Glioblastoma is the most maligant and prevalent human brain tumor of astroglial origin. It is considered as the death sentence in patients due to its poor response or resistance to existing therapeutic agents. Therefore, the search continues for an effective therapy for controlling the growth of this devastating cancer. Recent studies revealed the important roles of dietary phytochemicals for chemoprevention against various cancers and the development of natural diet-derived chemotherapeutic agents is an emerging area in cancer research. Previous studies have indicated that curcumin (CCM), a polyphenolic compound from Curcuma longa, possesses anti-proliferative properties because of cell-cycle arrest and induction of apoptosis in a variety of cancer cells. CCM inhibits skin, forestomach, mammary, liver and colon carcinogenesis in animal models of these cancers and indicates its anti-cancer activity. In the present investigation, we evaluated the chemotherapeutic efficacy of CCM against human malignant glioblastoma U87MG cells by inducing apoptosis and examining the molecular mechanisms, especially the proteolytic activities, involved in this process. Trypan blue dye exclusion test showed decreased viability of U87MG cells with increasing dose of CCM. Under light microscopy, Wright staining and ApopTag assay, respectively, showed the morphological and biochemical features of apoptosis in U87MG cells treated with 25 μM and 50 μM of CCM for 24 h. Western blot analyses showed that CCM treatment activated extrinsic pathway of apoptosis by activation of caspase-8, cleavage of Bid to tBid, down regulation of anti-apoptotic nuclear factor kappa B (NFκB) and upregulation of its inhibitor IκBα expression, favoring the process of apoptosis. CCM also caused activation of mitochondria-mediated intrinsic pathway of apoptosis with changes in expression of Bax and Bcl-2 proteins resulting in an increase in Bax:Bcl-2 ratio, release of cytochrome c from mitochondria followed by activation of caspase-9 and caspase-3 for apoptosis. Increased activities of calpain and caspase-3 were confirmed in the cleavage of 270 kD α-spectrin at specific sites generating 145 kD spectrin break down product (SBDP) and 120 kD SBDP, respectively. In course of CCM-induced apoptosis, increased expression of Smac/Diablo suggested suppression of the inhibitor-of-apoptosis proteins (IAPs), also called the Baculovirus IAP Repeat Containing (BIRC) proteins, as we confirmed their down regulation in U87MG cells following treatment with CCM. Taken together, our results indicated activation of multiple molecular mechanisms and involvement of proteolytic activities of calpain and caspases for apoptosis in human malignant glioblastoma U87MG cells following exposure to CCM. This research was supported in parts by the grants from the National Cancer Institute, National Institute of Neurological Diseases and Stroke, and State of SC.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]