Abstractp53 plays a key role in mediating cell response to various stresses, mainly by inducing or repressing a number of genes involved in cell cycle arrest, senescence, apoptosis, DNA repair, and angiogenesis. According to this important function, p53 activity is controlled in a very complex manner, including several auto-regulatory loops, through the intervention of dozens of modulator proteins (the 'p53 interactome'). p53 mutations are observed in a significant minority of breast tumours. In the remaining cases, alterations of interactome components or target genes could contribute, to some extent, to reduce the ability of p53 to efficiently manage stress events. While the prognostic and predictive value of p53 is still debated, there is an increasing interest for p53-based therapies. The present paper aims to provide updated information on p53 regulation and function, with specific interest on its role in breast cancer.

We show here that the neurotrophin nerve growth factor (NGF), which has been shown to be a mitogen for breast cancer cells, also stimulates cell survival through a distinct signaling pathway. Breast cancer cell lines (MCF-7, T47-D, BT-20, and MDA-MB-231) were found to express both types of NGF receptors: p140 trkA and p75 NTR . The two other tyrosine kinase receptors for neurotrophins, TrkB and TrkC, were not expressed. The mitogenic effect of NGF on breast cancer cells required the tyrosine kinase activity of p140 trkA as well as the mitogen-activated protein kinase (MAPK) cascade, but was independent of p75 NTR . In contrast, the anti-apoptotic effect of NGF (studied using the ceramide analogue C2) required p75 NTR as well as the activation of the transcription factor NF-kB, but neither p140 trkA nor MAPK was necessary. Other neurotrophins (BDNF, NT-3, NT-4/5) also induced cell survival, although not proliferation, emphasizing the importance of p75 NTR in NGF-mediated survival. Both the pharmacological NF-B inhibitor SN50, and cell transfection with IkBm, resulted in a diminution of NGF anti-apoptotic effect. These data show that two distinct signaling pathways are required for NGF activity and confirm the roles played by p75 NTR and NF-B in the activation of the survival pathway in breast cancer cells. Nerve growth factor (NGF)1 is the archetypal member of the neurotrophin superfamily, which also includes brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), NT-4/5, and NT-6 (1). NGF interacts with two classes of membrane receptor: the TrkA proto-oncogene product p140 trkA , which possesses intrinsic tyrosine kinase activity, and a secondary receptor, p75 NTR , that belongs to the tumor necrosis factor (TNF) receptor family (2). The stimulation of cell survival and cell differentiation by NGF and other neurotrophins have been described primarily in neuronal cell systems (3). Although the neurotrophic effect through p140trkA is known to involve the MAPK cascade, the role of p75 NTR is still controversial; there is evidence that it can both positively and negatively regulate neuronal cell death and differentiation, depending on the cell type examined (4). In some cases, p75NTR is an inducer of apoptosis, even without NGF stimulation (5), whereas in other cases the activation of p75 NTR by NGF results in a protection from cell death (6). In addition to its neurotrophic function, other activities of NGF have been described. For example, NGF can modulate gene expression in monocytes (7), it is chemotactic for melanocytes (8), and its inhibition on p75 NTR can block the migration of Schwann cells (9). NGF also stimulates the proliferation of chromaffin cells (10), lymphocytes (11), and keratinocytes (12). We have previously shown that NGF is mitogenic for cancerous but not normal human breast cells (13), and these data, as well as others showing a role for NGF in the stimulation of prostatic cancer cells (14 -17), implicate NGF in non-neuronal carcinogenesis.Both cellular proliferation as well as tumor c...

The Trk family of neurotrophin tyrosine kinase receptors is emerging as an important player in carcinogenic progression in non-neuronal tissues. Here, we show that breast tumors present high levels of TrkA and phospho-TrkA compared to normal breast tissues. To further evaluate the precise functions of TrkA overexpression in breast cancer development, we have performed a series of biological tests using breast cancer cells that stably overexpress TrkA. We show that (1) TrkA overexpression promoted cell growth, migration and invasion in vitro; (2) overexpression of TrkA per se conferred constitutive activation of its tyrosine kinase activity; (3) signal pathways including PI3K-Akt and ERK/ p38 MAP kinases were activated by TrkA overexpression and were required for the maintenance of a more aggressive cellular phenotype; and (4) TrkA overexpression enhanced tumor growth, angiogenesis and metastasis of xenografted breast cancer cells in immunodeficient mice. Moreover, recovered metastatic cells from the lungs exhibited enhanced anoikis resistance that was abolished by the pharmacological inhibitor K252a, suggesting that TrkA-promoted breast tumor metastasis could be mediated at least in part by enhancing anoikis resistance. Together, these results provide the first direct evidence that TrkA overexpression enhances the tumorigenic properties of breast cancer cells and point to TrkA as a potential target in breast cancer therapy.

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