Low-density lipoprotein: an old substance with a new function?

Regulation of low-density lipoprotein receptors and assessment of their functional role in Burkitt's lymphoma cells

The status of low-density lipoprotein receptors (LDLR) in Daudi Burkitt's lymphoma (BL) cells was examined using a flow cytometric assay employing the fluorescent ligand DiI-LDL, and a radioligand-binding assay using [125I]LDL. The binding is concentration-and time-dependent; and is specific, as judged by its competitive displacement in the presence of unlabeled LDL, and inhibition by heparin, EGTA, and 4 degrees C incubation. The regulation of the receptor and its functional role were then explored. Our results suggest the following: (a) In sharp contrast to normal peripheral blood lymphocytes, the LDLR levels in BL cells are basally elevated when cultured in fetal bovine serum (FBS) medium. (b) In accord with normal peripheral blood lymphocytes, incubation in lipoprotein-deficient serum (LPDS) medium further up-regulates the level of the receptor in BL cells, and co-incubation with LDL or 25-hydroxycholesterol down-regulates the receptor level. The magnitude of the up-regulation is significantly smaller than in normal peripheral blood lymphocytes. (c) Northern blots using a plasmid-DNA probe for LDLR mRNA point to a similar pattern for message regulation as is observed in direct binding studies. (d) Although the LDLR level is constitutively high in BL cells, availability of LDL, unlike transferrin, is not a growth requirement since incubation of cells in LPDS medium does not prevent proliferation of these cells. (e) In contrast to anti-transferrin receptor antibody which results in apoptosis upon binding, anti-LDLR antibody does not inhibit growth or induce apoptosis. Our results suggest LDLR is expressed at a significantly higher level in BL cells than in normal peripheral blood lymphocytes. Although up-regulation and down-regulation of LDLR are observed, this applies only to a small population of LDLR. The bulk receptor population is significantly resistant to down-regulation. Furthermore, notable differences in the functional role of the LDLR are found relative to the transferrin receptor which is also up-regulated in the BL cells.

LDL and acetyl-LDL inhibit the NK activity and are taken up by CD56+ lymphocytes

The effect of LDL and modified LDL (acetyl-LDL) was studied on human natural killer cell-mediated cytotoxicity against K562 cells. Incubation for 24 h of peripheral blood lymphocytes (PBL) with a high concentration (200 micrograms/ml) of LDL decreased the NK activity in some donors. After acetylation of the LDL protein (apoB), the modified-LDL systematically inhibited the NK function of PBL in a time- and dose-dependent manner. Inhibition mediated by acetyl-LDL (AcLDL) was significantly greater than that of LDL, indicating that the apoB modification can mediate the inhibition of the NK function. AcLDL also inhibited the NK activity of peripheral blood mononuclear cells, suggesting that, under our experimental conditions, monocytes are not efficient enough to protect NK cells against the adverse effects of modified-LDL. With a cytofluorimetric analysis, the internalization of acetyl-LDL by PBL was demonstrated and was only 3-4 times lower than LDL internalization in lymphocytes. It appeared to be time, temperature and dose dependent, saturable and different from the internalization mediated by the known scavenger receptors. Finally, CD14- CD3+ lymphocytes and CD14- CD56+ lymphocytes were able to internalize AcLDL in the same way. Our results suggest that in some in vivo circumstances, when the LDL concentration and/or the modified-LDL/LDL ratio increase in tissues, lipoproteins are internalized by NK cells and also can induce adverse effects on the NK function.

Hsc70/uncoating ATPase activation--proposal of a new model

To the best of my knowledge, the signal for the clathrin removal from coated vesicles by the uncoating ATPase is still obscure. I presume that a specific configuration of preexisting, but separately ineffective uncoating ATPase binding sites, is responsible for the triggering of its action. Changes in the geometrical structure of clathrin that inevitably appear during the coated pit-coated vesicle conversion, place the preexisting uncoating ATPase recognition sites into the necessary disposition.

Novel inhibitory action of tunicamycin homologues suggests a role for dynamic protein fatty acylation in growth cone-mediated neurite extension

In neuronal growth cones, the advancing tips of elongating axons and dendrites, specific protein substrates appear to undergo cycles of posttranslational modification by covalent attachment and removal of long-chain fatty acids. We show here that ongoing fatty acylation can be inhibited selectively by long-chain homologues of the antibiotic tunicamycin, a known inhibitor of N-linked glycosylation. Tunicamycin directly inhibits transfer of palmitate to protein in a cell-free system, indicating that tunicamycin inhibition of protein palmitoylation reflects an action of the drug separate from its previously established effects on glycosylation. Tunicamycin treatment of differentiated PC12 cells or dissociated rat sensory neurons, under conditions in which protein palmitoylation is inhibited, produces a prompt cessation of neurite elongation and induces a collapse of neuronal growth cones. These growth cone responses are rapidly reversed by washout of the antibiotic, even in the absence of protein synthesis, or by addition of serum. Two additional lines of evidence suggest that the effects of tunicamycin on growth cones arise from its ability to inhibit protein long-chain acylation, rather than its previously established effects on protein glycosylation and synthesis. (a) The abilities of different tunicamycin homologues to induce growth cone collapse very systematically with the length of the fatty acyl side-chain of tunicamycin, in a manner predicted and observed for the inhibition of protein palmitoylation. Homologues with fatty acyl moieties shorter than palmitic acid (16 hydrocarbons), including potent inhibitors of glycosylation, are poor inhibitors of growth cone function. (b) The tunicamycin-induced impairment of growth cone function can be reversed by the addition of excess exogenous fatty acid, which reverses the inhibition of protein palmitoylation but has no effect on the inhibition of protein glycosylation. These results suggest an important role for dynamic protein acylation in growth cone-mediated extension of neuronal processes.

Epinephrine potentiates activation of human platelets by low density lipoproteins

Low density lipoproteins activate phosphoinositide turnover, increase free cytoplasmic calcium concentration and stimulate phosphorylation of 20- and 47-kDa proteins in blood platelets. All these effects are substantially potentiated by epinephrine.

Oxidized low density lipoproteins inhibit relaxations of porcine coronary arteries. Role of scavenger receptor and endothelium-derived nitric oxide

BACKGROUND

We studied the effects of low density lipoprotein (LDL) on endothelium function.

METHODS AND RESULTS

Porcine epicardial and intramyocardial coronary arteries suspended in organ chambers for isometric tension recording were exposed to LDL for 2 hours and were then washed. In epicardial coronary arteries, oxidized LDL (30-300 micrograms/ml) but not native LDL or lysolecithin inhibited endothelium-dependent relaxations to serotonin, thrombin, and aggregating platelets (5,000-75,000/microliter). Endothelium-dependent relaxations to bradykinin and A23187 and endothelium-independent relaxations to SIN-1 were unaffected by oxidized LDL. In intramyocardial coronary arteries, oxidized LDL had no appreciable effect on relaxations to serotonin. The effect of oxidized LDL on the response to serotonin in epicardial coronary arteries was completely prevented by dextran sulfate (10 micrograms/ml). The inhibitory effect of oxidized LDL persisted in the presence of pertussis toxin. Similar to the lipoproteins, L-NG-monomethyl arginine (L-NMMA) reduced relaxations to serotonin but not to bradykinin in epicardial coronary arteries. In the presence of L-NMMA, oxidized LDL further reduced the response to serotonin. In arteries in which relaxations to serotonin were inhibited by oxidized LDL, L-arginine but not D-arginine induced a full relaxation. Pretreatment with L-arginine potentiated relaxations to serotonin in arteries exposed to oxidized LDL.

CONCLUSIONS

Thus, oxidized LDL activates the scavenger receptor on endothelial cells and inhibits the receptor-operated nitric oxide formation in epicardial but not in intramyocardial coronary arteries. The mechanism is not related to dysfunction of a Gi protein but is related to a reduced intracellular availability of L-arginine. The reduced nitric oxide formation at sites of early atherosclerotic lesions may favor platelet aggregation and vasospasm, both of which are known clinical events in patients with coronary artery disease.

Effects of exercise on plasma lipoprotein levels and endothelium-dependent vasodilatation in young and old rats

Effects of treadmill exercise (1 h.day-1 for 12 weeks) on the mechanical properties of isolated aortae and plasma concentrations of low and high density lipoprotein cholesterols ([LDL-C]pl and [HDL-C]pl, respectively) were investigated in young (16 weeks) and old (98 weeks) rats. With aging, [LDL-C]pl increased and acetylcholine (ACh) elicited greater endothelium-dependent relaxations of the aorta in old rats than in young rats. Also, in all groups of rats tested (young, old, exercised and nonexercised), the increase in the [LDL-C]pl correlated with an increase in the sensitivity of relaxation of the aorta to ACh. On the other hand, a greater [HDL-C]pl was associated with a smaller relaxation response to ACh in young rats only when exercised and nonexercised groups were combined. The increase in [HDL-C]pl with aging, however, did not correlate with the extent of ACh-induced relaxation. Exercise in old rats reduced [LDL-C]pl and the extent of ACh-induced relaxation of the aorta. Therefore, [LDL-C]pl appeared to be closely related to the extent of endothelium-dependent relaxation in response to ACh in the aorta. The [LDL-C]pl also correlated with the gain in mass of the rat. Exercise in old rats reduced the body mass and was accompanied by a decrease in [LDL-C]pl.

Conditions that result in the mobilization and influx of Ca2+ into rat hepatocytes induce the rapid loss of 3-hydroxy-3-methylglutaryl-CoA reductase activity that is not reversed by phosphatase treatment

We investigated the effects of conditions that induce Ca2+ mobilization from intracellular stores and Ca2+ influx into hepatocytes on the expressed and total (fully dephosphorylated) activities of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase. Vasopressin and phenylephrine when added alone had small or negligible effects on the phosphorylation state of the enzyme, as judged from the expressed/total activity ratio. However, when added in combination with glucagon, they elicited appreciable increases in the phosphorylation of the enzyme. Glucagon on its own had no effect either on phosphorylation state or on total HMG-CoA reductase activity during 40 min of incubation. Under conditions of sustained Ca2+ influx (i.e. vasopressin or phenylephrine plus glucagon), there was a marked loss of total HMG-CoA reductase activity. This effect was more pronounced when vasopressin was used; 50% of the enzyme activity was lost within 40 min. The involvement of Ca2+ in these effects was verified directly by the use of ionophore A23187. Its addition to hepatocytes resulted both in a very pronounced increase in the phosphorylation state of the enzyme and in the loss of 50% of the total activity within 30 min. There was no correlation between the ability of any set of conditions to increase the phosphorylation of the enzyme and the subsequent loss of total HMG-CoA reductase activity. The latter parameter appeared to be directly related, however, to the maintenance of prolonged Ca2+ influx, as indicated by the continued activation of glycogen phosphorylase, measured in the same cells. The lack of a causal relationship between increased phosphorylation and loss of total activity was demonstrated directly by studies in which okadaic acid was used to induce phosphorylation of HMG-CoA reductase in hepatocytes by inhibition of phosphatase 1 and 2A activities. This was not accompanied by any loss of total enzyme activity. Neither did okadaic acid enhance the loss of reductase induced by A23187 when the two agents were added together. It is concluded that altered Ca2+ fluxes in hepatocytes in vivo, under conditions of acute or chronic stress (such as may be associated with trauma or diabetes respectively), may be involved in the regulation of the expression of HMG-CoA reductase activity through alteration of enzyme concentration in the liver.

Disturbed intracellular calcium-related processes of hepatocytes and neutrophils in human alcoholic liver disease

Isolated human hepatocytes and separated neutrophils of 11 patients with alcoholic liver disease (ALD) were used to study some aspects of cellular calcium-related processes compared to nonalcoholic controls. 45Ca2+ efflux from the cells decreased in ALD and the calmodulin-inhibitor trifluoperazine did not influence it further. The intracellular free calcium concentration [( Ca2+]i) of nonstimulated hepatocytes and neutrophils proved to be higher in ALD with the Quin2/AM loading technique. However, the [Ca2+]i rise in hepatocytes and neutrophils, with stimulation by low density lipoprotein (LDL) and N-formyl-methionyl-leucyl phenylalanine (FMLP), respectively, was diminished in ALD compared to appropriate controls. The slower 45Ca2+ extrusion rate, higher basal [Ca2+]i levels, and the diminished [Ca2+]i elevation of activated hepatocytes and neutrophils, suggest disturbed calcium-related intracellular processes in ALD, in particular, impaired regulation of the plasma membrane Ca2(+)-ATPase.

Rapid and reversible inhibition by low density lipoprotein of the endothelium-dependent relaxation to hemostatic substances in porcine coronary arteries. Heat and acid labile factors in low density lipoprotein mediate the inhibition

The effects of hemostatic substances on the vascular tone in porcine coronary arteries and the influence of low density lipoprotein on tension were investigated. Thrombin induced a marked concentration-dependent relaxation in prostaglandin F2 alpha-precontracted strips with intact endothelium, whereas it produced a modest constriction in endothelium-denuded arteries. Methylene blue abolished the relaxation, but indomethacin did not affect it significantly. An exposure of the intact strips to low density lipoprotein resulted in a marked inhibition of the relaxation to thrombin but did not interfere with vasodilation by sodium nitroprusside. The inhibition by low density lipoprotein was reversed completely by washing. In contrast, high density lipoprotein lacked such inhibitory effects. Adenosine diphosphate, calcium ionophore A23187, and platelet-activating factor also produced relaxation in the intact strips. An exposure of the strips to low density lipoprotein almost abolished relaxation to these substances. The inhibition was also reversible. Heat treatment or acid treatment of low density lipoprotein resulted in a complete loss of the inhibitory effects, but diisopropyl fluorophosphate treatment did not alter the effect. It is concluded that low density lipoprotein may play a new pathological role in promotion of coronary vasospasm through rapid and reversible inhibition in endothelium-dependent relaxation to hemostatic substances.