PSIII-22 Downregulations in the expression of insulin-like growth factor binding proteins in a temporal model of in utero hypothyroidism

Abstract The insulin-like growth factor (IGF) system is responsible for mediating the effects of growth hormone, which has important roles in regulating fetal development and growth throughout gestation. Prior research suggests a relationship between thyroid hormones and activity of the IGF system; however, the temporal association of this interaction is not well understood. As a result, the objective of the current study was to better characterize temporal dysregulations in the IGF system occurring as a result of induced fetal hypothyroidism in a porcine model. To accomplish this, pregnant gilts (n = 24) were divided evenly into a control (CON) and methimazole (MMI) group. The MMI group was treated daily with 5 mg/kg of oral MMI, a potent antithyroid drug, while the CON received an equivalent sham carrier. The two groups were further subdivided into four timepoints (n = 3/group at each timepoint), with treatment in each group beginning at either d 34, 45, 55, or 65 of gestation. Treatment was continued for 21 d, and gilts were subsequently euthanized to allow for fetal sample collection at d 55, 66, 76, and 86, respectively. Blood samples were collected from the axillary artery of all fetuses (n = 340), and extensive sampling of fetal thyroid, liver (LVR), and kidney (KID) performed on a subset of 4 fetuses per litter (n = 96). Fetuses from the MMI group were confirmed hypothyroid by the presence of goitrous thyroid histology and significantly decreased circulating thyroxine concentrations relative to CON (P < 0.001 for all timepoints). Following confirmation of fetal hypothyroidism, samples of LVR and KID were ground to a fine powder to allow for RNA extraction, reverse transcription, and analysis of gene expression by qPCR. Resultant Ct values were normalized within tissue based on the geometric mean of two stable reference genes, with fold changes calculated using the 2-∆∆Ct method. P-values were calculated within timepoint using a Wilcoxon signed-rank test, with the threshold for statistical significance P < 0.05. IGF2, the major IGF present throughout gestation, was significantly upregulated at d 86 in KID only (P = 0.005). Many significant changes were observed in the expression of the IGF binding proteins (IGFBPs), which modulate the activity of IGF1 and IGF2. In the LVR, both IGFBP1 and IGFBP3 were downregulated at d 76 (P = 0.006 and P = 0.002, respectively). Further, hepatic IGFBP2 was downregulated at all timepoints (P = 0.003, 0.028, < 0.001, and < 0.001, respectively), and hepatic IGFBP4 downregulated at d 66, 76, and 86 (P = 0.020, < 0.001, and < 0.001, respectively). These results indicate that fetal hypothyroidism impacts IGF function via downregulation of hepatic-derived IGFBPs. These changes in IGF activity may alter fetal growth and development, providing a possible explanation for the growth modulations observed in fetal hypothyroidism.