When Tumor Suppressor TGFβ Meets the HER2 (ERBB2) Oncogene

Despite its tumor suppressive role in normal mammary epithelial cells, TGF beta has been reported to promote the migration, invasion and survival in breast cancer cells overexpressing the HER2 (ERBB2; neu) oncogene, and to accelerate the metastasis of neu-induced mammary tumors in mice. A clearer understanding of the molecular mechanisms underlying the crosstalk between TGF beta and HER2 has started to emerge. In recent studies reviewed here, the synergistic effect of TGF beta and HER2 on tumor progression has been shown to likely be a combined result of two distinct features: (1) loss of TGF beta's tumor suppressive effect through functional alterations in the anti-mitogenic effect of Smad-mediated transcription, and (2) gain of pro-survival and pro-migratory function through HER2-dependent mechanisms. In HER2-overexpressing breast cancer, this crosstalk results in increased cancer cell proliferation, survival and invasion, accelerated metastasis in animal models, and resistance to chemotherapy and HER2-targeted therapy. Thus, the transformed cellular context imparted by constitutively active HER2 signaling, as a consequence of HER2 gene amplification or overexpression, aborts the tumor suppressive role of TGF beta and facilitated the oncogenic role of this pathway. In turn, TGF beta potentiates oncogenic HER2 signaling by inducing shedding of the ERBB ligands and clustering of HER2 with integrins. Here we discuss recent studies examining Smad-dependent and -independent mechanisms of crosstalk between TGF beta and HER2. Therefore, blockade of TGF beta:HER2 crosstalk may suppress breast cancer progression and metastasis, and enhance the efficiency of conventional therapies in patients with HER2-overexpressing breast cancer.