Introduction

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The MSR mediates the uptake and degradation of polyanionic macromolecules including chemically modified proteins, such as oxidized LDL and AcLDL [for review, see Krieger (1992) and Wu et al. (1992)]. Degradation of modified LDL results in elevated intracellular cholesterol levels. Unlike the LDL receptor, expression of the MSR is not down-regulated by high levels of intracellular cholesterol. Therefore, excess supply of modified LDL will result in the intracellular formation of droplets containing cholesterol esters. The subendothelial formation of these lipid-laden foam cells from macrophages is an early event in the pathogenesis of atherosclerosis (Ross, 1993). Treatment of Watanabe heritable hyperlipidemic rabbits with dextran sulfate, an antagonist of the MSR, inhibited the formation of foam cells as well as atherosclerotic lesions (Tsubamoto et al., 1994). In mice lacking the tumor necrosis factor receptor p55, MSR expression is increased and atherosclerosis accelerated (Schreyer et al., 1996). Expression of MSR is present in atherosclerotic plaques and increased in hyperlipidemic patients (Matsumoto et al., 1990; Villanova et al., 1996). Therefore, investigation in the regulation of the expression of MSR may provide new insights for the treatment of atherosclerosis. The expression of MSR is low in circulating monocytes, but is substantially increased during their differentiation into macrophages (Geng et al., 1994). Two subtypes of MSR exist that are produced by alternative splicing of a common gene transcript (Emi et al., 1993). The first eight exons are shared by both subtypes and encodes the binding site. In addition, MSR-I has a extracellular cysteine-rich carboxyl-terminal domain encoded by exons 10 and 11, whereas MSR-II has a short 6-amino acid domain in its carboxyl-terminal end. The ratio between isoforms changes during differentiation from monocyte to macrophage because MSR-I is up-regulated (Geng et al., 1994).

Several studies now suggest that increased expression of proteins, such as scavenger receptors, cytokines, inducible nitric oxide synthase, and cell adhesion molecules, is correlated in some way with increased levels of ROS and the oxidative modification of lipoproteins in the vessel wall (Lo et al., 1993; Kiener et al., 1995; Mietus-Snyder et al., 1997). The importance of oxidant stress leading to endothelial injury and atherosclerosis has recently been reviewed by McGorisk and Treasure (1996). A relationship clearly exists between oxidative stress and the formation of ROS, resulting in the activation of proatherogenic redox-sensitive genes and the generation of mediators leading to atherosclerotic lesion formation. Several critical signaling processes seem to involve ROS. For example, stimulation of vascular smooth muscle by platelet-derived growth factor requires intracellular generation of hydrogen peroxide (Sundaresan et al., 1995). Additionally, ROS mediate activation of the epidermal growth factor receptor induced by UV irradiation (Huang et al., 1996) and cytokine-induced activation of c-jun amino-terminal kinases (Lo et al., 1996). Therefore, ROS seem not only to be essential in signaling pathways maintaining cellular function, but also in cellular dysfunction leading to disease. We have investigated the role of ROS on the activity and expression of MSR in the human monocyte/macrophage cell line THP-1 differentiated by the phorbol ester PMA.

	Materials and Methods

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MSR activity. THP-1 cells were grown in suspension in RPMI-1640 medium supplemented with 2 mM L-glutamine, 50 µM -mercaptoethanol, and 10% fetal bovine serum. All incubations were performed at 37° in a humidified atmosphere containing 5% CO₂. Cells were seeded onto 6-well plates at 1.5 × 10⁶ cells per well. PMA (0.1 µM) was added to differentiate THP-1 cells into macrophage-like cells. Drugs were added 30 min before the addition of PMA. After 24 hr cells, which had adhered to the bottom of the well, were washed, lifted from the dish, and transferred to microcentrifuge tubes. After centrifugation (1600 × g for 3 min) the cells were incubated at 37° for 90 min in RPMI containing 30 µg/ml AcLDL labeled with DiI-AcLDL. Subsequently, cells were washed with phosphate-buffered saline and after centrifugation (1600 × g for 3 min) fixed in 1% paraformaldehyde in phosphate-buffered saline. To determine the fluorescence of the THP-1 cells, the suspension was measured by flow cytometry (FACScan, Becton Dickinson), and 10⁴ cells were analyzed using the Lysis software (Hassall, 1992; Tsubamoto et al., 1994). For every sample, the median of the fluorescence per cell was determined and expressed as percentage of cells treated with PMA alone.

Levels of MSR-I and II mRNA. The levels of mRNA for MSR were determined by the sensitive PCR after RT of total RNA (RT-PCR). THP-1 cells were cultured in T25 flasks at 4 × 10⁶ cells in 4 ml. PMA (0.1 µM) was added to differentiate THP-1 cells into macrophage-like cells. Drugs were added 30 min before the addition of PMA. After 24 hr, cells that had adhered to the bottom of the flask were washed and resuspended in 1 ml of RNazol B (Biogensis, Poole, UK) to isolate total RNA. A pilot experiment showed that, at 24 hr, mRNA levels for MSRs were not different from those at 48 and 72 hr. Chloroform (0.1 ml) was added to separate RNA from DNA and proteins by extraction. The aqueous phase was collected, and isopropanol (1:1 v/v) was added to precipitate RNA, which, after centrifugation, was washed in 70% ethanol and dissolved in 1 mM EDTA.

MSR mRNA stability. To investigate whether ROS influence the stability of MSR mRNA after the differentiation of THP-1 cells to macrophages, the influence of DMTU, sodium benzoate, and cycloheximide on mRNA levels was tested in the presence of the transcription inhibitor actinomycin D. THP-1 cells were cultured in 6-well plates at 1.5 × 10⁶ cells in 2 ml. PMA (0.1 µM) was used to differentiate THP-1 cells into macrophage-like cells and, after 20 hr, actinomycin D (7.5 µg/ml) was added followed by the different drugs. After 3 and 7 hr, cells were homogenized in 1 ml of RNazol B, total RNA was isolated, and levels of MSR mRNA were determined by RT-PCR reaction as described above.

Adhesion and viability of THP-1 cells. Differentiation of THP-1 cells with PMA into macrophage-like cells results in the adhesion of cells to the bottom of the well and the expression of MSR. To assess the specificity of any drug effects on the expression or activity of MSR, adhesion (DNA content) and viability (MTT conversion) of the THP-1 cells was determined. THP-1 cells were seeded onto 96-well plates at 1 × 10⁵ cells in 100 µl of medium per well. Drugs were added, and after 30 min the cells were differentiated using PMA (0.1 µM). After 24 hr, the wells were washed, and the adherent cells were solubilized by addition of water, shaking, and incubation at 37° for 1 hr. Hoechst 33258, a fluorescent dye for DNA, was added and incubated for another 20 min. Fluorescence was determined with excitation at 355 nm and emission at 460 nm. In another set, MTT was added to the cells (0.4 µg/ml final concentration) at 24 hr and incubated for 1 hr. In viable cells, MTT is converted to an insoluble purple formazan by dehydrogenase enzymes in the mitochondria. Subsequently, the wells were washed, and adherent cells were solubilized with dimethylsulfoxide. The absorbance of the converted dye was measured at 550 nm with background subtraction at 650 nm. In both tests, the result of adherent THP-1 cells treated with PMA alone was set at 100%.

Drugs. THP-1 cells were obtained from the European Type Culture Collection (Porton, UK). TEMPO and acetovanillone (apocynin) were purchased from Aldrich (Dorset, UK). Calpain inhibitor I was obtained from Calbiochem (Nottingham, UK). Enzymes and reagents for RT-PCR were purchased from Perkin-Elmer, except for the PCR primers, which were ordered from Severn Biotech Ltd. (Worcs, UK). DiI-AcLDL and RNazol B was obtained from Biogenesis (Poole, UK). All other drugs were purchased from Sigma (Dorset, UK).

Data analysis

All data are expressed as mean ± standard error of n observations. DiI-AcLDL internalization in THP-1 macrophages was determined in two separate samples per experiment. Viability and adherence of THP-1 cells was measured in triplicate, and the average was used in subsequent calculations. Statistical analysis was performed using one-way analysis of variance followed by Dunnet's test for multiple comparison of separate treatments with control (THP-1 cells differentiated with PMA).

	Results

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MSR activity. The uptake of DiI-AcLDL by differentiated THP-1 cells was concentration-dependent, inhibited by excess unlabeled AcLDL, and prevented by incubation at 4° (Fig. 1). Differentiation of THP-1 resulted in a 5-fold increase in the uptake of DiI-AcLDL. The uptake was prevented by treatment with the protein synthesis inhibitor cycloheximide (1 µg/ml, 9 ± 4%, n = 6, p < 0.01). To investigate the role of ROS in the induction of the uptake of AcLDL, cells were treated with various oxygen radical scavengers (Fig. 2). TEMPO (1 mM), an intracellular scavenger of superoxide anion (Samuni et al., 1990), DMTU (10 mM), an intracellular hydrogen peroxide scavenger (Parker et al., 1985; Curtis et al., 1988), and sodium benzoate (25 mM), a hydroxyl radical scavenger (Sagone et al., 1980), inhibited the enhanced DiI-AcLDL uptake. The iron chelator deferoxamine (10 µM) also prevented the increase in DiI-AcLDL uptake. Furthermore, inhibition of ROS generation in the mitochondria by respiratory chain complex I inhibitor rotenone (1 µM) attenuated the induction of DiI-AcLDL uptake (Fig. 3). The complex III inhibitor antimycin A (1 µM) had no significant effect. Acetovanillone (50 µM) (Stolk et al., 1994), an inhibitor of NADPH oxidase, also reduced the uptake of DiI-AcLDL. Two inhibitors of the activation of the stress response nuclear factor B (NFB, (Schreck and Baeuerle, 1991; Miyamoto et al., 1994), the radical scavenger and metal chelator, PDTC (25 µM), and the IB protease inhibitor calpain inhibitor I (3 µM), did not significantly influence the uptake of DiI-AcLDL (92 ± 5%, n = 9 and 103 ± 14%, n = 6).

View larger version (23K): [in this window] [in a new window]   Fig. 1.   The uptake of DiI-AcLDL in differentiated THP-1 macrophages. PMA (0.1 µM) was added to THP-1 cells, and after 24 hr macrophages were collected and incubated for 90 min with different concentrations of DiI-AcLDL at 37° (), with 30 µg/ml DiI-AcLDL at 4° () or with 30 µg/ml DiI-AcLDL in the present of excess unlabeled AcLDL (500 µg/ml, ). Subsequently, cells were analyzed by flow cytometry, and fluorescence (median) was determined. , Autofluorescence of differentiated THP-1 macrophages. Error bars, mean ± standard error (n = 6). *, p < 0.05 compared with incubation with 30 µg/ml DiI-AcLDL alone.

View larger version (32K): [in this window] [in a new window]   Fig. 2.   ROS mediate the induction of DiI-AcLDL uptake in THP-1 macrophages. THP-1 cells were differentiated by PMA (0.1 µM) in the absence () or presence of TEMPO (1 mM, ), DMTU (10 mM, ), sodium benzoate (25 mM, ), or deferoxamine (10 µM, ). After 24 hr, cells were incubated with 30 µg/ml DiI-AcLDL and analyzed by flow cytometry. , Uptake of DiI-AcLDL by nondifferentiated THP-1 cells. Mean ± standard error are presented as percentage of PMA control (n > 10). *, p < 0.05 compared with PMA control.

View larger version (35K): [in this window] [in a new window]   Fig. 3.   The induction of DiI-AcLDL uptake in THP-1 macrophages requires the generation of ROS and protein synthesis. THP-1 cells were differentiated by PMA (0.1 µM) in the absence () or presence of acetovanillone (50 µM, ), rotenone (1 µM, ), antimycin A (1 µM, ), or cycloheximide (1 µg/ml, ). After 24 hr, cells were incubated with 30 µg/ml DiI-AcLDL and analyzed by flow cytometry. , Uptake of DiI-AcLDL by nondifferentiated THP-1 cells. Mean ± standard error are presented as percentage of PMA control (n > 10). *p < 0.05 compared with PMA control.

Levels of MSR-I and II mRNA. Similarly, the levels of mRNA for MSR-I increased approximately 20-fold after the exposure of THP-1 cells to PMA. The levels of mRNA for the MSR-II receptor showed only a 3-fold increase. The increase in the levels of mRNA for MSR-I was significantly inhibited by the oxygen radical scavengers, TEMPO, DMTU, and sodium benzoate, and the metal chelator deferoxamine (Fig. 4). Cycloheximide, acetovanillone, and rotenone, but not antimycin A, also inhibited the levels of MSR-I mRNA (Fig. 5). The mRNA levels were not significantly affected by treatment with PDTC (83 ± 10%, n = 5) or calpain inhibitor I (95 ± 34%, n = 5).

View larger version (44K): [in this window] [in a new window]   Fig. 4.   ROS contribute to the increase in mRNA levels of MSR-I in THP-1 macrophages. THP-1 cells were differentiated by PMA (0.1 µM) in the absence () or presence of TEMPO (1 mM, ), DMTU (10 mM, ), sodium benzoate (25 mM, Benz, ), or deferoxamine (10 µM, Def, ). After 24 hr, total RNA was isolated, and RT-PCR was performed with primers specific for MSR-I. After electrophoresis on a 1% agarose gel, PCR products were stained by ethidium bromide (a) and quantified by scanning densitometry (b). , mRNA levels of MSR-I mRNA in nondifferentiated THP-1 cells (Ctrl). Mean ± standard error are presented as percentage of PMA control (n > 6). *, p < 0.05 compared with PMA control.

View larger version (40K): [in this window] [in a new window]   Fig. 5.   The increase in mRNA levels of MSR-I in THP-1 macrophages requires the generation of ROS and protein synthesis. THP-1 cells were differentiated by PMA (0.1 µM) in the absence () or presence of acetovanillone (50 µM, Acv, ), rotenone (1 µM, Rot, ), antimycin A (1 µM, AA, ) or cycloheximide (1 µg/ml, Cyh, ). After 24 hr, total RNA was isolated and RT-PCR was performed with primers specific for MSR-I. After electrophoresis on a 1% agarose gel, PCR products were stained by ethidium bromide (a) and quantified by scanning densitometry (b). , mRNA levels of MSR-I in nondifferentiated THP-1 cells (Ctrl). Mean ± standard error are presented as percentage of PMA control (n > 6). *, p < 0.05 compared with PMA control.

View larger version (56K): [in this window] [in a new window]   Fig. 6.   Effect of scavengers of ROS on the mRNA levels of MSR-II in THP-1 macrophages. THP-1 cells were differentiated by PMA (0.1 µM) in the absence () or presence of TEMPO (1 mM, ), DMTU (10 mM, ), sodium benzoate (25 mM, Benz, ) or deferoxamine (10 µM, Def, ). After 24 hr, total RNA was isolated and RT-PCR was performed with primers specific for MSR-II. After electrophoresis on a 1% agarose gel, PCR products were stained by ethidium bromide (a) and quantified by scanning densitometry (b). , Levels of MSR-II mRNA in nondifferentiated THP-1 cells (Ctrl). Mean ± standard error are presented as percentage of PMA control (n > 6). *p < 0.05 compared with PMA control.

View larger version (51K): [in this window] [in a new window]   Fig. 7.   The increase in mRNA levels for MSR-II in THP-1 macrophages is not dependent on protein synthesis or formation of ROS by NADPH oxidase or mitochondrial respiratory enzymes. THP-1 cells were differentiated by PMA (0.1 µM) in the absence () or presence of acetovanillone (50 µM, Acv, ), rotenone (1 µM, Rot, ), antimycin A (1 µM, AA, ), or cycloheximide (1 µg/ml, Cyh, ). After 24 hr, total RNA was isolated, and RT-PCR was performed with primers specific for MSR-II. After electrophoresis on a 1% agarose gel, PCR products were stained by ethidium bromide (a) and quantified by scanning densitometry (b). , Levels of MSR-II mRNA in nondifferentiated THP-1 cells. Mean ± standard error are presented as percentage of PMA control (n > 6). *, p < 0.05 compared with PMA control.

MSR mRNA stability. Post-treatment of differentiated THP-1 cells with DMTU, sodium benzoate, or cycloheximide together with the transcription inhibitor actinomycin D, resulted in a significant decrease in the levels of MSR-I mRNA at 7 hr, but not at 3 hr, compared with differentiated THP-1 cells receiving actinomycin D alone (Fig. 8). Again, the levels of MSR-II mRNA were not significantly affected by these post-treatments (Fig. 9).

View larger version (58K): [in this window] [in a new window]   Fig. 8.   ROS and protein synthesis contribute to the stabilization of MSR-I mRNA in THP-1 macrophages. THP-1 cells were differentiated by PMA (0.1 µM), and after 20 hr actinomycin D (7.5 µg/ml, Act. D) was added followed by DMTU (10 mM, , n = 5), sodium benzoate (25 mM, Benz, , n = 5), cycloheximide (1 µg/ml, Cyh, , n = 5), or vehicle (Ctrl, RPMI medium, n = 5). After an additional 3 or 7 hr, total RNA was isolated, and RT-PCR was performed with primers specific for MSR-I. After electrophoresis on a 1% agarose gel, PCR products were stained by ethidium bromide (a) and quantified by scanning densitometry (b). Mean ± standard error are presented as percentage of PMA-differentiated THP-1 macrophages treated with actinomycin D alone. *, p < 0.05 compared with PMA control.

View larger version (55K): [in this window] [in a new window]   Fig. 9.   ROS or protein synthesis do not contribute to the stabilization of MSR-II mRNA in THP-1 macrophages. THP-1 cells were differentiated by PMA (0.1 µM) and after 20 hr with actinomycin D (7.5 µg/ml, Act. D) was added followed by DMTU (10 mM, , n = 5), sodium benzoate (25 mM, Benz, , n = 5), cycloheximide (1 µg/ml, Cyh, , n = 5), or vehicle (Ctrl, RPMI medium, n = 5). After an additional 3 or 7 hr, total RNA was isolated, and RT-PCR was performed with primers specific for MSR-II. After electrophoresis on a 1% agarose gel, PCR products were stained by ethidium bromide (a) and quantified by scanning densitometry (b). Mean ± standard error are presented as percentage of PMA-differentiated THP-1 macrophages treated with actinomycin D alone.

Adhesion and viability of THP-1 cells. THP-1 cells grow in suspension and differentiate into macrophage-like cells in the presence of PMA, resulting in adhesion of most cells to the bottom of the well (82 ± 7% of total cells at 24 hr). Treatment with cycloheximide or rotenone partially reduced the adhesion of THP-1 cells elicited by PMA as measured by DNA content of the adherent cells (Table 1). None of the other drugs inhibited the adhesion. The viability of the adherent cells, as determined by the reduction of MTT, was not significantly affected by any of the treatments including cycloheximide or rotenone (Table 1).

	Discussion

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A number of studies have shown that an increase in expression of MSR mRNA and protein results in an elevated uptake and breakdown of modified LDL (Matsumoto et al., 1990; Geng et al., 1994; Wu et al., 1994). The present data in differentiated THP-1 cells support this notion, because the AcLDL uptake was (i) associated with an increase in mRNA levels of the MSR-I and II and (ii) inhibited by the protein synthesis inhibitor cycloheximide. Our results further indicate that ROS contribute to the increase in the uptake of AcLDL. Hydroxyl radicals are produced from superoxide anions and hydrogen peroxide in the presence of free iron via the Fenton reaction. Deferoxamine, an iron chelator, or scavengers of hydroxyl radicals (sodium benzoate), superoxide anions (TEMPO) and hydrogen peroxide (DMTU) inhibit the induction of AcLDL uptake. These results suggest an important role for hydroxyl radicals in modulation of MSR activity. Rotenone, but not antimycin A, decreased the uptake of AcLDL, which indicates that leakage of electrons in the mitochondria between enzyme complex I and III contributes to the generation of ROS-mediating MSR activity. Inhibition of ROS generation by NADPH oxidase by acetovanillone also decreased AcLDL uptake. Thus, several intracellular sources of ROS production contribute to the induction of MSR activity.

Recently, Mietus-Snyder et al. (1997) have demonstrated that scavenger receptor expression in smooth muscle cells elicited by phorbol ester treatment is mediated, in part, by ROS. They showed that after the activation of protein kinase C, intracellular levels of ROS rise, and that an elevated oxidative stress results in mRNA transcription and scavenger receptor activity. Here, the oxygen radical scavengers and inhibitors of ROS formation that attenuated MSR activity also decreased the levels of mRNA for the MSR-I receptor as detected by RT-PCR. This suggests that ROS regulate the uptake of AcLDL at the level of gene expression. Interestingly, the levels of mRNA for MSR-II was largely unaffected by the inhibitors of ROS formation or activity. As the two isoforms are produced by alternative splicing, the regulation of the respective isoform is post-transcriptional. This suggests that ROS affect MSR-I mRNA levels by a post-transcriptional mechanism. Indeed, post-treatment of differentiated THP-1 cells when transcription is blocked by actinomycin D, with the hydroxyl radical scavengers sodium benzoate and DMTU decreased the mRNA levels of MSR-I, but not MSR-II, when compared with differentiated cells receiving actinomycin D alone. The results indicate that ROS selectively contributes to the stabilization of MSR-I mRNA. Pre- and post-treatment with cycloheximide also attenuated the mRNA levels of MSR-I, but not the level of MSR-II. Inhibition of MSR-I mRNA levels by cycloheximide has also been reported by Dufva et al. (1995). Thus, protein synthesis seems to be required for the stabilization of MSR-I mRNA. Another similarity is that post-treatment with cycloheximide or the scavengers DMTU and sodium benzoate was effective only after an incubation of 7 hr but not 3 hr. Possibly, ROS induce a protein that can selectively stabilize MSR-I mRNA by binding to a domain encoding for the cysteine-rich carboxyl terminus, which is lacking in the MSR-II mRNA.

The transcription factor NF-B is oxidative stress sensitive (Schulze-Osthoff et al., 1995; Sen and Packer, 1996), and the intracellular redox state of the cell seems to be crucial for its activation, which occurs concomitantly with a rise in ROS. However, neither PDTC or calpain inhibitor I, two unrelated inhibitors of the activation of NFB, did significantly influence the levels of MSR mRNA or MSR activity. Therefore, other stress-responsive transcription factors than NFB are involved in the induction of MSR expression and activity, and the post-transcriptional regulation of both subtypes. This may include AP-1 as ROS can up-regulate its activation (Schulze-Osthoff et al., 1995; Lo et al., 1996) and there seems to be a requirement for AP-1 expression in scavenger receptor constructs (Wu et al., 1994).

In isolated human blood monocytes, the expression of MSR-I is similar or less to MSR-II (Geng et al., 1994; Dufva et al., 1995). During differentiation to macrophages, the expression of MSR increases, which is primarily attributed to an increase in the MSR-I isoform. This increase in MSR type I to type II ratio is maintained during the transformation of macrophages into foam cells (Geng et al., 1994). Therefore, regulation of the MSR-I receptor may be important in the generation of foam cells and development of atherosclerotic plaques. In this study, differentiation of THP-1 cells to macrophages resulted in a 20-fold increase in the levels of MSR-I mRNA, but only in a 3-fold increase for isotype II. This further supports the notion that ROS and an as yet unknown protein contribute to a selective stabilization of MSR-I mRNA. Furthermore, the uptake of AcLDL is directly related to the selective influence of ROS inhibitors on MSR-I mRNA. Thus, THP-1 macrophages are useful model to study the regulation of the MSR receptors.

In macrophages, inhibition of MSR expression and activity has been reported in response to interferon- (Fong et al., 1990; Geng and Hansson, 1992), transforming growth factor-₁ (Bottalico et al., 1991), tumor necrosis factor- (Van Lenten and Fogelman, 1992; Hsu et al., 1996), LPS (Van Lenten et al., 1985), granulocyte/macrophage colony-stimulating factor (Van Der Kooij et al., 1996), all-trans-retinoic acid, and dexamethasone (Moulton et al., 1992). Tumor necrosis factor- regulates MSR expression both by transcriptional and post-transcriptional mechanisms, but mainly by decreasing the half-life of both MSR-I and MSR-II mRNA via protein synthesis (Hsu et al., 1996). The granulocyte/macrophage colony-stimulating factor also inhibits the expression of both isoforms (Van Der Kooij et al., 1996). LPS inhibits MSR-I and MSR-II mRNA expression in human monocyte-derived macrophages to a different extent. Furthermore, the regulation of the expression of MSR-I, but not MSR-II, mRNA by LPS was cycloheximide sensitive. The present study shows that ROS mediates the stabilization of MSR-I, but not MSR-II, mRNA. Mietus-Snyder et al. (1997) have demonstrated that oxidative stress also results in an elevated mRNA. Thus, the regulation of MSR isoform expression is likely to be a complex transcriptional as well as post-transcriptional process involving ROS.

In conclusion, ROS contribute to an increase in the uptake of modified LDL in PMA differentiated THP-1 macrophages by stabilizing MSR-I mRNA. The rise in the levels of MSR-I mRNA involves post-transcriptional mechanisms independent of any regulation by NF-B-like stress responsive transcription factors. However, other redox sensitive factors such as AP-1 seem to be functional in MSR regulation (Moulton et al., 1992; Wu et al., 1994). The present study suggests that during atherogenesis, oxidant stress may contribute to the formation of macrophage foam cells, not only by oxidizing LDL, but also by increasing MSR-I mRNA and activity.