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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2007;27:219.)
© 2007 American Heart Association, Inc.

Atherosclerosis and Lipoproteins

Apolipoprotein CIII-Induced THP-1 Cell Adhesion to Endothelial Cells Involves Pertussis Toxin-Sensitive G Protein- and Protein Kinase C-Mediated Nuclear Factor-B Activation

Akio Kawakami; Masanori Aikawa; Noriko Nitta; Masayuki Yoshida; Peter Libby; Frank M. Sacks

From the Department of Nutrition (A.K., F.M.S.), Harvard School of Public Health, and the Cardiovascular Division (M.A., P.L., F.M.S.), Brigham and Women’s Hospital, Department of Medicine, Harvard Medical School, Boston, Mass; and the Department of Medical Biochemistry (N.N., M.Y.), Graduate School of Medicine, Tokyo Medical and Dental University, Japan. Current affiliation for A.K.: Geriatrics and Vascular Medicine, Tokyo Medical and Dental University, Japan.

Correspondence to Frank M. Sacks, MD, Department of Nutrition, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115. E-mail fsacks{at}hsph.harvard.edu

	Abstract

Top Abstract Introduction Methods Results Discussion References

 
Objective— Plasma apolipoprotein CIII (apoCIII) independently predicts risk for coronary heart disease (CHD). We recently reported that apoCIII directly enhances adhesion of human monocytes to endothelial cells (ECs), and identified the activation of PKC as a necessary upstream event of enhanced monocyte adhesion. This study tested the hypothesis that apoCIII activates PKC in human monocytic THP-1 cells, leading to NF-B activation.

Methods and Results— Among inhibitors specific to PKC activators, phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor D609 limited apoCIII-induced PKC activation and THP-1 cell adhesion. ApoCIII increased PC-PLC activity in THP-1 cells, resulting in PKC activation. Pertussis toxin (PTX) inhibited apoCIII-induced PC-PLC activation and subsequent PKC activation, implicating PTX-sensitive G protein pathway. ApoCIII further activated nuclear factor-B (NF-B) through PKC in THP-1 cells and augmented ß1-integrin expression. The NF-B inhibitor peptide SN50 partially inhibited apoCIII-induced ß1-integrin expression and THP-1 cell adhesion. ApoCIII-rich VLDL had similar effects to apoCIII alone.

Conclusions— PTX-sensitive G protein pathway participates critically in PKC stimulation in THP-1 cells exposed to apoCIII, activating NF-B, and increasing ß1-integrin. This action causes monocytic cells to adhere to endothelial cells. Furthermore, because leukocyte NF-B activation contributes to inflammatory aspects of atherogenesis, apoCIII may stimulate diverse inflammatory responses through monocyte activation.

This study showed that apoCIII alone or VLDL containing apoCIII activates PKC through the PTX-sensitive G protein pathway in human monocytic cells, leading to NF-B activation and increased ß1-integrin expression. Leukocyte NF-B activation contributes importantly to inflammatory aspects of atherogenesis. Thus, apoCIII may stimulate diverse inflammatory responses through monocyte activation.

Key Words: apolipoproteins • atherosclerosis • adhesion molecules • leukocytes • signal transduction

	Introduction

Top Abstract Introduction Methods Results Discussion References

 
Previous clinical studies recognized apoCIII as an independent risk factor for coronary heart disease (CHD),^1–3 especially as a component of apoB lipoproteins.^1,4 Excessive apoCIII delays lipolysis of apoB lipoproteins⁵ and inhibits their uptake by normal high-affinity receptors on hepatocytes,⁶ causing hypertriglyceridemia. However, the direct effect of apoCIII on vascular cells or leukocytes remained untested. We recently reported that apoCIII alone or apoCIII-rich VLDL or LDL augment THP-1 cell adhesion to vascular ECs under static or flow conditions.⁷ Protein kinase C (PKC) participated critically in this process. Ca²⁺, phospholipids, and diacylglycerol (DAG) activate PKC, one of the conventional PKC isoforms.^8,9 The literature currently provides little information regarding possible direct effects of apoCIII on these molecules, and the mechanism for PKC activation by apoCIII remains undetermined. PKC participates importantly in several mechanisms in atherogenesis including monocyte-endothelial interaction.^8,10 However, the downstream pathway of PKC in apoCIII-treated THP-1 cells remains incompletely understood, although activation of RhoA partially contributes to the effect of apoCIII.⁷

The present study demonstrates that pertussis toxin (PTX)-sensitive G protein and phosphatidylcholine-specific phospholipase C (PC-PLC) mediate apoCIII-induced PKC activation, and that apoCIII-induced PKC activation induces activation of nuclear factor-B (NF-B), a key regulator for inflammation in atherogenesis. Indeed, such activation leads to increased ß1-integrin expression in THP-1 cells, enhancing their adhesion to ECs. VLDL rich in apoCIII (VLDL CIII+) also produced similar effects, providing new mechanistic insights into the atherogenicity of apoCIII.

	Methods

Top Abstract Introduction Methods Results Discussion References

 
Lipoprotein Preparation
Blood was drawn in tubes containing EDTA from 12 apparently healthy volunteers at 12-hours fasting. The subjects had not taken cardiovascular medications, pharmacological doses of antioxidant drugs, vitamins, or estrogen for more than 14 days. VLDL CIII+ or VLDL without apoCIII (VLDL CIII–) were isolated from plasma as described previously.⁴ In some experiments, apoCIII or VLDL CIII+ was pretreated with anti-apoCIII antibody (50 µg/mL) or isotype-matched goat IgG (50 µg/mL) for 30 minutes before the addition to THP-1 cells. Endotoxin levels in the lipoprotein fractions measured using a Limulus amebocyte lysate test (Associates of Cape Cod) were less than 0.03 EU/mL. The use of human plasma for lipoprotein isolation was approved by the Institutional Review Board of Harvard School of Public Health.

Cell Culture and Reagents
THP-1 cells, a human monocytic cell line, were obtained from American Type Culture Collection. Human saphenous vein endothelial cells (HSVECs) were collected under a protocol approved by the Human Research Committee of the Brigham and Women’s Hospital. Human apoCIII was purchased from Academy Biomedical. Antibodies used in the present study include rat anti-NF-B p65 antibody, rat anti-IB antibody, fluorescein isothiocyanate (FITC)-conjugated goat anti-mouse antibody (Santa Cruz Biotechnology), mouse anti-PKC antibody (BD Biosciences), goat anti-apoCIII antibody (Academy Biomedical), and mouse anti-ß1-integrin antibody (JB1A) (Chemicon International). Other reagents used in the present study include 1-butanol (Fisher Chemical), D609 (Acros Organics), U73122 (Biomol Research Laboratories), bromoenol lactone, A23187 (Calbiochem), pertussis toxin (List Biological Laboratories), cholera toxin (List Biological Laboratories), platelet glycoprotein (GP)-antagonist 2A (Biomol Research Laboratories), and SN50 and its control peptide (Calbiochem).

Static Adhesion Assay
HSVEC monolayer was stimulated in a 96-well plate for 4 hours with interleukin (IL)-1ß (10 ng/mL) (Genzyme) before starting the adhesion assay. THP-1 cells (1x10⁶/mL) were labeled with BCECF-AM (Calbiochem), placed on an HSVEC monolayer (6 wells/condition) at 1x10⁵ THP-1 cells/well, and allowed to adhere for 10 minutes. After removing nonadherent cells, the fluorescent intensity of adhered cells and total cells applied to the well was measured by CytoFlour II (Perceptive Biosystems). The ratio of adherent to total cells was expressed as adhesion (%).

Immunoblotting
To detect PKC activation, total cell lysates and the membrane fraction of THP-1 cells (1x10⁶/mL) were prepared as described previously.¹¹ To detect NF-B nuclear translocation and IB cytosol degradation, cytosol and nuclear fraction of THP-1 cells (1x10⁶/mL) were prepared using Nuclear and Cytoplasmic Extraction Reagents (Pierce Biotechnology). An equal amount of protein (30 µg) from each fraction was subjected to 12% SDS-PAGE. Immunoreactive proteins were detected using indicated antibodies with enhanced chemiluminescence (ECL) plus (Amersham Biosciences).

PC-PLC Activity Assay
The activity of the PC-PLC enzyme of THP-1 cells was determined with Amplex Red phosphatidylcholine-specific phospholipase C assay kit (Molecular Probes), following the manufacturer’s instruction.

Measurement of Intracellular Calcium of THP-1 Cells
THP-1 cells (2x10⁶ cells/mL) were incubated with PBS containing 1.2 mmol/L Ca²⁺ and fura 2-AM (5 µg/mL) for 1 hour. The cells were then washed and resuspended in PBS containing 1.2 mmol/L Ca²⁺ at a density of 10⁶ cells/mL. To measure intracellular calcium ([Ca²⁺]_i), the cell suspension (1 mL) was placed in the cuvette of a CAF-110 fluorescence spectrophotometer (Jasco). ApoCIII (100 µg/mL) was directly injected into the cuvette, and [Ca²⁺]_i was measured by excitation at 340 and 380 nm and fluorescence emission at 500 nm.

Flow Cytometry
THP-1 cells (1x10⁶/mL) were treated with mouse anti-ß1-integrin antibody (JB1A) or isotype-matched IgG for 10 minutes, followed by incubation with FITC-conjugated goat anti-mouse antibody. Cell surface ß1-integrin expression was analyzed using a fluorescence-activated cell sorter (FACS) Caliber (BD Biosciences).

Statistical Analysis
Results are presented as mean±SD. Data were analyzed using an unpaired t test, and a value of P<0.05 was considered significant.

	Results

Top Abstract Introduction Methods Results Discussion References

 
Involvement of PC-PLC in ApoCIII-Induced PKC Activation
Phospholipases participate importantly in the activation of conventional PKC isoforms.^9,10 Experiments with selective inhibitors examined their potential involvement in apoCIII-induced PKC activation, (Figure 1A). The selective PC-PLC inhibitor D609 inhibited apoCIII-induced PKC activation. The phosphatidyinositol-specific phospholipase C (PI-PLC) inhibitor U73122, phospholipase D (PLD) inhibitor 1-butanol, or phospholipase A₂ (PLA₂) inhibitor bromoenol lactone minimally affected PKC activation. D609 inhibited PKC activation in a concentration-dependent manner (Figure 1B). In line with PKC inhibition, D609 inhibited apoCIII-induced THP-1 cell adhesion to ECs (Figure 1C).

View larger version (31K): [in this window] [in a new window]   Figure 1. PC-PLC mediates apoCIII-induced PKC activation. A, THP-1 cells were pretreated with PI-PLC inhibitor U73122 (50 µmol/mL), PC-PLC inhibitor D609 (50 µg/mL), PLD inhibitor 1-butanol (0.5% vol), or PLA2 inhibitor bromoenol lactone (50 µmol/mL) for 30 minutes, and then incubated in the presence of PBS (Control) or apoCIII (100 µg/mL) for 2 hours. Cytosol and membrane fractions were subjected to 12% SDS-PAGE for immunoblotting. Blots represent 4 independent experiments using apoCIII from 4 different donors that provided similar results. B, THP-1 cells were pretreated in the presence of indicated concentrations of D609, and then incubated in the presence of PBS (Control) or apoCIII (100 µg/mL) for 2 hours. Cytosol and membrane fractions were subjected to 12% SDS-PAGE for immunoblotting. Blots represent 4 independent experiments using apoCIII from 4 different donors that provided similar results. C, THP-1 cells were pretreated in the presence of indicated concentrations of D609, and then incubated in the presence of PBS (Control) or apoCIII (100 µg/mL) for 8 hours. Static adhesion assays were carried out. The data are from 4 independent experiments using apoCIII from 4 different donors that provided similar results. *P<0.01 vs Control, #P<0.05, ##P<0.01 vs apoCIII alone.

ApoCIII Activates PC-PLC
To support a role for PC-PLC in apoCIII-induced PKC activation, we measured PC-PLC activity of THP-1 cells. ApoCIII treatment of THP-1 cells increased PC-PLC activity in a concentration-dependent manner (Figure 2A). When incubated with a blocking anti-apoCIII antibody, apoCIII did not increase PC-PLC activity (Figure 2B).

View larger version (16K): [in this window] [in a new window]   Figure 2. ApoCIII increases PC-PLC activity in THP-1 cells. A, THP-1 cells were pretreated in the presence of indicated concentrations of apoCIII for 2 hours. PC-PLC activity was measured as in Methods. The data are from 3 independent experiments apoCIII from 3 different donors that provided similar results. *P<0.05, ** P<0.01 vs 0 µg/mL. B, THP-1 cells were incubated in the presence of PBS (Control) or indicated apoCIII preparations (100 µg /mL) for 8 hours. Static adhesion assays were carried out. The data are from 3 independent experiments apoCIII from 3 different donors that provided similar results. *P<0.01 vs Control, #P<0.01 vs apoCIII alone.

Effect of ApoCIII on [Ca²⁺]_i
We examined the effect of apoCIII on [Ca²⁺]_i of THP-1 cells. [Ca²⁺]_i were monitored for up to 30 minutes after the addition of apoCIII to THP-1 cells. ApoCIII did not increase [Ca²⁺]_i (1.1-fold increase of baseline), whereas the Ca²⁺ ionophore A23187 promptly increased [Ca²⁺]_i by 1.9-fold of baseline, indicating that apoCIII-induced PKC activation does not depend on Ca²⁺ (see supplemental Figure I, available online at http://atvb.ahajournals.org).

PTX-Sensitive G Protein Mediates ApoCIII-Induced PKC Activation
Heterotrimeric G proteins regulate PLC activity.^12,13 To elucidate the signal transduction pathway that mediates apoCIII-induced PKC activation, we examined the participation of heterotrimeric G proteins. The G_i protein inhibitor pertussis toxin (PTX) inhibited apoCIII-induced PKC activation (Figure 3A). Neither the G_s protein inhibitor cholera toxin (CTX) nor the G_q protein inhibitor GP-antagonist 2A (GP) affected PKC activation. PTX abolished PC-PLC activity induced by apoCIII and apoCIII-induced THP-1 cell adhesion (Figure 3B and 3C).

View larger version (24K): [in this window] [in a new window]   Figure 3. ApoCIII activates PC-PLC via PTX-sensitive G protein in THP-1 cells. A, THP-1 cells were pretreated with Gi protein inhibitor pertussis toxin (PTX; 100 ng/mL), Gs protein inhibitor cholera toxin (CTX; 100 ng/mL), or Gq protein inhibitor GP-antagonist 2A (GP; 100 ng/mL) for 8 hours, and then incubated in the presence of PBS (Control) or apoCIII (100 µg/mL) for 2 hours. Cytosol and membrane fractions were subjected to 12% SDS-PAGE for immunoblotting. Blots represent 4 independent experiments using apoCIII from 4 different donors that provided similar results. B, THP-1 cells were pretreated in the presence of indicated concentrations of PTX for 8 hours, and then incubated in the presence of PBS (Control) or apoCIII (100 µg/mL) for 2 hours. PC-PLC activity was measured as in Methods. The data are from 3 independent experiments using apoCIII from 3 different donors that provided similar results. *P<0.01 vs Control, #P<0.01 vs apoCIII alone. C, THP-1 cells were pretreated in the presence of indicated concentrations of PTX for 8 hours, and then incubated in the presence of PBS (Control) or apoCIII (100 µg/mL) for 8 hours. Static adhesion assays were carried out. The data are from 3 independent experiments using apoCIII from 3 different donors that provided similar results. *P<0.01 vs Control, #P<0.05, ##P<0.01 vs apoCIII alone.

ApoCIII Activates NF-B in THP-1 Cells
Our recent study showed that PKC activated ß1-integrin through RhoA in apoCIII-treated THP-1 cells.⁷ However, RhoA inhibition by C3 exoenzyme only inhibited ß1-integrin activation and THP-1 cell adhesion partially, suggesting that another mechanism participates in the apoCIII-mediated increase in ß1-integrin expression and THP-1 cell adhesion. PKC induces activation of NF-B, which plays a pivotal role in vascular inflammation including augmentation of ß1-integrin.¹⁴ We therefore examined whether apoCIII affects NF-B activation in THP-1 cells. ApoCIII treatment of THP-1 cells induced nuclear translocation of NF-B p65 and degradation of I-B in the cytosol (Figure 4A), indicating NF-B activation.¹⁵ The PKC inhibitor Go6976 attenuated apoCIII-induced NF-B activation, although it did not affect baseline activity (Figure 4B), indicating that NF-B activation by apoCIII depends partly on PKC. Incubation of THP-1 cells with apoCIII increased expression of ß1-integrin (Figure 4C). Pretreatment of THP-1 cells with the NF-B inhibitor peptide SN50¹⁶ or Go6976 reduced augmentation of ß1-integrin by apoCIII (Figure 4C and 4D). These inhibitors did not affect baseline ß1-integrin expression in THP-1 cells (data not shown). SN50 attenuated apoCIII-induced THP-1 cell adhesion, although it did not affect baseline adhesion (Figure 4E). The control peptide for SN50 did not affect apoCIII-induced THP-1 cell adhesion (data not shown). D609 also reduced augmentation of ß1-integrin by apoCIII (supplemental Figure II). These results indicate that PC-PLC- and PKC-induced NF-B activation participates in THP-1 cell adhesion.

View larger version (38K): [in this window] [in a new window]   Figure 4. ApoCIII induces NF-B activation and ß1-integrin expression in THP-1 cells. A, THP-1 cells were pretreated in the presence of indicated concentrations of apoCIII for 4 hours. Cytosol and nuclear fractions were subjected to 12% SDS-PAGE for immunoblotting. Blots represent 4 independent experiments using apoCIII from 4 different donors that provided similar results. B, THP-1 cells were pretreated in the presence or absence of PKC inhibitor Go6976 (5 nmol/mL) for 30 minutes, and then incubated in the presence of PBS (Control) or apoCIII (100 µg/mL) for 4 hours. Cytosol and nuclear fractions were subjected to 12% SDS-PAGE for immunoblotting. Blots represent 4 independent experiments using apoCIII from 4 different donors that provided similar results. C and D, THP-1 cells were pretreated in the presence or absence of SN50 (20 µmol/mL) or Go6976 (5 nmol/mL) for 30 minutes, and then incubated in the presence of PBS (Control) or apoCIII (100 µg/mL) for 8 hours. ß1-integrin expression was examined with flow cytometry. Data represent 3 independent experiments using apoCIII from 3 different donors that provided similar results. E, THP-1 cells were pretreated in the presence or absence of SN50 (20 µmol/mL) for 30 minutes, and then incubated in the presence of PBS (Control) or apoCIII (100 µg/mL) for 8 hours. Static adhesion assays were carried out. The data are from 3 independent experiments using apoCIII from 3 different donors that provided similar results. *P<0.01 vs Control, #P<0.01 vs apoCIII alone.

VLDL CIII+ Activates PC-PLC and PKC Though PTX-Sensitive G Protein Pathway
We recently showed that VLDL CIII+ activates PKC in an apoCIII-dependent manner.⁷ Thus, we examined whether VLDL CIII+ activates PKC through PC-PLC pathway, as did apoCIII. Indeed, VLDL CIII+ activated PC-PLC activity in THP-1 cells (Figure 5A). In contrast, VLDL CIII– did not affect PC-PLC activity. When VLDL CIII+ was pretreated with anti-apoCIII antibody, VLDL CIII+ did not increase PC-PLC activity. PTX limited VLDL CIII+–induced PC-PLC activation by 80% (Figure 5B). In accord with experiments using apoCIII alone, pretreating THP-1 cells with PTX or D609 inhibited VLDL CIII+–induced PKC activation (Figure 5C), indicating the involvement of PTX-sensitive G protein and PC-PLC in PKC activation by VLDL CIII+ as we have shown for apoCIII itself.

View larger version (30K): [in this window] [in a new window]   Figure 5. VLDL CIII+ activates PC-PLC and PKC though PTX-sensitive G protein pathway. A, THP-1 cells were incubated in the presence of PBS (Control) or indicated VLDL preparations (100 µg apoB/mL) for 8 hours. Static adhesion assays were carried out. The data are from 3 independent experiments using VLDL preparations from 3 different donors that provided similar results. *P<0.01 vs Control. B, THP-1 cells were pretreated in the presence or absence of pertussis toxin (PTX; 500 ng/mL) for 8 hours, and then incubated in the presence of PBS (Control) or VLDL CIII+ (100 µg apoB/mL) for 8 hours. Static adhesion assays were carried out. The data are from 3 independent experiments using VLDLCIII+ from 3 different donors that provided similar results. *P<0.01 vs Control, #P<0.01 vs VLDL CIII+. C, THP-1 cells were pretreated with PTX (500 ng/mL) for 8 hours, or D609 for 30 minutes, and then incubated in the presence of PBS (Control) or VLDL CIII+ (100 µg apoB/mL) for 2 hours. Cytosol and membrane fractions were subjected to 12% SDS-PAGE for immunoblotting. Blots represent 4 independent experiments using VLDLCIII+ from 4 different donors that provided similar results.

VLDL CIII+ Activates NF-B and Increases ß1-Integrin in THP-1 Cells
Finally, we tested whether VLDL CIII+ activates NF-B and induces ß1-integrin expression. VLDL CIII+ activated NF-B in THP-1 cells. The PKC inhibitor Go6976 attenuated this activation, whereas it did not affect baseline activity (Figure 6A). Further, pretreatment of THP-1 cells with SN50 or Go6976 inhibited the ability of VLDL CIII+ to increase ß1-integrin expression (Figure 6B and 6C). SN50 significantly inhibited THP-1 cell adhesion induced by VLDL CIII+, although it did not alter baseline adhesion (Figure 6D). D609 also attenuated the augmentation of ß1-integrin by VLDLCIII+ (supplemental Figure II).

View larger version (31K): [in this window] [in a new window]   Figure 6. VLDL CIII+ -induced NF-B activation increases ß1-integrin in THP-1 cells. A, THP-1 cells were pretreated in the presence or absence of Go6976 (5 nmol/mL) for 30 minutes, and then incubated in the presence of PBS (Control) or VLDL CIII+ (100 µg apoB/mL) for 4 hours. Cytosol and nuclear fractions were subjected to 12% SDS-PAGE for immunoblotting. Blots represent 4 independent experiments using VLDLCIII+ from 4 different donors that provided similar results. B and C, THP-1 cells were pretreated in the presence or absence of SN50 (20 µmol/mL) or Go6976 (5 nmol/mL) for 30 minutes, and then incubated in the presence of PBS (Control) or VLDL CIII+ (100 µg apoB/mL) for 8 hours. ß1-integrin expression was examined with flow cytometry. Data represent 3 independent experiments using VLDLCIII+ from 3 different donors that provided similar results. D, THP-1 cells were pretreated in the presence or absence of SN50 (20 µmol/mL) for 30 minutes, and then incubated in the presence of PBS (Control) or VLDL CIII+ (100 µg apoB/mL) for 8 hours. Static adhesion assays were carried out. The data are from 3 independent experiments using VLDLCIII+ from 3 different donors that provided similar results. *P<0.01 vs Control, #P<0.01 vs VLDL CIII+ without inhibitor.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
We recently reported that apoCIII alone or as a component of apoB lipoproteins activated ß1-integrin through PKC in THP-1 cells and increased their adhesion to ECs under flow condition.⁷ The present study further examined both the mechanism of apoCIII-induced PKC activation in THP-1 cells and the downstream pathway of PKC that leads to THP-1 cell adhesion.

ApoCIII increased PC-PLC activity, and selective inhibition of PC-PLC by D609 abolished apoCIII-induced PKC activation. ApoCIII resides on VLDL and other lipoproteins in the circulation, and we determined that PC-PLC participates in PKC activation induced by VLDL CIII+. PC-PLC hydrolyzes PC to generate phospholylcholine and DAG, and DAG activates PKC. Unlike other phospholipases, PC-PLC does not affect [Ca²⁺]_i in cells. Ca²⁺ potently activates conventional PKCs including PKC.⁹ However, apoCIII treatment of THP-1 cells did not increase [Ca²⁺]_i in THP-1 cells. These results indicate that PC-PLC rather than other phospholipases dominantly activates PKC in THP-1 cells exposed to apoCIII.

Heterotrimeric G proteins regulate PC-PLC activity.^12,13 In the present study, PTX inhibited PC-PLC activation by apoCIII, suggesting that apoCIII itself can activate PC-PLC through PTX-sensitive G protein. Heterotrimeric G proteins couple with various types of membrane receptors, and their G subunit mediates signal transduction.¹⁷ Zhao et al reported that ßVLDL activated smooth muscle cell mitogen-activated protein (MAP) kinase via PTX-sensitive G protein-mediated transactivation of the epidermal growth factor (EGF) receptor.¹⁸ We showed previously that apoCIII-rich remnant lipoproteins activated PKC in rat smooth muscle cells, and that PTX inhibited PKC activation,¹⁹ suggesting that specific components of VLDL or VLDL remnants interact with PTX-sensitive G protein or its membrane receptors. Our current results support these observations, pointing to apoCIII as one of these components. Determining how apoCIII activates the PTX-sensitive G protein pathway will require further investigation.

Our previous study demonstrated that apoCIII activates RhoA though PKC, leading to ß1-integrin activation.⁷ However, RhoA inhibition did not completely inhibit THP-1 cell adhesion, implying that another pathway downstream of PKC also participates in this process. Thus, we examined the mechanism that links PKC activation and apoCIII-induced ß1-integrin expression, and showed that apoCIII activates NF-B though PKC. NF-B activation by apoCIII increased ß1-integrin expression, contributing also to enhanced THP-1 cell adhesion (Figure 4C and 4E). Several studies reported that cytokines, pathogens, or high glucose activates NF-B via PKC,^20–22 and NF-B, in turn induces adhesion molecules including ß1-integrin.¹⁴ Our present study provides new evidence that apoCIII in apoB lipoproteins activates NF-B via PKC, independently of their lipid moieties.

In conclusion, we demonstrated that apoCIII, alone or in association with VLDL, activates PKC through the PTX-sensitive G protein pathway in human monocytic cells, leading to NF-B activation and increase in ß1-integrin expression. We recently reported that apoCIII activates PKC in vascular endothelial cells causing them to produce adhesion molecules.²³ Thus, apoCIII activates both monocytic and endothelial components that participate in adhesion, a key step in atherogenesis. Finally, leukocyte NF-B activation participates importantly in inflammatory aspects of atherogenesis.²⁴ Thus, the apoCIII pathway may promote diverse inflammatory responses through monocyte activation, contributing to atherogenesis.

	Acknowledgments

 
We thank Jeremy Furtado for his technical assistance. We also acknowledge Karen E. Williams for her editorial assistance.

Sources of Funding

This study was supported in part by grants from the National Heart, Lung, and Blood Institute (HL69376 to F.M.S.; HL48743 and HL80472 to P.L.; HL56985 to P.L. and M.A.), the Donald W. Reynolds Foundation (to P.L.), the Japan Heart Foundation/Pfizer Grant for Research on Hypertension, Hyperlipidemia, and Vascular Metabolism, the Japan research foundation for clinical pharmacology, and the Takeda Science Foundation (to A.K.).

Disclosures

None.

	Footnotes

 
Original received April 18, 2006; final version accepted September 20, 2006.

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