Pleiotrophin (PTN,Ptnangiogenesis ͉ collagen ͉ polyoma virus middle T ͉ scirrhous carcinoma ͉ stromal fibroblasts B reast cancers progress through stages of increasing malignancy triggered by genetic and epigenetic mutations that promote their growth, invasiveness, and metastasis. Because mortality from breast cancer is most often due to growth of distant metastases that are not controlled by existing therapies, there is a vital need to better understand the molecular pathogenesis of breast cancer and to identify mutations in breast cancer cells that promote breast cancer progression and lead to metastasis (1).Pleiotrophin [PTN (the protein), Ptn (the gene)] is a 136-aa heparin-binding cytokine (2, 3) frequently detected in human breast cancers (4). Its expression is constitutive in human breast cancer cells in which it has been tested (5). Furthermore, PTN stimulates functional responses that are known to stimulate breast cancer progression, such as proliferation (2, 3), cytoskeletal rearrangements, and loss of cell-cell adhesion (6, 7), transformation (8), induction of an epithelial to mesenchymal transition (6), and stimulation of tumor angiogenesis (9, 10), in cultured cells or in cells that ectopically express Ptn. These functional responses to PTN are likely to be significant in progression of breast cancers, because interruption of constitutive PTN signaling in human breast cancer cells that inappropriately express Ptn reverses their transformed phenotype in vitro and in vivo (5); these PTN-stimulated functional responses thus support the possibility that mutations in breast cancer cells, which deregulate endogenous Ptn expression, may initiate similar functional responses and lead to breast cancer progression.In the following studies, three models were tested to determine whether inappropriate expression of Ptn alone is sufficient to induce breast cancer or whether inappropriate expression of Ptn cooperates with different pathogenic mechanisms that stimulate breast cancer progression. Together, the studies demonstrate both in vivo and in vitro that inappropriate expression of Ptn promotes breast cancer progression; the studies also demonstrate that PTN secretion from human breast cancer cells alone is sufficient to promote progression of breast cancer to a more aggressive breast cancer phenotype through activation of stromal cells and extensive remodeling of the microenvironment.
Results
Mouse Mammary Tumor Virus (MMTV)-Ptn Fails to Induce BreastCancer in MMTV-Ptn Transgenic Mice. To test whether inappropriate expression of Ptn alone is sufficient to induce breast cancers in mice, MMTV-long terminal repeat-driven-Ptn (MMTV-Ptn) transgenic mice were generated [see supporting information (SI) Materials and Methods and SI Fig. 5]. Female MMTV-Ptn mice had normal mammary gland development and lactation and developed pups in normal numbers and in size equal to that of control mouse pups. Expression of the MMTV-Ptn transgene was detected in high levels in breast epithelial cells. In extensive microscopic exami...
