OPEN ACCESS PEER-REVIEWED * Published: October 22, 2012 https://doi.org/10.1371/journal.pone.0047910
The objectives of this study are to examine whether supplementation of selenium stimulates mitochondrial biogenesis signaling pathways and whether selenium enhances mitochondrial functional performance. Murine hippocampal neuronal HT22 cells were treated with sodium selenite for 24 hours.
Mitochondria are both the powerhouse and source of ROS (Reactive oxygen species) production in cells. Pathological conditions that cause increased free radical production instigate mitochondrial damage, resulting in release of proapoptotic factors that subsequently activate intrinsic apoptotic cell death pathways. Mitochondrial biogenesis, the process by which new mitochondria are formed, is activated in response to cellular stress. Peroxisome proliferator-activated receptor (PPAR) gamma coactivator-1 (PGC-1α and PGC-1β) and nuclear respiratory factors (NRF1 and NRF2) are master regulators of mitochondrial biogenesis .We hypothesize that selenium may stimulate the mitochondrial biogenesis signaling pathway and enhance mitochondrial functional performance. To test this hypothesis, we measured nuclear mitochondrial biogenesis regulating factors PGC-1α and NRF1, levels of mitochondrial proteins, and functions of mitochondria and activities of respiratory complexes in selenite- and non-selenite-treated mural hippocampal HT22 neuronal cells. Our results demonstrate that supplementation of selenium significantly increases the levels of mitochondrial biogenesis markers and mitochondrial protein levels, and improves mitochondrial functional performance and respiratory complex activities.
Selenium increases mitochondrial biogenesis markers and mitochondrial proteins
The two key nuclear transcriptional factors, PGC-1α and NRF1, were used to evaluate the effects of selenium on mitochondrial biogenesis. As shown in Fig. 1, treatment of HT22 cells with 100 nM selenite for 24 h resulted in a 50% increase of protein levels of PGC-1α and NRF1 in the nuclear fraction.
Figure 1. Supplementation of selenium enhances protein levels of mitochondrial biogenesis markers, PGC-1α and NRF1, in the nuclear fractions.
To verify whether elevation of nuclear PGC-1α and NRF1 increases mitochondrial mass, we measured two mitochondrial proteins, cytochrome c and COX IV. As demonstrated in Fig. 2, selenite treatment increased both proteins in the mitochondrial fraction.
Figure 2. Selenite increases mitochondrial proteins.
Selenium improves mitochondrial respiration and complex activities
To determine whether the stimulation of mitochondrial biogenesis by selenium leads to any functional gain of the mitochondria, we measured mitochondrial oxygen consumption and calculated mitochondrial respiratory rate. Our results showed that selenium treatment of HT22 cells resulted in increased oxygen consumption, thereby improving the mitochondrial respiratory rate compared with non-selenium treated cells. Thus, selenium treatment increased the respiration rate by ∼36% in HT22 cells (Fig. 3).
Figure 3. Selenite enhances mitochondrial respiration in HT22 cells.
To further studied whether increased mitochondrial respiration is related to an increase in the activity of mitochondrial complexes, we measured oxygen utilization using complex specific substrates and calculated the activities of each mitochondrial respiratory complex from the difference in oxygen content reduction in the presence of specific respiratory complex inhibitors. As shown in Fig.4, selenite treatment not only increased activity of complex I, II+III and IV in cells by 50, 60, and 85%, respectively, but also increased the activities per milligram of protein, suggesting that besides mitochondrial biogenesis, selenium may also increase the efficiency of each complex.
Figure 4. Selenite increases the activities of mitochondrial respiratory complexes I, II+III and IV.
It is generally believed that selenium exerts most or all of its protective effects by incorporating into selenoproteins as the amino acid selenocysteine. Although the functions of many selenoproteins remain to be characterized, some of them, such as GPXs, TRXRs, and selenoprotein P regulate redox signaling –. Our data show that selenite increases translation of PGC-1α and NRF1, two key regulators stimulating mitochondrial biogenesis. Subsequently, levels of two mitochondrial proteins, cytochrome c and COX IV, are increased in Se-treated cells comparing with non-treated controls, suggesting that increased mitochondrial biogenesis signal has led to enhancement of mitochondrial protein synthesis.
Research results revealed that treatment of selenium to the HT22 cells elevated the levels of nuclear mitochondrial biogenesis regulators PGC-1α and NRF1, as well as mitochondrial proteins cytochrome c and cytochrome c oxidase IV (COX IV). Supplementation of selenium significantly increased mitochondrial respiration and improved the activities of mitochondrial respiratory complexes. We conclude that selenium activates mitochondrial biogenesis signaling pathway and improves mitochondrial function.
Mitochondria play key roles in cell survival and death. Mitochondrial deterioration is a major pathophysiology of the aging process. Mitochondria initiate an intrinsic cell death pathway after being stressed. Therefore, preserving mitochondrial function by selenium may slow down the aging process and ameliorate cell death induced by various stress factors such as hypoxia, stroke, and inflammation. Indeed, published studies have reported that dietary selenium protects against selected signs of ageing , enhances ATP production in traumatic brain injury , increases the activities of mitochondrial respiratory chain complexes II, III, and IV  and inhibits mitochondria-initiated activation of the capase-9 and caspase-3 cell death pathway  after oxidative stress or hypoxic/ischemic injuries.
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