IGFBP2 stimulates osteoblast differentiation via regulating RPTPβ/AKT pathway

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: SUN 199-233-Bone Biology
Sunday, June 16, 2013: 1:45 PM-3:45 PM
Expo Halls ABC (Moscone Center)

Poster Board SUN-206
Gang Xi*1, Xinchun Shen1, Christine Wai1, Clifford J Rosen2 and David R Clemmons1
1University of North Carolina at Chapel Hill, Chapel Hill, NC, 2Maine Medical Center Research Institute, Scarborough, ME
The heparin-binding domain (HBD) localized in the linker region (HBD1), is unique to IGFBP2, and has IGF-I binding-independent skeletal growth stimulating activity. IGFBP-2 also contains a C-terminal HBD (HBD2) which shares the sequence similarity with HBDs contained in other forms of IGFBPs. This study investigated the roles of IGFBP-2 and its HBDs in altering osteoblast differentiation. Overexpression of IGFBP-2 in MC3T3 cells not only significantly enhanced the total number of cells that differentiated but it also accelerated the time course of differentiation by at least 3 days. Both osteocalcin expression and Alizarin staining were accelerated compared to control cells. To confirm this result, the calvarial osteoblasts were isolated from IGFBP-2 null and wild type mice. Differentiation was significantly impaired in the cells isolated from IGFBP-2 null mice, compared to the cells isolated from wild type controls. Consistently, addition of exogenous wild type IGFBP-2 was able to rescue the IGFBP-2 -/- cell differentiation but it had no effect on IGFBP-2 +/+ cells. Importantly, peptides containing the HBD1or HBD2 sequences had an ability to rescue IGFBP-2 -/- cell differentiation. Since AKT activation is essential for osteoblast differentiation and IGFBP-2 has been shown to enhance AKT activation via inhibition of the receptor tyrosine phosphataseβ (RPTPβ)/PTEN, we hypothesized that the effect of IGFBP-2 on osteoblast differentiation could be mediated via this pathway. To support this hypothesis, IGF-I, a differentiation promoter, stimulated AKT activation. Consistent with the differentiation results, overexpression of IGFBP-2 significantly enhanced IGF-I-stimulated AKT activation, especially at the early stage of cell differentiation. A similar observation was detected when IGFBP-2 was added exogenously to the primary calvarial osteoblasts isolated from IGFBP-2 -/- mice. To further test this hypothesis, RPTPβ was overexpressed or knocked down in MC3T3 cells. Overexpression of RPTPβ was associated with reduced AKT activation in response to IGF-I and significantly impaired cell differentiation whereas knock down of RPTPβ was associated with enhanced AKT activation and significant stimulation of cell differentiation. Taken together, the present study indicates that, IGFBP-2 is a potent stimulant and accelerator of osteoblast differentiation, both HBD domains have a similar function and the RPTPβ/PTEN/AKT pathway mediates this response.

Nothing to Disclose: GX, XS, CW, CJR, DRC

*Please take note of The Endocrine Society's News Embargo Policy at http://www.endo-society.org/endo2013/media.cfm