Basic Hormone Action

Introduction
  • Hormones coordinate their physiological effects by changing the behavior of target cells through a number of mechanisms. These include changes to the membrane permeability of different ions, changes to the activities of intracellular enzymes, and changes to the transcription of genes. What all of these changes require is that the presence of the hormone result in modifications to factors within target cell. Therefore, the primary question of hormone action is how the presence of a hormone can modify factors within target cell.
Hormone Receptors
  • All hormones modify intracellular functions by first binding to a specific hormone receptor. After binding its cognate hormone, the hormone receptor undergoes a change in activity. Any changes in target cell membrane permeability, intracellular enzyme activity, or transcription that result from the presence of a hormone are ultimately the direct result of the action of the hormone-bound hormone receptor within the target cell. However, a wide variety of structurally diverse hormone receptors exist which employ vastly different strategies in changing cellular behavior. Roughly, hormone receptors can be divided into two classes, Membrane Hormone Receptors and Nuclear Receptors.
Membrane Hormone Receptors
  • Polar hormones such as peptide hormones and hydrophilic amine hormones, cannot cross the cellular plasma membrane. Consequently, their cognate hormone receptors must be membrane proteins which can bind the extracellular hormone. This poses a mechanistic problem as membrane proteins are stuck in the membrane and thus have a geographically limited reach within the cell; for example, how can a membrane receptor modify gene transcription which occurs within the nucleus.
  • The molecular solution to this issue is provided by Second Messenger systems which allow communication of membrane-hormone receptors with the rest of the cell. Second messengers are considered to be intermediate molecules whose activity or molecular composition changes when membrane-bound receptors are bound by their cognate hormones. These second messengers are typically soluble molecules which can diffuse throughout the entire cell and can thus communicate the signal of hormone binding to membrane-bound receptors throughout the cell. Second messenger systems also possess complex regulatory networks which ensure that the signals generated by hormone binding are sufficient to cause cellular behavior changes but not result in hyperactive responses that can lead to pathology. Second messenger systems are diverse and complex; consequently, a comprehensive discussion of these systems even for the hormones discussed is far beyond the scope of this work.
Nuclear Receptor Systems
  • In some cases, hormones are hydrophobic and thus can cross the cellular plasma membrane. The primary hormones that display this property are the steroid hormones and thyroid hormones. In such cases, the hormone receptors, known as Nuclear Receptors, are soluble proteins present within the cytosol or nucleus of their target cells. Since these nuclear receptors can access the entire geography of the target cells, they generally affect processes such as gene transcription directly and not via second messenger systems.