December 20, 2006
CONTACT: Karen Kreeger
karen.kreeger@uphs.upenn.edu
How Blood Flow Dictates Gene Expression
Implications for Treating Atherosclerosis
(PHILADELPHIA) – Researchers at the University
of Pennsylvania School of Medicine have pinpointed a key
regulatory protein that translates blood flow into gene expression.
The investigators showed that in a model of mouse embryonic development
a transcription
factor called Klf2, which resides in cells that line blood vessels,
is activated by rapid, pulsed blood flow, as reported in the December
issue of Developmental
Cell. Understanding Klf2’s role in blood vessel and muscle
biology could help with fighting atherosclerosis.
“We always knew that there had to be this line of communication
from the vessel lining, or endothelium, to the smooth muscles, which never
sees a blood cell,” says senior author Mark
Kahn, MD, Associate Professor of Medicine. “That’s
where Klf2 fits: This is the first time, at a molecular level, that this
chain has been demonstrated in an animal.”
Swirling eddies of blood form when vessels branch, much like when a river
divides. Atherosclerosis typically forms at these sites of so-called disturbed
flow as opposed to regions of rapid blood flow through the main vessels.
This relationship between atherosclerosis and flow has been known for
decades. More recently, tissue-culture studies have shown that Klf2 is
activated by increased blood flow, or “fluid sheer stress.”
Indeed, in this study Kahn; first author John S. Lee, MD, PhD,
Instructor in the Department
of Medicine; and colleagues show that the expression of Klf2 in a
developing mouse embryo mirrors events in previous tissue-culture studies.
They found that Klf2 is expressed on the high-flow side of developing
mitral
and aortic
valves in the heart of a 14-day-old embryo.
The researchers surmise that the mechanical stimulus of blood flowing
in a vessel leads to the upregulation
of Klf2, which either activates or represses genes that control smooth
muscle tone, that is the caliber of the vessel. (Tone is governed by how
much a muscle contracts or relaxes.) These genes encode
proteins that are either secreted or are on the cell surface of the endothelium
and so influence how smooth-muscle cells contract or relax.
The researchers suggest that when Klf2 is expressed, smooth muscle cells
lining the blood vessels maintain their ability to regulate vessel tone.
However, when Klf2 is genetically deleted, or “knocked out,”
from the blood vessels of mouse embryos, they had an abnormally high cardiac
output, as measured by ultrasound,
while the overall structure of their blood vessels was normal. These findings
implicate loss of vessel tone as the primary defect in Klf2 knockout mice.
Now that Klf2 has been established as an important regulator of blood
flow in live animals and is required for the development of a healthy
cardiovascular system, the next step is to elucidate the role of Klf2
in normal adult blood vessels and in the pathogenesis
of vascular
diseases, such as atherosclerosis.
Co-authors are Eric Sebzda, Cara Bertozzi, Mei Chen, Patti Mericko, Diane
Zhou, Lan Chen, and John J. Lepore from Penn; Qing Yu and Cecelia W. Lo
from the National
Heart, Lung and Blood Institute; Jordan T. Shin and Calum A. MacRae
from Massachusetts
General Hospital, Charlestown, Mass., and Matthias Stadtfeld and Thomas
Graf from Albert
Einstein College of Medicine, New York. This research was funded by
the National Heart,
Lung and Blood Institute.
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This release is available online at
http://www.uphs.upenn.edu/news/News_Releases/dec06/blood-flow-gene-expression.htm