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Am J Physiol Endocrinol Metab 271: E1029-E1035, 1996;
0193-1849/96 $5.00
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AJP - Endocrinology and Metabolism, Vol 271, Issue 6 E1029-E1035, Copyright © 1996 by American Physiological Society

ARTICLES

Effects and fate of human IGF-binding protein-5 in rat osteoblast cultures

C. Schmid, I. Schlapfer, M. A. Gosteli-Peter, E. R. Froesch and J. Zapf
Department of Internal Medicine, University Hospital, Zurich, Switzerland.

Osteoblasts prepared from calvaria of newborn rats produce insulin-like growth factors (IGF) and IGF-binding proteins (IGFBP), IGFBP-5 was discovered in bone extracts. However, we could not detect IGFBP-5 in the medium of newborn rat osteoblasts, although we found mRNA expression. To find an explanation for this discrepancy and to learn more about the physiological role of IGFBP-5 in these cells, we studied the biological activity and the fate of recombinant human (rh) IGFBP-5 in comparison to rhIGFBP-3. IGFBP-5 but not IGFBP-3 stimulated thymidine incorporation into DNA both in the absence and presence of IGF-I. However, IGFBP-5 did not enhance uridine incorporation into RNA and glucose incorporation into glycogen. 125I-rhIGFBP-5 but not 125I-rhIGFBP-3 rapidly disappeared from the culture medium consistent with the observation that endogenous (rat) IGFBP-3 but not IGFBP-5 accumulated in the medium. However, intact 125I-labeled or unlabeled rhIGFBP-5 was associated with the cell-layer matrix, whereas IGFBP-5 fragments appeared in the medium. Trapping of IGFBP-5 in the cell layer matrix may enhance local availability of IGF.


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