Transcriptional regulation by extracellular signals: mechanisms and specificity

GATA-1 reprograms avian myelomonocytic cell lines into eosinophils, thromboblasts, and erythroblasts

The transcription factor GATA-1 is expressed in early hematopoietic progenitors and specifically down-regulated in myelomonocytic cells during lineage determination. Our earlier observation that the differentiation of Myb-Ets-transformed chicken hematopoietic progenitors into myeloblasts likewise involves a GATA-1 down-regulation, whereas expression is maintained in erythroid, thrombocytic, and eosinophilic derivatives, prompted us to study the effect of forced GATA-1 expression in Myb-Ets-transformed myeloblasts. We found that the factor rapidly suppresses myelomonocytic markers and induces a reprogramming of myeloblasts into cells resembling either transformed eosinophils or thromboblasts. In addition, we observed a correlation between the level of GATA-1 expression and the phenotype of the cell, intermediate levels of the factor being expressed by eosinophils and high levels by thromboblasts, suggesting a dosage effect of the factor. GATA-1 can also induce the formation of erythroblasts when expressed in a myelomonocytic cell line transformed with a Myb-Ets mutant containing a lesion in Ets. These cells mature into erythrocytes following temperature-inactivation of the Ets protein. Finally, the factor can reprogram a v-Myc-transformed macrophage cell line into myeloblasts, eosinophils, and erythroblasts, showing that the effects of GATA-1 are not limited to Myb-Ets-transformed myeloblasts. Our results suggest that GATA-1 is a lineage-determining transcription factor in transformed hematopoietic cells, which not only activates lineage-specific genetic programs but also suppresses myelomonocytic differentiation. They also point to a high degree of plasticity of transformed hematopoietic cells.

Conversion of Xenopus ectoderm into neurons by NeuroD, a basic helix-loop-helix protein

Basic helix-loop-helix (bHLH) proteins are instrumental in determining cell type during development. A bHLH protein, termed NeuroD, for neurogenic differentiation, has now been identified as a differentiation factor for neurogenesis because (i) it is expressed transiently in a subset of neurons in the central and peripheral nervous systems at the time of their terminal differentiation into mature neurons and (ii) ectopic expression of neuroD in Xenopus embryos causes premature differentiation of neuronal precursors. Furthermore, neuroD can convert presumptive epidermal cells into neurons and also act as a neuronal determination gene. However, unlike another previously identified proneural gene (XASH-3), neuroD seems competent to bypass the normal inhibitory influences that usually prevent neurogenesis in ventral and lateral ectoderm and is capable of converting most of the embryonic ectoderm into neurons. The data suggest that neuroD may participate in the terminal differentiation step during vertebrate neuronal development.

Raf1 interaction with Cdc25 phosphatase ties mitogenic signal transduction to cell cycle activation

The Ras and Raf1 proto-oncogenes transduce extracellular signals that promote cell growth. Cdc25 phosphatases activate the cell division cycle by dephosphorylation of critical threonine and tyrosine residues within the cyclin-dependent kinases. We show here that Cdc25 phosphatase associates with raf1 in somatic mammalian cells and in meiotic frog oocytes. Furthermore, Cdc25 phosphatase can be activated in vitro in a Raf1-dependent manner. We suggest that activation of the cell cycle by the Ras/Raf1 pathways might be mediated in part by Cdc25.

Integrins and signal transduction pathways: the road taken

Adhesive interactions play critical roles in directing the migration, proliferation, and differentiation of cells; aberrations in such interactions can lead to pathological disorders. These adhesive interactions, mediated by cell surface receptors that bind to ligands on adjacent cells or in the extracellular matrix, also regulate intracellular signal transduction pathways that control adhesion-induced changes in cell physiology. Though the extracellular molecular interactions involving many adhesion receptors have been well characterized, the adhesion-dependent intracellular signaling events that regulate these physiological alterations have only begun to be elucidated. This article will focus on recent advances in our understanding of intracellular signal transduction pathways regulated by the integrin family of adhesion receptors.

Specificity of receptor tyrosine kinase signaling: transient versus sustained extracellular signal-regulated kinase activation

A number of different intracellular signaling pathways have been shown to be activated by receptor tyrosine kinases. These activation events include the phosphoinositide 3-kinase, 70 kDa S6 kinase, mitogen-activated protein kinase (MAPK), phospholipase C-gamma, and the Jak/STAT pathways. The precise role of each of these pathways in cell signaling remains to be resolved, but studies on the differentiation of mammalian PC12 cells in tissue culture and the genetics of cell fate determination in Drosophila and Caenorhabditis suggest that the extracellular signal-regulated kinase (ERK-regulated) MAPK pathway may be sufficient for these cellular responses. Experiments with PC12 cells also suggest that the duration of ERK activation is critical for cell signaling decisions.