IGF binding protein 3 exerts its ligand-independent action by antagonizing BMP in zebrafish embryos

• Published in: Journal of cell science Vol. 124; no. Pt 11; pp. 1925 - 1935
• Main Authors: Zhong, Yingbin, Lu, Ling, Zhou, Jianfeng, Li, Yun, Liu, Yunzhang, Clemmons, David R, Duan, Cunming
• Format: Journal Article
• Language: English
• Published: England 01-06-2011
• Subjects: Amino Acid Sequence; Animals; Binding Sites; Bone Morphogenetic Protein 2 - antagonists & inhibitors; Bone Morphogenetic Protein 2 - metabolism; Cell Line; Conserved Sequence; Danio rerio; Freshwater; Gene Expression; Glycoproteins - genetics; Glycoproteins - metabolism; Humans; Insulin-Like Growth Factor Binding Protein 3 - metabolism; Intercellular Signaling Peptides and Proteins - genetics; Intercellular Signaling Peptides and Proteins - metabolism; Mice; Molecular Sequence Data; Nuclear Localization Signals - metabolism; Protein Structure, Tertiary; Protein Transport; Sequence Alignment; Transcriptional Activation; Up-Regulation; Zebrafish - embryology; Zebrafish - metabolism
• ISSN: 0021-9533; 1477-9137
• DOI: 10.1242/jcs.082644

IGFBP3 is a multi-functional protein that has IGF-dependent and IGF-independent actions in cultured cells. Here we show that the IGF binding domain (IBD), nuclear localization signal (NLS) and transactivation domain (TA) are conserved and functional in zebrafish Igfbp3. The in vivo roles of these domains were investigated by expression of Igfbp3 and its mutants in zebrafish embryos. Igfbp3, and its NLS and TA mutants had equally strong dorsalizing effects. Human IGFBP3 had similar dorsalizing effects in zebrafish embryos. The activities of IBD and IBD+NLS mutants were lower, but they still caused dorsalization. Thus, the IGF-independent action of Igfbp3 is not related to NLS or TA in this in vivo model. We next tested the hypothesis that Igfbp3 exerts its IGF-independent action by affecting Bmp signaling. Co-expression of Igfbp3 with Bmp2b abolished Bmp2b-induced gene expression and inhibited its ventralizing activity. Biochemical assays and in vitro experiments revealed that IGFBP3 bound BMP2 and inhibited BMP2-induced Smad signaling in cultured human cells. In vivo expression of Igfbp3 increased chordin expression in zebrafish embryos by alleviating the negative regulation of Bmp2. The elevated level of Chordin acted together with Igfbp3 to inhibit the actions of Bmp2. Knockdown of Igfbp3 enhanced the ventralized phenotype caused by chordin knockdown. These results suggest that Igfbp3 exerts its IGF-independent actions by antagonizing Bmp signaling and that this mechanism is conserved.