Pancreatic CSCs and microenvironment

Pancreatic ductal adenocarcinoma (PDAC) is the most common malignant neoplasm of the pancreas and presents with dismal prognosis. Key hallmarks of these tumors are chemo resistance and early metastasis, leaving few effective options for curative treatment. In part, treatment resistance might be spawned by tumor cell plasticity within pancreatic cancers, and specifically by therapy resistant pancreatic cancer stem cells. The concept of cancer stem cells (CSCs) postulates that tumors are heterogeneous and hierarchically organized, with CSCs at their apex, aggregating conspicuous abilities such as self-renewal, recapitulation of the primary tumor and its propagation, pronounced generalized therapy resistance, and metastasis formation. Thus, understanding the nascency and regulation of pancreatic CSCs might be critical to find more effective treatment options for PDAC. The operational definition of CSCs relies on their ability to initiate tumor growth in xenograft models and thus recapitulate original tumor architecture. The cell surface marker CD133, and more efficiently the combination of CD133 and CXCR4, seemed to enrich for such CSCs that also showed high metastatic potential [1]. Moreover, functional properties, such as aldehyde dehydrogenase (ALDH) activity, anchorageindependent growth in serum-depleted media or, more recently, intracellular autofluorescence [2], have proven to effectively enrich for tumor initiating CSCs. However, these biomarkers remain unreliable and, despite many research efforts, CSC regulation still is incompletely understood. In this context, current research focuses on the question if cancer stemness is a rigid predefined or a fluctuating state with susceptible “stemness-prone” cells that are able to switch between CSCs and non-CSCs due to tumor cell plasticity. Additionally, certain subtypes of CSCs might be more relevant for metastasis (M-CSCs) or confer chemo resistance (CR-CSCs), while both properties may overlap (Figure 1A and B) [3]. CSC properties may depend on certain signaling pathway activities and in this context, WNT signaling has been attributed a role in colorectal cancer [4]. Although canonical WNT signaling is generally not affected by driver mutations in PDAC, recent evidence suggests that it might be required for early tumorigenesis in PDAC [5]. Most likely, this (temporary) activation of WNT is ligand-receptor mediated and limited to physiological levels of WNT in contrast to WNT-driven tumorigenesis in colorectal cancer. Recently, we were able to show that contextual WNT signals do influence cancer stemness-associated traits in susceptible PDAC cells [6]. To investigate WNT activity on the single cell level, we used a model system of lentivirally transduced PDAC cells with WNT-reporter constructs and isolated WNTnegative and WNTpositive populations. The latter showed more of a mesenchymal phenotype, suggesting features of epithelial to mesenchymal transition (EMT). Moreover, WNTpositive cells were more tumorigenic, metastatic, and chemo resistant compared to their WNTnegative counterparts. R-Spondin 2 (RSPO2), a critical and novel enhancer of preexisting WNT signals that binds to receptors of the