Influence of thyroxine in vivo on preadipocyte development and insulin-like growth factor-I and IGF binding protein secretion in fetal stromal vascular cell cultures

Studies were conducted to determine the influence of thyroxine (T-4) in vivo on preadipocyte development and insulin-like growth factor-I (IGF-I) and IGF binding proteins (IGFBPs) secretion in stromal-vascular (S-V) cultures. Fetal pigs were hypophysectomized (hypox) at 70 days of gestation, implanted with T-4 pellets, and fetuses from the dam at 75 days of gestation, In a second experiment, hyper and T-4 implantation were performed at 75 days and fetal pigs removed at 95 days of gestation. Primary cultures of stromal vascular (S-V) cells derived from fetal adipose tissue were established. Cultures were stained for morphological analysis and conditioned media were collected for IGF-I determination by radioimmunoassay (RIA) and IGFBP analysis by Western blotting. After only 5 days of T-4 treatment, fat cell cluster number and size and lipid deposition in cultures were significantly increased compared to cultures from untreated hyper fetuses. Fetal hypox reduced IGF-I secretion by preadipocytes at both ages and T-4 treatment completely normalized IGF-I secretion (p<0.05). Four IGFBPs (BP-1, BP-2, BP-3 & BP-4) detected in S-V cultures derived from 95-day fetuses were decreased in concentration by hypox by 44 +/- 9.4%, 32 +/- 9.7%, 42 +/- 12% and 53 +/- 6.9%. In cultures derived from T-4 treated hypox fetuses, the levels of these four IGFBPs were increased by 187 +/- 25%, 239 +/- 38%, 190 +/- 5% and 347 +/- 43% over control values, respectively. In cultures from 75-day fetuses, only IGFBP-2 (major one) and BP-1 (minor one) were detected and their secretion was also decreased by hypox and elevated by T-4 treatment (190 +/- 49.5%, 156 +/- 30%, respectively, of controls). The results provide direct evidence that T-4 has a major influence on fetal preadipocyte development. T-4 stimulated production of IGF-I and IGFBP in fetal S-V cultures, which in turn, may have mediated the capability of T-4 to enhance fetal adipose tissue development.