Endocrine Abstracts

Several studies demonstrated that IGFs stimulate aromatase activity in ovary but no data on bone are available. In the present study the role of IGF system components on aromatase action has been characterized during osteogenic differentiation in a model of rat tibial osteoblasts. At confluence (day 0) cells have been transferred to differentiating medium supplemented with 1% FCS and delta4androstenedion with or without IGF-II 3 nM, IGFBP-2 1 nM and IGFBP-3 1 nM. Cells have been treated with test substances continuously for 9 days or at intervals corresponding to the stages of osteogenesis: Stage 1=proliferation; Stage 2=extracellular matrix deposition; Stage 3=mineralization of extracellular matrix. The aromatase activity has been evaluated by measuring in the conditioned medium the concentration of estradiol by a competitive chemiluminescent enzyme immunoassay. The differentiating effect has been evaluated by the measurement of alkaline phosphatase, which is an early marker of osteogenesis and of calcium incorporation, which is a late marker of osteogenesis. The secretion of the metalloproteinases by means of zymogram has been evaluated in different stages. The results showed that the continous treatment for 9 days with IGF-II and IGFBP-2 alone or in combination, inhibits the physiologic decrease of aromatase and stimulates the differentiation markers, including the metalloproteinase activation. Conversely, treatment with IGFBP-3 inhibits both aromatase activity and cellular differentiation. IGF-II, IGFBP-2 and IGFBP-3 exerted their action on aromatase activity and cellular differentiation also when added in S1 stage. IGF-II resulted ineffective when added alone in S2 or S3 but in S2 addition of IGFBP-2 restored the effect. IGFBP-3 and IGFBP-2 exerted their action also in S2 but not in S3.

In conclusion, these preliminary data suggest that in our cell system the aromatase activity is related to the osteogenic differentiation stages. Moreover, the IGF system plays an important role in the regulation of both bone aromatase activity and osteogenesis.