Research Article: Induction of ErbB-3 Expression by α6β4 Integrin Contributes to Tamoxifen Resistance in ERβ1-Negative Breast Carcinomas

Date Published: February 13, 2008

Publisher: Public Library of Science

Author(s): Valentina Folgiero, Paolo Avetrani, Giulia Bon, Selene E. Di Carlo, Alessandra Fabi, Cecilia Nisticò, Patrizia Vici, Elisa Melucci, Simonetta Buglioni, Letizia Perracchio, Isabella Sperduti, Laura Rosanò, Ada Sacchi, Marcella Mottolese, Rita Falcioni, Dong-Yan Jin.

Abstract: Tamoxifen is still the most widely used drug in hormone therapy for the treatment of breast cancer. Its benefits in adjuvant treatment are well documented in controlled and randomized clinical studies, which have demonstrated an increase in disease-free intervals of patients with positive hormonal receptors. However, the mechanisms involved in endocrine resistance are not clear. Laboratory and clinical data now indicate that bi-directional molecular cross-talk between nuclear or membrane ER and growth factor receptor pathways may be involved in endocrine resistance. We recently found a functional interaction between α6β4 integrin and ErbB-3 receptor to maintain the PI3K/Akt survival pathway of mammary tumour cells. We sought to improve understanding of this process in order to provide the involvement of both receptors insight into mechanism of Tamoxifen resistance.

Partial Text: In many breast cancer (BC), activation of the phosphatidylinositol 3-kinase (PI3K) pathway may deeply reduce the efficacy to targeted therapies [1]–[3]. In the last few years, a strong activation of the PI3-K/Akt signaling pathway was observed in tumor cells that express high levels of integrin α6β4, a laminin receptor implicated in tumor progression and invasion [4]–[9]. The involvement of this integrin in tumor progression is supported by large experimental evidence. In mammary and ovary carcinoma cell lines, α6β4 integrin associates with ErbB-2 overexpression and co-operates to promote a PI3K-dependent invasion and survival [10], [6]. In MMTV-Neu mice, the introduction of a targeted deletion of the β4 cytoplasmic domain revealed that β4 integrin promotes tumor progression cooperating with ErbB-2 signaling [11]. Inactivation of α6β4 integrin by RNA interference inhibits tumor growth both in vitro and in vivo[12]–[14] and strongly reduces the activity of the PI3K pathway inducing apoptosis upon hormone deprivation and TAM treatment in MCF7 BC cells [12]. In addition, we have recently evidenced that the α6β4-induced PI3K-dependent survival pathway of two different BC cell lines is due to the capability of α6β4 integrin to enhance ErbB-3 expression. This enhancement results in an increase of ErbB-2/ErbB-3 heterodimerization and consequently in the activation of the PI3K survival pathway [15]. Collectively, these studies suggest a strong cooperation between α6β4 integrin and EGFR family members in mammary tumors and highlight a pathway by which this integrin might contribute to BC tumorigenicity and responsiveness to treatments.

It is widely known that α6β4 integrin expression and signaling are involved in the mechanisms that regulate tumor progression and resistance to apoptotic stimuli [4]–[9], [13], [15]. One of these mechanisms involves the ability of α6β4 integrin to regulate the translation of ErbB-3 receptor in a manner which is eIF-4E-dependent [15]. The ErbB-3 up-regulation associated with α6β4 integrin over-expression results in an increase of ErbB-2/ErbB-3 heterodimerization and consequent Akt phosphorylation favoring the survival of BC cells [15]. In the present work, we extended our previous study [15] to a novel panel of human BC cell lines which express different levels of α6β4 integrin confirming that the integrin expression correlates with ErbB-3 protein positivity. We also showed that β4 integrin depletion inhibits ErbB-3 translation and strongly reduces Akt activity while, ErbB-3 depletion abrogates Akt phosphorylation. Furthermore, the involvement of ErbB-3 in tumor progression was also supported by the finding that its depletion, in the absence of hormone stimuli, induces apoptosis, inhibits the in vitro invasion and favors TAM responsiveness. Given that ErbB-3 protein binds the regulatory subunit of PI3K but lacks kinase activity [24], our observations imply that ErbB-2/ErbB-3 sustains the survival of BC cells in the absence of ERs signaling through the activation of PI3K pathway. This hypothesis is strongly supported by recent findings which demonstrate that ErbB3 down-regulation by RNA interference abrogates ErbB-2-mediated TAM resistance in BC cells [25].