Autocrine Human Growth Hormone Inhibits Placental Transforming Growth Factor-β Gene Transcription to Prevent Apoptosis and Allow Cell Cycle Progression of Human Mammary Carcinoma Cells

Abstract

Multiple cellular effects of human growth hormone (hGH) are mediated by an indirect mechanism requiring transcriptional activation of genes encoding protein effector molecules such as insulin-like growth factor-1. Such protein effector molecules then act directly to mediate the cellular functions of hGH. We report here that autocrine hGH production by mammary carcinoma cells specifically results in the transcriptional repression of the p53-regulated placental transforming growth factor-␤ (PTGF-␤) gene. Transcriptional repression of the PTGF-␤ gene does not require the p53-binding sites in the PTGF-␤ promoter, and autocrine hGH also desensitized the response of the PTGF-␤ promoter to p53 overexpression. Transcriptional repression of the PTGF-␤ gene is accompanied by consequent decreases in its protein product, Smad-mediated transcription, and its cellular effects that include cell cycle arrest and apoptosis. PTGF-␤ specifically inhibited the autocrine hGH-stimulated expression of cyclin D1 required for autocrine hGH-stimulated mammary carcinoma cell cycle progression. Thus, one mechanism by which autocrine hGH promotes an increase in mammary carcinoma cell number is by transcriptional repression of protein effector molecules that promote cell cycle arrest and apoptosis. Such transcriptional repression of negative regulatory factors, such as PTGF-␤, may also be requisite for direct stimulation of mammary carcinoma cell mitogenesis by hGH. The human growth hormone (hGH)1 gene is expressed in epithelial cells of the normal human mammary gland.2 Increased epithelial expression of the hGH gene is associated with the acquisition of pathological proliferation, and the highest level of hGH gene expression is observed in metastatic mammary carcinoma cells. 2 hGH receptor gene expression per mammary epithelial cell remains constant throughout the process of neoplastic progression (1), and therefore changes in the local concentration of ligand are likely to be pivotal to determine the effects of hGH on the behavior of the mammary epithelial cell. We have recently generated a model system to study the role of autocrine-produced hGH in mammary carcinoma by stable transfection of either the hGH gene or a translation-deficient hGH gene into mammary carcinoma (MCF-7) cells (2). The autocrine production of hGH by mammary carcinoma cells results in a hyperproliferative state with marked synergism observed between trophic agents such as IGF-1 (2). The increase in mammary carcinoma cell number as a consequence of autocrine production of hGH is a result of both increased mitogenesis and decreased apoptosis (3). Autocrine hGH production also results in enhancement of the rate of mammary carcinoma cell spreading on a collagen substrate (4), suggesting that it may affect cell motility and dissemination of the carcinoma. All of the studied effects of autocrine hGH on mammary carcinoma cell behavior are mediated via the hGH receptor (3). Thus, autocrine production of hGH by mammary carcinoma cells may direct mammary carcinoma cell behavio...

Human Growth Hormone-regulated HOXA1 Is a Human Mammary Epithelial Oncogene

“…First, autocrine hGH increases transcription of the CHOP gene, and increased p38 mitogen-activated protein kinase-dependent CHOP-mediated transcription results in mammary carcinoma cell survival (23). Autocrine hGH production in mammary carcinoma cells also conversely results in transcriptional repression of the p53-regulated placental transforming growth factor-␤ gene with subsequent decreases in its protein product, Smad-mediated transcription, and its cellular effects, which include cell cycle arrest and apoptosis (36). Several avenues exist for antagonism of the somatotropic axis.…”

“…However, Bcl-2 is not the only mechanism utilized by hGH to prevent apoptotic cell death of mammary carcinoma cells, and simple antagonism of hGH signal transduction may represent a more efficacious approach to enhance the response of mammary carcinoma cells to chemotherapy. We have previously described that autocrine hGH production promotes mammary carcinoma cell survival by at least two other mechanisms (23,36). First, autocrine hGH increases transcription of the CHOP gene, and increased p38 mitogen-activated protein kinase-dependent CHOP-mediated transcription results in mammary carcinoma cell survival (23).…”

“…To verify the autocrine hGH-dependent up-regulation of HOXA1 mRNA observed by cDNA array screening, we first examined the level of HOXA1 mRNA in MCF7-MUT and MCF7-hGH cells by semi-quantitative RT-PCR. The validation that the conditions of RT-PCR used here yielded semi-quantitative estimates of mRNA is described under "Experimental Procedures" and has been utilized previously by us (23,36). One amplified fragment of the predicted size (359 bp) appropriate for HOXA1 mRNA was detected in MCF7-MUT and MCF7-hGH cells (Fig.…”

αCP1 Mediates Stabilization of hTERT mRNA by Autocrine Human Growth Hormone

“…Transient transfection was performed by use of Effectene (Qiagen) as described (38). Luciferase assays were performed as described previously (57). The results were normalized to the level of ␤-galactosidase activity and protein concentration in the samples.…”

The oncogenic potential of autocrine human growth hormone in breast cancer

“…Second, autocrine GH up-regulates gadd153 (CHOP), which was found to provide enhanced protection from apoptosis (11), and down-regulates expression of transcripts for p53-regulated placental transforming growth factor-␤ (PTGF-␤). The latter is known to inhibit GH-stimulated cyclin D1 expression and to promote apoptosis (12). Most recently, this group reported at the International Congress of Endocrinology † that autocrine hGH increases telomerase catalytic subunit (hTERT) transcript levels by stabilizing the message, and increased TERT is known to immortalize human mammary epithelial cells.…”