CD44v6: A Key Protein in Cancer Growth and Metastasis

CD44v6 is a variant form of the CD44 cell-surface protein that acts as a co-receptor for growth signals, a migration promoter, and a marker of the most dangerous subpopulations of tumor cells. Its presence on cancer cells has been linked to worse survival, higher rates of metastasis, and resistance to chemotherapy across a range of tumor types. The protein is not itself an oncogene that causes cancer to start, but once a tumor exists, CD44v6 helps it do nearly everything that makes cancer lethal: invade surrounding tissue, seed distant organs, build its own blood supply, and survive treatment.

How CD44v6 Is Made

The CD44 gene contains 20 exons, of which the first five and the last five are “constant” and appear in every version of the protein. Sandwiched between them are ten “variant” exons, labeled V1 through V10, that can be mixed and matched through a process called alternative splicing. The simplest version of the protein, called CD44 standard, skips all ten variant exons. CD44v6 is any isoform that includes exon V6 in the final product.

The splicing machinery that decides whether V6 gets included is still not fully mapped out. Researchers have identified specific regulatory proteins that promote its inclusion. One such protein, SC35, binds to response elements within the V6 exon and its flanking sequences to encourage its incorporation into the finished molecule.¹ Another family of splicing regulators, the SR proteins, also influences V6 inclusion.² The practical consequence is that tumor cells can ramp up production of CD44v6 by altering the activity of these splicing regulators, and that splice-switching is one reason the protein appears in aggressive cancers but not in normal tissue nearby.

Where CD44v6 Appears in Normal Tissue

CD44v6 is not exclusive to cancer. In the healthy colon, for example, it shows up in the base of intestinal crypts, the tiny invaginations where stem cells live and divide to replenish the gut lining.³ That location is a clue: CD44v6 seems to be part of the toolkit stem cells use to maintain and regenerate tissue. The trouble starts when cancer cells co-opt that toolkit. In tumors, CD44v6 expression is no longer confined to a small stem-cell niche. It can spread across the tumor bulk and light up in the most aggressive cell populations, which is precisely what makes it useful as a marker and a target.

How CD44v6 Drives Tumor Progression

CD44v6 does not do one thing. It plugs into several different signaling networks that, together, make a tumor more invasive, better fed with blood, and harder to kill.

Boosting Growth Signals Through c-Met

One of the best-studied roles of CD44v6 is as a co-receptor for MET, a receptor that responds to hepatocyte growth factor (HGF). MET activation drives cell growth, survival, and movement. Research has shown that MET requires CD44v6 for its ligand-induced activation, meaning that without CD44v6 on the cell surface, MET signaling is impaired even when HGF is present.⁴ This partnership is especially relevant in colorectal and liver cancers, where MET signaling is frequently overactive.

Feeding Tumors Through Angiogenesis

Tumors cannot grow beyond a few millimeters without recruiting their own blood supply. CD44v6 assists in this process too. It acts as a co-receptor for VEGFR-2, the main receptor that drives new blood-vessel formation. Blocking CD44v6 in animal models impairs blood-vessel sprouting both in the lab and inside growing tumors, suggesting that the co-receptor function of CD44v6 for both c-Met and VEGFR-2 makes it a two-for-one target against tumor growth and its blood supply.⁵

Promoting Migration

Cancer cells need to move to metastasize, and CD44v6 helps them do it. In breast cancer cells, the bone-matrix protein osteopontin stimulates migration through interactions with CD44v6 on the cell surface. Blocking CD44v6 with function-blocking antibodies reduced that migration.⁶ CD44v6 has also been linked to the activity of matrix metalloproteinases (MMPs), enzymes that chew through surrounding tissue to clear a path for invading cells. In undifferentiated thyroid carcinoma, CD44v6 expression tracked alongside MMP-2 and MMP-9 levels, and knocking down the upstream regulator that controlled all three reduced the cells’ ability to invade.⁷

CD44v6 as a Cancer Stem Cell Marker

Cancer stem cells are the small subpopulation within a tumor that can self-renew, seed new tumors, and survive conventional chemotherapy. Across several cancer types, CD44v6 has emerged as one of the clearest surface markers for these cells. In colorectal cancer, all cancer stem cells that drive metastasis express CD44v6, and that expression is required for their migration and ability to generate secondary tumors.⁸

A similar picture holds in liver cancer. Hepatocellular carcinoma cells positive for CD44v6 show higher levels of self-renewal and tumor-growth ability, express elevated MET, and activate stemness-related transcription factors.⁹ In gastric cancer, CD44v6 enhances self-renewal, proliferation, migration, and resistance to the chemotherapy drug cisplatin. Knocking down CD44v6 suppressed all of those traits in laboratory and animal experiments.¹⁰

In pancreatic cancer, CD44v6-positive cells dominated among the tumor cells that survived, expanded, and traveled to distant organs in mice, making CD44v6 one of several biomarkers for the migrating cancer-initiating cells that seed metastases.¹¹

Prognostic Significance Across Cancer Types

If CD44v6 marks the most aggressive cells, it follows that tumors with high CD44v6 expression should have worse outcomes. Multiple meta-analyses confirm this.

In colorectal cancer, a pooled analysis of published studies found that CD44v6 overexpression was linked to lower five-year survival, more lymph-node involvement, more advanced disease staging, and a higher likelihood of distant metastasis.¹² In breast cancer, another meta-analysis found that CD44v6 expression correlated with higher tumor grade, lymph-node spread, and poorer overall survival.¹³ In osteosarcoma (bone cancer), a meta-analysis of eight studies with nearly 500 patients found that CD44v6-positive tumors were roughly 75 percent more likely to metastasize and had significantly poorer survival.¹⁴

In pancreatic cancer, new research has moved beyond tissue samples to look at tiny membrane-enclosed particles called extracellular vesicles that tumors shed into the bloodstream. Patients whose blood contained a high proportion of CD44v6-positive extracellular vesicles before surgery had shorter recurrence-free and overall survival, and were more likely to develop early liver metastases. This made CD44v6-positive vesicles an independent predictor of poor outcome.¹⁵ That finding is exciting because it suggests a possible blood-based test to identify the highest-risk patients before they even have surgery.

The Immune Evasion Connection

Beyond directly helping tumor cells grow and migrate, CD44 (including its v6 variant) may also help tumors evade the immune system. In papillary thyroid carcinoma, higher CD44 levels correlated with increased levels of immunosuppressive signals such as TGF-β and IL-10, as well as greater numbers of regulatory T cells (Tregs) in the tumor microenvironment.¹⁶ Tregs suppress anti-tumor immune responses, so a CD44-driven increase in their numbers could essentially give the tumor a cloak against immune attack. This connection is still being worked out, but it adds another layer to why CD44v6-high tumors are so difficult to control.

Therapeutic Approaches Targeting CD44v6

Because CD44v6 sits on the outside of the cell and marks the most dangerous tumor subpopulations, it has attracted attention as a therapeutic target. Several strategies have been tried, with mixed results so far.

Antibody-Drug Conjugates and the Bivatuzumab Lesson

The most prominent early attempt was bivatuzumab mertansine, an antibody-drug conjugate that linked an anti-CD44v6 antibody to a cell-killing toxin. The idea was straightforward: the antibody would home in on CD44v6-positive cells and deliver a lethal payload. In a phase I trial in patients with head and neck or esophageal squamous cell carcinoma, the drug was escalated through multiple dose levels. At the highest dose tested, one patient developed toxic epidermal necrolysis, a severe and life-threatening skin reaction, and died. The likely cause was that normal skin cells, particularly keratinocytes, also express CD44v6 at low levels. After a review of skin toxicity across four clinical trials involving 70 patients total, the developers concluded that the risk-benefit profile was unfavorable and discontinued the program.¹⁷ A parallel trial in metastatic breast cancer also found mild-to-moderate skin reactions in about three-quarters of patients, and the entire clinical program was halted after the fatal skin reaction in the head-and-neck trial.¹⁸

The bivatuzumab story illustrates a fundamental challenge: because CD44v6 is expressed at low levels in some healthy tissues, especially the skin, any therapy that kills all CD44v6-positive cells can cause collateral damage. Future antibody-based approaches will need to find ways to widen the gap between the dose that kills tumor cells and the dose that harms skin.

CAR T-Cell Therapy

A newer strategy is to engineer a patient’s own immune cells to recognize CD44v6. Researchers have created chimeric antigen receptor (CAR) T cells that specifically target the v6 region. In laboratory studies with primary head and neck squamous cell carcinoma cells, these CAR T cells showed high specificity and a direct correlation between the amount of CD44v6 on a target cell and the killing power of the T cell.¹⁹ Other groups have worked on optimizing the architecture of the CAR itself, tailoring a spacer region to improve both the T cells’ memory phenotype and their anti-tumor activity against blood and solid cancers in mice.²⁰ CAR T-cell therapy against CD44v6 has not yet produced large clinical-trial data, so it remains an experimental approach, but the specificity results are encouraging.

Peptide Inhibitors

Rather than killing CD44v6-expressing cells outright, another approach is to block CD44v6’s signaling function with small peptides. Two peptides identified in screening studies, known as NLN and NEW, were shown to trigger CD44v6 internalization (pulling it off the cell surface), block MET signaling, and reduce cell migration and invasion.²¹ The appeal of peptide-based approaches is that they may avoid the severe toxicity problems seen with antibody-drug conjugates by blocking function rather than killing cells wholesale.

CD44v6 and Drug Resistance

One of the most clinically relevant aspects of CD44v6 is its link to treatment resistance. In gastric cancer, CD44v6-positive cells show enhanced resistance to cisplatin, a widely used chemotherapy drug.¹⁰ In liver carcinoma, researchers found that inhibiting CD44v6 with peptides increased the effectiveness of both tyrosine kinase inhibitors and chemotherapy drugs in cell lines and patient-derived organoids.²² This raises the possibility of using CD44v6 blockade not as a standalone treatment but as something that sensitizes tumors to drugs they would otherwise shrug off. For patients whose cancers stop responding to first-line therapy, a combination approach that disables CD44v6 could reopen the door to treatment options that had seemingly failed.

The connection to plasticity adds further complexity. In colorectal cancer, CD44v6 and MET signaling appear to control a “molecular switch” that governs cell plasticity, the ability of non-stem cancer cells to revert back into stem-like cells. Blocking CD44v6 disrupted this reversion in tumor organoids, potentially cutting off a tumor’s ability to replenish its stem-cell pool after treatment wipes out the first wave.²³ If confirmed in clinical studies, this would mean that CD44v6 blockade does not just make chemotherapy more effective in the short term; it could prevent the tumor from rebuilding the very cells that drive relapse.

Imaging CD44v6 to Find and Track Tumors

Beyond therapy, CD44v6 is being explored as a target for molecular imaging. The goal is to inject a patient with a labeled molecule that binds CD44v6, then use PET or PET-CT scanning to light up tumors and metastases that express the protein. Several antibody-based imaging agents have shown promise in animal models. Small single-chain antibody fragments labeled with radioactive iodine-124 clearly visualized CD44v6-expressing tumors in mice.²⁴ A different antibody fragment format showed superior imaging properties compared to the standard PET sugar tracer, FDG, for CD44v6-high squamous cell carcinomas.²⁵

A more recent approach uses a two-step “pretargeting” strategy. First, a full-size anti-CD44v6 antibody is injected and given time to find the tumor. Then, a small radioactive molecule designed to click onto the antibody is injected separately. This reduces the amount of time the radioactive label circulates in the body, lowering radiation exposure to healthy organs. The approach successfully imaged head-and-neck squamous cell carcinoma tumors in mice using zirconium-89 PET.²⁶ If these imaging tools reach the clinic, they could serve two purposes: helping surgeons identify tumor margins before or during an operation, and helping oncologists pick patients whose tumors are CD44v6-positive and therefore might benefit from CD44v6-targeted therapy. That kind of patient-selection step would be particularly valuable given the bivatuzumab experience, where an inability to predict which patients had enough CD44v6 in their normal skin to be at risk contributed to the fatal toxicity.