This is how dense breast tissue may lead to cancer
· Medical Xpressby Levi Gadye, University of California, San Francisco
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Breast cancer often grows in tissue that is unusually dense. That's why clinicians flag women with this kind of breast tissue for follow-up after a mammogram.
A UC San Francisco study has uncovered a mechanism that could help explain how this dense environment promotes breast cancer. Dense tissue, which is stiffer than normal tissue, attracts immune cells to the breast and prompts them to release chemicals that mutate DNA. The chemicals penetrate nearby breast cells and damage their DNA, raising the risk of cancer.
"For decades, we've used this relationship between breast tissue density and cancer risk to more readily find cancer," said Valerie M. Weaver, director of the UCSF Center for Bioengineering and Tissue Regeneration and senior author of the paper, which appeared in Cancer Cell. "Our research could enable us to prevent this cancerous domino effect from occurring."
Dense tissue attracts macrophages
Weaver's team, led by UCSF postdoctoral scholar Mary Kate Hayward, began by analyzing publicly available genomic data from breast tumors. They found that patients with worse outcomes had more breast tissue fibrosis, or scarring; more macrophages, a type of immune cell often involved in cancer; and more DNA mutations.
To understand how these factors were linked, the team looked more closely at breast tumor samples from UCSF Health patients in the lab. They directly measured tumor stiffness and took microscopic images of collagen, a protein that can make tissues stiffer when it sticks together.
The stiffer parts of tumors had more collagen and the same cancer hallmarks—fibrosis, macrophages and DNA mutations—they had seen in the publicly available genomic data. They also had more activity from STAT3, a signaling pathway known to drive cancer.
"When we saw the elevated STAT3 signal, we thought it could be an important step linking the stiffness to the increase in macrophages we were seeing," Hayward said.
Lab tumors grown in gels of varying squishiness revealed the first step of that sequence, which was later confirmed in mouse models of cancer. The stiff environment prompted breast cells to turn on STAT3, which led to the release of other signals that lured macrophages from the bloodstream. Those macrophages, the scientists suspected, might be the same macrophages that would eventually help cancer grow.
Macrophages misbehave, unleashing DNA damage nearby
Macrophages normally eat dangerous things in the body—pathogens, debris and dead cells—but in cancer, they do the tumor's bidding, earning the title of "tumor-associated."
Hayward and Weaver wanted to know how the macrophages that were drawn into denser breast tissue by STAT3 might provoke cancer. If grown on stiff gels but not soft gels, the macrophages made chemicals called reactive oxygen species (ROS)—a sign of stress.
This wasn't a huge surprise because elevated ROS is a well-known hallmark of cancer. But ROS are short-lived and highly reactive, making them unlikely to travel far enough from the macrophages to damage nearby cells.
The researchers instead found that ROS caused oxidative damage to fats in the macrophages, turning them into aldehydes—a class of chemicals capable of damaging DNA. Unlike ROS, the aldehydes could escape the macrophages and travel to nearby cells.
"What was striking was that the macrophages were producing chemicals that could travel to nearby cells and damage their DNA," Hayward said. "We tend to think of chemicals in the environment as DNA-damaging and cancer-causing. Yet we found macrophages could also produce DNA-damaging chemicals right within the tissue."
Experiments in Petri dish models of tumor cells and mouse models of breast cancer, plus an additional analysis of human breast cancer biopsies, confirmed it. Dense, stiff breast tissue had more macrophages, more aldehydes and more DNA damage—and more advanced disease.
Hayward sees an opportunity to explore drugs that prevent aldehydes from forming in macrophages. But the bigger picture is that dense breast tissue doesn't simply hide the growth of tumors, she said.
"We've uncovered an explanation for why dense breast tissue may make it easier for new tumors to form—maybe it could help us get ahead of the cancer, before it has the opportunity to grow."
Publication details
Mary-Kate Hayward et al, Tissue tension fosters macrophage-driven lipid peroxidation-induced DNA damage, Cancer Cell (2026). DOI: 10.1016/j.ccell.2026.03.022
Journal information: Cancer Cell
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OncologyWomen's healthObstetrics & gynecologyCommon illnesses & Prevention Provided by University of California, San Francisco Who's behind this story?
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