Oxaloacetate regulates complex II respiration in brown fat: dependence on UCP1 expression

Abstract

We previously found that skeletal muscle mitochondria incubated at low membrane potential (DW) or interscapular brown adipose tissue (IBAT) mitochondria, wherein DW is intrinsically low, accumulate oxaloacetate (OAA) in amounts sufficient to inhibit complex II respiration. We proposed a mechanism wherein low DW reduces reverse electron transport (RET) to complex I causing a low NADH/NAD ratio favoring malate conversion to OAA. To further assess the mechanism and its physiologic relevance, we carried out studies of mice with inherently different levels of IBAT mitochondrial inner membrane potential. Isolated complex II (succinate)-energized IBAT mitochondria from obesity-resistant 129SVE mice compared with obesity-prone C57BL/6J displayed greater UCP1 expression, similar O2 flux despite lower DW, similar OAA concentrations, and similar NADH/NAD . When GDP was added to inhibit UCP1, 129SVE IBAT mitochondria, despite their lower DW, exhibited much lower respiration, twofold greater OAA concentrations, much lower RET (as marked by ROS), and much lower NADH and NADH/NAD ratios compared with the C57BL/6J IBAT mitochondria. UCP1 knock-out abolished OAA accumulation by succinate-energized mitochondria associated with markedly greater DW, ROS, and NADH, but equal or greater O2 flux compared with WT mitochondria. GDP addition, compared with no GDP, increased DW and complex II respiration in wild-type (WT) mice associated with much less OAA. Respiration on complex I substrates followed the more classical dynamics of greater respiration at lower DW. These findings support the abovementioned mechanism for OAA- and DW-dependent complex II respiration and support its physiological relevance.