Johns Hopkins scientists have discovered that a
deceptively simple sugar is in fact a critical regulator of
cells' natural life cycle.
The discovery reveals that, when disturbed, this
process could contribute to cancer or other diseases by
failing to properly control the steps and timing of cell
division, the researchers say. The findings are described
in the Sept. 23 issue of the Journal of Biological
The sugar, known as O-GlcNAc (pronounced
oh-GLUCK-nack), is used inside cells to modify proteins,
turning the proteins off or on, helping or preventing their
interactions with other proteins, keeping them from
destruction or allowing their destruction. The comings and
goings of the sugar on proteins seem to be important
controllers of cell division, the researchers say.
"The dogma for decades has been that the cycle of cell
division is controlled by the appearance and disappearance
of certain proteins called cyclins, but experiments have
shown that you can knock out any of these and still get
perfectly normal cell division," said the study's first
author, Chad Slawson, a postdoctoral fellow in
chemistry in Johns Hopkins' Institute for Basic
Biomedical Sciences. "In contrast, our experiments show
that by increasing or decreasing the amount of sugar
attached to proteins, the cell cycle is disrupted and isn't
salvageable unless O-GlcNAc levels are fixed."
In experiments with human cells and mouse cells,
Slawson and his colleagues showed that preventing a cell
from removing the sugar from proteins causes the cell to
copy its genetic material and make new nuclei but to fail
to divide in two. The end result is cells with more than
one nucleus--a situation fairly common in cancer cells.
"Cells with more than one nucleus can survive, but
they are dysregulated--things just don't go right," Slawson
said. "The longer they survive, the worse it gets."
On the other hand, cells that had higher than normal
amounts of the enzyme that removes the sugar from proteins
ended up with nuclei that didn't look right under a
powerful microscope. Instead of being disseminated fairly
uniformly through the entire nucleus, the genetic material
of these cells was bunched up, giving the contents of the
nucleus a "wrinkly" appearance.
Exactly what is going wrong is still unclear, said
Gerald Hart, professor and director of Biological
Chemistry. He's been studying O-GlcNAc since his lab
discovered it attached to proteins inside cells 20 years
ago. His team now knows which enzymes put the sugar onto
proteins and which enzymes take it off, and knocking out or
blocking these enzymes allowed the researchers to control
whether proteins were sugar-laden or sugar-free.
"Normally, the enzyme that adds the sugar to proteins
is enriched at the hub of activity during cell division,"
Slawson said. "When we knock it out or block it with a
chemical, the cell cycle lengthens and cell division
doesn't happen properly. Clearly the enzyme is there for a
But understanding what the sugar itself is doing and
how its presence on or absence from proteins affects the
cell depends solely on what protein it's being attached to
or removed from.
"Whether it's turning something on or off depends on
the protein to which the sugar is attached," Hart said.
"It's harder than having discovered an enzyme that does
just one thing. To figure out the sugar's effect, we have
to look at what it's modifying, and the extent and the
location of the modification."
The sugar seems to modify as many proteins as the
ubiquitous phosphate groups widely recognized as protein
controllers, and it frequently seems to compete with
phosphate groups for the same spots on proteins. Hart
suggests that a particular balance between O-GlcNAc and
phosphates on proteins may help fine-tune their
The researchers' next step is to examine select
proteins modified by O-GlcNAc and found at locations
important for various steps in cell division to figure out
why an imbalance of O-GlcNAc on the cells' proteins has
such a dramatic effect on the process.
The researchers were funded by the National Institute
of Child Health and Human Development, the National
Institute of Diabetes and Digestive and Kidney Diseases and
the National Cancer Institute.
Authors on the paper are Slawson, Natasha Zachara,
Keith Vosseller, Win Den Cheung, Daniel Lane and Gerald
Hart, all at Johns Hopkins while working on this project.
Vosseller is now at Drexel University.
O-GlcNAc modification of proteins is detected using an
antibody developed at Johns Hopkins. Under a licensing
agreement between Covance Research Products, Sigma Chemical
Company and The Johns Hopkins University School of
Medicine, Hart receives a percentage of royalties received
by the university on sales of this antibody, CTD 110.6. The
terms of this arrangement are being managed in accordance
with the university's conflict-of-interest policy.