Interferon Regulatory Factor-1 (IRF1) Signaling Regulates Apoptosis and Autophagy to Determine Endocrine Responsiveness and Cell Fate in Human Breast Cancer

Abstract

Interferon regulatory factor-1 (IRF1) is a nuclear transcription factor and pivotal regulator of cell fate in cancer cells. While IRF1 is known to possess tumor suppressive activities, the role of IRF1 in mediating apoptosis and autophagy in breast cancer is largely unknown. Here, we show that IRF1 inhibits antiapoptotic B-cell lymphoma 2 (BCL2) protein expression, whose overexpression often contributes to antiestrogen resistance. We proposed that directly targeting the antiapoptotic BCL2 members with GX15-070 (GX; obatoclax), a BH3-mimetic currently in clinical development, would be an attractive strategy to overcome antiestrogen resistance in some breast cancers. Inhibition of BCL2 activity, through treatment with GX, was more effective in reducing the cell density of antiestrogen resistant breast cancer cells versus sensitive cells, and this increased sensitivity correlated with an accumulation of autophagic vacuoles. While GX treatment promoted autophagic vacuole and autolysosome formation, p62/SQSTM1, a marker for autophagic degradation, levels accumulated. Moreover, GX exposure resulted in a reduction of cathepsin D and L protein expression that would otherwise digest autolysosome cargo. Taken together, these data highlight a new mechanism of GX-induced cell death that could be used to design novel therapeutic interventions for antiestrogen resistant breast cancer.

In addition to establishing a functional role for BCL2 members downstream of IRF1, we also provided evidence that this IRF1 mediates antiestrogen sensitivity in breast cancer through autophagy signaling. We demonstrated that IRF1 and autophagy-related protein 7 (ATG7) expression were inversely correlated in vivo and in human breast cancer tissues. We then identified both ATG7 and the functionally related protein, beclin-1 (BECN1), as negative regulators of IRF1. Conversely, loss of IRF1 promoted antiestrogen resistance by increasing prosurvival autophagy, whereas combined ATG7 and IRF1 knockdown restored sensitivity to the antiestrogen fulvestrant (Faslodex, ICI 182780, ICI). Thus, inhibition of autophagy proteins, ATG7 and BECN1, enables IRF1-dependent and -independent signaling pathways and provides a novel therapeutic approach for inhibiting breast cancer cell proliferation.