Spatial constraints govern competition of mutant clones in human epidermis.

Deep sequencing can detect somatic DNA mutations in tissues permitting inference of clonal relationships. This has been applied to human epidermis, where sun exposure leads to the accumulation of mutations and an increased risk of skin cancer. However, previous studies have yielded conflicting conclusions about the relative importance of positive selection and neutral drift in clonal evolution. Here, we sequenced larger areas of skin than previously, focusing on cancer-prone skin spanning five decades of life. The mutant clones identified were too large to be accounted for solely by neutral drift. Rather, using mathematical modelling and computational lattice-based simulations, we show that observed clone size distributions can be explained by a combination of neutral drift and stochastic nucleation of mutations at the boundary of expanding mutant clones that have a competitive advantage. These findings demonstrate that spatial context and cell competition cooperate to determine the fate of a mutant stem cell.

TWEAK induces angiogenesis and proliferation of endothelial cells.

TWEAK is a recently described member of the Tumor Necrosis Factor (TNF) ligand family whose transcripts are present in a wide variety of human tissues (Chicheportiche, Y., Bourdon, P. R., Xu, H., Hsu Y. M., Scott, H., Hession, C., Garcia, I., and Browning, J. L. (1997) J. Biol. Chem. 272, 32401-32410). TWEAK is a weak inducer of apoptosis in transformed cells when administered with interferon-gamma or cycloheximide (Chicheportiche, Y., Bourdon, P. R., Xu, H., Hsu Y. M., Scott, H., Hession, C., Garcia, I., and Browning, J. L. (1997) J. Biol. Chem. 272, 32401-32410; Masters, S. A., Sheridan, J. P., Pitti, R. M., Brush, A. G., and Ashkenazi, A. (1998) Curr. Biol. 8, 525-528) and also promotes IL-8 secretion in cultured cells. We report here that picomolar concentrations of recombinant soluble TWEAK induce proliferation in a variety of normal human endothelial cells and in aortic smooth muscle cells and reduce culture requirements for serum and growth factors. Blocking antibodies to Vascular Endothelial Growth Factor (VEGF) do not significantly inhibit TWEAK-induced proliferation, indicating that TWEAK does not function indirectly through up-regulation of VEGF. Pellets containing TWEAK induce a strong angiogenic response when implanted in rat corneas, suggesting a role for TWEAK in vasculature formation in vivo.

Monocyte chemoattractant protein-2 is a potent agonist of CCR2B.

The binding and functional activity of the CC chemokines monocyte chemoattractant protein-1 (MCP-1), MCP-2, and MCP-3 have been characterized using Chinese hamster ovary DXB-11 cells transfected with the chemokine receptor CCR2B. Receptor binding studies demonstrated that 125I-labeled MCP-1 bound to a single class of high-affinity receptors with a Kd of 0.14 (0.07-0.32) nM. In competition studies MCP-1, MCP-2, and MCP-3 completely inhibited 125I-labeled MCP-1 binding with Ki values of 0.3 (0.16-0.46), 8.8 (3.4-26), and 12.2 (0.6-22) nM, respectively. In calcium mobilization studies, MCP-1 and MCP-3 induced robust elevations in intracellular calcium concentrations, whereas MCP-2 was only weakly active. In contrast, using changes in extracellular acidification rate as a functional readout, all three chemokines were identified as potent agonists of CCR2B. These data demonstrate that MCP-2, in addition to MCP-1 and MCP-3, is a potent agonist of CCR2B and furthermore that MCP-2 activates either different or a subset of the signaling pathways activated by MCP-1 and MCP-3.