Abstract P4-06-05: NF-κB Upregulates β1-integrin via Increased Transcriptional Activity in Three-dimensional Culture: a Mechanism by which Malignant Breast Cells Acquire Radioresistance

Abstract

Abstract The molecular mechanisms giving rise to tumor viability after therapeutic radiation remain elusive. We showed previously that β1-integrin is associated with enhanced cell survival of breast cancer cells after exposure to high dose ionizing radiation (IR). Recently, we discovered that a typical binding site for Nuclear factor-κB (NF-κB), a stress-sensitive heterodimeric transcription factor, was located in human β1-integrin promoter, which indicates that β1-integrin-mediated resistance to radiation may be functionally regulated by NF-κB. The aim of the present study was to investigate the relationship between NF-κB and β1-integrin pathways in radioprotection of malignant T4-2 breast cells in three-dimensional laminin-rich extracellular matrix (3D lrECM). We show that NF-κB is required for radiation-induced β1-integrin transactivation. Inhibition of NF-κB activity significantly reduced clonogenic survival with the inhibition of IR-induced total and phosphorylated (Thr 788/789) β1-integrins, correlated with increased DNA binding activity at the β1- integrin promoter. Treatment of T4-2 colonies with NF-κB activation inhibitor before IR resulted in a reduction of the size of colonies, associated with a decrease in proliferation and increase in apoptosis, indicating an increased susceptibility to IR. Further studies revealed that a direct interaction of β1-integrin with α5-integrin was highly induced, but not with NF-κB p65, in T4-2 cells surviving IR. These interactions were found to be oppositely regulated in radiosensitive S1 cells, suggesting that IR induces the formation of α5β1-integrin complexes leading to increased survival post-IR. Together these results provide the first evidence that NF-κB-induced β1-integrin transactivation is responsible for increased survival post IR in breast cancer. Since β1-integrin is shown to activate NF-κB, our data further suggest a loop-like β1-integrin–NF-κB–β1-integrin pathway in tumor radioresistance. This novel pathway may serve as an efficient drug target to re-sensitize radioresistant tumor cells. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-06-05.