HCC is the fifth most common cancer worldwide affecting one million individuals annually. Despite advances in the detection and treatment of the disease, mortality rate remains high because current therapies are limited by the emergence of chemotherapy-resistant cancer cells. Recent research efforts in stem cell biology suggest that tumors are organized in a hierarchy of heterogeneous cell populations and that the capability to maintain tumor formation and growth specifically resides in a small population of cells called cancer stem cells (CSCs). The stem cell-like characteristics of these cells and their limited number within the bulk of the tumor are believed to account for their capability to escape conventional therapies. There is accumulating evidence showing that normal and cancer stem cells share common properties, thus we hypothesized that the identification of normal stem cells may lend insight into the understanding of the events that regulate these cells and consequences of their subversion. Liver regeneration is believed to rely on a subset of stem/progenitor cells, thus we began by studying what normal stem cells are involved in the liver regeneration process using a severe partial hepatectomy (PH) model where over 70% of the liver mass was removed. cDNA microarray analysis on RNA samples from mouse liver that was subjected to PH found Prominin-1, the mouse homologue of human CD133, to be dramatically up-regulated during the early restoration of the liver (day 3) with its expression diminishing at the final stages of liver regeneration (day 7). CD133 expression in cultured cell lines is found to correlate with the ability of the cells to develop tumor in vivo. Functional studies on isolated CD133+ and CD133- cells from human HCC cell lines and xenograft tumors found CD133+ cells to be inherently more tumorigenic than CD133- cells, showing a greater colony-forming efficiency, higher proliferative output and greater ability to form tumor in vivo. As few as 1000 CD133+ cells are sufficient for consistent tumor development in SCID mice while at least 50x as many CD133- cells are necessary to generate a tumor in the same model. Moreover, CD133+ cells are found to be endowed with characteristics similar to those of stem cells including the preferential expression of “stemness” genes; the ability to self-renew and the ability to differentiate into non-hepatocyte-like lineages. Further, CD133 is found to be maintained in very few numbers in CD133+ induced xenograft tumors and is also found to represent only a minority of the tumor cells in human HCC specimen. In addition, CD133+ cells are more resistant to doxorubicin (DOX) and fluorouracil (5-FU), anticancer drugs commonly used in treating HCC patients. The identification of tumorigenic liver CSCs in this study should not only provide new insight into the process of hepatocarcinogenesis but also open new avenues to the development of more effective cancer therapies.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA