Introduction and Overview of Receptor Systems

Receptors are defined as proteins that recognize a molecule or molecules (ligands) with some degree of specificity to initiate a signaling cascade that couples to secondary messenger systems. These messenger systems 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. The 'business end' of virtually all receptor systems is often mediated by modulation of DNA and ultimately regulation of protein/cellular functions. This introductory chapter provides highlights of 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. As appropriate, the molecular biology and human disease implications will be emphasized.