Increased mammary epithelial expression of the human growth hormone (hGH) gene is associated with the acquisition of pathological proliferation. We report here that autocrine hGH production by human mammary carcinoma cells increased the expression and transcriptional activity of the homeobox domain containing protein HOXA1. Forced expression of HOXA1 in human mammary carcinoma cells resulted in increased total cell number primarily by the promotion of cell survival mediated by the transcriptional up-regulation of Bcl-2. HOXA1 also abrogated the apoptotic response of mammary carcinoma cells to doxorubicin. Forced expression of HOXA1 in mammary carcinoma cells, in a Bcl-2-dependent manner, resulted in dramatic enhancement of anchorage-independent proliferation and colony formation in soft agar. Finally, forced expression of HOXA1 was sufficient to result in the oncogenic transformation of immortalized human mammary epithelial cells with aggressive in vivo tumor formation. Herein, we have therefore provided a molecular mechanism by which autocrine hGH stimulation of human mammary epithelial cells may result in oncogenic transformation.

We report here that autocrine production of human growth hormone (hGH) results in a phenotypic conversion of mammary carcinoma cells such that they exhibit the morphological and molecular characteristics of a mesenchymal cell, including expression of fibronectin and vimentin. Autocrine production of hGH resulted in reduced plakoglobin expression and relocalization of E-cadherin to the cytoplasm, leading to dissolution of cell-cell contacts and decreased cell height. These phenotypic changes were accompanied by an increase in cell motility, elevated activity of specific matrix metalloproteinases, and an acquired ability to invade a reconstituted basement membrane. Forced expression of plakoglobin significantly decreased mammary carcinoma cell migration and invasion stimulated by autocrine hGH. In vivo, autocrine hGH stimulated local invasion of mammary carcinoma cells concomitant with a prominent stromal reaction in comparison with well delineated and capsulated growth of mammary carcinoma cells lacking autocrine production of hGH. Thus, autocrine production of hGH by mammary carcinoma cells is sufficient for generation of an invasive phenotype. Therapeutic targeting of autocrine hGH may provide a mechanistic approach to prevent metastatic extension of human mammary carcinoma. M ammary epithelial carcinoma is one of the leading causes of cancer-related mortality of female residents in Western countries, predominantly due to a high metastatic frequency (1). Identification of specific molecular determinants for metastatic mammary carcinoma and subsequent development of concordant biologic therapies are therefore of primary importance. During metastatic conversion, mammary carcinoma cells acquire the ability to invade surrounding tissue with subsequent dissemination to secondary organs. The acquisition of a migratory and invasive phenotype by cells of epithelial origin is associated with gain of mesenchymal characteristics concomitant with loss of the epithelial phenotype, a phenomenon referred to as epithelial-mesenchymal transition (2). This transition from epithelial to mesenchymal cell phenotype involves specific morphological and molecular alterations, including the loss of E-cadherin-mediated cell adhesion (2-4).The human growth hormone (hGH) gene is expressed in the normal and neoplastic human mammary epithelial cell (5, 6). Elevated hGH gene expression is observed to be associated with metastatic mammary carcinoma cells (5), suggestive of a functional contribution of autocrine hGH to the metastatic process. We have concordantly previously demonstrated that autocrine hGH dramatically enhances spreading of mammary carcinoma cells on a collagen substrate (7). These observations suggest that autocrine production of hGH may contribute to conversion of human mammary epithelial cells to a mesenchymal phenotype. We demonstrate herein that in mammary carcinoma cells with epithelial morphology, autocrine production of hGH promotes mesenchymal cellular morphology, increased cell migration, and increased matrix metallop...

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