PTEN and lung cancer: The phosphatase and tensin homolog (PTEN) / phosphoinositide 3-kinase (PI3K) / protein kinase B (Akt) / mammalian target of rapamycin (mTOR) pathway is one of the most important signaling pathways that plays a crucial role not only in physiological cellular functions (e.g., cell cycle, proliferation, survival, metabolism and motility) but also in cancer development, progression, and treatment (e.g., breast cancer, hepatocellular carcinoma, prostate cancer, and lung cancer) [18–22].