Effects of IGF-I on 1,25-dihydroxyvitamin D3 synthesis by human placenta in culture

The aim of the present study was to assess the effects of insulin-like growth factor I (IGF-I) upon the synthesis of 1,25-dihydroxyvitamin D-3 [1,25-(OH)(2)D-3] by human placenta trophoblasts in culture. Cytotrophoblastic cells obtained from normal term human placentae were cultured in Dulbecco's modified Eagle's medium with HEPES and glucose (DMEM-HG) during 72 h and further incubated in serum-free DMEM-F12 in the presence of IGF-I prior to the addition of [H-3]-25-(OH)D-3 used as a precursor. The results showed that 2 h preincubation time with IGF-I was required for maximal production of [H-3]-1,25-(OH)(2)D-3. Cultures in the presence of increasing concentrations of IGF-I (0-6.5 nmol/l), added 2 h before incubation with the labelled substrate, resulted in a dose-dependent response increment of [H-3]-1,25-(OH)(2)D-3 production with a maximal conversion rate at the dose of 2.6 nmol/l. Higher doses of IGF-I did not result in further stimulatory effects. Go-incubations in the presence of cycloheximide significantly (P < 0.0001) inhibited the IGF-I-mediated effects upon [H-3]-1,25(OH)(2)D-3 production. Identity of putative [H-3]-1,25-(OH)(2)D-3 produced by human placenta was confirmed by spectral and receptor binding analysis. These results demonstrate the ability of cultured human syncytiotrophoblast cells to convert 25-(OH)D-3 to 1,25-(OH)(2)D-3 and suggest a local protein-dependent regulatory effect of IGF-I upon this biotransformation.