MicroRNAs in Medicinal Plants

Could the effects of some medicinal plants also be due to plant microRNAs? In see.nf/herbalmirnas I provide an overview of the findings on microRNAs in ginseng, licorice, red-root sage (danshen), and another traditional Chinese herb, the Chinese honeysuckle, which seemed to work remarkably well in hospitalized Covid-19 patients. As I explain in the video, unfortunately the study—like so many improvised trial series during the pandemic—left a lot to be desired. The bottom line is that microRNAs can provide insight into why certain plants are so effective (and why some poisonous plants are so poisonous!).

Bone of Contention

The concept that microRNAs from food can be therapeutic has been described as “compelling, cutting-edge, and revolutionary.” The first reports were met with considerable skepticism, and meanwhile this has expanded into a fierce controversy. Many later attempts to reproduce the original findings could not confirm them unequivocally and littered the medical literature with editorials like “Dietary microRNAs: Unicorn or Panacea?” and “Noncoding RNAs from plant-based diets: Fairy tale or treasure?” In see.nf/discord I discuss this tug-of-war. MicroRNAs remain an exciting field of research, but the biological role of dietary microRNAs is still far from clearly established.

Eating Animal microRNAs

What happens when we eat or drink animal microRNAs from meat, milk, and eggs? MicroRNAs from animals may, under some circumstances, be taken up in much larger quantities than plant ones. The problem is that it is very difficult to distinguish ingested animal microRNAs from those produced by our own animal body, because they are very similar or completely identical.

Scientists tried to solve this dilemma with genetically modified “knockout” mice in which the gene for a particular microRNA had been “knocked out,” meaning deactivated or removed. Then they gave microRNA-451 knockout mice, for example, blood from wild-type mice, chickens, and pigs to drink. When they found that microRNA-451 then circulated in these mice’ bloodstream and carried out its regulatory function, they knew that animal microRNAs from food can indeed affect physiology.

Setting aside vampire mice, how can such results be confirmed by studies in humans? This is an important question, because there are a number of pro-inflammatory and cancer-promoting microRNAs in animal products that match the same microRNAs in humans 100 percent. Even if the meat microRNA cannot be distinguished from one’s own microRNA, one could at least test whether microRNA levels rise after eating meat. This was tracked for three microRNAs shared by cattle and humans, and after eating beef there was no increase in the relevant blood levels, although—as you may remember—the rectal biopsy after consuming red meat at least revealed microRNA changes in the colon. Chicken microRNAs after eating eggs, on the other hand, can at least be detected in the human bloodstream.

In a study funded by the U.S. Department of Agriculture, published under the title “MicroRNAs from Chicken Eggs Are Bioavailable in Healthy Adults and Can Modulate MicroRNA Expression in Peripheral Blood Mononuclear Cells,” volunteers were given hard-boiled eggs to eat. Within nine hours, their microRNA-181a and microRNA-181b levels rose to about 150 percent and 300 percent above baseline, respectively, reflecting their relatively large amounts in eggs. This was accompanied by suppression of the confirmed target gene of miR-181b in the subjects’ white blood cells. To verify that chicken microRNAs after eating eggs actually enter the human bloodstream—and do not merely indirectly increase endogenous microRNA levels—the researchers were able to track the entry of a chicken microRNA into the participants’ circulation.

Drinking Animal microRNAs

The most compelling evidence for the possibility of cross-kingdom gene regulation is found in the literature on dairy products. Of all the body fluids studied, milk has the highest microRNA content. Milk is a secretion of mammary gland epithelial cells, which release microRNA-filled exosomes into the milk. The literature on human breast milk indicates that most of these are immunomodulatory, especially in the first six months of lactation. We have long known that breast milk contains antibodies and other protective agents that provide passive immunity and support development of the immune system. These agents are absent in industrially produced infant formula, and in addition, microRNAs could lend even more weight to the oft-invoked statement that breast is best.

Babies are not only breastfed, but also programmed. Milk is no longer viewed today merely as infant nutrition, but rather as a highly developed communication system that steers early development. For example, we have known for more than ten years that there is something in milk that prevents allergies. Natural rat milk can prevent allergies in rat pups, but artificial milk cannot. MicroRNAs could be an explanation for why breastfeeding, compared with feeding formula, seems to protect better against asthma and infectious childhood diseases and leads to higher intelligence. If microRNAs in milk can manipulate an infant’s physiology like that, what happens when, after weaning, we drink milk as adults—possibly that of another species?

The milk of pandas and pigs, humans, cows, and water buffalo shares some highly expressed microRNAs, but cow’s milk additionally contains large amounts of hundreds of others—up to 1500 different microRNAs. Since most of the microRNAs in milk are encapsulated in exosomes, they are heat-stable. While most exosomes and their contents are destroyed by boiling or ultra-high-temperature processing (as in the manufacture of shelf-stable coffee creamer), a substantial portion of microRNAs in milk remains intact with industrial pasteurization. Most of these then also survive adult digestion.

To demonstrate that microRNAs from the milk of one species can enter the bloodstream of another that drinks the milk, milk microRNAs were outfitted with a fluorescent label as a tracker. These microRNAs added to cow’s milk distributed to and accumulated in the spleen, liver, heart, and brain of mice. In vitro, human cells also took them up, and as a result multiple genes were up- and downregulated. Of course, it is ridiculous to imagine mice suckling from cows. Primates, on the other hand …

Cow’s Milk microRNAs

Government-funded researchers at the University of Nebraska had men and women drink different amounts of milk—one, two, or four cups. Substantial, dose-dependent milk microRNAs appeared in their blood, peaking four hours after consumption and influencing expression of the target genes. But the bovine microRNAs tested were identical to human microRNAs. How do we know that drinking milk does not somehow spur the production of the body’s own microRNAs, rather than those from milk crossing from the digestive tract into the bloodstream? The levels of a control microRNA that does not occur in milk remained unchanged, but even more convincing evidence came from later studies that, using highly sensitive PCR techniques, could detect tiny differences between bovine and human microRNAs. And indeed: hours after milk consumption, cattle-specific microRNAs circulate throughout the body, compellingly demonstrating that exosomes from industrially pasteurized supermarket milk can enter the tissues of human consumers. What follows from that?