Cyclic AMP stimulates the synthesis and function of the low-density lipoprotein receptor in human vascular smooth-muscle cells and fibroblasts

Chloroquine increases low-density lipoprotein removal from plasma in systemic lupus patients

Low-density lipoprotein (LDL) pathway in systemic lupus erythematosus (SLE) patients taking chloroquine diphosphate (CDP) was evaluated through the kinetic behavior of a radioactive cholesterol-rich nanoemulsion (LDE) that resembles the LDL lipidic structure. LDE was labeled with (14)C-cholesteryl ester ((14)C-CE), then IV injected in inactive female SLE patients: 10 taking CDP (CDP), 10 without therapy (NO THERAPY); and 10 normal subjects (CONTROL). Groups were age-matched and followed rigorous selection criteria of conditions that interfere in the lipid profile. Blood samples were collected in pre-established intervals after infusion for radioactivity measurement. Fasting lipoproteins were determined in the beginning of kinetic studies. Fractional clearance rate (FCR) of (14)C-CE was significantly different in the three groups (P = 0.03). In fact, a greater FCR of (14)C-CE was observed in CDP compared to NO THERAPY (0.076 +/- 0.037 versus 0.046 +/- 0.021 h(-1); P < 0.05) and to CONTROL (0.0516 +/- 0.0125 h(-1); P < 0.05). Accordingly, a significant lower total and LDL cholesterol were observed in CDP (156 +/- 16 and 88 +/- 16 mg/dl) compared to NO THERAPY (174 +/- 15 and 108 +/- 17 mg/dl; P < 0.05) and to CONTROL (200 +/- 24 and 118 +/- 23 mg/dl; P < 0.05). In contrast, no difference in (FCR) of (14)C-CE of NO THERAPY and CONTROL groups was observed. This is the first in vivo demonstration that LDE removal by LDL receptor from plasma is increased in SLE patients taking CDP with a consequent beneficial decrease in LDL-c levels.

Role of the cyclic AMP-dependent pathway in free radical-induced cholesterol accumulation in vascular smooth muscle cells

We have previously reported that free radical-treated vascular smooth muscle cells (SMC) lead to cholesterol accumulation in vitro. In the current study, we investigated the effects of oxidative stress on cyclic AMP concentration and cAMP-dependent enzymes involved in cholesterol homeostasis in A7r5 cells. Under our conditions of a mild oxidative stress, namely with no change in cell viability, we found that free radicals, initiated using azobis-amidinopropane dihydrochloride (AAPH), resulted in a dose-dependent decrease in cellular cAMP which was opposed by vitamin E preincubation. Although the addition of adenylate cyclase activators (carbacyclin and forskolin) increased cAMP levels it did not succeed in restoring the AAPH-induced decrease. The oxidative stress-induced increase in activities of 3-hydroxy-3-methylglutaryl coenzyme A reductase and of acyl coenzyme A: cholesterol acyltransferase and the decrease in neutral cholesteryl ester hydrolase activity were suppressed by addition of dibutyryl cAMP. Taken together, these results strongly suggest that free radicals reduce cAMP concentrations by altering cell membrane adenylate cyclase activity. The changes of cAMP-dependent enzymes induced by oxidative stress resulting in cholesterol accumulation might be one of the processes leading to SMC-derived foam cells depicted in atheroma plaque. Moreover, if extrapolated to in vivo, these data may explain in part the beneficial effects of antioxidants in the reduction of cardiovascular diseases.

Increased uptake of LDL by oxidized macrophages is the result of an initial enhanced LDL receptor activity and of a further progressive oxidation of LDL

Iron ions were recently shown to induce cellular lipid peroxidation in macrophages, and these oxidized cells can convert native low-density lipoprotein (LDL) to oxidized LDL (Ox-LDL). The present study demonstrates that deoxycholic acid (DCA) and angiotensin II (ANG-II) can also induce oxidative modification of macrophages via metal ions independent mechanisms. Furthermore, incubation of LDL (200 micrograms of protein/ml) for 24 h at 37 degrees C with DCA, ANG-II, as well as FeSO4-induced oxidized macrophages, resulted in oxidative modification of the lipoprotein as evidenced by increased TBARS formation in LDL (by 50, 105, and 258%, respectively), decreased TNBS reactivity (by 45, 56, and 42%, respectively), and increased cellular uptake (by 60, 166, and 230%, respectively). A positive correlation (n = .88) was found between the extent of the cellular lipid peroxidation and the increment in the cellular uptake of the LDL. The oxidative modification of LDL by oxidized macrophages was found to be a progressive process. Incubation of LDL with oxidized macrophages for increasing periods of time up to 24 h resulted in progressive increment in: (1) the electrophoretic mobility of the LDL; (2) the TBARS formation in LDL; (3) the cellular uptake of LDL by the oxidized macrophages via the Ox-LDL receptor. Upon fractionation on a heparin-sepharose column of LDL that was incubated for different periods of time with oxidized macrophages, a gradual increment in the unbound LDL fraction was obtained, up to 72% after 24 h of incubation. During the first hour of LDL incubation with the oxidized macrophages a twofold increase in the cellular uptake of LDL by these cells was detected, although no significant oxidation of the lipoprotein occurred during this short time period. This effect could be attributed to an increased number of LDL receptors on the cell surface of the oxidized macrophages. In conclusion, increased uptake of LDL by oxidized macrophages results from two routes: (1) enhanced uptake via the LDL receptor due to increased LDL receptor activity; (2) lipoprotein uptake via the Ox-LDL receptors due to cellular modification of LDL. Both of these processes lead to macrophage cholesterol accumulation and foam cell formation, and thus contribute to accelerated atherosclerosis under oxidative stress.

Calcium-dependent second-messenger regulation of low-density lipoprotein oxidation by human aortic smooth muscle cells

BACKGROUND

We postulated that the response of vascular smooth muscle cells (SMCs) to oxidized low-density lipoprotein (LDL) may be modulated through calcium and 3', 5' cyclic adenosine monophosphate (Ca+2-cAMP) second-messenger activity.

METHODS

Changes in cytosolic calcium [Ca+2]i in aortic SMCs exposed to native (N-) and oxidized (Ox-) LDL were measured with a Fura 2-AM indicator. The influence of cAMP on this response was determined by incubating the cells with either forskolin or 8-bromo-cAMP (stimulatory) or galanin (inhibitory). The cells were then activated by an initial preincubation with N- or Ox-LDL, and the subsequent cellular oxidation of N-LDL was measured. The effect of cAMP mediators alone or in conjunction with calcium antagonism was studied.

RESULTS

Exposure of SMCs to Ox-LDL resulted in a marked elevation of [Ca+2]i(306 +/- 12 nmol/L) compared with that in the control group (192 +/- 15 nmol/Liter; p < 10(-6)). This response was augmented by cAMP stimulation (406 +/- 8 nmol/L; p < 10(-6)) but reduced by cAMP inhibition (247 +/- 8 nmol/L; p < 10(-6)). The activation of intracellular signaling by initial Ox-LDL priming increased the subsequent oxidation of N-LDL (0.40 +/- 0.02 nmol malondialdehyde versus 0.24 +/- 0.02 nmol MDA control; p < 10(-4)). This response was enhanced by cAMP (0.45 +/- 0.03 nmol MDA; p < 10(-4)) and inhibited by galanin (0.26 +/- 0.02; p < 10(-4)). The cAMP effect was reversed by the blockade of calcium mobilization via membrane channels and reticular release.

CONCLUSIONS

Oxidized LDL-induced Ca+2-cAMP signaling modulates the cellular oxidation of N-LDL. This finding suggests a mechanism through which the scavenger uptake of modified LDL may potentially be regulated.

Elevation of cyclic AMP in human skin fibroblasts results in increased capacity for HDL binding

Pre-incubation of cultured human skin fibroblasts, lung fibroblasts and vascular smooth muscle cells, for 24 h with cAMP-elevating agents resulted in a significant increase (40-60%) of the cells' capacity to bind HDL. The increase was due to enhancement of the maximal binding capacity of a high affinity saturable site which binds HDL in preference to LDL. The effect was dependent upon the concentration of the cAMP-elevating agents and required more than 4 h to become evident. Cyclic AMP-mediated elevation of HDL binding occurred in cells with access to an exogenous source of cholesterol, which could be the physiological donor LDL or non-lipoprotein in origin. The observed effects were not subsequent to changes in cellular balance of cholesterol to cholesterol ester and were not due to inhibition of cellular proliferation.

Altered cholesterol trafficking in herpesvirus-infected arterial cells. Evidence for viral protein kinase-mediated cholesterol accumulation

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-2 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.

The immunosuppressive substance 2-chloro-2-deoxyadenosine modulates lipoprotein metabolism in a murine macrophage cell line (P388 cells)

A recently developed immunosuppressive substance, 2-chloro-2-deoxyadenosine (2-CdA), was reported to inhibit monocyte functions at low concentration. Because macrophages play a key role in the formation of atherosclerotic plaques, it was of interest to study the effect of 2-CdA on cellular lipid metabolism. For this purpose we have used a macrophage cell line (P388) to perform incubation studies in the presence of acetylated low density lipoprotein (Ac-LDL) and 2-CdA. The addition of 2-CdA, in concentrations ranging from 5-20 nM, induced a dose-dependent decrease in cellular cholesterol content and in the amount of extracellular [14C]oleic acid (OA) incorporated into the cholesteryl ester (CE) fraction. The effect was maximized at 20 nM 2-CdA with an 86% reduction in cholesterol esterification compared to controls (P < 0.008). To evaluate the mechanism of interaction of 2-CdA with cellular lipid metabolism, deoxycytidine (dCyt) and 3-methoxybenzamide (3-MOB), substances known to antagonize the effect of 2-CdA in different ways, were co-administered with 2-CdA. dCyt, a competitive inhibitor of dCyt kinase, which catalyzes phosphorylation to the active metabolite, antagonized the effects of 20 nM 2-CdA, producing significantly greater incorporation of extracellular [14C]OA into the CE fraction than in the presence of 2-CdA alone (P < 0.0086). Co-incubation with 2-CdA and the poly-ADP-ribose synthetase inhibitor 3-MOB, which is known to render cells resistant to 2-CdA toxicity by preventing cellular nicotinamide adenine dinucleotide (NAD)- and adenosine triphosphase-depletion, also reversed the effect of 2-CdA on lipid accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of low-density lipoprotein uptake in HepG2 cells by epidermal growth factor via a tyrosine kinase-dependent, but protein kinase C-independent, mechanism

Epidermal growth factor (EGF), a potent mitogenic polypeptide, stimulated the uptake and degradation of [3H]sucrose-labelled low-density lipoprotein (LDL) by HepG2 cells. The increase in LDL uptake was prevented by the presence of the tyrosine kinase inhibitor genistein. Activation of protein kinase C with phorbol 12-myristate 13-acetate (PMA) also stimulated the uptake of [3H]LDL by HepG2 cells. When EGF and PMA were added together, PMA increased the response to EGF in an additive manner. The protein kinase C inhibitor Ro-31-8220 prevented the increase in LDL uptake caused by PMA, but did not affect EGF stimulation of LDL uptake. Similarly, down-regulation of protein kinase C activity by chronic treatment with PMA also did not affect the EGF stimulation of LDL uptake. These results suggest that the EGF stimulation of LDL uptake and degradation by HepG2 cells is mediated by a tyrosine kinase-dependent, but protein kinase C-independent, mechanism.

Oxytocin receptors and prostaglandin release in rabbit amnion

Besides stimulating uterine myometrial and mammary myoepithelial cell contraction, oxytocin (OT) causes the release of prostaglandins (PGs) from uterine endometrium/decidua and amnion cells. Lacking information about OT receptors eliciting PG release, we don't know how they are related to OT receptors involved in smooth muscle contraction. The amnion offers great potential for characterizing OT receptors associated with PG release, as the amount of iodinated OT antagonist ([125I]OTA) bound to rabbit amnion membranes during labor is among the greatest of any tissue yet studied, reaching about 10 pmol/mg membrane protein. The relative affinities of several OT analogues for binding sites on amnion membranes are the same as those on decidual membranes. There are differences in the ligand profile between amnion and myometrium, but they could be due to the additional presence of vasopressin receptors on myometrial membranes. An increase in the sensitivity of PGE2 release from amnion cells in culture to OT and analogues accompanies the rise in OT receptor concentration at the end of gestation. Increases in [125I]OTA binding in vivo can be mimicked with cultured amnion cells by addition of agents that elevate intracellular cAMP levels. Based on the time course and inhibition of the increase with cycloheximide, cAMP might induce OT receptor gene expression. The increase also is reflected by a marked elevation in the covalent labeling of a 50-kDa electrophoretic band with a photoactivated derivative of [125I]OTA. Because of the homogeneity of cell types in the amnion, the ease of culturing amnion cells, and the high concentration of OT receptors that can be induced, this tissue should be very useful in characterizing OT receptors associated with PG synthesis.

Stimulation of rat hepatic low density lipoprotein receptors by glucagon. Evidence of a novel regulatory mechanism in vivo

We studied the influence of glucagon on hepatic LDL receptors and plasma lipoproteins in rats. A dose-dependent (maximum, threefold) increase in LDL-receptor binding was evident already at a dose of 2 x 4 micrograms, and detectable 3 h after injection; concomitantly, cholesterol and apolipoprotein (apo) B and apoE within LDL and large HDL decreased in plasma. LDL receptor mRNA levels were however unaltered or reduced. Hepatic microsomal cholesterol was increased and the enzymatic activities of 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7 alpha-hydroxylase in hepatic microsomes were reduced. Insulin alone increased receptor binding and receptor mRNA levels twofold, but plasma cholesterol was unchanged and plasma apoE and apoB increased. Administration of insulin to glucagon-treated animals reduced the LDL-receptor binding to control levels and apoB appeared in LDL particles. Estrogen treatment increased LDL-receptor binding and mRNA levels five- and eightfold, respectively. Combined treatment with glucagon and estrogen reduced the stimulation of LDL-receptor mRNA levels by 80% although LDL-receptor binding was unchanged. Immunoblot analysis showed that glucagon increased the number of hepatic LDL receptors. We conclude that glucagon induces the number of hepatic LDL receptors by a mechanism not related to increased mRNA levels, suggesting the presence of a posttranscriptional regulatory mechanism present in the liver in vivo.

Transcriptional activation of low density lipoprotein receptor gene by angiotensin-converting enzyme inhibitors and Ca(2+)-channel blockers involves protein kinase C isoforms

The pharmacological potency of angiotensin-converting enzyme (ACE) inhibitors (lisinopril and enalaprilat) on the transcription of low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl-CoA reductase genes was examined in human vascular smooth muscle cells and compared with the action of Ca(2+)-channel blockers (manidipine, verapamil, and diltiazem). Analogous to Ca(2+)-channel blockers, nanomolar concentrations of enalaprilat or lisinopril stimulated the synthesis of low density lipoprotein receptor mRNA and amplified the transcription induced by recombinant platelet-derived growth factor BB. In contrast to Ca(2+)-channel blockers, ACE inhibitors did not alter the transcription of the 3-hydroxy-3-methylglutaryl-CoA reductase gene. Platelet-derived growth factor BB stimulated the translocation of delta and epsilon isoforms of protein kinase C. Similar to Ca(2+)-channel blockers, ACE inhibitors reduced the translocation of delta and epsilon isoforms of protein kinase C. Furthermore, ACE inhibitors and Ca(2+)-channel blockers inhibited platelet-derived growth factor BB-induced transcription of c-fos and c-jun genes. The findings suggest that increased de novo synthesis of mRNA low density lipoprotein receptor apparently involves the participation of delta and epsilon isoforms of protein kinase C and transcription factors c-Fos and c-Jun.

Evidence for up-regulated low density lipoprotein receptor in human lung adenocarcinoma cell line A549

Human lung adenocarcinoma cell line A549 was studied with respect to the metabolism of human low density lipoprotein (LDL) and 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGR) activity. After incubation in medium containing lipoprotein-deficient serum (LPDS) for 24 h, the A549 cell line expresses a single class of high affinity LDL binding sites (KD at 37 degrees C of 15.1 +/- 0.7 nM and capacity of 118 +/- 2.8 ng/mg cell protein) and an HMGR activity of 111.4 +/- 7 pmol/min/mg cell protein. After binding, the LDLs were internalized and degraded by a common saturable process. The HMGR activity was higher in A549 cells than in fibroblasts but LDL affinity and binding capacity were similar in both cell types. However, in the presence of lipoproteins, A549 cells showed a two-fold higher binding capacity than fibroblasts. When the cells were deprived of cholesterol, the amount of LDLR sites increased but the extent of stimulation was lower in A549 than in fibroblast cells (2.5-fold versus six-fold respectively). This increase was accompanied by a similar increase in the specific LDLR mRNA cellular levels (two-fold versus six-fold respectively). When cells were deprived of exogenous and endogenous cholesterol (biosynthesis blocked by compactin), the binding capacity and the LDLR mRNA levels were yet again increased in A549 cells but not in fibroblasts. Taken together these results suggest that the level of expression of the LDLR is up-regulated in A549 cells compared to fibroblasts.