Johns Hopkins researchers have discovered
a previously unrecognized role played by
the gene HIF-1 as it helps cells survive
when a lack of oxygen decreases production
of an energy-rich molecule called ATP and
increases production of toxic molecules.
ATP supplies energy the cell needs to
perform each of its many chemical reactions
and tasks, and in this way acts as the "currency"
for the cell's energy economy.
A report on the work, done with mouse
cells genetically altered to lack the HIF-1
gene, appears in the March 8 issue of Cell
A cell's energy demands are met by two
major types of sugar (glucose) using machines
similar to the two types of engines in a hybrid
car. One machine, the mitochondrion, is an
organelle that breaks down the glucose-using
oxygen and produces ATP; the other does the
same thing—albeit less efficiently—without
using oxygen, in a process called glycolysis.
Like the hybrid car, cells use oxygen and the
internal combustion engine at higher speeds
and rely on an electric engine without need
for oxygen consumption at lower speeds. Cells
consume glucose through its main energyproducing
machine, the mitochondrion, when
oxygen is ample. But like the internal combustion
engine, this process generates pollutants
or toxic oxygen molecules.
At lower oxygen levels, when cells are
starved for oxygen—as during exertion or
trauma—the genetic switch that the Johns
Hopkins researchers found deliberately
shuts off the cell's mitochondrial combustion
engine, which scientists had long—and
erroneously—believed ran down on its own
due to lack of oxygen.
"The unexpected discovery is that this
genetic switch actively shuts off the mitochondrion
under low oxygen conditions,
apparently to protect cells from mitochondrial
toxic oxygen pollutants," said Chi Van
Dang, professor of medicine, cell biology,
oncology and pathology, and vice dean for
research at the School of Medicine.
Dang says the switch may be a target for
cancer drugs because a cancer cell's survival
depends on it to convert glucose to lactic
acid through glycolysis even in the presence
of ample oxygen. Disruption of the switch
by a drug may cause cancer cells to pollute
themselves with toxic oxygen molecules and
undergo apoptosis or cell death.
The new finding, made by graduate student
Jung-whan Kim and the team led by
Dang, showed that during oxygen deprivation,
or hypoxia, the HIF-1 gene cuts the
link between two ATP-making biochemical
pathways: glycolysis, which makes modest
amounts of ATP by breaking down the glucose
without using oxygen; and the TCA
cycle in the mitochondrion, which normally
uses oxygen to produce large amounts of ATP
by processing a by-product of glycolysis.
The disruption of this link blocks the
tendency of the mitochondrion to make
toxic molecules as it struggles to produce
ATP during hypoxia. These toxic molecules,
called reactive oxygen species, or ROS, damage
molecules in the cell and even cause the
cell to undergo apoptosis.
The target of HIF-1 is the conversion of
pyruvate—the by-product of glycolysis—into
another molecule called acetyl co-enzyme
A, according to Dang. When oxygen levels
are normal, the cell produces acetyl CoA
and feeds it into the TCA cycle within the
mitochondrion. The mitochondrion then
processes acetyl CoA using oxygen to obtain
large amounts of ATP.
It was already known that during hypoxia
HIF-1 accelerates the output of ATP by glycolysis,
Dang noted. But until now, he said,
researchers thought that HIF-1 simply turned
up glycolysis and let the mitochondrion slow
down on its own and produce less ATP.
Because the mitochondrion runs on oxygen,
it doesn't work properly in hypoxic
conditions, Dang said. Instead, glycolysis is
left to shoulder the burden of making ATP
by being prodded into overdrive by HIF-1.
And left to itself during hypoxia, the mitochondrion
produces reactive oxygen species
that threaten the life of the cell.
"But our discovery clearly shows that hypoxia
doesn't simply trigger a passive shutdown of
the mitochondrion," Dang said. "Instead, HIF-
1 acts as a genetic switch to actively shut
down mitochondrial function and prevent the
production of reactive oxygen species."
The Johns Hopkins team demonstrated
that HIF-1 shuts down the TCA cycle by preventing
an enzyme called PDH from converting
pyruvate made by glycolysis into acetyl
CoA. Specifically, HIF-1 blocks the ability
of PDH to make this conversion. HIF-1
does this by activating a protein called PDK,
which binds to PDH and prevents it from
performing this critical task. This starves the
TCA cycle of acetyl CoA and shuts it down.
The Hopkins researchers made their discovery
using mouse embryo fibroblast cells
that were genetically altered to lack HIF-1.
When the investigators exposed these socalled
HIF-1 null MEFs to hypoxic conditions,
the cells were unable to activate PDK
to block mitochondrial function. This showed
that HIF-1 is required to activate PDK.
The team then genetically engineered
HIF-1 null MEFs and forced PDK to work—
even in the absence of the HIF-1 gene. The
hypoxic cells once again accelerated glycolysis
and produced increased amounts of
ATP; and with the PDK forced to work, the
cells were also able to shut down the TCA
cycle. This showed that PDK is the protein
activated by HIF-1 to prevent the mitochondrion
from producing ROS.
The other authors of this paper are Kim,
Irina Tchernyshyov and Gregg L. Semenza,
who discovered HIF-1 a decade ago.
This work was supported in part by the
National Institutes of Health, National Cancer
Institute and National Heart, Lung and
Blood Institute. Kim is a Howard Hughes
Medical Institute Predoctoral Fellow; Dang
is the Johns Hopkins Family Professor in