Hypoxia is a common physiological and pathophysiological phenomenon. A series of complicated mechanisms are developed in human body to accommodate hypoxia. For example, glycolysis increases to compensate the energy deficiency due to compromised oxidative phosphorylation under the hypoxic condition 7
; angiogenesis increases under the hypoxic condition to increase the blood vessel density and subsequently elevate the oxygen supply; systemic hypoxia may increase the expression of erythropoietin as a physiological response to accommodate the hypoxic environment. In addition, hypoxia is also a common pathology. For example, hypoxia is a common phenomenon in cancers 8
Previous studies has shown that hypoxia may induce the expression of hypoxia-inducible factors (HIF-1α), which may regulate the expression of different target genes affecting the growth and proliferation of cancer cells, and angiogenesis in cancers 9-11
. In recent years, results also reveal that hypoxia is related to the maintenance of undifferentiated state of stem cells and the proliferation and differentiation of stem cells in the neural crest and central nervous system 12, 13
. The hypoxia related regulation of stem cells has involvement of some stem cell related signaling molecules. To date, the signaling molecules involving in the normal self-renew and differentiation of stem cells include Wnt, BMP, Notch and Sonic hedgehog (Shh). Recent studies indicate that hypoxia may activate HIF-1α to up-regulate the down-stream molecules in Notch signaling pathway, which is crucial for the maintenance of undifferentiated state of neural stem cells 14
. Covello et al 15
confirmed that OCT-4, a stem cell transcription factor, is a target of HIF-2α. HIF-2α can activate OCT-4 to regulate the self-renew and differentiation of stem cells. Kaidi et al 16
also showed that Wnt/β-catenin signaling pathway could interact with HIF-1α. Thus, there is definite evidence showing that hypoxia is related to the signaling pathways involving in the regulation of stemness of normal stem cells. Moreover, findings also demonstrate that the molecules in these signaling pathways also involve in the self-renew and differentiation of CSCs 17,18
. Axelson et al 19
found hypoxia could alter the expression of differentiation related genes in neuroblastoma cells and breast cancer cells, which promotes the de-differentiation of differentiated cancer cells and the acquisition of stem cell like features in these cells. Tavaluc et al 20
confirmed that hypoxia could increase the side population cancer cells. However, few studies have been conducted to investigate the influence of hypoxia on glioma cells and CSCs in glioma.
In our previous studies, results showed CAIX expression was related to the markers, invasion of stem cells and the prognosis of cancers. On the basis of these findings, we proposed that hypoxia could elevate the stemness of CSCs in glioblastoma, and our study comprehensively demonstrated that hypoxia was a niche of CSCs. Firstly, the influence of hypoxia on the growth of glioma cells was detected. Results showed hypoxia maintained the undifferentiated state, and normoxia promoted the differentiation of these cells. The growth curve was delineated, and cell cycle was detected. Results showed that hypoxia slowed down the growth and increased the percentage of the cells in the quiescent stage. Then, the biological behaviors of glioma cells were detected followed hypoxic treatment. The influence of hypoxia on the proliferation and migration of glioma cells was explored. Results indicated that hypoxia could promote the self-renewal and migration of these cells. Finally, western blot assay, PCR and immunofluorescence staining were employed to investigate the influence of hypoxia on the markers of CSCs in glioma and their differentiation. Findings indicated that hypoxia increased the expression of CD133 and nestin, markers of CSCs, but reduced the proportion of cells positive for GFAP, a marker for differentiation of stem cells. On the basis of influence of hypoxia on the function of glioblastoma cells, self-renew and differentiation of CSCs and expression of markers of stem cells, we concluded that hypoxia could de-differentiate the differentiated glioma cells and promote the acquisition of stemness in these cells. However, whether the elevation of stemness is attributed to the increase in CSCs in glioma due to de-differentiation of differentiated glioma cells or to the increase in proliferation of CSCs following hypoxia is still unclear, and more studies are required to elucidate it.
Taken together, our findings comprehensively demonstrate that hypoxia elevates the stemness of glioma cells in vitro, but in vivo studies are required and the specific mechanism underlying this effect is still poorly understood. In our future studies, in vivo studies and those on mechanism of this effect will be conducted aiming to identify the target for the treatment of glioma. Currently, the studies on CSCs are still in its infancy stage, and more studies are needed to elucidate the interaction between CSCs and their microenvironment.