Adipose tissue may be a reservoir for SARS-CoV-2, Brazilian researchers suggest

A study showed that the novel coronavirus can infect human adipocytes and that the viral load is three times higher in aged adipose cells. The results may help us understand why people with obesity and those who are older are more at risk of developing the severe form of COVID-19.

By Karina Toledo  |  Agência FAPESP – Experiments conducted at the University of Campinas (UNICAMP) in the state of São Paulo, Brazil, have confirmed that the novel coronavirus SARS-CoV-2 can infect human adipose cells (which store fat) and remain inside them. This finding may help scientists understand why individuals with obesity are more at risk of developing the severe form of COVID-19.

According to the hypothesis tested by the UNICAMP group, subjects with obesity have larger reservoirs for the virus, in addition to being susceptible to chronic diseases such as diabetes, dyslipidemia and high blood pressure, which in themselves are significant coronavirus risk factors.

“We have fat cells in all parts of our body. People with obesity have many more and much larger adipose cells. Our hypothesis is that adipose tissue is a reservoir for SARS-CoV-2. With adipocytes that are larger and more abundant, patients with obesity tend to have higher viral loads. However, we have yet to confirm whether the virus, after replicating, can exit fat cells with the capacity to infect other cells,” Marcelo Mori, a professor in UNICAMP’s Institute of Biology (IB) and principal investigator for the study, told Agência FAPESP.

The experiments in human adipocytes are being conducted in vitro with FAPESP’s support at IB-UNICAMP’s Emerging Virus Research Laboratory (LEVE), a Biosafety Level III (BSL-3) facility. LEVE is headed by José Luiz Proença Módena, a professor at the institute and joint chair alongside Mori of the task force set up by UNICAMP to address the pandemic (read more at agencia.fapesp.br/32998). The results are still preliminary, and they have not yet been published.

As Mori explained, SARS-CoV-2 is not able to enter any type of human cell and replicate efficiently. Certain favorable conditions need to be present, including a membrane protein called ACE-2 (angiotensin-converting enzyme 2), to which the virus must bind to invade a cell.

The in vitro comparisons conducted by UNICAMP researchers showed that SARS-CoV-2 infects adipocytes better than epithelial cells in the intestines or lungs, for example. Furthermore, the virus takes over fat cells more easily when the cellular aging process is accelerated with ultraviolet radiation.

When the researchers measured viral load at 24 hours after UV exposure, they found that the viral load in the aged adipose cells was three times that of the viral load in “young” cells.

“We used UV radiation to induce a phenomenon known as senescence in adipocytes. Cellular senescence occurs naturally with aging. When cells enter senescence, they express molecules that recruit immune cells to the site. This mechanism is important to protect the organism from tumors, for example,” Mori explained.

The problem, he went on, is that in elderly patients and those with obesity, as well as in people with chronic diseases, senescent cells build up in adipose tissue until it becomes dysfunctional, potentially resulting in the development or aggravation of metabolic disorders.

The accelerated aging of adipocytes induced by UV radiation mimics what usually occurs in the adipose tissue of elderly patients and those with obesity. “Testing of senolytic drugs, compounds capable of killing senescent cells, recently began in humans. In experiments with animals, these compounds proved capable of prolonging life and reducing the development of chronic diseases associated with aging,” Mori said.

The UNICAMP group then had the idea of testing the effect of senolytic drugs in the context of infection by SARS-CoV-2. In experiments with human intestinal epithelial cells, they found that the treatment reduced the viral load in cells subjected to UV radiation.

“Some compounds reduced viral load by 95%,” Mori said. “We now plan to repeat the experiment using adipocytes.”

The tests have thus far involved adipocytes differentiated in vitro from a type of mesenchymal (preadipocyte) stem cell isolated from uninfected patients subjected to bariatric surgery. After differentiation, the cells were exposed to a strain of SARS-CoV-2 isolated from Brazilian patients and cultured in the laboratory by researchers at the University of São Paulo’s Biomedical Sciences Institute (ICB-USP) (read more at agencia.fapesp.br/32699).

The next steps include an analysis of adipocytes obtained by biopsy from confirmed COVID-19 patients. “One of the goals is to see if these cells are in fact infected by SARS-CoV-2 and if the virus is replicating inside them,” Mori said.

Proteomic analysis will also be performed to determine whether infection by SARS-CoV-2 affects the functioning of adipocytes and whether it causes any long-term damage to cells. This part of the research will be conducted in collaboration with Daniel Martins-de-Souza, a professor in IB-UNICAMP.

“We plan to compare all proteins expressed in cells with and without the virus to identify the signaling pathways that are altered by the infection and see how this affects cellular functioning,” Mori said.

Premature aging

In the Biochemistry and Tissue Biology Department at IB-UNICAMP, Mori has devoted himself to studying the biology of aging for several years. In his current project, he is investigating why elderly people and patients with aging-related diseases are more susceptible to complications of COVID-19.

“This finding that senescent adipocytes have higher viral loads points to the possibility of a link between metabolic disorders, aging and severe COVID-19,” Mori said.

However, it is not yet known whether the viral load is higher in these cells because they are more easily infectable when they are exposed to SARS-CoV-2 in the laboratory or the quantity of viruses that invade them is the same, but the rate of viral replication is higher. “We need to do more experiments and track the progress of viral load over time,” Mori said.

If the experiments confirm that the virus has some kind of metabolic effect on adipocytes, the implications could be considerable. “Fat cells play a key role in regulating the metabolism and the communication between different kinds of tissue,” Mori said. “They tell the brain when we should stop eating, tell muscles when to uptake blood sugar, and tell the body when to expend or store energy, rather like a metabolic thermostat. The virus may interfere with these processes, although for now this is speculation.”

Mori is conducting these investigations in partnership with Luiz Osório Silveira Leiria, a professor at the University of São Paulo’s Ribeirão Preto Medical School (FMRP-USP). Leiria is the principal investigator of a project supported by FAPESP to determine what role certain lipids play in the inflammatory process caused by SARS-CoV-2.

“I also collaborate in my research with a large network of scientists who belong to the UNICAMP Task Force Against COVID-19,” Mori said.