Abstract 403: Mitochondrial Complex II is a Source of the Reserve Respiratory Capacity that is Regulated by Metabolic Sensors via Sirtuin 3

Abstract

A cell’s survivability depends on its ability to meet its energy requirements. We hypothesized that the cells’ mitochondrial reserve respiratory capacity (RRC) is a critical component of its bioenergetics that can be utilized during an increase in energy demand, thereby, enhancing viability. Our goal was to identify the elements that regulate and contribute to the development of RRC and its involvement in cell survival. Our results show that development of RRC is dependent on metabolic substrate availability in a cell type-dependent manner. While the neonatal rat cardiac myocytes (NRCM) utilize glucose as the main substrate, developing a RRC [1.4-2.5 fold higher than basal oxygen consumption rate (OCR)] required fatty acids in addition to glucose. Accordingly, inhibition of either glucose or fatty acid oxidation separately, completely abrogated RRC, while having little impact on basal OCR, which is sustainable with either substrate or glutamate in the medium. Conversely, RRC was enhanced (1.4-1.8 fold) through increasing glucose oxidation via inhibiting pyruvate dehydrogenase kinase with dichloroacetate, or through increasing fatty acid oxidation via activation of AMP-activated kinase (AMPK). The latter was partly mediated through peroxisome proliferator-activated receptor alpha. These results suggested that RRC is an independently regulated entity of the cells’ bioenergetics. An electron flow activity assay revealed that the increase in RRC correlated with a specific increase in complex II (cII) activity. Inhibiting or disassembling holo cII completely abolished RRC, accompanied by a slight decrease in basal OCR (0.82-0.9 fold), thus confirming it as the source of RRC. Moreover, the development of RRC required Sirtuin (Sirt)3. Functionally, we show that enhancing RRC via fatty acid oxidation with 5-Aminoimidazole-4-carboxamide 1-β-Dribofuranoside in NRCM results in a burst of cII-dependent oxidative phosphorylation accompanied by reduced superoxide production and enhanced cell survival post-energy deprivation conditions. Thus, for the first time, we show that metabolic sensors increase the cells’ RRC via activating cII in a Sirt3-dependent manner, and that this mechanism can be exploited for increasing cell survival after hypoxia.