Antitumor Efficacy of the Dual PI3K/mTOR Inhibitor PF-04691502 in a Human Xenograft Tumor Model Derived from Colorectal Cancer Stem Cells Harboring a PIK3CA Mutation

PIK3CA (phosphoinositide-3-kinase, catalytic, alpha polypeptide) mutations can help predict the antitumor activity of phosphatidylinositol-3-kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway inhibitors in both preclinical and clinical settings. In light of the recent discovery of tumor-initiating cancer stem cells (CSCs) in various tumor types, we developed an in vitro CSC model from xenograft tumors established in mice from a colorectal cancer patient tumor in which the CD133+/EpCAM+ population represented tumor-initiating cells. CD133+/EpCAM+ CSCs were enriched under stem cell culture conditions and formed 3-dimensional tumor spheroids. Tumor spheroid cells exhibited CSC properties, including the capability for differentiation and self-renewal, higher tumorigenic potential and chemo-resistance. Genetic analysis using an OncoCarta™ panel revealed a PIK3CA (H1047R) mutation in these cells. Using a dual PI3K/mTOR inhibitor, PF-04691502, we then showed that blockage of the PI3K/mTOR pathway inhibited the in vitro proliferation of CSCs and in vivo xenograft tumor growth with manageable toxicity. Tumor growth inhibition in mice was accompanied by a significant reduction of phosphorylated Akt (pAKT) (S473), a well-established surrogate biomarker of PI3K/mTOR signaling pathway inhibition. Collectively, our data suggest that PF-04691502 exhibits potent anticancer activity in colorectal cancer by targeting both PIK3CA (H1047R) mutant CSCs and their derivatives. These results may assist in the clinical development of PF-04691502 for the treatment of a subpopulation of colorectal cancer patients with poor outcomes.     

Inhibition of DNMT suppresses the stemness of colorectal cancer cells through down-regulating Wnt signaling pathway

Cancer stem cell (CSC) theory reveals a new insight into the understanding of tumorigenesis and metastasis. Recently, DNA methylation is suggested to be a potential epigenetic mechanism for maintenance of CSCs. What's more, studies have shown that DNA methyltransferase (DNMT) is essential for CSCs and deletion of DNMT can reduce tumorigenesis by limiting CSC pool. Therefore, targeting the epigenetic modifiers especially DNA methylation offers an optional strategy for treating human cancers. In the present study we found that DNMT inhibitor 5-Aza-2′-deoxycytidine (5-AzaDC) markedly reduced colorectal CSC abundance in vitro and suppressed liver metastatic tumor growth in vivo. And 5-AzaDC inhibited the expression of active β-catenin and down-regulated the Wnt signaling pathway. The Wnt inhibitors were frequently inactivated by promoter methylation in colorectal cancer; however analysis of TCGA data base showed that only the expression of SFRP1 was significantly reduced in tumors compared to normal tissues. In addition, restoring of SFRP1 expression inhibited the stem cell-like potential of colorectal cancer cells. Our results indicated that inhibition of DNMT blocked the self-renewal of colorectal CSCs and SFRP1 was essential for the maintenance of colorectal CSCs.     

Crystal structures of human CtBP in complex with substrate MTOB reveal active site features useful for inhibitor design

The oncogenic corepressors C-terminal Binding Protein (CtBP) 1 and 2 harbor regulatory d-isomer specific 2-hydroxyacid dehydrogenase (d2-HDH) domains. 4-Methylthio 2-oxobutyric acid (MTOB) exhibits substrate inhibition and can interfere with CtBP oncogenic activity in cell culture and mice. Crystal structures of human CtBP1 and CtBP2 in complex with MTOB and NAD⁺ revealed two key features: a conserved tryptophan that likely contributes to substrate specificity and a hydrophilic cavity that links MTOB with an NAD⁺ phosphate. Neither feature is present in other d2-HDH enzymes. These structures thus offer key opportunities for the development of highly selective anti-neoplastic CtBP inhibitors.     

Roles of Wnt/β-catenin signaling in the gastric cancer stem cells proliferation and salinomycin treatment

The Wnt1 protein, a secreted ligand that activates Wnt signaling pathways, contributes to the self-renewal of cancer stem cells (CSCs) and thus may be a major determinant of tumor progression and chemoresistance. In a series of gastric cancer specimens, we found strong correlations among Wnt1 expression, CD44 expression, and the grade of gastric cancer. Stable overexpression of Wnt1 increased AGS gastric cancer cells'' proliferation rate and spheroids formation, which expressed CSC surface markers Oct4 and CD44. Subcutaneous injection of nude mice with Wnt1-overexpressing AGS cells resulted in larger tumors than injection of control AGS cells. Salinomycin, an antitumor agent, significantly reduced the volume of tumor caused by Wnt1-overexpressing AGS cells in vivo. This is achieved by inhibiting the proliferation of CD44+Oct4+ CSC subpopulation, at least partly through the suppression of Wnt1 and β-catenin expression. Taken together, activation of Wnt1 signaling accelerates the proliferation of gastric CSCs, whereas salinomycin acts to inhibit gastric tumor growth by suppressing Wnt signaling in CSCs. These results suggest that Wnt signaling might have a critical role in the self-renewal of gastric CSCs, and salinomycin targeting Wnt signaling may have important clinical applications in gastric cancer therapy.