The Characterization of Mitochondrial Permeability Transition in Clonal Pancreatic β-Cells

Abstract

Mitochondrial permeability transition (MPT), which contributes substantially to the regulation of normal mitochondrial metabolism, also plays a crucial role in the initiation of cell death. It is known that MPT is regulated in a tissue-specific manner. The importance of MPT in the pancreatic ␤-cell is heightened by the fact that mitochondrial bioenergetics serve as the main glucose-sensing regulator and energy source for insulin secretion. In the present study, using MIN6 and INS-1 ␤-cells, we revealed that both Ca 2؉ -phosphate-and oxidant-induced MPT is remarkably different from other tissues. Ca 2؉ -phosphate-induced transition is accompanied by a decline in mitochondrial reactive oxygen species production related to a significant potential dependence of reactive oxygen species formation in ␤-cell mitochondria. Hydroperoxides, which are indirect MPT co-inducers active in liver and heart mitochondria, are inefficient in ␤-cell mitochondria, due to the low mitochondrial ability to metabolize them. Direct cross-linking of mitochondrial thiols in pancreatic ␤-cells induces the opening of a low conductance ion permeability of the mitochondrial membrane instead of the full scale MPT opening typical for liver mitochondria. Low conductance MPT is independent of both endogenous and exogenous Ca 2؉ , suggesting a novel type of nonclassical MPT in ␤-cells. It results in the conversion of electrical transmembrane potential into ⌬pH instead of a decrease in total protonmotive force, thus mitochondrial respiration remains in a controlled state. Both Ca 2؉ -and oxidant-induced MPTs are phosphate-dependent and, through the "phosphate flush" (associated with stimulation of insulin secretion), are expected to participate in the regulation in ␤-cell glucose-sensing and secretory activity.Mitochondrial permeability transition (MPT) 1 is a permeability increase of the inner mitochondrial membrane to solutes of molecular mass up to ϳ1500 Da, which is caused by an opening of specific nonselective proteinaceous pores in the inner mitochondrial membrane (1, 2). Increased permeability of the inner membrane is initiated by an increased level of intramitochondrial Ca 2ϩ and is regulated by multiple effectors, including inorganic phosphate, the redox state of pyridine nucleotides and thiols (oxidative stress), membrane potential, cyclosporin A, and other factors (3). MPT pores are thought to have at least two open conformations and a variable degree of reversibility. Most importantly, MPT in its different forms contributes to both normal cell physiology (regulation of oxidative phosphorylation and Ca 2ϩ metabolism) and to regulatory processes leading to apoptosis (2-4). Specific features of MPT show significant tissue-specific variability (5-8). The most significant and well documented deviation from classical MPT described in liver mitochondria was observed in brain tissue (9). Brain mitochondria demonstrate high resistance to MPT opening (10), low sensitivity to cyclosporin A (5), different responses to MPT modifiers, and a distinctive...

Free Fatty Acid–Induced Reduction in Glucose-Stimulated Insulin Secretion

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α-Lipoic acid regulates AMP-activated protein kinase and inhibits insulin secretion from beta cells

“…Oxidative phosphorylation was not affected by α-LA (200 μmol/l), but mitochondrial Ca 2+ handling, which is another important mitochondrial function, was clearly dependent on α-LA. Incremental addition of Ca 2+ to respiring mitochondria causes acceleration of oxygen consumption and membrane depolarisation due to Ca 2+ uptake into the matrix [25], both of which were more pronounced in the presence of α-LA (Fig. 7a,b,d).…”

“…Mitochondrial membrane potential (ΔΨ m ), oxygen consumption and reactive oxygen species (ROS) production Measurements were performed as previously described [25], except that α-LA was added to the cells prior to measurements. Oxygen consumption measurements were performed using a Clark-type electrode coupled to an Oxygraph unit (Hansatech, Pentney, UK).…”

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