Response To Stress By The Tumor Microenvironment
Fibroblasts are typically quiescent and tumor suppressive in adult mammals but secrete factors upon activation in benign and malignant diseases such as wound healing and tumor growth1. As residents of the tumor microenvironment (TME), fibroblasts are subjected to the same stresses that tumor cells experience including nutrient deprivation that activates the integrated stress response (ISR) pathway. This eventually leads to either apoptosis due to intense or prolonged stress or autophagy to promote cellular survival in the absence of essential nutrients2–5. Studies have shown that nutrient deprivation also activates the tumor suppressor p53 in fibroblasts. However, little is known about the role of other tumor suppressors such as the INK4a locus of the CDKN2A gene, the second most commonly mutated tumor suppressor gene in human cancers6–8. The INK4a locus encodes two tumor suppressor genes including the p19 Alternative reading frame (p19Arf) that activates p53 by sequestering the ubiquitin ligase MDM2. While P19Arf regulation has been well characterized in cancer cells, its role in the TME has not been investigated. Endothelial cells in the TME are also subjected to stress such as shear stress that is altered upon changes in blood flow as a consequence of tumor vascularization as well as activities such as aerobic exercise. Exercise is commonly prescribed to cancer patients to enhance quality of life however, the effect of exercise during chemotherapy has not been extensively investigated. My work shows that the growth of transplanted tumor cells in the flank of mice is significantly upregulated in p19Arf-/- mice as compared to wild-type littermate controls indicating a role for P19Arf in the TME. My studies show that primary murine adult lung fibroblasts (ALFs) induce P19Arf expression upon nutrient deprivation and that prolonged leucine deprivation triggers apoptosis in wild-type ALFs. However, p19Arf-/- ALFs demonstrate enhanced proliferation, migration and survival in response to long-term leucine deprivation due in part to upregulation of the ISR pathway and increased autophagic flux. My data also suggests that loss of p19Arf in fibroblasts promotes survival during nutrient deprivation through increased proliferation and autophagy. My studies investigating the effect of acute aerobic exercise on endothelial cell activation and subsequent tumor vascular normalization show that a single round of acute exercise enhances chemotherapeutic efficacy when administered after exercise in melanoma xenograft tumor models independent of tumor vascular normalization. My data suggests that acute exercise may provide an opportunity to enhance chemotherapeutic efficacy.