J Mol Cell Cardio123,
1339-1342 (1991)
Can Altering R. Preston
Serum Maso~&~r*,
Cholesterol
Affect Neurologic
Leo G. Herbette1*2p3*4*5,
and David
Function? I. Sihrerman5
From the Departments of Radiology’, Biochemist$, the Centerfor Cardiovascular Membrane Research3, the Biomolecular Structure Analysis Center”, and the Cardiology Division of the Department of Medicine5, University of Connecticut Health Center, Fawnington, CT 06030, USA (Received 9 October 1990, accepted in revisedform 10 May 1991) Despite their success in establishing that cholesterol lowering reduces cardiovascular mortality, the major prospective primary prevention trials of the last decade have failed to produce a significant difference in overall mortality between both treatment and control groups [l-3]. This disappointing result has led to considerable skepticism about the value of pharmacologic cholesterol lowering therapy as a useful strategy to reduce overall mortality [4-61. In several of these trials, the lack of a difference in overall mortality may be attributed to an unexpected excess in the number of non-medical deaths in the treatment group, deaths which are usually attributable to violence or traumatic events [1, 31. Decreased serum cholesterol levels in populations prone to violent behavior [ 7, 81 and in children with aggressive conduct disorder [ 91, have also been reported, although these findings have not been replicated in another, smaller series [IO]. As Oliver notes in this issue [II], a recent meta-analysis of six primary prevention trials, including drug and dietary interventions, finds a significant increase in non-medical deaths in the drug treatment groups examined in the aggregate [12]. Oliver raises the intriguing question of whether there might be a biological basis for this troubling and hitherto unexplained phenomenon. Could altering serum cholesterol in large populations produce alterations in behavior, which in a few susceptible individuals, would result in increased mortality?
In order to answer this question, one must test two hypotheses; one, that altering cholesterol levels within the blood alters cholesterol content within the brain (particularly within brain cell membranes), and two, that altering cholesterol within the brain alters neurological function. While neither hypothesis has been adequately explored, the available data to date suggest that both may be true. Within the blood, transport of cholesterol into cells primarily occurs through binding of low density lipoprotein (LDL) to its cell surface receptor. The LDL receptor hypothesis suggests that lowering extracellular cholesterol, either through inhibition of intracellular cholesterol synthesis, or through interference with enterohepatic reabsorption, increases cell surface LDL receptor number. Increased LDL receptors in turn lead to increased transport of cholesterol into cells, where it is stored as cholesteryl ester after re-esterification, exported from the cell, or redistributed between cell and nuclear membranes [13]. Low density lipoprotein receptors have been widely found in the central nervous system [ 14-161, and astrocytes within the central nervous system have been shown to synthesize apolipoprotein E [17], one of the LDL receptor’s two ligands. The logical implication of these studies is that uptake of cholesterol via LDL receptors is an important component of cholesterol transport within the central nervous system. However, the relationship between LDL metabolism within the plasma and LDL metabolism within
This project was supported by an American Heart Association the John A. Hartford Foundation (RPM), and the American Established Investigator of the American Heart Association.
Fellowship from the Connecticut State Affiliate (RPM), Health Assistance Foundation (LGH). Dr Herbette is an
Please Center,
address all correspondence Farmington, CT 06030,
0022-2828/91/111339
+ 04 $03,00/o
to: David
I. Silverman,
Cardiology
Division,
University
of Connecticut
Health
USA. @ 1991 Academic
Press
Limited
1340
R. P. Mason
the brain remains unestablished, and is likely to be complex. For instance, LDL receptors are not an absolute requirement for normal neurologic function, since patients who are hom*ozygou$ for familial hypercholesterolemia do not exhibit central nervous system abnormalities [ 181. Similarly, patients with familial hypobetalipoproteinemia (apoliproprotein B deficiency resulting in low LDL cholesterol levels), have normal central nervous system function [19]. Therefore, the LDL receptor pathway is probably not the only mediator of brain cholesterol transport. The concentration of cholesterol within the cell membrane, commonly expressed as the membrane cholesterol to phospholipid (C :Pl) m,ole ratio, appears to be highly influenced by cholesterol content in the surrounding environment. In vitro equilibrium and kinetic membrane binding experiments in our laboratory have shown that cholesterol is rapidly absorbed into the membrane bilayer from the surrounding medium, and has an extremely high partition coefficient (mass ratio of cholesterol in the membrane compared to the surrounding aqueous medium). In cell culture experiments, the plasma membrane C:Pl mole ratio in cardiac and smooth muscle cells can be efficiently and reproducibly modified in culture with cholesterol/phospholipid liposomes (20, 211. Cholesterol is rapidly transferred, in a concentration dependent manner, from the high cholesterolcontaining liposomes to the cell plasma membrane, or in the reverse direction when the concentration gradient is reversed. Animal models also provide data that support the hypothesis that altering serum cholesterol content may affect cell membrane cholesterol content. In New Zealand rabbits fed a high cholesterol (2%) diet for 10 weeks, arterial smooth muscle cell plasma membrane C:Pl mole ratio was elevated from 0.38: 1 to 0.68:1, an increase of 80% [22]. In marmosets, 22 weeks of 0.5% wfw dietary cholesterol supplementation results in a 15% increase in myocardial membrane cholesterol:phospholipid mole ratio [23], and 2% cholesterol supplementation in rats results in as much as a 45 % increase [24]. Therefore, alterations in both dietary and serum cholesterol content may significantly change cell membrane cholesterol content.
et al. Membrane cholesterol content in turn importantly affects membrane function. Within cells, cholesterol is divided into intracellular and plasma membrane components. The cell membrane contains approximately 90% of the cell’s total cholesterol, and includes a number of proteins which regulate cellular function [25]. Both increases and decreases in cell membrane cholesterol have been shown to perturb the function of integral membrane proteins, including ion channels [20-221, neurotransmitter receptor binding [Z&28], and peripheral adrenergic receptor activity [23, 291. Molecular structure studies in our laboratory, using small angle X-ray scattering techniques, have also shown that alterations in the C:Pl mole ratio substantially affect the membrane bilayer time averaged location of a phenylalkylamine calcium blocker [30]. These findings demonstrate that interactions of exogenous drugs and endogenous neurotransmitters with membrane sites of activity are highly selective [31], involve the lipid bilayer of cell membranes [32], and may be significantly affected by alterations in membrane cholesterol. In the brain, membrane cholesterol content appears to be highly reproducible, site specific, and highly regulated. Changes in brain membrane C:Pl ratio are associated both with aging [33] and disease 1341. Finally, lowering serum cholesterol in a non-human primate model has been reported to affect behavior [35]. Cyanomologous monkeys fed a low fat diet had significantly lower serum cholesterol levels compared to controls, and exhibited a significant increase in episodes of aggressive behavior, as evaluated by a standard behavioral tool, The above findings, in the aggregate, suggest that indeed, serum and brain cholesterol may be linked metabolically, and that subtle perturbations in brain membrane cholesterol content caused by alterations in serum cholesterol concentration, such as pharmacologic or dietary cholesterol lowering, could conceivably affect neurologic function. At present, the National Cholesterol Education Program (NCEP) guidelines suggest pharmacologic therapy for patients with elevated LDL cholesterols who have not responded to dietary cholesterol lowering, and have two or more risk factors for coronary
Serum
Cholesterol
and
artery disease, or one other risk factor besides male sex [36]. While the salutary effect of such a strategy on progression of ischemic heart disease is established, the possibility that alterations in serum cholesterol might
Neurologic
Function
1341
influence brain membrane cholesterol, and ultimately affect behavior, is inadequately explored, and further research in this area is eagerly awaited.
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