Pancreatic ductal adenocarcinoma (PDA) is the 3rd leading cause of cancer death in the U.S. and is generally refractory to chemotherapy. We discovered that the harsh PDA microenvironment primes cancer cells against additional cytotoxic insults (e.g., chemotherapy) and promotes PDA aggressiveness. A better understanding of the molecular underpinnings behind this adaptive program would expose PDAs metabolic vulnerabilities. We identified the RNA binding protein, HuR (ELAVL1), as a major player in the acute pro-survival response. Upon stress, HuR translocates from the nucleus to the cytoplasm with key survival transcripts, like IDH1 (an NADPH generating enzyme). HuR enhances RNA stability and protein translation of target mRNAs, to rapidly adjust the transcriptome in response to stress. Our research highlights two metabolic processes in the HuR adaptive program: a) antioxidant defense and b) mitochondrial performance. HuR silencing in PDA cells produced excessive ROS and NADPH depletion under low glucose or chemotherapy stress. An unbiased RNA seq analysis of antioxidant genes in HuR deficient cells identified IDH1 as the leading antioxidant enzyme under HuR control. RNA binding and RNA stability assays showed that HuR regulates IDH1 post-transcriptionally, and HuR deficient PDA cells had markedly reduced IDH1 mRNA and protein expression. HuR-deficient cells failed to engraft in nude mice, while IDH1 overexpression rescued PDA engraftment. HuR-deficient cells also had dysfunctional mitochondria, reflected by reduced oxygen consumption, ATP, and mitochondrial abundance. Based on this body of work, we hypothesize that PDAs reliance on HuR under low nutrient conditions exposes new therapeutic opportunities.
In Aim 1, we establish the survival impact of the HuR- IDH1 axis, by editing out HuR binding sites (CRISPR) in the IDH1 3'UTR. We generated a conditional IDH1 knockout mouse, and will cross it with an established PDA model to validate IDH1 as a therapeutic target. We will test an allosteric modulator of mutant IDH1 (GSK-321) as a novel wild type IDH1 inhibitor in PDA, and launch hit-to-lead optimization to improve drug properties.
In Aim 2, we identify specific aspects of mitochondrial biology under HuR control through studies of mitochondrial structure and function in HuR- deficient PDA cells. The importance of the HuR-IDH1 axis on mitochondrial ROS levels will be demonstrated. Additional transcripts will impact HuR's regulation of mitochondrial performance will be identified. A novel mitochondrial inhibitor, CPI-613, will be combined with HuR or IDH1 inhibition as a new synthetic lethal approach against PDAs adaptive metabolic program.
In Aim 3, we will use a diabetic mouse model to show that a hyperglycemic state suppresses the HuR pro-survival network, and sensitizes PDA to chemotherapy. Successful engraftment of HuR-deficient cells in hyperglycemic mice would suggest that HuR is less important under these conditions. Our studies of HuR biology will improve understanding of PDA metabolic tendencies, and reveal therapeutic opportunities relevant to PDAs nutrient deprived microenvironment.

Public Health Relevance

Pancreatic cancer cells rely on the RNA binding protein, HuR, to regulate a pro-survival response to low nutrient conditions in the tumor microenvironment. In this proposal, we show that antioxidant defense (via the IDH1 enzyme) and mitochondrial function are the principal effector arms of this adaptive program. Insights gained from this work should expose therapeutic opportunities to target deadly pancreatic cancer cells, and spare normal tissue.

National Institute of Health (NIH)
National Cancer Institute (NCI)
Method to Extend Research in Time (MERIT) Award (R37)
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Tumor Cell Biology Study Section (TCB)
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Timmer, William C
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Case Western Reserve University
Schools of Medicine
United States
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