Mechanism studies on the combination of Spatholobi Caulis and Glycyrrhizae Radix et Rhizoma for breast cancer treatment

Abstract

Lacking an effective strategy for treating triple-negative breast cancer (TNBC) and drug resistance of chemotherapeutic drugs are two significant problems in breast cancer clinical treatment. As traditional Chinese medicine (TCM) has more than thousands of years of clinical experience for treating tumors, it is promising to seek these problems' solutions in TCM. Spatholobi Caulis (SS) is one of the Chinese medicinal herbs and used for treating unregular menstruation. According to the theory of TCM, SS is suitable for treating breast cancer. After optimizing the extractive method and establishing quality control, SS's extracts were produced in the pharmaceutical factory and named SSP. SSP shows a remarkable anti-TNBC effect in vitro and in vivo. The mechanism of the anti-TNBC effect of SSP was investigated by proteomics and found SSP inhibits the growth of TNBC through downregulating the K-Ras-Raf signaling pathway. SSP also synergizes Doxorubicin to inhibit the proliferation of drug-resistant breast cancer cells, MCF-7/Adr. Further study revealed that SSP both decreases P-glycoprotein's expression and activity and promotes the Doxorubicin into the nucleus to strengthen the inhibitory effect of Doxorubicin to MCF-7/Adr cells.

However, while inhibiting the growth of TNBC, SSP reduces the weight of mice. NIH mice were used to evaluate the side effect of SSP. SSP reduces the weight of NIH mice but does not show toxicity to the liver and kidney. Metabolomics was used to investigate the metabolism change of NIH mice caused by SSP. The result demonstrated that SSP increases the β-oxidation of fatty acid and tricarboxylic acid cycle (TCA). SSP also inhibits the synthesis of fatty acid in the liver of NIH mice by qPCR analysis. As Glycyrrhizae Radix et Rhizoma (Licorice) is famous for detoxification in TCM, Licorice was used to reduce SSP's side effects in breast cancer treatment. In vitro, the anti-TNBC effect of SSP and Licorice's combination is more robust than Licorice or SSP alone. In vivo, Licorice indeed reduces the side effect of SSP on the weight of mice. Further studies have proved that Licorice reverses the increase of fatty acid oxidation and energy dissipation and the decrease of fatty acid synthesis, which are induced by SSP.

Licorice not only reduces the side effect of SSP in treating breast cancer, its active component, Glycyrrhetinic Acid (GA), but also induces the ferroptosis related death of TNBC. GA increases reactive oxygen species (ROS) and reactive nitrogen species (RNS) by upregulating NADPH oxidase's expression and activity and the expression of inducible nitric oxide synthase (iNOS). Excessive ROS and RNS lead to lipid peroxidation. Moreover, GA reduced GSH by downregulating the expression of SLC7A11 to further raising the lipid peroxidation. A large amount of lipid peroxidation caused by GA triggered ferroptosis related death in MDA-MB-231 cells.

In conclusion, this study has identified an effective strategy by combining SSP and Licorice to treat TNBC and found SSP sensitizes MCF-7/Adr to Doxorubicin. Meanwhile, it has elucidated the mechanism of the anti-breast cancer effect of SSP and GA and the detoxification effect of Licorice.