Published: April 22, 2014. By Johns Hopkins Medicine
http://www.hopkinsmedicine.org
Working with mice and rabbits, Johns Hopkins scientists have found a
way to block abnormal cholesterol production, transport and breakdown,
successfully preventing the development of atherosclerosis, the main
cause of heart attacks and strokes and the number-one cause of death
among humans. The condition develops when fat builds inside blood
vessels over time and renders them stiff, narrowed and hardened, greatly
reducing their ability to feed oxygen-rich blood to the heart muscle
and the brain.
In a series of experiments, described April 7 in the journal Circulation,
the Johns Hopkins team says it identified and halted the action of a
single molecular culprit responsible for a range of biological glitches
that affect the body's ability to properly use, transport and purge
itself of cholesterol — the fatty substance that accumulates inside
vessels and fuels heart disease.
The offender, the researchers say, is a fat-and-sugar molecule called
glycosphingolipid, or GSL, which resides in the membranes of all cells,
and is mostly known for regulating cell growth. Results of the
experiments, the scientists say, reveal that this very same molecule
also regulates the way the body handles cholesterol.
The Johns Hopkins team used an existing man-made compound called
D-PDMP to block the synthesis of the GSL molecule, and by doing so,
prevented the development of heart disease in mice and rabbits fed a
high-fat, cholesterol-laden diet. The findings reveal that D-PDMP
appears to work by interfering with a constellation of genetic pathways
that regulate fat metabolism on multiple fronts — from the way cells
derive and absorb cholesterol from food, to the way cholesterol is
transported to tissues and organs and is then broken down by the liver
and excreted from the body.
"Current cholesterol-lowering medications tackle the problem on a
single front — either by blocking cholesterol synthesis or by preventing
the body from absorbing too much of it," says lead investigator Subroto
Chatterjee, Ph.D., a cardio-metabolic expert at the Johns Hopkins
Children's Center. "But atherosclerosis is a multi-factorial problem
that requires hitting the abnormal cholesterol cycle at many points. By
inhibiting the synthesis of GSL, we believe we have achieved exactly
that."
Specifically, the experiments showed that treatment with D-PDMP led to:
- a drop in the animals' levels of so-called bad cholesterol or low-density lipoprotein, LDL;
- a drop in oxidized LDL, a particularly virulent form of fat that
forms when LDL encounters free radicals. Oxidized LDL easily sticks to
the walls of blood vessels, where it ignites inflammation, damaging the
vessel walls and promoting the growth of fatty plaque; - a surge in good cholesterol or high-density lipoprotein, HDL, known to counteract the effects of LDL by mopping it up; and
- a significant drop in triglycerides, another type of plaque-building fat.
building up inside the animals' vessels. These effects were observed in
animals on a daily D-PDMP treatment even though they ate a diet made up
of 20 percent triglycerides — the human equivalent of eating a greasy
burger for breakfast, lunch and dinner. In addition, the researchers
say, D-PDMP appears to precision-target the worst byproducts of aberrant
cell growth signaling, such as oxidized LDL and the activity of certain
chemicals that fuel vessel inflammation, without altering cell growth
itself.
D-PDMP, which is already widely used in basic research to
experimentally block and study cell growth and other basic cell
functions, is deemed safe in animals, the investigators say. For
example, animals in the current study had no side effects even when
given D-PDMP doses 10 times higher than the minimum effective dose, the
study found. The research team is currently designing a compound drug
with D-PDMP, which they soon plan to test in other animals and,
eventually, in humans.
Mice used in the experiments were genetically engineered to lack a
protein essential in the breakdown of fats and thus were predisposed to
atherosclerosis. The researchers fed the animals a high-fat diet over
the course of several months, but also gave a third of the animals a
low-dose of D-PDMP. They gave a double dose of the same inhibitor to
another third and placebo to the rest.
When scientists measured the thickness of the animals' aortas — the
body's largest vessel and one that carries blood from the heart to the
rest of the body — they found striking differences among the groups. As
expected, the aortas of mice that got placebo had grown thicker from the
accumulation of fat and calcium deposits inside them. The aortas of
mice on low-dose D-PDMP, however, were significantly thinner with little
to no obstruction. To the researchers' surprise, Chatterjee says, mice
eating high-fat foods and treated with high-dose D-PDMP had nearly
pristine arteries free of obstruction, indistinguishable from those of
healthy mice.
Next, the researchers measured how well and how fast blood traveled
through the animals' blood vessels. Slower blood flow signals clogging
of the vessel and is a marker of atherosclerosis. The vessels of mice
fed a high-fat diet plus D-PDMP had normal blood flow. Mice receiving a
high-fat diet without D-PDMP predictably had compromised blood flow.
When researchers examined cells from the animals' livers — the main
site of fat synthesis and breakdown — they noticed marked differences in
the expression of several genes that regulate cholesterol metabolism.
The activity of these genes is heralded by the levels of enzymes they
produce, Chatterjee says. Mice treated with D-PDMP had notably higher
levels of two enzymes responsible for maintaining the body's delicate
fat homeostasis by regulating the way cells take in and break down
cholesterol. Specifically, the scientists say, the inhibitor appeared to
stimulate the action and efficacy of a class of protein pumps in the
cell responsible for maintaining healthy cholesterol levels by
transporting cholesterol in and out of the bloodstream. In addition,
mice treated that way had higher levels of lipoprotein lipase, an enzyme
responsible for the breakdown of triglycerides. A deficiency in this
enzyme causes dangerous buildup of blood triglycerides.
Treatment with a D-PDMP also boosted the activity of an enzyme
responsible for purging the body of fats by converting these fats into
bile, the fat-dissolving substance secreted by the liver.
In a final set of experiments, researchers compared the effects of
treatment with D-PDMP in two groups of healthy rabbits, both fed
high-fat diets, with half of them receiving treatment. Rabbits that ate
high-fat food alone developed all the classic signs of
atherosclerosis—fatty plaque buildup in the arteries and stiff, narrowed
blood vessels. Their cholesterol levels shot up 17-fold. By contrast,
rabbits treated with D-PDMP never developed atherosclerosis. Their
cholesterol levels also remained normal or near-normal.
The World Health Organizations estimates that high cholesterol claims
2.6 million lives worldwide each year. More than 70 million Americans
have high cholesterol, according to the U.S. Centers for Disease Control
and Prevention. Current cholesterol-lowering drugs, such as statins, do
not work in about one-third of people who take them, experts say.
