Abstract
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 levels 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. In this study, we successfully identified the STAT3 was the direct target of Nar through the combination strategies of connectivity map and drug affinity responsive target stability. In MCF7 cells, Nar could suppress phosphorylation, activation, dimerization, and nuclear translocation of STAT3 by directly binding with the STAT3 SH2 domain. In addition, Nar could specifically degrade total STAT3 via the proteasome pathway in MCF-7/TR (tamoxifen-resistant MCF-7) cells. This distinct mechanism of Nar-targeting STAT3 was mainly attributed to the various levels of reactive oxygen species in regular and tamoxifen-resistant ER-positive breast cancer cells. Meanwhile, Nar-loaded nanoparticles could markedly decrease the protein levels of STAT3 in tumors, resulting in significantly increased MCF-7/TR xenograft tumor regression without obvious toxicity. 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.
Highlights
Narciclasine (Nar), a plant growth inhibitor, was first isolated from bulbs of several Narcissus species in 1967, which has been shown to exhibit antitumor, anti-inflammatory, anti-Alzheimer’s disease, and obesity-suppressing activities.[1,2,3,4,5] In the US National Cancer Institute NCI-60 human tumor cell lines screen, Nar displayed broad cytotoxicity against various cancer cell lines.[2]
Potential target of Nar predicted by connectivity map (CMAP)-drug affinity responsive target stability (DARTS)/MS strategy Based on strong cytotoxic effects of Nar on cancer cells, the antitumor targets were needed to discover in this study
We examined the levels of phosphorylated signal transducer and activator of transcription 3 (STAT3) (p-STAT3) and total STAT3 in other breast cancer cell lines, namely T47D (ER-positive breast cancer cells) and MDA-MB231 (ER-negative breast cancer cells) treated with Nar, the results showed that Nar significantly inhibited STAT3 phosphorylation in a concentration-dependent manner without significantly affecting the total level of STAT3 in T47D and MDA-MB-231 cells (Figure S2)
Summary
We combined this CMAP analysis with DARTS/MS strategy to resolve the molecular targets of Nar
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