Abstract 2460: Tumorigenic potential is independent of sphere phenotype in liver cancer
Authors: Raggi Chiara , Marquardt Jens U. , Andersen Jesper B. , Factor Valentina M. , Thorgeirsson Snorri S. 
Published: June 2012

Background: A phenotypic plasticity, a hallmark of embryonic development and cancer progression/metastasis, is dynamically regulated by extrinsic signals produced by niche and intrinsic cell autonomous mechanisms. Consequently, both cell microenvironment and genetic/epigenetic machinery play important roles in cell fate reprogramming.

Objective: In this study we aimed to evaluate the effects of environmental and/or epigenetic modulation on cancer stem cell (CSC) properties using a panel of human HCC cell lines.

Methods: Seven human hepatoma cell lines, including one primary HCC cell line, were plated at high (HD) and low density (LD) and grown in the presence or absence of DNA-methyltransferase inhibitor zebularine (Zeb) for 3 days. Thereafter cells were propagated in serum-free condition for evaluation of sphere-forming ability. Primary tumor-spheres were passaged every 10 days to generate passage 1 (P1) trough P7 spheres. Gene expression was examined by qRT-PCR and microarray analysis. Sphere tumorigenic capacity was evaluated in vivo after subcutaneous (s.c.) transplantation into NOD/SCID mice.

Results: Cells grown under LD failed to form spheres when placed in non-adherent growth conditions. In contrast, Zeb-treated LD (LDZ) cells were capable of forming clonal-expanding spherical colonies. Cells cultured at HD possessed a comparable sphere forming ability regardless of Zeb pre-treatment. However, despite the similar phenotype and size, spheres formed by LDZ and HD/HDZ cells possessed different self-renewal and tumorigenic potential in vitro and in vivo. Thus, LDZ spheres exhibited a considerably stronger induction of the stem cell-related genes (NANOG, OCT4, and BMI1) as well as epithelial-mesenchymal transition markers (β-catenin, ZEB1, VIM and SNA1) as compared to HD spheres. Furthermore, in serial sphere formation assay, only LDZ-spheres could retain a long term self-renewal while the sphere-forming ability of both HD- and HDZ-spheres was exhausted within the first three passages in vitro. In addition, cells dissociated from LDZ-spheres at P1 possessed higher tumorigenicity upon s.c. transplantation into NOD/SCID mice as compared to P1 HD-spheres, and only LDZ-spheres could propagate the tumor growth in vivo at later passages.

Conclusion: We describe functional tumor-sphere approach which offers an efficient tool to enrich for cells with CSC properties. By modulating either cell density and/or epigenome in hepatoma cell lines we selected for sphere-forming phenotype possessing a high tumor-initiating potential including a long-term ability for self-renew during serial sphere passaging and propagation of tumor growth in vivo. The global gene expression analysis is underway to characterize the molecular mechanisms responsible for maintaining CSC compartment in sphere-derived tumor clones.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2460. doi:10.1158/1538-7445.AM2011-2460