Phorbol esters as signal transducers and tumor promoters.

Extracellular ligands transfer information into the cell through various signaling pathways which operate in an integrated way. Oncogene proteins and tumor promoters cooperate to constitutively turn on the signaling pathways and lead to irreversible cell activation. The evidence that supports this concept is reviewed, with emphasis on the role of phorbol esters and protein kinase C in signal transduction and oncogenesis.