Heterogeneity and targeting of pancreatic cancer stem cells

Abstract

Cancer stem cells (CSC) have been identified in an ever-increasing number of human malignancies on the basis of their ability to recapitulate tumors in the ectopic setting and maintain long-term tumorigenic potential. In addition, in pancreatic adenocarcinoma, CSCs may display additional properties, such as relative drug resistance and enhanced invasive and migratory potential that implicate a role in disease pathogenesis spanning initial tumor formation to metastatic disease progression. Importantly, these findings also indicate that the development of novel therapeutic strategies capable of inhibiting or eliminating CSCs will improve clinical outcomes. Preclinical studies have already described a wide array of potential approaches that target CSC-specific surface antigens and cellular pathways involved in cell survival, adhesion, self-renewal, and differentiation. Further, progress in this area should continue to move forward as the unique biology of CSCs is better understood. All preclinical studies to date have focused on targeting specific and phenotypically defined CSCs, but multiple cell populations with the ability to form tumors and self-renew have been identified in pancreatic carcinoma. As the clinical efficacy of CSC-directed therapies will depend on the inhibition of all sources of tumor self-renewal, better understanding of how specific CSC populations are related to one another and whether each possesses specific functional properties will be critical. In this CCR Focus article, we discuss the potential relationships between different pancreatic CSC populations and strategies to identify novel targeting approaches.

Figure 1. Potential relationships between CSCs and mature tumor cells

(A) A linear organization with a single phenotypically distinct CSC giving rise to the other CSC populations and ultimately non-clonogenic mature cells. (B) Each phenotypic CSC represents a distinct state of the same clonogenic cell that gives rise to the mature tumor cell. (C) Each CSC population is unrelated to another and parallel lines of mature tumor cell production exist. (D) A plastic system in which non-clonogenic mature cells give rise to CSC displaying a variety of phenotypes.

Figure 2. Potential functional relationships between CSCs

(A) Distinct CSCs may give rise to macroscopic tumors in distinct anatomical locations. Each population is responsible for tumor growth and resistance to therapy in different organs. (B) A primary CSC population is responsible for tumor initiation and growth at the primary site. Additional populations are responsible for initiation and maintenance at metastatic sites and for resistance to chemotherapy.