Abstract SY19-01: Cell to cell variability in the responses of tumor cells to death ligands

Abstract

Abstract Cell to cell variability in phenotype and fate is important for tissue homeostasis, the emergence of drug resistance and the evolution of tumors. I will discuss cell to cell variability in the responses of clonal populations of human cells to TRAIL, a prototypical inducer of receptor-mediated (extrinsic) apoptosis and an investigational therapeutic. Some TRAIL-treated human cells die within ~40 min, others only after 12 hr and some live indefinitely. During this variable delay initiator caspases (ICs) are active in processing substrates such as Bid and downstream effector caspases (ECs) but mitochondrial membrane permeablization (MOMP) does not occur and ECs are catalytically inactive. We have explored three explanations for cell to cell differences among members of a clonal and notionally uniform population: (i) genetic or epigenetic variation; (ii) the involvement of one or more biochemical processes subject to stochastic fluctuation; (iii) difference in cell state such as position in the cell cycle or protein concentrations. To distinguish among these three possibilities we compared the fates of sister cells exposed to TRAIL using live-cell microscopy. Genetic and epigenetic factors are transmitted with near-perfect fidelity; reaction rates dominated by stochastic fluctuations are no more similar in sisters than in any two cells chosen at random and variability arising from differences in mRNA or protein levels should be transiently heritable (because division is a binomial process in which sister cells inherit roughly equal numbers of abundant cytosolic and nuclear factors but drift towards the population average over time). The later explanation is correct; natural fluctuations in protein levels dominate phenotypic heterogeneity in extrinsic apoptosis and cause transient phenotypic heritablility. Variability can be manipulated by protein over-expression and drugs and I will describe analytic methods for modeling these perturbations. Finally, I will show that natural variation is sufficient to cause some cells to follow one biochemical pathway to death (a MOMP-dependent Type II pathway) and another set of genetically identical cells to follow a fundamentally different pathway (a MOMP-independent Type I pathway). Citation Format: Peter K. Sorger, Sabrina Spencer, Deborah Flusberg, Suzanne Gaudet. Cell to cell variability in the responses of tumor cells to death ligands [abstract]. In: Proceedings of the AACR 101st Annual Meeting 2010; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr SY19-01