Cancer Stem Complex, Not a Cancer Stem Cell, Is the Driver of Cancer Evolution

Here, we put forward the hypothesis on the mechanism of functioning of cancer stem cells, provided that they exist. The hypothesis is based on the following postulates. 1) Paracrine exchange between cancer and stromal cells is efficient only if they are in a close contact and form a synapse-like cleft between them for the cell-cell crosstalk. The concentration of paracrine signaling molecules in the cleft is high because of the cleft small volume. 2) Cancer stem cells per se do not exist. Instead, there are cancer stem complexes formed by cancer cells tightly bound to stromal cells (portable niches) that exchange paracrine signals. 3) Cancer stem complex is a complex system with newly emerged properties, such as a stemness and resistance to external impacts, including therapeutic interventions. 4) The stemness manifests itself as the ability of cancer cells within the complex to divide asymmetrically: one daughter cell remains in the complex forming a renewed stem complex, whereas the other daughter cell detaches from the complex and transforms in a non-stem cell capable of differentiation. 5) An increased resistance of a cancer stem complex is due to the integration of its intrinsic defense systems through the exchange of paracrine signals, i.e., represents a microresistance at the cell level. 6) Cancer stem complexes can stochastically dissociate with the formation of non-stem cancer cells. Partially differentiated non-stem cancer cells are able to stochastically bind to the stromal component, dedifferentiate under the action of paracrine signals, and form new cancer stem complexes. Therefore, a tumor is a flexible system existing in the pseudo-equilibrium state. Such systems comply with the Le Chatelier's principle stating that an equilibrium system under the action of external factors activates the processes antagonistic to the changes (homeostasis). This promotes tumor resistance at the level of cell populations, i.e., the macroresistance. 7) The portable niche travels with the cancer cell during metastasis. We propose a general therapeutic strategy targeting the contacts between cancer and stromal cells. The disruption of these contacts should lead to the destruction of cancer stem complexes and elimination of tumors.