Adult stem cells and cancer.

Rubio et al. (1) reported in Cancer Research that adipose tissue– derived stem cells acquired a transformed phenotype after longterm culture in vitro . This article has been cited together with our article published in the same issue (2) as evidence for the malignant transformation potential of stem cells by the news media. Whereas our study showed the dangers of having a continuous high level of telomerase activity during long-term in vitro culture of adult stem cells, the study of Rubio et al. employed what seemed to be ‘‘normal’’ cells. We have previously cultured several strains of normal human bone marrow–derived mesenchymal stem cells and we have not observed ‘‘immortalization’’ of the cells (3). Unfortunately, the authors refer to our cells as another example of the human adiposederived MSC, which is not correct. The differences between our study and the study by Rubio et al., therefore, need clarification because they may have an important effect on the stem cell field. It is possible that the donors employed by Rubio et al. were harboring a disease and the clinical data related to these donors need to be provided as well as the reason for undergoing the surgical procedure. Furthermore, growth curves of the individual cell lines established should be provided and the authors should provide data showing at which population doubling level the cells entered crisis. Finally, the high percentage of the transformation events and the similarity between the cell lines described require proof that the cells that formed the tumors were the cells present at the beginning of the culture by using DNA fingerprinting. These issues are important to resolve because they are important for the stem cell community as a whole.