Platelet glucose metabolism-impact of diabetes

Abstract

Glucose metabolism is crucial for providing platelets with adequate ATP to do their functions properly. Little is known about platelet glucose uptake and the role of the first rate-limiting glycolytic enzyme, hexokinase (HK), in the regulation of platelet activation. Glucose uptake in response to platelet agonists, thrombin and convulxin (CVX), increased in a dose and time-dependent manner. Focusing on signalling downstream of glycoprotein VI (GPVI), it was found that inhibition of signalling enzymes SFKs, Syk, or PI3K linked to this receptor significantly decreased platelet glucose uptake, combined with reduced P-selectin expression. Glucose uptake was found to be independent to integrin activation, but interestingly required ADP signalling and, to a lesser extent, TxA2 signalling. Using immunoblotting and immunoprecipitation, we found that HK I and II are expressed in human and murine platelets, with HK I equally localised in cytosol and mitochondria and HK II was exclusively found in membrane fraction. Blocking HK activity with three different HK inhibitors revealed that HK plays an important role in glucose transport into platelets upon platelet activation with CVX. Functional studies have shown that HK may play a role in GPVI-mediated platelet aggregation and activation, αIIbβ3, and P-selectin expression. Platelet activation with CVX caused an increase in HK activity, which required SFKs, Syk, and PI3K/Akt pathways. These data suggest that HK plays a key, but poorly defined, role in GPVI-mediated platelet activation. Next, the effect of diabetes on platelet glucose uptake was examined. Type1 diabetes mellitus was induced in mice using Streptozotocin (STZ) injection. C57BL/6J male mice aged 8-12 weeks received STZ intraperitoneally (50 mg/kg body weight;3 days). After confirming diabetes, glucose uptake studies demonstrated that glucose uptake was basally high in diabetic mice compared to vehicle control. Further, this increase in glucose uptake was also associated with increased mitochondrial membrane potential in diabetic mice compared to vehicle group (hyperpolarisation). Correlation analysis revealed a strong positive association between fasting blood glucose levels and platelet glucose uptake upon activation in STZ mice. These data suggest that glucose metabolism is altered in diabetes. In conclusion, these studies suggest that platelet activation regulates platelet glucose metabolism by regulating GLUT3 expression and HK activity.