The Hidden Architects of Colorectal Cancer: How Fibroblasts Drive Treatment Failure

Deep inside every colorectal tumor, beyond the rapidly dividing cancer cells that dominate textbooks and treatment plans, sits a population of cellular accomplices that researchers are only now learning to interrogate. These are cancer-associated fibroblasts, or CAFs, the most abundant cells in the stroma that surrounds and infiltrates solid tumors. A new review published in Cancer Cell International by a team at the Brazilian National Cancer Institute in Rio de Janeiro synthesizes the most recent evidence on how these cells shape colorectal cancer progression and, crucially, how they help tumors shrug off chemotherapy and radiotherapy. The message emerging from the literature is uncomfortable but clarifying: targeting the cancer cells alone has never been the whole story, and in colorectal cancer the stroma may be one of the main reasons so many patients eventually relapse.

Fibroblasts are, in healthy tissue, the quiet maintenance workers of the body. They secrete structural proteins, maintain the extracellular matrix, and coordinate wound healing. In the tumor microenvironment, however, they become reprogrammed. Signals from malignant epithelial cells, immune cells, and inflammatory mediators push resident fibroblasts into an activated state reminiscent of the myofibroblasts that close a healing wound, but in cancer this activation never switches off. The result is a chronic, tumor-supportive state in which fibroblasts proliferate, deposit matrix, secrete growth factors, and communicate with nearly every other cell type in the tumor. The review’s authors emphasize that CAFs are instrumental in tumor progression precisely because of these multifaceted interactions with tumor cells, other stromal components, and the extracellular matrix.

One of the most important conceptual shifts in recent years is the recognition that CAFs are not a uniform population. The review details how single-cell transcriptomics and lineage-tracing studies have revealed a bewildering heterogeneity of fibroblast states within colorectal tumors. Myofibroblastic CAFs, characterized by alpha-smooth muscle actin expression and contractile properties, tend to sit adjacent to invasive fronts and deposit dense matrix. Inflammatory CAFs secrete cytokines such as interleukin-6 and interleukin-8 that fuel pro-tumor signaling and immune evasion. Antigen-presenting CAFs express major histocompatibility complex molecules and can modulate immune surveillance, while other subtypes with lipogenic or developmental programs have been described in various contexts. Markers such as FAP, fibroblast activation protein; PDGFR-alpha and PDGFR-beta; FSP1; and TAGLN are used to distinguish these populations, though no single marker cleanly defines a functional state, a limitation the authors highlight as a persistent obstacle to both research and therapy.

Where these CAFs come from matters as much as what they become. The review surveys evidence that colorectal CAFs can arise from resident tissue fibroblasts, from stellate-like cells, from mesenchymal stem cells recruited to the tumor, from epithelial cells undergoing epithelial-to-mesenchymal transition, and even from endothelial cells transitioning through endothelial-to-mesenchymal processes. Each origin may imprint distinct behaviors and sensitivities on the resulting fibroblast, which helps explain why CAFs in different patients, or even in different regions of the same tumor, can behave so differently. This plasticity also means that therapies aimed at eliminating CAFs must contend with the possibility that new CAFs will be recruited or converted from untapped precursor pools, a phenomenon that has undermined several otherwise promising stromal-targeting strategies in other cancers.

A central theme of the review is extracellular matrix remodeling. CAFs in colorectal cancer secrete collagen I, fibronectin, laminins, and tenascin C, and they cross-link and stiffen this matrix through enzymes such as lysyl oxidase and matrix metalloproteinases. The consequences are twofold. Mechanically, a stiffened, dense matrix raises interstitial fluid pressure and compresses blood vessels, creating a physical barrier that prevents chemotherapeutic drugs from penetrating the tumor at adequate concentrations. Biologically, matrix stiffness itself is a signaling input: integrin-mediated mechanotransduction in both cancer cells and fibroblasts activates pro-survival pathways, promotes invasion, and reinforces the activated state of the CAFs themselves. In colorectal cancer, this stromal barricade has been repeatedly linked to poor prognosis and to the desmoplastic phenotype that pathologists recognize as abundant pale stroma surrounding malignant glands.

The mechanistic core of the review concerns how CAFs drive therapeutic resistance through defined signaling pathways. The TGF-beta pathway stands out as a master regulator: CAFs both produce and respond to TGF-beta, which maintains their activated state, promotes epithelial-to-mesenchymal transition in neighboring cancer cells, and suppresses anti-tumor immune activity. The Wnt and beta-catenin axis, so central to colorectal carcinogenesis through APC loss, is further amplified by stromal Wnt ligands secreted by CAFs, sustaining stem-like properties in cancer cells that are associated with recurrence after treatment. The Hedgehog pathway, Notch signaling, the PI3K-AKT-mTOR cascade, and JAK-STAT signaling downstream of interleukin-6 all feature in the resistance mechanisms catalogued by the authors. Through these circuits, CAFs can shield cancer cells from apoptosis induced by 5-fluorouracil, oxaliplatin, and irinotecan, the backbone drugs of colorectal cancer chemotherapy, and can blunt the effectiveness of radiotherapy by buffering oxidative stress and enhancing DNA damage repair in adjacent tumor cells.

Resistance is not only biochemical but also metabolic and immunological. The review describes how CAFs rewire their metabolism toward aerobic glycolysis, exporting lactate and other metabolites that cancer cells can take up and use, a metabolic symbiosis that sustains tumor growth under the nutrient- and oxygen-poor conditions created by the abnormal stromal vasculature. At the same time, CAFs sculpt the immune landscape by secreting chemokines that recruit immunosuppressive myeloid cells and regulatory T cells, by expressing checkpoint ligands such as PD-L1 and PD-L2, and by physically excluding cytotoxic T lymphocytes from tumor nests through their dense matrix. This has direct implications for immunotherapy: colorectal cancers with high stromal content, particularly the microsatellite-stable majority of cases, respond poorly to immune checkpoint inhibitors, and CAF-mediated immune exclusion is increasingly viewed as a key contributor to that failure.

What can be done about it? The review surveys preclinical agents and clinical trials aimed at disrupting the CAF compartment in colorectal cancer. Strategies fall into several broad categories: depleting or inhibiting CAFs directly, for example with FAP-targeted approaches including antibodies, CAR-T cells, and radioligands; blocking CAF-activating signals such as TGF-beta, with small-molecule kinase inhibitors and ligand traps, some of which have reached clinical testing in gastrointestinal cancers; disrupting matrix deposition and cross-linking, notably with lysyl oxidase inhibitors; and reprogramming CAFs from a tumor-promoting to a quiescent or even tumor-inhibiting state, an approach that avoids the toxicity of wholesale depletion. Vitamin D receptor agonists and retinoids have shown stromal-reprogramming effects in preclinical models of pancreatic and colorectal cancer, and the authors discuss how such agents might restore the differentiated, matrix-suppressive phenotype of quiescent fibroblasts.

The authors are careful to note the hard lessons from the field. In pancreatic cancer, the depletion of alpha-smooth muscle actin-positive myofibroblasts in mouse models once accelerated tumor progression, a result that shocked the community and underscored that some CAF subsets may restrain, rather than promote, tumor growth. The emerging consensus, reflected throughout this review, is that the goal should not be to eliminate all fibroblasts but to selectively target the pro-tumor subsets while preserving or enhancing protective ones. This demands better markers, a deeper understanding of CAF ontogeny, and combination strategies that pair stromal targeting with chemotherapy, radiotherapy, or immunotherapy so that dismantling the stromal shield sensitizes tumors to existing treatments rather than being used alone.

For patients with colorectal cancer, particularly the substantial fraction who develop resistance to standard chemotherapy and radiotherapy, the stakes of this research are high. Colorectal cancer remains one of the most common and lethal malignancies worldwide, and the review’s authors argue that the advances they summarize, from CAF subtype characterization to stromal-targeted therapeutics, could contribute to improving survival in tumors that currently defy classic therapies. The tumor microenvironment has moved from the margins of cancer biology to its center, and the fibroblast, once dismissed as passive scaffolding, is now understood as an active engineer of malignancy. Learning to outmaneuver these cellular architects, the review concludes, may be one of the decisive steps toward finally breaking through the wall of therapeutic resistance in colorectal cancer.

Subject of Research: The role of cancer-associated fibroblasts in therapeutic resistance and tumor progression in colorectal cancer

Article Title: Cancer-associated fibroblasts as key mediators of therapeutic resistance and tumor progression in colorectal cancer

Article References: de Lima Coelho, B. M., Murad, L. B., Costa, R. M. B., Castillo-Medina, Y. K., Lopes-Rios, L. L., & Morgado-Diaz, J. A. (2026). Cancer-associated fibroblasts as key mediators of therapeutic resistance and tumor progression in colorectal cancer. Cancer Cell International. https://doi.org/10.1186/s12935-026-04474-1

Image Credits: AI Generated

DOI: 10.1186/s12935-026-04474-1

Keywords: cancer-associated fibroblasts, colorectal cancer, tumor microenvironment, therapeutic resistance, extracellular matrix, TGF-beta, epithelial-mesenchymal transition, chemotherapy, radiotherapy, immunotherapy, CAF subtypes, tumor stroma