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  • Tirapazamine We aimed to gain insight

    2019-07-24

    We aimed to gain insight into the consequences of a Ndufs4−/− on skeletal muscle bioenergetics and metabolism by combining biochemical strategies (respiratory chain enzyme assays) and multi-platform metabolic profiling (LC–MS/MS, GC–TOF–MS, and NMR analyses). Since the metabolic properties and subpopulations of muscle mitochondria depend on muscle fiber type composition [16], both glycolytic (white quadriceps; primarily type IIB myofibers) and oxidative (soleus; primarily type I myofibers) muscles were investigated. To the best of our knowledge, this is the first report of fiber type-specific enzymatic and metabolic data in Ndufs4−/− skeletal muscles. We report a metabolic signature consisting of 48 compounds in white quadriceps and 34 compounds in soleus muscles that distinguish whole-body Ndufs4 knockout mice from wild types. These metabolic alterations provide insight into skeletal muscle-specific changes in metabolic pathways that result from whole-body mitochondrial dysfunction. These pathways could possibly be targeted in therapeutic interventions to promote muscle health in mitochondrial disorders and improve locomotion, metabolism, and quality of life.
    Materials and methods
    Results and discussion
    Conclusions We report the first fiber type-specific enzymatic and metabolic data in Ndufs4−/− skeletal muscles. Enzyme assays revealed a severe reduction (80%) in CI activity in both glycolytic (white quadriceps) and oxidative (soleus) muscles from whole-body Ndufs4−/− mice. Such an extreme reduction in CI activity would greatly reduce the electron flux to the rest of the respiratory chain (RC). As an adaptive response in both tissues, our metabolic data suggest that alternative fuels and non-classical pathways serve to sustain the RC ubiquinol (QH2) pool, thereby restoring electron flux to CIII through the ubiquinone (Q) cycle. However, despite these metabolic adaptions in skeletal muscle, a disturbed redox balance still seems to result in congested mitochondrial pathways. Metabolic and bioenergetic disturbances due to CI deficiency seem to be more evident in glycolytic fibers, possibly due to their innately lower mitochondrial content. Some of the most significant metabolic alterations we observed in Ndufs4−/− muscles implicate the involvement of the glycerol-3-phosphate (GP) shuttle, the electron transfer flavoprotein (ETF/ETF-QO) system, complex II of the RC, and the proline Tirapazamine in electron delivery to the Q pool. Some of these mechanisms might also serve to restore NAD+ levels (GP shuttle and proline cycle), while others could contribute to redox imbalance (NAD+ consuming reactions fueling ETF/ETF-QO and the Krebs cycle) in CI deficiency. Altogether, the observed adaptive mechanisms could explain the apparent lack of an obvious muscle phenotype in Ndufs4−/− mice, as well as the reported ability of skeletal muscle to maintain normal ATP production, despite CI deficiency [9,29]. However, the hypotheses generated in this study need to be followed up/validated in more targeted metabolic studies on Ndufs4−/− mice. For now, it remains unclear whether these adaptive responses are capable of sustaining energy homeostasis when challenged with exercise or nutrient deprivation. A comparison of these and related metabolic pathways in other tissues (e.g. the brain) could possibly provide answers to tissue-specific phenotypes and novel insight in targeted therapeutic interventions.
    Transparency document
    Introduction The skeleton is one of the key metastatic target tissue for many types of tumors [1]. Almost 80% of breast cancer patients with advanced malignancy show bone metastases, leading to pathological fractures, hypercalcemia, intolerable bone pain, and a series of bone-related deficiencies that seriously impact their quality of life [2]. Bone destruction caused by tumor metastasis is a complex process [3] with excessive activation of osteoclasts the crucial element in tumor-induced osteolysis [4]. Breast cancer cells secretes a number of growth factors including receptor activator of nuclear factor-κB ligand (RANKL) that potently promotes osteoclast formation and activation, resulting in excessive bone resorption [5,6]. The resulting cancer cell-induced bone destruction leads to the release of cytokines from the bone matrix which in turn enhances breast cancer cell proliferation and survival, thus forming a vicious cycle of positive induction [7,8]. Hence, agents that can inhibit both osteoclasts and breast cancer cells, such as bisphosphonates have been shown to be effective for treating breast cancer-induced bone diseases [9,10]. However, the high dose and frequent usage of bisphosphonates have been shown to be associated with serious side effects including non-classical bone fractures and osteonecrosis [11]. Thus, it is imperative that newer novel compounds and molecular targets are identified for the safe and effective treatment of breast cancer-induced osteolysis.