IGF-independent regulation of breast cancer growth by IGF binding proteins

The human IGFBP family consists of at least seven proteins, designated as IGFBP-1, -2, -3, -4, -5, -6, and-7. IGFBPs 1-6 bind IGF-I and IGF-II with high affinity whereas IGFBP-7, a newly identified IGFBP, binds IGFs with lower affinity and constitutes a low-affinity member of the IGFBP family. IGFBPs serve to transport the IGFs, prolong their half-lives, and modulate their biological action. At the cellular level, IGFBPs can either potentiate or inhibit the mitogenic effects of IGFs, depending upon cell types and IGFBP species (IGF-dependent action of IGFBPs). However, recent studies have indicated that IGFBPs, especially IGFBP-3, potently inhibit breast cancer cell growth in an IGF-independent manner. The IGF-independent action of IGFBP-3 requires interaction with cell-surface association proteins, presumably putative IGFBP-3 specific receptors, and is responsible for growth inhibitory action of the known growth suppressing factors such as TGF-beta, retinoic acid, and antiestrogens in breast cancer cells. Thus, IGFBP-3 appears to be a major factor in a negative control system involved in regulating human breast cancer cell growth in vitro. IGFBP-7, representing a low affinity IGFBP, appears to function as an IGF-independent cell growth regulator in breast cancer cells. Overall structural similarity between IGFBP-7 and classical high affinity IGFBPs 1-6 suggests that the mechanisms of action and signaling pathways used by IGFBP-7 may provide insight into the IGF-independent actions of the high affinity IGFBPs. A fuller understanding of the IGF-independent action of IGFBPs will allow us to understand how the growth of neoplastic cells can be modulated by the IGF/IGFBP system, and how other growth factors or pharmacological agents can interface with this system.