Flavonoid found in citrus slows lung cancer cell growth in lab tests

"Our results suggest that a quercetin-rich diet could support treatment for lung cancer," says Finotto. "We also think it may have preventive effects. We want to investigate that next."

Quercetin is found in citrus fruits, onions and buckwheat and is known for its anti-inflammatory properties. Now researchers at Universitätsklinikum Erlangen at Friedrich-Alexander Universität Erlangen-Nürnberg (FAU) have investigated how the plant compound could help treat lung cancer. In the lab, experiments showed that quercetin slows the division of lung cancer cells and almost completely stops their migration. The results have appeared in the journal Cell Death Discovery.

Why is quercetin interesting for the field of cancer research?

Lung cancer is the most common form of cancer worldwide—no other cancer causes more deaths. Although there are various treatment options available, they do not work for all patients. Researchers in the Department of Molecular Pneumology at Universitätsklinikum Erlangen have now shown that quercetin acts on lung cancer cells at several sites.

"Many drugs against lung cancer target a single molecule," explains Dr. Susetta Finotto. "Quercetin, by contrast, attacked at several points simultaneously in our experiments." For the study, the researchers examined the effect of quercetin on lung cancer cells in the lab, on mouse lung sections and in living mice.

How does quercetin slow down lung cancer cells?

Quercetin led to the tumor cells dividing more slowly in the experiments. In one of the two types of lung cancer cells studied, the plant compound also reduced oxidative stress. This stress is caused by reactive oxygen compounds in cells and can lead to the development of cancer.

Quercetin had a particularly pronounced effect on the migration of cancer cells—a key prerequisite for them to form metastases. "The higher the quercetin dose, the fewer cancer cells migrated. At the highest dose, the migration almost completely stopped," reports Christine Carvalho, lead author of the study.

Where does quercetin act?

Quercetin acted on proteins that are important for tumor growth and immune defense. In the studied cancer cells, the amounts of EGFR and PD-L1 decreased. EGFR takes up growth signals and transmits them into the cell. PD-L1 can help cancer cells hide from the body's own immune defenses.

An effect on the immune system was also seen in the lung sections from mice: quercetin increased the amount of interferon-alpha-2. This messenger substance helps the immune system defend against tumors. "In the mice that received quercetin, the gut microbiome also changed," explains Finotto. "Bacteria that produce certain short-chain fatty acids were more common." These fatty acids travel via the bloodstream from the gut to the lungs and can inhibit the growth of cancer cells there.

Can quercetin help people with lung cancer?

In the mouse model, the researchers found indications of fewer tumor foci with quercetin. However, since only a few animals developed a tumor, further studies are needed to confirm the result. Whether the effects can be transferred to humans is also unclear.

"Our results suggest that a quercetin-rich diet could support treatment for lung cancer," says Finotto. "We also think it may have preventive effects. We want to investigate that next."