mTOR-dependent cell survival mechanisms.

The mechanistic target of rapamycin (mTOR) kinase is a conserved regulator of cell growth, proliferation, and survival. In cells, mTOR is the catalytic subunit of two complexes called mTORC1 and mTORC2, which have distinct upstream regulatory signals and downstream substrates. mTORC1 directly senses cellular nutrient availability while indirectly sensing circulating nutrients through growth factor signaling pathways. Cellular stresses that restrict growth also impinge on mTORC1 activity. mTORC2 is less well understood and appears only to sense growth factors. As an integrator of diverse growth regulatory signals, mTOR evolved to be a central signaling hub for controlling cellular metabolism and energy homoeostasis, and defects in mTOR signaling are important in the pathologies of cancer, diabetes, and aging. Here we discuss mechanisms by which each mTOR complex might regulate cell survival in response to metabolic and other stresses.

PTEN loss in the Myf5 lineage redistributes body fat and reveals subsets of white adipocytes that arise from Myf5 precursors.

The developmental origin of adipose tissue and what controls its distribution is poorly understood. By lineage tracing and gene expression analysis in mice, we provide evidence that mesenchymal precursors expressing Myf5--which are thought to give rise only to brown adipocytes and skeletal muscle--also give rise to a subset of white adipocytes. Furthermore, individual brown and white fats contain a mixture of adipocyte progenitor cells derived from Myf5(+) and Myf5(neg) lineages, the number of which varies with depot location. Subsets of white adipocytes originating from both Myf5(+) and Myf5(neg) precursors respond to ß(3)-adrenoreceptor stimulation, suggesting "brite" adipocytes may also have multiple origins. We additionally find that deleting PTEN with myf5-cre causes lipomatosis and partial lipodystrophy by selectively expanding the Myf5(+) adipocyte lineages. Thus, the spectrum of adipocytes arising from Myf5(+) precursors is broader than previously thought, and differences in PI3K activity between adipocyte lineages alter body fat distribution.

Sarcomas induced in discrete subsets of prospectively isolated skeletal muscle cells.

Soft-tissue sarcomas are heterogeneous cancers that can present with tissue-specific differentiation markers. To examine the cellular basis for this histopathological variation and to identify sarcoma-relevant molecular pathways, we generated a chimeric mouse model in which sarcoma-associated genetic lesions can be introduced into discrete, muscle-resident myogenic and mesenchymal cell lineages. Expression of Kirsten rat sarcoma viral oncogene [Kras(G12V)] and disruption of cyclin-dependent kinase inhibitor 2A (CDKN2A; p16p19) in prospectively isolated satellite cells gave rise to pleomorphic rhabdomyosarcomas (MyoD-, Myogenin- and Desmin-positive), whereas introduction of the same oncogenetic hits in nonmyogenic progenitors induced pleomorphic sarcomas lacking myogenic features. Transcriptional profiling demonstrated that myogenic and nonmyogenic Kras; p16p19(null) sarcomas recapitulate gene-expression signatures of human rhabdomyosarcomas and identified a cluster of genes that is concordantly up-regulated in both mouse and human sarcomas. This cluster includes genes associated with Ras and mechanistic target of rapamycin (mTOR) signaling, a finding consistent with activation of the Ras and mTOR pathways both in Kras; p16p19(null) sarcomas and in 26-50% of human rhabdomyosarcomas surveyed. Moreover, chemical inhibition of Ras or mTOR signaling arrested the growth of mouse Kras; p16p19(null) sarcomas and of human rhabdomyosarcoma cells in vitro and in vivo. Taken together, these data demonstrate the critical importance of lineage commitment within the tumor cell-of-origin in determining sarcoma histotype and introduce an experimental platform for rapid dissection of sarcoma-relevant cellular and molecular events.

DNA damage-induced cell death is enhanced by progression through mitosis.

Progression through the G(2)/M transition following DNA damage is linked to cytokinesis failure and mitotic death. In four different transformed cell lines and two human embryonic stem cell lines, we find that DNA damage triggers mitotic chromatin decondensation and global phosphorylation of histone H2AX, which has been associated with apoptosis. However, extended time-lapse studies in HCT116 colorectal cancer cells indicate that death does not take place during mitosis, but 72% of cells die within 3 days of mitotic exit. By contrast, only 11% of cells in the same cultures that remained in interphase died, suggesting that progression through mitosis enhances cell death following DNA damage. These time-lapse studies also confirmed that DNA damage leads to high rates of cytokinesis failure, but showed that cells that completed cytokinesis following damage died at higher rates than cells that failed to complete division. Therefore, post-mitotic cell death is not a response to cytokinesis failure or polyploidy. We also show that post-mitotic cell death is largely independent of p53 and is only partially suppressed by the apical caspase inhibitor Z-VAD-FMK. These findings suggest that progression through mitosis following DNA damage initiates a p53- and caspase-independent cell death response that prevents propagation of genetic lesions.