Cancer stem-like cells reside in hypoxic and slightly acidic tumor niches. Such microenvironments favor more aggressive undifferentiated phenotypes and a slow growing "quiescent state" which preserves them from chemotherapeutic agents that essentially target proliferating cells. Our objective was to identify compounds active on glioblastoma stem-like cells, including under conditions that mimick those found in vivo within this most severe and incurable form of brain malignancy. We screened the Prestwick Library to identify cytotoxic compounds towards glioblastoma stem-like cells, either in a proliferating state or in more slow-growing "quiescent" phenotype resulting from non-renewal of the culture medium in vitro. Compound effects were assessed by ATP-level determination using a cell-based assay. Twenty active molecules belonging to different pharmacological classes have thus been identified. Among those, the stimulant laxative drug bisacodyl was the sole to inhibit in a potent and specific manner the survival of quiescent glioblastoma stem-like cells. Subsequent structure-function relationship studies led to identification of 4,4'-dihydroxydiphenyl-2-pyridyl-methane (DDPM), the deacetylated form of bisacodyl, as the pharmacophore. To our knowledge, bisacodyl is currently the only known compound targeting glioblastoma cancer stem-like cells in their quiescent, more resistant state. Due to its known non-toxicity in humans, bisacodyl appears as a new potential anti-tumor agent that may, in association with classical chemotherapeutic compounds, participate in tumor eradication.

SUMMARYPrevious studies have shown that Nicotiana tabacum contains three Agrobacterium-derived T-DNA sequences inherited from its paternal ancestor Nicotiana tomentosiformis. Among these, the TB locus carries an intact mannopine synthase 2 0 gene (TB-mas2 0 ). This gene is similar to the Agrobacterium rhizogenes A4-mas2 0 gene that encodes the synthesis of the Amadori compound deoxyfructosyl-glutamine (DFG or santhopine). In this study we show that TB-mas2 0 is expressed at very low levels in N. tomentosiformis and in most N. tabacum cultivars; however, some cultivars show high TB-mas2 0 expression levels. The TB-mas2 0 promoter sequences of low-and high-expressing cultivars are identical. The low/high level of expression segregates as a single Mendelian factor in a cross between a low-and a high-expression cultivar. pTBmas2 0 -GUS and pA4-mas2 0 -GUS reporter genes were stably introduced in N. benthamiana. Both were mainly expressed in the root expansion zone and leaf vasculature. Roots of tobacco cultivars with high TBmas2 0 expression contain detectable levels of DFG.