Milk microRNAs—Lots of Potential Functions but Conflicting Evidence

  • Milk contains microRNAs that are protected from degradation by a lipid coat.
  • Milk microRNAs are altered in quantity and type by milk processing.
  • There is conflicting evidence whether milk microRNAs can change offspring tissue functions.

 

Controversy in science is good. It invites additional investigation generating new data, and ultimately there is either adequate proof or a lack of substantiation of an idea. The scientific idea then either swims into the future or sinks into oblivion. It is a proven but brutal, survival- of-the-fittest evolutionary process. This is how science works. In the intriguing case of the functions of milk microRNAs, this laborious scientific process is still underway.

What are microRNAs and what do they do?

A genome is the complete DNA code of an organism containing the instructions for the program of life and enabling the exquisitely efficient and accurate transfer of this coded information from one generation to the next. Most of the scientific focus on genomes over the last few decades relates to the identification of genes that encode for proteins within each species. Only about 1.5% of the genome codes for proteins. The neglected remainder of the genome is now revealing its many secrets.

The production of a protein from a gene is complex and involves an intermediary called messenger RNA, the first cousin of DNA. Proteins form components of large cellular structures and make myriad biochemicals that are essential for everyday life. Changing the quantity of a protein often alters cellular and tissue functions. Only recently have scientists discovered that the genome also makes a huge variety of very small and very large RNAs that do not encode proteins. The functions of these RNAs are only partially known.

One group of very small RNAs is called microRNAs. Each microRNA functions by putting a brake on the amount of a specific protein being made within a cell, thus microRNAs are ultimately regulators of the quantities of target proteins being synthesized, and through that mechanism, microRNAs affect cell function. MicroRNAs have big reputations as they regulate many cellular processes, particularly the development of functionally specialized cells like muscle, nerve, and immune cells.

Dietary microRNAs

MicroRNAs are present in many foods [1, 2]. In particular, strong evidence from multiple sources shows that microRNAs are enriched compared to mammary cells in milk from cattle, humans, pigs, and rodents [1, 3–10]. This conclusion is not in dispute as it has jumped the rigorous hurdle of multiple detailed scientific investigations. It was initially thought that microRNAs should be vulnerable to destruction outside of cells, especially in milk; however, they are remarkably stable in body fluids [11]. This stability is due to a sphere of protective fatty acids surrounding multiple microRNAs [1, 7]. Perhaps microRNA stability is only a relative term as several investigations indicate that milk processing radically changes the quantities and types of microRNAs within milk [3, 7–9].

The biologically intriguing question is whether ingested milk microRNAs are stable in the gut and then regulate tissue development and function in the offspring. This is where the controversy begins.

Are dietary microRNAs present in offspring tissues?

The journey of ingested microRNAs to their target cells in the offspring is wrought with extreme difficulties. They not only have to survive the digestive activities of the gut, but microRNAs must pass through the gut cell barrier, enter into the blood circulation and then be absorbed by specific cells in a tissue. Here, sufficient quantities of the absorbed microRNAs must be present to regulate the quantity of the target protein within a cell and therefore its functions. This may be a tall order!

Several scientific investigations concluded that ingested microRNAs are stable and function in offspring. One study directly investigated the stability of two microRNAs under simulated gastrointestional conditions, and the investigators concluded that they can survive the perils of the gastrointestinal tract [12]. In addition, it is well established that the digestive functions of newborns are immature, which would also assist the survival of microRNAs in the gut. Other investigators demonstrated that bovine milk microRNAs are taken into human kidney, intestinal and immune cells grown in laboratory culture [10, 13, 14]. The cellular specificity and functional implications of this action are unclear. There is also scattered evidence that ingested milk microRNAs are taken up by offspring tissues where they alter tissue function, particularly immune system function [1, 2, 4, 10, 14–16].

The opposing conclusion that dietary microRNAs ingested by another species do not have biological effects after ingestion has been reached by a raft of recent scientific publications (summarized in [17–20]). Moreover, specific studies of humans ingesting bovine milk could not detect the transfer of bovine microRNAs into the blood circulation [20].

The contradictory information in these collective publications, however, cannot be easily reconciled. At this time, only more research using smart experimental designs can settle this scientific controversy. The urgency of this task is emphasized by preliminary reports that milk microRNAs may help some medical conditions, such as arthritis [14].

Conclusions

MicroRNAs are present in milk and have potential to promote the development of tissues in newborn offspring—a maternal inheritance offering a helping hand. However, the scientific jury has not yet received sufficient evidence to enable a decision regarding the effects of ingested microRNAs on an individual.

 

1. Zempleni J., Baier S.R., Howard K.M., Cui J. Gene regulation by dietary microRNAs. Can J Physiol Pharmacol. 2015;93(12):1097–102.
2. Alsaweed M., Hartmann P.E., Geddes D.T., Kakulas F. MicroRNAs in breastmilk and the lactating breast: potential immunoprotectors and developmental regulators for the infant and the mother. Int J Environ Res Public Health. 2015;12(11):13981-4020.
3. Chen X., Gao C., Li H., Huang L., Sun Q., Dong Y., et al. Identification and characterization of microRNAs in raw milk during different periods of lactation, commercial fluid, and powdered milk products. Cell Res. 2010;20(10):1128-37.
4. Sun Q., Chen X., Yu J., Zen K., Zhang C.Y., Li L. Immune modulatory function of abundant immune-related microRNAs in microvesicles from bovine colostrum. Protein Cell. 2013;4(3):197-210.
5. Gu Y., Li M., Wang T., Liang Y., Zhong Z., Wang X., et al. Lactation-related microRNA expression profiles of porcine breast milk exosomes. PLoS One. 2012;7(8):e43691.
6. Xi Y., Jiang X., Li R., Chen M., Song W., Li X. The levels of human milk microRNAs and their association with maternal weight characteristics. Eur J Clin Nutr. 2016;70(4):445-9.
7. Alsaweed M., Hepworth A.R., Lefèvre C., Hartmann P.E., Geddes D.T., Hassiotou F. Human milk microRNA and total RNA differ depending on milk fractionation. J Cell Biochem. 2015;116(10):2397–407.
8. Howard K.M., Jati Kusuma R., Baier S.R., Friemel T., Markham L., Vanamala J., et al. Loss of miRNAs during processing and storage of cow’s (Bos taurus) milk. J Agric Food Chem. 2015;63(2):588–92.
9. Kirchner B., Pfaffl M.W., Dumpler J., von Mutius E., Ege M.J. microRNA in native and processed cow’s milk and its implication for the farm milk effect on asthma. J Allergy Clin Immunol. 2016;137(6):1893-5.e13.
10. Baier S.R., Nguyen C., Xie F., Wood J.R., Zempleni J. MicroRNAs are absorbed in biologically meaningful amounts from nutritionally relevant doses of cow milk and affect gene expression in peripheral blood mononuclear cells, HEK-293 kidney cell cultures, and mouse livers. J Nutr. 2014;144(10):1495–500.
11. Izumi H., Kosaka N., Shimizu T., Sekine K., Ochiya T., Takase M. Bovine milk contains microRNA and messenger RNA that are stable under degradative conditions. J Dairy Sci. 2012;95(9):4831–41.
12. Benmoussa A., Lee C.H., Laffont B., Savard P., Laugier J., Boilard E., et al. Commercial dairy cow milk microRNAs resist digestion under simulated gastrointestinal tract conditions. J Nutr. 2016;146(11):2206–15.
13. Izumi H., Tsuda M., Sato Y., Kosaka N., Ochiya T., Iwamoto H., et al. Bovine milk exosomes contain microRNA and mRNA and are taken up by human macrophages. J Dairy Sci. 2015;98(5):2920–33.
14. Arntz O.J., Pieters BC, Oliveira MC, Broeren MG, Bennink MB, de Vries M, et al. Oral administration of bovine milk derived extracellular vesicles attenuates arthritis in two mouse models. Mol Nutr Food Res. 2015;59(9):1701–12.
15. Zempleni J, Aguilar-Lozano A, Sadri M, Sukreet S, Manca S, Wu D, et al. Biological activities of extracellular vesicles and their cargos from bovine and human milk in humans and implications for infants. J Nutr. 2017;147(1):3–10.
16. Zempleni J., Baier S.R., Hirschi K. Diet-responsive microRNAs are likely exogenous. J Biol Chem. 2015;290(41):25197.
17. Kang W., Bang-Bertelsen C.H., Holm A., Houben A., Müller A.H., Thymann T., et al. Survey of 800+ datasets from human tissue and body fluid reveals XenomiRs are likely artifacts. RNA. 2017.
18. Bağcı C., Allmer J. One step forward, two steps back; xeno-microRNAs reported in breast milk are artifacts. PLoS One. 2016;11(1):e0145065.
19. Witwer K.W., Hirschi K.D. Transfer and functional consequences of dietary microRNAs in vertebrates: concepts in search of corroboration: negative results challenge the hypothesis that dietary xenomiRs cross the gut and regulate genes in ingesting vertebrates, but important questions persist. Bioessays. 2014;36(4):394–406.
20. Auerbach A, Vyas G, Li A, Halushka M, Witwer K. Uptake of dietary milk miRNAs by adult humans: a validation study. F1000Res. 2016;5:721.

 

Contributed by
Dr. Ross Tellam (AM)
Research Scientist
Brisbane, Australia