1.  INTRODUCTION

       Steroid hormones play an essential role in the proliferation and differentiation of the mammary gland. As a consequence steroid hormones make the mammary epithelium prone to oncogenic derailment. In addition they may stimulate the promotion and progression of mammary tumors Studies using progesterone receptor knockout mice

(PRKO) have revealed the importance of progesterone in mammary development. Progesterone is essential for the lobulo-alveolar outgrowth of mammary epithelium. The highest DNA labeling indices are found in the progesterone-dominated phase of the sexual cycle. Progesterone exerts its effect through activation of type A- or type B-progesterone receptors, which bind to specific DNA response elements after dimerization. In addition specific co-activator or co-repressor proteins are essential for interaction with the general transcription complex and modulation of the transcription process. When wild-type mammary epithelium is transplanted into mammary tissue of PR-/­mice, progesterone stimulates the normal outgrowth of the neighbouring PR­/- epithelium. This indicates that progesterone stimulates epithelial proliferation in part by a paracrine factor.

2.   MAMMARY GROWTHHORMONE

         Synthetic progestins as well as endogenous progesterone may induce excessive plasma GH concentrations in the dog resulting in acromegaly and insulin resistance. The elevated plasma GH concentrations were characterized by the absence of a pulse pattern and insensitivity to stimulation and inhibition tests, with the exception of an inhibition by the progesterone antagonist RUU 486¹¹. These autonomous characteristics, and the absence of a

decrease of plasma GH concentrations after complete hypophysectomy was the impetus for a search for an extra-pituitary source. This resulted in the finding that the mammary gland was the source of progestin-induced plasma GH concentrations. This was confirmed by an arterio-venous gradient over the mammary gland, an immediate decline of plasma GH concentrations after complete mammectomy and the immunohistochemical staining of GH positive cells in foci of hyperplastic mammary   tissue.The expression of the gene encoding GH was also proven by RT-PCR, and Northern blot in the canine mammary gland. The stimulation of GH mRNA expression after progestin treatment in the cat and the presence of GH mRNA in the human mammary gland did prove that mammary GH expression is not a dog specific phenomenon.

3.  REGULATION OF MAMMARY GH GENE EXPRESSION

      Improved insights in the regulation of mammary GH synthesis may lead to specific treatments aimed to interfere with mammary GH production.Analysis of the mammary GH mRNA revealed that the mammary transcript is identical to that of the pituitary, including the 5’UTR. This suggests that also the same promoter is used. An essential element in the GH gene promoter is a Pit-1 response element. Mutations in Pit-1 (also called POU1F1) result into dwarfism due to disturbances in the development of GH (and prolactin and TSH) producing cells in the pituitary. Comparison of the canine GH gene promoter with that of the human GH gene showed the presence of a Pit-1 and a progesterone receptor response element (Fig. 1). However, analysis of Pit-1 mRNA within the mammary gland revealed that mammary GH expression occurred in the absence of Pit-1 expression. In addition the finding that German shepherd dwarfs, which have a pituitary anomaly resulting in GH deficiency, produce GH after progesterone stimulation further strengthen the differences in GH gene expression between mammary and pituitary tissue.Immunohistochemically all canine GH producing mammary cells stain for the PR but not all PR containing cells express GH. In malignant tumors GH mRNA can be found in the absence of detectable PR by ligand binding assays. These findings indicate that additional transcription factors are required  for mammary  GH  expression.

4.   BIOLOGICAL FUNCTIONS OF MAMMARY GH

       The release of mammary produced GH into the circulation results in the dog into clinical features of GH excess. However, mammary GH may have also local autocrine or paracrine effects on proliferation and differentiation of mammary epithelium (Fig. 2). Apart form direct effects on recruitment of stem cells, GH may also stimulate the local expression of IGF-I in stromal cells of the mammary fat pad.


                            

Fig. 2 Schematic representation of the contribution of mammary GH to the endocrine effects of pituitary produced GH, local para/autocrine tissue effects, and exocrine effects via milk.It has been shown that especially in colostrum supra-physiological GH concentrations higher than 1000 µg/1 can be found. The effects on immune function or postnatal development of the gastro-intestinal system of the neonate remain to be elucidated.

5. MAMMARY GH AND BREAST CANCER

     The local production of GH has implications for the insights in mammary gland tumor formation and progression, and treatment of mammary cancer. Expression of mammary GH is associated with local expression of IGFs and their binding proteins (IGFBPs), thus creating a proliferative environment for the glandular epithelium. Also treatment with GH may induce mammary gland hyperplasia as has been shown in aging primates. The stimulated   cell   proliferation   may   increase   the   possibilities   for   tumor initiation  by  mutagenic   compounds,   or  stimulate  tumor  promotion   and progression.Against the background of these considerations treatment of mammary cancer with progesterone agonists may need to be re-evaluated for potential adverse effects. Whether progestins have beneficial or adverse effects may be dependent upon the specificity of the progestin. Frequently used compounds such as MPA and megestrol acetate have also profound glucocorticoid and androgen agonistic effects, which may in part be responsible for the observed beneficial effects in patients with mammary cancer.