Tumor cells, with stem-like properties, are highly aggressive and often display drug resistance. Here, we reveal that integrin αvβ3 serves as a marker of breast, lung, and pancreatic carcinomas with stem-like properties that are highly resistant to receptor tyrosine kinase inhibitors such as erlotinib. This was observed in vitro and in mice bearing patient-derived tumor xenografts or in clinical specimens from lung cancer patients that had progressed on erlotinib. Mechanistically, αvβ3, in the unligated state, recruits KRAS and RalB to the tumor cell plasma membrane, leading to the activation of TBK-1/NFκB. In fact, αvβ3 expression and the resulting KRAS/RalB/NFκB pathway were both necessary and sufficient for tumor initiation, anchorage-independence, self-renewal, and erlotinib resistance. Pharmacological targeting of this pathway with Bortezomib reversed both tumor stemness and erlotinib resistance. These findings not only identify αvβ3 as a marker/driver of carcinoma stemness but they reveal a therapeutic strategy to sensitize such tumors to RTK inhibition.

Human embryonic stem cells (hESCs) hold great promise for cell-based therapies and drug screening applications. However, growing and processing large quantities of undifferentiated hESCs is a challenging task. Conventionally, hESCs are passaged as clusters, which can limit their growth efficiency and use in downstream applications. This study demonstrates that hESCs can be passaged as single cells using Accutase, a formulated mixture of digestive enzymes. In contrast to trypsin treatment, Accutase treatment does not significantly affect the viability and proliferation rate of hESC dissociation into single cells. Accutase-dissociated single cells can be separated by FACS and proliferate as fully pluripotent hESCs. An Oct4-eGFP reporter construct engineered into hESCs was used to monitor the pluripotency of hESCs passaged with Accutase. Compared to collagenase-passaged hESCs, Accutase-treated cultures contained a larger proportion of undifferentiated (Oct4-positive) cells. Additionally, Accutase-passaged undifferentiated hESCs could be grown as monolayers without the need for monitoring and/or selection for quality hESC colonies.

The ErbB2 (Neu) receptor tyrosine kinase is frequently overexpressed in human breast cancers, and this phenotype correlates with a poor clinical prognosis. We examined the effects of the mammalian target of rapamycin inhibitor, rapamycin, on mammary tumorigenesis in transgenic mice bearing an activated ErbB2 (NeuYD) transgene in the absence or presence of a second transgene encoding vascular endothelial growth factor (VEGF). Treatment of NeuYD or NeuYD Â VEGF mice with rapamycin dramatically inhibited tumor growth accompanied by a marked decrease in tumor vascularization. Two key events that may underlie the antitumor activity of rapamycin were decreased expression of ErbB3 and inhibition of hypoxia-inducible factor-1-dependent responses to hypoxic stress. Rapamycin exposure caused only a modest inhibition of the proliferation of tumor-derived cell lines in standard monolayer cultures, but dramatically inhibited the growth of the same cells in three-dimensional cultures, due in part to the induction of apoptotic cell death. These studies underscore the therapeutic potential of mammalian target of rapamycin inhibitors in ErbB2-positive breast cancers and indicate that, relative to monolayer cultures, three-dimensional cell cultures are more predictive in vitro models for studies of the antitumor mechanisms of rapamycin and related compounds. (Cancer Res 2005; 65(12): 5325-36)

Maternal Embryonic Leucine zipper Kinase (MELK) is expressed in several developing tissues, in the adult germ line, and in adult neural progenitors. MELK expression is elevated in aggressive undifferentiated tumors correlating with poor patient outcome in human breast cancer. To investigate the role of MELK in mammary tumorigenesis in vivo we used a MELK-GFP reporter mouse, which allows prospective isolation of MELK expressing cells based on GFP fluorescence. We found that in the normal mammary gland, cells expressing high levels of MELK were enriched for proliferating cells, expressing markers of mammary progenitors. The isolation of cells with high levels of MELK in mammary tumors from MMTV-Wnt1/MELK-GFP bitransgenic mice resulted in a significant enrichment of tumorsphere formation in culture and tumor initiation after transplantation into mammary fat pads of syngeneic mice. Furthermore, using lentiviral delivery of MELK-specific shRNA and limiting dilution cell transplantations we demonstrated that MELK function is required for mammary tumorigenesis in vivo. Our findings identify MELK as potential target in breast tumor initiating cells.