As described earlier in this chapter, the receptors for lipid-soluble messengers, once activated by hormone binding, act in the nucleus as transcription factors to increase or decrease the rate of gene transcription. We now emphasize that there are many other transcription factors inside cells and that the signal transduc-tion pathways initiated by plasma-membrane receptors
Homeostatic Mechanisms and Cellular Communication CHAPTER SEVEN
often result in the activation, by phosphorylation, of these transcription factors. Thus, many first messengers that bind to plasma-membrane receptors can also alter gene transcription via second messengers. For example, at least three of the proteins phosphorylated by cAMP-dependent protein kinase function as transcription factors.
Some of the genes influenced by transcription factors activated in response to first messengers are known collectively as primary response genes, or PRGs (also termed immediate-early genes). In many cases, especially those involving first messengers that influence the proliferation or differentiation of their target cells, the story does not stop with a PRG and the protein it encodes. In these cases, the protein encoded by the PRG is itself a transcription factor for other genes (Figure 7-20). Thus, an initial transcription factor activated in the signal transduction pathway causes the synthesis of a different transcription factor, which in turn causes the synthesis of additional proteins, ones particularly important for the long-term biochemical events required for cellular proliferation and differentiation. A great deal of research is being done on the transcription factors encoded by PRGs because of their relevance for the abnormal growth and differentiation typical of cancer.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.