The Role of Plectin in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is an intractable clinical problem. It is the fourth leading cause of cancer deaths in the United States and the five-year survival rate of PDAC is only 6%. The prognosis remains poor due to its highly aggressive nature and resistance to extant therapies. Unlike other cancer types such as prostate, colon, and breast cancer, there have been no significant improvements in PDAC survival over the past 40 years despite a large number of clinical trials. This is because most patients are diagnosed with metastatic disease due to the lack of specific symptoms and the absence of suitable biomarkers for early detection. Because early detection and complete surgical resection offer the best hope for long term survival, there has been significant focus in recent years on the development of improved diagnostic methods.

Using phage display and functional proteomics, we previously demonstrated that the cytoskeletal linker protein plectin is a robust biomarker for PDAC. We showed that 100% of human PDAC specimens overexpressed plectin, whereas plectin was absent in normal pancreata. Because the phage display technique identifies cell surface antigens, we hypothesized that plectin, normally cytoplasmic, is aberrantly localized to the cell surface in PDAC cells. Aberrant protein localization has been associated with the pathogenesis of many human diseases. Given its abnormal localization and expression in PDAC, mislocalized plectin could have direct functional roles in the transition from pre-invasive to invasive disease and/or be of importance for the established primary and metastatic tumors. Although plectin has been studied extensively in normal physiology and other diseases such as epidermolysis bullosa, the biological role of plectin in pancreatic cancer has not been studied. Understanding the biology of aberrant localization of plectin to the cell surface is important because it provides insights into a functional role for this important biomarker of PDAC.

In this dissertation, we confirmed the abnormal cell surface localization of plectin and identified a mechanism for plectin deregulation in cancer. We demonstrated that plectin is secreted through exosomes (nanometer-sized membrane particles) in PDAC cell lines but not in non-transformed human pancreatic ductal epithelial cells. We also showed that integrin β4, a known binding partner of plectin, is found in the PDAC exosomes and necessary for plectin inclusion in the exosomes. Remarkably, plectin-positive exosomes could also be isolated from serum of tumor-bearing mice. Plectin-positive exosomes derived from PDAC were taken up by cells that were devoid of cell surface plectin and induced migration and invasion. Moreover, plectin-positive exosomes were able to enhance the growth tumors that were unable to produce exosomes. In vitro shRNA and overexpression studies as well as in vivo models collectively demonstrated a positive role of plectin for tumor growth, migration, and invasion. Taken together, this series of studies establishes a novel and critical role for plectin expression and mislocalization in PDAC progression.