Scientists in Germany have discovered that colon cancers contain two different cell types, characterized by mitogen-activated protein kinase (MAPK) and NOTCH signaling, respectively, which can regenerate each other when one type is killed by a targeted therapeutic. The studies, headed by David Horst, Ph.D., at the Charité University Hospital in Berlin, suggest that this cell plasticity allows the tumor to sidestep drugs directed against one or other of the two signaling pathways, and that combination strategies for treating colorectal cancer may improve patient outcomes. Early tests in mice confirmed that a combination of NOTCH and MAPK inhibitors more effectively blocked cancer growth and improved survival than treatment using only one or the other type of targeted therapeutic. 

“…when combining both therapies to target both cell populations, we found strong repressive effects on tumor cell proliferation and increased cell death, resulting in slower tumor growth and prolonged survival times compared to either treatment alone,” Dr. Horst comments. “Our data support a new concept for cancer therapy that advocates specific and simultaneous targeting of several different tumor cell populations to strongly improve therapy response.”

Human colon cancers contain two populations of cancer cells, one at the tumor edge in which the MAPK pathway is highly active (indicated by green staining and the white arrowhead), and one in the center of tumor in which the NOTCH pathway is activated (indicated by red staining and the white arrow). [Schmidt et al., 2018]
Human colon cancers contain two populations of cancer cells, one at the tumor edge in which the MAPK pathway is highly active (indicated by green staining and the white arrowhead), and one in the center of tumor in which the NOTCH pathway is activated (indicated by red staining and the white arrow). [Schmidt et al., 2018]

The research team, including scientists at Ludwig-Maximilians-Universität München, the German Cancer Consortium (DKTK), and German Cancer Research Center (DKFZ), report their findings today in the Journal of Experimental Medicine, in a paper entitled “Targeting Tumor Cell Plasticity by Combined Inhibition of NOTCH and MAPK Signaling in Colon Cancer.” 

Colon cancer is the third most common cause of cancer death in men and women, and while early-stage disease can be cured by removing the tumor, later-stage disease relies on systemic therapy, including drugs that target signaling pathways, such as MAPK, that promote tumor progression, say the authors. Unfortunately, as Dr. Horst points out, such treatments aren’t always effective: “Targeting MAPK signaling has limited effects and usually prolongs patient survival by only a few months. We therefore urgently need radical improvements in targeted therapy for patients with colorectal cancer.”

Prior research has linked NOTCH signaling with poor prognosis in colorectal cancer, suggesting that NOTCH may also contribute to tumor progression and thus highlighting NOTCH signaling as a promising therapeutic target. However, other studies have found that NOTCH inhibits MAPK signaling, and initial trials of NOTCH pathway inhibitors have been “disappointing,” the authors write.

In colon cancer, MAPK signaling is typically high in cells at the edge of the tumor, “where putative colon cancer stem cells reside and where tumor cells undergo EMT [epithelial-mesenchymal transition],” but is much lower in cells at the center of the tumor that also have a more epithelial phenotype. In contrast, the distribution of NOTCH activity and cell phenotype associated with NOTCH signaling in colon tumors hasn’t been well characterized.

The researchers examined more than 300 colorectal tumor specimens to investigate in more detail where the different tumor cell types might reside. They found that the tumors comprised at least two, “distinct” cell populations. While cells with high MAPK signaling were, as expected, located at the tumor edge, and undergo EMT, NOTCH signaling was confined principally to cells at the center of the colorectal tumors, which exhibited a more epithelial phenotype. In fact, NOTCH signaling was “unexpectedly downregulated at the infiltrative tumor edge,” the authors state.

High NOTCH and/or MAPK signaling was also associated with relatively poor patient outcomes. Conversely, cases with low levels of both MAPK and NOTCH signaling markers demonstrated the best outcomes, and these patients “almost perfectly survived the follow-up period, with no event of cancer-specific death and only one event of tumor progression.” Higher levels of MAPK, NOTCH, or both were also more commonly found in liver metastases. None of the tumors that were negative for MAPK signaling and demonstrated low NOTCH signaling had metastasized.

Interestingly, when the researchers chemically inhibited MAPK signaling in colon cancer xenograft-bearing mice, the population of NOTCH-signaling cells expanded, and the cells migrated to the tumor edge. “These findings suggest that colon cancer cell subpopulations switched form MAPK to NOTCH activity upon MAPK repression,” the team states. Conversely, inhibiting NOTCH signaling led to an increase in MAPK signaling cells.

When the team then more closely analyzed tumors that had been treated with either the MAPK or NOTCH signaling inhibitors, they found that “astonishingly,” the original distributions and relative numbers of MAPK- and NOTCH-signaling colon cancer cells was the same as it would have been pretreatment. “…  these data indicated that colon cancers may evade targeted treatment against MAPK or NOTCH signaling by a reversible shift in predominating pathway activity,” the reseachers note.

Further studies in tumor-bearing mice indicated that each cell type can in fact respond to inhibition of the other by regenerating the lost population once treatment has been withdrawn. “Collectively, these findings demonstrate that tumor cell subsets with high MAPK or NOTCH activity can be restored from remaining tumor cells during recovery,” the team writes, which provides “evidence for plasticity of signaling pathway activity in colon cancer cells.”

In a final set of experiments, the researchers demonstrated that treatment of colorectal tumor-bearing mice with both a MAPK and a NOTCH signaling inhibitor more effectively slowed tumor growth and improved survival, than treatment with either inhibitor alone. “Collectively, these data demonstrate superior therapeutic effects upon combined targeting of different tumor cell subpopulations with high MAPK and high NOTCH signaling in colon cancer,” they conclude. “…when combining both therapies, we found strong repressive effects on tumor cell proliferation and increased apoptosis, resulting in slowed tumor growth and prolonged tumor-specific survival. Given that these effects significantly outweighed those of single agent treatments, our findings denote that combined treatments mainly succeeded by restricting tumor cell plasticity.”

The researchers suggest that more detailed examination of the different targetable phenotypes and pathways found in different tumor cell subpopulations may offer up new opportunities for improving treatment options for patients with different types of cancer. They are advocating continued studies to evaluate whether combination treatment strategies could improve outcomes for patients with colorectal cancer. “Further preclinical and clinical trials may therefore reveal if combined MAPK and NOTCH inhibition, in addition to established chemotherapeutic protocols, can improve therapy response in patients with colorectal cancer,” Dr. Horst states.

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