Consumption of Mediterranean versus Western Diet Leads to Distinct Mammary Gland Microbiome Populations

Abstract

Recent identification of a mammary gland-specific microbiome led to studies investigating bacteria populations in breast cancer. Malignant breast tumors have lower Lactobacillus abundance compared with benign lesions, implicating Lactobacillus as a negative regulator of breast cancer. Diet is a main determinant of gut microbial diversity. Whether diet affects breast microbiome populations is unknown. In a non-human primate model, we found that consumption of a Western or Mediterranean diet modulated mammary gland microbiota and metabolite profiles. Mediterranean diet consumption led to increased mammary gland Lactobacillus abundance compared with Western diet-fed monkeys. Moreover, mammary glands from Mediterranean diet-fed monkeys had higher levels of bile acid metabolites and increased bacterial-processed bioactive compounds. These data suggest that diet directly influences microbiome populations outside the intestinal tract in distal sites such as the mammary gland. Our study demonstrates that diet affects the mammary gland microbiome, establishing an alternative mechanistic pathway for breast cancer prevention.

Keywords: bile acid; breast; diet; hippurate; mammary gland; microbiome; oxidative stress.

Figure 1.. Habitual Diet Shifts Mammary Gland Bacterial Populations with No Effect on Quantity or Diversity

(A) Diagram of mammary gland sections aseptically obtained and used for analysis. (B) Non-metric multidimensional scaling (NMDS) analysis to visualize microbiome similarities in ordination plots. Western diet 16S samples (n = 19) shown as mustard green circles and Mediterranean diet 16S samples (n = 11) shown as purple triangles. Variation in community structure was assessed with permutational multivariate ANOVA (PERMANOVA) with treatment group as the main fixed factor. Microbiota from Western diet-fed monkey mammary gland samples significantly varied from Mediterranean diet-fed monkey mammary gland microbiota (R² = 0.1087, p = 0.001). (C) Bacterial load was determined via qPCR targeting the V4 region of the 16S gene. Results are expressed in copy number per microliter of DNA extract. No significant differences were observed by diet groups. (D) Alpha diversity was estimated with the Shannon index on raw OTU abundance tables after filtering out contaminants. No significant differences were observed by diet groups. Error bars show the min to max distribution.

Figure 2.. Diet Modulates Mammary Gland Microbiota Clustering and Taxonomic Profiles

(A) Taxonomic profiles of the mammary gland tissue from monkeys consuming a Western or Mediterranean diet at phylum level. *p < 0.05. (B) Community composition between Western diet-fed and Mediterranean diet-fed monkey mammary gland samples at family level. *p < 0.05. (C) Relative abundance of bacterial genera in different breast tissue samples is visualized by bar plots. Each bar represents a subject and each colored box a bacterial taxon. The height of a color box represents the relative abundance of that organism within the sample. “Other” represents lower abundance taxa. n = 11–18, *p < 0.05. Error bars show the min to max distribution.

Figure 3.. Differential Taxa between Western Diet-Fed and Mediterranean Diet-Fed Monkey Breast Tissue Microbiota

Taxa with nominal p values < 0.05 at the genus level are shown by * and their mean percentage proportional abundances in each diet type. Consumption of Mediterranean diet led to an approximate 10-fold upregulation of Lactobacillus abundance in the mammary gland. n = 11–18, *p < 0.05. Error bars show the min to max distribution.

Figure 4.. Diet Differentially Modulates Bile Acid Metabolite Levels in the Mammary Glands

Bile acid metabolites were determined by global untargeted metabolomics analysis of mammary gland samples from monkeys consuming a Western or Mediterranean diet for 31 months. (A) Cholate (CA). (B) Glycocholate (GCA). (C) Taurocholate (TCA). (D) Deoxycholate (DCA). (E) Chenodeoxycholate (CDCA). (F) Glycochenodeoxycholate (GCDCA). (G) Glycodeoxycholate (GDCA). (H) Taurodeoxycholate (TDCA). n = 17–21, *p < 0.05. Error bars show the min to max distribution.

Figure 5.. Diet Differentially Modulates Bacterial-Processed Bioactive Compounds

(A) Hippurate. (B) p-Cresol glucuronide. (C) 3-Hydroxy-3-phenylpropionate. (D) Cinnamoylglycine. (E) 3-Indoxyl sulfate. (F) Indole propionate. (G) Indolin-2-one. (H) Trimethylamine N-oxide. n = 17–21, *p < 0.05. Error bars show the min to max distribution.

Figure 6.. Diet Shifts Oxidative Stress Metabolites in the Mammary Gland Tissue

(A) Heatmap of oxidative stress, pro-inflammatory,antioxidant, anti-inflammatory, and reactive oxygen species (ROS)-scavenging metabolites in the mammary glands (MG). (B) Reduced glutathione (GSH)-to-oxidized glutathione (GSSG) metabolite ratio in mammary glands of Western and Mediterranean diet-fed monkeys. n = 17–21, *p < 0.05. Error bars show SD.