particularly for the progression of structural remodeling of the left atrium (LA), remain elusive. This study examined the association between the characteristics of circulating and local monocytes and extent of structural remodeling in LA, gauged by LA size, in AF patients. METHODS: First, 161 AF patients who were referred for catheter ablation were enrolled and divided into two groups according to the median of LA diameter (≤39 mm: normal LA group, >39 mm: enlarged LA group). As a control group, 22 patients underwent catheter ablation for paroxysmal supraventricular tachycardia (PSVT) without history of AF were analyzed. Blood samples were collected for flow cytometric analyses to evaluate monocyte subsets based on the levels of CD14 and CD16. Moreover, monocytes were isolated from blood to measure CC chemokine receptor 2 (CCR2) transcripts and protein levels, and migratory activity toward monocyte chemoattractant protein 1 (MCP-1). Second, to characterize the local monocytes in the atrial wall in AF, the resected left atrial appendages (LAA) in AF patients underwent cardiac surgery were histologically evaluated (n = 20). RESULTS: The proportions of monocyte subsets based on CD14 and CD16 expressions were not significantly different between the normal and enlarged LA group. Both transcripts and total protein levels of CCR2 in monocytes were higher in the enlarged LA group compared to those in the normal LA group. In the enlarged LA group, monocytes exhibited more enhanced migratory activity than the normal LA group. Moreover, we found a significantly higher number of CCR2-positive monocytes/macrophages in the LAA in the enlarged LA group. CONCLUSION: Enhanced migratory activity in circulating and local monocytes may play a pivotal role in the pathogenesis of progression in atrial remodeling in AF patients.

Increased risk of cardiovascular mortality by strict glycemic control (pre-procedural HbA1c < 6.5%) in Japanese medically-treated diabetic patients following percutaneous coronary intervention: a 10-year follow-up study.

BACKGROUND: In the secondary prevention of cardiovascular (CV) disease in patients with diabetes, an optimal level of HbA1c, the most widely-used glycemic control indicator, for favorable clinical consequences still remains to be established. This study assessed the association between preprocedural HbA1c level and CV mortality in Japanese diabetic patients undergoing percutaneous coronary intervention (PCI). METHODS: This is a retrospective observational study using a single-center prospective PCI database involving consecutive 4542 patients who underwent PCI between 2000 and 2016. Patients with any antidiabetic medication including insulin at PCI were included in the analysis (n = 1328). We divided the patients into 5 and 2 groups according to HbA1c level; HbA1c: < 6.5% (n = 267), 6.5-7.0% (n = 268), 7.0-7.5% (n = 262), 7.5-8.5% (n = 287) and ≥ 8.5% (n = 244), and 7.0% > and ≤ 7.0%, respectively. The primary outcome was CV mortality including sudden death. The median follow-up duration was 6.2 years. RESULTS: In the follow-up period, CV and sudden death occurred in 81 and 23 patients, respectively. While unadjusted Kaplan-Meier analysis showed no difference in cumulative CV mortality rate between patients binarized by preprocedural HbA1c 7.0%, analysis of the 5 groups of HbA1c showed significantly higher cumulative CV death in patients with HbA1c < 6.5% compared with those with 7.0-7.5% (P = 0.042). Multivariate Cox hazard analysis revealed a U-shaped relationship between preprocedural HbA1c level and risk of CV death, and the lowest risk was in the HbA1c 7.0-7.5% group (Hazard ratio of HbA1c < 6.5% compared to 7.0-7.5%: 2.97, 95% confidence interval: 1.33-7.25, P = 0.007). Similarly, univariate analysis revealed the lowest risk of sudden death was in the HbA1c 7.0-7.5% group. CONCLUSION: The findings indicate an increased risk of CV mortality by strict glycemic control (HbA1c < 6.5%) in the secondary prevention of CV disease in Japanese patients with medically-treated diabetes. Trial registration This study reports the retrospective analysis of a prospective registry database of patients who underwent PCI at Juntendo University Hospital, Tokyo, Japan (Juntendo Physicians' Alliance for Clinical Trials, J-PACT), which is publicly registered (University Medical Information Network Japan-Clinical Trials Registry UMIN-CTR 000035587).

Reduced Number of Platelets During Intra-Aortic Balloon Pumping Counterpulsation Predicts Higher Cardiovascular Mortality After Device Removal in Association with Systemic Inflammation.

Thrombocytopenia is a frequent complication in patients requiring intra-aortic balloon pumping (IABP) counterpulsation. However, its prognostic impact has not been fully addressed. The objective of this study is to evaluate the impact of the change in the platelet number during IABP use on the prognosis after device removal.This is a retrospective observational study. Patients in the intensive cardiac care unit at three Juntendo University hospitals who underwent percutaneous implantation of IABP with or without veno-arterial extracorporeal membrane oxygenation (V-A ECMO), since 2012-2016, were enrolled in the study (n = 439). Patients who died during mechanical circulatory support (n = 47) were excluded. We evaluated the prognostic impact of the ratio of platelet reduction from the baseline (% PLT reduction) during IABP use on cardiovascular mortality after device removal.The median and the range of follow-up period were 298 days and 0-1,869 days, respectively. Unadjusted Kaplan-Meier analysis demonstrated that patients with a higher % PLT reduction had higher cardiovascular (CV) mortality. An adjusted Cox proportional hazard analysis demonstrated that a 10% higher % PLT reduction was associated with higher cardiovascular (CV) mortality (Hazard ratio: 1.3, 95% Confidence interval: 1.1-1.6, P < 0.001). Moreover, % PLT reduction and the maximum C-reactive protein (CRP) level during IABP use were positively correlated (r = 0.326, P < 0.001).The reduced number of platelets during IABP use was associated with an increased risk of CV mortality.

Promotion of oxidative stress is associated with mitochondrial dysfunction and muscle atrophy in aging mice.

AIM: We examined the changes in oxidative stress, mitochondrial function and muscle atrophy during aging in mice. METHODS: We used 6-, 12- and 24-month (6 M, 12 M and 24 M)-old C57BL/6J mice. Skeletal muscles were removed from the lower limb and used for quantitative real-time polymerase chain reaction, immunoblotting and histological analyses. RESULTS: The muscle weight and myocyte cross-sectional area were significantly decreased in the 12 M and 24 M mice compared with those of the 6 M mice. The levels of the oxidative stress markers, nicotinamide adenine dinucleotide phosphate oxidase 2, nicotinamide adenine dinucleotide phosphate oxidase 4, mitochondrial 4-hydroxy-2-nonenal and 3-nitrotyrosine, were significantly higher in the 24 M mice compared with those of the 6 M mice. Furthermore, the 24 M mice had lower levels of mitochondrial markers, peroxisome proliferator-activated receptor gamma coactivator 1 (PGC)-α, peroxisome proliferator-activated receptor gamma coactivator-1ß, sirtuin-1, adenosine triphosphate synthase mitochondria F1 complex α subunit 1 and mitochondrial cytochrome c oxidase 1. The ubiquitin-proteasome pathway genes muscle ring finger-1 and atrogin-1 were significantly upregulated in the 12 M and 24 M mice, and protein synthesis markers (phosphorylated-Akt and -p70 ribosomal S6 kinase) were significantly lower in the 24 M mice compared with the 6 M mice (all P < 0.05). CONCLUSIONS: These findings have important implications for the mechanisms that underlie sarcopenia and frailty processes. Geriatr Gerontol Int 2020; 20: 78-84.

A Novel Nutritional Index Serves as A Useful Prognostic Indicator in Cardiac Critical Patients Requiring Mechanical Circulatory Support.

BACKGROUND: A poor nutritional status has been gathering intense clinical interest recently as it has been suggested to associate with adverse outcomes in patients in the intensive care unit (ICU). However, there is still no established nutritional index dominantly used in clinical practice. We have previously proposed a novel nutritional index, which can be calculated using serum levels of triglycerides, total cholesterol, and body weight (TCBI). In this study, to expand the application of TCBI for critical patients, we investigated the usefulness of TCBI to predict prognosis in hemodynamically unstable patients with percutaneously implantable mechanical circulatory support (MCS) devices in the ICU. PATIENTS AND METHODS: This is a retrospective analysis of a multicenter registry consisting of three Juntendo University hospitals in Japan involving patients who received MCS devices, including intra-aortic balloon pumping (IABP) with or without veno-arterial extracorporeal membrane oxygenation (VA-ECMO), between 2012 and 2016 (n = 439). The median follow-up period was 298 days. RESULTS: Spearman's correlation coefficient between TCBI and the geriatric nutritional risk index (GNRI) was 0.44 (p < 0.0001), indicating a moderate positive correlation for these two variables. Unadjusted Kaplan-Meier analysis demonstrated reduced risks of all-cause and cardiovascular mortalities in patients with higher tertiles of TCBI. Furthermore, adjusted multivariate Cox proportional hazard analyses revealed that the highest tertile TCBI was an independent predictor for the reduced risk of all-cause mortality (hazard ratio (HR): 0.22, 95% confidence interval: 0.10-0.48, p < 0.0001) and cardiovascular mortality (0.20, 0.09-0.45, p < 0.0001). CONCLUSION: A novel and simple to calculate nutritional index, TCBI, can be applicable as a prognostic indicator in hemodynamically unstable patients requiring MCS devices.

Lack of IκBNS promotes cholate-containing high-fat diet-induced inflammation and atherogenesis in low-density lipoprotein (LDL) receptor-deficient mice.

BACKGROUND: IκBNS, a nuclear IκB protein, regulates a subset of Toll-like receptor (TLR) dependent genes. A cholate-containing high-fat diet (HFD(CA(+))) induces TLR4 mediated early inflammatory response. The present study aims to clarify that the lack of IκBNS promotes atherogenesis in low-density lipoprotein receptor-deficient (LDLr-/-) mice fed HFD(CA(+)) compared with those fed a cholate-free HFD (HFD(CA(-))). METHODS AND RESULTS: Mice that lacked IκBNS (IκBNS-/-) were crossed with LDLr-/- mice and formation of atherosclerotic lesions was analyzed after 6-week consumption of HFD(CA(+)) or HFD(CA(-)). IκBNS-/-/LDLr-/- mice fed HFD(CA(+)) (IκBNS-/-/LDLr-/-(CA(+))) showed a 3.5-fold increase of atherosclerotic lesion size in the aorta compared with LDLr-/-(CA(+)) mice (p < 0.01), whereas there was no difference between LDLr-/-(CA(-)) and IκBNS-/-/LDLr-/-(CA(-)) mice. Immunohistochemical analysis of the aortic root revealed that HFD(CA(+)) significantly increased Mac-3 (macrophage)-positive area by 1.5-fold (p < 0.01) and TLR4, interleukin-6 (IL-6) expression by 1.7-fold (p < 0.05) and 1.5-fold (p < 0.05), respectively, in IκBNS-/-/LDLr-/-(CA(+)) compared with LDLr-/---(CA(+)) mice. Furthermore, active STAT3 (pSTAT3)-positive cells were significantly increased by 1.7-fold in the lesions of IκBNS-/-/LDLr-/-(CA(+)) compared with LDLr-/-(CA(+)) mice (p < 0.01). These findings suggest that IκBNS deficiency and HFD(CA(+)) promote atherogenesis in LDLr-/- mice via TLR4/IL-6/STAT3 pathway. Finally, we showed that the monocytes from peripheral blood of IκBNS-/-/LDLr-/-(CA(+)) mice were found to contain the highest proportion of Ly6Chi monocytes among the four groups, suggesting that lack of IκBNS enhanced inflammation in response to HFD(CA(+)) feeding. CONCLUSIONS: The present study is the first to demonstrate that the activation of innate immune system using HFD(CA(+)) induced significant inflammation and atherogenesis in IκBNS-/-/LDLr-/- compared with LDLr-/- mice.

Mass spectrometry meets the challenge of understanding the complexity of the lipoproteome: recent findings regarding proteins involved in dyslipidemia and cardiovascular disease.

Despite the fact that link between dyslipidemia and atherosclerosis was made over 100 years ago, atherosclerosis remains a major cause of morbidity and mortality worldwide. Major efforts focus towards understanding lipid metabolism, particularly by studying its particle compartments in circulation: the lipoproteins. In recent years, mass spectrometry has played an integral role in the deep sequencing of the lipoproteome and in metabolism studies conducted in vivo. This review highlights the path of lipoprotein research towards state-of-the-art mass spectrometry with special emphasis on the method of selected reaction monitoring and its impact on apolipoprotein metabolism studies. Also presented is what is expected for the lipoprotein field with the recent advent of high resolution/accurate mass parallel reaction monitoring mass spectrometry. The benefits of high resolution/accurate mass measurements are demonstrated by example instrument workflows and by detailing a novel method to quantify very low levels of circulating proprotein convertase subtilisin-kexin type 9 in rabbit. It is anticipated that future clinical studies or clinical trials aimed to treat dyslipidemia by manipulating key regulatory proteins will benefit from the new and exciting opportunities afforded by the latest technologies in mass spectrometry.

New CETP inhibitor K-312 reduces PCSK9 expression: a potential effect on LDL cholesterol metabolism.

Despite significant reduction of cardiovascular events by statin treatment, substantial residual risk persists, driving emerging needs for the development of new therapies. We identified a novel cholesteryl ester transfer protein (CETP) inhibitor, K-312, that raises HDL and lowers LDL cholesterol levels in animals. K-312 also suppresses hepatocyte expression of proprotein convertase subtilisin/kexin 9 (PCSK9), a molecule that increases LDL cholesterol. We explored the underlying mechanism for the reduction of PCSK9 expression by K-312. K-312 inhibited in vitro human plasma CETP activity (IC50; 0.06 µM). Administration of K-312 to cholesterol-fed New Zealand White rabbits for 18 wk raised HDL cholesterol, decreased LDL cholesterol, and attenuated aortic atherosclerosis. Our search for additional beneficial characteristics of this compound revealed that K-312 decreases PCSK9 expression in human primary hepatocytes and in the human hepatoma cell line HepG2. siRNA silencing of CETP in HepG2 did not compromise the suppression of PCSK9 by K-312, suggesting a mechanism independent of CETP. In HepG2 cells, K-312 treatment decreased the active forms of sterol regulatory element-binding proteins (SREBP-1 and -2) that regulate promoter activity of PCSK9. Chromatin immunoprecipitation assays demonstrated that K-312 decreased the occupancy of SREBP-1 and SREBP-2 on the sterol regulatory element of the PCSK9 promoter. PCSK9 protein levels decreased by K-312 treatment in the circulating blood of cholesterol-fed rabbits, as determined by two independent mass spectrometry approaches, including the recently developed, highly sensitive parallel reaction monitoring method. New CETP inhibitor K-312 decreases LDL cholesterol and PCSK9 levels, serving as a new therapy for dyslipidemia and cardiovascular disease.

Low expression of cell-surface thromboxane A2 receptor beta-isoform through the negative regulation of its membrane traffic by proteasomes.

Human thromboxane A(2) receptor (TP) consists of two alternatively spliced isoforms, TP alpha and TP beta, which differ in their cytoplasmic tails. To examine the functional difference between TP alpha and TP beta, we searched proteins bound to C termini of TP isoforms by a yeast two-hybrid system, and found that proteasome subunit alpha 7 and proteasome activator PA28 gamma interacted potently with the C terminus of TP beta. The binding of TP beta with alpha 7 and PA28 gamma was confirmed by co-immunoprecipitation and pull-down assays. MG-132 and lactacystin, proteasome inhibitors, increased cell-surface expression of TP beta, but not TP alpha. Scatchard analysis of [(3)H]SQ29548 binding revealed that the B(max) was higher in transiently TP alpha-expressing cells than TP alpha-expressing cells. In addition, TP-mediated phosphoinositide hydrolysis was clearly observed in TP alpha-, but not TP beta-expressing cells. These results suggest that TP beta binds to alpha 7 and PA28 gamma, and the cell-surface expression of TP beta is lower than that of TP alpha through the negative regulation of its membrane traffic by proteasomes.

Different pathways for activation of extracellular signal-regulated kinase through thromboxane A2 receptor isoforms.

Thromboxane A2 receptor (TP) consists of two alternatively spliced isoforms, TPalpha and TPbeta, which differ in their cytoplasmic tails. In the present study, we examined the difference in signal transduction of TPalpha and TPbeta, using stably expressing cells of TPalpha and TPbeta. The cells expressing TPalpha (TPalpha-SC2) and TPbeta (TPbeta-SC15) were selected based on the similar binding sites of [3H]-SQ29548, a TP antagonist. U46619, a TP agonist, elicited phosphoinositide hydrolysis in TPalpha-SC2 and TPbeta-SC15 cells with a similar concentration-dependency. U46619 also caused the phosphorylation of extracellular signal-regulated kinase (ERK1/2) in both TPalpha-SC2 and TPbeta-SC15 cells. While the peak of the phosphorylation of ERK1/2 was observed 5 min after addition of U46619 in TPalpha-SC2 cells, the long lasting phosphorylation up to 60 min was in TPbeta-SC15 cells. U46619-induced phosphorylation of ERK1/2 at 5 min was inhibited by pertussis toxin in both cells, suggesting that G(i) is involved in the phosphorylation mediated via both TP isoforms. Interfering G(12/13) activity by overexpression of p115-RGS reduced U46619-induced ERK1/2 phosphorylation in TPbeta-SC15 cells, but not in TPalpha-SC2 cells. H89, an inhibitor of protein kinase A (PKA), reduced U46619-induced ERK1/2 phosphorylation in TPalpha-SC2 cells, but not in TPbeta-SC15 cells. These results indicate that G(i) may be involved in TP-mediated ERK1/2 phosphorylation in both isoforms. In addition, H89-sensitive kinase and G(12/13) may be involved in TP-mediated ERK1/2 phosphorylation in TPalpha and TPbeta, respectively.

Physiological significance of thromboxane A(2) receptor dimerization.

The thromboxane A(2) receptor (TP), one of the G protein-coupled receptors (GPCRs), consists of two splicing variants, TPalpha and TPbeta, which differ in their C-terminal regions. In the present study, we investigated whether TPalpha and TPbeta formed homo- or hetero-dimers and whether the dimerization changed the function of TP. The immunofluorescent analysis using human embryonic kidney (HEK) 293 cells expressing either FLAG-tagged TPalpha or TPbeta showed that TPalpha is mainly distributed on plasma membranes and TPbeta existed on plasma membranes and within the cells. Co-immunoprecipitation analysis using HEK293 cells expressing both TPalpha and TPbeta showed that TPalpha and TPbeta formed homo- and hetero-dimers. U46619, a TP agonist, caused phosphoinositide hydrolysis and elevation of [Ca(2+)](i) in a concentration-dependent manner in Chinese hamster ovary (CHO) cells expressing TPalpha or TPbeta. The responses were observed to a greater extent in the cells expressing TPalpha than TPbeta. In the cells expressing both TPalpha and TPbeta, U46619-induced responses were observed to a lesser extent than in the cells expressing TPalpha alone. Furthermore, [(3)H]SQ29548 binding showed that the level of the cell surface expression of TP was the following order: the cells expressing TPalpha > TPalpha and TPbeta > TPbeta. These results indicate that TPalpha and TPbeta formed homo- and hetero-dimers, and TP-mediated signaling may be regulated by the hetero-dimer.