ENGINEERING OF GLYCEROL-STIMULATED INSULIN-SECRETION IN ISLET BETA-CELLS - DIFFERENTIAL METABOLIC FATES OF GLUCOSE AND GLYCEROL PROVIDE INSIGHT INTO MECHANISMS OF STIMULUS-SECRETION COUPLING

Insulin secretion from beta cells in the islets of Langerhans can be s timulated by a number of metabolic fuels, including glucose and glycer aldehyde, and is thought to be mediated by metabolism of the secretago gues and an attendant increase in the ATP:ADP ratio. Curiously, glycer ol fails to stimulate insulin secretion, even though it has been repor ted that islets contain abundant glycerol kinase activity and oxidize glycerol efficiently, We have reinvestigated this point and find that rat islets and the well differentiated insulinoma cell line INS-1 cont ain negligible glycerol kinase activity. A recombinant adenovirus cont aining the bacterial glycerol kinase gene (AdCMV-GlpK) was constructed and used to express the enzyme in islets and INS-1 cells, resulting i n insulin secretion in response to glycerol. In AdCMV-GlpK-treated INS -1 cells a greater proportion of glycerol is converted to lactate and a lesser proportion is oxidized compared with glucose. The two fuels a re equally potent as insulin secretagogues, despite the fact that oxid ation of glycerol at its maximally effective dose (2-5 mM) occurs at a rate that is similar to the rate of glucose oxidation at its basal, n onstimulatory concentration (3 mM). We also investigated the possibili ty that glycerol may signal via expansion of the glycerol phosphate po ol to allow enhanced fatty acid esterification and formation of comple x lipids. Addition of Triacsin-C, an inhibitor of long-chain acyl-CoA synthetase, to AdCMV-GlpK-treated INS-1 cells did not inhibit glycerol -stimulated insulin secretion despite the fact that it blocked glycero l incorporation into cellular lipids. We conclude from these studies t hat glycerol kinase expression is sufficient to activate glycerol sign aling in beta cells, showing that the failure of normal islets to resp ond to this substrate is due to a lack of this enzyme activity. Furthe r, our studies show that glycerol signaling is not Linked to esterific ation or oxidation of the substrate, but is likely mediated by its met abolism in the glycerol phosphate shuttle and/or the distal portion of the glycolytic pathway, either of which can lead to production of ATP and an increased ATP:ADP ratio.