The insulin-like growth factor pathway in lung cancer.

The insulin-like growth factor (IGF) pathway is involved in the normal control of fetal development, tissue growth, and metabolism. Two distinct ligands (insulin-like growth factor-1 [IGF-1] and IGF-2) plus insulin, and two receptors (insulin-like growth factor receptor-1 [IGF-1R] and the insulin receptor) capable of both homo- and heteropolymerization mediate the actions of this pathway. Cellular functions of IGF-regulated signaling are influenced by the expression of a variety of receptor docking proteins, including four different insulin receptor substrate proteins. Downstream signaling is primarily through the phosphatidylinositol-3 kinase-Akt pathway and the mitogen-activated protein kinase pathway, resulting in increased cell proliferation and apoptosis inhibition. Ligand-driven activation is influenced by upstream endocrine factors (particularly for IGF-1), imprinting (for IGF-2), by multiple circulating and tissue-based IGF-binding proteins/proteases, and by the expression of the IGF-2 clearance receptor (IGF-2R). Deregulation of IGF signaling has been described in several cancer types, including both small cell and non-small cell lung cancer. A number of IGF receptor inhibitors, including monoclonal antibodies and small molecule inhibitors are currently undergoing testing in clinical trials as both monotherapy, and in combination with chemotherapy, or with other targeted agents. Preliminary results from a randomized phase II trial of an anti-IGF-1R monoclonal antibody in combination with carboplatin/paclitaxel already suggest a potential efficacy benefit from targeting this pathway in the first line advanced non-small cell lung cancer setting.