BDNF and neoplasm: Various tumor microenvironmental stressors, such as endoplasmic reticulum (ER) stress, nutrient deprivation, and mechanical abnormality, can synergistically regulate neurotrophic factors, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), precursor for BDNF (proBDNF), and other related factors, thereby activating neural growth pathways [26–28].