Effect of cyclic AMP on low density lipoprotein binding and internalization by cultured human fibroblasts

Lovastatin enhances the photocytotoxicity of UVA radiation towards cultured N.C.T.C. 2544 human keratinocytes: prevention by cholesterol supplementation and by a cathepsin inhibitor

The effect of the hydroxymethylglutaryl-CoA (HMG-CoA) inhibitor lovastatin on the UVA-induced photocytotoxicity has been investigated in cultured human N.C.T.C. 2544 keratinocytes. In the absence of irradiation, 5 x 10(-7) M lovastatin did not exhibit any significant cytotoxic effect towards this cell line. Although the drug cannot act as a photosensitizer, because it does not absorb in the UVA range, it markedly increased the UVA-induced cellular damage (about 70% reduction in cell viability at 5 x 10(-7) M). This effect was not accompanied by an increase in the lipid peroxidation product content of cells as compared with treatment with UVA alone. Medium supplementation with 0.01 mg/ml free cholesterol totally prevented the enhancement of UVA photocytotoxicity induced by lovastatin. A protective effect was also observed when cells were supplemented with an amount of low-density lipoprotein giving the same cholesterol concentration in the culture medium. Finally, E64 [L-trans-epoxysuccinyl-leucylamido-(4-guanidino)-butane], a lysosomal cathepsin inhibitor, also prevents the cell death induced by UVA in cells treated with lovastatin. These results suggest that HMG-CoA reductase inhibitors could increase the sensitivity of skin cells to UVA radiation, and that this phenomenon is related to lysosomal enzyme release.

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

1. Cyclic AMP-elevating agents stimulate low-density lipoprotein (LDL) receptor activity in human vascular smooth-muscle cells by increasing the rate of receptor protein synthesis. The stimulation is not secondary to the decrease in the regulatory pool of free cholesterol, since it is unaffected, or even enhanced, by inhibition of cholesterol synthesis and esterification, or inhibition of the conversion of cholesterol into its repressor metabolites. The cyclic AMP-mediated up-regulation of the receptor is maintained at low concentrations of inhibitory sterols, but is eventually over-ridden at high concentrations of these sterols. 2. Cyclic AMP-elevating agents also stimulate the hydrolysis of lysosomal cholesterol esters, thus increasing the cellular cholesterol pool and repressing the expression of the LDL receptor. This cholesterol-mediated repressive effect of cyclic AMP can be prevented by chloroquine, which inhibits lysosomal actions, or by ketoconazole, which inhibits conversion of free cholesterol into its repressor metabolite. Thus the cyclic AMP stimulation of the LDL receptor can be masked by the rapid mobilization of free cholesterol from existing cholesterol esters within cultured cells. 3. We have observed that elevated cyclic AMP concentrations will up-regulate the LDL receptor in cholesterol-depleted human vascular smooth-muscle cells, skin fibroblasts and foetal-lung fibroblasts. We propose that our results are evidence for a cyclic AMP-stimulated, sterol-independent, control of LDL-receptor synthesis which is of widespread occurrence in human cells.

Lysosome inhibitors enhance the ability of cyclic AMP-elevating agents to induce the LDL receptor in human vascular smooth muscle cells

In human vascular smooth muscle cells cyclic AMP elevation by forskolin increases synthesis of the LDL receptor by a mechanism which appears independent of sterol control. This increased receptor synthesis is further enhanced by chloroquine. Both forskolin and prostaglandin E1 increase the number of cell surface LDL receptors indicating that prostaglandins could exert physiological control over LDL metabolism. This effect is enhanced synergistically by chloroquine. The stimulation by forskolin of LDL receptor synthesis and expression leads to increased metabolism of apo-B and increased hydrolysis of LDL-borne cholesteryl ester. These effects of cyclic AMP on the activity of the LDL pathway are enhanced more than additively by preincubation with the reversible lysosomal inhibitor NH4Cl. Thus cyclic AMP causes up-regulation of the LDL receptor pathway resulting in increased rates of LDL metabolism but this effect can be damped or masked in cell culture by a cyclic AMP-sensitive lysosomal event, probably the acute stimulation of lysosomal cholesterol ester hydrolase.

Verapamil enhances high-density lipoprotein processing in Hep G2 cells preloaded with cholesterol

The effects of the calcium channel blocker of the arylalkylamine series verapamil have been investigated on high-density lipoprotein (HDL3) catabolism in the human hepatoma cell line Hep G2. It was found that verapamil markedly enhanced HDL3 binding, uptake and degradation in Hep G2 cells preloaded with nonlipoprotein cholesterol. This effect was dose-dependent, and a 1.5-2-fold increase of the three studied parameters was observed in cells pretreated 24 h with 100 microM verapamil. No significant effect of the drug was found in cells not preincubated with cholesterol. Verapamil induced an increase in the cellular cholesterol content in preloaded cells. Other calcium antagonists such as diltiazem, nifedipine, nitrendipine or amphiphilic drugs such as phenothiazines and propranolol also enhanced HDL3 uptake by Hep G2 cells. These effects of verapamil on HDL3 metabolism could be related to its amphiphilic characteristics, and to its calcium antagonist properties.

Agents which increase cyclic AMP have diverse effects on low-density-lipoprotein-receptor function in human vascular smooth-muscle cells and skin fibroblasts

Receptor-mediated binding and metabolism of low-density lipoproteins (LDL) in cultured human vascular smooth-muscle cells and skin fibroblasts are altered by increased cellular cyclic AMP concentrations. However, the LDL receptor does not respond to changes in cyclic AMP concentration in a simple manner. The activation of adenylate cyclase with forskolin, or the addition of membrane-permeant cyclic AMP analogues, initially decreases the expression of the LDL receptor, but is followed by a substantial increase in receptor expression after 24 h. This increase does not occur in the presence of inhibitors of RNA or protein synthesis, and is due to doubling of the Bmax. of the LDL receptor, without alteration of its affinity for LDL. By contrast, elevation of cyclic AMP concentration by inhibition of phosphodiesterases results in decreased receptor expression throughout the 24 h period. These two response patterns are reproducible phenomena, consistently observed in low-passaged cells derived from seven unrelated individuals.

Abnormal cholesterol metabolism in imipramine-treated fibroblast cultures. Similarities with Niemann-Pick type C disease

Addition of low-density lipoprotein (LDL) to cholesterol-deprived human skin fibroblast cultures treated by imipramine at a 20 microM concentration induced a significant intracellular accumulation of unesterified cholesterol. Intracytoplasmic inclusions were already visible by histochemical filipin staining after 2 h of LDL uptake and were progressively mobilized towards the perinuclear region within 24 h. At this concentration of the drug, the rate of proteolytic 125I-LDL hydrolysis was similar in treated and untreated cells. Treated cells maintained in lipoprotein-deficient medium showed no abnormality, indicating the exogenous origin of the accumulated sterol. Further, the drug induced a drastic dose-dependent impairment of LDL-stimulated cholesterol esterification, not related to an inhibition of acyl CoA:cholesterol acyltransferase, and a significant delay in down-regulation of de novo cholesterol synthesis. However, imipramine did not affect 25-hydroxycholesterol-mediated regulation of the two latter processes. These results resemble those observed in Niemann-Pick type C disease and suggest an impaired mobilization of LDL-derived cholesterol in imipramine-treated cells.

The antihypertensive drug propranolol enhances LDL catabolism and alters cholesterol metabolism in human cultured fibroblasts

The effects of 3 beta-blockers with different pharmacological properties (non-selective: propranolol; beta 1-selective: metoprolol; and with intrinsic sympathomimetic activity: pindolol) were comparatively studied on LDL and lipid metabolism in human fibroblasts. At 10(-4) M, propranolol increased low density lipoprotein binding, uptake and degradation by 1.5-, 2.2- and 1.8-fold, respectively, whereas metoprolol and pindolol had no effect. This effect of propranolol is mainly due to an increase in LDL receptor number. Propranolol also enhanced sterol, triacylglycerol, fatty acid and phospholipid synthesis by 2-3-fold from sodium acetate. Cholesterol esterification by oleic acid was significantly and specifically decreased 4-fold by propranolol. Metoprolol and pindolol affect neither sterol synthesis nor cholesterol esterification. Pretreatment of cultured fibroblasts with propranolol induced an increase in hydroxymethyl-glutaryl-coenzyme A reductase activity and a decrease in acyl-coenzyme A: cholesterol-O-acyltransferase (ACAT) activity. Propranolol inhibited the induction of ACAT activity by exogenous cholesterol. Preincubation of a cell-free extract with propranolol also induced inhibition of ACAT activity. Propranolol decreased the cholesteryl ester content of cultured cells. These effects of propranolol on LDL and cholesterol metabolism might be related to the amphiphilic properties of the drug and suggest an effect on the cholesterol intracellular traffic. The decrease in cholesterol esterification and in the cholesteryl ester cellular level induced by propranolol may be involved in its antagonizing effect on experimental atherogenesis.

Glucagon, cyclic AMP and adrenaline stimulate the degradation of low-density lipoprotein by cultured rat hepatocytes

Rat hepatocytes were preincubated for 16 h with hormones or drugs and then for a further 8 h with 125I-human low-density lipoprotein (LDL). Glucagon (via cyclic AMP) and adrenaline (via cyclic AMP and alpha-effects) increased the binding of 125I-LDL to the LDL receptor, and the degradation of LDL to [125I]iodotyrosine. The effects on degradation were antagonized by dexamethasone, and the action of cyclic AMP on binding and degradation was inhibited by actinomycin D. The results are discussed in relation to the control of lipoprotein metabolism in diabetes.

Cyclic AMP decreases LDL catabolism and cholesterol synthesis in the human hepatoma cell line HepG2

A 24h pretreatment of the human hepatoma cell line HepG2 with dibutyryl cyclic AMP in the presence of theophylline induced a dose dependent decrease in low density lipoprotein binding, uptake and degradation. This effect is most likely due to a reduction of the LDL receptor number. Sterol synthesis from sodium acetate is markedly inhibited, either in the presence or absence of LDL, whereas synthesis from mevalonic acid is unchanged. Cyclic AMP also induced a decrease in hydroxy methyl glutaryl coenzyme A reductase activity. These effects of cyclic AMP might be involved in some hormonal regulation of the LDL pathway and cholesterol metabolism in the liver.

Lipophilic beta-adrenoceptor antagonists stimulate cholesterol biosynthesis in human skin fibroblasts

The effect of a series of beta-adrenoceptor antagonists on cholesterol biosynthesis was studied in vitro in normal human skin fibroblasts. Some, but not all, of the drugs studied stimulated the incorporation of [2-14C]-acetate into cell sterols in a dose-dependent manner. This effect was unrelated to beta-blocking potency, selectivity for beta 1 or beta 2 adrenoceptors and partial agonistic activity of the drugs, thus ruling out a beta-receptor mediated mechanism. A positive, statistically significant correlation was found, however, between the drug lipophilicity and the stimulation of sterol biosynthesis. Propranolol, the most effective agent in increasing [2-14C]-acetate incorporation into cellular sterols, also enhanced the conversion of 3-hydroxy-3-methylglutaryl CoA (HMGCoA) into mevalonic acid, suggesting an interference of lipophilic beta-adrenoceptor antagonists with HMHCoA-reductase, the feed-back regulated rate limiting step of cholesterol biosynthesis.

Comparative study of the effect of beta-blockers with different pharmacological properties on cholesteryl ester formation in mouse peritoneal macrophages

The effect of three beta-blockers: non-selective (propranolol), beta 1-selective (metoprolol), and with intrinsic sympathomimetic activity (pindolol), was investigated on 14C-oleic acid incorporation into cholesteryl esters in mouse peritoneal macrophages. Incorporation of 14C-oleic acid into cholesteryl esters was reduced about 10-fold by propranolol at 10(-4) M while incorporation into triacylglycerols was only 30% decreased at the same concentration. Metoprolol and pindolol had no significant effect on 14C-oleic incorporation into cholesteryl esters or triacylglycerols. Finally, propranolol inhibited the acyl-coenzyme A: cholesterol-O-acyltransferase activity, measured in vitro on macrophages homogenates, while the other studied beta-blockers were ineffective. These results suggest that propranolol could antagonize cholesteryl ester accumulation by macrophages, one of the main processes involved in atherogenesis.

Dibutyryl-cyclic AMP inhibits cholesterol esterification in J 774 monocyte-like cells

The effect of dibutyryl-cyclic AMP (dbcAMP) and theophylline was investigated on oleic acid incorporation into cholesteryl esters and triacylglycerols in the mouse monocyte-macrophage cell line J 774. 24h pretreatment of macrophages with dbcAMP decreased cholesteryl ester formation in a dose-dependent manner (about 4 fold reduction for dbcAMP 10(-4)M + theophylline 10(-3)M), while oleic acid incorporation into triacylglycerols was markedly (2 to 3 fold) enhanced. The catabolism of acetylated LDL was only slightly affected (about 15-20% reduction with dbcAMP 5 X 10(-4)M + theophylline 10(-3)M). Acyl Coenzyme A: cholesterol-O-acyl-transferase activity, measured in vitro on cell homogenates, was reduced in dbcAMP-treated cells, whereas diacylglycerol acyltransferase activity was increased. These results suggest that cyclic AMP can modulate cholesteryl ester and triacylglycerol formation in macrophages, and that these metabolisms are inversely regulated. Agents which increase cyclic AMP intracellular level could be of interest for reducing cholesteryl ester accumulation in macrophages.

Cyclic AMP increases incorporation of exogenous fatty acids into triacylglycerols in hamster fibroblasts

Incorporation of various exogenous saturated or unsaturated [14C]labeled fatty acids (palmitic, stearic, oleic, linoleic and arachidonic) into triacylglycerols by hamster fibroblasts was markedly enhanced (two- to fourfold) in the presence of theophylline or dibutyryl cyclic adenosine monophosphate (dbcAMP). This effect was observed for short-term (1-6 hr) as well as long-term (15-24 hr) preincubation with dbcAMP. In the presence of sodium fluoride, a phosphoprotein phosphatase inhibitor, measurement of diacylglycerol acyltransferase (DGAT) activity in cells pretreated with dbcAMP pointed out a marked increase (3 X) in specific activity. The results suggest that DGAT activity in fibroblasts could be activated by a cAMP-dependent phosphorylation process.

Epinephrine decreases low density lipoprotein processing and lipid synthesis in cultured human fibroblasts

The effect of epinephrine on 125I-low density lipoprotein (LDL) uptake and cholesterol metabolism was investigated after a 24 hours pretreatment of cultured human fibroblasts. Epinephrine decreased LDL uptake (binding + internalization) and degradation in a dose-dependent manner. Cholesterol synthesis from 14C sodium acetate and cholesterol esterification measured by 14C oleic acid incorporation into cholesteryl esters were also decreased. These results are in agreement with the general view that epinephrine increases cyclic AMP intracellular level, as it was previously demonstrated that dibutyryl cyclic AMP or isoproterenol treatment of cultured fibroblasts had similar effect on these pathways. The decrease in LDL processing induced by epinephrine could be involved in the worsening effect of epinephrine on atherosclerosis.

Effects of phenothiazines on low density lipoprotein metabolism in cultured human fibroblasts

Treatment of cultured human fibroblasts with trifluoperazine or chlorpromazine resulted in a biphasic effect on low density lipoprotein (LDL) catabolism, depending upon the dose. At up to 10(-5) M, a marked increase in LDL binding, internalization and degradation was observed. This phenomenon took place within the first hours of incubation with the drugs, suggesting a direct effect on cell membrane physical characteristics, probably related to the lipophilic properties of phenothiazines. Concentrations above 2 X 10(-5) M resulted in a relative decrease in LDL binding and internalization, and in a dramatic decrease in LDL degradation, which may be related to an inhibition of calmodulin-dependent processes.