Lack of Insulin-Like Growth Factor I Exaggerates the Effect of Calcium Deficiency on Bone Accretion in Mice

Recent studies provide evidence that the GH/IGF-I axis plays a critical role in the regulation of bone accretion that occurs during puberty and that the peak bone mineral density (BMD) is dependent on the amount of dietary calcium intake during the active growth phases. To evaluate whether IGF-I deficiency exaggerates the effect of calcium deficiency on bone accretion during active growth phases, IGF-I knockout (KO) and wild-type (WT) mice were fed with low calcium (0.01%) or normal calcium (0.6%) for 2 wk during the pubertal growth phase and were labeled with tetracycline. The low calcium diet caused significant decreases in endosteal bone formation parameters and a much greater increase in the resorbing surface of both the endosteum and periosteum of the tibia of IGF-I KO mice compared with WT mice. Accordingly, femur BMD measured by dual energy x-ray absorptiometry or peripheral quantitative computed tomography increased significantly in IGF-I WT mice fed the low calcium diet, but not in IGF-I KO mice. IGF-I-deficient mice fed the normal calcium diet showed elevated PTH levels, decreased serum 1,25-dihydroxyvitamin D and serum calcium levels at baseline. Serum calcium changes due to calcium deficiency were greater in IGF-I KO mice compared with WT mice. PTH levels were 7-fold higher in IGF-I KO mice fed normal calcium compared with WT mice, which was further elevated in mice fed the low calcium diet. Treatment of IGF-I-deficient lit/lit mice with GH decreased the serum PTH level by 70% (P < 0.01). Based on these and past findings, we conclude that: 1) IGF-I deficiency exaggerates the negative effects of calcium deficiency on bone accretion; and 2) IGF-I deficiency may lead to 1,25-dihydroxyvitamin D deficiency and elevated PTH levels even under normal calcium diet.