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Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000.

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Developmental Biology. 6th edition.

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Cross-Talk between Pathways

We have been representing the major signal transduction pathways as if they were linear chains, through which information flows in a single conduit. However, these pathways are just the major highways of information flow. Between them, avenues and streets connect one pathway with another. (This may be why there are so many steps between the cell surface and the nucleus. Each step is a potential regulatory point as well as a potential intersection.) This cross-talk can be seen in numerous tissues, wherein two signaling pathways reinforce each other. We must remember that a cell has numerous receptors and is constantly receiving many signals simultaneously.

In some cells, gene transcription requires two signals. This pattern is seen during lymphocyte differentiation, for which two signals are needed, each one producing one of the two peptides of a transcription factor needed for the production of interleukin 2 (IL-2, also known as T cell growth factor). One peptide, c-Fos, is produced by the binding of the T cell receptor to an antigen (Figure 6.40). This signal activates the Ras pathway, creating a transcription factor, Elk-1, that activates the c-fos gene to synthesize c-Fos. The second signal comes from the B7 glycoprotein on the surface of the cell presenting the antigen. This signal activates a second cascade of kinases, eventually producing c-Jun. The two peptides, c-Fos and c-Jun, can join to make the AP-1 protein, a transcription factor that binds to the IL-2 enhancer and activates its expression (Li et al. 1996).

Figure 6.40. Two signals are needed to effect the differentiation of the T lymphocyte.

Figure 6.40

Two signals are needed to effect the differentiation of the T lymphocyte. The first signal comes from the receptors that bind the antigen. The second signal comes from the binding of the CD28 protein to the B7 protein on the surface of the antigen-presenting (more...)

We also have seen that one receptor can activate several different pathways. The fibroblast growth factors, for instance, can activate the RTK pathway, the STAT pathway, or even a third pathway that involves lipid turnover and increases the levels of calcium ions in the cell.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, Sinauer Associates.
Bookshelf ID: NBK10087

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