Involvement of ligand occupancy in Insulin-like growth factor-I (IGF-I) induced cell growth in osteoblast like MC3T3-E1 cells

Article type: Research Article

Authors: Kim, Seok-Kwun | Kwon, Ji-Young | Nam, Taek-Jeong

Affiliations: Department of Plastic and Reconstructive Surgery, College of Medicine, Dong-A University, Busan, South Korea | Department of Food and Life Science, Pukyong National University, Busan, South Korea

Note: [] Address for correspondence: Dr.S.-K. Kim, Department of Plastic and Reconstructive Surgery, College of Medicine, Dong-A University, #1, 3ga, Dongdaesin-dong, Seo-gu, Busan 602-715, Korea. Tel.: +82 51 240 5411; Fax: +82 51 248 1527; E-mail: sgkim1@daunet.donga.ac.kr

Abstract: Growth factors and matrix proteins regulate the proliferation and differentiation of osteoblasts. The insulin-like growth factor (IGF) system comprises IGF-I, IGF-II, and six high-affinity IGF-binding proteins (IGFBPs). IGFs stimulate cell growth in many types of tissue; IGF-binding proteins regulate cellular actions and can affect cell growth. IGF-I is involved in differentiation, proliferation, and matrix formation in osteoblasts; IGFBP-5 is associated with the extracellular matrix (ECM) and can potentiate the actions of IGF-I. We investigated the effect of ECM proteins on the responses of MC3T3-E1 osteoblast cells to IGF-I and IGFBP-5. In addition, because extracellular signal-regulated kinases 1 and 2 (Erk 1/2) affect cell growth, we evaluated the effects of IGFBP-5 on Erk 1/2 phosphorylation in MC3T3-E1 cells. IGF-I caused an increase in IGFBP-5 expression in cultured MC3T3-E1 cells, and IGF-I plus IGFBP-5 significantly increased cell growth. Likewise, the addition of IGF-I and IGFBP-5 to cultured MC3T3-E1 cells increased the synthesis of the ECM proteins osteopontin (OPN) and thrombospondin-1 (TSP-1), which can bind to αVβ3 integrin receptors on the cell surface. By contrast, the addition of an antibody against ECM proteins inhibited the effects of OPN and TSP-1 on IGFBP-5 expression. The stimulatory effect of IGFBP-5 was mediated via Erk 1/2 activation. These data suggest that IGFBP-5 regulates Erk 1/2 phosphorylation in cultured MC3T3-E1 cells via ECM proteins that may ultimately stimulate the growth of osteoblasts. We determined whether occupation of the αVβ3 integrin receptor affects IGF-I receptor (IGF-IR)-mediated signaling and function in MC3T3-E1 osteoblast cells. Occupation of the αVβ3 integrin receptor with ECM proteins induced IGF-I-stimulated IGF-IR phosphorylation. Conversely, in the presence of the αVβ3-specific disintegrin echistatin, IGF-I-stimulated IGF-IR activation was inhibited. IGF-I-stimulated IGF-IR phosphorylation was accompanied by IRS-1 phosphorylation and MAPK activation. However, these effects were attenuated by echistatin. Thus, occupancy of the αVβ3 disintegrin receptor modulates IGF-I-induced IGF-IR activation and IGF-IR-mediated function in MC 3T3-E1 osteoblasts.

Keywords: Extracellular matrix, extracellular signal-regulated kinases 1 and 2, insulin-like growth factor (IGF)-I, IGF-binding protein-5, MC3T3-E1 osteoblast cells

Journal: BioFactors, vol. 29, no. 4, pp. 187-202, 2007