Limited Mitochondrial Permeabilization Is an Early Manifestation of Palmitate-induced Lipotoxicity in Pancreatic β-Cells

Abstract

Involvement of the mitochondrial permeability transition (MPT) pore in early stages of lipotoxic stress in the pancreatic ␤-cell lines MIN6 and INS-1 was the focus of this study. Both long term (indirect) and acute (direct) effects of fatty acid (FA) application on ␤-cell susceptibility to Ca 2؉ -induced MPT induction were examined using both permeabilized and intact ␤-cells. Long term exposure to moderate (i.e. below cytotoxic) levels of the saturated FA palmitate sensitized ␤-cell mitochondria to MPT induced by Ca 2؉ . Long term exposure to palmitate was significantly a more efficient inducer of MPT than the unsaturated FA oleate, although upon acute application both caused similar MPT activation. Application of antioxidants, inhibitors of the ceramide pathway, or modifiers of membrane fluidity did not protect ␤-cell mitochondria from FA exposure. However, significant protection was provided by co-application of the unsaturated FA oleate in a phosphatidylinositol 3-kinase-dependent manner. Characterization of MPT pore opening in response to moderate palmitate treatment revealed the opening of a unique form of MPT in ␤-cells as it encompassed features of both low and high conductance MPT states. Specifically, this MPT showed solute selectivity, characteristic of a low conductance MPT; however, it affected mitochondrial respiration and membrane potential in a way typical of a high conductance MPT. Activation of the full-size/high conductance form of MPT required application of high levels of FA that reduced growth and initiated apoptosis. These findings suggest that in the ␤-cell, MPTs can act as both initiators of cell death and as versatile modulators of cell metabolism, depending on the mode of the MPT pore induced. Mitochondrial permeability transition (MPT)2 is the calcium (Ca 2ϩ )-dependent opening of a nonspecific pore in the mitochondrial inner membrane in response to a variety of inducers and co-inducers, including inorganic phosphate (P i ), thiol oxidants, fatty acids (FA), and others. Opening of the "full-size" or "high conductance" MPT causes equilibration across the inner membrane of ions and all solutes up to 1.5 kDa, which induces mitochondrial swelling and the release of mitochondrial proteins capable of apoptosis activation (1-5). MPT can also operate in a more "limited size" or "low conductance" mode, which allows passage of only small ions across the inner mitochondrial membrane (6, 7). Because opening of the low conductance MPT does not allow passage of large molecules, but does allow the passage of small ions, mitochondrial ion gradients and energy production are affected. As such, it is thought that the low conductance MPT functions not to induce cell death but to aid in the fine regulation of cell metabolism (6). Extensive evidence documents the high conductance MPT as a cause of generalized mitochondrial dysfunction and cell death, and an important mechanism in disease pathologies such as hepatotoxicity (8), cardiac ischemia/reperfusion (9, 10), and neuronal injury (10, 11). In contrast,...

Coordinate Changes in Histone Modifications, mRNA Levels, and Metabolite Profiles in Clonal INS-1 832/13 β-Cells Accompany Functional Adaptations to Lipotoxicity

“…At the same time, the INS-1 832/13 ␤-cells adapted to the exposure to palmitate by increasing their capacity to oxidize lipids. There is some disagreement with previous studies, which have shown increased respiration upon prolonged palmitate treatment (19,58). It should be kept in mind that we examined glucose-induced, not lipid-induced, respiration.…”

Increased Lipogenesis and Stearate Accelerate Vascular Calcification in Calcifying Vascular Cells

“…The data suggest that changes in stearate-CoA or its metabolite levels control mineralization and osteogenic differentiation in vascular cells, through a mechanism yet to be determined. In a number of previous studies using cell culture systems, saturated fatty acids have been shown to have lipotoxic effects via several cell signaling pathways including 1) endoplasmic reticulum stress (36,37), 2) Toll-like receptor activation (38 -41), 3) an increase in reactive oxygen species (42, 43), 4) de novo ceramide synthesis (44 -46), 5) modification of mitochondrial structure and function (44,47), and 6) suppression of anti-apoptotic components (46,48). In addition, unsaturated fatty acid supplementation has been known to block the lipotoxic effects of saturated fatty acids.…”

Fatty Acids Suppress Autophagic Turnover in β-Cells

“…5b) (as previously shown in Refs. 23,32,33). The combination of depolarization and reduced oxygen consumption suggested a respiratory chain defect.…”