Hypoxic stress promotes astrocyte infiltration-like growth via HIF-1α/GDNF/LOXL2 axis

Abstract

Elevated levels of glial cell line-derived neurotrophic factor (GDNF) are implicated in the transformation of astrocytes into astrogliomas, but the underlying mechanisms are not fully understood. In this study, we found that hypoxia led to a significant increase in GDNF expression in primary rat astrocytes from various brain regions, including the cortex, hippocampus, and corpus callosum. This was accompanied by the activation of astrocytes, particularly those of the A2 subtype, and a concurrent increase in hypoxia-inducible factor 1-alpha (HIF-1α) expression. The elevated levels of HIF-1α enhanced its binding to the GDNF promoter, resulting in increased GDNF expression. Interestingly, this process formed a positive feedback loop, as elevated GDNF further activated HIF-1α in primary rat and human astrocytes. Furthermore, lysyl oxidase-like protein 2 (LOXL2), a novel downstream oncogene of GDNF, showed a significant increase following hypoxia treatment and exhibited a positive correlation with GDNF expression. Inhibiting GDNF signaling effectively suppressed this expression. Hypoxia-induced GDNF also increased the phosphorylation of ERK, P38, and CREB through the classical GDNF receptors, GFRα1 and RET. This led to increased binding of phosphorylated CREB to the LOXL2 promoter, resulting in enhanced LOXL2 expression. Consequently, rat astrocytes under hypoxic stress exhibited increased cell viability, migration, and epithelial-mesenchymal transition, which were mitigated by inhibiting GDNF signaling or silencing LOXL2. This phenomenon was also observed in C6 cells. Our findings suggest that hypoxia induces astrocyte activation and upregulates LOXL2 expression through the HIF-1α/GDNF/P-CREB signaling axis, facilitating the infiltration-like growth of astrocytes and the infiltrative growth of C6 astroglioma cells.