Sandra Hess, Michael Pfreundschuh and Beate Gleissner
Yuichi Hirose, Makoto Katayama, Mitchel S. Berger and Russell O. Pieper
Object. The Chk1 and p38 mitogen-activated protein kinase (MAPK) pathways play key roles in the G2 arrest caused by exposing glioma cells to temozolomide (TMZ). Although inhibition of either pathway sensitizes glioma cells to TMZ-induced cytotoxicity, the relative contributions of these pathways to TMZ-induced G2 arrest and to TMZ resistance conferred by G2 arrest have not been defined.
Methods. The authors pharmacologically inhibited the Chk1 and/or p38 pathways in U87MG human glioma cells prior to and/or after exposure to TMZ; thereafter, effects on the TMZ-induced G2 arrest pathway and toxicity were monitored. The p38 inhibitor SB203580 or the Chk1 inhibitor UCN-01 or their combination blocked TMZ-mediated inactivation of cdc25C and cdc2, suggesting that p38 and Chk1 pathways work cooperatively and are both necessary to inactivate cdc25C and cdc2. Consistent with this idea, the inhibition of both Chk1 and p38 pathways did not lead to greater bypass of TMZ-induced G2 arrest or greater cytotoxicity than inhibition of either pathway alone. Inhibition of p38 did not alter TMZ-induced Chk1 activation/phosphorylation and vice versa, suggesting that p38 and Chk1 do not cooperatively bring about G2 arrest by reciprocal activation/phosphorylation. The two pathways, however, are not functionally identical; the Chk1 pathway was required for both the initiation and maintenance of TMZ-induced G2 arrest, whereas the p38 pathway played a role only in the initiation.
Conclusions. The Chk1 and p38 pathways cooperate to bring about TMZ-induced G2 arrest, and the inhibition of either pathway alone is sufficient to sensitize U87MG glioma cells to TMZ-induced cytotoxicity.
Yuichi Hirose, Emiko L. Kreklau, Leonard C. Erickson, Mitchel S. Berger and Russell O. Pieper
Object. Temozolomide (TMZ)-induced O6-methylguanine (MG) DNA lesions, if not removed by MG—DNA methyltransferase (MGMT), mispair with thymine, trigger rounds of futile mismatch repair (MMR), and in glioma cells lead to prolonged G2—M arrest and ultimately cell death. Depletion of MGMT by O6-benzylguanine (BG) sensitizes tumor cells to TMZ, and this combination is currently used in clinical trials. The use of the TMZ+BG combination in gliomas, however, is complicated by the prolonged TMZ-induced G2—M arrest, which may delay activation of poorly defined cell death pathways and allow for MGMT repletion and reversal of toxicity.
Methods. To address these issues, the actions of TMZ were monitored in DNA MMR-proficient SF767 glioma cells depleted of MGMT by BG, and in cells in which BG was removed at various times after TMZ exposure. In MGMT-depleted cells, TMZ exposure led to DNA single-strand breaks and phosphorylation of cdc2, followed by G2—M arrest, induction of p53/p21, and DNA double-strand breaks. Although DNA single-strand breaks, phosphorylation of cdc2, and G2—M arrest could be reversed by repletion of MGMT up to 5 days after TMZ exposure, TMZ-induced cytotoxicity could only be prevented if MGMT was replenished within 24 hours of the onset of G2—M arrest, and before the creation of DNA double-strand breaks.
Conclusions. These results indicate that although SF767 glioma cells undergo a prolonged G2—M arrest in response to TMZ, their ability to escape TMZ-induced cytotoxicity by MGMT repletion is limited to an approximately 24-hour period after the onset of G2—M arrest.
John H. Chi, Amith Panner, Kristine Cachola, Courtney A. Crane, Joseph Murray, Russell O. Pieper, C. David James and Andrew T. Parsa
Despite recent advances in cancer immunotherapy, cellular mechanisms controlling expression of tumor-associated antigens are poorly understood. Mutations in cancer cells, such as loss of PTEN, may increase expression of tumor-associated antigens. The authors investigated the relationship between PTEN status and the expression of a glioma-associated antigen, adenosine diphosphate–ribosylation factor 4–like (ARF4L) protein.
Human glioma cell lines with confirmed PTEN status were examined by Northern blot analysis and quantitative polymerase chain reaction. Western blot analysis was used to measure ARF4L protein levels across multiple cell lines.
The loss of PTEN was shown to lead to increased levels of ARF4L protein but no change in transcript levels. Cell lines with serial mutations, including activation of Ras and Akt pathways, also demonstrated increased levels of ARF4L protein, which decreased after treatment with rapamycin. The ARF4L transcript preferentially localized to the polysomal compartment after PTEN loss in glioma or activation of Akt in human astrocytes.
Expression of ARF4L is controlled by the activated Akt/mTOR pathway, which is a downstream effect of the loss of PTEN function. Mutations leading to oncogenesis may impact the regulation and expression of tumor specific antigens. Screening of mutation status in glioma may be helpful in selecting patients for immunotherapy trials in the future.
Masayuki Kanamori, Tomohiro Kawaguchi, Janice M. Nigro, Burt G. Feuerstein, Mitchel S. Berger, Lucio Miele and Russell O. Pieper
Because activation of Notch receptors has been suggested to be critical for Ras-mediated transformation, and because many gliomas exhibit deregulated Ras signaling, the authors measured Notch levels and activation in primary samples and cell lines derived from glioblastoma multiforme (GBM) as well as the contribution of Notch pathway activation to astrocytic transformation and growth.
Western blot analysis of Notch 1 expression and activation showed that Notch 1 protein was overexpressed and/or activated in Ras-transformed astrocytes, in three of four GBM cell lines, and in four of five primary GBM samples. Expansion of these studies to assess mRNA expression of components of the Notch signaling pathway by cDNA expression array showed that cDNAs encoding components of the Notch signaling pathway, including the Notch ligand Jagged-1, Notch 3, and the downstream targets of Notch (HES1 and HES2), were also overexpressed relative to non-neoplastic brain controls in 23, 71, and 51% of 35 primary GBMs, respectively. Furthermore, inhibition of Notch signaling by genetic or pharmacological means led to selective suppression of the growth and expression of markers of differentiation in cells exhibiting Notch pathway deregulation.
Notch activation contributes to Ras-induced transformation of glial cells and to glioma growth, survival, or both and as such may represent a new target for GBM therapy.