Mesenchymal stem cells (MSCs), which potentially transdifferentiate into multiple cell types, are increasingly reported to be beneficial in models of organ system injury. However, the molecular mechanisms underlying interactions between MSCs and host cells, in particular endothelial cells (ECs), remain unclear. We show here in a matrigel angiogenesis assay that MSCs are capable of inhibiting capillary growth. After addition of MSCs to EC-derived capillaries in matrigel at EC: MSC ratio of 1:1, MSCs migrated toward the capillaries, intercalated between ECs, established Cx43-based intercellular gap junctional communication (GJC) with ECs, and increased production of reactive oxygen species (ROS). These events led to EC apoptosis and capillary degeneration. In an in vivo tumor model, direct MSC inoculation into subcutaneous melanomas induced apoptosis and abrogated tumor growth. Thus, our findings show for the first time that at high numbers, MSCs are potentially cytotoxic and that when injected locally in tumor tissue they might be effective antiangiogenesis agents suitable for cancer therapy. IntroductionIntense interest in the therapeutic application of bone marrowderived mesenchymal stem cells (MSCs) arises from the possibility that MSCs promote vascular repair. In animal models, intravenous injections of MSCs protected against heart failure by enhancing cardiac myocyte survival 1 and blocked lipopolysaccharide-induced acute lung injury by reducing total cell and proinflammatory cytokines in the lung. 2 In a collagen gel model, MSCs promoted survival of capillaries grown from human umbilical vein endothelial cells (HUVECs). 3 Despite these findings, the lack of conclusive evidence supporting a beneficial effect of MSCs in the clinical setting 4 indicates that mechanisms underlying MSC-endothelial cell (EC) interactions require better understanding.Several reports indicate that these interactions result from direct contact between MSCs and host cells. The MSC-induced responses include induction of gene transcription in ECs, 3 mitochondrial transfer in A549 cells, 5 and interleukin-10 (IL-10) secretion in alveolar macrophages. 6 In the context of tumor growth, MSCs recruit ECs to induce angiogenesis in stable tissue 7 as well as in tumors, 8 raising the possibility that MSCs might promote tumor growth. By contrast, intravenously injected MSCs are capable of abrogating growth of the Kaposi sarcoma, 9 suggesting that MSCs potentially possess cytotoxic properties. However, the mechanisms by which MSCs engage ECs are not understood and might involve gap junctional communication (GJC), as proposed for MSCcardiomycyte interactions. 10 Here, we addressed MSC-EC interactions in a capillary culture with the expectation that MSCs would enhance angiogenesis. However, surprisingly, addition of MSCs caused dose-dependent EC cytotoxicity that was attributable to the formation of MSC-EC GJC and the production of MSC-derived reactive oxygen species (ROS). The combined effect of these responses was capillary destruction. Further...

Infrared spectra of myeloid leukemia (ML-1) cells are reported for cells derived from an asynchronous, exponentially growing culture, as well as for cells that were fractionated according to their stage within the cell division cycle. The observed results suggest that the cells' DNA is detectable by infrared spectroscopy mainly when the cell is in the S phase, during the replication of DNA. In the G1 and G2 phases, the DNA is so tightly packed in the nucleus that it appears opaque to infrared radiation. Consequently, the nucleic acid spectral contributions in the G1 and G2 phases would be mostly that of cytoplasmic RNA. These results suggest that infrared spectral changes observed earlier between normal and abnormal cells may have been due to different distributions of cells within the stages of the cell division cycle.

Mitomycin C (MC) is a cytotoxic chemotherapeutic agent that causes DNA damage in the form of DNA crosslinks as well as a variety of DNA monoadducts and is known to induce p53. The various DNA adducts formed upon treatment of mouse mammary tumor cells with MC as well as 10-decarbamoyl MC (DMC) and 2,7-diaminomitosene (2,7-DAM), the major MC metabolite, have been elucidated. The cytotoxicity of DMC parallels closely that of MC in a number of rodent cell lines tested, whereas 2,7-DAM is relatively noncytotoxic. In this study, we investigate the ability of MC, DMC, and 2,7-DAM to activate p53 at equidose concentrations by treating tissue culture cell lines with the three mitomycins. Whereas MC and DMC induced p53 protein levels and increased the levels of p21 and Gadd45 mRNA, 2,7-DAM did not. Furthermore, MC and DMC, but not 2,7-DAM, were able to induce apoptosis efficiently in ML-1 cells. Therefore the 2,7-DAM monoadducts were unable to activate the p53 pathway. Interestingly, DMC was able to initiate apoptosis via a p53-independent pathway whereas MC was not. This is the first finding that adducts of a multiadduct type DNA-damaging agent are differentially recognized by DNA damage sensor pathways.

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