Determining the Role of Discoidin Domain Receptors in the Pathogenesis of Pancreatic Ductal Adenocarcinoma

Abstract

Pancreatic ductal adenocarcinoma (PDA) is an aggressive and devastating malignancy characterized by a collagen-rich, fibroinflammatory stroma. In the presence of a persistent injury as seen in chronic pancreatitis, a risk factor for the development of PDA, or oncogenic KRAS, the pancreas undergoes an initial morphological event where healthy acinar cells transdifferentiate into a ductal-like phenotype in a process called acinar-ductal metaplasia (ADM). ADM can advance into pre-cancerous lesions with a concomitant increase in collagen deposition. Due to the lack of clinical symptoms, limitations in early diagnosis and the excess collagen production that creates a barrier for treatment options, most patients are diagnosed with metastatic PDA leaving them with a 10% 5-year survival rate. Therefore, understanding the initiating molecular events of PDA and the crosstalk between the collagen-dense ECM and tumor cells is essential in providing early diagnostic methods with the potential for treatment options of this deadly disease.

To understand the molecular interactions between tumor-derived epithelial cells and the collagen dense stroma, this project focuses on a set of receptor tyrosine kinases called Discoidin Domain Receptors, DDR1 and DDR2, that bind to fibrillar collagens. This facilitates cell proliferation, migration, adhesion, and extracellular matrix remodeling. DDR1 has been shown to be expressed in epithelial cells, while DDR2 is found in the mesenchymal compartment such as fibroblasts and connective tissue. The aim of this project was to define the roles of DDRs, independently of each other, in the pathogenesis of pancreatic disease where overproduction of collagen can serve as a natural reservoir for DDR activation. To determine the significance of DDR1 in a model of experimental pancreatitis, I utilized a DDR1-ablated mouse (DDR1-/-). DDR1-/- mice were subjected to cerulein, a cholecystokinin ortholog that induces acinar cell stress to mimic the events of ADM and increased fibrosis seen in human pancreatitis. DDR1-/- mice were also crossed into the established KrasG12D/+; Ptf1aCre/+ (KC) model of tumorigenesis and the KrasLSL-G12D/+; Trp53LSL-R172H/+; Ptf1aCre/+ (KPC) of metastasis. A common phenotype observed among all models in the absence of DDR1 was significant tissue atrophy, acinar cell dropout, and perturbation in proliferation, suggesting DDR1 is necessary for pancreatic tissue homeostasis and recovery following extended injury.

To study the role of DDR2 in PDA development, we used a conditional DDR2 knockout mouse (DDR2fl/fl) and bred them into the KC and KPC model. However, this only allows for DDR2 depletion within the pancreatic epithelium. To address the role of DDR2 in both the epithelial and mesenchymal compartments, we crossed DDR2fl/fl into the dual recombinase system using global B-actinCreERT2 to knockout DDR2 and the KrasFSF-G12D/+; Ptf1aFlpo/+ (KF) to induce tumorigenesis and KrasFSF-G12D/+; Trp53+/- Ptf1aFlpo/+ (KPF) for studies in stages, including metastases. The preliminary data gathered from these models show that depletion of DDR2 promotes tumorigenesis and decreases survival in the metastatic models, suggesting DDR2 is necessary in the tumor-stroma response during the progression of PDA.

Collectively, the role of DDRs in the context of fibrosis and cancer are complex. Further analysis is necessary to determine the role of the DDRs in an organ and cell specific manner. However, the results from these studies and my observations have helped define a prospective point of regulation between the tumor cells and the overactive collagen-dense microenvironment surrounding the pancreas throughout disease development.