THROMBOSIS RESEARCH 60; 431-432,199O 0049-3848/90 $3.00 + .OO Printed in the USA. Copyright (c) 1990 Pergamon Press pk. All rights reserved.
LETTER
TO THE EDITORS-IN-CHIEF
HDL, A PARTIAL AGONIST OF PLATELET ACTIVATION ‘7 A. Pletscher?cf,
F. Ferracin and H. Block
Department of Research, Kantonsspital, CH-403 1 Basel, Switzerland (Received
19.7.1990;
acCepted
in original form 21.9.1990 by Editor-in-Chief
A.L. Copley)
There is general agreement that low density lipoprotein (LDL) exerts a proaggregatory effect on blood platelets, whereas high density lipoprotein (HDL), depending on the investigators, has been reported to decrease or increase platelet aggregability or to exert no effect at all (1). We have carried out further investigations on the action of HDL on platelets and the results are presented here. Human platelets were isolated by centrifugation and resuspended in Tyrode buffer devoid of albumin (since albumin inhibits lipoprotein-induced shape change) as previously described (2). For the aggregation experiments 0.4 g/l human fibrinogen was added to the buffer. HDL (total fraction),
HDL2 and HDL3 were prepared from human plasma as reported earlier (3). Gel
electrophoresis and HPLC-analysis did not reveal the presence of detectable amounts of LDL in the HDL fraction. We found that HDL, like LDL (EC50 3.5 + 0.9 and 15.4 + 3.2 ug/ml respectively;
fig. 1) as well as HDL2 and HDL3 (EC50 14.1 f 1.9 and 12.2 f 1.7)‘caused a
marked shape change reaction.* However, the HDL fractions in concentrations up to 500 ug/ml, in contrast to LDL (300 &ml)
did not induce platelet aggregation (measured by maximal
aggregation and slope). Also, when HDL was combined with non-aggregatory concentrations of adrenaline and other platelet agonists (5hydroxytryptamine
or vasopressin) no platelet aggregation
occured, whereas combinations of non-aggregatory concentrations of LDL and adrenaline (or the other agonists) caused aggregation. When platelets were preincubated with the HDL fraction the aggregatory effect of LDL or of combinations LDL-adrenaline was inhibited. The IC5o values of the HDL fractions varied between 100-300 &ml, HDL;? being slightly more active than HDL3. Key tlxzn3s:
Highdensity
lipoprotein (HDL), platelet activation
* Previous results (2) according to which HDL did not cause a shape change reaction were due to an artifact, because of the presence of sodium azide (contained in the lipoprotein stock solution) in the platelet suspensions. wt
&dress:
Swiss Acadcq u-I-4051
Basel,
of Medical Sciences, lwxzsplatz13 switzerland 431
432
HDL, A PARTIAL
AGONIST...
Vol. 60, No. 5
The site of action of HDL at the plasma membrane is not yet clear. Provided that LDL and HDL have a common site (e.g. the apo B/ape E receptor) HDL might be considered as a partial agonist causing only shape change but not aggregation. On the other hand, LDL would be a full agonist since it induces aggregation in addition to shape change. This hypothesis (which needs to be confirmed) would explain the antagonistic effect of HDL on LDL-induced
aggregation, since
partial agonists are known to antagonize the action of full agonists. Furthermore the present results indicate that I-IDL might not only protect the arterial wall from the damaging influence of LDL (as generally assumed) but also have a direct effect at the platelet level which could be of interest in the pathogenesis of atherosclerosis. Shape change
Aggregation
(b) HDL LDL
I
HDL + Adrenahe
HDL
TT HDL + LDL + Adr.
LDL + Adredine
Fig. 1 Shape change of human platelets was induced by 5 &ml LDL and 20 @ml aggregation experiments were carried out with 500 @ml HDL, 300 &ml LDL (a) and 150 &ml LDL plus 0.3 uM adrenaline (b). HDL was administered 1 min. before LDL. In (b) LDL or adrenaline alone in the concentrations used did not cause aggregation. REFERENCES 1. DESAJ, K.; BRUCKDORFER, K.R.; HUTTON, R.A.; OWEN, S.O. Binding of apo E-rich high density lipoprotein particles by saturable sites on human blood platelets inhibits agonistinduced platelet aggregation. LnQ,831-84Ql989 ’’ 2. KNORR M.; LOCHER, R.; VOGT, E.; VETI’ER, W.; BLOCK, L.H.; FERRACIN, F.; LEVKOVITS, H.; PLETSCHER, A. Rapid activation of human platelets by low concentrations of low density lipoprotein via phosphatidylinositol cycle. Eur., 753-759, 1988 3. HAVEL, R.J.; EDER, H.A.; BRAGDEN, J.H. The distribution and chemical composition of ultracentrifugally separated lipoproteins in human serum. ,I. Clin. Invest. 34, 1345- 1353, 1955
LETTER
TO THE EDITORS-IN-CHIEF
HDL, A PARTIAL AGONIST OF PLATELET ACTIVATION ‘7 A. Pletscher?cf,
F. Ferracin and H. Block
Department of Research, Kantonsspital, CH-403 1 Basel, Switzerland (Received
19.7.1990;
acCepted
in original form 21.9.1990 by Editor-in-Chief
A.L. Copley)
There is general agreement that low density lipoprotein (LDL) exerts a proaggregatory effect on blood platelets, whereas high density lipoprotein (HDL), depending on the investigators, has been reported to decrease or increase platelet aggregability or to exert no effect at all (1). We have carried out further investigations on the action of HDL on platelets and the results are presented here. Human platelets were isolated by centrifugation and resuspended in Tyrode buffer devoid of albumin (since albumin inhibits lipoprotein-induced shape change) as previously described (2). For the aggregation experiments 0.4 g/l human fibrinogen was added to the buffer. HDL (total fraction),
HDL2 and HDL3 were prepared from human plasma as reported earlier (3). Gel
electrophoresis and HPLC-analysis did not reveal the presence of detectable amounts of LDL in the HDL fraction. We found that HDL, like LDL (EC50 3.5 + 0.9 and 15.4 + 3.2 ug/ml respectively;
fig. 1) as well as HDL2 and HDL3 (EC50 14.1 f 1.9 and 12.2 f 1.7)‘caused a
marked shape change reaction.* However, the HDL fractions in concentrations up to 500 ug/ml, in contrast to LDL (300 &ml)
did not induce platelet aggregation (measured by maximal
aggregation and slope). Also, when HDL was combined with non-aggregatory concentrations of adrenaline and other platelet agonists (5hydroxytryptamine
or vasopressin) no platelet aggregation
occured, whereas combinations of non-aggregatory concentrations of LDL and adrenaline (or the other agonists) caused aggregation. When platelets were preincubated with the HDL fraction the aggregatory effect of LDL or of combinations LDL-adrenaline was inhibited. The IC5o values of the HDL fractions varied between 100-300 &ml, HDL;? being slightly more active than HDL3. Key tlxzn3s:
Highdensity
lipoprotein (HDL), platelet activation
* Previous results (2) according to which HDL did not cause a shape change reaction were due to an artifact, because of the presence of sodium azide (contained in the lipoprotein stock solution) in the platelet suspensions. wt
&dress:
Swiss Acadcq u-I-4051
Basel,
of Medical Sciences, lwxzsplatz13 switzerland 431
432
HDL, A PARTIAL
AGONIST...
Vol. 60, No. 5
The site of action of HDL at the plasma membrane is not yet clear. Provided that LDL and HDL have a common site (e.g. the apo B/ape E receptor) HDL might be considered as a partial agonist causing only shape change but not aggregation. On the other hand, LDL would be a full agonist since it induces aggregation in addition to shape change. This hypothesis (which needs to be confirmed) would explain the antagonistic effect of HDL on LDL-induced
aggregation, since
partial agonists are known to antagonize the action of full agonists. Furthermore the present results indicate that I-IDL might not only protect the arterial wall from the damaging influence of LDL (as generally assumed) but also have a direct effect at the platelet level which could be of interest in the pathogenesis of atherosclerosis. Shape change
Aggregation
(b) HDL LDL
I
HDL + Adrenahe
HDL
TT HDL + LDL + Adr.
LDL + Adredine
Fig. 1 Shape change of human platelets was induced by 5 &ml LDL and 20 @ml aggregation experiments were carried out with 500 @ml HDL, 300 &ml LDL (a) and 150 &ml LDL plus 0.3 uM adrenaline (b). HDL was administered 1 min. before LDL. In (b) LDL or adrenaline alone in the concentrations used did not cause aggregation. REFERENCES 1. DESAJ, K.; BRUCKDORFER, K.R.; HUTTON, R.A.; OWEN, S.O. Binding of apo E-rich high density lipoprotein particles by saturable sites on human blood platelets inhibits agonistinduced platelet aggregation. LnQ,831-84Ql989 ’’ 2. KNORR M.; LOCHER, R.; VOGT, E.; VETI’ER, W.; BLOCK, L.H.; FERRACIN, F.; LEVKOVITS, H.; PLETSCHER, A. Rapid activation of human platelets by low concentrations of low density lipoprotein via phosphatidylinositol cycle. Eur., 753-759, 1988 3. HAVEL, R.J.; EDER, H.A.; BRAGDEN, J.H. The distribution and chemical composition of ultracentrifugally separated lipoproteins in human serum. ,I. Clin. Invest. 34, 1345- 1353, 1955
