RESUMO
Background: Hepatocellular carcinoma (HCC) incidence is increasing worldwide due to the obesity epidemic, which drives metabolic dysfunction-associated steatohepatitis (MASH) that can lead to HCC. However, the molecular pathways that lead to MASH-HCC are poorly understood. We have previously reported that male mice with global haploinsufficiency of hypoxia-associated factor, HAF ( SART1 +/ - ) spontaneously develop MASH/HCC. However, the cell type(s) responsible for HCC associated with HAF loss are unclear. Results: SART1 -floxed mice were crossed with mice expressing Cre-recombinase within hepatocytes (Alb-Cre; hepS -/- ) or macrophages (LysM-Cre, macS -/- ). Only hepS -/- mice (both male and female) developed HCC suggesting that HAF protects against HCC primarily within hepatocytes. HAF-deficient macrophages showed decreased P-p65 and P-p50 and in many major components of the NF-κB pathway, which was recapitulated using HAF siRNA in vitro . HAF depletion increased apoptosis both in vitro and in vivo , suggesting that HAF mediates a tumor suppressor role by suppressing hepatocyte apoptosis. We show that HAF regulates NF-κB activity by controlling transcription of TRADD and RIPK1 . Mice fed a high-fat diet (HFD) showed marked suppression of HAF, P-p65 and TRADD within their livers after 26 weeks, but manifest profound upregulation of HAF, P-65 and TRADD within their livers after 40 weeks of HFD, implicating deregulation of the HAF-NF-κB axis in the progression to MASH. In humans, HAF was significantly decreased in livers with simple steatosis but significantly increased in HCC compared to normal liver. Conclusions: HAF is novel transcriptional regulator of the NF-κB pathway that protects against hepatocyte apoptosis and is a key determinant of cell fate during progression to MASH and MASH-HCC.
RESUMO
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-1α. 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 hypoxia, particularly in pVHL-deficient cells. Intriguingly, HAF-mediated resistance was HIF-2α-dependent in normoxia, but HIF-2α-independent in hypoxia indicating two potential mechanisms of HAF-mediated resistance: a HIF-2α-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.