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. 2016 Sep 19;8(1):1814–1844. doi: 10.18632/oncotarget.12123

Figure 1. Overview of the IGF system.

Figure 1

The IGF system consists of four receptors: IR, IGF-IR, IGF-IIR, and hybrid receptors. IR is expressed as two isoforms - IR-A and IR-B. IR-A has oncogenic potential, expressed predominantly in fetal tissues, and its expression declines during adulthood. IR-B is the physiologically expressed isoform in adult tissues. The IR-A or IR-B receptor makes one half of the hybrid receptors along with one half of the IGF-IR. The IGF system receptors interact mainly with three ligands: insulin, IGF-I, and IGF-II. Excluding IGF-IIR, these receptors possess tyrosine kinase activity. At the other hand, IGF-IIR (also known as mannose-6-phosphate [M6P] receptor) binds and removes circulating IGF-II to keep its free form at very low levels. The figure depicts IGF system ligands in order of their binding affinities to the different receptors. Ligands shown within the same rectangle have almost similar affinities to bind with a specific receptor. Ligands shown in separate yet close rectangles have slightly different receptor binding affinities. When the rectangles are widely separated, the ligands binding affinities are remarkably different.