"It was very surprising to find an enzyme whose inhibition lessened platelet aggregation without abnormal bleeding, and we immediately realized that it could have very important implications for the treatment of cardiovascular disease," said Shafi Kuchay, a graduate student in pharmacology and first author of the paper.
When clots form, small blood cells called platelets begin to clump together. Aspirin and other anti-clotting agents reduce the risk of heart attack and stroke by blocking the biochemical pathway that causes platelets to become sticky. But all these drugs put patients at risk of excessive bleeding.
The UIC researchers made a mouse model that lacked a gene for a protease enzyme most commonly found in blood cells called calpain-1, in order to determine its function. They found that mice lacking calpain-1 had reduced platelet aggregation but did not have any abnormal bleeding.
The mice lacking calpain-1 (called "knockout" mice) had increased levels of another enzyme, known as protein tyrosine phosphatase-1B. When the mice were given a PTP1B inhibitor, the reduced platelet aggregation was restored. When calpain-1 knockout mice and mice lacking PTP1B were crossed to create double-knockout mice, platelet aggregation was restored in the offspring that lacked the genes for both enzymes. The researchers were thus able to establish that PTP1B turns off the signal for platelet aggregation and that calpain-1 regulated the amount and activity of this "off switch."
"Because of the danger of excessive bleeding, people taking anti-clotting medications are monitored carefully and warned not to exceed their recommended doses," said Dr. Athar Chishti of the UIC Cancer Center, and senior author of the study. "Our research unveils a new molecular target for anti-platelet drugs, which may avoid the dangerous side-effects of the current drugs."
In a secondary, serendipitous finding, the fact that the calpain-1 knockout mice have elevated PTP1B levels may prove important to research into diabetes and obesity.
"Mice that lack the gene for PTP1B have been known for some time to display increased insulin sensitivity and resistance to diet-induced obesity," said Chishti, who is professor of pharmacology. He noted that PTP1B inhibition has already been identified as a therapeutic goal by many researchers in diabetes and obesity.
"Our calpain-1 knockout mice with their elevated PTP1B levels offer a good model system for testing the potency of novel PTP1B inhibitors," he said.