Most cancer-related deaths are caused by metastasis, as cancers spreads to sites throughout the body. Breast cancer metastasis is strongly associated with tumor size, and scientists want to better understand how these factors are linked. Now researchers have developed a new way to grow human microscopic tumors in 3-D under conditions that faithfully mimic the early stages of cancer progression. The work could lead to new ways to stop cancers at various stages in their development. The scientists detailed their findings online in May in the journal Cancer Research.
Scientists often investigate breast tumor progression with experiments on lab dishes, but cancer cells grown in flat 2-D layers can act much differently than real tumors, which develop in a 3-D context in the body. Human breast cancer cells implanted in lab animals such as mice grow to resemble tumors more closely than do 2-D layers, but this has limited applicability to people. So scientists have begun creating 3-D models of breast cancer using biodegradable scaffolds and microfluidic systems, attempting to sidestep the problems associated with using both xenografts and 2D models.
Study senior author Shilpa Sant, a pharmaceutical scientist and bioengineer at the University of Pittsburgh, and her colleagues wanted to develop a 3-D model of breast cancer in order to analyze, in real time, how microscopic tumors change as they grow. Previous microtumor growth techniques produced cancers in a hodgepodge of sizes; Sant and her colleagues wanted to more precisely control microtumor size.
The researchers generated microtumors by growing a variety of breast cancer cell lines in hundreds of microscopic wells only 150 to 600 microns wide. Some of these cells possessed proteins known as receptors that, when attached to hormones such as estrogen or progesterone, made those cells grow.
The scientists found that these microtumors mimicked qualities of natural tumors, such as diversity of cells, the distribution of dying and proliferating cells, and gradients in their levels of oxygen available to them. They also discovered that by precisely controlling well size, they could link breast tumor size to aggressive migratory tumor cell behavior. For instance, large microtumors about 600 microns wide that possessed receptors to estrogen and progesterone migrated out of their wells and generated molecules linked with invasive behavior, while smaller hormone-receptor-positive microtumors did not.
Sant was excited to find that tweaking size alone triggered aggressive microtumor behavior. In contrast, experiments with 2-D layers and xenografts often trigger aggressive behavior artificially using gene manipulation or by reducing oxygen levels available to the cancer cells – but neither approach necessarily mimics natural breast tumor progression, Sant says.
“This study further confirms how critically important it is to study tumor cells in appropriate microenvironmental context in vitro,” says bioengineer Claudia Fischbach-Teschl at Cornell University, who did not take part in this research. “It would be interesting to test how the described observations would be regulated if the differently-sized micro tumors were incorporated into a context that contains stromal — that is, non-tumor — cells.”
While noting the need for further studies that confirm the findings are relevant to actual patient cells, Fischbach-Teschl suggests these micro tumors might be useful for testing anti-cancer drugs. Sant agrees, saying the 3-D model will be a “powerful tool to test different therapeutics targeting different stages in tumor progression.”
She and her colleagues now hope to extend this platform to other cancer types such as head and neck, ovarian and prostate cancer. And they’d like their model to include other microenvironment factors that are important in tumor progression, such as the extracellular matrix and supportive tissue.