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Mitogenic inhibition by phorbol esters is associated with decreased phosphatidylinositol-3 kinase activation
Abstract:
In contrast to their role as potent tumor promoters, phorbol esters can cause inhibition of cell growth. Because the effect of phorbol esters occurs through activation of protein kinase C (PKC) and because activated PKC is translocated to the membrane placing it in a position to act on the intracellular portion of the growth factor receptor, we asked whether this inhibitory effect is mediated through the action of phorbol 12-myristate 13-acetate (PMA) on receptor association with the signal transfer proteins. When added to rat vascular smooth muscle (VSM) cells concurrently with basic fibroblast growth factor (bFGF), PMA at 100 ng/ml completely inhibits bFGF-stimulated DNA synthesis. Under the same growth-inhibitory conditions of PMA addition, aggregation of phosphatidylinositol 3-kinase (PI3K) to the fibroblast growth factor receptor and tyrosine phosphorylation of the 85-kDa regulatory component of the signal transfer protein PI3K are reduced by 94 and 79%, respectively. PI3K catalytic activity, as measured by conversion of phosphatidylinositol to phosphatidylinositol 3-phosphate, is decreased 88% by PMA addition. This effect is not specific to PI3K, since aggregation of phospholipase C-gamma 1 to the activated bFGF receptor is also decreased by PMA treatment. In addition, the PI3K inhibitor wortmannin markedly attenuates bFGF-stimulated VSM cell growth in a dose-dependent manner. These data suggest that the site of growth inhibition by PMA in VSM cells lies upstream of signal transfer particle aggregation and that such growth arrest may be mediated through inhibition of activation of PI3K.
Insights
Phorbol 12-myristate 13-acetate (PMA) inhibits vascular smooth muscle cell growth by blocking fibroblast growth factor receptor signaling. PMA reduces phosphatidylinositol 3-kinase activation, a key step in cell proliferation.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Phorbol esters, like phorbol 12-myristate 13-acetate (PMA), are known tumor promoters but can also inhibit cell growth.
- Protein kinase C (PKC) activation by phorbol esters leads to its translocation to the cell membrane, where it can interact with growth factor receptors.
- The precise mechanism by which PMA inhibits cell growth, particularly its effect on signal transduction pathways, requires further elucidation.
Purpose of the Study:
- To investigate whether the growth inhibitory effect of PMA in vascular smooth muscle (VSM) cells is mediated by its action on receptor association with signal transfer proteins.
- To determine the impact of PMA on the activation of key signaling molecules downstream of the fibroblast growth factor receptor (FGFR).
Main Methods:
- Rat vascular smooth muscle (VSM) cells were treated with PMA concurrently with basic fibroblast growth factor (bFGF).
- Analysis of bFGF-stimulated DNA synthesis, receptor-associated phosphatidylinositol 3-kinase (PI3K) aggregation, and tyrosine phosphorylation of PI3K.
- Measurement of PI3K catalytic activity and phospholipase C-gamma 1 (PLCγ1) aggregation to the FGFR.
- Assessment of the effect of the PI3K inhibitor wortmannin on bFGF-stimulated VSM cell growth.
Main Results:
- PMA (100 ng/ml) completely inhibited bFGF-stimulated DNA synthesis in VSM cells.
- PMA significantly reduced the aggregation of PI3K to the FGFR (94%) and tyrosine phosphorylation of PI3K (79%).
- PI3K catalytic activity was decreased by 88% in the presence of PMA, and PLCγ1 aggregation was also reduced.
- Wortmannin, a PI3K inhibitor, attenuated bFGF-stimulated VSM cell growth in a dose-dependent manner.
Conclusions:
- The growth inhibitory effect of PMA in VSM cells occurs upstream of signal transfer protein aggregation.
- PMA-induced growth arrest is likely mediated by the inhibition of phosphatidylinositol 3-kinase (PI3K) activation.
- These findings highlight a novel mechanism by which phorbol esters can suppress cell proliferation via modulation of FGFR-PI3K signaling.