A Multipotent PROX1+ Tumor Stem/Progenitor Cell Population Emerges during Intestinal Tumorigenesis and Mediates Radioresistance in Colorectal Cancer

Abstract Colorectal cancer progression is associated with an increase in PROX1+ tumor cells, which exhibit features of colorectal cancer stem cells and contribute to metastasis. Here, we aimed to provide a better understanding of the function of PROX1+ cells in colorectal cancer, investigating their progeny and their role in therapy resistance. PROX1+ cells in intestinal adenomas of ApcMin/+ mice expressed intestinal epithelial and colorectal cancer stem cell markers, and cells with high PROX1 expression could both self-renew tumor stem/progenitor cells and contribute to differentiated tumor cells. Most Prox1 lineage–traced tumor cells were stem/progenitor cells, which can supply cells to multiple intestinal tumor cell lineages, whereas most lineage-traced Lgr5+ tumor cells were enterocytes, indicating that Prox1+ and Lgr5+ tumor stem cells have distinct differentiation programs. Although the PROX1+ tumor cells proliferated slower than PROX1− cells, irradiation increased the proportion of PROX1+ cells in human colorectal cancer cell lines, patient-derived organoids, and tumor xenografts. Furthermore, transcripts related to DNA damage repair (DDR) were enriched in PROX1+ versus PROX1− cells in adenomas and in colorectal cancer tumor cells from patients. Experiments with PROX1 silencing and overexpression indicated that PROX1 expression enhances colorectal cancer cell colony formation following irradiation. PROX1 interacted with DDR proteins, including components of nonhomologous end joining (NHEJ) and base excision repair, and inhibition of NHEJ repair led to a decreased proportion of PROX1+ cells following irradiation. In conclusion, PROX1+ cells are irradiation-resistant tumor stem/progenitor cells capable of self-renewal and differentiation. DDR inhibitors could represent a strategy to target the treatment-resistant PROX1+ tumor stem cells. Significance: Colorectal cancer stem/progenitor cells expressing the PROX1 transcription factor display increased radiation resistance, which may be targeted by small-molecule inhibitors of the nonhomologous end-joining DNA damage repair pathway.