Human milk miRNAs primarily originate from the mammary gland resulting in unique miRNA profiles of fractionated milk

Abstract

Human milk (HM) contains regulatory biomolecules including miRNAs, the origin and functional significance of which are still undetermined. We used TaqMan OpenArrays to profile 681 mature miRNAs in HM cells and fat, and compared them with maternal peripheral blood mononuclear cells (PBMCs) and plasma, and bovine and soy infant formulae. HM cells and PBMCs (292 and 345 miRNAs, respectively) had higher miRNA content than HM fat and plasma (242 and 219 miRNAs, respectively) (p < 0.05). A strong association in miRNA profiles was found between HM cells and fat, whilst PBMCs and plasma were distinctly different to HM, displaying marked inter-individual variation. Considering the dominance of epithelial cells in mature milk of healthy women, these results suggest that HM miRNAs primarily originate from the mammary epithelium, whilst the maternal circulation may have a smaller contribution. Our findings demonstrate that unlike infant formulae, which contained very few human miRNA, HM is a rich source of lactation-specific miRNA, which could be used as biomarkers of the performance and health status of the lactating mammary gland. Given the recently identified stability, uptake and functionality of food- and milk-derived miRNA in vivo, HM miRNA are likely to contribute to infant protection and development.

Roles of MicroRNA across Prenatal and Postnatal Periods

“…More than 1400 mature miRNAs are expressed in human milk [71]. These miRNAs are thought to originate from mammary gland epithelial cells [72]. Their resistance to degradation stems from their assembly with proteins (Ago2 and other RNA-binding proteins) in skim milk [72,73], packaged inside milk cells [74] and inside the heterogeneous population of milk vesicles, which include milk fat globules [75] and exosomes [76,77].…”

“…These miRNAs are thought to originate from mammary gland epithelial cells [72]. Their resistance to degradation stems from their assembly with proteins (Ago2 and other RNA-binding proteins) in skim milk [72,73], packaged inside milk cells [74] and inside the heterogeneous population of milk vesicles, which include milk fat globules [75] and exosomes [76,77]. Human milk somatic cells or exosomes which carry miRNA [74] are thought to survive in the acidic environment of the stomach, to be transferred into the circulation and may then be integrated in various tissues [78].…”

“…Interestingly, breast milk is rich with immune-related miRNAs [71,77,88] and miRNAs are implicated in multiple physiological processes [75], including cellular differentiation and proliferation, tissue identity, metabolism, and developmental programming [72]. Some miRNAs present in milk are involved in nervous system pathways and may mediate brain development.…”

MicroRNAs in the development and neoplasia of the mammary gland

“…The study of miRs in mammary gland development is essential, as miRs in milk are derived from the mammary gland and can be a biomarker of a healthy lactating gland, as well as protecting infants and promoting their development 3 . The role of miRs in milk is still being debated.…”

Potential relevance of microRNAs in inter-species epigenetic communication, and implications for disease pathogenesis

“…39,40 The largest amount of milk miRNAs is derived from the lactating mammary epithelium. 41 If miRNAs in the milk can be absorbed, these could have epigenetic regulatory roles in the offspring, i.e., miRNAs might be involved in the "epigenetic priming" of the baby (so-called functional hypothesis). 42,43 The functional activity of milkderived miRNAs is not universally accepted, and the other, nutritional hypothesis argues that miRNAs are not absorbed into the circulation, but they are degraded to nucleotides, which merely serve as nutrients.…”