Necrotic core — even the name sounds creepy. But what it does in the realm of cancer spread is creepier still.
New research by Fred Hutchinson Cancer Center molecular biologist Ami Yamamoto in the Cheung Lab, shows that a tumor’s necrotic core — a mass of dying and dead cells previously thought to be inconsequential or perhaps even beneficial — contains factors that appear to promote metastasis, or the seeding of tumors cells throughout the body.
Metastatic, or stage 4 cancers, are treatable but still not curable. Yamamoto, first author of a study published this week in PNAS, Proceedings of the National Academy of Sciences, hopes to change that by better understanding the process of metastasis itself.
Tumor necrotic cores are a fairly common phenomenon, Yamamoto said, but they haven’t been linked to cancer metastasis until recently.
“Our research put together observations other people have made into the specific context of breast cancer metastasis. We’re looking at it from the angle of ‘Where do cancer cells disseminate from?’” she said. “Our work shows a pretty direct link between necrosis, circulating tumor cells and cancer metastasis.”
What exactly are necrotic cores?
As the name implies (necrosis being the death of the cells in your body tissues), they’re tumors that are dying from the inside out. Past research has linked necrosis with the dissemination of cancer cells but the how and why of this association hasn’t been well understood.
“The necrotic part of the tumor is like a perfect environment for metastatic spread,” Yamamoto explained. “There’s leaky blood vessels, hypoxia (low levels of oxygen) and the recruitment of immune cells, some of which have been shown to help cancer cells spread.”
Surgeons, pathologists, radiologists, clinicians and researchers often come across necrotic cores in their line of work and they’re usually not a good sign.
“Necrosis is a clinical finding seen in aggressive tumors that grow quickly,” said physician-researcher Kevin Cheung, MD, senior author on the study. “When we see it at tumor board, it means this is a dangerous tumor that needs to be treated aggressively.”
But necrosis isn’t only something you see in large, late-stage tumors.
“Necrosis can happen early, as well,” Cheung said. “You can see it in DCIS (ductal carcinoma in situ, or Stage 0 breast cancer). You can see it in small tumors. It’s not necessarily directly coupled to tumor size.”
And now, he said, “Ami’s research suggests that circulating tumor cell dissemination correlates best with necrosis, not the size of the tumors.” (The dissemination of circulating tumor cells, or CTCs, is what leads to metastasis.)
Also, the necrotic core of a tumor isn’t necessarily a complete dead zone.
“Even though it is a necrotic region overall, not all the cells are dead,” Yamamoto said. “If they were all entirely dead, then tumor cells wouldn’t disseminate. What we think is happening is that it’s mostly dead, a very necrotic region, but there are surviving living tumor cells in the necrotic region and it’s an environment that makes it easier for those surviving tumor cells to disseminate.”
— Fred Hutch molecular biologist Ami Yamamoto
Yamamoto’s research into the necrotic core of tumors began with the creation of a new rat model of breast cancer metastasis – rats make better models than mice for advanced human disease – which she followed for several weeks in order to track the progression of circulating tumor cells.
“We counted CTCs at four time points after the rats developed palpable tumors,” Yamamoto said, adding that the researchers found zero CTCs at the first two time points and a handful at the third time point.
At the fourth time point, however, everything changed.
“Suddenly, we found hundreds of CTCs,” she said. “Naturally, we wanted to know what was changing between the third and fourth time point. What we found was that the primary tumor developed a large central area of necrosis, or inflammatory cell death, concurrent to the rise in CTC count.”
Further investigation showed a stark difference in gene expression between the necrotic and the non-necrotic regions of the tumor.
“We found that a gene which encodes angiopoietin-like 7, a secreted protein, was the most enriched tumor-derived gene in the necrotic and peri-necrotic regions (next to necrotic regions) of the tumor,” Yamamoto said.
This single protein, angiopoietin-like-7, remodels the tumor microenvironment, somehow encouraging the tumor cells to grow past their nutrient limits, undergo necrosis and start spreading to other parts of the body.
At least in their rat models. The finding stunned the researchers.
“This was a surprise,” Cheung said. “We thought necrosis was entirely unregulated, not something you can control.”
Yamamoto then did experiments to see how controlling the protein would impact necrosis.
“When we suppressed the expression of this protein in the tumors, there was a dramatic reduction of necrotic tumor area,” she said. “Suppression of angiopoietin-like 7, or A-7, also reduced circulating tumor cells to almost zero and reduced distant metastases and dilated, large blood vessels.”
Yamamoto said their finding was exciting and completely unexpected.
“Usually when we are trying to solve problems in biology, it’s a combination of different genes,” said. “Suppressing a single gene doesn’t do much a lot of the time. It was really surprising that just suppressing this one gene could reduce necrosis and CTC counts by so much. But when it’s suppressed, we have less necrosis.”
Yamamoto said their research not only showed A-7 regulates the development of central necrosis in the primary tumor, but also the development of dilated blood vessels which could be helping the dissemination of circulating tumor cells and metastasis.
Beyond the surprise of such an important mechanism to necrosis, their findings unveiled the potential for a new targeted treatment for patients.
“Our ultimate goal is to develop a therapeutic antibody against A-7 that will prevent or reduce metastasis in patients with metastatic breast cancer,” she said. “That’s the next critical next step in our research.”
Toward that end, the team has already come up with over 200 candidates for an anti-A-7 antibody but need to screen through all of them to see which ones do the best job of blocking the function. Cheung said he and his team also want to analyze the data of large patient cohorts to better understand the role between necrosis in the tumor and the risk of metastatic dissemination.
“Some patients have evidence of markers of necrosis in their blood,” he said. “That suggests this is potentially happening not just in preclinical models but in patients.”
Funding from the donor-backed Evergreen Fund and the Vancouver, Wash.-based Kuni Foundation will help the researchers conduct a trial to screen through their antibody candidates. The top contender will then be tested in the lab and if all goes well, will move into clinical trials with metastatic breast cancer patients.
Cheung and Yamamoto also want to delve into the unanswered questions that remain.
“We need to find out how the necrotic core is formed, how the cells are getting into the blood circulation and how best to target cells in these necrotic zones,” Cheung said. “We also need to develop models that can help us dissect this chicken and egg question of whether dilated vessels are the result of dissemination or the instigator of the process.”
In addition to demonstrating that one overexpressed protein is triggering dissemination and metastasis, their work shows the benefit of researching areas once thought to be fruitless.
“Before we started this, we probably considered the necrotic core to be the least interesting part of the tumor,” Cheung said. “We’re now confident the inside is a really interesting place for studying the source of metastatic cancer. There’s more cancer biology there that awaits.”
Diane Mapes is a staff writer at Fred Hutchinson Cancer Center. She has written extensively about health issues for NBC News, TODAY, CNN, MSN, Seattle Magazine and other publications. A breast cancer survivor, she blogs at doublewhammied.com and tweets @double_whammied. Email her at dmapes@fredhutch.org. Just diagnosed and need information and resources? Visit our Patient Care page.
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