An age-related decrease in the concentration of insulin-like growth factor binding protein-5 in human cortical bone

The skeletal contents of insulin-like growth factor-2 (IGF-II), insulin-like growth factor binding protein-5 (IGFBP-5), and insulin-like growth factor binding protein-3 (IGFBP-3) were determined in duplicate samples of human femoral cortical bone obtained from 64 subjects (44 males and 20 females) between the ages of 20 and 64 years. The results of these quantitative measurements revealed an age-related decrease in the femoral cortical content of IGFBP-5 (r=-0.272, P=0.031) in the total population. Although the femoral cortical content of IGF-II did not show a similar decrease with age, it could be correlated to the femoral cortical content of IGFBP-5 (r=0.442, P<0.001). In constrast, the femoral cortical content of IGFBP-3 did not decrease with age and could not be correlated to the femoral cortical contents of either IGFBP-5 or IGF-II. Comparisons of these results with previous measurements of insulin-like growth factor-1 (IGF-I) and transforming growth factor- (TGF-), in extracts of the same bones, showed significant cross-correlations between the femoral cortical contents of each of these growth factors and the femoral cortical contents of IGFBP-5 (r=0.625 for IGF-I versus IGFBP-5, r=0.554 for TGF- versus IGFBP-5, P<0.001 for each) but not IGFBP-3. Together, these data indicate average net losses of 60% and 29% of the femoral cortical contents of IGF-I and IGFBP-5, respectively, and apparent net losses (i.e., nonsignificant decreases) of 21% and 25% of the femoral cortical contents of IGF-II and TGF-, respectively, between the third and the sixth decades (i.e., decreases from young adult values of 75.1 pmol/g of bone for IGF-I, 124.7 pmol/g of bone for IGF-II, 0.71 pmol/g of bone for TGF-, 115.6 pmol/g of bone for IGFBP-5, and 26.2 pmol/g of bone for IGFBP-3).