FoxO3a Inhibits Tamoxifen-Resistant Breast Cancer Progression by Inducing Integrin α5 Expression

SIMPLE SUMMARY: Tamoxifen is a mainstay of adjuvant treatment for estrogen receptor α-positive (ERα+) breast tumors, which account for over 70% of all the diagnosed breast malignancies. Unfortunately, ~30% of patients initially responding to the therapy will eventually relapse with an endocrine-resistant disease. Here, we show that the transcription factor FoxO3a can overcome tamoxifen resistance by inhibiting cell motility and invasiveness, the main features of tumor progression. The underlying mechanism could be ascribed to FoxO3a-dependent transcriptional up-regulation of the integrin α5 subunit of the α5β1 fibronectin receptor, a well-known membrane heterodimer controlling cell adhesion. Indeed, FoxO3a and integrin α5 expression is significantly correlated in ERα+ tumors, and FoxO3a protective effects are lost in cells with reduced levels of integrin α5. Therefore, a pharmacological increase in FoxO3a levels could represent an effective approach to be exploited in combination with tamoxifen treatment in order to restore the sensitivity to the therapy in refractory tumors. ABSTRACT: Resistance to endocrine therapy is still a major clinical challenge in the management of estrogen receptor α-positive (ERα+) breast cancer (BC). Here, the role of the Forkhead box class O (FoxO)3a transcription factor in tumor progression has been evaluated in tamoxifen-resistant BC cells (TamR), expressing lower levels of FoxO3a compared to sensitive ones. FoxO3a re-expression reduces TamR motility (wound-healing and transmigration assays) and invasiveness (matrigel transwell invasion assays) through the mRNA (qRT-PCR) and protein (Western blot) induction of the integrin α5 subunit of the α5β1 fibronectin receptor, a well-known membrane heterodimer controlling cell adhesion and signaling. The induction occurs through FoxO3a binding to a specific Forkhead responsive core sequence located on the integrin α5 promoter (cloning, luciferase, and ChIP assays). Moreover, FoxO3a failed to inhibit migration and invasion in integrin α5 silenced (siRNA) cells, demonstrating integrin α5 involvement in both processes. Finally, using large-scale gene expression data sets, a strong positive correlation between FoxO3a and integrin α5 in ERα+, but not in ER-negative (ERα−), BC patients emerged. Altogether, our data show how the oncosuppressor FoxO3a, by increasing the expression of its novel transcriptional target integrin α5, reverts the phenotype of endocrine-resistant BC toward a lower aggressiveness.