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Vol. 54, Issue 2, 241-248, August 1998
Institut für Pharmakologie, Heinrich-Heine-Universität,
Düsseldorf, Germany
The triazolopyrimidine trapidil has been found in controlled clinical
trials to prevent restenosis after vascular injury. Although trapidil
is widely regarded as a platelet-derived growth factor receptor (PDGF)
antagonist, its precise mode of action is still unknown. This study was
designed to investigate the inhibition of mitogenesis by trapidil in
cultured bovine coronary artery smooth muscle cells (SMC) and to
identify major signal transduction pathways involved. Trapidil
inhibited PDGF-BB-induced mitogenesis in SMC in a
concentration-dependent manner. Comparable inhibitory effects were
obtained after stimulation of smooth muscle cells by phorbol ester,
which suggests that the action of trapidil was not restricted to PDGF
receptor-mediated mechanisms. Trapidil also inhibited PDGF- and phorbol
ester-induced mitogen-activated protein kinase as well as Raf-1 kinase
activity. As a possible target of trapidil, stimulation of cellular
protein kinase A (PKA) activity was detected. Trapidil also induced the
phosphorylation of vasodilator-stimulated phosphoprotein in SMC.
Antimitogenic effects of trapidil were completely abolished by PKA
inhibitors. Neither a direct stimulation of cAMP formation nor a
phosphodiesterase inhibition was observed at antimitogenic
concentrations of trapidil. However, trapidil directly activated
purified PKA holoenzyme in a cAMP-independent manner. In conclusion,
trapidil exerts its antimitogenic effects on SMC by direct activation
of PKA. Thus, PKA-mediated inhibition of the Raf-1/MAP kinase pathway
may be involved in the antimitogenic actions of the compound.
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