CLEVELAND (11/15/04)- Not all HDL (high density lipoprotein), often
referred to as "good cholesterol", protects against plaque build-up
in artery walls. That's because in some people HDL is dysfunctional, or
malformed,
and unable to do its job properly, according to researchers from the Cleveland
Clinic. The fact that sometimes HDL doesn't do its job helps explain why
some people who have high HDL levels still get heart attacks and suffer
from
other cardiovascular disease, said Stanley Hazen, MD, PhD, head of preventive
cardiology and rehabilitation at the Cleveland Clinic.
"Its
a bit of a misconception, even among physicians, that having a high HDL completely
counteracts having an elevated LDL (low-density lipoprotein)
cholesterol level, he said. LDL is often referred to as "bad" cholesterol.
While this seems to put a damper on having faith in what cholesterol tests
tell you about risk for cardiovascular disease (CVD), all is not lost.
For one, dysfunctional HDL is not that common, plus population studies show
that people with high HDL usually have lower risk for heart disease. Still,
close to half of people who get heart attacks have healthy cholesterol levels,
he said. Now there is also a test under development that one day could be
used to help identify people with respectable HDL levels but who may be still
at risk, and Dr. Hazen and his research team are the ones working on it.
The test will be for a chemical signature showing that a person's HDL is
dysfunctional.
Cholesterol is a steroid normally present in the body and is produced by
virtually every living cell, especially those
in the liver and kidney. It is not all bad and serves
several important functions in the body. Cholesterol is an important component
of
cell membranes and
serves as
a building block for the production of other steroids and hormones such as
testosterone. Some cholesterol is absorbed into the body from dietary sources.
One of the functions of HDL is to carry excess cholesterol from peripheral
tissues either onwards to the liver for reuse at another time, or to
the bile for excretion. LDL, on the other hand, the major carrier of "bad"
cholesterol,
helps to deliver cholesterol to various tissues throughout the body where
it is used as part of cell membranes during cell growth.
HDL is made of proteins and lipids.
Apolipoprotein A-1 (apoA-1) is the main protein component that gives HDL
its shape and helps it perform its functions
- such as the shuttling of excess cholesterol away from artery walls and
preventing plaque build-up. What researchers found is that HDL becomes dysfunctional
when an enzyme from white blood cells, called myeloperoxidase (MPO), binds
to apoA-1 and modifies it. Once modified, apoA-1 blocks HDL's ability to
perform its normal tasks. Normally, MPO helps fight infection and "people
with high MPO levels are better at fighting infection," Dr.
Hazen said.
A chemical fingerprint for MPO-modified HDL was discovered by the researchers
and levels of this marker could be a useful indicator for determining which
people with elevated HDL levels may still be at risk for
CVD. Likely,
it would be a "complimentary test to traditional lipid screening," Dr.
Hazen said. However, people should still follow lifestyle advice when it
comes
to preventing heart disease. Eat smart, exercise and aim for lower LDL levels.
"Focus on
the basics," he said. The study was published in The Journal
of Clinical Investigation.
end
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