STUDY OF THE STILL ENIGMATIC ROLE OF PYRUVATE DEHYDROGENASE KINASE TYPE 4 (PDK4) IN THE CONTROL OF GLUCAGON SECRETION BY ALPHA CELLS

Details

Supervisors

Gilon, Patrick ; Butaye, François

Faculty

Faculté de pharmacie et des sciences biomédicales

Degree label

Master [120] en sciences biomédicales, à finalité approfondie

Abstract

enThe islets of Langerhans, the endocrine part of the pancreas, play a key role in glucose homeostasis. They are composed of three main cell types: α, β and δ cells. Glucagon secreted by α-cells is the principal hyperglycemic hormone of the body. Two defects of glucagon secretion occur in diabetes: an impaired glucagon response to hypoglycemia and a relative chronic hyperglucagonemia. The causes of these defects are unknown. However, the mechanisms responsible for the control of glucagon secretion remain unclear. Pyruvate dehydrogenase kinases (PDKs) are enzymes that phosphorylate and inhibit pyruvate dehydrogenase (PDH), thereby reducing PDH activity and limiting the entry of pyruvate into the Krebs cycle. Four isoforms of PDKs have been identified (PDK1–4). Diabetes, insulin resistance, and fasting have been shown to increase the expression of PDK4 expression in various tissues. PDK4 plays a central role in metabolic flexibility allowing cells to adapt their metabolism according to the available substrate (such as glucose or fatty acids). PDK4 is strongly expressed in α-cells, but its role is unknown. The study of PDK4 within α-cells by the host laboratory showed that in α cell specific PDK4 knockout (αPDK4KO ) mice, there is an increase in glucagon secretion in response to a low glucose concentration. Based on these findings, the present study investigates the enigmatic role of α cell’s PDK4 in glucagon secretion. Using murine αPDK4KO and wild type (WT) we (1) assessed the impact of PDK4 inhibition on glucagon release, (2) determined whether the modulation of glucagon secretion in αPDK4KO mice originates from β-oxidation or glycolysis, (3) quantified ATP/ADP dynamics in response to glucose and fatty acids, (4) examined liver α cell cross talk under control diet with transcriptomic profiling of gluconeogenic, β oxidative and lipogenic genes. Our data confirms a significant increase in glucagon secretion at low glucose levels in αPDK4KO mice compared to WT mice; acute exposure to palmitate significantly increased glucagon secretion in αPDK4WT islets. Transitioning from 1 mM to 10 mM glucose significantly decreased intracellular ATP/ADP ratio in α-cells of αPDK4KO mice, and a strong inhibition of ATP synthesis with 2 mM sodium azide significantly reduced glucagon secretion in WT mice, confirming the ATP dependence of its release. While the PDK inhibitor DCA increased glucagon output, other pharmacologic inhibitors of PDK did not. Overall, we suggest that the absence of PDK4 forces α-cell metabolism to rely on β-oxidation.