| Cells sense, respond to, and integrate a multiplicity of signals from their environment. Although some of these signals may be mediated by cell-cell contact, in multicellular organisms many signal molecules, such as hormones, originate in organs distant from their site of action and must be carried in the blood to their target effector cells. Likewise, immune cells such as phagocytes are recruited from the blood to sites of inflammation by migrating along chemo-attractant gradients. Signals generated in these ways are sensed and processed by cellular signal transduction cassettes that comprise specific cell-surface membrane receptors, effector signaling elements, and regulatory proteins. These signaling cassettes serve to detect, amplify, and integrate diverse external signals to generate the appropriate cellular response (Fig. 38.1). In this chapter, we first discuss how cell-surface receptors sense and transduce their specific hormone signal by transmembrane coupling to effector enzyme systems, generating low-molecular-weight molecules termed second messengers. We then discuss the diversity of these second messengers and how they influence the activity of a range of key protein kinases with distinct substrates that ultimately determine the type of obtained biological response.
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