Cover caption

Abstract

 

Numerous cellular stresses activate p53, which results in stopping the cell cycle to allow for repair or induction of apoptosis if the stress is too overwhelming for the cell. Among the stresses that can activate p53 is the fluorescent dye Hoescht 33342, which is currently widely used to identify putative cancer stem cells (CSCs). The stem cell hypothesis maintains that a small population of stem cells is responsible for tumor growth. Hoescht 33342 is effluxed from the stem cells (thereby forming a “side population” in flow cytometry assays) while the majority of the population is unable to efflux the dye. Implanting the two populations has shown that fewer side population cells are required to give rise to tumors than non-side population (non-SP) cells-but clearly, the dye is affecting the non SP cells. Further confounding the equation is that p53 is inactivated in many tumors, thereby possibly skewing the populations in many tumor populations. Given the toxicity of Hoescht, it is desirable to be able to enrich CSC populations in vivo using non-toxic methods that will not kill cells that retain the dyes. In the current issue of Cancer Biology & Therapy, Allen and colleagues explore the use of Calcein AM to identify dye-effluxing putative cancer stem cell populations. The cover of this month’s issue shows cells stained with Calcein AM (green), D-luciferin (blue), and Mitotracker Red FM (red). Putative cancer stems cells are able to efflux both Calcein AM and D-luciferin and therefore stain only red. To learn more about using Calcein AM for identifying and following cancer stem cells, see the paper by Allen and colleagues.