To clarify the mechanisms of
purvalanol A in the induction of apoptosis, we investigated whether
purvalanol A influenced the
RNA synthesis and expression of
RNA polymerase II and
signal transducer and activator of transcription 3 (STAT3). When MKN45 cells were treated with 30 micromol/l
purvalanol A,
mitochondrial dysfunction occurred before the induction of the apoptosis and the expression of antiapoptotic
proteins survivin, Bcl-XL, and Bcl-2 was reduced. The treatment with parvalanol A was also shown to reduce not only
mRNA for these
proteins but also global
RNA synthesis. The phosphorylation of the carboxy-terminal domain of
RNA polymerase II, which was involved in transcriptional regulation, was strongly inhibited by
purvalanol A, followed by the partial inhibition of the expression of
RNA polymerase II. Furthermore, the phosphorylation at Tyr705 of STAT3, which is known to be a phosphorylation site for
Janus kinase 2 (JAK2), was completely inhibited by
purvalanol A early (3 h) after
drug treatment, although the phosphorylation of STAT3 at Ser727, which is a phosphorylation site for Ras/Raf/
MEK and extracellular signal-regulated
protein kinase 1/2, was still detectable until late (12 h)
after treatment. In addition, the
tyrosine phosphorylation of JAK2 was efficiently inhibited by
purvalanol A. These results suggest that the inhibition of JAK2/STAT3 and
RNA polymerase II is crucial in the downregulation of antiapoptotic
proteins leading to the apoptotic cell death induced by parvalanol A.