Dependence receptors: the trophic theory revisited.

Most transmembrane receptors are believed to be inactive in the absence of their cognate ligand and initiate downstream signal transduction only upon ligand binding. However, research has shown that some receptors are also active in the absence of their ligand and induce a "negative" signal to trigger apoptosis. Thus, these receptors, dubbed dependence receptors, create a cellular state of dependence on ligand availability. The dependence receptor theory postulates key roles for these receptors during embryonic development, neurodegenerative diseases, or cancer progression and metastasis and is bolstered by new evidence.