Cholesterol, the characteristic steroid alcohol of animal tissues, performs a number of essential functions in the body.
OVERVIEW
Cholesterol, the
characteristic steroid alcohol of animal tissues, performs a number of
essential functions in the body. For example, cholesterol is a structural
component of all cell membranes, modulating their fluidity, and, in specialized
tissues, cholesterol is a precursor of bile acids, steroid hormones, and
vitamin D. It is, therefore, critically important that the cells of the body be
assured an appropriate supply of cholesterol. To meet this need, a complex
series of transport, biosynthetic, and regulatory mechanisms has evolved. The
liver plays a central role in the regulation of the body’s cholesterol
homeostasis. For example, cholesterol enters the liver’s cholesterol pool from
a number of sources including dietary cholesterol as well as that synthesized
de novo by extrahepatic tissues and by the liver itself. Cholesterol is
eliminated from the liver as unmodified cholesterol in the bile, or it can be
converted to bile salts that are secreted into the intestinal lumen. It can
also serve as a component of plasma lipoproteins that carry lipids to the
peripheral tissues. In humans, the balance between cholesterol influx and
efflux is not precise, resulting in a gradual deposition of cholesterol in the
tissues, particularly in the endothelial linings of blood vessels. This is a
potentially life-threatening occurrence when the lipid deposition leads to
plaque formation, causing the narrowing of blood vessels (atherosclerosis) and
increased risk of cardio-, cerebro-, and peripheral vascular disease. Figure
18.1 summarizes the major sources of liver cholesterol and the routes by which
cholesterol leaves the liver.
Figure 18.1 Sources of liver
cholesterol (influx) and routes by which cholesterol leaves the liver (efflux).
HDL = highdensity lipoprotein; VLDL = very-lowdensity lipoprotein.
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