Johns Hopkins scientists have discovered that a deceptively simplesugar is in fact a critical regulator of cells' natural life cycle.
The discovery reveals that, when disturbed, this process couldcontribute to cancer or other diseases by failing to properly controlthe steps and timing of cell division, the researchers say. Thefindings are described in the Sept. 23 issue of the Journal ofBiological Chemistry, available online now.
The sugar, known as O-GlcNAc (pronounced oh-GLUCK-nack), isused inside cells to modify proteins, turning the proteins off or on,helping or preventing their interactions with other proteins, keepingthem from destruction or allowing their destruction. The comings andgoings of the sugar on proteins seem to be important controllers ofcell division, say the researchers.
"The dogma for decades has been that the cycle of cell divisionis controlled by the appearance and disappearance of certain proteinscalled cyclins, but experiments have shown that you can knock out anyof these and still get perfectly normal cell division," says thestudy's first author, Chad Slawson, Ph.D., a postdoctoral fellow inbiological chemistry in Johns Hopkins' Institute for Basic BiomedicalSciences. "In contrast, our experiments show that by increasing ordecreasing the amount of sugar attached to proteins, the cell cycle isdisrupted and isn't salvageable unless O-GlcNAc levels are fixed."
In experiments with human cells and mouse cells, Slawson andhis colleagues showed that preventing a cell from removing the sugarfrom proteins causes the cell to copy its genetic material and make newnuclei, but to fail to divide in two. The end result is cells with morethan one nucleus -- a situation fairly common in cancer cells.
"Cells with more than one nucleus can survive, but they aredysregulated -- things just don't go right," says Slawson. "The longerthey survive, the worse it gets."
On the other hand, cells that had higher than normal amounts ofthe enzyme that removes the sugar from proteins ended up with nucleithat didn't look right under a powerful microscope. Instead of beingdisseminated fairly uniformly through the entire nucleus, the geneticmaterial of these cells was bunched up, giving the contents of thenucleus a "wrinkly" appearance.
Exactly what is going wrong is still unclear, adds Gerald Hart,Ph.D., professor and director of biological chemistry. He's beenstudying O-GlcNAc since his lab discovered it attached to proteinsinside cells 20 years ago. They now know which enzymes put the sugaronto proteins and which enzymes take it off -- and knocking out orblocking these enzymes allowed the researchers to control whetherproteins were sugar-laden or sugar-free.
"Normally, the enzyme that adds the sugar to proteins isenriched at the hub of activity during cell division," notes Slawson."When we knock it out or block it with a chemical, the cell cyclelengthens and cell division doesn't happen properly. Clearly the enzymeis there for a reason."
But understanding what the sugar itself is doing and how itspresence on or absence from proteins affects the cell depends solely onwhat protein it's being attached to or removed from.
"Whether it's turning something on or off depends on theprotein to which the sugar is attached," says Hart. "It's harder thanhaving discovered an enzyme that does just one thing. To figure out thesugar's effect, we have to look at what it's modifying, and the extentand the location of the modification."
The sugar seems to modify as many proteins as the ubiquitousphosphate groups widely recognized as protein controllers, and itfrequently seems to compete with phosphate groups for the same spots onproteins. Hart suggests that a particular balance between O-GlcNAc andphosphates on proteins may help fine-tune their activities.
The researchers' next steps are to examine select proteinsmodified by O-GlcNAc and found at locations important for various stepsin cell division to figure out why an imbalance of O-GlcNAc on thecells' proteins has such a dramatic effect on the process.
The researchers were funded by the National Institute of ChildHealth and Human Development, the National Institute of Diabetes andDigestive and Kidney Diseases and the National Cancer Institute.
Authors on the paper are Slawson, Natasha Zachara, KeithVosseller, Win Den Cheung, Daniel Lane and Gerald Hart, all at JohnsHopkins while working on this project. Vosseller is now at DrexelUniversity.
O-GlcNAc modification of proteins is detected using an antibodydeveloped at Johns Hopkins. Under a licensing agreement between CovanceResearch Products, Sigma Chemical Company and The Johns HopkinsUniversity School of Medicine, Hart receives a percentage of royaltiesreceived by the university on sales of this antibody (CTD 110.6). Theterms of this arrangement are being managed in accordance with theuniversity's conflict of interest policy.
On the Web:http://www.jbc.org/cgi/reprint/M503396200v1
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