| Abstract |
STI-571 (Gleevec, imatinib mesylate) is a kinase inhibitor that
specifically targets the activated Abl oncoprotein and several membrane
tyrosine kinase receptors, including PDGF-R. Both PDGF ligands and
receptors are often overexpressed in gliomas, suggesting the existence of
an autocrine stimulatory loop. It is known that PDGF-R signals mainly
through PI3K/Akt and Ras/MAPK pathways. PI3K/Akt is a major cell survival
pathway and constitutively active AKT2 has been demonstrated to confer
cisplatin resistance. In a previous study, it has been shown that STI-571
inhibits the growth of the human glioblastoma cell line U87-MG in nude
mice. In the present study we compared STI-571 antiproliferative activity
on U87-MG and on its cisplatin-resistant variant U87-MG/Pt. U87-MG/Pt was
selected in our laboratory by culturing cells in stepwise increasing
concentrations of cisplatin (resistance index ~ 10) and displays a lower
level of PDGF-R protein expression than U87-MG. Unexpectedly, U87-MG/Pt
cells were significantly more sensitive to STI-571 than U87-MG (IC50
values = 18.6 +- 3.9 muM vs. 32. 6 +- 3.9 muM, respectively; P<0.0005,
two-tailed Student's t test) in a 3-day growth inhibition assay. Similar
results were obtained in clonogenic survival assays. Treatment of both
cell lines with STI-571 resulted in a dose-and time-dependent down-
regulation of PDGF-R protein expression, starting as soon as 8 hrs from
cell exposure. Interestingly, STI-571 (25 muM) increased p21 protein
expression in a p53-independent manner and inhibited phosphorylation of
Akt, without affecting Akt level of expression, only in U87-MG/Pt cells.
By contrast, no inhibition of Akt phosphorylation was observed in U87-MG,
not even when higher concentrations of STI-571 (up to 50 muM) were used.
We are currently evaluating whether the inhibition of Akt phosphorylation
results in a higher induction of apoptosis in U87-MG/Pt. In both cell
lines, STI-571 increased the level of phosphorylation of p42/44 MAPK, thus
suggesting that STI-571 efficacy may be potentiated by co-treatment with
MEK 1/2 inhibitors. These findings confirm that STI-571 is active in
glioblastoma cell lines. More interestingly a cisplatin-resistant variant
shows a clear-cut higher sensitivity to STI-571, and the data available so
far suggest that Akt might be involved in the mechanisms underlying this
phenomenon. We are presently studying whether cisplatin-resistant cell
lines from other tumors are also more sensitive to STI-571, and the
resultant clinical implications.
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