XPO1 (CRM1) inhibition represses STAT3 activation to drive a survivin-dependent oncogenic switch in triple negative breast cancer

Inhibition of XPO1 (CRM1)-mediated nuclear export of multiple tumor suppressor proteins has been proposed as a novel cancer therapeutic strategy to turn off oncogenic signals and enhance tumor suppression. Survivin is a multifunctional protein with oncogenic properties when expressed in the cytoplasm that requires the XPO1-RanGTP complex for its nuclear export. We investigated the anti-tumor mechanisms of the drug-like selective inhibitors of nuclear export (SINE) XPO1 antagonists, KPT-185, KPT-251 KPT-276, and KPT-330 in estrogen-receptor positive and triple negative breast cancer (TNBC) cell lines and xenograft models of human breast tumors. KPT compounds significantly inhibited breast cancer cell growth and induced tumor cell death, both in vitro and in vivo. These drugs initially promoted survivin accumulation within tumor cell nuclei. However, their major in vitro effect was to decrease survivin cytoplasmic protein levels, correlating with the onset of apoptosis. XPO1 inhibition repressed Survivin transcription by inhibiting CBP-mediated STAT3 acetylation, and blocking STAT3 binding to the Survivin promoter. Additionally, caspase-3 was activated to cleave survivin, rendering it unavailable to bind XIAP and block the caspase cascade. Collectively, these data demonstrate that XPO1 inhibition by SINE compounds represses STAT3 transactivation to block the selective oncogenic properties of survivin and supports their clinical use in triple negative breast tumors.