Disruption of a Quorum Sensing mechanism triggers tumorigenesis: a simple discrete model corroborated by experiments in mammary cancer stem cells

Abstract

Background: The balance between self-renewal and differentiation of stem cells is expected to be tightly controlled in order to maintain tissue homeostasis throughout life, also in the face of environmental hazards. Theory, predicting that homeostasis is maintained by a negative feedback on stem cell proliferation, implies a Quorum Sensing mechanism in higher vertebrates.

Results: Application of this theory to a cellular automata model of stem cell development in disrupted environments shows a sharply dichotomous growth dynamics: maturation within 50-400 cell cycles, or immortalization. This dichotomy is mainly driven by intercellular communication, low intensity of which causes perpetual proliferation. Another driving force is the cells' kinetic parameters. Reduced tissue life span of differentiated cells results in uncontrolled proliferation. Model's analysis, showing that under the Quorum Sensing control, stem cell fraction within a steady state population is fixed, is corroborated by experiments in breast carcinoma cells. Experimental results show that the plating densities of CD44+ cells and of CD44+/24lo/ESA+ cells do not affect stem cell fraction near confluence.

Conclusions: This study suggests that stem cell immortalization may be triggered by reduced intercellular communication, rather than exclusively result from somatic evolution, and implies that stem cell proliferation can be attenuated by signal manipulation, or enhanced by cytotoxics targeted to differentiated cells. In vivo verification and identification of the Quorum Sensing mediating molecules will pave the way to a higher level control of stem cell proliferation in cancer and in tissue engineering.

Figure 1

Geometric structure of the modelled tissue. The intensity of signal reaching the considered SC (black circle) from SCs in the first, most proximal, layer (blue), second layer (green) or third layer (red) is determined by the respective signal intensity coefficients K₁, K₂ or K₃. Note that the drawn cell sizes are insignificant and that nodes may be occupied by differentiated cells.

Figure 2

Types of SC development. SC development in disrupted environment shows two alternative types of behavior: transient SC proliferation (triangles), production of DCs (squares) and disappearance (A), or rapidly achieved proliferative SC state (B); a case of large DC production and maintenance of a small SC population is rarely achieved (C).

Figure 3

Factors affecting SC fate. (A) Cells with various kinetic parameters were simulated under different levels of intercellular communication. Three different effects are depicted at steady state. (a) Under a relatively high intercellular communication, K₁ = K₂ = K₃ = 6, K₄ = ... = 0, in about half of the cases all SCs transiently replicate and differentiate into end cells, and in about half of the cases SCs uncontrollably proliferate; (b) Under a lower intercellular communication, K₁ = 6, K₂ = ... = 0, most of the cases evolve to SC immortalization. (c) An even lower intercellular communication, K₁ = K₂ = K₃ = 1, K₄ = ... = 0, or K₁ = 1, K₂ = ... = 0, preserves all SCs in the proliferative state (note two overlapping curves). (B) Effect of cell kinetic parameters in environments of high intercellular intensity. Transient SC proliferation and transition to differentiation is dominant when F, Ψ, T obey condition (a) in Table 1. Intermediate probability of full differentiation is obtained under case (b) in Table 1. Immortalization is always obtained under case (c) in Table 1. (C) Proportion of immortalized SCs as a function of initial SC density. The parameters Φ, Ψ, Θ obey case (a) in Table I (open squares), case (b) (filled squares), case (c)(circles).

Figure 4

Experimental results. (A) MCF7 cells were sorted to isolate CD44+ cells and seeded at different proportions (0, 2.5, 5, 10, 32 and 100%) and mixed with the remaining cell population. Cells were cultured in monolayer for 7 days and trypsinised to single cells which were cultured in non-adherent suspension for 7 days. The percentage of mammosphere formation units (%MFU) was counted to determine whether the population achieved a steady state (light grey bars). (B) The percentage of cells expressing CD44 after 7 days culture in monolayer was examined using flow cytometry analysis (open bars) (C) MCF7 cells were sorted to isolate CD44+/24lo/ESA+ "stem-like" cells and seeded at different proportions (0, 1, 3, 6, 9 and 12%) admixed with the total cell population. Cells were cultured for 2 days (dark grey bars), (D) 4 days (diagonal striped bars) or (E) 7 days (dotted bars). Cells were trypsinised to single cells which were cultured in non-adherent suspension for 7 days. The percentage mammosphere formation units (%MFU) was counted. Data was analyzed using analysis of variance (ANOVA) and P values ≤ 0.05 were assumed to be significant.