The role of growth hormone (GH), insulin-like growth factor (IGF)-II and the IGF-I receptor (IGF-Ir) in the regulation of the in vivo expression of Na(+)-coupled anionic [System X-AG; GLAST1 (EAAT1), GLT1 (EAAT2), EAAC1 (EAAT3), EAAT4; where the human homologues of amino acid transport proteins first cloned in the rat are given in parentheses] and Na(+)-independent cationic (System y(+);CAT1) amino acid transport proteins was evaluated by comparing transporter expression in day 17 placentae of mice that overexpressed bovine GH (GH+) or that carried null gene mutations for IGF-II or IGF-Ir. Northern analysis revealed no apparent difference in the mRNA content of GLAST1 (EAAT1), EAAC1 (EAAT3), or EAAT4, in homogenates of GH+ placentae, but levels of GLT1 (EAAT2) and CAT1 mRNA were increased. Immunoblot analysis revealed that whole-placental steady-state GLAST1 (EAAT1), EAAC1 (EAAT3), and EAAT4 protein levels were not affected by GH+, whereas GLT1 (EAAT2) levels were increased. Immunohistochemical analysis showed that the cell-specific expression of the anionic and CAT1 transporters was not affected by overexpression of GH. Similar analyses of null IGF-II placentae demonstrated increases in GLAST1 (EAAT1), EAAT4 and CAT1 mRNAs. Parallel immunoblot analysis demonstrated decreased expression of GLT1 (EAAT2), GLAST1 (EAAT1) and EAAC1 (EAAT3) protein, but an increased expression of EAAT4. In null IGF-II and IGF-Ir placentae, however, GLT1 (EAAT2) and EAAC1 (EAAT3) protein content was decreased in junctional zone cells, whereas CAT1 content was increased in junctional and labyrinth zone cells. These data indicate that an excess level of GH stimulates GLT1 (EAAT2) expression and that a normal level of IGF-II is required for typical expression of GLT1 (EAAT2), GLAST1 (EAAT1) and EAAC1 (EAAT3), but that IGF-II downregulates the expression of EAAT4 and CAT1.
Copyright 1999 Harcourt Publishers Ltd.