Activation of the phosphatidylinositol-3'-kinase/protein kinase B-dependent antiapoptotic pathway plays an important role in the development of radioresistance of human glioma cells

PI3K signaling in glioma--animal models and therapeutic challenges

The PI3 kinase (PI3K) family plays a complex role in cell biology and metabolism. Signaling through the PI3Ks is frequently activated in many human cancers, including glioblastoma, because of gain-of-function mutations in PIK3CA or loss of PTEN. Experiments involving genetic mouse models and small molecule inhibitors have helped to elucidate the roles of the regulatory and catalytic subunits of PI3K in metabolism and cancer. Downstream of PI3K is Akt, a critical effector of growth, proliferation and survival. The suggested dependence of glioblastoma tumors on PI3K signaling implies that PI3K inhibitors should lead to effective killing of these cancer cells, but that has been shown not to be the case. The engagement of other survival pathways in response to PI3K inhibition prompts the need to develop combination therapies that promote cytotoxicity in cancer cells.

Characterization of structurally distinct, isoform-selective phosphoinositide 3'-kinase inhibitors in combination with radiation in the treatment of glioblastoma

The phosphoinositide 3'-kinase (PI3K)-mediated signaling pathway plays a key role in fundamental cellular functions important in normal cellular homeostasis and malignant transformation. Deregulated signaling through this pathway contributes to development of gliomas and their resistance to radiation and chemotherapy. Targeting the PI3K signaling pathway has thus emerged as a promising approach to successful treatment of gliomas. We assessed the radiosensitizing potential of four small-molecule inhibitors that differ in their activities against specific isoforms of the PI3K 110-kDa catalytic subunit (p110). p110alpha inhibitors blocked phosphorylation of both protein kinase B/Akt and S6 in all cell lines examined, effectively decreased cellular proliferation, and produced additive cytotoxic effects in combination with radiation therapy. The p110beta inhibitor exhibited limited biochemical effects and failed to decrease cellular proliferation or viability as either a single agent or in combination with radiation or rapamycin. In vivo studies examining the effects of the p110alpha inhibitor in combination with radiation indicated a significant reduction in tumor growth rate induced by the combined treatment compared with each treatment modality alone. This translated into a trend toward prolonged time-to-failure for mice in the combination treatment group. In conclusion, PI3K inhibitors are promising agents in the treatment of glioblastomas, especially when used in combination with ionizing radiation.

Inhibition of Akt by the alkylphospholipid perifosine does not enhance the radiosensitivity of human glioma cells

Akt has been implicated as a molecular determinant of cellular radiosensitivity. Because it is often constitutively activated or overexpressed in malignant gliomas, it has been suggested as a target for brain tumor radiosensitization. To evaluate the role of Akt in glioma radioresponse, we have determined the effects of perifosine, a clinically relevant alkylphospholipid that inhibits Akt activation, on the radiosensitivity of three human glioma cell lines (U87, U251, and LN229). Each of the glioma cell lines expressed clearly detectable levels of phosphorylated Akt indicative of constitutive Akt activity. Exposure to a perifosine concentration that reduced survival by approximately 50% significantly reduced the level of phosphorylated Akt as well as Akt activity. Cell survival analysis using a clonogenic assay, however, revealed that this Akt-inhibiting perifosine treatment did not enhance the radiosensitivity of the glioma cell lines. This evaluation was then extended to an in vivo model using U251 xenografts. Perifosine delivered to mice bearing U251 xenografts substantially reduced tumor phosphorylated Akt levels and inhibited tumor growth rate. However, the combination of perifosine and radiation resulted in a less than additive increase in tumor growth delay. Thus, in vitro and in vivo data indicate that the perifosine-mediated decrease in Akt activity does not enhance the radiosensitivity of three genetically disparate glioma cell lines. These results suggest that, although Akt may influence the radiosensitivity of other tumor types, it does not seem to be a target for glioma cell radiosensitization.

Phorbol 12-myristate 13-acetate and serum synergize to promote rapamycin-insensitive cell proliferation via protein kinase C-eta

Previously, we have shown that PKC-eta (protein kinase C-eta) positively regulates glioblastoma proliferation and confers resistance to irradiation-induced apoptosis. In this study, we investigated the efficacy of rapamycin in inhibiting cell proliferation in two glioblastoma cell lines U-251MG (PKC-eta expressing) and U-1242MG (PKC-eta deficient) following PKC-eta activation. In U-251MG cells, rapamycin (10 nM) treatment was less effective as an antiproliferative agent when cells were concurrently stimulated with 10% serum and phorbol 12-myristate 13-acetate (PMA, 100 nM), a potent activator of PKC isozymes. Rapamycin-insensitive growth was owing to PKC-eta, as U-1242MG and U-251MG cells infected with a kinase-dead form of PKC-eta (U-251kr) were susceptible to rapamycin-induced inhibition of cell proliferation. Furthermore, U-251MG cells transfected with PKC-eta antisense oligonucleotides were sensitive to rapamycin. PKC-eta-expressing cells stimulated with PMA maintained p70S6K phosphorylation on Thr389 and phosphorylation of rpS6 (ser235/36), suggesting p70S6K kinase activity was still intact. Inhibition of p70S6K expression with small interfering RNA oligonucleotides inhibited cell proliferation greater than 50% in the presence of a combination of PMA and serum. Additionally, p70S6K co-precipitated with PKC-eta, suggesting a physical interaction between PKC-eta and p70S6K regulates the observed phosphorylation. Taken together, these data demonstrate that rapamycin-insensitive glioblastoma proliferation involves PKC-eta signaling.

Antitumor properties of dimethyl-celecoxib, a derivative of celecoxib that does not inhibit cyclooxygenase-2: implications for glioma therapy

✓ Celecoxib (Celebrex) appears to be unique among the class of selective COX-2 inhibitors (coxibs), because this particular compound exerts a second function that is independent of its celebrated ability to inhibit COX-2. This second function is the potential to inhibit cell proliferation and stimulate apoptotic cell death at much lower concentrations than any other coxibs. Intriguingly, these two functions are mediated by different moieties of the celecoxib molecule and can be separated. The author, as well as others, have generated and investigated analogs of celecoxib that retain only one of these two functions. One derivative, 2,5-dimethyl-celecoxib (DMC), which retains the antiproliferative and apoptosis-inducing function, but completely lacks the COX-2 inhibitory activity, is able to mimic faithfully all of the numerous antitumor effects of celecoxib that have been investigated so far, including reduction of neovascularization and inhibition of experimental tumor growth in various in vivo tumor models. In view of the controversy that has recently arisen regarding the life-threatening side effects of this class of coxibs, it may be worthwhile to pursue further the potential benefits of drugs such as DMC for anticancer therapy. Because DMC is not a coxib yet potently maintains celecoxib's antitumor potential, one may be inclined to speculate that this novel compound could potentially be advantageous in the management of COX-2–independent cancers. In this summary, the implications of recent findings with DMC will be presented and discussed.

Immunohistochemically determined total epidermal growth factor receptor levels not of prognostic value in newly diagnosed glioblastoma multiforme: Report from the Radiation Therapy Oncology Group

PURPOSE

The Radiation Therapy Oncology Group (RTOG) performed an analysis of patterns of immunohistochemically detected total epidermal growth factor receptor (EGFR) protein expression levels and their prognostic significance on archival tissue in newly diagnosed glioblastoma multiforme (GBM) patients from prior prospective RTOG clinical trials.

METHODS AND MATERIALS

Patients in this study had been treated on previous RTOG GBM trials (RTOG 7401, 7918, 8302, 8409, 9006, 9305, 9602, and 9806). Tissue microarrays were prepared from 155 patients enrolled in these trials. These specimens were stained using a mouse monoclonal antibody specific for the extracellular binding domain of EGFR to detect total EGFR (including both wild-type phosphorylated and wild-type unphosphorylated isoforms with some cross-reactivity with EGFRvIII). The intensity of total EGFR protein expression was measured by computerized quantitative image analysis using the SAMBA 4000 Cell Image Analysis System. The parameters measured were the mean optical densities over the labeled areas and the staining index, which represents the proportion of stained area relative to the mean stain concentration. Both parameters were correlated with the clinical outcome.

RESULTS

No differences in either overall or progression-free survival could be demonstrated by the mean optical density class or mean optical density quartile or the staining index of total EGFR immunostaining among the representative RTOG GBM cases.

CONCLUSION

Total EGFR protein expression levels, as measured immunohistochemically, do not appear to be of prognostic value in newly diagnosed GBM patients. Given the accumulating clinical evidence of the activity of anti-EGFR agents in GBM and the preclinical data suggesting the important role of downstream mediators as effectors of EGFR signaling, the RTOG is conducting additional investigations into the prognostic value of activation patterns of EGFR signaling, both at the level of the receptor (e.g., EGFRvIII, phospho-EGFR) and at the level of downstream signal transduction pathways (e.g., PI3K, Ras/MAPK pathways).