Investigators from the Univ. of California, San Francisco, C. Kwon et al., reported in the June 26th issue of PNAS that the results of their study demonstrated that canonical Wnt ligands were positive regulators for the development of cardiac progenitors as well as subsequent induction of the proliferative expansion of these progenitor cells. The authors noted that others have reported that "canonical Wnt ligands negatively regulate cardiomyogenesis in several species." However, the researchers' study showed, using both tissue-specific in vitro and in vivo genetic manipulation of mouse embryos, that canonical Wnt signaling and subsequent upregulation of β-catenins resulted in the expansion of cardiac progenitors. The authors noted that proliferation of the committed progenitors was required prior to the cells differentiating into cardiomyocyates during embryonic development.

In yesterday's post, we noted that University of Pittsburgh researchers reported that their data suggested that the microenvironment may provide extrinsic differentiation signals which induce muscle-derived stem cells into cancer stem cells. Similarly, researchers from The Scripps Research Institute, Y. Chang et al., published in the June 19th online edition of PNAS their study on the secretory cytokine, pleiotrophin (PTN), that appears to remodel the tumor microenvironment in breast cancers. Overexpression of Ptn (MMTV promoter-driven Ptn) in mouse breast cancers resulted in extensive remodeling of the tumor microenvironment in which there was a dramatic increase in angiogenesis as well as mouse protocollagens Ia2, IV a5, and XI a1, and elastin. In a xenograft model, a human breast cancer cell line (MFC-7) which overexpressed Ptn also resulted in extensive remodeling of the tumor in nude mice. Using a coculture model, the experimental results demonstrated that when NIH 3T3 stromal fibroblasts were cultured with overexpressed MCF-7-Ptn cells, the secreted PTN produced a more aggressive breast cancer cell phenotype which was reflected by upregulation of PKCd and matrix metalloproteinases-9 (MMP-9) in both cell types. When the cells were implanted into nude mice, the microenvironment of the resultant tumor recapitulated the morphology of the microenvironment of the tumors in mice implanted with only the MCF-7-Ptn cells. The authors concluded that inappropriate expression of Ptn results in stimulating a reactive stromal microenvironment driving breast cancer progression. (The study results imply that secretory PTN stimulates stromal fibroblasts which in turn produce factors such as MMP-9 that may activate quiescent cancer stem cell in the breast tumor.)

Scientists, I. Zucchi et al., from the National Research Council in Milan, Italy reported in the June 12th online edition of PNAS on the properties of a cancer stem cell from a rat mammary adenocarcinoma. In this study, the investigators found a cancer-initiating cell from a rodent tumor cell line (LA7) which fulfilled the current criteria of a cancer stem cell: i) ability to self-renew; ii) unlimited proliferative potential; iii)tumorogenic; and, iv) multipotential property for forming different cell lineages. The researchers found that the LA7 cell in vitro were able to recapitulate morphologically and functionally the ductal-alveolar-like architecture of the mammary tree. The LA7 cells derived from cultures were found to be tumorigenic and they were also able to recapitulate the ductal-alveolar-like architecture when the cells were injected into NOD-SCID mice. Due to the difficulty of isolating cancer stem cells from primary tumors, the authors proposed that "LA7 cells can be used as a model to study the dynamics of tumor formation at the single-cell level."

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In the June 14th online edition of Stem Cells, J. B. Pollett et al. reported the results of their study on concomitant differentiation signals that may lead to malignant transformation in a population of multipotent muscle-derived stem cells (MDSCs). The University of Pittsburgh researchers found that by subjecting pre-committed MDSCs to a variety of concomitant differentiation signals, they were able to induce cancer stem cells. Experimental results demonstrated that a subpopulation MDSCs which were committed to either an osteogenic or myogenic lineage would form tumors if they were simultaneously exposed to other extrinsic differentiation signals. For example, MDSCs pretreated with BMP-4 and predisposed to myogenesis would form tumors in 25% of mice following implantation. If these cells were transduced to express BMP-4 and implanted into the bone they would form bone. However, if these same cells were implanted into the skeletal muscle of a mouse they formed tumors in 100% of the mice receiving the transplant. With siRNA to knockdown MyoD1 expression, the transduced MDSCs lost their tumorigenicity. The authors postulated that "the initial stages of transformation may be due to changes in the balance between inherent 'nature' of the cell and extrinsic signaling pathways." The investigators also noted from their experimental data that a "potential link between somatic stem cells and cancer may involve the microenvironment stem cell niche during the transformation process."

Researchers from Stanford University School of Medicine N. Hosen et al., reported in the June 18th online edition of PNAS on the isolation of human leukemic stem cells expressing the surface marker CD96. Using a signal sequence trap strategy, the investigators found a subpopulation of CD34+CD38- acute myeloid leukemic cells expressing a surface molecule of the Ig gene superfamily, CD96 (74.0 ± 25.3% in 19 of 29 cases). It was also noted that only a few (4.9 ± 1.6%) of the cells in hematopoietic stem cells in blood of normal individuals weakly expressed the CD96 surface epitope. The CD96+ and CD96- fractions were transplanted into irradiated newborn Rag2^-/-γ_c^-/- mice. The investigators reported that only the CD96+ fraction were able to engraft the bone marrow of the recipient mice. The authors concluded that CD96 is a novel leukemic stem cell marker which may serve as a drug target for treating patients with acute myeloid leukemia.

At the Tissue Engineering and Regenerative Medicine International Society meeting held in Toronto from June 13-16, researchers from the University of Pittsburgh, V. Donnenberg et al., reported the results of their study comparing cancer stem cells to normal stem cells derived from human lung tissues. With tumor samples from freshly isolated untreated primary non-small cell lung cancer, the normal lung parenchyma, and lung tissues from organ donors, the investigators analyzed the characteristics of the each subpopulation of cells found in the respective tissues. The cell surface markers used to analyze the different cell subpopulations were CD90, CD117, CD133, CD45, cytokeratin, Human Epithelial Antigen (HEA), and MUC-1. From the tumors, the researchers identified a constitutive MDR (multi-drug resistant), non-hematopoietic, side population of resting cancer cells that had both Hoescht 33342 and R123 (rhodamine) efflux activities. It was further noted that the cells were found within the HEA+ and CD90+ subpopulation of cells by flow cytometry. Similarly, the authors previously reported a CD90+ subpopulation of tumorigenic stem cells in breast cancer tissues. In the current study, the investigators found these resting cells possess some of the same characteristics of adult tissue stem cells. The authors concluded that in order to have more effective therapies against various forms of cancers, drugs need to be developed with differential toxicities that target these rare resting cancer stem cells and not the cancer stem cell which share many of the same characteristics as adult stem cells in normal tissues.

Researchers from the Univ. of Michigan School of Medicine, P. Dalerba et al., published online in the June 4th edition of PNAS the results of their study on characterizing the cancer stem cells in human colorectal cancers. With flow cytometry, the investigators sorted the dissociated cells of the tumor displaying specific surface markers (EpCAM and CD44) and tested their tumorigenicity in NOD/SCID mice. When the separated cells were injected in the immunocompromised mice, it was found that all six of the human tumor tissues tested formed tumors from a subpopulation of cells with the phenotype EpCAM^high/CD44⁺. The investigators concluded that these EpCAM^high/CD44⁺ were the cancer-initiating cells which gave rise to the "full morphologic and phenotypic heterogeneity of their partial lesions." Another marker, CD166, was found on the surface of these cancer stem cells. The authors concluded that the three surface markers used in the study for characterizing human cancer-initiating cells provide a "highly robust surface marker profile for colorectal cancer stem cell isolation."