Cancer cells produce small amounts of their own form of type 1 collagen (Col1), creating a unique extracellular matrix that affects the tumor microbiome and protects against immune responses, according to the results of a study by researchers at the University of Texas MD Anderson Cancer Center. The research indicated that this abnormal homotrimer variant of collagen has oncogenic properties, and its structure is fundamentally different from that of normal collagen made in the human body, providing a highly specific target for therapeutic strategies.

The reported preclinical research indicated that deletion of Col1 homotrimers increased overall survival of mice with pancreatic ductal adenocarcinoma (PDAC), and was associated with improved efficacy of anti-PD-1 immunotherapy.

The study, published in Cancer Cell, builds upon previously published findings from the laboratory of Raghu Kalluri, MD, PhD, chair of cancer biology and director of operations for the James P. Allison Institute, to bring a new understanding of the unique roles of collagen made by fibroblasts and cancer cells.

“Cancer cells make an atypical collagen to create their own protective extracellular matrix that helps their proliferation and their ability to survive and repel T cells. It also changes the microbiome in a way that helps them thrive,” said Kalluri, the senior study author. “Uncovering and understanding this unique adaptation can help us target more specific treatments to combat these effects.” Kalluri and colleagues published their findings in a paper titled, “An oncogenic collagen I homotrimer from cancer cells binds to α3ß1 integrin and affects tumor microbiome and immunity to promote pancreatic cancer.”

Col1, the most abundant protein in the body, is produced by fibroblasts and found mostly in bones, tendons, and skin. It is also found in association with pancreatic cancer. “Col1 is the most abundant protein in the human body and accumulates extensively in the microenvironment of pancreatic ductal adenocarcinoma (PDAC),” the authors wrote. Previously, collagen in tumors was believed to promote cancer development, but Kalluri’s laboratory showed that it likely plays a protective role in suppressing pancreatic cancer progression. “Recent studies suggest that Col1 produced by myofibroblasts restrains PDAC, but the structure and function of Col1 produced by cancer cells (cancer-Col1) in PDAC remain unknown,” the team continued.

In its normal form, collagen is a heterotrimer consisting of two α1 chains (encoded by the Col1a1 gene) and one α2 chain (encoded by the Col1a2 gene), which assemble to form a triple-helix structure as part of the extracellular matrix. However, when studying human pancreatic cancer cell lines, the researchers discovered that the cells expressed only the α1 gene (Col1a1), whereas fibroblasts expressed both genes. Further analysis revealed that cancer cells have silenced the α2 gene (Col1a2) through epigenetic hypermethylation, resulting in a cancer-specific collagen “homotrimer” made up of three α1 chains. The researchers noted, “We demonstrate that PDAC cancer cells produce Col1 homotrimer of a1 chains due to the suppression of the Col1a2 gene via DNA hypermethylation of the promoter … Collectively, the production of Col1 homotrimer consisting of only the Col1a1 chain is likely an overarching feature of both human and mouse pancreatic cancer cells.”

To investigate the real-world effects of this observation, the researchers created knockout mouse models of pancreatic cancer with Col1a1 deleted only in cancer cells. Loss of this cancer-specific homotrimer reduced cancer cell proliferation and reprogrammed the tumor microbiome. This led to lower immunosuppression, which was associated with increased T-cell infiltration and elimination of cancer cells.

Additionally, these knockout mice responded more favorably to anti-PD1 immunotherapy, suggesting that targeting this cancer-specific collagen could help boost the anti-tumor immune response. “Deletion of Col1 homotrimers enhances T-cell infiltration and enables efficacy of anti-PD-1 immunotherapy,” the scientists pointed out.

Raghu Kalluri, MD, PhD [MD Anderson Cancer Center]

“This discovery illustrates the importance of mouse models, as it was only when we noticed a difference in their survival that we found this abnormal collagen variant existed and was produced specifically by the cancer cells,” Kalluri said. “Because it is generated in such small amounts relative to normal collagen, the homotrimer would have otherwise gone undetected without specific tools to differentiate them.”

Given the relationship between the gut and tumor microbiomes and immune responses, the researchers also explored the microbiome in their mouse models. Interestingly, loss of the cancer-specific collagen led to changes in the bacterial composition within the tumor. There was a corresponding decrease in myeloid-derived suppressor cells (MDSCs) and an increase in T cells, contributing to favorable survival outcomes.

These effects were completely reversed by disrupting the microbiome with antibiotics, suggesting that cancer-specific collagen promotes cancer progression by enhancing a tumor-promoting microbiome. This is early evidence that the extracellular matrix directly influences the tumor microbiome, which could help researchers to understand how cancer cells have developed these adaptations against an immune response.

“Deletion of Col1 homotrimers increases overall survival of mice with PDAC, associated with reprogramming of the tumor microbiome with increased microaerophilic Campylobacterales, which can be reversed with broad-spectrum antibiotics,” the investigators noted. “Our studies suggest that tumor-promoting Col1 homotrimers likely repel T cells and recruit MDSCs through the Col1 homotrimer-regulated cancer cell secretome,” the team stated. “Such immune modulation was associated with unique intratumoral microbiome. Our results provide insights into the connection between cancer cell-produced collagen homotrimer and its impact on the tumor-promoting microbiome.”

Further investigation clarified some of the mechanisms behind these findings, demonstrating that loss of the cancer-specific collagen increased levels of CXCL16, which attracts T cells, and reduced expression of CXCL5, which attracts MDSCs. Loss of the collagen homotrimer also increased the amount of normal fibroblast collagen in the stroma, which, as Kalluri’s lab previously showed, inhibits tumor progression. These results provide additional evidence that cancer-produced homotrimers affect signal pathways that can alter the tumor immune profile. “Our mechanistic studies demonstrate that cancer cells produce their own small amounts of a specific oncogenic Col1 homotrimer variant that facilitates cancer cell proliferation and survival,” the investigators noted. “Deletion of cancer cell-derived Col1 does not affect the total Col1 content of the PDAC stroma of established tumors but still has significant biological effect on PDAC progression.”

The study, in addition, found that the abnormal collagen upregulates signal pathways associated with cancer cell proliferation by binding to a surface protein called integrin α3. Indeed, suppressing integrin α3 in vivo increased T-cell infiltration and prolonged survival, highlighting this interaction as a very specific target for potential therapeutic strategies. “Our results also underscore the role of cancer cell-associated α3ß1 integrin in mediating Col1 homotrimer-induced signaling transduction.”

While the current study looked specifically at pancreatic cancer, Kalluri noted that collagen homotrimers also are seen in other cancer types, including lung and colon cancers, signifying a possible unifying principle with broad implications for cancer treatment. As the investigators pointed out, “There is no current evidence supporting the presence of Col1 homotrimers in normal tissue, and therefore its specific production by cancer cells constitutes a unique oncogenic variant with cancer-promoting properties, similar to oncogenic proteins generated by mutated oncogenes in cancer cells.”

“No other cell in the normal human body makes this unique collagen, so it offers tremendous potential for the development of highly specific therapies that may improve patient responses to treatment,” Kalluri sated. “On many levels, this is a fundamental discovery and a prime example of how basic science unravels important findings that could later benefit our patients.”

The authors further concluded, “In summary, our results establish a function for Col1 in PDAC progression with the identification of an oncogenic Col1 homotrimer variant that is specifically produced by cancer cells and that affects tumor immunity and microbiome, with implications for development of new therapeutic strategies.”