Date of Award

Spring 5-12-2017

Level of Access

Campus-Only Thesis

Language

English

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical Sciences

Advisor

Clifford J Rosen

Second Committee Member

Gregory Cox

Third Committee Member

Thomas Gridley

Additional Committee Members

Robert Friesel

Calvin Vary

Abstract

Insulin-like Growth Factor (IGF) regulates bone growth and induces adipogenesis. Interestingly, IGF Binding Protein 4 (IGFBP4) is highly expressed in adipocytes and osteoblasts and was found to be inhibitory of IGF in vitro. However, studies in mice suggested that it does not act as an inhibitor in vivo. The purpose of our study is to clarify how IGFBP4 mediates adipose and skeletal development in vivo in Igfbp4 null (Igfbp4-/-) mice. Adult Igfbp4-/- mice suffered from growth retardation with reductions in body weight, body length and femur length. At 8 and 16 weeks of age, DXA showed that both sexes of Igfbp4-/- mice had reduced fat proportion and lean mass. The weights of inguinal and gonadal adipose depots were significantly reduced. Pparγ expression was significantly decreased in inguinal fat from Igfbp4-/- male and female mice indicating that lipid metabolism may be altered with the deletion of IGFBP4. Indeed, cultures of ear mesenchymal stem cells (eMSC) and bone marrow stromal cells (BMSC) from Igfbp4-/- mice showed decreased adipogenesis. Moreover, control eMSC increased their levels of phosphorylated Akt (p-Akt) and Igfbp4 expression during adipogenesis. However, Igfbp4-/- eMSC had decreased levels of p-Akt suggesting that IGF signaling may be impaired with loss of Igfbp4. When mice were challenged with a high fat diet (HFD), Igfbp4-/- females were protected against HFD-induced obesity. However, Igfbp4-/- males gained as much weight than littermate controls marked with adipocyte hypertrophy, liver steatosis and brown fat lipid accumulation. After high fat feeding, inguinal Igfbp4 expression of control mice was downregulated in females but not in males suggesting a clear sex-specific regulation of Igfbp4. At 16 weeks of age and on a regular diet, marked reductions in bone mineral density and bone mineral content were observed in Igfbp4-/- females, but not in males. Micro-computed Tomography (μCT) of femurs revealed reductions in trabecular bone volume and trabecular and cortical thicknesses in Igfbp4-/- females. Surprisingly, males had significantly more trabecular bone with higher connectivity density and trabecular number. Concordantly, Sost, an inhibitor of bone formation, had decreased gene expression in Igfbp4-/- males but not in females. In summary, our results indicate that loss of Igfbp4 impairs mesenchymal differentiation and IGF downstream signaling. Our data shows that IGFBP4 modulates bone and fat development with a clear gender and tissue specificity.

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