P1-04-06: Ionizing Radiation Reprograms Non-Tumorigenic Cancer Cells into Cancer Stem Cells

Breast cancers are thought to be organized hierarchically with a small number of breast cancer stem cells (BCSCs) able to re-grow a tumor while their progeny lack this feature. BCSCs in breast cancer have been found to be relatively resistant to radiation and several groups reported enrichment for BCSCs when breast cancers are subjected to classical anticancer treatment. Differentiation of BCSCs is thought to be unidirectional but an alternative model assumes that stemness can be obtained by clonal evolution.

In this study, we quantified the number of BCSCs surviving after radiation treatment. We compared the number of surviving BCSCs to the expected number and found an increase in BCSCs after irradiation that could not be explained by current models. We propose that radiation induces a BCSC phenotype in previously non-BCSCs and show that this transition is Notch-dependent and coincided with up-regulation of the transcription factors Oct4, Sox-2, Nanog, and Klf4.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P1-04-06.

Abstract PD02-10: Radiation Induces Notch-Dependent De Novo Generation of Breast Cancer Stem Cells

Like in normal tissues, the self-renewal of cancer stem cells (CSCs) might be also under tight control of developmental pathway like the Notch, Wnt, Sonic Hedgehog or TGFβ pathways. The Notch pathway plays an important role in normal breast development, cell fate, and normal stem cell self-renewal, and its deregulation has been shown to play a role in cancer. Aberrant Notch signaling has been implicated in the development and progression of both preinvasive ductal carcinomas in situ and invasive. Interestingly, in breast cancer, the Notch pathway plays major role for CSCs maintenance. We previously published that BCSCs (Breast Cancer Stem Cells) are more resistant to radiation. Here we demonstrate a link between BCSCs radio-resistance and the Notch pathway. We show that irradiation of MCF-7 cells increases CSC numbers and that this correlates with an induction of Notch signaling proteins expression in a dose and time specific manner. Jagged 1 was quickly (1h) increased 28-fold after 2Gy, DLL1 was increased 15-fold after 3 to 6h of 2 and 4 Gy irradiation, and Notch 2 was increased 16-fold 6h after 2, 4, 6 or 8 Gy, while DLL3 was increased 10-fold after the highest doses (6 to 12 Gy). Inhibition of Notch signaling pathway by the γ-secretase inhibitor prevents enrichment for CSCs and reduces radiation-induced overexpression of Notch proteins. More interestingly, we demonstrated that radiation-induced Notch signaling contributes to the phenotype plasticity of BCSCs and their progeny. Therefore, ionizing radiation induces de novo generation of BCSCs from non-tumorigenic cells. Moreover, we also identified, by FACS analysis, an induction of polyploid CSCs. expressed a relatively higher level of Oct4 and Sox2 than the unirradiated non-CSCs (two key transcription factors involved in stemness maintenance). In summary, we provide evidence that radiation-induced Notch signaling contributes to the plasticity of the BCSC phenotype, thereby generating BCSCs de novo from non-tumorigenic cells. Our data suggest that stochastic as well hierarchical CSC models apply to breast cancer.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr PD02-10.

Low proteasome activity as a means to track and target breast cancer stem cells in-vivo

Abstract #5055

Based on clinical observations, the existence of cancer stem cells (CSCs)/cancer initiating cells (CICs) in solid cancers has been postulated by radiation biologists and oncologists for decades. According to the stem cell hypothesis, only a small number of CSCs/CISs within a tumor have the ability to repopulate an entire tumor while their progeny does not. CSCs/CISs in solid tumors can now be identified prospectively in gliomas, head & neck cancers, pancreatic cancers, colon cancers, prostate cancers, melanomas, and in breast cancers. CSCs/CISs are thought to be mostly quiescent and relatively resistant to conventional anti-cancer therapy.
 Breast CSCs/CICs are enriched in cell populations with high CD44 and low or absent CD24 expression. Unfortunately, detection of breast CSCs/CISs, using CD44 and CD24 expression analysis, requires dissociation of the tumors and antibody staining for surface markers. We recently discovered that cancer stem cells, and specially breast CSCs/CICs, have low proteasome activity. In order to utilize this novel marker for the identification of breast CSCs/CICs, we developed retroviral vectors coding for fusion proteins between thymidine kinase (TK), the fluorescent protein ZsGreen and the c-terminal degron of murine ornithine decarboxylase (cODC). We stably transfected human breast cancer cells lines (MCF-7, MDA-MB-231, T47-D) with this reporter gene construct for 26S proteasome activity.
 Our reporter construct allowed us to track CSCs/CICs responses to radiation therapy in vivo via fluorescent imaging, but also to target them selectively using suicide gene (TK) therapy. Thus, the use of thymidine kinase in our fusion constructs allows for specific elimination of this cell population by ganciclovir preventing self-renewal in-vitro and causing T47-D and MDA-MB-231 tumor xenograft regression in vivo.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5055.