20 de Junho, 2009
Quando o colesterol HDL deixa de ser ‘bom’ para tornar-se em ‘mau’, ou mais alguns buracos de uma hipótese lipídica que, apesar de tudo, já se tornou num mito urbano
Autor: O Primitivo. Categoria: Saúde
Vídeo: HDL: When Good Cholesterol Goes Bad.
Mais um bocado de doutrinação lipídica, desta vez explorando a possibilidade de determinadas proteínas da fracção HDL nem sempre serem tão boazinhas quanto isso, em particular uma tal de proteína apo-A2, que é a segunda mais importante da fracção HDL e com capacidade para inibir a sua função anti-inflamatória e anti-oxidante.
When ‘Good’ Cholesterol Goes Bad
Date:8/22/2007
HDL analysis uncovers a dark side, but also a new way it fights heart disease
WEDNESDAY, Aug. 22 (HealthDay News) — It looks like HDL, the "good" cholesterol that supposedly protects against cardiovascular disease, might have a harmful side.
New research suggests that some people’s HDL is more protective for their hearts than others, and that certain proteins in HDL can exacerbate vessel damage, particularly in people with heart disease.
But there’s good news, too, as scientists uncover a new means by which HDL boosts cardiovascular health.
The findings were to be reported Wednesday at the American Chemical Society’s annual meeting in Boston.
So far, the cholesterol story has been a relatively simple one in which "bad" low-density lipoprotein (LDL) cholesterol formed plaques that eventually blocked arteries, while "good" high-density lipoprotein (HDL) cholesterol worked to carry away those deposits, explained Dr. Jay Heinecke, a professor of medicine at the University of Washington, Seattle.
"But in the last few years there has been growing evidence that HDL does other things," he said. "In particular, it may be inhibiting inflammation."
Inflammation is the major villain in the new picture. Arteries are not only blocked because of the gradual growth of plaque. Instead, there comes a moment when plaque ruptures, causing a clot to form and block blood flow, Heinecke said. Proteins called proteases play a major role in these ruptures.
"What we found in HDL is a whole series of proteins that inhibit proteases," Heinecke said, describing what he called the most detailed analysis to date of HDL’s protein composition. "So, part of [HDL's] protective effect is to prevent rupture."
The Seattle analysis also found a lot of previously unrecognized HDL proteins, including 22 that play roles in cholesterol metabolism.
One finding of particular significance is that HDL protein composition can be different in people who have heart disease and those who do not — meaning that some of the supposedly "good" HDL proteins are really bad.
"With LDL cholesterol it’s simple — the lower the better," Heinecke said. "With HDL, it’s much more complicated. The protein composition of people with and without heart disease is different."
So, measuring blood levels of LDL and HDL cholesterol is not as predictive of cardiac risk, as has been assumed, Heinecke stressed. "Protein composition [in HDL cholesterol] may be a better handle on whether someone is at risk," he added.
Animal studies have found "dysfunctional" HDL cholesterol, which works against coronary health, Heinecke said. "It is proposed that the same thing is going on in humans," he noted.
The finding of dysfunctional HDL proteins helps explain why a major pharmaceutical company ended work on an HDL-boosting drug when it was found to actually increase deaths and heart problems in a human trial.
A better understanding of the protein components of HDL could lead to more accurate tests for heart disease, Heinecke said. "Most people who have heart attacks have normal levels of HDL, so the composition of the HDL may tell who is vulnerable," he said.
Future cardiac therapy may include LDL-lowering statins and new drugs aimed at the damaging components of HDL cholesterol, Heinecke said.
SOURCES: Jay Heinecke, M.D., professor, medicine, University of Washington, Seattle; Aug. 22, 2007, American Chemical Society annual meeting, Boston
Ainda a este respeito, leia estes artigos muito interessantes:
The Amazing New World of HDL Cholesterol (Wellness Resources)
Some ”good cholesterol” isn’t actually good enough
HDL Not Always the Good Cholesterol We Think Says University of Chicago Study (Senior Journal)
Um comentário final: nesta hipótese lipídica das lipoproteínas "boas" e "más", como facilmente constatamos, é também pouco sólida a sua base de sustentação no que respeita ao HDL. A ideia simplista de que a fraçcão HDL-C é sempre boa e a LDL-C é sempre má, simplesmente está cheia de "buracos". Isto porque, por exemplo, uma fracção LDL elevada nem sempre conduz a eventos cardíacos. Aliás, até parece protectora pois, para valores de LDL acima de 130 mg/dl, o risco coronário é DECRESCENTE e diminuto acima de 180 mg/dl . Sim, você leu muito bem, e estou a citar o estudo observacional que demonstra isso, é descrescente!
Em resultado disso, num dos artigos mais acima pode ler-se esta frase cómica, típica de uma teoria mesmo muito frágil:
"It is now apparent that subjects with high HDL are not necessarily protected from heart problems and should ask their doctor to find out whether their HDL is good or bad."
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Artigos relacionados:
Synthesis, transport, and processing of apolipoproteins of high density lipoproteins
Apolipoprotein A-containing lipoprotein particles: physiological role, quantification, and clinical significance
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