title: Eyes wide shut: protein kinase C isozymes are not the only receptors for the phorbol ester tumor promoters.
abstract: In addition to the well-characterized interaction with classical and novel protein kinase C (PKC) isozymes, the phorbol ester tumor promoters bind to other receptors lacking kinase activity. Among these novel phorbol ester receptors, two families of proteins may play a role in the regulation of cell growth and malignant transformation: chimaerins and ras guanyl-releasing protein (ras-GRP). These proteins possess a single copy of the C1 domain that is involved in binding of phorbol esters and the lipid second messenger diacylglycerol. Four isoforms of chimaerins (alpha1-, alpha2-, beta1-, and beta2-chimaerins) have been isolated to-date, all of them possessing GTPase-activating protein activity for Rac, a small GTP-binding protein that controls actin cytoskeleton organization, cell-cycle progression, adhesion, and migration. Ras-GRP is a guanine nucleotide exchange factor for ras and promotes malignant transformation in fibroblasts in a phorbol ester-dependent manner. The C1 domain in Ras-GRP may, therefore, have a dominant role in Ras-GRP activation and is essential for phorbol ester-dependent activation of downstream effectors of ras, i.e., the mitogen-activated protein kinase cascade. Thus, a novel concept emerges in which phorbol esters may exert cellular responses through pathways not involving phorbol ester-responsive PKC isozymes. The discovery of "nonPKC" phorbol ester receptors adds an additional level of complexity to the understanding of phorbol ester effects and the molecular mechanisms of carcinogenesis.
authors.affiliation: Center for Experimental Therapeutics and Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104-6160, USA.
authors.forename: M G
authors.initials: MG
authors.lastname: Kazanietz
mesh_terms: D000818:Animals; D002273:Carcinogens / Q000633:toxicity; D002471:Cell Transformation, Neoplastic / Q000187:drug effects; D004268:DNA-Binding Proteins / Q000502:physiology; D020662:Guanine Nucleotide Exchange Factors; D006801:Humans; D007527:Isoenzymes / Q000502:physiology; D010703:Phorbol Esters / Q000633:toxicity; D011493:Protein Kinase C / Q000502:physiology*; D011956:Receptors, Cell Surface / Q000502:physiology*; D015398:Signal Transduction
chemical_list: D002273:Carcinogens; D004268:DNA-Binding Proteins; D020662:Guanine Nucleotide Exchange Factors; D007527:Isoenzymes; D010703:Phorbol Esters; C491790:RASGRP1 protein, human; D011956:Receptors, Cell Surface; D011493:Protein Kinase C