GDNF and cancer: Nerves in cancerogenesis were reported as a rich source of neurotrophic factors like nerve growth factor (NGF), glial-cell-derived neurotrophic factor (GDNF), artemin; of neuronal chemokines like fractalkine; and of autonomic neurotransmitters like norepinephrine which can all enhance the invasiveness of cancer cells via matrix-metalloproteinase (MMP) upregulation, trigger neural invasion (NI) and activate pro-survival signaling pathways [43].