Ubiquitin-specific protease 8 links the PTEN-Akt-AIP4 pathway to the control of FLIPS stability and TRAIL sensitivity in glioblastoma multiforme.

The antiapoptotic protein FLIP(S) is a key suppressor of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in human glioblastoma multiforme (GBM) cells. We previously reported that a novel phosphatase and tensin homologue (PTEN)-Akt-atrophin-interacting protein 4 (AIP4) pathway regulates FLIP(S) ubiquitination and stability, although the means by which PTEN and Akt were linked to AIP4 activity were unclear. Here, we report that a second regulator of ubiquitin metabolism, the ubiquitin-specific protease 8 (USP8), is a downstream target of Akt, and that USP8 links Akt to AIP4 and the regulation of FLIP(S) stability and TRAIL resistance.

A novel PTEN-dependent link to ubiquitination controls FLIPS stability and TRAIL sensitivity in glioblastoma multiforme.

Phosphatase and tensin homologue (PTEN) loss and activation of the Akt-mammalian target of rapamycin (mTOR) pathway increases mRNA translation, increases levels of the antiapoptotic protein FLIP(S), and confers resistance to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in glioblastoma multiforme (GBM). In PTEN-deficient GBM cells, however, the FLIP(S) protein also exhibited a longer half-life than in PTEN mutant GBM cells, and this longer half-life correlated with decreased FLIP(S) polyubiquitination. FLIP(S) half-life in PTEN mutant GBM cells was reduced by exposure to an Akt inhibitor, but not to rapamycin, suggesting the existence of a previously undescribed, mTOR-independent linkage between PTEN and the ubiquitin-dependent control of protein stability.

Honokiol-mediated inhibition of PI3K/mTOR pathway: a potential strategy to overcome immunoresistance in glioma, breast, and prostate carcinoma without impacting T cell function.

Inhibition of the phosphoinositide 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway is an appealing method for decreasing the immunoresistance and augmenting T cell-mediated immunotherapy. A major impediment to this strategy is the impact of conventional PI3K/mTOR pathway inhibitors on T cell function. In particular, rapamycin, is a well-known immunosuppressant that can decrease the activity of the PI3K/mTOR pathway in tumor cells, but also has a profound inhibitory effect on T cells.

Increased expression of the glioma-associated antigen ARF4L after loss of the tumor suppressor PTEN. Laboratory investigation.

Object: 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.

Heat shock protein 90alpha recruits FLIPS to the death-inducing signaling complex and contributes to TRAIL resistance in human glioma.

Heat shock protein 90 (HSP90) is a molecular chaperone that contributes to the proper folding and stability of target proteins. Because HSP90 has been suggested to interact with FLIP(S), the key regulator of tumor necrosis factor-alpha-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in glioma cells, we examined the role HSP90 played in controlling TRAIL response. HSP90alpha was found to associate with FLIP(S) in resting cells in a manner dependent on the ATP-binding NH2-terminal domain of HSP90alpha.

Loss of tumor suppressor PTEN function increases B7-H1 expression and immunoresistance in glioma.

Cancer immunoresistance and immune escape may play important roles in tumor progression and pose obstacles for immunotherapy. Expression of the immunosuppressive protein B7 homolog 1 (B7-H1), also known as programmed death ligand-1 (PD-L1), is increased in many pathological conditions, including cancer. Here we show that expression of the gene encoding B7-H1 increases post transcriptionally in human glioma after loss of phosphatase and tensin homolog (PTEN) and activation of the phosphatidylinositol-3-OH kinase (PI(3)K) pathway.

Use of APO2L/TRAIL with mTOR inhibitors in the treatment of glioblastoma multiforme.

The mammalian target of rapamycin (mTOR) plays a critical role in the regulation of cell growth, proliferation and survival. Components of the mTOR pathway are activated in a variety of tumors, including glioblastoma multiforme (GBM), and we have found that one surprising consequence of mTOR pathway activation is resistance of GBMs to the proapoptotic effects of agents such as APO2L/TRAIL. mTOR inhibition has become feasible following the development of rapamycin and comparable analogs with improved pharmacological properties, including CCI-779, RAD001 and AP23573.

mTOR-independent translational control of the extrinsic cell death pathway by RalA.

Oncogenic potential is associated with translational regulation, and the prevailing view is that oncogenes use mTOR-dependent pathways to up-regulate the synthesis of proteins critical for transformation. In this study, we show that RalA, a key mediator of Ras transformation, is also linked to the translational machinery. At least part of this linkage, however, is independent of mTOR and acts through RalBP1 to suppress cdc42-mediated activation of S6 kinase and the translation of the antiapoptotic protein FLIP(S).

T-type calcium channels and tumor proliferation.

The role of T-type Ca2+ channels in proliferation of tumor cells is reviewed. Intracellular Ca2+ is important in controlling proliferation as evidenced by pulses, or oscillations, of intracellular Ca2+ which occur in a cell cycle-dependent manner in many tumor cells. Voltage-gated calcium channels, such as the T-type Ca2+ channel, are well suited to participate in such oscillations due to their unique activation/inactivation properties.

Translational regulation of TRAIL sensitivity.

TNF-related apoptosis-inducing ligand (TRAIL) is a peptide that induces apoptosis to varying degrees in tumor cells. While TRAIL sensitivity in tumors has been linked to c-myc- and MEK/Erk-induced enhancement of caspase activation, our recent study identified a third input controlling TRAIL sensitivity, namely the Akt-mTOR pathway. We showed that instead of enhancing TRAIL sensitivity, Akt expression, acting through mTOR and the mTOR targets S6 kinase and eIF-4E, selectively enhances translation of the anti-apoptotic protein FLIP(S) and confers TRAIL resistance.

mTOR controls FLIPS translation and TRAIL sensitivity in glioblastoma multiforme cells.

The tumor-selective, proapoptotic, death receptor ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a mediator of antitumor drug activity and in itself is a promising agent for the treatment of human malignancies. Like many tumors, however, glioblastoma multiforme (GBM), the most fatal form of glioma, exhibits a range of TRAIL sensitivity, and only a small percentage of GBM tumors undergo TRAIL-induced apoptosis. We here show that TRAIL resistance in GBM is a consequence of overexpression of the short isoform of the caspase-8 inhibitor, c-FLICE inhibitory protein (FLIP(S)), and that FLIP(S) expression is in turn translationally enhanced by activation of the Akt-mammalian target of rapamycin (mTOR)-p70 S6 kinase 1 (S6K1) pathway.

Variation of T-type calcium channel protein expression affects cell division of cultured tumor cells.

In this study we investigated the T-type calcium channel and its involvement in the cell division of U87MG cultured glioma cells and N1E-115 neuroblastoma cells. Using Western blot analysis, we found that expression of both alpha1G and alpha1H subunits of the T-type calcium channel decreased during conditions associated with a decrease in proliferation as evidenced by increased expression of cyclin D1, a marker for non-proliferating cells. Both serum starvation and application of mibefradil, a selective T-type calcium channel antagonist, resulted in a 50% decrease in the expression of alpha1G and alpha1H and a 700-900% increase in levels of cyclin D1 in U87MG and N1E-115 cells, respectively.

Convection-enhanced delivery of tumor necrosis factor-related apoptosis-inducing ligand with systemic administration of temozolomide prolongs survival in an intracranial glioblastoma xenograft model.

Although tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potent activator of cell death, preferentially killing neoplastic cells over normal cells, the efficacy of TRAIL for the treatment of glioma might be limited due to cellular resistance and, importantly, poor distribution after systemic administration. TRAIL and temozolomide (TMZ) were recently shown to have a synergistic antitumor effect against U87MG glioma cells in vitro. Convection-enhanced delivery (CED) can effectively distribute TRAIL protein throughout a brain tumor mass.