Tumor heterogeneity is in part determined by the existence of cancer stem cells (CSCs) and more differentiated tumor cells. CSCs are considered to be the tumorigenic root of cancers and suggested to be chemotherapy resistant. Here we exploited an assay that allowed us to measure chemotherapy-induced cell death in CSCs and differentiated tumor cells simultaneously. This confirmed that CSCs are selectively resistant to conventional chemotherapy, which we revealed is determined by decreased mitochondrial priming. In agreement, lowering the anti-apoptotic threshold using ABT-737 and WEHI-539 was sufficient to enhance chemotherapy efficacy, whereas ABT-199 failed to sensitize CSCs. Our data therefore point to a crucial role of BCLXL in protecting CSCs from chemotherapy and suggest that BH3 mimetics, in combination with chemotherapy, can be an efficient way to target chemotherapy-resistant CSCs. Cell Death and Differentiation (2014) 21, 1170-1177 doi:10.1038/cdd.2014 published online 28 March 2014 Colorectal cancer is the third most common cancer worldwide.1,2 Patients with advanced stage colorectal cancer are routinely treated with 5-fluorouracil (5-FU), leucovorin and oxaliplatin (FOLFOX), or with 5-FU, leucovorin and irinotecan (FOLFIRI), often in combination with targeted agents such as anti-VEGF or anti-EGFR at metastatic disease.3-6 Despite this intensive treatment, therapy is still insufficiently effective and chemotherapy resistance occurs frequently. Although still speculative, it has been suggested that unequal sensitivity to chemotherapy is due to an intratumoral heterogeneity that is orchestrated by cancer stem cells (CSCs) that can self-renew and give rise to more differentiated progeny. 7,8 When isolated from patients, CSCs efficiently form tumors upon xenotransplantation into mice which resemble the primary tumor from which they originated. [9][10][11] In addition, many xenotransplantation studies have emphasized the importance of CSCs for tumor growth.9-12 Colon CSCs were originally isolated from primary human colorectal tumor specimens using CD133 as cell surface marker and shown to be highly tumorigenic when compared with the non-CSCs population within a tumor. 9,10 Later, other cell surface markers as well as the activity of the Wnt pathway have been used to isolate colon CSCs from tumors.12,13 Spheroid cultures have been established from human primary colorectal tumors that selectively enrich for the growth of colon CSCs,11,12 although it is important to realize that these spheres also contain more differentiated tumor cells.12 In agreement, we have shown that the Wnt activity reporter that directs the expression of enhanced green fluorescent protein (TOP-GFP) allows for the separation of CSCs from more differentiated progeny in the spheroid cultures.
12CSCs are suggested to be responsible for tumor recurrence after initial therapy, as they are considered to be selectively resistant to therapy.11,14 Conventional chemotherapy induces, among others, DNA damage and subsequent activation of the mito...
