Circulating levels of IGF-1 directly regulate bone growth and density

IGF-1 is a growth-promoting polypeptide that is essential for normal growth and development.In serum, the majority of the IGFs exist in a 150-kDa complex including the IGF molecule, IGF binding protein 3 (IGFBP-3), and the acid labile subunit (ALS).This complex prolongs the half-life of serum IGFs and facilitates their endocrine actions.Liver IGF-1-deficient (LID) mice and ALS knockout (ALSKO) mice exhibited relatively normal growth and development, despite having 75% and 65% reductions in serum IGF-1 levels, respectively.Double gene disrupted mice were generated by crossing LID+ALSKO mice.These mice exhibited further reductions in serum IGF-1 levels and a significant reduction in linear growth.The proximal growth plates of the tibiae of LID+ALSKO mice were smaller in total height as well as in the height of the proliferative and hypertrophic zones of chondrocytes.There was also a 10% decrease in bone mineral density and a greater than 35% decrease in periosteal circumference and cortical thickness in these mice.IGF-1 treatment for 4 weeks restored the total height of the proximal growth plate of the tibia.Thus, the double gene disruption LID+ALSKO mouse model demonstrates that a threshold concentration of circulating IGF-1 is necessary for normal bone growth and suggests that IGF-1, IGFBP-3, and ALS play a prominent role in the pathophysiology of osteoporosis.