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Effects of Recombinant Insulin-Like Growth Factor-Binding Protein-4 on Bone Formation Parameters in Mice¹

Musculoskeletal Disease Center, J. L. Pettis Veterans Administration Medical Center (N.M., C.R., X.Q., D.J.B., S.M.), Loma Linda, California 92357; and the Departments of Medicine (X.Q., D.J.B., S.M.), Biochemistry (S.M.), and Physiology (S.M.) Loma Linda University, Loma Linda, California 92350

Address all correspondence and requests for reprints to: Subburaman Mohan, Ph.D., Musculoskeletal Disease Center (151), J. L. Pettis Veterans Administration Medical Center, 11201 Benton Street, Loma Linda, California 92357. E-mail: mohans{at}llvamc.va.gov

Insulin-like growth factor (IGF)-binding protein-4 (IGFBP-4), one of the most abundant IGFBPs produced by bone cells, is a potent inhibitor of IGF actions in vitro. To evaluate the modulation of IGF actions on bone formation in vivo by IGFBP-4, we produced intact and fragment (50- to 100-fold reduced IGF affinity) forms of BP-4 and examined their local and systemic effects using biochemical markers. Local administration of IGF-I over the right parietal bone significantly increased bone extract alkaline phosphatase activity; this was completely blocked by an equimolar dose of intact IGFBP-4, but not IGFBP-4 fragment. A single sc administration of IGF-I (2 µg/g BW) significantly increased bone formation markers in both serum and skeletal extracts; surprisingly, so did intact IGFBP-4, but not fragment IGFBP-4. Subcutaneous administration of an equimolar dose of IGFBP-4 along with IGF-I did not significantly block the IGF-I effect. Administration of intact IGFBP-4 significantly increased the serum 50-kDa IGF pool and decreased the 150-kDa IGF pool without significantly changing total IGF-I. We postulate that the increase in the 50-kDa IGF pool might enhance IGFs bioavailability via a mechanism involving IGFBP-4-specific protease. This study demonstrates for the first time that a single local administration of IGFBP-4 inhibits IGF-I-induced increases in bone formation, whereas systemic administration of IGFBP-4 alone increases serum levels of bone formation markers.

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