• The EMT-activator Zeb1 is a key factor for cell plasticity and promotes metastasis in pancreatic cancer.
• Posted by 東京大学 医科学研究所 分子シグナル制御分野　Jane Weng
• 投稿日 2017/06/28

Metastasis is a critical indicator of tumor burden and a major cause of cancer-associated mortality. The activation of epithelial-to-mesenchymal transition (EMT) is a major driver of tumorigenesis and metastasis. However, the roles of EMT in inducing metastasis and the significance of EMT in tumorigenesis have been challenged. Two independent groups have reported that EMT is not required for metastasis but contributes to chemoresistance [1,2]. In particular, Zheng et al. have demonstrated that the EMT induced by EMT transcription factors (EMT-TFs), Snail and Twist, is dispensable in pancreatic cancer by using Pdx1-cre;LSL-KrasG12D;P53R172H/+ (KPC) mouse model.

In this study, in contrast to depletion of Snail and Twist in pancreatic cancer, Krebs et al. used the same KPC mouse model and depleted EMT-activator Zeb1 to generate KPC;Zeb1fl/fl (KPCZ) mice in order to investigate the roles of Zeb1 in metastasis.
(1) Zeb1 depletion reduces invasion and metastasis.
First, the authors showed that depletion of Zeb1 in KPC mice did not affect the onset and only insignificantly reduced the growth rate of primary tumors. However, Zeb1 depletion strongly reduced the differentiation, invasion and metastasis in pancreatic cancer (Figure 1).
(2) Zeb1 depletion fixes the tumor cells in epithelial state and reduces cell stemness, tumorigenic and colonization capacity.
Next, they isolated the primary tumor cells from control KPC and Zeb1-depleted KPCZ mice to further examine the effect of Zeb1 depletion. As expected, KPC primary tumors with strong heterogeneity displayed highly variable cell phenotypes from mesenchymal to mixed and epithelial. In contrast, the authors showed that all tumor lines derived from KPCZ mice were fixed in an epithelial state with strongly reduced mesenchymal gene expression (Figure 2). Furthermore, they also demonstrated that complete depletion of Zeb1 eradicated the lung colonization capacity after intravenous injection (Figure 3a-d). Cancer stemness is associated with not only tumorigenic and colonization capacity, therefore, the authors also examined the stemness of these tumor lines by sphere-formation assay and the expressions of stemness markers. As the results, they demonstrated that Zeb1 depletion reduced the stemness and expressions of stemness marker, Sox2 (Figure 3e-h).
(3) Zeb1 depletion reduces cellular plasticity.
Cell plasticity of cancer cells is an important driving force of malignant tumor progression, and allowing the cells to adapt to the changing tumor environment. Based on the findings above, next, Krebs et al. carried out a global gene-expression analysis to examine the impact of Zeb1 on phenotypic variability and cellular plasticity. Similarly, they showed that Zeb1-depleted KPCZ cells only displayed homogeneous epithelial state (Figure 5). In addition, Zeb1 depletion reduced cellular plasticity in response to TGFβ in global gene expression changes (Figure 6). Not only the plasticity in phenotypic variability, Zeb1 depletion also affected the metabolic plasticity as KPCZ cells displayed reduced OXPHOS and plasticity in switching between energy pathways (Figure 7).

Overall, Krebs et al. demonstrated a critical role of Zeb1 in pancreatic cancer from early initiation towards metastasis in contrast to other transcription factors, Snail and Twist. Depletion of Zeb1 not only affects the stemness and colonization capacity, but also alters the cellular plasticity (both phenotypic and metabolic), which is required for driving tumor progression. In contrast to Snail and Twist that were shown to be dispensable for metastasis in pancreatic cancer, here, the authors strongly argued that different EMT-TFs possibly have different subfunctions that are tissue specific.
Consequently, therapeutic strategies targeting these EMT transcription factors should consider the specificities under variable circumstances.

Comment:
In this study, Krebs et al. demonstrated the essential role of Zeb1 in the activation of EMT program, leading to tumor progression. In particular, the authors described how Zeb1 depletion affects the grading and invasiveness in pancreatic cancer. However, they also showed that KPCZ mice did not display any significant difference in the onset, tumor-free survival, and primary tumor growth rate. It is surprising Zeb1-depletion significantly reduced the invasiveness and malignancy of tumor without delaying the onset or improving the survival.
According to previous study done by Zheng et al., although both EMT induced by Snail and Twist is dispensable for metastasis, Snail- and Twist-mediated EMT induces chemoresistance in KPC pancreatic cancer mouse model. From the findings described in this study, it is curious whether Zeb1 contributes to the induction of chemoresistance in the same model.
Despite these unanswered questions, Krebs et al. have clearly described the importance of EMT induced by Zeb1 and its role in regulating cellular plasticity in pancreatic cancer. Combining these findings, the future therapeutic strategy targeting EMT is expected to consider cellular plasticity and tissue specificities in order to improve the effectiveness of therapies.