Identification and characterization of growth hormone receptor mRNA in the mammary gland.

The present report describes the first characterization of growth hormone (GH) receptor (GH-R) mRNA in the rabbit mammary gland. Northern blot analysis of poly(A)+ RNA isolated from several tissues of rabbit probed with a rabbit liver GH-R cDNA fragment revealed hybridization to only one transcript of 4.2 kb. A specific hybridizing signal appears in the mammary gland mRNA during gestation, when three different probes derived from liver GH-R cDNA and encoding respectively for extracellular, transmembrane and intracellular regions, were used. The signal is lower than in the liver but highly significant. These results indicate that the three regions are present and well conserved in the GH-R transcript found in the mammary gland. By S1 nuclease mapping analysis we demonstrated that the extracellular and transmembrane domains of mammary gland GH-R mRNA are strongly homologous to the liver GH-R mRNA. In addition, mammary gland GH-R mRNA is probably generated by mammary epithelial cells as demonstrated by the hybridization signal obtained using mRNA extracted from purified acini. The increase in the concentration of GH-R mRNA occurs during epithelial cell proliferation associated with a decrease in the proportion of adipocytes and connective cells at late gestation. The 4.2 kb GH-R mRNA species was also detected in ovine and porcine mammary glands during gestation, suggesting a probable expression of the related form of GH-R in these species.