A map of the coronary arteries that pump blood to the heart lookslike the dense foliage of a treetop. The main blood vessels arecrisscrossed with branches, sub-branches, twigs and leaves.
Atherosclerotic plaque buildup narrows, and eventually blocks, bloodflow, mainly at the forks where the arteries bifurcate. In contrast, thelong, straight stretches are largely devoid of these life-threateningobstructions.
The reason may be physical as much as biological. When a movingliquid encounters a sharp divergence or curvature in its flow, itsmolecules riot in a chaotic, milling jumble that collides head-on withthe walls or banks _ blood vessels or rivers _ through which theycourse. (See BioWorld Today, Nov. 22, 1995, p. 1.)
It's these mauled sites of turbulence that are likeliest to form plaque.But the stresses aren't all physical.
Besides those hemodynamic forces, biological factors _ such ashigh-cholesterol low-density lipoproteins (LDL), diabetes andcigarette smoke _ also can damage the artery's delicate inner lining.
That lining consists of a single layer of flat, seamlessly joinedendothelial cells, which separate the moving blood from the smoothmuscle cells that constrict and relax blood vessels in maintaining thebody's optimum blood pressure.
Stretches of many arteries trace a straight line, like the trunk of thattree below its multi-branching crown. These linear lengths of bloodvessel are relatively plaque-free, even though the oxidized LDL andtobacco risk factors (to name only two of many) contact their walls inpassing as they do the tumultuous branch points.
Along these calm straight-line sections, the endothelial cells expressgenes that protect their immediate laminar-flow neighborhood fromatherogenic attack. Three such known genes have now had whattriggers their expression identified.
* Manganese superoxide dismutase (MnSOD) "detoxifies reactivefree oxygen radical species that are in the cell," explained molecularbiologist Dean Falb, of Millennium Pharmaceuticals Inc. inCambridge, Mass.
* Endothelial cell nitric oxide synthase (EcNOS) releases nitricoxide, which causes blood vessels to relax.
* Cyclooxygenase-2 (COX-2) makes prostacyclin (a form ofprostaglandin) in the cell. When released it prevents adherence ofplatelets, which are key components in clotting and plaque.
Fluid Dynamics Drive Arterial Genes
"We've known that these three arterial genes," Falb told BioWorldToday, "were protective against plaque, but we didn't know why orhow." A collaborative research project between Millennium andHarvard Medical School has come up with an answer, reported in thecurrent Proceedings of the National Academy of Sciences (PNAS)dated Sept. 17, 1996.
Its title: "Identification of vascular endothelial cells differentiallyresponsive to fluid mechanical stimuli: COX-2, MnSOD, and EcNOSare selectively up-regulated by steady laminar shear stress."
In other words, cells lining unkinked stretches of artery, but not thebranched areas, continually express protective enzymes from threeanti-atherogenic genes.
Harvard's Michael Gimbrone, the PNAS paper's co-senior authorwith Falb, spent 14 years perfecting a device that subjects endothelialcells to one of three hemodynamic forces: buffeted by turbulenceshear stresses; subject only to laminar shear stress, and static.
The Harvard co-authors obtained their endothelial cells from humanumbilical cord veins, a common arterial surrogate.
They grew their dissociated primary cells in flat round tissue-cultureplates about six inches in diameter. Hovering in the medium justabove the cultures rotated a stainless-steel cone with an extremelyobtuse-angle point. As it spun, it reproduced the hemodynamic shearstresses in the straight and branched arterial regions, respectively.
Falb's atherosclerosis biology group at Millennium then singled outthe three protective genes, which all came from the laminar-flow-stress cellular contingent.
Using a proprietary PCR-based display technique called RADE(rapid analysis of differential gene expression), they amplified thethousands of messenger RNAs in those cells, and compared theresting controls with laminar and turbulence stresses. "Besides thethree known genes," Falb said, "we identified a whole list of novelprotective genes _ such as G-coupled protein receptors, iontransporters, things in signal transduction pathways.
"It's very striking," he observed, "that the force of the flow can havethis dramatic effect on expression of certain genes. Correlating thatwith where lesions form in vivo, we think, might be very significantin the disease process. If we could induce the pathways throughoutthe vasculature in aortic or coronary endothelial cells, we mightreduce atherosclerotic lesion formation."
The goal is to "find small molecules, for example, that mimic thosepathways, and turn on these kinds of activities."
Falb added, "We have a large $50 million collaboration in this areawith Eli Lilly & Co. [of Indianapolis]. Targets that we pull out wouldgo into their small-molecule discovery program."
Among the novel genes his group has turned up, some are receptors,Falb pointed out. "If we could find a ligand for such a receptor, we'dhave a great drug. You could put it in and turn on thoseatheroprotective mechanisms."
The Millennium team now is "making transgenic mice thatcontinually overexpress MnSOD, COX-2 and our novel genes," Falbsaid, "and a knockout animal that lacks the atherogenic gene forapolipoprotein E, so it develops atherosclerosis.
"We have crossed these transgenic animals with the knockoutatherosclerosis model," he concluded, "and will begin analysis in acouple of months from now, to see what effect the expression has onlesion formation." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.