Cancer stem cells--the current status of an old concept: literature review and clinical approaches

Abstract

As regards their morphology and biology, tumours consist of heterogeneous cell populations. The cancer stem cell (CSC) hypothesis assumes that a tumour is hierarchically organized and not all of the cells are equally capable of generating descendants, similarly to normal tissue. The only cells being able to self-renew and produce a heterogeneous tumour cell population are cancer stem cells. CSCs probably derive from normal stem cells, although progenitor cells may be taken into consideration as the source of cancer stem cells. CSCs reside in the niche defined as the microenvironment formed by stromal cells, vasculature and extracellular matrix. The CSC assays include FACS sorting, xenotransplantation to immunodeficient mice (SCID), incubation with Hoechst 33342 dye, cell culture in non-adherent conditions, cell culture with bromodeoxyuridine. CSCs have certain properties that make them resistant to anticancer therapy, which suggests they may be the target for potential therapeutic strategies.

Figure 1

Two features defining cancer stem cells. A – Self-renewal: the ability to generate descendants retaining stemness characteristics. Stem cells undergo symmetric division to give two stem cells or asymmetric division, where one descendant remains a stem cell, whereas the other cell loses stemness features. B - Restoration of the heterogeneous cancer cell population. The secondary tumour is composed of the same cell types as the primary tumour.

Figure 2

Basic tumour heterogeneity models. A - Clonal evolution model. High proliferation and genomic instability result in a large number of cells differing in genotype and thus phenotype. The best fitted cells are selected by Darwinian processes to generate clonal variants of the tumour. B - Cancer stem cell model. CSC population is capable of unlimited number of divisions. Tumour heterogeneity results from existence of phenotypically diverse populations of different stages of cell maturation.

Figure 3

Clonal evolution and CSCs model are not exclusive. The population of CSCs may undergo clonal evolution. Tumour heterogeneity results from existence of both clonal variants and different stages of cell maturation.

Figure 4

The historical concepts of CSCs origin. A - Embryonal rest theory. The pluripotent embryonal cells remain in the adult organism in the form of “embryonal rest”. They are the origin of CSCs. B - Dedifferentiation theory. Somatic stem cells of adult tissue gain pluripotency through dedifferentiation.

Figure 5

The stages of CSCs isolation by FACS. A - Solid tumour is digested by enzymes to cell suspension. B - Tumour cell suspension is incubated with antibodies directed against antigens specific for CSCs conjugated with fluorescent dye. C - The opsonized cell suspension is let through a narrow tunnel to form a single-cell-diameter stream. The vibration produces droplets containing single cells at the mouth of the tunnel. D - The droplets pass through a laser beam. The fluorescent dye is excited to emit the light identified by a detector. E - The detector is paired with the device which gives electrostatic charge to light-emitting cells. The “dark” cells remain uncharged. F - The flow of electrostatically charged cells is bent by the electric field. The cells are collected in the tube. The uncharged cells fall by gravitation.

Figure 6

CSCs xenotransplantation into SCID mouse. A - Cell suspension containing FACS-isolated CSCs is injected subcutaneously or intraperitoneally into SCID mouse. B - Generation of a secondary tumour with heterogeneous population, analogous to the primary tumour, is highly conclusive of CSCs existence in cell suspension.