Cancer stem cells: the lessons from pre-cancerous stem cells

Abstract

How a cancer is initiated and established remains elusive despite all the advances in decades of cancer research. Recently the cancer stem cell (CSC) hypothesis has been revived, challenging the long-standing model of "clonal evolution" for cancer development and implicating the dawning of a potential cure for cancer [1]. The recent identification of precancerous stem cells (pCSCs) in cancer, an early stage of CSC development, however, implicates that the "clonal evolution" is not contradictory to the CSC hypothesis, but is rather an aspect of the process of CSC development [2]. The discovery of pCSC has revealed and will continue to reveal the volatile properties of CSC with respects to their phenotype, differentiation and tumorigenic capacity during initiation and progression. Both pCSC and CSC might also serve as precursors of tumor stromal components such as tumor vasculogenic stem/progenitor cells (TVPCs). Thus, the CSC hypothesis covers the developing process of tumor-initiating cells (TIC) --> pCSC --> CSC --> cancer, a cellular process that should parallel the histological process of hyperplasia/metaplasia (TIC) --> precancerous lesions (pCSC) --> malignant lesions (CSC --> cancer). The embryonic stem (ES) cell and germline stem (GS) cell genes are subverted in pCSCs. Especially the GS cell protein piwil2 may play an important role during the development of TIC --> pCSC --> CSC, and this protein may be used as a common biomarker for early detection, prevention, and treatment of cancer. As cancer stem cell research is yet in its infancy, definitive conclusions regarding the role of pCSC can not be made at this time. However this review will discuss what we have learned from pCSC and how this has led to innovative ideas that may eventually have major impacts on the understanding and treatment of cancer.

1

Self-renewal of stem cells and development of CSCs. HSCs maintain their homeostasis via asymmetry (A) and symmetry division (B). In homeostatic environment, HSCs divide asymmetrily to supply progenitors required for replenishment of blood. Once the injured HSCs lose the capacity of self-renewal (C), the healthy HSCs divide symmetrily. Once homeostasis is recovered, HSCs turn off programs for symmetry division. The mutating HSCs (TICs) may develop into pCSCs and CSCs (D), whereas the mutating progenitors (TICs) can acquire the capacity of self-renewal, developing into pCSCs and CSCs (E). Eventually the progenies of CSCs lose control in proliferation, occupying the space for normal HSCs and progenitors. HSC: haematopoietic stem cell; TIC: tumor-initiating cell.

2

The cancer stem cell hypothesis. Cancer is developed from CSCs that are derived from a TIC insulted by car-cinogens. The TIC can be a stem cell or a progenitor cell and the latter can acquire stemness when progressing to pre-cancerous lesion. With the accumulation of epigenetic and genetic alterations, TICs develop into pCSCs, which have the potential for benign and malignant differentiation depending on environment cues. A qualitative mutation of onco-genes or tumor suppressor genes in pCSCs may render the loss of benign potential of differentiation and commitment to CSCs. The CSC can develop into a tumour with cellular heterogeneity because of its capacity of self-renewal and multi-potency of differentiation, as well as reconstitute a new tumour distant from original tumours. Histologically the proliferation of TICs may result in hyperplasia and metaplasia, the progression of TICs to pCSCs may be responsible for dysplasia or pre-cancer, and the commitment of CSCs ultimately leads to irreversible adenocarcinoma or carcinoma. Moreover, pCSCs and CSCs may also serve as the precursors of tumour stromal components, such as the pre-cursors for tumour vasculogenesis: TVPCs. The cancer stem cell hypothesis explains the whole process of tumour development from a TIC to tumour at the histological, cellular, and molecular levels. Here gastric intestinal cancer is used as a cartoon model [4, 139, 140]. TICs: tumour initiating cells; LOH: Loss of heterozygosity; MIN: micro-satellite instability; CIN: chromosomal instability; TVPCs: tumour vasculogenic stem/progenitor cells.

3

The model of cancer stem cell development. TICs that are adult stem cells or proliferating progenitors insulted by carcinogens may develop into pCSCs or die of programmed cell death during accumulation of epigenetic and genetic alterations. The fates of pCSCs are determined by environmental cues, and they either develop into CSCs or differentiate into benign tissue cells. Both pCSCs and CSCs are susceptible to differentiation-induced cell death (DICD). Piwil2 and piwil2-regulated genes (PRG) such as ES cell genes are subverted in pCSCs, contributing to the stemness of pCSCs, albeit abnormal.

4

Establishment of clonal pCSC and CSC lines. Single tumour cells are cultured in regular or conditioned medium until tumour cells grow out and demonstrate a stable cell line. Then the cells are cloned by limiting dilution. The cloned cell lines are analysed phenotypically by flow cytometry, and the Lin – CD44⁺ cells with ambiguous stem cell markers are subjected to in vitro CFC assay and in vivo functional assay. SCID: SCID mice; BMR: lethally irradiated bone marrow-reconstituted mice; IC: immunocompetent mice.