Mechanism: Understanding the role of cell plasticity in cancer

Adaptive cell fate decisions in response to altered environmental cues including treatment-imposed pressure are critical for tumors to adapt. Such plastic cellular behaviors drive tumor initiation, delamination of tumor cells from the primary site, and metastatic outgrowth. Furthermore, plasticity contributes to tumor heterogeneity, immune escape, and resistance to therapy.

Prrx1a and Prrx1b isoforms execute distinct functions in PDAC progression and metastasis

We have discovered the transcription factor Paired-related homeobox 1 (Prrx1) orchestrating cellular plasticity in numerous physiologic and pathophysiologic conditions in the pancreas, including PDAC (Reichert, et al., Genes & Development 2013). We identified novel roles for both Prrx1 isoforms, Prrx1a and Prrx1b, in the metastatic cascade using complementary in vitro and in vivo models (Takano, Reichert,…, Rustgi, Genes & Development 2016). Specifically, we found that Prrx1b promotes invasion, tumor dedifferentiation, and EMT. In contrast, Prrx1a stimulates metastatic out-growth, tumor differentiation, and MET. We further demonstrated that the switch from Prrx1b to Prrx1a governs EMT plasticity in both mouse models of PDAC and human PDAC. Lastly, we identify hepatocyte growth factor (HGF) as a novel transcriptional target of Prrx1b. Targeted therapy of HGF in combination with gemcitabine in a preclinical model of PDAC reduces primary tumor volume and eliminates metastatic disease. This preclinical trial prompted AVEO Pharmaceuticals to initiate a phase 1b clinical trial (NCT03316599).

Regulation of epithelial plasticity determines metastatic organotropism in pancreatic cancer

A central concept of epithelial plasticity is the ability of a cell to polarize as well as to establish and release cell-to-cell contacts. A crucial protein for the formation of cell-cell contacts is membranous E-cadherin. E-cadherin is stabilized at the cell membrane by p120catenin (p120ctn). Using multiple complementary mouse models, we demonstrated that liver and lung metastatic organotropism in pancreatic cancer is dependent on p120ctn-mediated epithelial identity (Reichert, …, Rustgi, Developmental Cell 2018). Mono-allelic p120ctn loss accelerates KrasG12D-driven PDAC formation and metastasis to the liver as well as being sufficient for E-cadherin-mediated cell adhesion. In contrast, cells with bi-allelic p120ctn loss demonstrate marked lung organotropism, although rescue with p120ctn restores liver metastasis. In a p120ctn-independent PDAC model, mosaic E-cadherin knockout shows selective pressure for E-cadherin-positive liver metastasis and E-cadherin-negative lung metastasis. Taken together, p120ctn-mediated epithelial plasticity may be added to the list of emerging concepts underlying metastatic organotropism. Importantly, these findings might explain why PDAC patients that solely suffer from lung metastases have a better prognosis than patients with liver metastases since the tumor cells are more plastic in the latter case.

Cancer-associated fibroblast plasticity regulated by Prrx1 drives aggressive pancreatic cancer biology

Importantly, these dynamic cell fate decisions are not restricted to tumor cells but are also present in cellular components of the tumor microenvironment (TME), specifically cancer-associated fibroblasts (CAFs). The latter are believed to exert tumor-supporting or tumor-restraining functions depending on their state of differentiation. Again, these distinct phenotypes are interconvertible increasing the complexity of PDAC. Interestingly, histological analysis of PRRX1 expression in murine and human PDAC revealed that PRRX1 expression is not limited to the tumor cell compartment, but is also found in the tumor stroma including CAFs. We found that high stromal expression levels of Prrx1 are associated with the squamous subtype, whereas low stromal Prrx1 expression is found in classical PDAC, thereby indicating a potential functional role of Prrx1 in CAFs. Consequently, we developed a conditional knock-out allele of Prrx1 in order to specifically ablate Prrx1 in fibroblasts, both in vitro and in vivo. Mechanistically, we describe that loss of Prrx1 mediates CAF activation, leads to increased amounts of ECM, improved tumor differentiation, fewer circulating tumor cells, and reduced metastasis. At the same time, Prrx1 expression in CAFs promotes EMT and chemotherapeutic resistance in tumor cells through paracrine HGF signaling. Interestingly, both in orthotopic implanted tumors as well as in an endogenous PDAC mouse model, CAFs lacking Prrx1 displayed a significant increase in CD3+, CD8+, and CD4+ T-cells, B-cells, as well as macrophages and dendritic cells in the TME, compared to controls. These data indicate that the Prrx1-proficient CAFs might shape an immunosuppressive TME and thereby trigger a more aggressive PDAC phenotype. Taken together, our results indicate that targeting the plasticity of fibroblasts has a significant impact on tumor biology. These data highlight the possibilities of new treatment strategies through the conversion of tumor-promoting CAFs into tumor-restraining CAFs (Feldmann,…, Reichert, Gastroenterology 2021).