Primary colorectal tumors affect distant organs before cancer cells arrive on site
by Katharine Hendrix
It is critical to better understand the mechanics of colorectal cancer (CRC) metastasis, as it is the second leading cause of cancer deaths in the nation. Patients with advanced cases often die because current treatments for widely metastasized disease are not effective.
Recent cancer research shows that premetastatic niches form at sites far from the original tumor before new tumors occur. In CRC, these supportive microenvironments form in preferred secondary organs, such as the liver and lung, and facilitate the colonization, survival, and growth of metastasizing tumor cells. However, the mechanisms responsible for the formation of these premetastatic niches, including what role(s) the primary tumor may play, are not well understood.
MUSC investigators led by Raymond N. DuBois, M.D., Ph.D., dean of the MUSC College of Medicine and professor of biochemistry and molecular biology, have now illuminated how primary CRC tumors contribute to premetastatic niche formation.
In an April 28, 2017 Cancer Research article (doi: 10.1158/0008-5472.CAN-16-3199), they report that primary colorectal tumors secrete vascular endothelial growth factor (VEGF)-A, inducing CXCL1- and CXCR2-positive myeloid-derived suppressor cell (MDSC) recruitment at distant sites and establishing niches for future metastases. Liver-infiltrating MDSCs help bypass immune responses and facilitate tumor cell survival in the new location. This research suggests CXCR2 antagonists may reduce metastasis.
“The idea that some sort of ‘priming’ needs to take place for metastasis to occur in distant organs — that there is some sort of activity in the future tumor location — is not new. But most research has focused on growth factors, chemokines and proinflammatory cytokines. There hasn’t been much work looking at immune cell activity in distant organs prior to metastasis,” explains DuBois. “We knew that the type and density of immune cells in the primary tumor play a role in progression. For example, when more immature myeloid cells are present in the tumor, it becomes resistant to immune attack. But we didn’t know what to expect in a metastatic model.”
To explore this area, the team first evaluated whether the presence of a primary tumor affected immune cell profiles in premetastatic liver and lung tissues of mice. They found that the presence of a primary cecal tumor caused MDSCs to begin infiltrating the liver before metastasis began. Working backward from this finding, they used a series of experiments to reveal the chain of events that led up to MDSC infiltration.
Because CXCR2 is essential for drawing MDSCs out of the bloodstream and toward CRC tumors and colonic mucosa, the team began looking for CXCR2 and its ligands in mouse liver tissue. The team not only found that the ligand CXCL1 attracted MDSCs from the bloodstream into premetastatic liver tissue, but also that administering a CXCR2 antagonist inhibited CXCL1 chemotaxis. This demonstrated that CXCR2 is required for CXCL1 to induce MDSC liver infiltration. Of importance, they also found that liver-infiltrating MDSCs secrete factors that promote cancer cell survival and metastatic tumor formation without invoking the innate and adaptive immune responses.
Next, because VEGF is known to induce CXCL1 expression in lung cancer, the research team examined whether VEGF secreted by CRC tumors also regulated CXCL1 expression. Their results demonstrated that VEGF-A secretion by primary CRC tumor cells stimulates macrophages to produce CXCL1. Interestingly, although VEGF-A knockdown inhibited liver metastasis, it did not affect the growth of the primary tumor.
“We did not expect to find that a primary tumor could affect a distant organ before any of the cancer cells arrived on site,” says DuBois. “We were surprised to see these changes before a single metastatic cell took up residence.”
Together, these studies reveal that VEGF-A secreted by the primary CRC tumor stimulates macrophages to produce CXCR1, which recruits CXCR2-expressing MDSCs from the bloodstream into healthy liver tissue. The MDSCs then create a premetastatic niche or microenvironment where cancer cells can grow to form new tumors. These results demonstrate for the first time that cells in the primary tumor contribute to forming distant premetastatic niches, which facilitate the spread of disease.
“Now that we know the primary tumor puts things in motion remotely prior to metastasis, we should be able to inhibit this process and have a positive impact on survival,” explains DuBois. “We now know which molecules and immune cells are involved and that, if we disrupt the CXCL1-CXCR2 axis, we can possibly reduce the spread of disease. Both antibodies and small molecules can inhibit this pathway, but they have not yet been optimized. I hope these findings will speed up the development of inhibitors of the CXCR2 pathway.”