Mild Hypoxia Promotes Survival and Proliferation of SOD2-Deficient Astrocytes via c-Myc Activation

Abstract

Mouse astrocytes deficient in the mitochondrial form of manganese superoxide dismutase (SOD2) do not survive in culture under atmospheric air with 20% oxygen (O 2 ), which is a common condition for cell cultures. Seeding the cells and maintaining them under mild hypoxic conditions (5% O 2 ) circumvents this problem and allows the cells to grow and become confluent. Previous studies from our laboratory showed that this adaptation of the cells was not attributable to compensation by other enzymes of the antioxidant defense system. We hypothesized that transcriptional activity and upregulation of genes other than those with an antioxidant function are involved. Our present study shows that c-Myc was significantly induced and that it inhibited p21 and induced proteins such as cyclindependent kinases, cyclin D, and cyclin E, which are involved in the cell cycle process, along with phosphorylation of the retinoblastoma protein and Cdc2 (cell division cycle 2). These mechanisms contribute to cell proliferation. Small interfering RNA of c-Myc, however, blocked proliferation of SOD2 homozygous (SOD2Ϫ/Ϫ) astrocytes under mild hypoxia consisting of 5% O 2 , whereas it did not affect the growth of wild-type astrocytes. Our results indicate that c-Myc plays a critical role in hypoxia-induced proliferation and survival of SOD2Ϫ/Ϫ astrocytes by overcoming injury caused by oxidative stress.

Hypoxia induces differential translation of enolase/MBP-1

“…Since glucose deprivation results in mitochondrial dysfunction and enhances cellular sensitivity to oxidative stress, robust quantities of ROS are produced [43,44] resulting in higher levels of c-myc mRNA. Previous studies show that c-myc is markedly upregulated in response to superoxide dismutase deficiency suggesting that early upregulation of c-myc may play a role in helping cells overcome oxidative stress [45]. In our study, when cells were exposed to 25 mM glucose in a hypoxic environment, ROS production was markedly attenuated.…”

Moderate Hypoxia Down-Regulates Interleukin-6 Secretion and TLR4 Expression in Human Sw.71 Placental Cells

“…Interestingly, LPS stimulated ROS production only under normoxia in trophoblast cells. Similarly, although there was no significant difference in ROS production between 20% and 5% O 2 conditions in the wildtype astrocytes, ROS production was much higher under 20% O 2 conditions compared with that under 5% O 2 conditions in the manganese superoxide dismutas-/-astrocytes [29]. Therefore, we suggest that under normoxic conditions, ROS production in cells is better primed for responding to an inflammatory signal and/or cellular stresses.…”

Hypoxic preconditioning enhances neural stem cell transplantation therapy after intracerebral hemorrhage in mice

“…Hypoxic preconditioning, or exposure to sublethal levels of hypoxia, has been demonstrated to enhance cell proliferation and viability in the ischemic stroke model; 18,19 however, the effects of hypoxic preconditioning on NSCs prior to transplantation in the ICH model are yet to be explored. In this present study, we demonstrate for the first time that (1) mild hypoxia can enhance NSC proliferation and resilience against Hb cytotoxicity via Akt activation; (2) hypoxic preconditioning upregulates pAkt via HIF-1a and enhances VEGF production in NSCs, which subsequently increases expression of VEGF found around the bleed after transplantation; (3) hypoxic preconditioning increases survival of transplanted cells; and (4) transplantation of hypoxic preconditioned NSCs enhances neuroprotection and facilitates functional recovery in mice with ICH.…”