p neurotrophin receptor p NTR or CD
p75 neurotrophin receptor (p75NTR or CD271), a member of the tumor necrosis factor receptor superfamily, has been implicated in several steps of glioma tumorgenesis . p75NTR mediates glioma invasion and progression through γ-secretase-dependent and -independent mechanisms , , and its expression has been linked to stemness both in glioma and other cancers such as melanoma, breast cancer, esophageal squamous cell carcinomas, and hypopharyngeal carcinoma , , , , . The expression of p75NTR with regards to intratumoral heterogeneity and the possible regulation of p75NTR by microenvironmental factors, are largely unknown in the context of oicr sale tumors. However, both γ-secretase-mediated cleavage of p75NTR to generate a p75 intracellular domain (p75-ICD) as well as expression of p75NTR itself has been shown to be induced by hypoxia in other cellular contexts .
Here, we investigate the expression and regulation of p75NTR in perinecrotic tumor areas and hypoxic glioma cells. We found that p75NTR expression is restricted primarily to perinecrotic, hypoxic tumor areas, and that p75NTR protein levels are upregulated at hypoxia in primary human glioma cells. The p75NTR itself regulated HIF protein levels, and was required for hypoxia-induced stemness, migration, and invasion in glioma. Our data highlight the interplay between p75NTR and HIFs, and suggest that p75NTR may represent a potential molecular target to interfere with hypoxia-induced aggressive glioma phenotypes.
Material and methods
Discussion In this study, we discovered that the potential therapeutic target p75NTR regulates HIF signaling and glioma cell stemness in response to hypoxia. In vivo, we demonstrated increased p75NTR expression in mouse glioma samples, specifically enriched in hypoxic perinecrotic tumor areas. Forced expression of p75NTR resulted in increased HIF protein levels at hypoxia both in vitro and in vivo. While regulation of HIF-1α and HIF-2α by p75NTR has been previously reported in other cellular contexts , less is known about the dramatic induction of p75NTR protein levels at hypoxia, as reported here. RNAi-mediated knockdown of either HIF-1α or HIF-2α did not significantly affect p75NTR levels at hypoxia, suggesting that the hypoxic induction of p75NTR was not HIF-dependent, and that p75NTR is not a classical HIF downstream target gene. In agreement with these findings, NGFR mRNA levels were not induced by hypoxia in the majority of primary glioma cultures tested (data not shown). It is noteworthy that the induction of p75NTR protein levels at hypoxia was most clear when using primary cultures of human glioma, while the classical U251MG cell line displayed generally lower p75 levels at both normoxia and hypoxia. The hypoxic induction seen in primary human glioma cell cultures in vitro was supported by the finding that p75NTR levels were restricted to perinecrotic, hypoxic, tumor areas in murine gliomas in vivo. These findings together further show that primary glioma cultures may more accurately represent glioma biology in vivo. A growing literature has implicated HIFs as central regulators of stemness both in glioma and other tumor types , , , , , . While it appears that stem-like tumor cells frequently have an amplified response to hypoxia as compared to non-stem tumor bulk, the mechanisms underlying this discrepancy remain poorly understood. We have previously demonstrated that one marker of stem-like glioma cells – CD44 – can actively contribute to HIF stabilization in hypoxic and pseudohypoxic glioma cells , . While p75NTR may be controversial as a stem cell marker in glioma, its expression has been linked to stem-like tumor cells in several other malignanices , , , . Our data thus add p75NTR to the growing list of potential cell surface stem cell markers that can actively contribute to the stem cell state, rather than simply being correlated markers of stemness.