Hypoxia inducible factor-1 and KEAP1-NRF2, stress response mechanisms for hypoxia and oxidative stress, respectively, contribute to the aggressive behaviors of pancreatic cancer. These key molecules for stress response mechanisms are activated, both in pancreatic cancer cells and in pancreatic stellate cells. Both factors are involved in the mutual activation of cancer cells and stellate cells, by inducing cancer-promoting signals and their mediators. Therapeutic interventions targeting these pathways are promising approaches for novel therapies.
NRF2 activation contributes to malignant phenotype of pancreatic cancer cells. The human pancreatic cancer cell line MIAPaCa-2 was exposed to low-dose gemcitabine for 6 months, and a gemcitabine-resistant cell line was established. This cell line showed increased intracellular ROS and NRF2 accumulation and elevated the expression of NRF2 target genes [69][37]. NRF2 knockdown by RNA interference sensitized pancreatic cancer cells to gemcitabine, suggesting that NRF2 activation is essential for acquiring resistance. Crosstalk between NRF2 and other cancer-promoting signals also contributes to the malignant phenotype. The inducer of EMT, transforming growth factor-β1 signaling, was attenuated by knockdown of NRF2 in pancreatic cancer cells [70][38]. PanIN lesions of surgically resected human pancreas tissue showed increased expression of nuclear NRF2 and decreased expression of E-cadherin, compared to normal pancreatic duct epithelium [70][38]. Accumulation of p62 also activated NRF2 in pancreatic cancer, leading to accelerated carcinogenesis. Pancreas-specific mutant K-ras expression and deletion of IκB kinase α promoted pancreatic cancer by increasing inflammation. The inflamed pancreatic parenchyma revealed p62 accumulation, and the deletion of p62 attenuated cancer progression [71][39]. The major oncogene K-ras, frequently mutated in pancreatic cancer, also activated Nrf2. Pancreas-specific expression of K-ras, together with other oncogenic B-raf mutations or Myc overexpression, resulted in the activation of Nrf2, which reduced intracellular ROS and increased cellular proliferation [72][40]. Introduction of Nrf2-null background into the KPC mouse, a pancreatic cancer model driven by pancreas-specific mutant K-ras/p53 expression [73][41], delayed pancreatic cancer development via the attenuation of mRNA translation [74][42]. In this study, pancreatic organoids from KPC mice and Nrf2-null KPC mice were established, and Nrf2-null organoids showed vulnerability to AKT inhibition. Another study compared the development of precancerous lesions, pancreatic intraepithelial neoplasm (PanIN), and progression to invasive cancer between KPC mice and Nrf2-null KPC mice [75][43].