Bone Vol. 21, No. 3 September 1997:217-223 ELSEVIER ORIGINAL ARTICLES Circulating and Skeletal Insulin-like Growth Factor-I (IGF-I) Concentrations in Two Inbred Strains of Mice With Different Bone Mineral Densities C. J. ROSEN, 1 H. P. DIMAI, 2 D. VEREAULT, 1 L. R. DONAHUE, 3 W. G. BEAMER, 3 J. FARLEY, 2 S. LINKHART, 2 T. LINKHART, 2 S. MOHAN, 2 and D. J. BAYLINK 2 St. Joseph Hospital, Bc~ngor, ME, USA 2 j. L. Pettis VA Hospital Medical Center, Loma Linda, CA, USA 3 Jackson Laboratory, Bar Harbor, ME, USA Recent work has demonstrated differences in femoral bone mineral density between two common inbred strains of mice, C3H/HeJ (C3H) and C57BL/6J (B6), across a wide age range. To investigate one possible mechanism that could affect acquisition and maintenance of bone mass in mice, we stud- ied circulatory and skeletal insulin-like growth factor-I (IGF-I) and femoral bone mineral density (F-BMD) by pQCT in C3H and ]36 progenitor strains, as well as serum IGF-I obtained from matings between these two strains and mice bred from subsequent F I intercrosses (F2). Serum IGF-I measured by radioimmunoassay was more than 35% higher in virgin progenitor C3H than virgin B6 at I, 4, 8, and I0 months of age, and iin 8-month-old C3H compared with B6 retired breeders (p < 0.001). In the progenitors, there was also a strong correlation between serum IGF-I and serum alkaline phosphatase (r - 0.51,p = 0.001). In the 4 month F 1 females IGF-I levels and F-BMD were intermediate between C3H and B6 progenitors. In contrast, groups of F 2 mice with the highest or lowest BMD also had the highest or lowest serum IGF-I (p = 0.0001). IGF-I accounted for >35% of the variance in F-BMD among the F 2 mice. Conditioned media from newborn C3H calvarial cultures had higher concentra- tions of IGF-I than media from B6 cultures, and cell layer extracts from C3H calvariae exhibited greater alkaline phos- phatase activity than cultures from B6 calvarial cells (p < 0.0001). The skeletal content of IGF-I in C3H tibiae, femorae, and calvariae (6-14 weeks of age) was also significantly higher than IGF-I content in the same bones of the B6 mice (p < 0.05). These data suggest that a possible mechanism for the difference in acquisition and maintenance of bone mass between these two inbred strains is related to systemic and skeletal IGF-I synthesis. (Bone 21:217-223; 1997) © 1997 by Elsevier Science ][nc. All rights reserved. Key Words: Insulin-]like growth factors; Femoral bone density. Address for correspondence and reprints: C. J. Rosen, St. Joseph Hospital, 360 Broadway, Bangor, ME 04401. E-mail:crosen@maine. maine.edu Introduction Recent studies from our laboratory have demonstrated large differences (50%) in femoral bone density, measured by periph- eral quantitative computed tomography (pQCT), between C3H/ HeJ(C3H) and C57BL/6J(B6), two common inbred strains of mice. 2 These strain differences are noted as early as 4 weeks of age, a time during peak bone mass acquisition, and are main- tained through at least 12 months of age. 2't° Moreover, differ- ences in bone density are not limited to the femur, since C3H mice compared to B6 mice have consistently higher bone mass (both cortical and trabecular) at other skeletal sites including the tibia, vertebrae, and phalanges. 2 Although the mechanisms re- sponsible for a greater bone mineral density in C3H mice have yet to be clearly defined, changes in bone resorption or bone formation must underlie differences in the acquisition of peak bone mass between these two strains. Preliminary histomorpho- metrical and biochemical evidence from our laboratory suggests that bone formation is increased significantly at the time of acquisition of peak bone mass in C3H mice (Baylink et al., unpublished observation). Osteoblasts mediate bone formation through a series of events that are regulated by hormonal and skeletal growth factors. 1 Several of these regulatory factors are also operative during acquisition of bone mass. These include insulin-like growth factors (IGF-I and -II), transforming growth factor-13 (TGF-13), bone morphogenic proteins (BMPs), fibroblast growth factors, numerous interleukins, tumor necrosis factor (TNF), and various prostaglandins, t Several lines of evidence suggest that IGFs are important mitogenic and differentiative factors for bone cells. First, these peptides are found in very high concentrations within the skeletal matrix, t Second, calciotropic hormones such as parathyroid hormone (PTH), 1,25-dihydroxyvitamin D, and growth hormone may mediate their effects on osteoblasts in part by regulating the synthesis of IGFs and/or their binding pro- teins. 14'19 Third, studies of growth hormone-deficient patients have found that serum IGF-I concentrations closely correlate with reduced bone density, and that growth hormone replacement increases both serum IGF-I and bone mineral density. 4"13Fourth, age-associated declines in circulating IGF-I parallel similar drops in skeletal levels of IGF-I. 3'17 Fifth, several cross-sectional studies have demonstrated a strong linear relationship between serum IGF-I and bone mineral density in postmenopausal wom- en.5"16 These points suggest that IGF-I may play an important © 1997by ElsevierScience Inc. 217 8756-3282/97/$17.00 All rightsreserved. PII $8756-3282(97)00143-9