Acetoacetate and β-hydroxybutyrate in combination with other metabolites release insulin from INS-1 cells and provide clues about pathways in insulin secretion

Abstract

Mitochondrial anaplerosis is important for insulin secretion, but only some of the products of anaplerosis are known. We discovered novel effects of mitochondrial metabolites on insulin release in INS-1 832/13 cells that suggested pathways to some of these products. Acetoacetate, beta-hydroxybutyrate, alpha-ketoisocaproate (KIC), and monomethyl succinate (MMS) alone did not stimulate insulin release. Lactate released very little insulin. When acetoacetate, beta-hydroxybutyrate, or KIC were combined with MMS, or either ketone body was combined with lactate, insulin release was stimulated 10-fold to 20-fold the controls (almost as much as with glucose). Pyruvate was a potent stimulus of insulin release. In rat pancreatic islets, beta-hydroxybutyrate potentiated MMS- and glucose-induced insulin release. The pathways of their metabolism suggest that, in addition to producing ATP, the ketone bodies and KIC supply the acetate component and MMS supplies the oxaloacetate component of citrate. In line with this, citrate was increased by beta-hydroxybutyrate plus MMS in INS-1 cells and by beta-hydroxybutyrate plus succinate in mitochondria. The two ketone bodies and KIC can also be metabolized to acetoacetyl-CoA and acetyl-CoA, which are precursors of other short-chain acyl-CoAs (SC-CoAs). Measurements of SC-CoAs by LC-MS/MS in INS-1 cells confirmed that KIC, beta-hydroxybutyrate, glucose, and pyruvate increased the levels of acetyl-CoA, acetoacetyl-CoA, succinyl-CoA, hydroxymethylglutaryl-CoA, and malonyl-CoA. MMS increased incorporation of (14)C from beta-hydroxybutyrate into citrate, acid-precipitable material, and lipids, suggesting that the two molecules complement one another to increase anaplerosis. The results suggest that, besides citrate, some of the products of anaplerosis are SC-CoAs, which may be precursors of molecules involved in insulin secretion.

The Pancreatic β-Cell: A Bioenergetic Perspective

“…These rates are comparable to those for the utilization of 2.8 mM glucose by these cells (332). It should be noted that INS-1 832/13 cells can produce acetoacetylCoA and utilize ␤-hydroxybutyrate and acetoacetate (318,320), raising the possibility that ketone bodies could be produced by ␤-oxidation, in which case label from 14 Clabeled palmitate would not be liberated stoichiometrically as 14 CO 2 .…”

“…The amino acid can be trans-deaminated to ␣-ketoisocaproate (␣KIC) which can ultimately be metabolized to AcCoA and acetoacetate and is thus a potential bioenergetic substrate, in addition to ability to activate glutamate dehydrogenase. ␣KIC is an effective secretagogue in combination with methyl succinate (318,321), although it has been reported to be less effective than glucose in increasing NAD(P)H reduction or hyperpolarizing the mitochondria (400).…”

Medium-Chain Triacylglycerol Suppresses the Decrease of Plasma Albumin Level through the Insulin-Akt-mTOR Pathway in the Livers of Malnourished Rats

“…Medium-chain fatty acids, generated by MCT hydrolysis, are metabolized rapidly in mitochondria and yield a large amount of acyl-CoA, which is converted to ketone bodies and released to the blood (30). MacDonald et al reported that b-hydroxybutyrate potentiated glucose-induced insulin release in fresh rat pancreatic islets (31). In the present study, the plasma glucose level might be higher than for fasting rats because the rats were killed without food deprivation.…”

Plasma Membrane Potential Oscillations in Insulin Secreting Ins-1 832/13 Cells Do Not Require Glycolysis and Are Not Initiated by Fluctuations in Mitochondrial Bioenergetics

“…Pyruvate has long been recognized to be an effective secretagogue in insulinoma cells possessing increased monocarboxylate transporter levels (17,45,46). As well as pyruvate, the cellpermeant methyl succinate (37,45), dihydroxy acetone (17), ␤-hydroxybutyrate (47), acetoacetate (47), ␣-ketoisocaproate (46), and glycerol (17), separately or in combination, are effective secretagogues.…”