Colorectal cancer (CRC) remains one of the most common and deadly cancers worldwide. A major challenge in treating CRC is that cancer cells can develop resistance to standard chemotherapy drugs, most notably 5-fluorouracil (5-FU), the backbone of many CRC treatment regimens.
A protein called Src kinase plays a central role in promoting cancer cell migration, invasion, and growth. When Src is overly active, it drives cancer progression and has been linked to poor patient outcomes. This makes Src an attractive target for drugs called Src kinase inhibitors.
Saracatinib is one such Src kinase inhibitor. It works by blocking Src's activity, which in turn disrupts several signaling pathways that cancer cells use to survive and spread. These pathways include the MAPK pathway (which controls cell growth and division) and the EGFR pathway (triggered by growth signals from outside the cell).
This study used two CRC cell lines to investigate how saracatinib behaves under different conditions: standard cells (SNU-C5) and cells that had already acquired resistance to 5-FU (SNU-C5/5-FUR). The goal was to understand saracatinib's anti-cancer effects and whether a stem cell marker called Sox2 could limit those effects.
Within tumors, a subpopulation of cells called cancer stem cells (CSCs) are thought to drive tumor regrowth, metastasis, and resistance to treatment. These cells are identified by markers including CD44, Oct4, Sox2, cMyc, Klf4, and Nanog.
A process called epithelial-mesenchymal transition (EMT) is believed to convert normal cancer cells into CSC-like cells, giving them the ability to move, invade other tissues, and resist therapy. Researchers use a method called spheroid culture, where cells grow in 3D clusters without attachment, to study CSC behavior in conditions that more closely resemble a real tumor.
Sox2 is a transcription factor normally involved in cell identity and development. In cancer, elevated Sox2 levels are associated with stemness, drug efflux (pumping drugs out of cells), and resistance to treatment. Understanding how Sox2 interacts with Src kinase inhibitors is clinically important.
Earlier research showed that cells acquiring 5-FU resistance have enhanced drug efflux through a pump called ABCG2, and that their ERK signaling is lower than in sensitive cells. This prior knowledge set the stage for exploring whether saracatinib could bypass 5-FU resistance in CRC.
Researchers used two human CRC cell lines: the standard SNU-C5 line and the 5-FU-resistant variant SNU-C5/5-FUR. Cells were grown both in flat 2D monolayers (conventional laboratory dishes) and in 3D spheroid cultures (floating clusters mimicking tumor tissue), with conditions supplemented by either fetal bovine serum (FBS) or specific growth factors (GF).
Cell viability after saracatinib treatment was measured using the MTT assay, which gauges how many cells are alive. Cell death patterns (specifically whether cells died by programmed death called apoptosis or by necrosis) were analyzed using flow cytometry, a technique that identifies and counts cells based on their surface markers.
Cancer cell migration, a key step in metastasis, was assessed using the wound healing (scratch) assay. A scratch is made in a layer of cells, and researchers measure how quickly cancer cells fill in the gap. Saracatinib's ability to slow this process was measured over time.
Protein expression levels were measured using Western blotting, which allows researchers to detect specific proteins in cell samples. This was used to track changes in Src-related signaling proteins (p38, ERK, EGFR, Akt) and stem cell markers (Sox2, CD44, Oct4, Klf4, Nanog, cMyc, pan-Ras) following saracatinib treatment.
Saracatinib significantly reduced the viability of both CRC cell lines, although higher doses were needed to achieve the same effect in the 5-FU-resistant cells. At 10 micromolar concentration, standard SNU-C5 cells were reduced to about 55% viability, while resistant cells remained at about 79% viability, confirming that resistance affects drug potency.
Flow cytometry revealed that saracatinib dramatically increased cell death through apoptosis (programmed cell death). In standard cells, the proportion of apoptotic cells rose from 6.3% to 22.8%, and in resistant cells from 4.7% to 21.9%. This suggests saracatinib effectively triggers cell death regardless of 5-FU resistance status.
The scratch assay showed that saracatinib substantially blocked cell migration. While untreated cancer cells rapidly filled the scratch, saracatinib-treated cells maintained most of the gap, indicating reduced metastatic potential. This effect was seen in both the standard and resistant cell lines.
Saracatinib also reduced spheroid (3D cluster) formation significantly in both cell types when cells were grown in FBS-supplemented conditions. However, the drug was less effective at reducing spheroid formation in growth factor (GF)-supplemented conditions, particularly in the standard SNU-C5 cells, hinting at how the tumor microenvironment can influence drug response.
One of the most striking findings was that while saracatinib suppressed most cancer stem cell markers, it paradoxically increased the expression of Sox2 in both CRC cell lines grown in flat 2D conditions. All other stem cell markers (CD44, Oct4, cMyc, Klf4) were reduced by saracatinib, making Sox2 the outlier.
When cells with elevated Sox2 were re-treated with saracatinib and cultured in growth factor-supplemented conditions, they formed significantly larger spheroids than wild-type cells that had not undergone Sox2 upregulation. This was especially pronounced in the 5-FU-resistant cell line, suggesting that Sox2 amplifies cancer stem cell behavior when growth factors are present.
The larger spheroids formed by Sox2-upregulated cells indicate that these cells may have enhanced ability to survive, expand, and potentially resist treatment when the tumor microenvironment is rich in growth factors. This mirrors conditions that can occur in real tumors where growth factors are abundant.
Protein analysis showed that saracatinib reduced the activation of key signaling pathways: p38 and ERK were both suppressed in both 2D and 3D culture conditions. EGFR protein levels were reduced under both conditions as well, and Akt (a cell survival protein) was suppressed in 2D but not in 3D cultures, suggesting that 3D environments confer additional protection to cancer cells.
The paradoxical increase in Sox2 after saracatinib treatment may be explained by the relationship between Src signaling and Sox2 regulation. Normally, Src activates the Src-EGFR-Akt signaling cascade, which can promote Sox2 expression. When saracatinib blocks Src and EGFR, it disrupts this cascade, but Sox2 may be maintained or even upregulated through alternative mechanisms such as reduced p38 activation.
Research in other cancers has shown that when p38 is inactivated, Sox2 can become more active. Specifically, p38-mediated modification of Sox2 (phosphorylation at a specific site) has been linked to resistance to targeted therapies by increasing expression of ABCG2, a drug efflux pump. This is particularly relevant for the 5-FU-resistant cells, which already have elevated ABCG2 activity.
The study also found that growth factor-rich microenvironments amplify the effects of Sox2 upregulation. In real tumors, growth factors like EGF and FGF are commonly present in the tumor microenvironment. This means the Sox2-mediated resistance mechanism observed in the laboratory could be clinically relevant.
These findings suggest that using saracatinib alone may not be sufficient in all cases. Combination strategies, such as pairing saracatinib with p38 inhibitors or targeting Sox2 directly, may be needed to fully overcome drug resistance in CRC patients whose tumors are enriched in growth factors or cancer stem cell populations.
This study confirms that saracatinib has genuine anti-cancer activity in CRC cells, including those already resistant to standard 5-FU chemotherapy. It achieves this by inducing apoptosis, inhibiting cell migration, suppressing spheroid growth, and blocking key signaling pathways including MAPK and EGFR.
However, the paradoxical upregulation of Sox2 following saracatinib treatment is a significant concern. When growth factors are present, Sox2-upregulated cells form larger spheroids than untreated cells, suggesting that saracatinib might inadvertently enhance certain cancer stem cell properties under specific microenvironmental conditions.
For CRC patients, this research highlights that the tumor microenvironment matters. The same drug can have different outcomes depending on what growth factors or nutrients are available to cancer cells in the tumor. This is an important consideration when designing clinical trials and treatment protocols.
Future research should explore combination therapies that include saracatinib alongside p38 inhibitors or approaches that directly target Sox2 activity. The development of Sox2-targeting agents, though challenging because Sox2 is a transcription factor rather than an enzyme, represents a promising avenue for preventing drug resistance and recurrence in CRC.