Recently, considerable research attention has been focused on identification of naturally occurring chemopreventive substances capable of inhibiting, retarding, or reversing the multi-stage carcinogenesis. A wide array of plant derived polyphenolic compounds has been reported to own substantial anti-mutagenic and anti-carcinogenic activities. The majority of these naturally occurring polyphenolic compounds have potent anti-inflammatory properties, which appear to contribute to their chemopreventive or chemotherapeutic actions. Human malignant glioblastoma is a deadly brain cancer that continues to defy all current therapeutic strategies. So, innovative therapeutic strategies need to be explored for treating this deadly disease. We induced apoptosis in human malignant glioblastoma U87MG cells following exposure to apigenin (APG, a flavonoid present in fruits), (−)-epigallocatechin (EGC, a polyphenol from green tea), (−)-epigallocatechin-3-gallate (EGCG, another polyphenol from green tea), and genistein (GST, an isoflavonoid found in soyabin). Dose-response studies indicated a decrease in cell viability with an increasing dose of APG, EGC, EGCG, or GST. Wright staining and light microscopy for morphology detected predominantly apoptotic features in U87MG cells following exposure to 100 μM APG, 50 μM EGC, 50 μM EGCG, or 100 μM GST for 24 h. Determination of intracellular free Ca2+ levels by fura-2 assay suggested an association of flavonoid induced increase in intracellular free Ca2+ level with apoptosis in U87MG cells. Levels of transcriptional expression (mRNA) of the inhibitor-of-apoptosis proteins (IAPs), also currently designated as the baculovirus IAP repeat containing (BIRC) proteins, were determined by the reverse transcriptase-polymerase chain reaction (RT-PCR). In addition to the RT-PCR experiments, Western blot analyses were performed to determine the protein level expression of the selective BIRC proteins. Western blot analyses also showed alterations in expression of pro-apoptotic Bax and anti-apoptotic Bcl-2 proteins resulting in an increase in Bax:Bcl-2 ratio, and overexpression of calpain and caspase-3 that were also activated to cleave 270 kD α-spectrin at the specific tsites for generation of 145 kD spectrin break down product (SBDP) and 120 kD SBDP, repectively, in course of flavonoid induced apoptosis in U87MG cells. A colorimetric assay using synthetic peptide substrate specific for caspase-3 activity further confirmed the activation of caspase-3 in apoptotic cells. Taken together, all the results from this investigation strongly suggest that the selective plant derived flavonoids are potential therapeutic agents for the induction of apoptotic death in human glioblastoma cells. This investigation was supported in part by the R01 grants from the NCI and NINDS (Bethesda, MD), and also a grant from the State of SC.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]