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The IGF system plays critical roles in somatic growth in an endocrine fashion (somatomedin hypothesis) as well as proliferation and differentiation of normal and malignant cells in a paracrine/autocrine fashion. IGFBP-3 is known to modulate the actions of IGFs in circulation as well as the immediate extracellular environment. Interestingly, apart from the ability to inhibit or enhance IGF actions, IGFBP-3 also exhibits very clear, distinct biological effects independent of the IGF/IGF-I receptor axis. Over the past decade it has become widely appreciated that IGF/IGF-IR-independent actions of IGFBP-3 (antiproliferative and proapoptotic effects) contribute to improving the pathophysiology of a variety of human diseases, such as cancer, diabetes, and malnutrition. Recent studies have implicated interaction of IGFBP-3 with a variety of proteins or signaling cascades critical to cell cycle control and apoptosis; however, the actual mechanism of IGFBP-3 action is still unclear. This review reinforces the concept in support of the IGF/IGF-IR axis-independent actions of IGFBP-3 and delineates potential underlying mechanisms involved and subsequent biological significance, focusing in particular on functional binding partners and the clinical significance of IGFBP-3 in the assessment of cancer risk.

The mac25 cDNA was originally cloned from leptomeningial cells and subsequently reisolated through differential display as a sequence preferentially expressed in senescent human mammary epithelial cells. The deduced amino acid sequence of the human mac25 propeptide shares a 20 -25% identity to human insulin-like growth factor-binding proteins (IGFBPs), suggesting that mac25 could be another member of the IGFBP family.In the present study, we have generated recombinant human mac25 (rh-mac25) in a baculovirus expression system and assessed its affinity for IGFs and have eval- The IGFBP 1 family is a critical component of the IGF system.

one of the authors regrets that she inadvertently omitted references to the computer program and protein potential function that the authors used for their simulations of barnase cited above. The following sentence should have been the first sentence of the Methods section: Molecular dynamics simulations were performed with the program ENCAD (44) and the potential energy function of Levitt et al. (45).

), also binds insulin with high affinity and blocks insulin action. IGFBPs with enhanced affinity for insulin might contribute to the insulin resistance of pregnancy, type II diabetes mellitus, and other pathological conditions.Insulin-like growth factors (IGFs)-I 1 and -II are structurally related to insulin, sharing approximately 50% amino acid homology with insulin in the A-and B-chain regions but retaining a connecting peptide, as well as a carboxyl-terminal extension (1). The anabolic and mitogenic actions of IGFs are mediated largely through the type 1 IGF receptor, which, like the insulin receptor, is a heterotetrameric, membrane-spanning tyrosine kinase (2).It has been accepted dogma that, unlike insulin, the IGFs bind to a family of binding proteins (IGFBPs) with high affinity and specificity (3-5). The six well characterized IGFBPs have significant sequence homology, including a GCGCCXXC motif as part of 10 -12 conserved cysteines at the amino terminus and six conserved cysteines at the carboxyl terminus. We have recently characterized two additional secreted proteins with lower affinity for IGFs (IGFBP-7/mac25 and IGFBP-8/CTGF) and two additional potential members of the IGFBP superfamily (6 -9). The structure of IGFBP-7 revealed the presence of amino-terminal conserved sequences, including 11 cysteines, but lower sequence homology in the normally well-conserved COOH terminus. IGFBPs 1-6 are believed to function as carrier proteins for IGFs, delaying degradation of IGF peptides and increasing their half-lives and inhibiting IGF access to receptors (10 -14).Proteolysis of IGFBPs, as observed during pregnancy, results in IGFBP fragments with decreased affinity for IGFs and, thereby, promotes enhanced access of IGFs to their receptors (15,16). Recent studies have demonstrated that several of these IGFBPs, especially IGFBP-3, may also be capable of directly inhibiting cell growth in an IGF-independent manner (17-23). Both intact IGFBP-3 and IGFBP-3 proteolytic fragments have been shown to be capable of blocking the mitogenic effect of insulin (24,25). Whether these effects reflect inhibition of insulin signaling pathways or a direct effect of IGFBP-3 and its fragments on cell growth remains unclear.We have, therefore, reevaluated the ability of IGFBPs 1-6, IGFBP-7, and IGFBP-3 proteolytic fragments to bind insulin and have demonstrated that IGFBP-7 and NH 2 -terminal fragment of IGFBP-3 (IGFBP-3 ) not only bind insulin specifically but also modulate insulin binding to its receptor and subsequently inhibiting insulin-stimulated autophosphorylation of the insulin receptor ␤ subunit and phosphorylation of IRS-1. EXPERIMENTAL PROCEDURESMaterials-High performance liquid chromatography-purified hIG-FBP-1 from human amniotic fluid was kindly provided by Dr. D. R. Powell (Baylor College of Medicine, Houston, TX) (26). Recombinant human IGFBP-3 (rhIGFBP-3), a nonglycosylated 29 kDa core protein expressed in Escherichia coli cells, was the generous gift of Celtrix, Inc. (Santa Clara, CA) (27). Recombinant human IG...

Most estrogen receptor-negative breast cancer cells, including Hs578T cells, express mRNAs encoding insulin-like growth factor-binding protein (IGFBP)-3, as well as transforming growth factor (TGF)-beta receptors. Our previous studies (Oh, Y., Muller, H. L., Lamson, G., and Rosenfeld, R. G. (1993) J. Biol. Chem. 268, 14964-14971; Oh, Y., Muller, H. L., Pham, H. M., and Rosenfeld, R. G. (1993) J. Biol. Chem. 268, 26045-26048) have demonstrated a significant inhibitory effect of exogenous IGFBP-3 on Hs578T cell growth and existence of IGFBP-3-specific receptors that may mediate those direct inhibitory effect of IGFBP-3. TGF-beta is also a potent growth inhibitor in human breast cancer cells in vitro and regulates IGFBP-3 production in different cell systems, suggesting that IGFBP-3 is a major anti-proliferative factor and a key element for TGF-beta-induced growth inhibition in human breast cancer cells. In support of this hypothesis, we have demonstrated using Hs578T cells that: 1) TGF-beta stimulates IGFBP-3 gene expression and production prior to its inhibition of cell growth, 2) treatment with an IGFBP-3 antisense oligodeoxynucleotide selectively inhibits TGF-beta-induced IGFBP-3 synthesis and cell growth inhibition, and 3) treatment with IGF-II and IGF-II analogs diminish TGF-beta effects by blocking TGF-beta-induced binding of IGFBP-3 to the cell surface. These findings suggest that IGFBP-3 is a major anti-proliferative factor and a key element in TGF-beta-induced growth inhibition in human breast cancer cells.

These data demonstrate that an intense exercise bout in male adolescents leads to reductions in anabolic mediators and profound increases in inflammatory cytokines. This might explain the development of what seems to be a paradoxical catabolic state in the initial phases of exercise training programs.