Introduction
Aerobic cellular respiration refers to the process by which organisms
convert nutrients into chemical energy, adenosine triphosphate (ATP),
using oxygen, which is dominated in highly aerobic tissues, such as
cardiac and neural systems (Hall,
Klein-Flugge, Howarth & Attwell, 2012;
Hickey et al., 2009;
Neubauer, 2007). During glucose aerobic
metabolism, glucose with 6-carbon is broken down into 3-carbon pyruvate
by multiple key enzymes such as hexokinase-I/II (HK-I/II),
phosphofructokinase-1 (PFK-1), and pyruvate kinase (PK) in the
cytoplasm. Then pyruvate is transported into the mitochondria through
the mitochondrial pyruvate carrier (MPC)
(Bricker et al., 2012) and metabolized to
synthesize acetyl-coenzyme A (CoA) by pyruvate dehydrogenase (PDH)
(Sugden & Holness, 2003). After the
metabolism of tricarboxylic acid (TCA) cycle, nicotinamide adenine
dinucleotide (NADH) and flavin adenine dinucleotide (FADH2) from
acetyl-CoA can pass their electrons into the mitochondrial electron
transport chain (ETC) to produce ATP molecules, with the help of oxygen
(Fernie, Carrari & Sweetlove, 2004).
During this process, respiratory chain protein complexes I-V embedded in
the inner mitochondrial membrane is involved in ATP production
(Wallace, Fan & Procaccio, 2010).
Abnormal energy metabolism is a critical target during the progression
of disease, including myocardial ischemia, heart failure
(Fillmore & Lopaschuk, 2013), stoke
(Narne, Pandey & Phanithi, 2017), or
neurodegenerative diseases (Butterfield &
Halliwell, 2019). During the exposure to hypoxia, cells display
decreased carbon flux into the TCA cycle and diminished electron flux
through ETC (Papandreou, Cairns, Fontana,
Lim & Denko, 2006; Wheaton & Chandel,
2011). Moreover, oxygen-glucose deprivation (OGD) can cause
mitochondrial damage and ATP depletion in cardiomyocytes and neurons
(Almeida, Delgado-Esteban, Bolanos &
Medina, 2002; Kalogeris, Baines, Krenz &
Korthuis, 2012). Therefore, the
enhancements of glucose oxidation and mitochondrial bioenergy have been
potential strategies against various disorders, such as ischemic
conditions.
Oxidized nicotinamide adenine
dinucleotide (NAD+) is a well-known small molecule
that drives energy metabolism through its electron transfer function
(Hopp, Gruter & Hottiger, 2019).
Increased NAD+ level promotes the activity of sirtuin
1 (SIRT1), a strong deacetylase of class III histone deacetylase, which
heavily implicates in a number of physical and pathological processes,
such as aging-related diseases (Imai &
Guarente, 2014). SIRT1 deacetylates its substrate, peroxisome
proliferator initiated receptor gamma and coactivator 1 alpha (PGC-1α),
to regulate mitochondrial biosynthesis and energy metabolism
(Aquilano, Baldelli, Pagliei & Ciriolo,
2013). PGC-1α can bind and co-activate different transcription factors,
such as nuclear respiratory factor-1 (Nrf-1) and Nrf-2, in the promoter
of mitochondrial transcription factor A (TFAM), which increases the
transcription of key mitochondrial enzymes, such as cytochrome c oxidase
subunits (COXII, COXIV) and ATP synthase β subunit, and drives
mitochondrial DNA transcription and replication through the interaction
of TFAM (Irrcher, Adhihetty, Sheehan,
Joseph & Hood, 2003; Wu et al., 1999).
Previous studies have shown that the activation of SIRT1-PGC-1α pathway
plays a protective role in hypoxia or OGD-induced injuries
(Ma et al., 2017;
Zhou, Wang, Li, Yu & Zhao, 2017), which
improves mitochondrial function to prevent and treat ischemic heart or
brain disorders (Yang, Mukda & Chen,
2018; Zhao et al., 2019).
Ginsenosides are major active components of Panax ginseng C.A.
Meyer , a medicinal herb as new-resource health food approved by the
Ministry of Health, China. They are extensively used for diverse
beneficial effects on myocardial infarction, stroke, and
ischemia/reperfusion (I/R) injury
(Aravinthan et al., 2015;
Jia, Zhang, Huang & Leung, 2012;
Liu, Anderson, Fernandez & Dore, 2019).
Currently, about 250 ginsenosides have been isolated and identified,
which are mainly divided into three groups, protopanaxadiol (PPD, such
as Rb1, Rb2, Rc, Rd, Rg3), protopanaxatriol (PPT, such as Re, Rf, Rg1,
Rh1), and oleanolic acid (OA, such as Ro), based on their chemical
structures (Wu, Kwaku, Li, Yang, Ge & Xu,
2019). Proteomic analysis has shown that total ginsenosides (GS) can
increase the expression of proteins in the TCA cycle to enhance cardiac
energy metabolism in the ischemic rat
(Wang, Zhou, Yi, Jiang & Liu, 2012).
Meanwhile, ginsenoside Rb1, Rb3, Rg3, Rg1, Rg5, and compound K exert
cardioprotective and neuroprotective effects by increasing energy
metabolism (Chen et al., 2019b;
Xu, Ma, Fan, Chen, Zhang & Tang, 2019;
Yang et al., 2017), improving
mitochondrial dynamics and quality (Dong
et al., 2016; Sun et al., 2013), or
inhibiting mitochondrial apoptosis (Huang
et al., 2020; Li et al., 2018).
Importantly, several ginsenosides, such as Rb1, Rc, F1, Rg3, Rh2, and
their metabolites M1-M4 have been identified as SIRT1 activators
(Ma, Zhou & Yang, 2015;
Wang, Liang, Chen & Zhao, 2016;
Yang, Ma, Zhou, Xu & Zhang, 2017), which
play protective effects on a series of pathological injuries, such as
oxidative stress, inflammation, senescence, hypoxia, or OGD
(Cheng et al., 2019;
Song et al., 2014;
Wang et al., 2019). However, the
molecular mechanism underlying the effects of GS and each ginsenoside
monomer on mitochondrial aerobic metabolism and biosynthesis in
cardiomyocytes and neurons remains unclear and needs to be further
investigated.
In this study, we first investigated the effect of GS on mitochondrial
oxygen consumption rate (OCR) in a series of cell lines, including
aerobic respiration-dominated cardiomyocytes, neuronal cells, and
skeletal myoblasts (Di Lisa, Canton,
Menabo, Kaludercic & Bernardi, 2007;
Zeiger, Stankowski & McLaughlin, 2011),
as well as anaerobic respiration-dominated vascular endothelial,
epithelial, and stem cells (De Bock et
al., 2013; Tang et al., 2016). Then ATP
production and mitochondrial respiration capacity (MRC) were examined in
untreated and GS-treated cardiomyocytes and neurons. Targeted
metabolomics and Western blot analysis were performed to observe the
effect of GS on the contents of major metabolites and the levels of key
multiple enzymes from glycolysis and TCA cycle. Furthermore,
NAD+ level, SIRT1-PGC-1α pathway, and mitochondrial
biosynthesis in the cell, Drosophila melanogaster, and mice models were
further evaluated to explore the molecular mechanism of GS. In addition,
we observed the effect of GS on SIRT1-mediated mitochondrial function in
hypoxia- and OGD-induced cell models. Of note, the ginsenoside monomer
in GS for promoting SIRT1 activation and mitochondrial energy production
was preliminarily identified in cardiomyocytes. This is the first study
to explore the molecular mechanism by which GS promote mitochondrial
energy metabolism, which may provide new insights into the clinical
application of ginseng for the prevention and treatment of cardiac and
neurological disorders.