Knockdown of Ubiquitin Ligases in Glioblastoma Cancer Stem Cells Leads to Cell Death and Differentiation

The cancer stem cell (CSC) hypothesis proposes that a subpopulation of CSCs is frequently responsible for chemotherapy resistance and metastasis and is now a point of attack for research into the next generation of therapeutics. Although many of these agents are directed at inducing CSC apoptosis (as well as the bulk tumor), some agents may also decrease cell "stemness" possibly through induction of differentiation. Ubiquitin ligases, critical to virtually all cellular signaling systems, alter the degradation or trafficking of most proteins in the cell, and indeed broad perturbation of this system, through inhibition of the proteosome, is a successful cancer treatment. The authors examined several glioblastoma stem cell isolates pre- and postdifferentiation to elucidate the phenotypic effects following shRNA knockdown of ubiquitin ligases. The results were analyzed using high-content imaging (HCI) and identified ubiquitin ligases capable of inducing both CSC differentiation and apoptosis. Quite often these effects were specific to CSCs, as ubiquitin ligase knockdown in terminally differentiated progeny yielded markedly different results. The resolution of HCI at the subpopulation level makes it an excellent tool for the analysis of CSC phenotypic changes induced by shRNA knockdown and may suggest additional methods to target these cells for death or differentiation.