RESUMO
Human tissues express growth hormone receptors (hGHR) by the 3rd mo of gestation. We assessed developmental changes in hGHR function in fibroblasts and liver, testing binding and hormonal response. Fetal cells showed low but reproducible hGH binding. No age-related changes occurred in fibroblasts (9 wk-34 yr). In contrast, there was a fourfold increase in hGH binding in postnatal liver, with a sixfold increase in hGHR mRNA. Both full-length and truncated hGHR mRNAs were detected in all livers. Cross-linking revealed a larger hGH/receptor complex in fetal liver. Fetal hepatocytes produced 10 times more insulin-like growth factor (IGF)-II than IGF-I, and responded to hGH (150 ng/ml) with a significant increase in IGF-II. Fetal hepatocytes secreted three IGF-binding proteins (IGFBPs), including IGFBP1, but not IGFBP3. hGH did not alter fetal hepatocyte IGFBPs but stimulated glucose uptake. Exposure of fibroblasts to hGH decreased hGH binding only in >1-yr postnatal fibroblasts, whereas treatment with dexamethasone (100-400 nM) increased binding only in postnatal cells. Thus, although fetal hepatocytes and fibroblasts possess functional hGHR, these receptors (and/or their signaling pathways) are immature or have adapted to the in utero environment.
