CLDN1 and lung adenocarcinoma: Low CLDN1 expression in response to TNF-α is regulated by the PKC/iPLA2/PGE2/PPAR signaling cascade and is associated with lung adenocarcinoma rather than lung squamous cell carcinoma, whereas increased CLDN1 expression is associated with a better prognosis and tumor suppressive activity (24, 71, 72).