Overview of Receptor Systems

Receptors are defined as proteins that recognize a molecule or molecules (ligands) with some degree of specificity to initiate a biochemical signaling cascade that couples to secondary messenger systems. Receptor molecules transduce a signal from the exterior of the cell to the intracellular compartment, or reside within the cytosolic or nuclear compartment and convey signals that regulate gene expression. As such, receptor systems play critical roles in the regulation of development, cellular homeostasis, and disease onset and progression, to list a few. The ‘business end’ of virtually all receptor systems is often mediated by modulation of DNA functions and ultimately the regulation of protein/cellular functions. This article highlights major receptor systems that can be regulated by both endogenous and exogenous ligands, with a primary focus on xenobiotic receptor systems (XRSs) that modulate transcriptional events critical to the understanding the biology of the toxicological response. As appropriate, the molecular biology and human disease implications will be emphasized.