Hypoxia-Associated Factor (HAF) Mediates Neurofibromin Ubiquitination and Degradation Leading to Ras–ERK Pathway Activation in Hypoxia

Abstract

Low oxygen or hypoxia is a feature of all solid tumors and has been associated with aggressive disease. Here, we describe a novel mechanism for the hypoxia-dependent degradation of the Ras-GTPase-activating protein neurofibromin, by hypoxia-associated factor (HAF). We have previously characterized HAF as an oxygen-independent ubiquitin ligase for HIF-1a.Here, we show that HAF promotes neurofibromin ubiquitination and degradation independently of oxygen and pVHL, resulting in Ras-ERK pathway activation. Hypoxia enhanced HAF:neurofibromin binding independently of HAF-SUMOylation, whereas HAF knockdown increased neurofibromin levels primarily in hypoxia, supporting the role of HAF as a hypoxia-specific neurofibromin regulator. HAF overexpression increased p-ERK levels and promoted resistance of clear cell kidney cancer (ccRCC) cells to sorafenib and sunitinib in both normoxia and hypoxia. However, a greater-fold increase in sorafenib/sunitinib resistance was observed during hypox-ia, particularly in pVHL-deficient cells. Intriguingly, HAFmediated resistance was HIF-2a-dependent in normoxia, but HIF-2a-independent in hypoxia indicating two potential mechanisms of HAF-mediated resistance: a HIF-2a-dependent pathway dominant in normoxia, and the direct activation of the Ras-ERK pathway through neurofibromin degradation dominant in hypoxia. Patients with ccRCC with high HAF transcript or protein levels showed significantly decreased overall survival compared with those with low HAF. Thus, we establish a novel, nonmutational pathway of neurofibromin inactivation through hypoxia-induced HAF-mediated degradation, leading to Ras-ERK activation and poor prognosis in ccRCC.Implications: We describe a novel mechanism of neurofibromin degradation induced by hypoxia that leads to activation of the prooncogenic Ras-ERK pathway and resistance to therapy.

Macrophage HIF-1α Is an Independent Prognostic Indicator in Kidney Cancer

“…5F-G; ref. 40). Thus, the data suggest that the combination of hypoxia and M2 polarizing conditions within the tumor microenvironment promotes the upregulation of HIF-1a protein in ccRCC TAMs, whereas intrinsic factors associated with pVHL loss inhibits the production of HIF-1a protein in tumor cells regardless of oxygen tension.…”

Hypoxia Transcriptomic Modifications Induced by Proton Irradiation in U87 Glioblastoma Multiforme Cell Line

“…Their roles in the response to hypoxia and radiation stress are described. As is known, Wnt and Ras signaling regulate proliferation, motility, and survival in a variety of cancers and several literature data report their activation after hypoxia as well as after radiation exposure [33][34][35][36].…”

Metabolic Features of Neurofibromatosis Type 1-Associated Tumors

“…Altogether, these data indicate that NF1-related tumors display a pseudo-hypoxic signature that contributes to tumor proliferation and transition towards malignancy. Indeed, neurofibromin inactivation occurs in certain cancers through hypoxia-induced degradation, independently of NF1 gene mutations [39]. These data suggest that the hypoxic response might affect the Ras/ERK signaling pathways downstream to neurofibromin loss and its genetic Metabolic Features of Neurofibromatosis Type 1-Associated Tumors DOI: http://dx.doi.org/10.5772/intechopen.98661 inactivation installs a hypoxic-like response that may provide cells with an equipped and prompt response to any possible drop in oxygen availability.…”