Canine mammary growth hormone gene transcription initiates at the pituitary-specific start site in the absence of Pit-1

Molecular Signaling of Progesterone, Growth Hormone, Wnt, and HER in Mammary Glands of Dogs, Rodents, and Humans: New Treatment Target Identification

Mammary tumors are the most common form of neoplasia in the bitch. Female dogs are protected when they are spayed before the first estrus cycle, but this effect readily disappears and is already absent when dogs are spayed after the second heat. As the ovaries are removed during spaying, ovarian steroids are assumed to play an essential role in tumor development. The sensitivity toward tumor development is already present during early life, which may be caused by early mutations in stem cells during the first estrus cycles. Later on in life, tumors arise that are mostly steroid-receptor positive, although a small subset of tumors overexpressing human epidermal growth factor 2 (HER2) and some lacking estrogen receptor, progesterone receptor (PR), and HER2 (triple negative) are present, as is the situation in humans. Progesterone (P₄), acting through PR, is the major steroid involved in outgrowth of mammary tissue. PRs are expressed in two forms, the progesterone receptor A (PRA) and progesterone receptor B (PRB) isoforms derived from splice variants from a single gene. The dog and the whole family of canids have only a functional PRA isoform, whereas the PRB isoform, if expressed at all, is devoid of intrinsic biological activity. In human breast cancer, overexpression of the PRA isoform is related to more aggressive carcinomas making the dog a unique model to study PRA-related mammary cancer. Administration of P₄ to adult dogs results in local mammary expression of growth hormone (GH) and wing less-type mouse mammary tumor virus integration site family 4 (Wnt4). Both proteins play a role in activation of mammary stem cells. In this review, we summarize what is known on P₄, GH, and Wnt signaling in canine mammary cancer, how the family of HER receptors could interact with this signaling, and what this means for comparative and translational oncological aspects of human breast cancer development.

The contribution of growth hormone to mammary neoplasia

While the effects of growth hormone (GH) on longitudinal growth are well established, the observation that GH contributes to neoplastic progression is more recent. Accumulating literature implicates GH-mediated signal transduction in the development and progression of a wide range malignancies including breast cancer. Recently autocrine human GH been demonstrated to be an orthotopically expressed oncogene for the human mammary gland. This review will highlight recent evidence linking GH and mammary carcinoma and discuss GH-antagonism as a potential therapeutic approach for treatment of breast cancer.

Autocrine growth hormone: effects on growth hormone receptor trafficking and signaling

GH and GH receptor are expressed in many extrapituitary tissues, permitting autocrine/paracrine activity. Autocrine GH has regulatory functions in embryonic development and cellular differentiation and proliferation and is reported to be involved in the development and metastasis of tumor cells. To understand the principles of transport and signaling of autocrine GH and GH receptor, we used a model system to express both proteins in the same cell. Our experiments show that GH binds the GH receptor immediately after synthesis in the endoplasmic reticulum and facilitates maturation of GH receptor. The hormone-receptor complexes arrive at the cell surface where exogenously added GH is unable to bind these receptors. Autocrine GH activates the GH receptors, but signal transduction occurs only after exiting the endoplasmic reticulum. This model study explains why autocrine GH-producing cells may be insensitive for GH (antagonist) treatment and clarifies autocrine signaling events.

The oncogenic potential of growth hormone

A growing body of recent literature indicates that in addition to an essential role in growth and development, growth hormone may also play a more sinister role in oncogenic transformation and neoplastic progression. Here we review the accumulating evidence implicating growth hormone in the development and progression of cancer and describe what is known of the mechanisms utilised by this hormone in neoplastic transformation.

Pit-1 is expressed in normal and tumorous human breast and regulates GH secretion and cell proliferation

BACKGROUND

The transcription factor pituitary-1 (Pit-1) is mainly expressed in the pituitary gland, where it has critical roles in cell differentiation and as a transcriptional factor for GH and prolactin (PRL). It is also expressed in human extrapituitary tissues (placenta, lymphoid and haematopoietic tissues) and cell lines (human breast adenocarcinoma cells, MCF-7). Despite the widely suggested roles of GH and PRL in the progression of proliferative mammary disorders, Pit-1 expression in human mammary gland has not yet been reported.

OBJECTIVE

To evaluate the expression of Pit-1 in human breast and, using the MCF-7 cell line, to investigate whether Pit-1 overexpression regulates GH expression and increases cell proliferation.

METHODS

Using real-time RT-PCR, western blotting and immunohistochemistry, we evaluated the expression of Pit-1 mRNA and protein in seven normal human breasts and 14 invasive ductal mammary carcinomas. GH regulation by Pit-1 in MCF-7 cells was evaluated using RT-PCR, western blotting, ELISA and transfection assays. Cell proliferation was evaluated using bromodeoxyuridine.

RESULTS

We found expression of Pit-1 mRNA and protein in both normal and tumorous human breast. We also found that Pit-1 mRNA levels were significantly increased in breast carcinoma compared with normal breast. In MCF-7 cells, Pit-1 overexpression increased GH mRNA and protein concentrations and significantly increased cell proliferation.

CONCLUSIONS

These findings indicate that Pit-1 is expressed in human breast, that it regulates endogenous human mammary GH secretion, and that it increases cell proliferation. This suggests that, depending on its level of expression, Pit-1 may be involved in normal mammary development, breast disorders, or both.

Cortisol stimulates growth hormone gene expression in rainbow trout leucocytes in vitro

Extrapituitary expression of the growth hormone (GH) gene has been reported for the immune system of various vertebrates. In the rainbow trout (Oncorhynchus mykiss), GH mRNA could be detected in several lymphoid organs and leucocytes by reverse transcriptase-polymerase chain reaction (RT-PCR). To understand the control of GH expression in the fish immune system, mRNA levels for two distinct GH genes (GH1 and GH2) in trout leucocytes isolated from peripheral blood were quantified using a real-time PCR method. Both GH mRNAs could be detected in trout leucocytes, although their levels were extremely low compared to those in pituitary cells. The levels of GH2 mRNA in leucocytes were several times higher than those of GH1, while no difference was observed between GH1 and GH2 mRNA levels in the pituitary. Administration of dibutyryl cyclic AMP and cortisol produced a significant elevation of GH mRNA levels in trout leucocytes, although the levels were unchanged by T3. GH1 and GH2 mRNA levels showed similarities in responses to those factors. The effect of cortisol on GH mRNA appears biphasic; a dose-depending elevation of GH gene expression was observed in leucocytes treated with cortisol at below 200 nM, however, cortisol had no effect at 2000 nM. Cortisol-treated leucocytes showed no significant change in the mRNA level of beta-actin or proliferative activity during the experiments. Our results thus show that, at the low levels, GH gene expression in trout leucocytes is regulated by cortisol, which has been known as a regulatory factor of GH gene expression in pituitary cells, and suggest a physiological significance of paracrine GH produced in the fish immune system.

Cloning and characterization of the 5'-flanking region of the canine growth hormone gene

The growth hormone (GH) gene is expressed in a variety of tissues outside the pituitary, including the mammary gland. GH expression in the mammary gland is stimulated by progestins. The local synthesis of mammary GH may provide a highly proliferative environment within the mammary gland that may contribute to the development or progression of mammary tumours. To elucidate the mechanism regulating mammary GH expression, we cloned the 5'-flanking region of the canine GH gene using inverse polymerase chain reaction. Gel-shift experiments showed that several sequences in the 5'-flanking region of the GH gene bind mammary nuclear proteins and may be involved in basal and progesterone-induced mammary GH expression. Sequence analysis and comparison with the GH promoters of human, rat, and mouse genes revealed a number of shared binding sites for transcription factors such as Pit-1, which is involved in pituitary GH expression, and for factors involved in the differentiation of lymphoid cells. Moreover, a putative binding site for the progesterone receptor (PR) was identified in all promoters, indicating that the progestin-induced expression of GH in mammary tissue is most probably a direct effect of activated PRs on the GH gene promoter and that this may occur in various species.

Growth hormone gene expression in canine normal growth plates and spontaneous osteosarcoma

The indirect growth-promoting action of pituitary-derived growth hormone (GH) on skeletal growth is thought to be mediated by systemically released insulin-like growth factor-I (IGF-I) and by locally produced IGF-I. The aim of the present study was to document whether GH is expressed locally in canine bone and spontaneous osteosarcoma. Using RT-PCR the expression of GH mRNA was demonstrated in the metaphyseal, but not in the majority of epiphyseal ends of the canine growth plate. GH mRNA was also present in 25% of the canine osteosarcoma specimens. The expression of GH mRNA in predominantly active osteoid forming areas was associated with the presence of immunoreactive GH in osteoblasts, as shown by immunohistochemistry. It is concluded that locally produced GH is involved in osteoid formation and may play a role in the growth of neoplastic bone lesions in the dog.

Locally produced growth hormone in canine insulinomas

The production and release of GH has been demonstrated in a variety of extra-pituitary tissues. In this respect insulin-producing pancreatic tumours are also of interest because it has been observed that GH may promote islet cell proliferation. However, these effects have only been related to GH of pituitary origin and the possibility of local production of GH with autocrine-/paracrine effects has not been considered. In this study, a reverse transcriptase polymerase chain reaction (RT-PCR) was used to demonstrate the presence of GH mRNA in pancreatic tissue of five healthy dogs and insulinomas of 14 dogs. After Southern blotting of the RT-PCR products, blots were hybridized using a canine-specific GH-probe and quantified using phosphor imaging. GH gene expression was further demonstrated by in situ hybridization using a canine digoxigenin-labelled GH-specific cDNA probe. In addition, GH immunohistochemistry was performed. In five samples of normal pancreatic tissue a weak hybridization signal was found. This signal was significantly higher in nine of 12 primary tumours. In ten of 11 metastases there was a positive hybridization signal, and this signal was also significantly higher than in the primary tumours. In situ hybridization in one sample demonstrated that GH mRNA was only produced in the tumour cells. The local production of GH was confirmed by positive staining of tumour tissue with anti-GH antibodies in ten of 12 samples. It is concluded that canine insulinomas express the gene encoding GH mRNA. The locally produced GH may have an autocrine/paracrine effect on tumour progression. The relatively high expression levels in metastases of these tumours may be related to the low inhibitory influence of somatostatin outside the pancreas.

Morphogenic and tumorigenic potentials of the mammary growth hormone/growth hormone receptor system

Due to the characteristics of the luteal phase of the ovarian cycle in the dog, which spans a prolonged time period, this species is a suitable model to study the role of progestins in both normal morphogenic and abnormal tumorigenic processes in the mammary gland. It has been convincingly shown that progestins, including endogenous progesterone, induce the synthesis of growth hormone (GH) in the normal and the tumorous canine mammary gland. The growth hormone receptor (GHR) is also expressed in normal and tumorous canine mammary tissues and in this concise overview we highlight recent advances in our understanding of the significance of the GH/GHR system for mammary gland (patho)biology. In an attempt to unravel the cellular and molecular mechanisms associated with the GH/GHR system, we were able to show that both GH and GHR are differentially expressed in normal canine mammary tissues. Maximum expression of both GH and GHR occurs during the proliferation phase of the tissue, which links the progestin-induced mammary GH synthesis to the progestin-associated proliferation of epithelial cells in the mammary gland. Expression of the GH/GHR system is also present in most canine mammary tumors, albeit that GHR expression may be downregulated in undifferentiated mammary carcinomas. Upon GH stimulation of the GHR-positive CMT-U335 canine mammary tumor cell line, the transcription factors STAT5A and STAT5B become phosphorylated on their tyrosine residues, which is likely to reflect the significance of mammary GH in vivo. Molecular analysis of the canine mammary GHR transcripts by RT-PCR provided evidence for normal and alternative processing of the GHR primary transcript encoding the full-length plasma membrane GHR and at least four putative GH binding proteins (GHBPs), respectively. The translation products from the alternatively spliced GHR transcripts indicate an intact N-terminal ligand binding domain and an unique C-terminal portion, lacking the transmembrane domain and cytoplasmic tail. Thus, these proteins are considered to be able to bind GH, but have lost their signaling potential. The exact biological role of these GHBPs remains to be established, but GHBPs may have a transport function in the endocrine route, regulate the level of biologically available GH locally, or dominant-negatively influence the full-length plasma membrane GHR. In dog mammary cancer specimens strongly reduced levels of alternatively spliced GHR transcripts were found compared to the non-malignant mammary tissue. Notably, expression of both GH and GHR in mammary cancer cells is not restricted to dogs. Recent experiments generated evidence for GH and GHR expression in human breast cancer cells, and also in human prostate cancer cells, which represents another highly prevalent hormone-sensitive human malignancy. In agreement with our findings in the dog, the expression of the hGH-N gene in human mammary cancer cells seemed to correlate positively with their progesterone receptor status, which warrants, in our opinion, a reconsideration of the role of progestins in breast cancer of women. In human prostate cancer cells four different hGH-N transcripts were detected, which encode classical 22 kDa GH and GH-related proteins. Consistent with the findings on the canine GHR, different GHR transcripts in human mammary cancer cells and prostate cancer cells were detected encoding the full-length plasma membrane GHR and putative GHBPs.

Autocrine human growth hormone (hGH) regulation of human mammary carcinoma cell gene expression. Identification of CHOP as a mediator of hGH-stimulated human mammary carcinoma cell survival

By use of cDNA array technology we have screened 588 genes to determine the effect of autocrine production of human growth hormone (hGH) on gene expression in human mammary carcinoma cells. We have used a previously described cellular model to study autocrine hGH function in which the hGH gene or a translation-deficient hGH gene was stably transfected into MCF-7 cells. Fifty two of the screened genes were regulated, either positively () or negatively (), by autocrine production of hGH. We have now characterized the role of one of the up-regulated genes, chop (gadd153), in the effect of autocrine production of hGH on mammary carcinoma cell number. The effect of autocrine production of hGH on the level of CHOP mRNA was exerted at the transcriptional level as autocrine hGH increased chloramphenicol acetyltransferase production from a reporter plasmid containing a 1-kilobase pair fragment of the chop promoter. The autocrine hGH-stimulated increase in CHOP mRNA also resulted in an increase in CHOP protein. As a consequence, autocrine hGH stimulation of CHOP-mediated transcriptional activation was increased. Stable transfection of human CHOP cDNA into mammary carcinoma cells demonstrated that CHOP functioned not as a mediator of hGH-stimulated mitogenesis but rather enhanced the protection from apoptosis afforded by hGH in a p38 MAPK-dependent manner. Thus transcriptional up-regulation of chop is one mechanism by which hGH regulates mammary carcinoma cell number.

Progestin-induced mammary growth hormone (GH) production

Toxicity studies using beagle dogs revealed in the 1980s that synthetic progestins may induce a syndrome of growth hormone (GH) excess, known as acromegaly, and the development of predominantly benign mammary hyperplasia. In the early 1990s is was discovered that progestin-induced GH excess in the dog originates within the mammary gland. This mammary-derived GH may have endocrine, para/autocrine as well as exocrine effects. The expression of GH mRNA is also found in cats and humans indicating that mammary GH expression is not unique for the dog. The mammary gene is identical to the pituitary-expressed gene and uses the same promoter. Nevertheless a striking difference exists in the mammary gland. Pit-1, which is a prerequisite factor for pituitary GH mRNA expression, is likely not involved in the mammary gene expression. These studies shed new light on the mechanism of progesterone-induced mammary hyperplasia and urges for further research on potential adverse effects of synthetic progestins.

Cloning and cellular localization of the canine progesterone receptor: co-localization with growth hormone in the mammary gland

The mammary gland has been found to express the gene encoding growth hormone (GH) in several species. Within the mammary gland, it may act as an autocrine/paracrine growth factor for cyclic epithelial changes, and may be a determinant in mammary carcinogenesis. In the dog, progestins enhance mammary GH expression. To elucidate the mechanism of progestin-induced mammary GH expression, the canine progesterone receptor (PR) is characterized and the cellular localization of the PR in normal and tumorous mammary tissues is examined. Sequence analysis of the canine PR revealed two in-frame ATG codons, encoding a putative PR-B protein of 939 amino acids and a putative PR-A protein of 765 amino acids. Western blot analysis indicated that both isoforms occur in uterus and mammary gland issues. Immunohistochemical analysis of the PR revealed that the PR was differentially expressed in mammary tissue, with many PR-positive epithelial cells in the proliferation phase of the glandular tissue and a low number of PR-positive cells in differentiated mammary tissue. Stromal and myoepithelial cells had no specific PR staining. Mammary tumours had a variety of staining patterns, including no staining, normal nuclear staining, marked heterogeneous immunoreactivity and perinuclear staining of tumorous epithelial cells and cytoplasmic-staining of spindle cells. Double staining showed that all GH-producing cells were positive for PR, whereas not all PR containing cells stained for GH. It is concluded that the activated PR may transactivate GH expression in the mammary gland within the same cell and functions as a pre-requisite transcription factor. However, during malignant transformation this regulation may be lost.

Pituitary and extrapituitary growth hormone: Pit-1 dependence?

Growth hormone (GH) is primarily produced in pituitary somatotrophs. The synthesis of this hormone is thought to be dependent upon a pituitary-specific transcription factor (Pit-1). However, many extrapituitary tissues are now known to express GH genes. The extrapituitary production of GH may therefore indicate an extrapituitary distribution of the Pit-1 gene. The extrapituitary production of GH may, alternatively, indicate that GH expression occurs independently of Pit-1 in extrapituitary tissues. These possibilities are considered in this brief review.Key words: growth hormone, pituitary, pituitary transcription factor 1.

Mammary growth hormone and tumorigenesis--lessons from the dog

The discovery in the early 1990s that progestin-induced growth hormone (GH) excess in the dog originates in the mammary gland can be seen as a hallmark in the research on the pathogenesis of mammary cancer in the dog. The local biosynthesis and release of GH may provide a highly proliferative environment in the mammary gland, which contributes to the development and/or progression of mammary tumours. Before final goals such as prevention of tumour formation or inhibition of tumour promotion can be achieved it is of eminent importance to elucidate the mechanism of progesterone-induced mammary GH production and the mechanism of local autocrine/paracrine action of GH. These local GH effects may be achieved through direct growth stimulating effects of GH as well as by indirect effects mediated by the stimulation of the biosynthesis of insulin-like growth factor-I (IGF-I). The biological effects of the IGFs largely depend on the presence of IGF binding proteins (IGFBPs) which may both enhance or inhibit the activity of the IGFs. This review concentrates on recent advances in the understanding of the local mammary GH-IGF axis and the lessons which can be drawn from the dog for mammary cancer research in other species.