Activation of human platelet protein kinase C-ß₂ in vivo in response to acute hyperglycemia

 

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Summary

Protein kinase C (PKC) is known to be activated in experimental model systems by elevated glucose and may play an important role in the pathogenesis of diabetic complications. Since there is no information about its role in humans in vivo we investigated the activation of PKC in human thrombocytes during infusion of glucose and insulin in normal controls and in 19 NIDDM patients by determining membrane and cytosol levels of PKCß₂ using immune blots.

In the 27 subjects investigated (8 controls, 19 NIDDM) membrane-associated levels of PKCß₂ increased significantly after 60 and 150 min (p < 0.005). In controls an increase of membrane and of cytosolic PKCß₂ occurred upon elevation of glucose by 5.5 mmol/L or more and the membrane association persisted for at least 60 min. In NIDDM glucose was elevated by 7.5–10 mmol/L during infusions. Increases of both membrane and cytosolic PKCß₂ (<20%–300%) occurred in 10 NIDDM patients suggesting that both, translocation and increased synthesis of PKCß₂ were stimulated by glucose. Nine other patients showed no alteration (i.e. <20%) of PKCß₂. The 2 groups were similar regarding parameters of diabetes control, baseline glucose and glucose elevation during the test. However, the PKCß₂ responsive group had lower levels of serum triglycerides (1.39 ± 0.19 vs. 2.32 ± 0.34 g/L; p = 0.038). To assess whether absolute levels of PKC were altered in human diabetes, platelet levels of PKC alpha, ßt and ß₂ were determined in 22 controls and 25 NIDDM subjects with poorly controlled diabetes (HbA_1c = 9.8 ± 0.36%). Cytosolic levels of PKC alpha were signifcantly decreased by 27% compared to controls in NIDDM but there was no change of PKCßt or PKCß₂.

We conclude that 1. acute elevation of glucose by 5.5 mmol/L or more can activate PKCß₂ translocation in controls and NIDDM patients in vivo irrespective of parameters of metabolic control. 2. NIDDM patients differ in their PKCß₂-responses to glucose and 3. poor metabolic control leads to moderate downregulation of PKC alpha suggesting continued activation.