title: Identification and characterization of novel IGFBP5 interacting protein: evidence IGFBP5-IP is a potential regulator of osteoblast cell proliferation.
abstract: Insulin-like growth factor binding protein-5 (IGFBP5) is a multifunctional protein, which acts not only as a traditional binding protein, but also functions as a growth factor independent of IGFs to stimulate bone formation. It has been predicted that the intrinsic growth factor action of IGFBP5 involves binding of IGFBP5 to a putative receptor to induce downstream signaling pathways and/or nuclear translocation of IGFBP5 to influence transcription of genes involved in osteoblast cell proliferation/differentiation. Our study indentified proteins that bound to IGFBP5 using IGFBP5 as bait in a yeast two-hybrid screen of the U2 human osteosarcoma cell cDNA library. One of the clones that interacted strongly with the bait under high-stringency conditions corresponded to a novel IGFBP5 interacting protein (IGFBP5-IP) encoded by a gene that resides in mouse chromosome 10. The interaction between IGFBP5-IP and IGFBP5 is confirmed by in vitro coimmunoprecipitation studies that used pFlag and IGFBP5 polyclonal antibody, and cell lysates overexpressing both IGFBP5-IP and IGFBP5. Northern blot and RT-PCR analysis showed that the IGFBP-IP is expressed in both untransformed normal human osteoblasts and in osteosarcoma cell lines, which are known to produce IGFBP5. To determine the roles of IGFBP5-IP, we evaluated the effect of blocking the expression of IGFBP5-IP on osteoblast proliferation. We found that using a IGFBP5-IP-specific small interfering-hairpin plasmid resulted in a decrease in both basal and IGFBP5-induced osteoblast cell proliferation. On the basis of these findings, we predict that IGFBP5-IP may act as intracellular mediator of growth promoting actions of IGFBP5 and perhaps other osteoregulatory agents in bone cells.
authors.affiliation: Musculoskeletal Disease Center, PO Box 151, Jerry L. Pettis Veterans Affairs Medical Center, 11201 Benton St., Loma Linda, CA 92357, USA.
authors.forename: Yousef G
authors.initials: YG
authors.lastname: Amaar
authors.forename: Blanca
authors.initials: B
authors.lastname: Tapia
authors.forename: Shin-Tai
authors.initials: ST
authors.lastname: Chen
authors.forename: David J
authors.initials: DJ
authors.lastname: Baylink
authors.forename: Subburaman
authors.initials: S
authors.lastname: Mohan
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D049109:Cell Proliferation; D018076:DNA, Complementary; D005786:Gene Expression Regulation; D020868:Gene Silencing; D006801:Humans; D008969:Molecular Sequence Data; D010006:Osteoblasts / Q000166:cytology* / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D018076:DNA, Complementary; C509134:IGFBP5-interacting protein, human