Flavopiridol Induces Apoptosis in Glioma Cell Lines Independent of Retinoblastoma and p53 Tumor Suppressor Pathway Alterations by a Caspase-independent Pathway1

Flavopiridol Induces Apoptosis in Glioma Cell Lines Independent of Retinoblastoma and p53 Tumor Suppressor Pathway Alterations by a Caspase-independent Pathway¹

Department of Pathology [M. A., C. T., D. C. M., E. W. N.] and Division of Neuropathology [D. C. M.], and New York University Cancer Institute and Kaplan Comprehensive Cancer Center [D. C. M., E. W. N.], New York University School of Medicine, New York, New York 10016

To whom requests for reprints should be addressed, at Department of Pathology, New York University School of Medicine and Kaplan Comprehensive Cancer Center, 550 First Avenue, New York, NY 10016. Phone: (212) 263-8757; Fax: (212) 263-8211; E-mail: newcoe01{at}med.nyu.edu

Abstract

Flavopiridol is a synthetic flavone, which inhibits growth in vitro and in vivo of several solid malignancies such as renal, prostate, and colon cancers. It is a potent cyclin-dependent kinase inhibitor presently in clinical trials. In this study, we examined the effect of flavopiridol on a panel of glioma cell lines having different genetic profiles: five of six have codeletion of p16^INK4a and p14^ARF; three of six have p53 mutations; and one of six shows overexpression of mouse double minute-2 (MDM2) protein. Independent of retinoblastoma and p53 tumor suppressor pathway alterations, flavopiridol induced apoptosis in all cell lines but through a caspase-independent mechanism. No cleavage products for caspase 3 or its substrate poly(ADP-ribose) polymerase or caspase 8 were detected. The pan-caspase inhibitor Z-VAD-fmk did not inhibit flavopiridol-induced apoptosis. Mitochondrial damage measured by cytochrome c release and transmission electron microscopy was not observed in drug-treated glioma cells. In contrast, flavopiridol treatment induced translocation of apoptosis-inducing factor from the mitochondria to the nucleus. The proteins cyclin D₁ and MDM2 involved in the regulation of retinoblastoma and p53 activity, respectively, were down-regulated early after flavopiridol treatment. Given that MDM2 protein can confer oncogenic properties under certain circumstances, loss of MDM2 expression in tumor cells could promote increased chemosensitivity. After drug treatment, a low Bcl-2/Bax ratio was observed, a condition that may favor apoptosis. Taken together, the data indicate that flavopiridol has activity against glioma cell lines in vitro and should be considered for clinical development in the treatment of glioblastoma multiforme.

Footnotes

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↵1 Supported by NIH Grant CA90290 and grants from the National Brain Tumor Foundation, Children’s Brain Tumor Foundation, and Elsa U. Pardee Foundation (to E. W. N.).
↵3 The abbreviations used are: GBM, glioblastoma multiforme; MDM2, mouse double minute-2; CDK, cyclin-dependent kinase; Rb, retinoblastoma; PARP, poly(ADP-ribose) polymerase; PI, propidium iodide; DAPI, 4′,6-diamidino-2-phenylindole; Z-VAD-fmk, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone; AIF, apoptosis-inducing factor.
- Accepted December 20, 2002.
- Received August 30, 2002.
- Revision received December 2, 2002.
Molecular Cancer Therapeutics