Abstract
The cell-permeable diacylglycerol, sn-1,2-dioctanoylglycerol (DiC8), is shown to mimic the effect of tumor promoting phorbol diesters on epidermal growth factor (EGF) binding and action in intact cells. DiC8 inhibited the binding of [3H]phorbol dibutyrate to A431 cell monolayers indicating that the diacylglycerol interacts with the phorbol diester receptor. At 0.3 microM, DiC8 half-maximally inhibited the high affinity binding of 125I-EGF to A431 human epidermoid carcinoma cells. Scatchard analysis indicated that the inhibition of 125I-EGF binding was very similar to that observed in the presence of 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA). DiC8 also mimicked the action of PMA to increase the phosphorylation state of the EGF receptor in 32P-labeled cells. Phosphoamino acid analysis demonstrated that DiC8 and PMA caused an increase in the level of EGF-receptor phosphoserine and phosphothreonine, whereas EGF caused an increase in the level of phosphoserine, phosphothreonine, and phosphotyrosine. Phosphopeptide mapping of the EGF receptor showed that DiC8 and PMA enhanced the phosphorylation of the same tryptic peptides. DiC8 inhibited the EGF-dependent tyrosine phosphorylation of the EGF receptor in A431 cells in a similar manner to that observed with PMA. In further experiments with quiescent Swiss 3T3 fibroblasts, DiC8 mimicked the ability of PMA to stimulate the incorporation of [methyl-3H]thymidine synergistically with low concentrations of EGF. This result indicates that DiC8 will mimic the long-term effects of PMA to regulate mitogenesis and raises the possibility that it may be active in two stage carcinogenesis. As both DiC8 and PMA stimulate the Ca2+- and phospholipid-dependent protein kinase (C-kinase) in vitro, the results support the hypothesis that the activation of C-kinase is a critical component of phorbol diester action on EGF receptor modulation and cell proliferation.
Highlights
A CELL-PERMEABLE DIACYLGLYCEROL THAT MIMICS PHORBOL DIESTER ACTION ON THE EP~DERMAL G R O ~ F~AHCTOR RECEPTOR AND MITOGENESIS*
This hypothesis predicts that exogenously added diacylnoma cells.Scatchard analysisindicated that theinhi- glycerols that can permeate cells should mimic the effects of bition of 121-epidermal growth factor (EGF) binding was very similar to that observed in the presenceof 4B-phorbol 12B-myristate
Phosphoamino acid analysis demonstrated that DiCS and PMA caused an increase in thelevel of EGF-receptor phosphoserine and phorbol diesters
Summary
A CELL-PERMEABLE DIACYLGLYCEROL THAT MIMICS PHORBOL DIESTER ACTION ON THE EP~DERMAL G R O ~ F~AHCTOR RECEPTOR AND MITOGENESIS*. The hypothesis has been proposed that many of the effects of phorbol diester p ~ D,iC8 half-maximally inhibited the high affinity tumor promoters are caused by the stimulation of C-kinase binding of '"I-EGF to A431human epidermoid carci- [7].
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