Both tumour-promoting and non-promoting phorbol esters inhibit [125I]EGF binding and stimulate the phosphorylation of an 80 kd protein kinase C substrate protein in intact quiescent swiss 3T3 cells.
Sapintoxin A (SAP A) and 12-deoxyphorbol 13-phenylacetate (DOPP), are two biologically active but non-tumour-promoting phorbol esters that potently bind to and activate the phorbol ester receptor, protein kinase C (PKC). SAP A and DOPP cause a dose-dependent increase in the phosphorylation of an 80 kd (80K) substrate protein for PKC in Swiss 3T3 cells. A similar dose-response effect was seen with sapintoxin D (SAP D), the stage 2 promoting analogue of 12-O-tetradecanoylphorbol-13-acetate and the complete promoter phorbol 12,13-dibutyrate (PDB). The doses resulting in a half maximal phosphorylation of this protein (Ka) were 20 nM (SAP A), 45 nM (DOPP), 23 nM (SAP D) and 37 nM (PDB). Both non-promoting and promoting phorbol esters induced a dose-dependent inhibition of [125I]epidermal growth factor (EGF) binding to its receptor in Swiss 3T3 cells. The doses required for 50% inhibition of binding (Ki) were: 8 nM (SAP A), 16 nM (DOPP), 14 nM (SAP D) and 17 nM (PDB). The results clearly demonstrate that induction of phosphorylation of the 80K phosphoprotein and inhibition of [125I]EGF binding in Swiss 3T3 cells following exposure to phorbol esters is independent of the tumour-promoting activity of these compounds. The fact that SAP A, DOPP, SAP D and PDB are mitogenic for a variety of cell types and that exposure to these compounds leads to 80K phosphorylation and inhibition of [125I]EGF binding, suggests that these early biological events may play a role in the mitogenic response induced by these compounds.
Brooks
G
G
Imperial Cancer Research Fund Laboratories, London, UK.
Brooks
S F
SF
D000818:Animals; D002460:Cell Line; D004815:Epidermal Growth Factor / Q000378:metabolism*; D051379:Mice; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology; D010703:Phorbol Esters / Q000494:pharmacology*; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*
D010703:Phorbol Esters; D010750:Phosphoproteins; D015240:Phorbol 12,13-Dibutyrate; C029704:12-deoxyphorbolphenylacetate; D004815:Epidermal Growth Factor; C032016:sapintoxin A; D011493:Protein Kinase C