Glucagon-Like Peptide 1 Stimulates Lipolysis in Clonal Pancreatic β-Cells (HIT)

Abstract

Glucagon-like peptide 1 (GLP-1) is the most potent physiological incretin for insulin secretion from the pancreatic ␤-cell, but its mechanism of action has not been established. It interacts with specific cell-surface receptors, generates cAMP, and thereby activates protein kinase A (PKA). Many changes in pancreatic ␤-cell function have been attributed to PKA activation, but the contribution of each one to the secretory response is unknown. We show here for the first time that GLP-1 rapidly released free fatty acids (FFAs) from cellular stores, thereby lowering intracellular pH (pH i ) and stimulating FFA oxidation in clonal ␤-cells (HIT). Similar changes were observed with forskolin, suggesting that stimulation of lipolysis was a function of PKA activation in ␤-cells. G lucagon-like peptide 1 (GLP-1) is the most potent potentiator of glucose-induced insulin secretion that has been described (1,2). This peptide causes the elevation of cAMP and the activation of protein kinase A (PKA) (3); however, it releases insulin only in the presence of stimulatory glucose (4) and thus serves as an incretin rather than a secretagogue (5). Activation of PKA leads to phosphorylation of multiple ␤-cell proteins, many of which have been hypothesized to play a role in insulin secretion (6-8). The nature of the endogenous substrates for PKA that may potentiate insulin secretion is unknown. Because the islet contains large stores of triglycerides (9), particularly in diabetes (10), another possible role of the normal rise in cAMP could be to stimulate lipolysis (via lipase activation), thereby providing the cell with free fatty acids (FFAs). Recent research on hormone-sensitive lipase (HSL) in ␤-cells yielded results consistent with that notion (11). The released FFAs may directly effect secretion, or they may do so indirectly via generation of other lipids, including the putative long-chain acyl CoA (LC-CoA) signal, diacylglycerol (DAG), and phosphatidic acid (PA) (12). The acute addition of exogenous FFAs is also known to enhance glucose-stimulated secretion (9,13-15).We have shown in previous studies that added FFAs cause acidification in ␤-cells (16) and fat cells (17) as a consequence of the flip-flop mechanism of diffusion across the plasma membrane (16,17). Furthermore, we have shown in adipocytes that the elevation of cAMP stimulates lipolysis, with a resulting decrease in the intracellular pH (pH i ) caused by the release of FFAs, which become partially ionized (17). Therefore, in this study we assessed whether GLP-1 has a similar effect on lipolysis in ␤-cells. Our data show a decrease in pH i and a release of FFAs by agents that increase cAMP, presumably via activation of HSL. Furthermore, the incretin effect was largely diminished by a lipase inhibitor, whereas glucose-stimulated secretion was less affected. These findings indicate that cAMP-mediated lipolysis may play an important role in ␤-cell signal transduction and the incretin effect of GLP-1. RESEARCH DESIGN AND METHODSGrowth and incubation of cells. Clonal pancr...

Beta cell compensation for insulin resistance in Zucker fatty rats: increased lipolysis and fatty acid signalling

“…While GLP-1 and forskolin, both activators of PKA, have been shown to increase lipolysis in clonal pancreatic beta cells (HIT cells) [37], neither agent enhanced lipolysis in ZF or ZL islets. This finding is in agreement with our study of Hsl wildtype and knockout mice [18].…”

Mechanisms of action of glucagon-like peptide 1 in the pancreas

“…GLP-1 has been shown to stimulate fatty acid synthesis from triglyceride stores in both clonal β cell lines and in rat islets. Yaney and colleagues showed that acute treatment of the HIT cell line with GLP-1 (10 nM for 1 hr) increased formation of FFA via a PKAdependent activation of hormone sensitive lipase which led to the rapid breakdown of internal stores of triglycerides (Yaney et al, 2001). Treatment with the acyl synthetase (acyl CoA) inhibitor Triacsin C (which will inhibit FFA conversion to long chain (LC)-acyl CoA) increased the efflux of FFA in response to GLP-1.…”

The Neuronal Ca2+ Sensor Protein Visinin-like Protein-1 Is Expressed in Pancreatic Islets and Regulates Insulin Secretion

“…These include increasing voltage-sensitive Ca 2ϩ channel opening (48) and Ca 2ϩ influx (49), activating ryanodine receptors in the endoplasmic reticulum (50), stimulating lipolysis (51), and directly acting on the exocytotic machinery (35). Most of these actions are mediated by the protein PKA pathway, but the direct effect on exocytosis is partly PKA-independent (34), being mediated by the cAMP-binding protein cAMP-regulated guanine nucleotide exchange factor II (53).…”