ABCA9, an ER cholesterol transporter, inhibits breast cancer cell proliferation via SREBP‐2 signaling

The association between cholesterol metabolism and cancer development and progression has been recently highlighted. However, the role and function of many cholesterol transporters remain largely unknown. Here, we focused on the ATP‐binding cassette subfamily A member 9 (ABCA9) transporter given that its expression is significantly downregulated in both canine mammary tumors and human breast cancers, which in breast cancer patients correlates with poor prognosis. We found that ABCA9 is mainly present in the endoplasmic reticulum (ER) and is responsible for promoting cholesterol accumulation in this structure. Accordingly, ABCA9 inhibited sterol‐regulatory element binding protein‐2 (SREBP‐2) translocation from the ER to the nucleus, a crucial step for cholesterol synthesis, resulting in the downregulation of cholesterol synthesis gene expression. ABCA9 expression in breast cancer cells attenuated cell proliferation and reduced their colony‐forming abilities. We identified ABCA9 expression to be regulated by Forkhead box O1 (FOXO1). Inhibition of PI3K induced enhanced ABCA9 expression through the activation of the PI3K–Akt–FOXO1 pathway in breast cancer cells. Altogether, our study suggests that ABCA9 functions as an ER cholesterol transporter that suppresses cholesterol synthesis via the inhibition of SREBP‐2 signaling and that its restoration halts breast cancer cell proliferation. Our findings provide novel insight into the vital role of ABCA9 in breast cancer progression.