Gender-Specific Changes in Bone Turnover and Skeletal Architecture in Igfbp-2-Null Mice

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Abstract
IGF-binding protein-2 (IGFBP-2) is a 36-kDa protein that binds to the IGFs with high affinity. To determine its role in bone turnover, we compared Igfbp2−/− mice with Igfbp2+/+ colony controls. Igfbp2−/− males had shorter femurs and were heavier than controls but were not insulin resistant. Serum IGF-I levels in Igfbp2−/− mice were 10% higher than Igfbp2+/+ controls at 8 wk of age; in males, this was accompanied by a 3-fold increase in hepatic Igfbp3 and Igfbp5 mRNA transcripts compared with Igfbp2+/+ controls. The skeletal phenotype of the Igfbp2−/− mice was gender and compartment specific; Igfbp2−/− females had increased cortical thickness with a greater periosteal circumference compared with controls, whereas male Igfbp2−/− males had reduced cortical bone area and a 20% reduction in the trabecular bone volume fraction due to thinner trabeculae than Igfbp2+/+ controls. Serum osteocalcin levels were reduced by nearly 40% in Igfbp2−/− males, and in vitro, both CFU-ALP+ preosteoblasts, and tartrate-resistant acid phosphatase-positive osteoclasts were significantly less abundant than in Igfbp2+/+ male mice. Histomorphometry confirmed fewer osteoblasts and osteoclasts per bone perimeter and reduced bone formation in the Igfbp2−/− males. Lysates from both osteoblasts and osteoclasts in the Igfbp2−/− males had phosphatase and tensin homolog (PTEN) levels that were significantly higher than Igfbp2+/+ controls and were suppressed by addition of exogenous IGFBP-2. In summary, there are gender- and compartment-specific changes in Igfbp2−/− mice. IGFBP-2 may regulate bone turnover in both an IGF-I-dependent and -independent manner.

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