Molecular Mechanisms of Stemness and Invasion in Pancreatic Cancer

Abstract

Pancreatic cancer remains a cancer with the lowest survival rate and shortest median survival. It is predicted to accelerate to the 2nd cause of cancer related death in the next 15 years. Poor survival rates stem from late disease detection, aggressive tumor biology, and unsuccessful treatments. These dismal statistics highlight the urgency for increased knowledge of the biology of pancreatic cancer and the need for effective therapies. Within the past decade, cancer stem cells, a population within the tumor, have been exposed for their role in tumor initiation, progression, chemoresistance, and metastasis. These studies focus to examine the role of CD133 positive cancer stem cells in stemness and metastasis within pancreatic cancer. We demonstrated that CD133 positive pancreatic cancer cells, from human pancreatic cancer cell lines and a spontaneous murine model of pancreatic cancer, are capable of tumor initiation at very low cell numbers in both immunocompromised and immunocompetent mice, respectively. These cells exhibited chemoresistance properties by the upregulation of drug transporters, active drug efflux capabilities, and resistance to cell death upon treatment with conventional chemotherapies, such as gemcitabine and 5-fluorouracil. CD133 positive cancer stem cell population was, however, sensitive to a novel therapy- Minnelide™. In both in vitro and in vivo studies, CD133 positive cells responded to Minnelide™ treatment and underwent apoptosis in addition to the CD133 negative population. These investigations led to questions regarding the function of this surface marker. CD133 played no functional role in conferring the cancer stem cell phenotype to this population. Through overexpression of CD133 in a pancreatic cancer cell line with very low endogenous CD133 expression, we determined that CD133 expression influences both stemness and invasiveness. Cells overexpressing CD133 were capable of initiating tumors at low cell numbers, as compared to control cells, and exhibited an upregulation in pluripotency and developmental signaling gene expression. Additionally, tumors derived from CD133 overexpressing cells demonstrated a marked increase in metastasis to several distant sites. This was shown to occur through the activation of NF-kB signaling and induction of the epithelial-mesenchymal transition; resulting in increased cellular invasiveness. Further, CD133 expressing cells displayed increased expression and secretion of the cytokine, interleukin-1 beta. Inhibition of IL-1 signaling through various methods established a significant role for IL-1 in the induction of the epithelial-mesenchymal transition and cellular invasiveness. Significantly, interleukin-1 beta positively correlates with CD133 gene expression in pancreatic cancer cell lines of varying aggressiveness. In cell lines with high levels of CD133 positive populations, inhibition of IL-1 signaling demonstrated its critical role in epithelial-mesenchymal transition induction and invasiveness. This exhibited that IL-1 signaling functions within CD133 positive populations during the metastasis process. Finally, these studies demonstrated the pivotal role of NF-kB activation in the induction of the epithelial-mesenchymal transition, cellular invasion, and metastasis. Using triptolide and NF-kB signaling inhibitor treatment, as well as, NF-kB signaling pathway modulation via constitutively active or inactive plasmid expression, this signaling pathway was decisively confirmed to mediate invasion and metastasis in pancreatic cancer. Taken together, this work establishes the functional role of the cancer stem cell marker, CD133; IL-1 signaling; and NF-kB activation in pancreatic cancer stemness and metastasis.