Growth hormone (GH) status regulates GH receptor and GH binding protein mRNA in a tissue- and transcript-specific manner but has no effect on insulin-like growth factor-I receptor mRNA in the rat

The effect of growth hormone-deficiency (GHD) and treatment with recombinant bovine GH (bGH) or human IGF-I (hIGF-I) for 10 days on the expression of GH receptor (GHR). GH binding protein (GHBP) and of insulin-like growth factor-I receptor (IGF-IR) mRNA was examined using dw/dw and normal Lewis rats. Hepatic GHR and GHBP mRNA expression was significantly lower in dw/dw rats in comparison to Lewis rats (P < 0.01) while specific ¹²⁵I-bGH binding to hepatic microsomal membranes was significantly higher (P < 0.01), suggesting a reduction in hepatic GHR turnover with GHD. Treatment with bGH reduced hepatic specific ¹²⁵I-bGH binding in dw/dw rats, but had no effect in Lewis rats. Treatment with hIGF-I increased hepatic specific ¹²⁵I-bGH binding in Lewis rats. Hepatic GHR and GHBP mRNA expression was not changed by bGH or hIGF-I treatment, suggesting that differences in hepatic specific ¹²⁵I-bGH binding may be due to posttranscriptional mechanisms. GHBP mRNA expression was higher in kidney, heart, and muscle of dw/dw rats in comparison to Lewis rats (P < 0.01), while GHR mRNA abundance was not changed. Treatment of dw/dw rats with hIGF-I or bGH resulted in a coordinate reduction of GHR and GHBP mRNAs in kidney (P < 0.01). IGF-IR mRNA was not detected in liver and despite reduced plasma IGF-I levels and IGF-I mRNA expression IGF-IR mRNA abundance was not changed in nonhepatic tissues by GHD. Our data suggest that changes in plasma IGF-I levels and local IGF-I mRNA do not influence IGF-IR mRNA expression, while GHR and GHBP mRNA expression in different rat tissues are regulated independently. The increased nonhepatic GHBP mRNA expression with GHD suggests that nonhepatic GHBP may have an important physiological function distinct from that of GHBP in liver or in plasma.