Divergent effects of leptin on tumor progression. In breast cancer, both circulating and paracrine leptin have a pro-tumor effect by enhancing cell transformation, proliferation, neo-angiogenesis, epithelial-to-mesenchymal transformation (EMT), and metastatic evolution. At the same time, leptin is able to activate the anti-tumor immune response by stimulating a metabolic switch and functional enhancement (secretion of cytokines and granzymes) of tumor-infiltrating CD8⁺ T cells, able to kill tumor cells and hamper tumor progression. Schematic figures were created with images adapted from Smart Servier Medical Art (http://www.servier.fr/servier-medical-art).