Accumulation of nonesterified fatty acids causes the sustained energetic deficit in kidney proximal tubules after hypoxia-reoxygenation

Abstract

Kidney proximal tubules exhibit decreased ATP and reduced, but not absent, mitochondrial membrane potential (Deltapsi(m)) during reoxygenation after severe hypoxia. This energetic deficit, which plays a pivotal role in overall cellular recovery, cannot be explained by loss of mitochondrial membrane integrity, decreased electron transport, or compromised F1F0-ATPase and adenine nucleotide translocase activities. Addition of oleate to permeabilized tubules produced concentration-dependent decreases of Deltapsi(m) measured by safranin O uptake (threshold for oleate = 0.25 microM, 1.6 nmol/mg protein; maximal effect = 4 microM, 26 nmol/mg) that were reversed by delipidated BSA (dBSA). Cell nonesterified fatty acid (NEFA) levels increased from <1 to 17.4 nmol/mg protein during 60- min hypoxia and remained elevated at 7.6 nmol/mg after 60 min reoxygenation, at which time ATP had recovered to only 10% of control values. Safranin O uptake in reoxygenated tubules, which was decreased 85% after 60-min hypoxia, was normalized by dBSA, which improved ATP synthesis as well. dBSA also almost completely normalized Deltapsi(m) when the duration of hypoxia was increased to 120 min. In intact tubules, the protective substrate combination of alpha-ketoglutarate + malate (alpha-KG/MAL) increased ATP three- to fourfold, limited NEFA accumulation during hypoxia by 50%, and lowered NEFA during reoxygenation. Notably, dBSA also improved ATP recovery when added to intact tubules during reoxygenation and was additive to the effect of alpha-KG/MAL. We conclude that NEFA overload is the primary cause of energetic failure of reoxygenated proximal tubules and lowering NEFA substantially contributes to the benefit from supplementation with alpha-KG/MAL.

Regulation of the mitochondrial permeability transition in kidney proximal tubules and its alteration during hypoxia-reoxygenation

“…There is little information on expression of the MPT and its regulation by endogenous metabolites in kidney proximal tubules, where mitochondria are especially critical for the cell energy supply given that glycolysis is limited or absent, depending on the segment, which will increase their susceptibility to ischemia-induced ATP depletion (76). Recent studies using freshly isolated proximal tubules and approaches optimized to dynamically follow changes of mitochondrial energetics in both intact and acutely permeabilized tubules have defined persistent partial mitochondrial deenergization mediated in a nondisruptive fashion by accumulated NEFA as the mechanism for the energetic failure that is a major early determinant of cellular recovery and survival after hypoxic insults relevant to understanding ischemia in vivo (20,22,24). NEFA and deenergization are promoters of PTP opening in isolated mitochondria (6, 32, 18a, 36, 39 -41).…”

Illuminating Mitochondrial Function and Dysfunction using Multiphoton Technology

“…⌬⌿ m is the larger of the two components of proton motive force under both normal and pathologic conditions 9 ; therefore, it serves as a valuable index of the state of the entire system, as shown in recent studies documenting a major role for nonesterified fatty acids in the persistent mitochondrial dysfunction seen in re-oxygenated proximal tubules. 9,10 As discussed by Hall et al, 1 when ⌬⌿ m is low and ATP is available, the F 1 F O -ATPase reverses the flow of protons by hydrolyzing ATP to extrude protons and restore ⌬⌿ m .…”

Glycemic Control and Critical Illness

“…Findings from a wide variety of models of AKI, including ischemia/reperfusion, cisplatin, endotoxemia, glomerulonephritis, and ureteral obstruction, provide support for a direct contribution of hyperglycemia and insulin resistance to renal injury. 21,29,30 Accumulation of nonesterified fatty acids seems to be a major factor contributing to mitochondrial dysfunction, influencing cellular recovery in renal tubular cells. 29,31 These events operate through a variety of molecular and signaling pathways.…”

“…21,29,30 Accumulation of nonesterified fatty acids seems to be a major factor contributing to mitochondrial dysfunction, influencing cellular recovery in renal tubular cells. 29,31 These events operate through a variety of molecular and signaling pathways. 8,28,29 On the basis of these AKI models, it seems that the same pathways may influence critically ill patients, 8 although experimental events as described here seem accelerated in contrast to the longer duration required for chronic insulin resistance to produce AKI (Figure 2).…”

“…29,31 These events operate through a variety of molecular and signaling pathways. 8,28,29 On the basis of these AKI models, it seems that the same pathways may influence critically ill patients, 8 although experimental events as described here seem accelerated in contrast to the longer duration required for chronic insulin resistance to produce AKI (Figure 2). A likely scenario is that stress-related hyperglycemia induces compensatory responses in the kidney with resulting glycosuria.…”