Insulin-like growth factor II and transforming growth factor β1 regulate insulin-like growth factor I secretion in mouse bone cells

Bone cells in culture produce and respond to growth factors, suggesting that local as well as systemic factors regulate bone volume. Previous studies have shown that IGF-I is the major mitogen produced by mouse bone cells and that its production is regulated by systemic agents such as PTH and estrogen. Because IGF-II and transforming growth factor β1 have been shown, respectively, to increase and decrease MC3T3-E1 cell proliferation, we tested the hypothesis that these two growth factors modulate the production of IGF-I in this cell line. In order to eliminate artifacts owing to IGF binding proteins, conditioned media samples were pretreated with IGF-II before measurement of IGF-I by RIA. After 24 h treatment at a density of 2.5 x 10⁴ cells/cm², IGF-II (10 μg/l) induced a 2.2-fold increase compared with untreated control (9.5±1.5 vs 4.2±0.44 pg/μg protein, p<0.001), whereas transforming growth factor β1(1 μg/l) caused a 66% decrease in IGF-I production (1.5±0.3 vs 4.2±0.44 pg/μg protein, p<0.001). Both IGF-II and transforming growth factor β1 regulated IGF-I production in a dose-, time- and cell density-dependent manner. The lowest effective doses for IGF-II and transforming growth factor β1 were 1 and 0.01 μg/l, respectively. These results support a role for IGF-II and transforming growth factor β1 as potent modulators of IGF-I secretion in mouse bone cells. Furthermore, regulation of IGF-I production in bone cells by IGF-II and transforming growth factor β1 in an autocrine/paracrine manner could represent a component part of the mechanism whereby the skeleton locally adapts in reponse to external stimuli.