Cell cycle heterogeneity and plasticity of colorectal cancer stem cells

Abstract

Cancer stem cells (CSCs) are a long-lived and self-renewing cancer cell population that drives tumor propagation and maintains cancer heterogeneity. They are also implicated in the therapeutic resistance of various types of cancer. Recent studies of CSCs in colorectal cancer (CRC) have uncovered fundamental paradigms that have increased understanding of CSC systems in solid tumors. Colorectal CSCs share multiple biological properties with normal intestinal stem cells (ISCs), including expression of the stem cell marker Lgr5. New evidence suggests that colorectal CSCs manifest substantial heterogeneity, as exemplified by the existence of both actively cycling Lgr5⁺ CSCs as well as quiescent Lgr5⁺ CSCs that are resistant to conventional anticancer therapies. The classical view of a rigid cell hierarchy and irreversible cell differentiation trajectory in normal and neoplastic tissues is now challenged by the finding that differentiated cells have the capacity to revert to stem cells through dynamic physiological reprogramming events. Such plasticity of CSC systems likely underlies both carcinogenesis and therapeutic resistance in CRC. Further characterization of the mechanisms underpinning the heterogeneity and plasticity of CSCs should inform future development of eradicative therapeutic strategies for CRC.

Keywords: cancer stem cell; colorectal cancer; intestinal stem cell; plasticity; quiescence.

FIGURE 1

Cell cycle heterogeneity among colorectal cancer (CRC) stem cell populations. (A) Normal intestinal stem cell (ISC) populations. Lgr5⁺ crypt base columnar cells (CBCs) reside at the crypt base and divide rapidly to drive homeostatic epithelial turnover. The existence of quiescent +4 ISCs has also been indicated by analysis of several markers. (B) Cell cycle heterogeneity of Lgr5⁺ CSCs that underlies therapeutic resistance of CRC. Quiescent CSCs expressing markers such as Mex3a, p27, and p57 persist during conventional anticancer therapies, and their subsequent activation initiates post‐therapeutic tumor regrowth. (C) Our scRNA‐seq analysis of mouse CRC. Cell cycle analysis shows that Lgr5⁺ CSCs are divided into actively cycling (cluster 2) and quiescent (cluster 4) populations.

FIGURE 2

Mechanisms underlying the quiescence and therapy resistance of colorectal cancer (CRC) stem cells. (A) The cyclin‐dependent kinase (CDK) inhibitors p21, p27, and p57 directly inhibit the activity of cyclin–CDK complexes and maintain cells in a quiescent state. All of these CDK inhibitors are thought to be particularly important to inhibit the G₁ cyclin–CDK complexes such as Cyclin C–CDK3, Cyclin D–CDK4/6 or Cyclin E–CDK2. (B) Mechanism for regulation of dormancy and reactivation of quiescent Lgr5⁺ CSCs by the cell–matrix interface. In the steady state, COL17A1, which anchors cells to the basement membrane, mediates constitutive inhibition of FAK activity and thereby maintains p27 expression in CSCs. Chemotherapy disrupts this cell–matrix interface and activates a FAK–YAP–TEAD signaling axis that breaks the dormancy of cancer step cell (CSC) and thereby promotes post‐therapeutic tumor regrowth. RTK, receptor tyrosine kinase. (C) Our scRNA‐seq analysis of mouse CRC revealed that the p57 gene is specifically expressed in the quiescent subpopulation of Lgr5⁺ CSCs. This expression has been shown by others to be upregulated by a Wnt‐TCF1‐PROX1 axis in Lgr5⁺ CSCs.

FIGURE 3

Plasticity of stem cell systems in the normal intestine and colorectal cancer (CRC). (A) p57⁺ enteroendocrine cells in normal intestinal crypts do not manifest stem cell activity in the steady state. However, they dedifferentiate after injury and acquire stem cell potential to drive postinjury epithelial regeneration. (B and C) Plasticity promotes therapy resistance and metastasis of CRC. Lgr5⁻ cells that persist after anticancer therapies (B) or that metastasize (C) replenish or generate Lgr5⁺ cancer step cells (CSCs) through dedifferentiation and thereby initiate new tumor growth.

FIGURE 4

Mechanisms underlying the plasticity of intestinal stem cell (ISC) and cancer step cell (CSC) systems. Fetal‐like as well as metaplasia‐like transcriptomic changes underlie injury‐induced dedifferentiation and RSC induction in the normal intestinal epithelium. Such regenerative programs are also activated in CRC. The balance between CBC‐like and RSC‐like CSCs is dynamically altered depending on various selective pressures such as anticancer therapies, mutations, and signaling pathways.