Abstract 3078: Role of HuR in metabolic reprogramming of BRAF mutant melanoma treated with dabrafenib
Authors: Loftus Alexander , Zarei Mehrdad , Graor Hallie , Hajihassani Omid , Boutros Christina , Winter Jordan M. , Rothermel Luke D. 
Published: March 2024

Abstract
Introduction: Oncogenic BRAF mutations are present in about 50% of melanoma and are clinically targetable by BRAF inhibitors such as dabrafenib. While BRAF inhibitors show high rates of clinical response, that response is transient and resistance inevitably develops. Previously we have demonstrated that BRAF inhibitors induce a metabolic shift towards the mitochondrial oxidative phosphorylation. We have shown that inhibition of wild type isocitrate dehydrogenase 1 (wtIDH1), a vital anaplerotic and anti-ROS enzyme, demonstrates synergy with dabrafenib in vitro and in vivo. IDH1 expression has been found in pancreatic cancer to be posttranscriptionally regulated by the RNA-binding and stress response protein, HuR (ELAVL1) under nutrient and chemotherapy stress. We hypothesize that HuR functions as a survival mechanism in melanoma under BRAF inhibition, regulating metabolic reprogramming including the expression of IDH1.
Methods: The human BRAF V600E mutant melanoma cell line, A375, was used for all experiments. HuR and IDH1 expression was suppressed by siRNA oilgos through lipofectamine transfection, as validated by immunoblot analysis. The bioenergetic assay was conducted using Seahorse XFp mini extracellular analyzer to measure the oxygen consumption rate (OCR) under nutrient stress, both with and without dabrafenib exposure. The impact of HuR on IDH1 expression was further explored using qPCR and Immunoblot. Ivosidenib was used for pharmacologic inhibition of wtIDH as previously described.
Results: Mitochondrial respiration (OCR) increased in parental A375 BRAF mutant melanoma cells following nutrient stress with low glucose conditions akin to the tumor microenvironment. OCR was further increased after dabrafenib exposure. Immunoblot analysis revealed that dabrafenib treated cells at 6-hour and 12-hour time points displayed HuR translocation from nucleus to cytoplasm, as detected in cytoplasmic lysates. Transient HuR silencing blocked the adaptive OCR response to both low glucose and BRAF inhibition. This suggests that HuR is involved in the metabolic response to acute BRAF inhibition in BRAF mutant melanoma. HuR cytoplasmic translocation is associated with increased IDH1 expression 24 hours after dabrafenib exposure. However, there is no increase in IDH1 levels under nutrient or treatment stress in HuR knockdown cells, indicating that IDH1 is regulated by HuR. Transient IDH1 knockdown, as well as pharmacologic IDH1 inhibition, blocks the OCR response of melanoma cells to BRAF inhibition and low glucose. This indicates that IDH1 is important in the HuR metabolic response to targeted therapy.
Conclusion: We find that BRAF mutant melanoma cells adapt to acute dabrafenib exposure via HuR mediated expression of IDH1 to upregulate mitochondrial metabolism. This demonstrates a targeted therapy induced metabolic vulnerability that can be exploited by wtIDH1 inhibition.
Citation Format: Alexander Loftus, Mehrdad Zarei, Hallie Graor, Omid Hajihassani, Christina Boutros, Jordan M. Winter, Luke D. Rothermel. Role of HuR in metabolic reprogramming of BRAF mutant melanoma treated with dabrafenib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3078.