Lithocholic Acid, a Metabolite of the Microbiome, Increases Oxidative Stress in Breast Cancer

Abstract

In breast cancer patients, the diversity of the microbiome decreases, coinciding with decreased production of cytostatic bacterial metabolites like lithocholic acid (LCA). We hypothesized that LCA can modulate oxidative stress to exert cytostatic effects in breast cancer cells. Treatment of breast cancer cells with LCA decreased nuclear factor-2 (NRF2) expression and increased Kelch-like ECH associating protein 1 (KEAP1) expression via activation of Takeda G-protein coupled receptor (TGR5) and constitutive androstane receptor (CAR). Altered NRF2 and KEAP1 expression subsequently led to decreased expression of glutathione peroxidase 3 (GPX3), an antioxidant enzyme, and increased expression of inducible nitric oxide synthase (iNOS). The imbalance between the pro- and antioxidant enzymes increased cytostatic effects via increased levels of lipid and protein oxidation. These effects were reversed by the pharmacological induction of NRF2 with RA839, tBHQ, or by thiol antioxidants. The expression of key components of the LCA-elicited cytostatic pathway (iNOS and 4HNE) gradually decreased as the breast cancer stage advanced. The level of lipid peroxidation in tumors negatively correlated with the mitotic index. The overexpression of iNOS, nNOS, CAR, KEAP1, NOX4, and TGR5 or the downregulation of NRF2 correlated with better survival in breast cancer patients, except for triple negative cases. Taken together, LCA, a metabolite of the gut microbiome, elicits oxidative stress that slows down the proliferation of breast cancer cells. The LCA–oxidative stress protective pathway is lost as breast cancer progresses, and the loss correlates with poor prognosis.

Nuclear Factor Erythroid 2-Related Factor 2 in Regulating Cancer Metabolism

“…NRF2 induction by the Txnrd1 inhibitor, auranofin, promotes spheroid growth, evidenced by proliferation and differentiation markers, rather than the quiescent (CSC-like) phenotype. In line with this, Kovács et al (149) showed that the 4-hydroxynonenal content in breast cancer tissue negatively correlated with the proliferation of cancer cells. On the contrary, another point of view was provided in a study by Bocci (21) showing that the knockdown of NRF2 induces mesenchymal markers and the transition from hybrid E/M to the mesenchymal phenotype.…”

“…Removing KEAP1 from the KEAP1-NRF2 complex leads to stabilization and activation of NRF2. The inhibition of the proteasome (e.g., by MG132) can prevent the degradation of NRF2 (149). A food additive and procarcinogen, butylated hydroxyanisole, can induce NRF2 (179) and NAD(P)H quinone dehydrogenase 1 (NQO1), which boosts the expression of 8-Oxoguanine glycosylase (OGG1) (268).…”

“…LCA reduces NRF2 expression, and the consequent reduction in antioxidant defenses exerts cytostatic properties in breast cancer. This mechanism is ineffective in nontransformed cells (149).…”

“…A recent study identified two single nucleotide polymorphisms of KEAP1 associated with shorter relapse-free survival in breast carcinoma (87). A study by Kovács et al (149) showed that NRF2 overexpression and the downregulation of downstream pro-oxidant genes negatively correlates with patient survival in nontriple negative breast cancer. Our search in the TCGA-BRCA dataset revealed prominent amplification of DPP3 and p62 genes, which suggests amplification of NRF2 activity in breast cancer by noncanonical mechanisms (Fig.…”

Small Molecule Inhibitors of the Response Regulator ArsR Exhibit Bactericidal Activity against Helicobacter pylori

“…LCA is formed by the 7-α-dehydroxylation of the primary bile acid, chenodeoxycholic acid (CDCA), as a result of the bacterial action in the colon. Beyond its function in the intestinal digestion and fat absorption, this secondary bile acid has demonstrated other biological properties including vitamin D receptor modulation [42,43], proteasome regulation [44] and anti-proliferative and pro-apoptotic effects on cancer cells in vitro and in vivo [45][46][47].…”