Human Milk MicroRNAs/Exosomes: Composition and Biological Effects

Human milk contains a wide variety of bioactive components, including long-chain fatty acids, complex oligosaccharides, and bioactive proteins. More recently, it was discovered that breast milk also contains exosomes, i.e., microvesicles containing microRNAs (miRNAs) with sizes of ∼22 nucleotides. Several of these miRNAs have been shown to originate from the mammary gland, and many of them are involved in cellular development and immune function. Exosome-mediated transfer of miRNAs is a novel mechanism of genetic exchange between cells. It is therefore possible that exosomes in milk may survive digestion and deliver miRNAs to intestinal cells, or, if transferred into the blood stream, to cells in other tissues. In vitro work has shown that exosomes and their miRNA cargo can survive proteolytic digestion and that intestinal epithelial cells take up the exosomes and deliver them to the nucleus. Research on human adults consuming cow milk has shown that major bovine milk miRNAs are found in the circulation postprandially, further suggesting that exosomes can resist conditions in the gastrointestinal tract and be delivered to the systemic circulation. Thus, it is possible that milk miRNAs may transfer genetic material to the infant and thereby affect gene transcription and regulation of cellular events in several tissues.