A computational model depicts a PFK1 enzyme with the sugar GlcNAc attached (left). Comparing this model to that showing PFK1 complexed to a molecule that activates the enzyme (right) suggests how addition of GlcNAc may inhibit enzymatic activity. Credit: Caltech/Yi et al. |
Behaving
something like ravenous monsters, tumors need plentiful supplies of
cellular building blocks such as amino acids and nucleotides in order to
keep growing at a rapid pace and survive under harsh conditions. How
such tumors meet these burgeoning demands has not been fully understood.
Now chemists at the California Institute of Technology (Caltech) have
shown for the first time that a specific sugar, known as GlcNAc
(“glick-nack”), plays a key role in keeping the cancerous monsters
“fed.” The finding suggests new potential targets for therapeutic
intervention.
The new results appear in this week’s issue of the journal Science.
The
research team—led by Linda Hsieh-Wilson, professor of chemistry at
Caltech—found that tumor cells alter glycosylation, the addition of
carbohydrates (in this case GlcNAc) to their proteins, in response to
their surroundings. This ultimately helps the cancerous cells survive.
When the scientists blocked the addition of GlcNAc to a particular
protein in mice, tumor-cell growth was impaired.
The
researchers used chemical tools and molecular modeling techniques
developed in their laboratory to determine that GlcNAc inhibits a step
in glycolysis (not to be confused with glycosylation), a metabolic
pathway that involves 10 enzyme-driven steps. In normal cells,
glycolysis is a central process that produces high-energy compounds that
the cell needs to do work. But Hsieh-Wilson’s team found that when
GlcNAc attaches to the enzyme phosphofructokinase 1 (PFK1), it
suppresses glycolysis at an early phase and reroutes the products of
previous steps into a different pathway—one that yields the nucleotides a
tumor needs to grow, as well as molecules that protect tumor cells. So
GlcNAc causes tumor cells to make a trade—they produce fewer high-energy
compounds in order to get the products they need to grow and survive.
“We
have identified a novel molecular mechanism that cancer cells have
co-opted in order to produce intermediates that allow them to grow more
rapidly and to help them combat oxidative stress,” says Hsieh-Wilson,
who is also an investigator with the Howard Hughes Medical Institute.
This
is not the first time scientists have identified a mechanism by which
tumor cells might produce the intermediates they need to survive. But
most other mechanisms have involved genetic alterations, or
mutations—permanent changes that lead to less active forms of enzymes,
for example. “What’s unique here is that the addition of GlcNAc is
dynamic and reversible,” says Hsieh-Wilson. “This allows a cancer cell
to more rapidly alter its metabolism depending on the environment that
it encounters.”
In
their studies, Hsieh-Wilson’s team found that this glycosylation—the
addition of GlcNAc to PFK1—is enhanced under conditions associated with
tumors, such as low oxygen levels. They also found that glycosylation
of PFK1 was sensitive to the availability of nutrients. If certain
nutrients were absent, glycosylation was increased, and the tumor was
able to compensate for the dearth of nutrients by changing the cell’s
metabolism.
When
the researchers analyzed human breast and lung tumor tissues, they
found GlcNAc-related glycosylation was elevated two- to fourfold in the
majority of tumors relative to normal tissue from the same patients.
Then, working with mice injected with human lung-cancer cells, the
researchers replaced the existing PFK1 enzymes with either the normal
PFK1 enzyme or a mutant form that could no longer be glycosylated. The
mice with the mutant form of PFK1 showed decreased tumor growth,
demonstrating that blocking glycosylation impairs cancerous growth.
The
work suggests at least two possible avenues for future investigations
into fighting cancer. One would be to develop compounds that prevent
PFK1 from becoming glycosylated, similar to the mutant PFK1 enzymes in
the present study. The other would be to activate PFK1 enzymes in order
to keep glycolysis operating normally and help prevent cancer cells from
altering their cellular metabolism in favor of cancerous growth.
Hsieh-Wilson’s
group has previously studied GlcNAc-related glycosylation in the brain.
They have demonstrated, for example, that the addition of GlcNAc to a
protein called CREB inhibits the protein’s ability to turn on genes
needed for long-term memory storage. On the other hand, they have also
shown that having significantly lower levels of GlcNAc in the forebrain
leads to neurodegeneration. “The current thinking is that there’s a
balance between too little and too much glycosylation,” says
Hsieh-Wilson. “Being at either extreme make things go awry, whether it’s
in the brain or in the case of cancer cells.”
Additional
Caltech coauthors on the paper, “Phosphofructokinase 1 Glycosylation
Regulates Cell Growth and Metabolism,” were lead author Wen Yi, a
postdoctoral scholar in Hsieh-Wilson’s group; Peter Clark, a former
graduate student in Hsieh-Wilson’s group; and William Goddard III, the
Charles and Mary Ferkel Professor of Chemistry, Materials Science, and
Applied Physics. Daniel Mason and Eric Peters of the Genomics Institute
of the Novartis Research Foundation and Marie Keenan, Collin Hill, and
Edward Driggers of Agios Pharmaceuticals were also coauthors.
The
work was supported by the National Institutes of Health, the Department
of Defense Breast Cancer Research Program, and a Tobacco-Related
Disease Research Program postdoctoral fellowship.
