| Summary: | We hypothesized that the role of microbiota in breast cancer relates to its influence on gut lipid metabolism. This was tested in an in vitro model combining MCF-7 and Caco-2 cells. A total of 32 women newly diagnosed for breast cancer before any treatment and 28 healthy women provided their stools. Bacterial DNA was amplified by qPCR targeting 16s rRNA specific to Bacteroidetes and Firmicutes phyla, <i>Lactobacillales</i> sp., <i>Clostridium</i> cluster IV, <i>Faecalibacterium prausnitzii</i>, <i>Clostridium</i> cluster XIVa, <i>Roseburia intestinalis</i>, <i>Blautia</i> sp., <i>Lactonifactor longoviformis</i>, <i>Bifidobacterium</i> sp., Coriobacteriaceae, <i>Eggertella lenta</i>, <i>Escherichia,</i> and <i>Shigella</i>. Fecal waters (FW) were quantified for short chain fatty acids (SCFA). Caco-2 cells grown on filter inserts were incubated apically with 10% FW for 24 h, and <i>LXR</i>, apolipoproteins <i>AIV,</i> and <i>E</i> gene expression were estimated by real time (RT) qPCR. Then, MCF-7 cells were incubated with the whole basolateral medium for 24 h, and their viability was estimated by 3-(4,5-Dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) test. Regression models were used to determine the correlation between MCF-7 viability and bacteria relative abundance, Caco-2 cells lipid metabolism gene expression and stool composition, as well as microbiota composition and short chain fatty acids. Logistic regression models established disease odds ratios (OR) for MCF-7 viability and Caco-2 gene expression. The OR of MCF-7 viability was 1.05 (1.01–1.10) (OR (5th–95th), <i>p</i> = 0.04), while that of <i>apo AIV</i> gene expression was 0.63 (0.39–1.01), <i>p</i> = 0.055). Viability correlated with % <i>Bifidobacterium</i> sp. (21.18 ± 7.66, <i>p</i> = 0.008) and valerate (−2.849 ± 1.048, <i>p</i> = 0.009) (β ± s.d.). This study suggests that microbiota interacts with intestine cell lipid metabolism. Since these metabolites can reach breast cells by systemic circulation, we hypothesized that they may influence cancer disease.
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