Molecular basis of α-tocopherol inhibition of smooth muscle cell proliferation in vitro

Proc. Natl. Acad. Sci. U.S.A.

Bartoli

et al. 1995

272

151

ABSTRACTd-a-Tocopherol, but not d-jJ-tocopherol, negatively regulates proliferation of vascular smooth muscle cells at physiological concentrations. d-c-Tocopherol inhibits protein kinase C (PKC) activity, whereas d-13-tocopherol is ineffective. Furthermore d-18-tocopherol prevents the inhibition of cell growth and of PKC activity caused by d-ctocopherol. The negative regulation by d-cv-tocopherol of PKC activity appears to be the cause and not the effect of smooth muscle cell growth inhibition. d-a-Tocopherol does not act by binding to PKC directly but presumably by preventing PKC activation. It is concluded that, in vascular smooth muscle cells, d-c-tocopherol acts specifically through a nonantioxidant mechanism and exerts a negative control on a signal transduction pathway regulating cell proliferation.Vascular smooth muscle cell (vascular SMC) proliferation represents a significant event in a number of diseases such as arteriosclerosis and hypertension (1-4). Smooth muscle proliferation is controlled by growth factors released from blood cells (1, 2, 5), by inhibitors or stimulants produced by the vessel wall cells (6, 7), by tocopherols, and by active oxygen species (8, 9). Evidence indicates that experimental atherosclerosis and foam cell formation can be effectively retarded by antioxidants (10-12). In addition, supplementation of human subjects with antioxidants has been shown to increase the resistance of their low density lipoproteins to oxidation and to protect against arteriosclerosis (13-16). As antioxidants, tocopherols may stimulate in some cases cell proliferation by removing inhibitory lipid peroxides (17-22). However, d-atocopherol has also a direct effect as cell-growth inhibitor, and this effect is not obviously mediated by its reduction-oxidation properties (23)(24)(25).PKC participates in one of the major signal transduction systems triggered by the external stimulation of cells by various ligands including hormones, neurotransmitters, and growth factors (26). Activation of PKC by phorbol esters may be responsible for their growth-promoting activity. d-aTocopherol has been shown to inhibit PKC activity in a number of cell lines and, in particular, in SMC. The mechanism of this inhibition has not yet been clarified (23)(24)(25).In the present study PKC inhibition has been found to be the basis of the inhibition of cell proliferation by d-a-tocopherol.Moreover PKC inhibition has been found to be cell cycle dependent, a result inconsistent with a direct interaction between PKC and d-a-tocopherol. Finally, the inhibitory specificity of d-a-tocopherol versus d-3-tocopherol and their mutual competition suggest a nonantioxidant mechanism to be at the basis of its action. MATERIALS AND METHODSGrowth media and serum were from GIBCO; A7r5 rat aortic SMC were from the American Type Culture Collection; phorbol 12-myristate 13-acetate (PMA) and streptolysin-O (25,000 units) were from Sigma; calphostin C, calyculin A, and okadaic acid were from LC Services (

Section: Discussioncontrasting

confidence: 55%

d-alpha-tocopherol inhibition of vascular smooth muscle cell proliferation occurs at physiological concentrations, correlates with protein kinase C inhibition, and is independent of its antioxidant properties.

Tasinato

Proc. Natl. Acad. Sci. U.S.A.

Bartoli

et al. 1995

272

151

“…The effects described previously (Boscoboinik et al, 1992;Boscoboinik et al, 1991a;Boscoboinik et al, 1991b) and in Table 3 prompted us to investigate the mechanism at the basis of hydrogen-peroxide-induced proliferation and Da-tocopherol-induced inhibition in foetal-calf-serum-stimulated cells. A detailed analysis of the differential effects of D-a-tocopherol and D-/?-tocopherol at the different levels of the signal-transduction elements will be reported elsewhere.…”

Section: Effect Of Hydrogen Peroxide D-a-tocopherol and D-/?-tocophementioning

confidence: 87%

“…D-a-Tocopherol has a direct inhibitory effect on smooth muscle cell growth but this effect is not obviously linked with its radical chain-breaking properties (Ozer et al, 1993;Chatelain et al, 1993;Boscoboinik et al, 1992;Boscoboinik et al, 1991a;Boscoboinik et al, 1991b). In fact, D-P-tocopherol, an equally potent antioxidant (Pryor et al, 1994) is not capable of inhibiting cell proliferation (Chatelain et al, 1993).…”

mentioning

confidence: 99%

Modulation of Activator Protein‐1 (AP‐1) Transcription Factor and Protein Kinase C by Hydrogen Peroxide and d‐α‐Tocopherol in Vascular Smooth Muscle Cells

Stäuble

European Journal of Biochemistry

Tasinato

et al. 1994

Self Cite

106

The effects of hydrogen peroxide, D-a-tocopherol and of D-P-tocopherol on proliferation, protein kinase C and activator protein-1 (AP-1) activation have been studied in vascular smooth muscle cells. Cell proliferation, when activated by foetal calf serum, was inhibited by D-a-tocopherol. Protein kinase C activity was stimulated by hydrogen peroxide in a manner similar to phorbol myristate acetate; in the latter case, but not in the former, D-a-tocopherol inhibited the reaction. Hydrogen peroxide prevented phorbol-myristate-acetate-stimulated AP-1 binding to DNA but stimulated it if protein kinase C was down-regulated or inhibited. D-a-Tocopherol promoted AP-1 activation in quiescent cells but prevented its activation by phorbol myristate acetate.None of the described effects of D-a-tocopherol were shared by D-P-tocopherol, suggesting a non-antioxidant mechanism as the basis of its action. The data show that hydrogen peroxide and Da-tocopherol affect more than one element in the cell signal-transduction cascade.

“…This event is associated with inhibition of cell proliferation. The site of cell regulation by a-tocopherol appears to be in the late G I phase of the cycle Boscoboinik et al, 1992). The regulation of cell growth by a-tocopherol may represent a physiological mechanism relevant to the onset of diseased states such as arteriosclerosis and cancer.…”

Section: Inhibitionmentioning

confidence: 99%

The protein kinase C family

Azzi

European Journal of Biochemistry

Hensey

1992

Self Cite

354

130

Protein kinase C represents a structurally homologous group of proteins similar in size, structure and mechanism of activation. They can modulate the biological function of proteins in a rapid and reversible manner. Protein kinase C participates in one of the major signal transduction systems triggered by the external stimulation of cells by various ligands including hormones, neurotransmitters and growth factors. Hydrolysis of membrane inositol phospholipids by phospholipase C or of phosphatidylcholine, generates sn-l,2-diacylglycerol, considered the physiological activator of this kinase. Other agents, such as arachidonic acid, participate in the activation of some of these proteins. Activation of protein kinase C by phorbol esters and related compounds is not physiological and may be responsible, at least in part, for their tumor-promoting activity. The cellular localization of the different calcium-activated protein kinases, their substrate and activator specificity are dissimilar and thus their role in signal transduction is unlike. A better understanding of the exact cellular function of the different protein kinase C isoenzymes requires the identification and characterization of their physiological substrates.The eukaryotic cell is a highly regulated entity, responding to its immediate intracellular environment as well as to external stimuli. Most regulations are mediated, either directly or indirectly, by conformational changes in proteins, and the equilibrium between active and inactive conformational states can be altered by both allosteric and covalent mechanisms. A most common covalent means of regulating protein activity is protein phosphorylation, which is particularly prominent for the role that it serves in signal transduction and is important in many other cellular responses (Edelman et al., 1987). Signals impinging on cells have their effects amplified and distributed by a network of protein phosphorylation and dephosphorylation reactions.