EVIDENCE OF A DIRECT ROLE FOR GROWTH-HORMONE (GH) IN MAMMARY-GLAND PROLIFERATION AND LACTATION

A panel of monoclonal antibodies to the growth hormone (GH) receptor/b inding protein was used to demonstrate the existence and detail the ex pression of GH receptors in ductal and alveolar epithelial cells from rat and rabbit mammary glands by immunohistochemistry. Intense immunor eactivity was present in membrane, cytoplasm and some nuclei of epithe lial cells during proliferation and lactation. Receptor expression dec reased during weaning and was absent or weak in regressive mammary gla nds. Immunoreactivity was weak in ductal epithelial cells from virgin adult animals. Pronounced expression of GH receptor/binding protein wa s observed with two monoclonal antibodies and lesser reactivity was se en with others, paralleling their affinities for the receptor. The cyt oplasmic presence of this putatively plasma membrane located GH recept or is accounted for by the existence of a soluble form on the GH recep tor, namely the growth hormone binding protein derived from the membra ne receptor by cleavage. Primary localization of the receptor in proli ferating and lactating epithelial cells suggests that the rat and rabb it mammary gland is a GH target tissue. This finding is in contradicti on to both classical GH action and the somatomedin hypothesis and chal lenges the widely held view that GH has no direct influence on mammary growth and function.