Narciclasine Targets STAT3 via Distinct Mechanisms in Tamoxifen-Resistant Breast Cancer Cells

Abstract Background: Signal transducer and activator of transcription 3 (STAT3) is constitutively activated in multiple malignant tumors. Compared with regular estrogen receptor (ER)-positive breast cancers, the patients with tamoxifen-resistant breast cancers often exhibit higher level of STAT3 phosphorylation. Narciclasine (Nar) possesses strong inhibiting effects against a variety of cancer cells, however, the underlying antitumor target(s)/mechanism(s) remains barely understood. Methods: Targets prediction of narciclasine was performed by combining connectivity map (CMAP) and drug affinity responsive target stability (DARTS) strategy. Molecular and biochemical methods were used to elucidate the distinct mechanisms of narciclasine targeting STAT3. The narciclasine nano-delivery system was synthesized by thin film hydration method. Xenograft models were established to determine antitumor activity of narciclasine and its liposome in vivo.Results: In this study, we successfully identified the STAT3 was the direct target of Nar through the combination strategies of CMAP and DARTS. In ER-positive breast cancer cells, Nar could suppress phosphorylation, activation, dimerization, and nuclear translocation of STAT3 by directly binding with the STAT3 SH2 domain. Additionally, Nar could also specifically promote total STAT3 degradation via proteasome pathway and reduce the STAT3 protein stability in tamoxifen-resistant breast cancer cells (MCF-7/TR). This distinct mechanism of Nar targeting STAT3 was mainly attributed to the various levels of reactive oxygen species (ROS) in regular and tamoxifen-resistant ER-positive breast cancer cells. Meanwhile, Nar loaded nanoparticles could markedly decrease the protein levels of STAT3 in tumor sites, resulting in significant MCF-7/TR xenograft tumor regression without obvious toxicity. Conclusions: Our findings successfully highlight the STAT3 as the direct therapeutic target of Nar in ER-positive breast cancer cells, especially Nar leaded STAT3 degradation as a promising strategy for the tamoxifen-resistant breast cancer treatment.