Abstract
Cancer chemoresistance is often attributed to the presence of cancer stem cell (CSC)-like cells, but whether they are homogeneously chemoresistant remains unclear. We previously showed that in colon tumors, a subpopulation of LGR5+ CSC-like cells driven by TCF1 (TCF7), a Wnt-responsive transcription factor, were responsible for tumorigenicity. Here we demonstrate that the tumorigenic subpopulation of mouse LGR5+ cells exists in a slow-cycling state and identify a unique 22-gene signature that characterizes these slow-cycling CSC. Seven of the signature genes are specifically expressed in slow-cycling LGR5+ cells from xenografted human colon tumors and are upregulated in colon cancer clinical specimens. Among these seven, four genes (APCDD1, NOTUM, PROX1, and SP5) are known to be direct Wnt target genes, and PROX1 was expressed in the invasive fronts of colon tumors. PROX1 was activated by TCF1 to induce CDKN1C and maintain a slow-cycling state in colon cancer organoids. Strikingly, PROX1 was required for recurrent growth after chemotherapeutic treatment, suggesting that inhibition of slow-cycling CSC by targeting the TCF1-PROX1-CDKN1C pathway is an effective strategy to combat refractory colon cancer in combination with conventional chemotherapy. SIGNIFICANCE: These findings illustrate the importance of a slow-cycling CSC subpopulation in colon cancer development and chemoresistance, with potential implications for the identified slow-cycling CSC signatures and the TCF1-PROX1-CDKN1C pathway as therapeutic targets.
Highlights
The presence of cancer stem cells (CSC) is frequently associated with malignant properties of refractory cancers, such as chemoresistance or metastasis [1]
In order to investigate the biological features of the tumorigenic Lgr5þ cells that we identified in our previous study [8], tumor epithelial cells that were obtained from Apcmin/þ mice–derived colon tumors and expressed detectable levels of Lgr5 (Lgr5þve cells) were subjected to single-cell RNA-seq (Supplementary Fig. S1A)
In accordance, when we searched for publicly available data from The Cancer Genome Atlas (TCGA) for the signature genes of human slow-cycling CSCs, we found that PROX1 as well as all the other signature genes was upregulated at the early stage of colorectal cancers (Fig. 4C)
Summary
The presence of cancer stem cells (CSC) is frequently associated with malignant properties of refractory cancers, such as chemoresistance or metastasis [1]. LGR5, initially discovered as a Wnt target gene that marks stem cells of the normal intestine [2], behaves as a stem cell marker for many digestive organs, including esophagus, stomach, colon, liver, and pancreas [3]. LGR5 serves as the most established functional marker of colon CSCs, and LGR5-positive cells. Initial reports indicated that Lgr5-positve cells behave as colon CSCs, recent reports suggest the more heterogeneous and dynamic nature of colon CSC-like cells. It is reported that LGR5-negative cells are capable of tumor formation [5,6,7], suggesting that LGR5-negative cells are capable of undergoing dedifferentiation back to LGR5-positive CSC-like cells. The same reports indicated that LGR5-positive CSCs are indispensable for outgrowth of metastatic foci [5, 7], indicating that LGR5-positve cells behave as indispensable CSCs under certain conditions. It is likely that essentiality of LGR5-positive cells as CSC-like cells is context dependent and affected by environmental cues
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