How lung cancer 'hijacks' immune cell metabolism

US scientists have discovered how lung adenocarcinoma cancer cells take the wheel and steer macrophage lipid metabolism to drive tumour progression. Their findings, published in the journal Cancer Discovery, provide new inspiration for lung adenocarcinoma interventions that disrupt this tumour cell–macrophage relationship.

Lung adenocarcinoma is the most common lung cancer and the cause of most cancer-related deaths in the United States. There are several ways in which lung adenocarcinoma can arise, one being a mutation in a protein called EGFR (epidermal growth factor receptor). Non-mutated EGFR helps cells grow in response to injury, but mutated EGFR promotes out-of-control growth that can turn into cancer.

Modern immunotherapies don’t work against EGFR-driven lung adenocarcinoma, and while some drugs exist to treat the cancer, patients typically develop a resistance to them within just a few years. This inspired researchers at the Salk Institute for Biological Studies and their collaborators to probe for weak spots in the cancer’s growth pathway.

The team discovered that EGFR-driven lung adenocarcinoma hijacks a specialised population of lung-resident immune cells called macrophages, which are designed to dispose of diseased and damaged cells, as well as maintain a delicate balance of protective lipids (fats) around lung alveoli, tiny bulbs which are essential for breathing. The lung cancer cells pull macrophages into the tumour microenvironment and alter their lipid metabolism to turn them into cancer fuel-suppliers. The newly energised tumour cells then spur further macrophage proliferation to supply more fuel — a novel self-perpetuating cancer mechanism.

“We have discovered a novel way that lung cancer cells manipulate their local environment and other cell types surrounding them to promote their own growth,” said Professor Susan Kaech, Director of Salk’s NOMIS Center for Immunobiology and Microbial Pathogenesis and senior author on the study. “In this case, the tumour cells exploit lung-resident macrophages — remodelling them to provide nutrients, like cholesterol, to the cancer cells and stimulate tumour growth.”

The unique ability of lung-resident macrophages to maintain lipid balance becomes more complicated when tumour cells begin to exploit those lipids to help themselves grow. A better understanding of the mechanisms macrophages use to regulate their metabolism and lipid production can provide insight into how tumour cells selfishly manipulate those mechanisms to help themselves.

“The tumour cells excrete even more of a growth factor called GM-CSF (granulocyte macrophage colony-stimulating factor), which then causes the macrophages to grow alongside them and change their metabolism, resulting in excess lipids that the tumour cells use to strengthen themselves,” said first author Alexandra Kuhlmann-Hogan, currently a postdoctoral researcher at UC Los Angeles. “The cancer was effectively hijacking this normal macrophage process of maintaining the lungs with healthy lipids in order to fuel itself.”

“Not only were the tumour cells metabolically reprogramming the macrophages, they were also instigating a feedback loop that encouraged an optimal metabolic state in the tumour cells themselves,” added co-corresponding author Professor Katerina Politi, Scientific Director of the Center for Thoracic Cancers at Yale Cancer Center.

When the EGFR-driven lung adenocarcinoma cells secreted GM-CSF, it stimulated a gene in the macrophages called PPARγ (peroxisome proliferator-activated receptor gamma), which jump-started their metabolic reprogramming and subsequent secretion of lipids. In addition to using these macrophage-curated lipids to grow, the tumour cells also use the lipids to power the continued activation of the EGFR-drive that helps the cancer grow.

Kaech predicts that disrupting this loop could be a novel intervention for slowing down EGFR-driven cancer growth. Exactly how the delivery of lipids like cholesterol to tumour cells powers the EGFR oncogenic pathway, the researchers aren’t yet sure.

“Our results reveal new therapeutic possibilities for immunotherapy-resistant EGFR-driven lung adenocarcinomas,” said Salk’s Professor Christian Metallo, co-corresponding author of the study. “We have identified a key metabolic relationship between macrophages and alveoli that is exploited by tumour cells to support the cancer’s metabolic demands — now we just have to disrupt that exploitation.”

In future clinical trials, the researchers recommend pairing PPARγ inhibitors, which would disrupt macrophage hijacking, with statins, which would limit available cholesterol along with the currently used EGFR inhibitors. They are also curious whether similar immunological hijacking occurs in other tumour microenvironments around the body, suggesting these results may prompt further discoveries across other cancer types and immune cells.

Image caption: Lung adenocarcinoma tumour cells (green) and macrophages (red) accumulate in the lungs of control mice (left), but expansion of the tumour cells is hindered in macrophage PPARγ knockout mice (right), since the macrophages can’t be metabolically co-opted by tumour cells. Cellular cholesterol is visualised in yellow. Image credit: Salk Institute.