With E. Dale Abel, MD, PhD, and Elena Christofides, MD
Metabolism of glucose plays a critical role in the production, activation, and clearance of platelets according to a recent study published in Cell Reports.1 These results suggest mechanisms by which people with diabetes are at increased risk of developing thrombosis. Patients with diabetes experience increased risk of thrombosis due to a variety of factors.
“Thrombosis occurs because either a platelet becomes activated because of events happening in the platelet or events happening in the blood vessel wall. In addition to the changes in glucose metabolism in platelets that we describe in this work, other changes in diabetes potentially might impact the platelet, such as increased inflammatory cytokines, increased levels of certain lipids and fatty acids,” E. Dale Abel, MD, PhD, professor of internal medicine and director of the Fraternal Order of Eagles Diabetes Research Center at the University of Iowa Carver College of Medicine in Iowa City, told EndocrineWeb.
Platelets are anucleated cellular fragments of megakaryocytes in the bone marrow that bud off and enter circulation. Under normal function, they stop bleeding by aggregating and clotting at vessel injuries.
Researchers used a mouse model to eliminate glucose transporters on platelets. Platelet counts were lower and platelets had shorter lifespans in mice in which the platelets’ transporters were eliminated.
Researchers knocked out glucose transporter (GLUT) 1 and GLUT3, in combination and alone, on platelets to assess differences in formation, metabolism, and clearance of platelets. The result was lower platelet counts and platelets that relied on mitochondrial metabolism of substrates such as glutamate, particularly in the combined knockouts.
This study showed that the decreased platelet count resulted from decreased production of platelets from megakaryocytes and from increased clearance of platelets before they reached normal lifespan. When researchers removed platelets from mice lacking both GLUT1 and GLUT3, megakaryocytes produced new platelets at a lower rate. Decreased platelet budding from megakaryocytes persisted when researchers examined the megakaryocytes in vitro.
Notably, recently budded, young platelets functioned normally in the absence of glucose but were destroyed and subsequently cleared from circulation much earlier than normal. Increased clearance of platelets resulted from an overload of calcium in the cytosol and from increased activation of a protein called calpain. Calpain is a protein involved in cellular lifecycles.
“When we engineered mice that had a complete inability to import glucose, the platelets, although slightly impaired, remained viable in the absence of any stressors because their mitochondria were able to take up other substrates from circulation to generate the energy they needed,” Dr. Abel said.
Next, researchers inhibited mitochondrial metabolism to determine the effects on platelets in mice lacking GLUT1 and GLUT3 on their platelets. Within 30 minutes of mitochondrial inhibition, platelet counts decreased to zero, indicating a reliance on mitochondrial metabolism in the absence of the ability to use glucose.
“The results were unexpected in certain ways,” said Dr. Abel. “If you look at the platelet, it has a relatively small number of mitochondria, suggesting platelets might derive most of their energy directly from glucose without the need for mitochondria. What our work showed was that both the uptake of glucose and its metabolism and the energy production of mitochondria play important but distinct roles. But everything wasn’t perfect because if we either inhibited the mitochondria or we looked very carefully, we could see that there were signs of stress in these platelets even though the mitochondria had tried to compensate for the inability to metabolize glucose.”
In people with diabetes, platelets use too much glucose and are hyperactive. Hyperactive platelets underlie many vascular problems that arise in diabetes. The potential to decrease platelets’ ability to use glucose could decrease platelet hyperactivity and provide therapeutic benefit.
These results support controlling glucose metabolism in diabetic patients. “It’s clear that glucose metabolism is critical for healthy platelets. It’s also quite clear that the substrate in which the platelets incubate impacts healthy platelets. Metabolic control for the diabetic does matter for healthy platelets,” Elena A. Christofides, MD, FACE, an endocrinologist at Endocrinology Associates, Inc., in Columbus, OH, told EndocrineWeb
These results showing glucose metabolism is critical for platelet activation harbor translational potential.
“We have some preliminary observations that if you take platelets and you culture them in high glucose or if you take platelets from diabetic animals, there is hyper-activation. The question then becomes, ‘If you reduce the amount of glucose these platelets take in, can you bring that level of activation down?’ We are particularly excited about the translational potential, more so on the GLUT3 single knock-out. When you knock out GLUT3 alone, you get some minor impairments of the platelet,” explained Dr. Abel. ”In the context of diabetes where the platelets are hyper-activated, particularly targeting GLUT3 might potentially have therapeutic benefit by turning down but not completely abolishing the glucose utilization in these cells.”
Indeed, the potential exists for therapeutics targeting GLUT3 to treat and/or prevent thrombosis. “I was really impressed by the data on the single knock-out mice for GLUT3 because it reduced thrombosis by 22% and it seemed to have no impact on the lifespan of the platelets or the production of healthy platelets. That struck me as a really interesting line of inquiry for human modeling,” said Dr. Christofides. “It seems like just isolating GLUT3 transport might be an intriguing way to get to disruption of platelet aggregation via a different path that might even be safer yet than what we currently have available.”
Grants from the National Institutes of Health supported this research.