Differential expression of the growth hormone receptor and growth hormone-binding protein in epithelia and stroma of the mouse mammary gland at various physiological stages

Increasing evidence suggests that GH is important in normal mammary gland d evelopment. To investigate this further, we studied the distribution and le vels of grow-th hormone receptor (GHR) and GH-binding protein (GHBP) in the mouse mammary gland. At three weeks of age, the epithelial component of th e right fourth inguinal mammary gland of female mice was removed. These ani mals were then either maintained as virgins until they were killed or they were mated. One group of the mated mice was killed on day 18 of pregnancy a nd the remaining mated animals were allowed to carry their pups until term and were killed on day 6 of lactation. At the time of death, both the intac t left and the de-epithelialized right mammary glands were collected from a ll three groups. Some of the intact glands served as a source of epithelial cells, free of stroma. The mRNA levels for GHR and GHBP were measured in i ntact glands, epithelia-cleared fat pads, and isolated mammary epithelial c ells. GHR and GHBP mRNAs were expressed in both the mammary epithelium and stroma. However, the levels of both GHR and GHBP mRNAs were significantly h igher in the stroma as compared with the epithelium component. This increas e for both mRNAs was from 3- to 12-fold at each physiological state examine d. In the intact gland, both GHR and GHBP transcripts were highest in virgi ns, declined during late pregnancy, and the lowest levels were found in the lactating gland. GHBP and GHR protein concentrations were also assessed in intact glands and epithelia-fi-ee fat pads. Similar to the mRNAs, GHR and GHBP protein levels (means +/- S.E.M.) in intact glands were highest in vir gin mice (0.891 +/- 0.15 pmoles/mg protein and 0.136 +/- 0.26 pmoles/mg pro tein respectively), declined during late pregnancy (0.354 +/- 0.111 pmoles/ mg protein and 0.178 +/- 0.039 pmoles/mg protein respectively), and were lo west during lactation (0.096 +/- 0.037 pmoles/mg protein and 0.017 +/- 0.00 6 pmoles/mg protein respectively). Immunocytochemistry utilizing specific a ntisera against mouse (rn) GHR and mGHBP revealed that the two proteins are localized to both the stroma and parenchyma of mouse mammary glands, with similar patterns of immunostaining throughout the different physiological s tages analyzed. GHR immunolocalized to the plasma membrane and cytosol of m ammary epithelial cells and adipocytes, whereas the GHBP immunostaining was nuclear and cytosolic. In conclusion, we report here that GHR and GHBP mRN As and proteins are expressed in both the epithelium and the stroma of mamm ary glands of virgin, pregnant, and lactating mice. In intact glands, GHR a nd GHBP proteins, as well as their transcripts are higher in abundance in v irgin relative to lactating mice. At all physiological stages, GHR and GHBP mRNA levels are higher in the stroma compared with the parenchyma. These f indings indicate that the actions of GH in the mammary gland are both direc t through its binding to the epithelia, and indirect by binding to the stro ma and stimulation of IGF-I production which, in turn, affects mammary epit helial development.