A distinct difference in the metabolic stimulus–response coupling pathways for regulating proinsulin biosynthesis and insulin secretion that lies at the level of a requirement for fatty acyl moieties

Abstract

The regulation of proinsulin biosynthesis in pancreatic beta-cells is vital for maintaining optimal insulin stores for glucose-induced insulin release. The majority of nutrient fuels that induce insulin release also stimulate proinsulin biosynthesis, but since insulin exocytosis and proinsulin synthesis involve different cellular mechanisms, a point of divergence in the respective metabolic stimulus-response coupling pathways must exist. A parallel examination of the metabolic regulation of proinsulin biosynthesis and insulin secretion was undertaken in the same beta-cells. In MIN6 cells, glucose-induced proinsulin biosynthesis and insulin release shared a requirement for glycolysis to generate stimulus-coupling signals. Pyruvate stimulated both proinsulin synthesis (threshold 0.13-0.2 mM) and insulin release (threshold 0.2-0.3 mM) in MIN6 cells, which was eliminated by an inhibitor of pyruvate transport (1 mM alpha-cyano-4-hydroxycinnamate). A combination of alpha-oxoisohexanoate and glutamine also stimulated proinsulin biosynthesis and insulin release in MIN6 cells, which, together with the effect of pyruvate, indicated that anaplerosis was necessary for instigating secondary metabolic stimulus-coupling signals in the beta-cell. A consequence of increased anaplerosis in beta-cells is a marked increase in malonyl-CoA, which in turn inhibits beta-oxidation and elevates cytosolic fatty acyl-CoA levels. In the beta-cell, long-chain fatty acyl moieties have been strongly implicated as metabolic stimulus-coupling signals for regulating insulin exocytosis. Indeed, it was found in MIN6 cells and isolated rat pancreatic islets that exogenous oleate, palmitate and 2-bromopalmitate all markedly potentiated glucose-induced insulin release. However, in the very same beta-cells, these fatty acids in contrast inhibited glucose-induced proinsulin biosynthesis. This implies that neither fatty acyl moieties nor beta-oxidation are required for the metabolic stimulus-response coupling pathway specific for proinsulin biosynthesis, and represent an early point of divergence of the two signalling pathways for metabolic regulation of proinsulin biosynthesis and insulin release. Therefore alternative metabolic stimulus-coupling factors for the specific control of proinsulin biosynthesis at the translational level were considered. One possibility examined was an increase in glycerophosphate shuttle activity and change in cytosolic redox state of the beta-cell, as reflected by changes in the ratio of alpha-glycerophosphate to dihydroxyacetone phosphate. Although 16.7 mM glucose produced a significant rise in the alpha-glycerophosphate/dihydroxyacetone phosphate ratio, 1 mM pyruvate did not. It follows that the cytosolic redox state and fatty acyl moieties are not necessarily involved as secondary metabolic stimulus-coupling factors for regulation of proinsulin biosynthesis. However, the results indicate that glycolysis and the subsequent increase in anaplerosis are indeed necessary for this signalling pathway, and therefore an extramitoc...

Mitochondrial regulation of insulin production in rat pancreatic islets

“…However, the kinetics of insulin release and of proinsulin biosynthesis, and the metabolic signals mediating these processes, are not identical [3,8]. In fact, the role of various glycolytic and TCA cycle intermediates in glucose-stimulated insulin secretion is controversial [7,[9][10][11][12][13][14][15][16], and even less is known about the metabolic signals for glucose-stimulated insulin production.…”

Chronic Exposure to Excess Nutrients Left-shifts the Concentration Dependence of Glucose-stimulated Insulin Secretion in Pancreatic β-Cells

“…Glucose and other secretagogues stimulate both insulin secretion and synthesis by enhancing the translation of insulin mRNA to proinsulin (26). In contrast, it is known that LC fatty acids can deplete insulin stores by promoting secretion of insulin while simultaneously inhibiting proinsulin synthesis (26,27). Additionally, multiple reports have documented the ability of excess lipid to inhibit insulin gene transcription both in vitro (28) and in vivo (29).…”

“…Normally, insulin synthesis and secretion are tightly coupled such that stores are maintained within a narrow range. Glucose and other secretagogues stimulate both insulin secretion and synthesis by enhancing the translation of insulin mRNA to proinsulin (26). In contrast, it is known that LC fatty acids can deplete insulin stores by promoting secretion of insulin while simultaneously inhibiting proinsulin synthesis (26,27).…”

Molecular mechanisms of glutamine action

“…Intracellular glutamine concentration varies between 2 and 20 mM (depending on cell type) whereas its extracellular concentration averages 0.7 mM (Newsholme et al, 2003b). Glutamine plays an essential role, promoting and maintaining function of various organs and cells such as kidney (Conjard et al, 2002), intestine (Lima et al, 1992;Ramos Lima et al, 2002), liver (de Souza et al, 2001), heart (Khogali et al, 2002), neurons (Mates et al, 2002), lymphocytes , macrophages , neutrophils Pithon-Curi et al, 2002a, 2003bPithon-Curi et al, 2003), pancreatic b-cells (Skelly et al, 1998), and white adipocytes (Curi et al, 1987;Kowalchuk et al, 1988). At the most basic level, glutamine serves as important fuel in these cells and tissues.…”