PURPOSE: Exposure to the polyphenolic plant lignan secoisolariciresinol diglucoside (SDG) and its metabolite enterolactone (ENL) has been associated with reduced breast cancer progression, particularly for estrogen receptor alpha (ERα)-negative disease, and decreased preclinical mammary tumor growth. However, while preclinical studies have established that SDG and ENL affect measures of progression in models of triple-negative breast cancer (TNBC, a subset of ERα-negative disease), the molecular mechanisms underlying these effects remain unclear. METHODS: C57BL/6 mice were fed a control diet (control, 10% kcal from fat) or control diet + SDG (SDG, 100 mg/kg diet) for 8 weeks, then orthotopically injected with syngeneic E0771 mammary tumor cells (a model of TNBC); tumor growth was monitored for 3 weeks. The role of reduced NF-κB signaling in SDG’s anti-tumor effects was explored in vitro via treatment with the bioactive SDG metabolite ENL. In addition to the murine E0771 cells, the in vitro studies utilized MDA-MB-231 and MCF-7 cells, two human cell lines which model the triple-negative and luminal A breast cancer subtypes, respectively. RESULTS: SDG supplementation in the mice significantly reduced tumor volume and expression of phospho-p65 and NF-κB target genes (P < 0.05). Markers of macrophage infiltration were decreased in the distal-to-tumor mammary fat pad of mice supplemented with SDG relative to control mice (P < 0.05). In vitro, ENL treatment inhibited viability, survival, and NF-κB activity and target gene expression in E0771, MDA-MB-231, and MCF-7 cells (P < 0.05). Overexpression of Rela attenuated ENL’s inhibition of E0771 cell viability and survival.
CONCLUSIONS: SDG reduces tumor growth in the E0771 model of TNBC, likely via a mechanism involving inhibition of NF-κB activity. SDG could serve as a practical and effective adjuvant treatment to reduce recurrence, but greater understanding of its effects is needed to inform the development of more targeted recommendations for its use.
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To further explore SDG lignan impact on breast cancer, preclinical studies have examined the effects of lignan exposure on animal models of both pre- and postmenopausal ERα-positive breast cancer, with the vast majority demonstrating significant reductions in mammary tumor growth or preneoplastic changes. These anti-tumor effects have been linked to decreased proliferation and angiogenesis as
well as increased apoptosis. Researchers have demonstrated that SDG metabolites reduce proliferation,
adhesion, migration, and invasion in the triple negative MDA-MB-231 breast cancer cell line and increase these cells’ response to radiation and chemotherapy but the molecular pathways responsible for these effects have not been established. The current study examined the impact of SDG supplementation on in vivo growth of orthotopically injected E0771 mouse mammary tumor cells, a syngeneic model of basal-like TNBC. After demonstrating that SDG inhibits E0771 tumor growth in association with decreased tumor activity of the inflammation-regulating transcription factor nuclear factor-kappa B (NF-κB), connections between these factors using in vitro models of multiple breast cancer subtypes were examined. The findings suggest that SDG may inhibit basal-like breast tumor progression via modulation of NF-κB activity. It was found that: (a) SDG supplementation in a mouse model of premenopausal basal-like breast cancer reduces tumor growth and NF-κB activity; (b) in vitro treatment with ENL, the primary bioactive metabolite of SDG, inhibits cell viability, survival, and NF-κB activity in models of basal-like, claudin-low, and luminal A breast cancer; and (c) ENL inhibits viability
and survival via modulation of NF-κB activity in the E0771 basal-like breast cancer model, the same model in which SDG inhibited in vivo tumor growth. This is the first study to identify reduced NF-κB
activity as a mediator of ENL’s anti-tumor effects. Elevated NF-κB activity has been found in many cancers and is particularly high in ERα-negative breast cancer. In this study, a significant decrease in phosphorylated (Ser276) p65, an NF-κB family member, as well as NF-κB target gene expression in the tumors of SDG-supplemented mice. No NF-κB target genes were significantly increased in the SDG-fed mice. ENL reduces cell viability, survival, and NF-κB activity in MDA-MB-231 and MCF-7 cells, models of triple-negative claudin-low and ERα-positive luminal A breast cancer, respectively. ENL treatment produced a twofold greater inhibition of viability in these 2 cell lines in comparison with the E0771 cells. The findings suggest that while ENL inhibits cell viability and survival across multiple breast cancer subtypes, biologically relevant ENL-induced inhibition of NF-κB activity may be limited to nonluminal breast cancers, perhaps specifically to the basal-like and claudin-low subtypes. SDG reduces tumor growth in the E0771 model of basal-like breast cancer, likely via a mechanism involving inhibition of NF-κB activity. SDG could serve as a practical and effective adjuvant treatment for the prevention of recurrence.