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Inhibition of cell proliferation by alpha-tocopherol. Role of protein kinase C
D Boscoboinik1, A Szewczyk, C Hensey
1Institut für Biochemie und Molekularbiologie, Universität Bern, Switzerland.
Abstract:
The effect of alpha-tocopherol (vitamin E) on the proliferation of vascular smooth muscle cells (A7r5), human osteosarcoma cells (Saos-2), fibroblasts (Balb/3T3), and neuroblastoma cells (NB2A) has been studied. The proliferation of vascular smooth muscle cells was inhibited by physiologically relevant concentrations of alpha-tocopherol, neuroblastoma cells were only sensitive to higher alpha-tocopherol concentrations, and proliferation of the other cell lines was not inhibited. The inhibition of smooth muscle cell proliferation was specific for alpha-tocopherol. Trolox, phytol, and alpha-tocopherol esters had no effect. Proliferation of smooth muscle cells stimulated by platelet-derived growth factor or endothelin was completely sensitive to alpha-tocopherol. If smooth muscle cells were stimulated by fetal calf serum, proliferation was 50% inhibited by alpha-tocopherol. No effect of alpha-tocopherol was observed when proliferation of smooth muscle cells was stimulated by bombesin and lysophosphatidic acid. The possibility of an involvement of protein kinase C in the cell response to alpha-tocopherol was suggested by experiments with the isolated enzyme and supported by the 2- to 3-fold stimulation of phorbol ester binding induced by alpha-tocopherol in sensitive cells. Moreover, alpha-tocopherol also caused inhibition of protein kinase C translocation induced by phorbol esters and inhibition of the phosphorylation of its 80-kDa protein substrate in smooth muscle cells. A model is discussed by which alpha-tocopherol inhibits cell proliferation by interacting with the cytosolic protein kinase C, thus preventing its membrane translocation and activation.
Insights
Alpha-tocopherol (vitamin E) inhibits vascular smooth muscle cell proliferation, but not other cell types. This vitamin E effect involves protein kinase C inhibition, impacting cell growth signaling pathways.
Area of Science:
- Cell Biology
- Biochemistry
- Pharmacology
Background:
- Alpha-tocopherol, a form of vitamin E, is an antioxidant with potential roles beyond free radical scavenging.
- Vascular smooth muscle cell (VSMC) proliferation is a key factor in cardiovascular diseases.
- The specific cellular mechanisms of alpha-tocopherol's non-antioxidant effects are not fully understood.
Purpose of the Study:
- To investigate the effect of alpha-tocopherol on the proliferation of various cell lines, with a focus on VSMCs.
- To determine the specificity of alpha-tocopherol's inhibitory effects and its interaction with growth factor-stimulated proliferation.
- To explore the potential involvement of protein kinase C (PKC) in mediating alpha-tocopherol's cellular responses.
Main Methods:
- Cell proliferation assays were performed on VSMCs (A7r5), osteosarcoma cells (Saos-2), fibroblasts (Balb/3T3), and neuroblastoma cells (NB2A).
- The effects of alpha-tocopherol, Trolox, phytol, and alpha-tocopherol esters were tested on basal and stimulated cell proliferation (using platelet-derived growth factor, endothelin, fetal calf serum, bombesin, and lysophosphatidic acid).
- Experiments involving isolated protein kinase C, phorbol ester binding assays, and Western blotting were used to assess PKC activity and translocation.
Main Results:
- Alpha-tocopherol inhibited VSMC proliferation at physiologically relevant concentrations, while neuroblastoma cells required higher concentrations, and other cell lines were unaffected.
- Inhibition was specific to alpha-tocopherol, as related compounds like Trolox and phytol showed no effect.
- Alpha-tocopherol completely inhibited VSMC proliferation stimulated by platelet-derived growth factor and endothelin, partially inhibited serum-stimulated proliferation, but had no effect on bombesin or lysophosphatidic acid-stimulated proliferation.
- Alpha-tocopherol stimulated phorbol ester binding to PKC, inhibited PKC translocation to the membrane, and reduced the phosphorylation of its 80-kDa substrate in VSMCs.
Conclusions:
- Alpha-tocopherol specifically inhibits vascular smooth muscle cell proliferation through mechanisms distinct from its antioxidant properties.
- The findings suggest that alpha-tocopherol modulates protein kinase C activity, interfering with its activation and downstream signaling pathways.
- A proposed model indicates that alpha-tocopherol interacts with cytosolic PKC, preventing its membrane translocation and subsequent activation, thereby inhibiting cell proliferation.