MCF10A mammary epithelial cells form growth-arrested structures when cultured in three-dimensional basement membrane gels. Activation of the receptor tyrosine kinase ErbB2 induces formation of proliferative structures that share properties with noninvasive early stage lesions. We conducted a genetic screen to identify cDNAs that can cooperate with ErbB2 to induce migration in these cells, with the hypothesis that they would represent candidate ''second hits'' in the development of invasive breast carcinomas. We found that expression of transforming growth factor (TGF)␤1 and TGF␤3 in cells expressing activated ErbB2 induces migration in transwell chambers and invasive behavior in both basement membrane cultures and invasion chambers. The ability of ErbB2 to cooperate with TGF␤ correlated with sustained, elevated activation of extracellular signal-regulated kinase (Erk)-mitogen-activated protein kinase. Pharmacological reduction of Erk activity inhibited the cooperative effect of TGF␤ and ErbB2 on migration and expression of activated Erk kinase was sufficient to cooperate with TGF␤ to induce migration and invasion, suggesting that sustained Erk activation is critical for ErbB2͞TGF␤ cooperation. In addition, we show that costimulation of ErbB2 and TGF␤ induces autocrine secretion of factors that are sufficient to induce migration, but not invasion, by means of both epidermal growth factor receptor-dependent and -independent processes. These results support the role of TGF␤ as a pro-invasion factor in the progression of breast cancers with activated ErbB2 and suggest that activation of the Erk and epidermal growth factor receptor pathways are key in mediating these events. E rbB2 (HER2͞Neu) is overexpressed in 20% to 30% of invasive breast tumors and up to 85% of comedo type ductal carcinoma in situ (DCIS), an early stage in breast cancer (1, 2). The ErbB receptor family has four members: ErbB1 (EGFR͞ HER1), ErbB2, ErbB3, and ErbB4. When these receptors are stimulated with epidermal growth factor (EGF) family ligands, receptor activation occurs by means of both homo-and heterodimerization among the receptor family members (3). However, under conditions where ErbB2 is amplified or overexpressed, activation can occur by ligand-independent homodimerization (4, 5).Previously, we investigated the consequences of activation of ErbB2 homodimers in the context of a nontransformed human mammary epithelial cell line, MCF10A (6, 7). When grown in basement membrane cultures, MCF10A cells form growth arrested three-dimensional structures, termed acini (8). These structures are comprised of a single layer of polarized epithelial cells surrounding a hollow lumen and resemble the mammary acini that form terminal ductal lobular units in the adult breast (7,8).To study the contributions of ErbB receptors in these cells, we generated chimeras of ErbB1 (p75.B1) and ErbB2 (p75.B2), which are inducibly activated by means of homodimerization by using the dimeric FKBP ligand AP1510 (ARIAD Pharmaceuticals, Cambridge, MA; refs. 9 and 10). T...

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Amplification and overexpression of ErbB2 (HER2/Neu) is one of the most common alterations associated with breast cancer. Activation of ErbB2 via homodimerization in a non-transformed human mammary epithelial cell line, MCF-10A, in basement membrane cultures leads to formation of proliferative structures that share properties with non-invasive early stage lesions. Recently, we have shown that activation of ErbB2 homodimers combined with expression of transforming growth factor (TGF)-β induces invasive and migratory activity in MCF-10A cells. In this system, migration requires inputs from numerous cellular pathways. We discuss this data and a model for migration induced by ErbB2 and TGF-β. Concurrent studies by other groups have also shown that ErbB2 and TGF-β can cooperate to increase metastatic and invasive behavior in murine mammary tumors. Here we discuss these studies and the potential implications of this research on breast cancer therapeutics. Recently, we performed a screen to identify candidate genes that could potentially induce invasion of tumor cells overexpressing ErbB2 using these MCF-10A cells engineered to inducibly activate ErbB2 homodimers. Two transforming growth factor (TGF)-β family members, TGF-β1 and TGF-β3, scored in this screen. These family members, as well as TGF-β2, were found to induce both migration in transwell chambers and invasion through basement membrane gels in response to inducible activation of ErbB2 homodimers (Fig. 1C). 9 The most significant clue relating to the mechanism whereby ErbB2 and TGF-β enhance cell migration was the discovery that conditioned medium from cells expressing TGF-β and activated ErbB2 is able to induce migration of naïve parental MCF-10A cells. This result indicated that all of the activities required for motility induced by ErbB2 plus TGF-β are derived from secreted extracellular factors. Investigation of the properties of the secreted factors revealed that both ErbB1 (epidermal growth factor receptor; EGFR)dependent and-independent soluble motogenic factors are present. To the best of our www.landesbioscience.com Cell Cycle 597 ERbB2 AND TGF-β IN BREAST CANCER

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