Liver cells from rats fed a high-fat diet retained signs of senescence in organoids, miniature tissue models grown outside the body
Liver cells from rats fed a high-fat diet retained signs of senescence in organoids, miniature tissue models grown outside the body
On October 16, Aging Cell published a paper by Japanese researchers describing experiments in male rats. Starting at four weeks of age, one group received a diet providing 45% of calories from fat for eight weeks, while the control group received standard chow.
In 2014, Steve Horvath and colleagues estimated liver age using chemical marks on DNA that change with age. In humans, this estimate was higher by an average of 3,3 years for every 10 units of body mass index and did not decrease shortly after weight-loss surgery. Liver biopsies from people with fatty liver disease had also revealed signs of senescence in hepatocytes, the mature cells of the liver. The authors of the new study tested whether cells responsible for liver renewal retain a lasting effect of the diet when grown outside the body.
By 12 weeks of age, young rats fed the high-fat diet had roughly three times the proportion of liver tissue positive for a senescence marker seen in controls. A senescent cell has stopped dividing but remains alive and releases inflammatory substances. When the liver was injured with a toxin, the regenerative response was weaker in rats fed the high-fat diet: they had almost half as many dividing cells as controls and more than twice as many cells containing p16, a protein that inhibits cell division.
To determine whether the diet had altered the cells themselves or their surroundings within the body, the authors grew organoids from a liver cell fraction they expected to contain many cells responsible for tissue renewal. At each passage, organoids are separated into individual cells and seeded again to test the cells’ capacity for self-renewal. All organoids receive the same culture medium, so a difference that persists across passages points to a lasting change within the cells themselves.
Organoid cultures from ten rats fed the high-fat diet progressively lost the ability to form new organoids: six cultures retained this ability at the third passage, four at the fifth, and none at the twelfth. During the first three passages, these cultures had more cells showing signs of senescence and DNA damage, and roughly one-third as many dividing cells as the controls. Three of the six control cultures retained the ability to form organoids through the end of the observation period.
A comparison of gene activity in organoids from rats fed the high-fat diet revealed changes related to inflammation, mitochondrial function, and the production of reactive oxygen species, which damage cellular molecules. Genes involved in transcribing the mitochondria’s own genes were less active, and oxygen consumption was about 60% of the control level. Each change could help explain the reduced ability to form organoids, so the authors tested a drug targeting each proposed cause: BAY 11-7082, an inhibitor of inflammatory signaling; MitoQ, an antioxidant whose positive charge causes it to accumulate in mitochondria; and additional N-acetylcysteine, an antioxidant that acts throughout the cell.
In organoids treated with MitoQ, oxygen consumption returned to the control level, while levels of reactive oxygen species and inflammatory proteins were lower than in untreated organoids. By the third passage, organoid formation was about 70% of the level at the first passage (about 36% without treatment, compared with 100% in controls). N-acetylcysteine reduced the reactive oxygen species signal, but neither it nor BAY 11-7082 increased organoid formation.
The authors describe the findings as consistent with “cellular memory of past metabolic stress” and suggest that “reducing the original metabolic burden may not fully restore tissue maintenance if abnormal states persist within the cells responsible for tissue renewal.”