Pollution increases the risk of atherosclerosis, even with a healthy diet

A study shows that mice exposed to São Paulo air during gestation and following birth developed 13 times more atheromatous plaques than the control group

By Karina Toledo

Agência FAPESP – The preliminary results of a study conducted at the Universidade de São Paulo School of Medicine (FMUSP) show that exposure to atmospheric pollution in large urban centers during gestation and soon after birth increases the risk of atherosclerosis in prone individuals, even if they consume a low-fat diet.

Atherosclerosis is an inflammatory disease characterized by the hardening and obstruction of the arterial blood vessels due to the formation of fatty plaques. The worsening of this condition over time can lead to a heart attack, stroke and thrombosis.

“Many studies show pollution to be a risk factor for the disease, but according to the scientific literature, animals that are prone will only develop atherosclerosis if they receive a diet rich in fats. That is why our results were surprising,” said biologist Mariana Matera Veras, the study coordinator and member of the INAIRA (National Institute for Integrated Analysis of Environmental Risk), one of the National Science and Technology Institutes (INCTs) funded by FAPESP and the National Council for Scientific and Technological Development (CNPq) in the State of São Paulo.

The study was performed on mice that had been genetically modified to develop atherosclerosis. The animals were divided into eight groups, and at the end of the experiment, the animals underwent ultrasound analyses to evaluate the sizes of the fatty plaques that had formed in their blood vessels.

The first four groups received a balanced low-fat diet from weaning at one month until the end of the fourth month after birth. The first, considered to be the control group, lived in a chamber with filtered air and had no exposure to air pollution during gestation or after birth.

The second group was exposed to polluted air only during gestation, and these animals were then placed in a filtered-air chamber for the first four months of life. The third group was exposed only after birth, and the fourth group was exposed during gestation and after birth.

The four remaining groups were subjected to the same pollution exposure patterns but unlike the above groups, were given a balanced diet only until the third month of life. Over the three ensuing months, they received a diet rich in fat.

“In the animals that received a fatty diet, the effect of the pollution was less significant and was obscured by the diet,” said Veras.

In the mice given a healthy diet, those exposed to the pollution during gestation exhibited atheromatous plaques that were three times larger than those in the control group, whereas the plaques were seven times greater in the mice exposed to pollutants only after birth. When the gestational and post-partum exposures were combined, the increase was 13-fold.

“The data suggest that pollution works as a factor to change the uterine environment. Pollution could program these animals in such a way that they have a higher risk of developing atherosclerosis, even if they eat a balanced diet and live in a place with clean air following birth,” said Veras.

The results were presented at the 31st Symposium of the Society of Toxicologic Pathology held in Boston from June 24-28.

Multiple generations

The INAIRA group is now investigating such factors as the amount of fat circulating in the animals’ blood, inflammatory markers and the expression of genes associated with the formation of atheromatous plaques to understand how pollution affects the progression of the disease.

“We know that pollutants worsen the inflammatory processes in the organism and this favors the progression of plaques, but we want to know whether there are other mechanisms involved,” said Veras.

Other studies for evaluating the impact of pollutants during gestation and on the reproductive system are being conducted in parallel at INAIRA under the coordination of FMUSP professor Paulo Saldiva.

“The scientific literature shows that the pollution in large cities is related to premature birth, neonatal and post-neonatal death, slow intrauterine growth and low birthweight. In addition, pollution alters gender ratios, causing more girls to be born than boys. Thus, we created an animal model to better study these outcomes,” explained Veras.

The experiments involving many generations of mice were performed in a garden at FMUSP, near a busy avenue in São Paulo. The animals in the control group were housed in chambers with filtered air, and the other group was exposed to the open-air environment.

“We already found reduced fertility in the first generation of the exposed animals. In other words, the number of baby mice born alive was smaller, and the birthweight was lower, results that we have also observed in humans,” said Veras.

These effects were also observed in the second generation of the animals exposed to the polluted air, along with some additional effects: the females demonstrated irregular reproductive cycles, and the couples took longer (on average) to copulate, as the males showed disinterest.

“We also found a reduced number of ovarian follicles in the females, which suggests that the following generations could have infertility problems. In the males, we saw alterations in the morphology of the spermatozoa,” recounted Veras.

Low birthweight

During the passage from the second to third generation of the mice, the INAIRA researchers found alterations in the placenta, which explains, at least in part, the reduced birthweight.

“We also observed that the umbilical cord in the fetuses exposed to pollution is thinner, favoring compression. The evaluations of changes in the placenta and umbilical cord would be valuable in humans and could provide us with important information on the effect that air pollution has on fetal health,” said Veras.

The researchers are currently evaluating the third generation of mice to understand whether a low birthweight impacts their health throughout their lifespans. It has already been noted that the animals are born with organs weighing 25-30% less than normal mice.

“There is a retarded maturation of the lungs, and apparently, a smaller number of alveoli. This could mean a compromised respiratory function. The number of glomeruli in the kidneys is also reduced, and there is a change in the volume of some cerebral compartments,” said Veras.

According to this researcher, these alterations in fetal development would not be so severe as to prevent the animals from leading normal lives. “But if they are challenged by an illness in the future, it could make a difference,” she said.

The results of the study with the first and second generation of mice have already been published. The article “Chronic exposure to fine particulate matter emitted by traffic affects reproductive and fetal outcomes in mice” (doi:10.1016/j.envres.2009.03.006) by Mariana Matera Veras and others can be read at: www.sciencedirect.com/science/article/pii/S001393510900053X.

The article “Increased levels of air pollution and a decrease in the human and mouse male-to-female ratio in São Paulo, Brazil” (doi:10.1016/j.fertnstert.2006.06.023) by Ana Julia F. C. Lichtenfels and others can be read at: www.fertstert.org/article/S0015-0282(06)03183-9/fulltext.

The article “Particulate Urban Air Pollution Affects the Functional Morphology of Mouse Placenta” (doi:10.1095/biolreprod.108.069591) by Mariana Matera Veras and others can be read at: www.biolreprod.org/content/79/3/578.full.pdf.