©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
Altered Cholesterol Trafficking in Herpesvirus-infected Arterial Cells
EVIDENCE FOR VIRAL PROTEIN KINASE-MEDIATED CHOLESTEROL ACCUMULATION (*)

(Received for publication, April 3, 1995; and in revised form, June 15, 1995)

Hsien-Yeh Hsu (3) Andrew C. Nicholson (2) Kenneth B. Pomerantz (3) Robert J. Kaner (4) David P. Hajjar (2) (1)(§)

From the  (1)Departments ofBiochemistry, (2)Pathology, and (3)Medicine, Cornell University Medical College and the (4)Department of Medicine, Memorial Sloan-Kettering Cancer Center, New York, New York 10021

ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
FOOTNOTES
ACKNOWLEDGEMENTS
REFERENCES

ABSTRACT

Herpesvirus infection of arterial smooth muscle cells has been shown to cause cholesteryl ester (CE) accumulation. However, the effects of human herpes simplex virus type 1 (HSV-1) infection on cholesterol binding and internalization, intracellular metabolism, and efflux have not been evaluated. In addition, the effects of viral infection on signal transduction pathways that impact upon cholesterol metabolism have not been studied. We show in studies reported herein that HSV-1 infection of arterial smooth muscle cells enhances low density lipoprotein (LDL) binding and uptake which parallels an increase in LDL receptor steady state mRNA levels and transcription of the LDL receptor gene. HSV-1 also increases CE synthesis and 3-hydroxy-3-methylglutaryl-CoA reductase activity but concomitantly reduces CE hydrolysis and cholesterol efflux. Interestingly, this viral infection was associated with a time-dependent decrease in protein kinase A activity and an increase in viral-induced protein kinase (VPK) activity commensurate with the accumulation of esterified cholesterol. The relationship between increased VPK activity and alterations in CE accumulation in virally infected cells was explored using an HSV-1 VPK mutant in which the portion of the HSV-1 genome encoding VPK had been deleted. Cholesteryl ester accumulation was significantly increased (>50-fold) in HSV-1-infected cells compared to uninfected cells. However, the HSV-1 VPK mutant had no significant effect on CE accumulation. The relationship between VPK activity and these alterations in cholesterol metabolism was further supported by the observation that staurosporine and calphostin C (protein kinase inhibitors) reduced protein kinase activity in HSV-1-infected cells. These results suggest several potential mechanisms by which alterations in kinase activities in response to HSV-1 infection of vascular cells may alter cholesterol trafficking processes that eventually lead to CE accumulation.


INTRODUCTION

Herpesvirus infection of arterial smooth muscle cells can lead to marked accumulation of cholesterol (1) due, in part, to decreased cholesteryl ester (CE) (^1)hydrolysis(2) . HSV-1 infection has also been linked to decreased intracellular levels of cyclic AMP, a second messenger known to activate cellular CE hydrolases(3) . To date, this second messenger system is the only system known to significantly alter cellular cholesterol metabolism following herpesviral infection. However, HSV-1 infection alters other signal transduction pathways that could potentially affect cellular metabolism of cholesterol. For example, viral activation of tyrosine phosphorylation has been related to viral cytopathologic effects(4) , and, HSV-1 infection can also increase phosphatidylinositol 4,5-bisphosphate levels (5) and inositol phospholipid turnover(6) . Whether these signal transduction pathways, or others, are linked to altered lipid metabolism in the infected cell remains to be defined.

Cholesterol homeostasis in mammalian cells is maintained by the activities of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, the rate-limiting step in cholesterol synthesis(7) , and the low density lipoprotein (LDL) receptor, which provides an exogenous source of cholesterol. Sterol-mediated repression of transcription of the genes encoding these proteins inhibits expression of the HMG-CoA reductase and of the LDL receptor genes under conditions in which cholesterol is sufficient for normal cellular maintenance(8, 9, 10, 11) . This prevents excessive accumulation of cholesterol and CE.

The involvement of specific signal transduction pathways and second messengers have been implicated recently in the regulation of cholesterol homeostasis in non-virally infected cellular systems. Activators of protein kinase C (e.g. phorbol ester)- and protein kinase A (e.g. cyclic AMP)-dependent pathways modulate the activities of HMG-CoA reductase (7, 12, 13) and the LDL receptor(14, 15) . Our laboratory has demonstrated that the activities of acidic and neutral cholesteryl ester hydrolases (ACEH and NCEH) are also cyclic AMP-dependent(16) . Cholesterol efflux from cholesterol-loaded cells is also enhanced by activation of protein kinase C and cyclic AMP-dependent protein kinases (17, 18) . In virally infected cells, the link among protein kinase activation, the activation of specific signal transduction systems, and the control of cholesterol trafficking also has not been defined.

HSV-1 DNA encodes, in part, viral structural proteins, envelope glycoproteins, and viral DNA polymerases. It also encodes several proteins that are functionally related to cellular enzymes including a viral protein serine/threonine kinase (VPK)(19, 20, 21, 22) . VPK has distinct substrate specificity but may share some substrates with protein kinases A and C as determined by their ability to phosphorylate synthetic oligopeptides. The cellular substrates for VPK are unknown, but several viral proteins have been identified as substrates(23, 24) . In this report, we provide experimental evidence supporting the hypothesis that a viral kinase (VPK) alters cellular protein kinase-dependent pathways, which regulate the control of cellular cholesterol trafficking and cellular CE accumulation.


EXPERIMENTAL PROCEDURES

Materials

Medium 199, L-glutamine, penicillin, streptomycin, fetal calf serum (FCS), and anti-PKC monoclonal antibody 1.9 were purchased from Life Technologies, Inc. NaI was obtained from ICN Biochemicals (Costa Mesa, CA). Fungizone® was obtained from Flow Laboratories, Inc. (McLean, VA). Calphostin C was purchased from Calbiochem-Novabiochem Corp. (La Jolla, CA). Staurosporine, EGTA, leupeptin, and aprotinin were obtained from Boehringer Mannheim. HEPES, NaCl, glycerol, Triton X-100, MgCl(2), phenylmethylsulfonyl fluoride, pepstatin A, soybean trypsin inhibitor, protamine sulfate, mouse IgM kappa, and bovine serum albumin (Fraction V) were purchased from Sigma. [9,10-^3H]Oleic acid (specific activity 10.0 Ci/mmol) was purchased from DuPont NEN.

Viruses

HSV-1(F) was obtained from the American Tissue Type Collection (ATCC) (Rockville, MD). The HSV-1(F) protein kinase deletion mutant (VPK, strain R7041) and the repaired HSV-1(F) mutant (VPK, strain R7050) were generous gifts of Dr. Bernard Roizman (University of Chicago). Strain R7041 contains a deletion in the viral US3 gene and exhibits no VPK activity. The infectivity and production of viral glycoproteins by this strain remains intact. Strain R7050 was constructed from R7041 by inserting a US3 gene. The properties of these strains have been characterized extensively(25) . All viruses were propagated in, and plaque assays performed with, Vero cells.

Cell Culture

Smooth muscle cells were propagated from explants of rat thoracic aorta in Medium 199 containing 10% FCS, 2 mM glutamine, 100 units of penicillin/streptomycin, and 2.5 µg/ml amphotericin B (Fungizone®) as described elsewhere(1) .

Virus Infection of Cells

Smooth muscle cells monolayers at 90% confluence were washed twice with Dulbecco's phosphate-buffered saline (PBS), then placed overnight in medium containing 0.05% FCS. Cells were infected with HSV-1 at a multiplicity of infection (m.o.i.) of 0.1-1.0 and harvested at various time points following virus infection. For the experiments that were done to study the effects of viral infection for times longer than 3 h, cells were washed twice at room temperature with PBS and then refed medium with 0.05% or 10% FCS depending on the experimental protocol.

Isolation and Labeling of LDL

Human LDL (d = 1.019-1.063 g/ml) was prepared as described(26) . LDL was labeled by the method of Bilheimer (27) as described by Goldstein et al.(28) , yielding I-LDL with specific activity between 100 and 200 cpm/ng protein.

LDL Binding, Internalization, and Degradation

The binding, internalization, and degradation of LDL were performed according to the methods of Goldstein et al.(28) using I-LDL.

RNA Isolation and Northern Analysis

Total RNA was isolated by the guanidium isothiocyanate method(29) . Northern blot analysis was performed as described(30) . The cDNAs for the LDL receptor and the HMG-CoA reductase were purchased from ATCC. RNA from Northern blots was quantified using a densitometer and normalized by comparison to mRNA of a constitutive gene glyceraldehyde-3-phosphate dehydrogenase (GAPDH).

LDL Receptor Promoter in Luciferase Reporter Gene Construct

Plasmid LDLRP-Luc was constructed by ligating the human LDL receptor promoter (5` sequences extending 1563 base pairs from the A (+) of the translation initiation codon ATG; the human LDL receptor promoter, a gift from Dr. D. Russell, University of Texas) into plasmid Luc containing the luciferase gene (31) and as described previously (32) .

Calcium Phosphate Transfection

Cells were sub-passaged the day before transfection and replaced with fresh medium. The transfection method has been described elsewhere(33) .

Luciferase Assay

Luciferase activities were measured according to the method developed by de Wet et al.(31) , or by using the Luciferase Assay System kit (Promega Corp., WI). The control plasmid (Luc) was supplemented to correct for nonspecific variations in the transfection assay. Luciferase activities were normalized to protein content.

Measurement of Cholesterol and Cholesteryl Ester Mass

Smooth muscle cells were grown to near-confluence and infected with HSV-1 (m.o.i. = 0.1-1.0) or without virus (mock) for 24 h. The methods for lipid extraction, cholesterol and CE mass quantitation by gas-liquid chromatography were performed as described previously(1, 26) .

Cholesteryl Ester Metabolizing Enzymes

(a) Acid (lysosomal) CE hydrolase (ACEH) activity was assayed at pH 3.9 (34) . (b) Neutral (cytoplasmic) CE hydrolase (NCEH) activity was assayed at pH 7.0(35) . (c) Acyl-CoA:cholesterol acyltransferase (ACAT) activity in homogenates was measured as described previously(36) . (d) HMG-CoA reductase:cholesterol synthetic activity was measured by the method of Goldstein and Brown (37) .

Esterification of [^3H]Oleic Acid into Cellular Cholesteryl Ester

Smooth muscle cells were exposed to a [^3H]oleic acid-albumin mixture (final concentration of 100 µM oleate, 20 µM albumin) in the absence of FCS and with LDL (25 µg/ml/well) for 24 h at 37 °C for studying esterification as described elsewhere(38) . Cell lipids were extracted, and radioactivity in CE was assessed after separation by TLC as described(39) .

Assay of Virus-induced Protein Kinase (VPK) Activity

Confluent smooth muscle cells were washed twice with 5 ml of ice-cold PBS (without Ca, Mg), harvested, and pelleted at 325 g, 4 °C. The cell pellet was resuspended in 5 mM NaCl, 5 mM Tris-HCl, 10 mM EGTA, 1 mM MgCl(2), 2 mM dithiothreitol, 10 mM beta-mercaptoethanol, 1% Triton X-100, 1 mM phenylmethylsulfonyl fluoride, 10 µg/ml leupeptin, 10 µg/ml pepstatin A, and 10 µg/ml soybean trypsin inhibitor. The assay for virus-induced protein kinase activity was performed as described elsewhere(19, 22, 25) . Anti-PKC monoclonal antibody or control antibody (mouse IgM kappa) were added to the cell lysates, then incubated with protein A beads at 4 °C for 2 h, and the supernatants were assayed for VPK activity. Lysates from PMA-stimulated cells served as a positive control for the anti-PKC monoclonal antibody.

Assay of Protein Kinase A and C Activities from Smooth Muscle Cells

To assay PKA activity in the cell, we followed methods described elsewhere (40, 41) or used the Life Technologies protein kinase A enzyme assay system. Protein kinase C activity in the cell was assayed by a method described elsewhere(42) .

Protein Assay

Protein content in the tested samples was determined by the method of Lowry (43) or the Bio-Rad protein assay.

Statistical Analysis

Statistical differences between the experimental groups were examined by analysis of variance, and statistical significance was determined at a p level < 0.05. All data are expressed as mean ± S.E.


RESULTS

Since no published study to date has addressed the effects of herpesviral infection on cholesterol delivery with its subsequent metabolic fate, we first evaluated the effects of HSV-1 on the binding and uptake of LDL-derived cholesterol and the influence of viral infection on expression of the LDL receptor. The specific binding of I-LDL (at 4 °C) to arterial smooth muscle cells increased 3-9 h post-infection by HSV-1, and decreased to basal levels by 24 h post-infection (Fig. 1A). Furthermore, studies conducted at 37 °C demonstrated that HSV-1 enhanced internalization (Fig. 1B) but decreased degradation of LDL (Fig. 1C).


Figure 1: A, effect of HSV-1 infection on specific I-LDL binding. Smooth muscle cells were grown to 90% confluence in 12-well plates (24 mm) and then placed in serum-free medium (control) or serum-free medium inoculated with HSV-1 (m.o.i. = 0.5) for 3, 6, 9, and 24 h prior to the addition of I-LDL (50 µg/ml) at 4 °C for 2 h. LDL binding was assessed in supernatants following the release of receptor-bound I-LDL by dextran sulfate. Nonspecifically bound I-LDL was determined by incubating smooth muscle cells with a 100-fold excess of unlabeled LDL. Specific LDL binding is determined by subtracting nonspecifically bound counts from total counts released. The data derived from the HSV-1 group are significantly different from the uninfected group at all but the 24-h time point (* = p < 0.05). Binding was normalized to protein content of the cell layer. Points represent the mean of triplicate wells ± S.E. and are representative of two separate experiments. B and C, effect of HSV-1 infection on I-LDL uptake and degradation. Control and HSV-1-infected cells (m.o.i. = 0.5) were incubated for the indicated times prior to the addition of I-LDL (50 µg/ml) at 37 °C for 5 h. Degraded I-LDL was assayed in the supernatant as described under ``Experimental Procedures.'' Surface-bound I-LDL was release by treatment with 4 mg/ml dextran sulfate in PBS, and cell-associated I-LDL was determined after the cells were solubilized in 0.2 N NaOH. Each point represents the mean of quadruplicate wells ± S.E. and is representative of two separate experiments (* = p < 0.05).



To test the hypothesis that HSV-1 infection increased LDL receptor activity by altering the rate of expression of its mRNA, we first examined the influence of HSV-1 on the steady state levels of LDL receptor mRNA. HSV-1 infection enhanced LDL receptor mRNA (normalized to mRNA of glyceraldehyde-3-phosphate dehydrogenase gene) 5-fold after 60 min when compared to control cells (Fig. 2A). Viral-induced degradation of GAPDH mRNA was not observed during the time course of this experiment. Second, since actinomycin D inhibited the HSV-1 induction of the LDL receptor gene (data not shown), we tested the hypothesis that increased mRNA of LDL receptor following HSV-1 infection was due to increased transcriptional activity of the LDL receptor gene. Using a construct composed of the luciferase gene driven by the LDL receptor gene promoter(32) , we observed that by 2 h, HSV-1 infection or PMA stimulation (as a positive control) significantly induced luciferase activity by 5- and 4-fold, respectively, as compared to control cells (Fig. 2B). Luciferase activity in virally infected cells increased 12-fold relative to uninfected cells by 12 h post-infection.


Figure 2: A, HSV-1 infection induces LDL receptor mRNA. Smooth muscle cells were grown in lipoprotein-deficient medium for 24 h prior to infection. Total RNA was isolated from control (uninfected) and HSV-1-infected (m.o.i. = 1.0) smooth muscle cells at the indicated time points (10, 30, 60, and 120 min of HSV-1 infection). Northern blots were hybridized with P-labeled LDL receptor cDNA. Histograms represent densitometric scanning of the LDL receptor mRNA band normalized to mRNA of GAPDH and are expressed as a percentage of control. Insets are the respective autoradiograms with LDL receptor and GAPDH bands labeled and indicated with an arrow. B, HSV-1 infection induces LDL receptor gene promoter activity. Smooth muscle cells were transfected by the calcium phosphate method with LDLRP-Luc, a plasmid luciferase construct consisting of the promoter region from human LDL receptor gene in plasmid Luc containing the luciferase gene. Cells were infected with HSV-1 (m.o.i. = 1.0) or stimulated with PMA (100 nM) as a positive control. The luciferase activities of transfected cells were measured at the indicated times. Data derived from the HSV-1 group and PMA-treated group that are significantly (p < 0.05) different from the control group are indicated by (*). Data are expressed as the mean of quadruplicate wells ± S.E. and are representative of four separate experiments.



Since increased de novo cholesterol biosynthesis can also contribute to the increased mass of cholesterol in HSV-1-infected cells, we tested the hypothesis that HSV-1 infection altered the activity of HMG-CoA reductase. HSV-1 modestly increased HMG-CoA reductase activity relative to uninfected cells (Fig. 3A). This was reflected in a significant increase in HMG-CoA reductase mRNA in infected cells when compared to control cells (Fig. 3B). As expected, HMG-CoA reductase activity and its mRNA level are markedly reduced in both infected and control cells in the presence of 25-hydroxycholesterol and mevalonic acid (Fig. 3, A and B). However, in the presence of 25-hydroxycholesterol and mevalonic acid, there was a significant increase in HMG-CoA reductase activity and its mRNA level in HSV-1-infected cells relative to uninfected cells (Fig. 3, A and B).


Figure 3: A, effects of HSV-1 Infection on activity of HMG-CoA reductase in smooth muscle cells. Smooth muscle cells were grown to 95% confluence in Medium 199 containing 10% fetal calf serum. Prior to HSV-1 infection (m.o.i. = 0.5), half of the flasks were replaced with serum-free medium containing lipoprotein-deficient serum (LPDS, 5 mg/ml) plus 25-hydroxycholesterol (5 µg/ml) and mevalonic acid (20 mM), and incubated 6 h at 37 °C. After 6 h, the cells were washed, harvested, homogenized, and assayed for HMG-CoA reductase activity as described in Experimental Procedures. The data are expressed as nmol/h/mg protein. Data derived from the HSV-1-infected group are significantly different (p < 0.05) from the uninfected group in both the presence and absence of 25-hydroxycholesterol and mevalonic acid. Sample1 (Control), control cells grown in Medium 199 containing LPDS (5 mg/ml); sample2 (HSV), HSV-1-infected cells grown in Medium 199 containing LPDS (5 mg/ml); sample3 (Control/CHOL+MEV), control cells grown in Medium 199 containing LPDS plus 25-hydroxycholesterol (CHOL) and mevalonic acid (MEV); sample4 (HSV/CHOL+MEV), HSV-1-infected cells grown in Medium 199 containing LPDS plus 25-hydroxycholesterol and mevalonic acid. Data represent the mean of quadruplicate wells ± S.E. This figure is representative of two such experiments. B, Northern blot analysis of mRNA of HMG-CoA reductase in HSV-1-infected smooth muscle cells in the presence or absence of 25-hydroxycholesterol and mevalonic acid. Smooth muscle cells were grown to 95% confluence in Medium 199 containing 10% fetal calf serum. Prior to HSV-1 infection (m.o.i. = 0.5), the medium was replaced with Medium 199 containing LPDS (5 mg/ml) or Medium 199 containing LPDS (5 mg/ml) plus 25-hydroxycholesterol (5 µg/ml) and mevalonic acid (20 mM). Total RNA was isolated following incubation at 37 °C for 6 h. Northern blots were hybridized with P-labeled cDNAs of HMG-CoA reductase and GAPDH. The position of the 4.7-kilobase mRNA of HMG-CoA reductase is marked. Histograms represent densitometric scanning of HMG-CoA reductase mRNA normalized to mRNA of GAPDH. Uninfected cells without 25-hydroxycholesterol and mevalonic acid are assigned (normalized to) a value of 1, and the other treatment groups are expressed relative to control cells. Inset is an audioradiogram of the Northern blot probed with HMG-CoA reductase as labeled. Sample1 (Control), control cells grown in Medium 199 containing LPDS (5 mg/ml); sample2 (HSV), HSV-1-infected cells grown in Medium 199 containing LPDS (5 mg/ml); sample3 (Control/CHOL+MEV), control cells grown in Medium 199 containing LPDS plus 25-hydroxycholesterol (CHOL) and mevalonic acid (MEV); sample4 (HSV/CHOL+MEV), HSV-1-infected cells grown in Medium 199 containing LPDS plus 25-hydroxycholesterol and mevalonic acid.



We next tested the hypothesis that HSV-1 infection altered CE-hydrolytic enzymes, namely ACEH and NCEH, since these enzymes have been documented to participate in the regulation of cholesterol/CE levels in the cell. Both ACEH and NCEH activities were significantly decreased (by 45% and 50%, respectively) in HSV-1-infected cells relative to control cells at 24 h post-HSV-1 infection (Fig. 4). These data are consistent with the decrease in LDL degradation observed in HSV-1-infected cells (Fig. 1C) and provide another mechanism by which HSV-1 promotes cellular CE accumulation.


Figure 4: Effects of HSV-1 infection on the cholesteryl ester hydrolytic (ACEH and NCEH) and synthetic (ACAT) enzyme activities in smooth muscle cells. Confluent smooth muscle cells were infected with HSV-1 (m.o.i. = 0.5). After a 24-h incubation, medium was removed. Cells were scraped from the flasks after two washes with ice-cold PBS, harvested, and homogenized, and samples were assayed for ACEH, NCEH, and ACAT as described under ``Experimental Procedures.'' ACEH, NCEH, and ACAT activities are expressed as nmol/h/mg protein. ACEH and NCEH activity in HSV-1-infected groups are significantly different (p < 0.05) from control groups. ACAT activity is not significantly different (p > 0.05) between HSV-1-infected and control groups. Data represent the mean of quadruplicate wells ± S.E. This figure is representative of two such experiments.



We also determined if HSV-1 infection altered the rate of CE synthesis as measured by the incorporation of isotopic fatty acid into nascent CE. At the initiation of the experiments, [^3H]oleic acid-albumin mixture in the absence of FCS was added to infected and uninfected cells. CE synthesis was increased in HSV-1-infected cells relative to uninfected cells (Fig. 5). However, HSV-1 infection did not alter intrinsic ACAT activity (Fig. 4). These data support the concept that HSV-1 infection increases cholesterol esterification by increasing substrate (cholesterol) availability.


Figure 5: Esterification of [^3H]oleic acid into cellular CE pool in HSV-1-infected and uninfected smooth muscle cells. Smooth muscle cells were grown to 95% confluence, and half the groups were infected with HSV-1 (m.o.i. = 0.5). Esterification of free cholesterol with [^3H]oleic acid into cellular CE was assessed in cells incubated with a [^3H]oleic acid-albumin mixture (final concentration of 100 µM oleate, 20 µM albumin) and LDL (25 µg/ml/well). Cell lipids were extracted, and radioactivity in cellular CE was measured after separation by thin layer chromatography. Data derived from the HSV-1-infected group (after 6 h post-infection) are significantly different (p < 0.05) from the uninfected group. Data represent the mean of quadruplicate wells ± S.E. This figure is representative of two experiments.



Next, since inhibition of efflux could contribute to cholesterol and CE retention within the virally infected cell (as it does in uninfected cells), we evaluated the effect of HSV-1 on cholesterol efflux in the presence of HDL as a plasma cholesterol acceptor particle. During a 30-h experimental period, HSV-1 infection significantly reduced cholesterol efflux compared to uninfected cells (Fig. 6).


Figure 6: Efflux of cholesterol from cellular CE in HSV-1-infected and uninfected smooth muscle cells exposed to HDL. Smooth muscle cells were grown to 95% confluence and then incubated for 24 h at 37 °C with a [^3H]oleic acid-albumin mixture (final concentration of 100 µM oleate, 20 µM albumin) and LDL (50 µg/ml). After 24 h, cells were washed and infected with HSV-1 (m.o.i. = 0.5) or mock infection. After 2 h, the cells were washed with medium and replaced with medium containing HDL (400 µg/ml). At the designated times, medium was removed and the cells were washed (three times with ice-cold PBS containing BSA, and three times with ice-cold PBS). Cell lipids were extracted, and radioactivity in the remaining cellular CE (cholesteryl [^3H]oleate) was measured over time after separation by thin layer chromatography. The data derived from the HSV-1 group are significantly (p < 0.05) different from the uninfected control group at each time point. Data represent the mean of quadruplicate wells ± S.E. This figure is representative of two similar experiments.



As noted earlier, protein kinases have been implicated in regulation of cellular cholesterol trafficking in vascular cells(13, 16, 32, 44, 45, 46, 47) and viral-induced protein kinase (VPK) has been identified in HSV-1-infected cells(19, 20, 21, 22) . To test the hypothesis that VPK activity mediates HSV-1 induced alterations in CE content, we tested a VPK mutant in which the portion of the HSV-1 genome encoding VPK had been deleted(25) . Arterial smooth muscle cells were inoculated (m.o.i. of 0.1) with wild-type HSV-1, HSV-1 VPK mutant, or an HSV-1 VPK ``repair'' mutant (in which the sequence encoding VPK is reinserted in the HSV-1 VPK mutant). CE accumulation was significantly and reproducibly increased (>50-fold) in HSV-1-infected cells compared to mock-infected smooth muscle cells. Infection by the HSV-1 VPK mutant did not effect CE accumulation, whereas smooth muscle cells infected with the HSV-1 VPK repair mutant had CE accumulation equivalent to wild-type HSV-1. Viral infection (HSV-1, VPK mutant, or VPK repair mutant) did not significantly alter free cholesterol content from levels observed in uninfected cells (Fig. 7). To rule out the possibility that failure of the VPK mutant to induce CE accumulation resulted from reduced infectivity of smooth muscle cells, infectivity and replication of the VPK mutant were determined by plaque assay. There were no significant differences in the number of plaques produced in smooth muscle cells by wild-type HSV-1 or the HSV-1 VPK mutant (data not shown).


Figure 7: Cholesterol and CE contents of arterial smooth muscle cells infected with wild-type HSV-1, HSV-1 VPK mutant, and HSV-1 VPK repair mutant and of uninfected smooth muscle cells. Arterial smooth muscle cells were cultured with or without HSV-1 or HSV-1 mutants for 24 h (m.o.i. = 0.1). Medium was then removed, cells were washed (three times with ice-cold PBS containing BSA, and three times with ice-cold PBS) and lipids extracted in hexane/isopropanol (3:2). Cholesterol (CH) and CE were quantified by GLC as described under ``Experimental Procedures.'' Data represent the mean of 5 T25 flasks ± S.E. The mean values derived from the HSV-1 and HSV-1 VPK repair mutant are significantly (* = p < 0.05) different from the mock-infected and HSV-1 VPK mutant.



Finally, we quantified VPK activity in HSV-1-infected smooth muscle cells. Relative to mock-infected cells, wild-type HSV-1 infection increased VPK activity approximately 4-fold (mock-infected; 375 ± 8.8 cpm/µg protein, HSV; 1410 ± 7 cpm/µg protein) at the 24-h time point (Fig. 8A). Kinase activity in VPK mutant-infected cells was similar to that in mock-infected cells (VPK mutant; 377 ± 6 cpm/µg protein). Kinase activity in VPK repair mutant-infected cells was increased 2.5-fold relative to mock-infected cells (VPK repair mutant; 948 ± 43 cpm/µg protein). VPK activity was not reduced by an anti-PKC monoclonal antibody, which does not recognize VPK, but was reduced by the protein kinase inhibitors staurosporine and calphostin C (Fig. 8B). Neither staurosporine nor calphostin C affected HSV-1 infectivity or replication as determined by plaque assay (data not shown). In contrast to the effects of HSV-1 on VPK activity, HSV-1 infection transiently increased PKA activity at 1-3 h post-infection but was reduced by 60% relative to uninfected cells after 24 h (Fig. 8C). This effect appears to parallel the trend in cyclic AMP reduction following HSV-1 infection(3) .


Figure 8: HSV-1 VPK activity, inhibition of VPK activity by protein kinase inhibitors, and PKA activity in HSV-1-infected smooth muscle cells. Smooth muscle cells grown to confluence in T-75 flasks were infected with HSV-1 (m.o.i. = 0.5) for 0.2, 1, 3, 6, 12, and 24 h. At the indicated times, medium was removed, cells were washed and assayed for activity of VPK and PKA as described under ``Experimental Procedures.'' Panel A, kinase activity in uninfected cells was assayed and assigned a relative value of 1. Relative VPK activity (mean ± S.E.) in HSV-1-infected cell lysates is not significantly (p < 0.05) different from HSV-1-infected cell lysates incubated with an anti-PKC monoclonal antibody at any time point as determined by analysis of variance. Panel B, protein kinase inhibitors staurosporine (0.5 nM) or calphostin C (100 nM) were added to HSV-1-infected cells. VPK activity was assayed at the indicated times. A relative value of 1 was assigned to VPK activity at the 6-h time point. The data (mean ± S.E.) derived from the HSV-1 group are significantly (* = p < 0.05) different from the HSV-1 group incubated with inhibitors. Panel C, PKA activity in uninfected (control) cells was assayed and assigned a relative value of 1. Relative PKA activities (mean ± S.E.) derived from the HSV-1-infected group are significantly (p < 0.05) different from the uninfected group at all time points except 0.2 h. These data are representative of three experiments.




DISCUSSION

Herpesviruses have been implicated as potential etiologic agents in the pathogenesis of human atherosclerosis(48, 49) . In animal models of this disease, there is a significant accumulation of cholesterol and CE within the arterial wall(2) ; this is also mimicked in vitro(2) . Viral infection results in reduced CE hydrolytic activities, which could explain, in part, one mechanism for the retention of lipid in these cells. Previously, we have shown that the cytoplasmic (neutral) CE hydrolase can be activated by a cyclic AMP-dependent protein kinase, and that a reduction in its activity is accompanied by reduced cellular cyclic AMP activity in response to viral infection(3) . However, cholesterol trafficking can also be regulated by other second messenger systems, especially those systems that activate cellular protein kinases. This study was undertaken in order to identify potential mechanisms by which herpesviral infection and replication can alter influx, intracellular metabolism, and efflux of cellular lipid.

We have shown for the first time that HSV-1 infection can increase binding and internalization of labeled I-LDL to arterial smooth muscle cells (Fig. 1, A and B). Increased surface expression of the LDL receptor by HSV-1 infection resulted from increased transcription of the LDL receptor gene (Fig. 2B) and, subsequently, an increase in steady state levels of LDL receptor mRNA (Fig. 2A). Transcription assays using an LDL receptor promoter/luciferase construct demonstrated that increased transcription occurred through activation of the promoter of the LDL receptor gene (Fig. 2B). Some cellular genes can be directly activated by HSV-1 infection(50, 51, 52) . Activation of these genes may result directly from viral entry or may require viral protein synthesis (53) . It has been speculated that the binding of HSV-1 to the cell surface could act as a ``mitogen-like stimulation'' (6) and that this mitogenic effect is mediated by virion-associated protein(s), which may be sufficient to induce expression of specific cellular genes. In this context, HSV-1 may be activating signal transduction pathways similar to those activated by growth factors and cytokines, which we (32, 54, 55) and others (56, 57, 58, 59, 60) have shown to induce surface and mRNA expression for the LDL receptor. HSV-1 infection may increase LDL receptor gene transcription through a mechanism involving phosphorylation of cellular proteins. This could involve phosphorylation of a protein that activates or contributes to the activation of LDL receptor gene transcription; alternatively, phosphorylation may inactivate an inhibitor of LDL receptor gene transcription. The mechanism by which LDL receptor activity is induced by other protein kinases (13, 46) is unknown.

Protein kinase A (PKA) constitutes an important cellular signal transduction pathway following the activation of adenylate cyclase. We determined if HSV-1 infection would alter PKA activity in the arterial cells and relate this to changes to cytoplasmic CE hydrolysis. HSV-1 infection transiently increased in PKA activity during early periods of infection, then decreased to 40% of the control samples at 24 h post-infection (Fig. 8C). A similar transient induction of adenylate cyclase activity, cyclic AMP levels, and PKA activity have also been reported after infection of various cells with other human herpesvirus, followed by an eventual decrease in PKA activity(61) . We also observed a 45% decrease in ACEH activity and a 50% decrease in NCEH at this 24-h time point (Fig. 4). This demonstrates a parallel and correlative reduction in PKA and CE hydrolase activities. The reduction in ACEH activity is reflected in a decreased degradation of I-LDL in HSV-1-infected cells (Fig. 1C), while the reduction of NCEH activity is paralleled by the results of our cholesterol efflux experiments, which show a reduced capacity of HSV-1-infected cells to release cholesterol (Fig. 6). A reduction in cytoplasmic hydrolysis of CE leads to less free cholesterol available for efflux and more CE remaining within the cell. The regulatory process involved in cytoplasmic CE hydrolysis is related, in part, to intracellular levels of cyclic AMP and the subsequent activation of cyclic AMP-dependent protein kinases. Cyclic AMP can activate CE hydrolase activities in a variety of cell types(2, 36, 44, 45) , and AMP-activated protein kinases can also regulate several key enzymes of lipid metabolism in normal cells(47) . Thus, such a metabolic sequence of events, in turn, may activate cytoplasmic CE hydrolase by covalent phosphorylation(62) .

The activity of HMG-CoA reductase, like that of the LDL receptor, is down-regulated by cholesterol, 25-hydroxycholesterol, and mevalonic acid(7, 9, 10, 11) . We found that there was a modest increase in HMG-CoA reductase activity in infected cells compared to uninfected cells cultured in serum-containing medium (Fig. 3A). As expected, 25-hydroxycholesterol and mevalonic acid in serum-free medium down-regulated HMG-CoA reductase activity (Fig. 3A) and the mRNA of HMG-CoA reductase in both infected and uninfected cells (Fig. 3B). However, in the presence 25-hydroxycholesterol and mevalonic acid, there was a significant increase in both HMG-CoA reductase activity and HMG-CoA reductase mRNA in HSV-1-infected cells compared to uninfected cells (Fig. 3, A and B). This suggests that viral infection may abrogate the repressive effect of sterols on HMG-CoA reductase activity and HMG-CoA reductase mRNA synthesis. HSV-1 infection increased the esterification of [^3H]oleic acid into the synthesis of nascent CE as compared to uninfected cells (Fig. 5). It is likely that this is the result of increased substrate (cholesterol) availability either from exogenous (LDL receptor) or from endogenous (HMG-CoA reductase) sources, since we observed no increase in ACAT activity in response to viral infection (Fig. 4).

HSV-1 infection also led to a significant accumulation of CE (Fig. 7). These results are consistent with our previous findings(1) . We hypothesized that VPK could be directly or indirectly involved in the modulation of cholesterol metabolism in infected cells. This viral protein kinase has been isolated(19, 20, 21, 22) , and a correlation between induction of VPK activity and herpesviral infection has been established(19) . To identify the biochemical mechanisms responsible for this large increase in CE, particularly the involvement of VPK, we tested the ability of HSV-1 VPK mutant to alter CE mass. Infection by both wild-type HSV-1 and the VPK repair mutant increased CE accumulation in the cell by >50-fold. However, the CE content of smooth muscle cells was equivalent to uninfected cells following infection with an HSV-1 VPK mutant (Fig. 7). This VPK activity could not be inhibited with an anti-PKC antibody, but could be inhibited with the kinase inhibitors staurosporine and calphostin C (Fig. 8, A and B).

In summary, we have identified altered signal transduction pathways (increased VPK and decreased cellular PKA activity) that may predispose to changes in cholesterol/CE binding, intracellular metabolism, and efflux following HSV-1 infection. We show that HSV-1 infection: 1) increases LDL binding and uptake, LDL receptor mRNA steady state levels, and transcription of the LDL receptor gene; 2) increases CE synthesis and HMG-CoA reductase activity but reduced CE hydrolysis and cholesterol efflux; 3) decreases both lysosomal and cytoplasmic CE hydrolytic (ACEH and NCEH) activities, where the latter enzyme is PKA-sensitive; and 4) reduces PKA activity by 6 h post-infection, which we believe explains the decreased ACEH and NCEH activity in these cells. Each one of these cellular mechanisms, working alone or in concert, may lead to CE accumulation.

Finally, we have demonstrated an association between VPK activity and CE accumulation utilizing HSV-1 VPK mutants. The mechanism by which VPK alters cellular host cholesterol trafficking is unknown. Although VPK possesses similarities to eukaryotic protein kinases A and C, the cellular substrates for VPK are unknown. Interestingly, it has been shown that VPK can phosphorylate viral substrates, but it has also been speculated that VPK may phosphorylate some host proteins(25) . This is the first demonstration that increased VPK activity is associated with altered activities of kinase-dependent proteins involved in cholesterol trafficking. However, we have not conclusively demonstrated that VPK specifically phosphorylates serine/threonine residues on those proteins involved in cholesterol metabolism. It is noteworthy, however, that CE accumulation following HSV-1 infection in vascular smooth muscle cells is abrogated in cells infected with an HSV-1 VPK mutant, which clearly implicates this kinase in altered lipid metabolism following herpesviral infection.


FOOTNOTES

*
This work was supported in part by National Institutes of Health Grants HL-07423, HL-46403, and HL-45343 (to D. P. H.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked ``advertisement'' in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

§
To whom reprint requests and correspondence should be addressed: Depts. of Biochemistry and Pathology, Cornell University Medical College, 1300 York Ave., New York, NY 10021. Tel.: 212-746-6470; Fax: 212-746-8789.

(^1)
The abbreviations used are: CE, cholesteryl ester; HSV-1, herpes simplex virus type-1; m.o.i., multiplicity of infection; ACEH, acid (lysosomal) cholesteryl ester hydrolase; NCEH, neutral (cytoplasmic) cholesteryl ester hydrolase; ACAT, acyl-CoA:cholesterol acyltransferase; HMG-CoA reductase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase; VPK, virus-induced protein kinase; PKC, protein kinase C; PKA, protein kinase A; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; LDL, low density lipoprotein; LPDS, lipoprotein-deficient serum; HDL, high density lipoprotein; FCS, fetal calf serum; PBS, phosphate-buffered saline.


ACKNOWLEDGEMENTS

We acknowledge the technical assistance of John E. Shuman, Eric Stolze, James Born, and Barbara Summers. We thank Dr. Bernard Roizman for generously providing an HSV-1 VPK mutant and an HSV-1 VPK mutant.


REFERENCES

  1. Hajjar, D. P., Pomerantz, K. B., Falcone, D. J., Weksler, B. B., and Grant, A. J. (1987) J. Clin. Invest. 80,1317-1321 [Medline] [Order article via Infotrieve]
  2. Hajjar, D. P. (1991) Am. J. Pathol. 139,1195-1211 [Abstract]
  3. Etingin, O. R., and Hajjar, D. P. (1990) J. Lipid Res. 31,299-305 [Abstract]
  4. Cohen, D. I., Tani, Y., Tian, H., Boone, E., Samelson, L. E., and Lane, H. C. (1992) Science 256,542-545 [Medline] [Order article via Infotrieve]
  5. Langeland, N., Haarr, L., and Holmsen, H. (1986) Biochem. J. 237,707-712 [Medline] [Order article via Infotrieve]
  6. Garcin, D., Masse, T., Madjar, J. J., and Jacquemont, B. (1990) Eur. J. Biochem. 194,279-286 [Abstract]
  7. Goldstein, J., and Brown, M. (1990) Nature 343,425-430 [CrossRef][Medline] [Order article via Infotrieve]
  8. Brown, M., and Goldstein, J. (1986) Science 232,34-47 [Medline] [Order article via Infotrieve]
  9. Sudhof, T. C., Russell, D. W., Brown, M. S., and Goldstein, J. L. (1987) Cell 48,1061-1069 [Medline] [Order article via Infotrieve]
  10. Dawson, P., Hofmann, S., van der Westhuyzen, D., Sudhof, T., Brown, M., and Goldstein, J. (1988) J. Biol. Chem. 263,3372-3379 [Abstract/Free Full Text]
  11. Smith, J. R., Osborne, T. F., Brown, M. S., Goldstein, J. L., and Gil, G. (1988) J. Biol. Chem. 263,18480-18487 [Abstract/Free Full Text]
  12. Beg, Z. H., Stonik, J. A., and Brewer, H. B. J. (1987) Metabolism 36,900-917 [Medline] [Order article via Infotrieve]
  13. Auwerx, J. H., Chait, A., and Deeb, S. S. (1989) Proc. Natl. Acad. Sci. U. S. A. 86,1133-1137 [Abstract]
  14. Middleton, B., and Middleton, A. (1992) Biochem. J. 282,853-861 [Medline] [Order article via Infotrieve]
  15. Stout, R. W., and Bierman, E. L. (1983) Atherosclerosis 46,13-20 [Medline] [Order article via Infotrieve]
  16. Hajjar, D. P., and Pomerantz, K. B. (1992) FASEB J 6,2933-2941 [Abstract/Free Full Text]
  17. Mendez, A. J., Oram, J. F., and Bierman, E. L. (1991) J. Biol. Chem. 266,10104-10111 [Abstract/Free Full Text]
  18. Hokland, B. M., Slotte, J. P., Bierman, E. L., and Oram, J. F. (1993) J. Biol. Chem. 268,25343-25349 [Abstract/Free Full Text]
  19. Purves, F. C., Katan, M., Stevely, W. S., and Leader, D. P. (1986) J. Gen. Virol. 67,1049-1057 [Abstract]
  20. Leader, D. P., and Purves, F. C. (1988) Trends Biochem. Sci. 13,244-246 [Medline] [Order article via Infotrieve]
  21. Cunningham, C., Davison, A. J., Dolan, A., Frame, M. C., McGeoch, D. J., Meredith, D. M., Moss, H. W. M., and Orr, A. C. (1992) J. Gen. Virol. 73,303-311 [Abstract]
  22. Purves, F. C., Spector, D., and Roizman, B. (1992) J. Virol. 66,4295-4303 [Abstract]
  23. Purves, F. C., Spector, D., and Roizman, B. (1991) J. Virol. 65,5757-5764 [Medline] [Order article via Infotrieve]
  24. Purves, F. C., and Roizman, B. (1992) Proc. Natl. Acad. Sci. U. S. A. 89,7310-7314 [Abstract]
  25. Purves, F. C., Longnecker, R. M., Leader, D. P., and Roizman, B. (1987) J. Virol. 61,2896-2901 [Medline] [Order article via Infotrieve]
  26. Hajjar, D. P., Falcone, D. J., Fabricant, C. G., and Fabricant, J. (1985) J. Biol. Chem. 260,6124-6128 [Abstract/Free Full Text]
  27. Bilheimer, D., Eisenberg, S., and Levy, R. (1972) Biochim. Biophys. Acta. 260,212-221 [Medline] [Order article via Infotrieve]
  28. Goldstein, J., Basu, S., and Brown, M. (1983) Methods Enzymol. 98,241-260 [Medline] [Order article via Infotrieve]
  29. Chirgwin, J. M., Przybyla, A., MacDonald, R., and Rutter, W. J. (1979) Biochemistry 18,5894-5898
  30. Davis, L., Dibner, M., and Battery, J. (1986) in Basic Methods in Molecular Biology , Elsevier Science Publishing Co., Inc., New York _
  31. de Wet, J. R., Wood, K. V., DeLuca, M., Helinski, D. R., and Subramani, S. (1987) Mol. Cell. Biol. 7,725-737 [Medline] [Order article via Infotrieve]
  32. Hsu, H.-Y., Nicholson, A. C., and Hajjar, D. P. (1994) J. Biol. Chem. 269,9213-9220 [Abstract/Free Full Text]
  33. Chen, C., and Okayama, H. (1987) Mol. Cell. Biol. 7,2745-2752 [Medline] [Order article via Infotrieve]
  34. Haley, N. J., Fowler, S., and deDuve, C. (1980) J. Lipid Res. 21,961-969 [Medline] [Order article via Infotrieve]
  35. Hajjar, D. P., and Weksler, B. (1983) J. Lipid Res. 24,1176-1185 [Abstract]
  36. Hajjar, D. P., Weksler, B. B., Falcone, D. J., Hefton, J. H., Tack-Goldman, K., and Minick, C. R. (1982) J. Clin. Invest. 70,479-488 [Medline] [Order article via Infotrieve]
  37. Brown, M., and Goldstein, J. (1974) Proc. Natl. Acad. Sci. U. S. A. 71,788-792 [Abstract]
  38. Pomerantz, K., and Hajjar, D. P. (1990) Biochemistry 29,1892-1899 [Medline] [Order article via Infotrieve]
  39. Hajjar, D. P., Wight, T., and Smith, S. (1980) Am. J. Pathol. 100,683-705 [Medline] [Order article via Infotrieve]
  40. Sugden, P. H., and Corbin, J. D. (1976) Biochem. J. 159,423-437 [Medline] [Order article via Infotrieve]
  41. Ginty, D. D., Glowacka, D., DeFranco, C., and Wagner, J. A. (1991) J. Biol. Chem. 266,15325-15333 [Abstract/Free Full Text]
  42. Leng, L., Yu, F., Dong, L., Busquets, X., Osada, S., Richon, V. M., Marks, P. A., and Rifkind, R. A. (1993) Cancer Res. 53,5554-5558 [Abstract]
  43. Lowry, O. H., Rosebrough, N. J., Farr, A. L., and Randall, R. J. (1951) J. Biol. Chem. 193,265-275 [Free Full Text]
  44. Colbran, R. J., Garton, A. J., Cordle, S. R., and Yeaman, S. J. (1986) FEBS Lett. 201,257-261 [CrossRef][Medline] [Order article via Infotrieve]
  45. Ghosh, S., and Grogan, W. M. (1989) Lipids 24,733-736 [Medline] [Order article via Infotrieve]
  46. Auwerx, J. H., Chait, A., Wolfbauer, G., and Deeb, S. (1989) Mol. Cell. Biol. 9,2298-2302 [Medline] [Order article via Infotrieve]
  47. Hardie, G., Carling, D., and Sim, A. T. R. (1989) Trends Biochem. Sci. 14,20-23 [CrossRef]
  48. Benditt, E. P., Barrett, T., and McDougall, J. K. (1983) Proc. Natl. Acad. Sci. U. S. A. 80,6386-6389 [Abstract]
  49. Melnick, J. L., Petrie, B. L., Dreisman, G. R., Burak, J., McColam, C. H., and Debakey, M. E. (1983) Lancet ii,644-647
  50. Everett, R. D. (1985) EMBO J. 4,1973-1985 [Abstract]
  51. Macnab, J. C., Orr, A., and LaThangue, N. B. (1985) EMBO J. 4,3223-3228 [Abstract]
  52. Offord, E. A., Leake, R. E., and Macnab, J. C. (1989) J. Virol. 63,2388-2391 [Medline] [Order article via Infotrieve]
  53. Kemp, L. M., Preston, C. M., and Latchman, D. S. (1986) Nucleic Acids Res. 14,9261-9270 [Abstract]
  54. Nicholson, A. C., and Hajjar, D. P. (1992) J. Biol. Chem. 267,25982-25987 [Abstract/Free Full Text]
  55. Stopeck, A. T., Nicholson, A. C., Mancini, F. P., and Hajjar, D. P. (1993) J. Biol. Chem. 268,17489-17494 [Abstract/Free Full Text]
  56. Chen, J. K., Hoshi, H., and McKeehan, W. L. (1988) In Vitro Cell. Dev. Biol. 24,199-204 [Medline] [Order article via Infotrieve]
  57. Mazzone, T., Basheerruddin, K., Ping, L., Frazer, S., and Getz, G. (1989) J. Biol. Chem. 264,1787-1792 [Abstract/Free Full Text]
  58. Grove, R., Mazzucco, C., Allegretto, N., Kiener, P., Spitalny, G., Radka, S., Shoyab, M., Antonaccio, M., and Warr, G. (1991) J. Lipid Res. 32,1889-1897 [Abstract]
  59. Moorby, C. D., Gherardi, E., Dovey, L., Godliman, C., and Bowyer, D. E. (1992) Atherosclerosis 97,21-28 [Medline] [Order article via Infotrieve]
  60. Hamanaka, R., Kohno, K., Seguchi, T., Okamura, K., Morimoto, A., Ono, M., Ogata, J., and Kuwano, M. (1992) J. Biol. Chem. 267,13160-13165 [Abstract/Free Full Text]
  61. Albrecht, T., Boldogh, I., Fons, M., Lee, C. H., AbuBakar, S., Russell, J. M., and Au, W. W. (1989) Subcell. Biochem. 15,157-202 [Medline] [Order article via Infotrieve]
  62. Hajjar, D. P. (1986) Arch. Biochem. Biophys. 247,49-56 [Medline] [Order article via Infotrieve]

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