Introduction

What is cholangiocarcinoma?

Cholangiocarcinoma (CCA), or bile duct cancer, is a rare, highly aggressive malignant tumour that develops from the epithelial cells of the bile ducts.¹ CCA accounts for approximately 3% of all gastrointestinal cancers. Although CCA is relatively uncommon, its incidence is increasing globally, especially in Southeast Asia, where liver fluke infections are common.²

Why is CCA difficult to detect in the early stages of infection?

In CCA, early detection is difficult because it often causes no or vague symptoms in its early stages. Since most patients cannot receive curative surgery, it is typically diagnosed at an advanced stage.³ For those patients, the standard treatment is combination chemotherapy with gemcitabine and cisplatin.⁴ However, recurrence is frequent, and overall survival rates remain quite low.⁵

A major reason for treatment failure is thought to be the persistence of cancer stem cells (CSCs), a small subset of tumour cells with unique survival advantages.^6,7

This article explores the role of CSCs in CCA progression and drug resistance, and highlights the latest treatment strategies that specifically target CSCs.

Understanding cholangiocarcinoma

CCA can be divided anatomically into three types based on its location within the bile ducts: intrahepatic (iCCA), perihilar (pCCA), and distal (dCCA).^2,3 

Risk factors in CCA

Risk factors of CCA include:^4,1,5

• Chronic inflammatory conditions, such as primary sclerosing cholangitis and hepatolithiasis
• Infections such as Opisthorchis viverrini or Clonorchis sinensis
• Hepatitis B and C
• Congenital bile duct abnormalities
• Alcohol consumption
• Smoking
• Diabetes
• Obesity
• A combination of genetic and environmental factors

Symptoms and diagnosis in CCA

Common symptoms include:

• Jaundice
• Abdominal pain
• Itching
• Fatigue
• Weight loss, which usually appears late

Additionally, imaging tests such as CT scans, along with biomarkers including CA 19-9, are used for diagnosing CCA. However, a tissue biopsy is required for a late-stage diagnosis due to the limited sensitivity and specificity of these techniques.⁸

Why does cancer often come back stronger?

For many CCA patients, even successful surgery or chemotherapy is only a temporary solution, with cancer often returning within two years. For decades, this continuous pattern has confused doctors and led scientists to search for the main cause. One of the understandable explanations is the CSC hypothesis.^9,7

What are cancer stem cells?

Cancer stem cells (CSC) are a rare group of tumour cells with properties similar to normal stem cells in our bodies. Their most significant abilities are self-renewal and the ability to differentiate into multiple cell types. 

Similarly to how normal stem cells maintain tissues, CSCs are essential to the development and maintenance of tumours. In CCA, CSCs have been identified using markers such as CD133, CD44, EpCAM, and ALDH1.^2,4,9,10 These markers are useful for isolating cells with stem-like behaviour, even though they are not specific to CSCs.^4,6 

In addition, even a small number of CSCs can regenerate a tumour, suggesting their potent tumour-initiating ability.^11,12 CSCs are not a fixed, unchanging group; instead, they exist in a dynamic balance with non-stem cancer cells.⁹ 

Environmental stresses, such as chemotherapy or low oxygen (hypoxia), promote reversion of non-CSC tumour cells into stem-like properties.^2,6,12 CSC plasticity presents a particularly challenging therapeutic target.^9,6,10

Where do CSCs come from?

The exact origin of CSCs is still unclear; however, researchers have proposed several hypotheses. According to one theory, hepatic progenitor cells (HPCs) (a type of stem cell) could develop into CSCs by undergoing malignant transformation.^6,12 Another theory is that mature cholangiocytes develop stem cell-like properties through genetic or epigenetic changes.^1,6,12

How do CSCs survive?

The tumour microenvironment (TME) has a major impact on the survival of CSCs. Signals from immune cells, extracellular matrix (ECM) proteins, cancer-associated fibroblasts (CAFs), and hypoxic environments assist CSCs in preserving their stem-like characteristics. For example, hypoxia-inducible factors (HIFs) are triggered by low-oxygen conditions in the tumour. They boost CSC survival, invasiveness, DNA repair, anti-apoptotic signalling, and stemness.^{2,3,4,6,7,11,12}

In addition, inflammatory cytokines, such as IL-6 and TGF-β, are known to support the maintenance of CSC phenotypes.^4,10

CSCs and tumour progression

CSCs are at the centre of tumour initiation and metastasis. They can produce both new CSCs through asymmetric cell division and differentiated tumour cells. These abilities promote tumour heterogeneity, a hallmark of aggressive cancers. In experimental models, even a small number of CSCs can recreate a tumour that closely resembles the patient’s original tumour in both structure and behavioural complexity.^2,4,9,11,12 

In addition, CSCs boost the tumour's capability to invade and spread. CSCs exhibit EMT-associated transcription factors, allowing cancer cells to detach from the original location and move to different tissues.^4,7 In addition, CSCs secrete angiogenic factors such as VEGF, which promote the formation of new blood vessels, ensuring the tumour has the blood supply it requires to proliferate and spread.^2,6,11

Mechanisms of treatment resistance

One of the most important features of CSCs is their resistance to treatment.^4,11 Recurrent tumours are often more aggressive and more drug-resistant than previous tumours. Unlike regular tumour cells, which are more differentiated and vulnerable to cytotoxic drugs, CSCs often divide slowly or remain in a quiescent state, making them less responsive to drugs that target actively dividing cells.^2,9,12

Additionally, in CCA, CSCs use multiple overlapping mechanisms to evade therapy:

• Efficient drug efflux: overexpressing ATP-binding cassette (ABC) transporters actively pumps anticancer drugs out of the cell, lowering the drug concentrations inside cells^4,9,7,13
• Detoxification enzymes: high expression of aldehyde dehydrogenases (ALDH) is a marker for CSCs. It metabolises and neutralises chemotherapy drugs, such as gemcitabine, thus contributing to drug resistance^2,9,6,13
• Enhanced DNA repair: CSCs activate strong DNA repair pathways to rapidly repair damage caused by radiation therapy or chemotherapy, preventing treatment-induced cell death^2,6
• Apoptosis resistance: increased activity of survival pathways, such as PI3K/Akt and Bcl-2 family proteins, blocks programmed cell death even under high stress^2,9,11 
• Protective TME and hypoxia: CSCs often exist in areas of the tumour with poor blood supply and low oxygen. Additionally, hypoxia provides additional protection, enabling CSCs to withstand therapy^2,11,12 
• Therapy-induced plasticity: CSC populations can be enhanced by the treatment itself. Studies have shown that chemotherapy can reprogram non-stem cancer cells into a stem-like state, effectively “recharging” the CSC pool after treatment^9,6,12,7

Because of these defences, standard treatments could reduce the tumour initially; however, CSCs can often survive, regenerate the tumour, and drive recurrence, sometimes with even greater drug resistance than before.^11,12 

New strategies to target cancer stem cells

As CSCs have been identified as a key cause of treatment failure, various targeted strategies are now under investigation.¹⁰ The main approaches include:

• Monoclonal antibodies and CAR-T cell therapy targeting surface antigens, such as CD133^2,9
• Inhibitors of key stem cell–maintenance pathways such as Notch, Wnt/β-catenin, and Hedgehog^2,12,7,13
• Epigenetic drugs to suppress stemness-related CSC gene expression^4,12
• CSC differentiation induction, increasing cells' chemosensitivity^12,7
• TME disruption via anti-angiogenic agents or immune modulation^12,7
• Immunotherapy combinations with CSC-targeted agents^2,7,10

Challenges of CSCs in CCA

There are still many challenges in developing treatments that target CSCs. One significant problem is that CSC markers are often expressed on normal stem cells, which means that treatments could damage healthy tissues.⁴

Additionally, due to the plasticity of cancer cells, CSCs can persist even after treatment has eliminated the tumour.^4,9,6

Future directions of CSCs in CCA

The proportion and characteristics of CSCs vary between patients, and even within the same tumour, significant heterogeneity exists; this highlights the need for personalised treatment plans. Current technologies, such as single-cell RNA sequencing, spatial transcriptomics, and organoid models, are helping researchers better understand CSC mechanisms and design more precise targeting strategies.^1,2,7,9

FAQs

If my tumour is removed, can I still have CSCs?

Yes. CSCs can remain in surrounding tissues or move through the blood, even if scans show no visible tumour.¹² Therefore, follow-up care and additional treatments are sometimes required after surgery.

Can a healthy diet or lifestyle changes eliminate CSCs?

No. While a healthy lifestyle supports your immune system and overall health, CSC survival is not solely controlled by diet. Medical treatments targeting CSC mechanisms are necessary to eliminate them.

Can CSCs metastasise to other parts of the body?

Yes. CSCs contribute to tumour metastasis by moving through the blood or lymphatic system and starting new tumours in other organs.^6,12

Can CSCs turn into normal cancer cells and vice versa?

Yes. CSCs can produce regular tumour cells, and under certain stresses, such as chemotherapy, some tumour cells can become CSC-like again. This “plasticity” is one reason CSC-targeted therapy is challenging.^6,7,9

Summary

Cholangiocarcinoma (CCA), or bile duct cancer, is a rare and aggressive form of cancer originating from the bile ducts. It makes up about 3% of gastrointestinal cancers, with rising cases globally, especially in Southeast Asia, due to liver fluke infections. Early detection is challenging, leading to late diagnoses and limited treatment options, often resulting in low survival rates. Standard treatment involves chemotherapy with gemcitabine and cisplatin, but cancer frequently recurs due to cancer stem cells (CSCs), which contribute to treatment failures. CCA can be categorised into intrahepatic, perihilar, and distal types. Risk factors include chronic inflammatory diseases, specific infections, hepatitis B and C, congenital issues, lifestyle choices like alcohol use and smoking, diabetes, obesity, and genetic-environmental factors. Common symptoms include jaundice, abdominal pain, itching, fatigue, and significant weight loss.

Imaging tests like CT scans and biomarkers such as CA 19-9 aid in diagnosis, but tissue biopsies are often essential for accurate results. CSCs are crucial in tumour growth, regeneration, and metastasis, possessing properties for self-renewal and differentiation. Factors in the tumour microenvironment support CSC survival and drug resistance. Despite ongoing treatment efforts, CSCs can evade therapies, leading to more aggressive recurrent tumours. Research is focusing on targeted treatments for CSCs, but challenges remain, including the risk of harming normal stem cells and CSC plasticity. Overall, personalised treatment plans are being developed to better engage with the varying characteristics of CSCs in CCA.