Cultured human embryonic stem (hES) cells can acquire genetic and epigenetic changes that make them vulnerable to transformation. As hES cells with cancer-cell characteristics share properties with normal hES cells, such as self-renewal, teratoma formation and the expression of pluripotency markers, they may be misconstrued as superior hES cells with enhanced 'stemness'. We characterize two variant hES cell lines (v-hESC-1 and v-hESC-2) that express pluripotency markers at high levels and do not harbor chromosomal abnormalities by standard cytogenetic measures. We show that the two lines possess some features of neoplastic progression, including a high proliferative capacity, growth-factor independence, a 9- to 20-fold increase in frequency of tumor-initiating cells, niche independence and aberrant lineage specification, although they are not malignant. Array comparative genomic hybridization reveals an amplification at 20q11.1-11.2 in v-hESC-1 and a deletion at 5q34a-5q34b;5q3 and a mosaic gain of chromosome 12 in v-hESC-2. These results emphasize the need for functional characterization to distinguish partially transformed and normal hES cells.

No abstract

The synthases that produce fatty acids in mammals (FASs) are arranged as large multidomain polypeptides. The growing fatty acid chain is bound covalently during chain elongation and reduction to the acyl carrier protein (ACP) domain that is then able to access each catalytic site. In this work we report the highresolution nuclear magnetic resonance (NMR) solution structure of the isolated rat fatty acid synthase apoACP domain. Fatty acid biosynthesis in mammals is important not only for energy homeostasis and development but also as a potential target for the treatment of obesity (1) and cancer (2). Type I fatty acid synthases (FASs) 3 that catalyze the biosynthesis of fatty acids in mammals, utilize simple acyl units, bound to the phosphopantetheine arm of a holo acyl carrier protein (ACP) domain, for chain initiation and elongation. The recent elucidation of the low resolution structure of the mammalian FAS by x-ray crystallography (3) has provided key structural and mechanistic insights into this important enzyme. Although the resolution of the crystal is insufficient to discern the backbone and side chains, the electron density has permitted the authors to propose a model based on the structures of individual domains and homologous enzymes. The authors have proposed a "head to head" dimeric model that comprises a central core consisting of the enol-reductase, dehydratase (DH), and ketosynthase with the malonyl transferase and ketoreductase domains being located peripherally. Dimerization occurs through association of the KS domains. Notable absences in the crystal structure are the locations of the peripheral ACP and thioesterase domains, suggesting that the positions of the ACP and thioesterase domains are relatively mobile compared with the core of the FAS. In comparison, bacterial Type II FASs consist of discrete monofunctional proteins (4). Structural studies of the Type II FAS ACP components are particularly well developed and have revealed the structural basis of acyl chain binding and the phenomenon of conformational switching. The crystal structures of Escherichia coli FAS butyryl (5), hexanoyl-, heptanoyl-, and decanoyl-ACPs (6) and nuclear magnetic resonance (NMR) structures of spinach FAS decanoyl-and stearoyl-ACPs have been reported (7). These structures reveal that during fatty acid biosynthesis, fully saturated acyl chains are sequestered within a central cavity in the ACP formed through conformational changes in the protein. The fatty acid chain is sequestered within the hydrophobic core of the ACP perhaps to protect the thioester moiety from hydrolysis. Binding of the acyl chain also influences the dynamics of the ACPs. Spinach FAS holo-ACP exists in equilibrium between a folded and largely disordered form, however, upon acylation this equilibrium is shifted toward the folded form. At present the physiological role of switching of this, and other ACPs, is unknown (8, 9). It has been suggested that switching confers allosteric regulation of the ACP, whereby its interaction with other enzymes...

When bone marrow stem cells were differentiated into chondrocytes, adipocytes, or osteocytes, nucleostemin expression was 70%-90% lower than in the undifferentiated cells retained in monolayer culture. We conclude that nucleostemin is a marker of undifferentiated human adult bone marrow stem cells and that it is involved in the regulation of proliferation of these cells. STEM CELLS