Apoptosis and dependence receptors: a molecular basis for cellular addiction.

Classical signal transduction is initiated by ligand-receptor interactions. We have described an alternative form of signal transduction that is initiated by the withdrawal of ligands from specific receptors referred to as dependence receptors. This process is widespread, featuring in developmental cell death, carcinogenesis (especially metastasis), neurodegeneration, and possibly subapoptotic events such as neurite retraction and somal atrophy. Initial mechanistic studies of dependence receptors suggest that these receptors form complexes that include specific caspases. Complex formation appears to be a function of ligand-receptor interaction, and dependence receptors appear to exist in at least two conformational states. Complex formation in the absence of ligand leads to caspase activation by a mechanism that in at least some cases is dependent on caspase cleavage of the receptor itself, releasing proapoptotic peptides. Thus these receptors may serve in caspase amplification, and in so doing create cellular states of dependence on their respective ligands.