STAT3 and cancer: Cancer cells metastasize to the brain and interact with the local tumor microenvironment to destroy the blood–brain barrier (BBB), leading to brain metastasis; at the same time, in tumor cells, the activation of JAK/STAT3 and PI3K/AKT signaling pathways and the downregulation of phosphatases and tensin homolog (PTEN) develop cell cycle progression, tumor cell proliferation, and, finally, reduced survival [39].