Detection of two growth hormone receptor mRNAs and primary translation products in the mouse

Two mouse growth hormone-receptor primary translation products of Mr 95,900 and 31,800 were identified from in vitro-translated late pregnant mouse liver mRNA. RNA isolated from mouse liver was translated in a rabbit reticulocyte lysate system containing [35S]methionine, and the growth hormone receptor primary translation products were identified by immunoprecipitation with anti-mouse growth hormone receptor antiserum followed by sodium dodecyl sulfate/PAGE and fluorography. Detectable amounts of the Mr 95,900 and 31,800 proteins were not present in in vitro-translated nonpregnant mouse liver mRNA. This result is consistent with previous observations of the up-regulation of growth hormone receptors in the liver during pregnancy in the mouse. Northern (RNA) blot analysis of mouse liver and adipose tissue RNA with a rabbit growth hormone receptor cDNA probe revealed two hybridizing mRNAs of approximately 3.9 and 1.2 kilobases. These two RNAs were greatly up-regulated in liver, but not in adipose tissue, during pregnancy. The sizes of these mRNAs closely matched predictions of the sizes of the mRNAs coding for the proteins of Mr 95,900 and 31,800 made by in vitro translation of size-fractionated late-pregnant mouse liver poly(A)+RNA. These results suggest a mechanism for the generation of both the heterogeneous forms of the growth hormone receptor identified in mouse liver membrane preparations and the mouse serum growth hormone-binding protein.