Poor chemosensitivity and the development of chemoresistance remain major obstacles to successful chemotherapy of malignant gliomas. GRP78 is a key regulator of the unfolded protein response (UPR). As a Ca²⁺-binding molecular chaperone in the endoplasmic reticulum (ER), GRP78 maintains ER homeostasis, suppresses stress-induced apoptosis, and controls UPR signaling. We report here that GRP78 is expressed at low levels in normal adult brain, but is significantly elevated in malignant glioma specimens and human malignant glioma cell lines, correlating with their rate of proliferation. Down-regulation of GRP78 by small interfering RNA leads to a slowdown in glioma cell growth. Our studies further reveal that temozolomide, the chemotherapeutic agent of choice for treatment of malignant gliomas, leads to induction of CHOP, a major proapoptotic arm of the UPR. Knockdown of GRP78 in glioblastoma cell lines induces CHOP and activates caspase-7 in temozolomide-treated cells. Colony survival assays further establish that knockdown of GRP78 lowers resistance of glioma cells to temozolomide, and, conversely, overexpression of GRP78 confers higher resistance. Knockdown of GRP78 also sensitizes glioma cells to 5-fluorouracil and CPT-11. Treatment of glioma cells with (−)-epigallocatechin gallate, which targets the ATP-binding domain of GRP78 and blocks its protective function, sensitizes glioma cells to temozolomide. These results identify a novel chemoresistance mechanism in malignant gliomas and show that combination of drugs capable of suppressing GRP78 with conventional agents such as temozolomide might represent a novel approach to eliminate residual tumor cells after surgery and increase the effectiveness of malignant glioma chemotherapy. [Cancer Res 2007;67(20):9809–16]

The endoplasmic reticulum (ER) is a multifunctional organelle required for lipid biosynthesis, calcium storage, and protein folding and processing. A number of physiological and pathological conditions, as well as a variety of pharmacological agents, are able to disturb proper ER function and thereby cause ER stress, which severely impairs protein folding and therefore poses the risk of proteotoxicity. Specific triggers for ER stress include, for example, particular intracellular alterations (e.g., calcium or redox imbalances), certain microenvironmental conditions (e.g., hypoglycemia, hypoxia, and acidosis), high-fat and high-sugar diet, a variety of natural compounds (e.g., thapsigargin, tunicamycin, and geldanamycin), and several prescription drugs (e.g., bortezomib/Velcade, celecoxib/Celebrex, and nelfinavir/Viracept). The cell reacts to ER stress by initiating a defensive process, called the unfolded protein response (UPR), which is comprised of cellular mechanisms aimed at adaptation and safeguarding cellular survival or, in cases of excessively severe stress, at initiation of apoptosis and elimination of the faulty cell. In recent years, this dichotomic stress response system has been linked to several human diseases, and efforts are underway to develop approaches to exploit ER stress mechanisms for therapy. For example, obesity and type 2 diabetes have been linked to ER stress-induced failure of insulin-producing pancreatic beta cells, and current research efforts are aimed at developing drugs that ameliorate cellular stress and thereby protect beta cell function. Other studies seek to pharmacologically aggravate chronic ER stress in cancer cells in order to enhance apoptosis and achieve tumor cell death. In the following, these principles will be presented and discussed.

The anticancer potency of green tea and its individual components is being intensely investigated, and some cancer patients already self-medicate with this “miracle herb” in hopes of augmenting the anticancer outcome of their chemotherapy. Bortezomib (BZM) is a proteasome inhibitor in clinical use for multiple myeloma. Here, we investigated whether the combination of these compounds would yield increased antitumor efficacy in multiple myeloma and glioblastoma cell lines in vitro and in vivo. Unexpectedly, we discovered that various green tea constituents, in particular (-)-epigallocatechin gallate (EGCG) and other polyphenols with 1,2-benzenediol moieties, effectively prevented tumor cell death induced by BZM in vitro and in vivo. This pronounced antagonistic function of EGCG was evident only with boronic acid–based proteasome inhibitors (BZM, MG-262, PS-IX), but not with several non–boronic acid proteasome inhibitors (MG-132, PS-I, nelfinavir). EGCG directly reacted with BZM and blocked its proteasome inhibitory function; as a consequence, BZM could not trigger endoplasmic reticulum stress or caspase-7 activation, and did not induce tumor cell death. Taken together, our results indicate that green tea polyphenols may have the potential to negate the therapeutic efficacy of BZM and suggest that consumption of green tea products may be contraindicated during cancer therapy with BZM.

The transient induction of c-fos mRNA and protein suggests that regulation occurs not only by transcriptional activation but also at the level of turnover of the gene product. Here we present evidence for the rapid turnover of c-fos mRNA and some of the requirements for its specific degradation. The half life of induced mature cytoplasmic c-fos mRNA is 9 min in both serum-starved and growing primary human fibroblasts and in NIH 3T3 cells. A structure present at the 3' end of the c-fos mRNA molecule is involved in its low stability since the substitution or the removal of the untranslated 3' portion prolongues the RNA life time. The rapid turnover of fos mRNA requires, in addition, continued protein synthesis. Treatment of cells with cycloheximide stabilizes c-fos mRNA. Washing out cycloheximide reestablishes the rapid turnover. Both changes occur with lag periods of less than 17 minutes.