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Cool-1 Therapy for Glioblastoma

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Cool-1 mediated inhibition of c-Cbl modulates multiple critical properties of glioblastomas, including the ability to generate tumors in vivo”from Stem Cells by Stuart P. Atkinson

 

Glioblastomas (GBMs) are not only one of the most deadly cancers known; they are also infamous for being difficult to treat due to resistance to therapy and the wide range of metastasis. A recent study by the laboratory of Mark Noble at the University of Rochester Medical Center, New York, USA has identified the redox/Fyn/cCbl (RFC) pathway as a potential therapeutic target [1]. In this pathway, cellular oxidation activates Fyn kinase and then the c-Cbl ubiquitin ligase [2] which leads to the ubiquitylation and degradation of c-Cbl’s target proteins, which include important cancer targets (e.g. EGFR). Inhibition of c-Cbl activation by Cdc42 promotes tumour initiating capacity in target cells and inhibition of Cdc42 can reverse this effect [1]. They group now show that in GBM, c-Cbl is inhibited by another mechanism – binding by Cool-1/ß-Pix – and may provide further targets for GBM therapies [3].

(BCNU), an inhibitor of glutathione reductase, was initially used to increase intracellular oxidative status of oligodendrocyte/type-2 astrocyte progenitor cells (O-2A/OPCs) where the RFC pathway was originally discovered [4]. While exposure of O-2A/OPCs BCNU led to the expected increases in Fyn kinase activity and phospho-c-Cbl and decreased EGFR levels, BCNU caused increased oxidation and activate Fyn kinase, but not c-Cbl phosphorylation or EGFR reduction in GBM cell lines. A comparison of lysates demonstrated that Cool-1, and not Cdc42, was strikingly different in GBM samples as compared to O-2A/OPCs; isoform expression was altered, phosphorylation of Cool-1 was higher and it formed complexes with c-Cbl only in GBM lysates. Reduction of Cool-1 levels by short hairpin RNA (shRNA) in GBM cells allowed BCNU-induced c-Cbl phosphorylation and decreases in EGFR. Furthermore, Cool-1 shRNA treatment of GBM cells increased the cytotoxic nature of BCNU and also temozolomide (TMZ) a common GBM therapeutic option with no pro-oxidant activities. Additional consequences of Cool-1 knockdown in GBM cells were a decrease in cell growth and division, shift in cell cycle towards an increased G0/G1 population, and cell migration. It did not however have an effect on primary glial progenitor cells apart from a decrease in migration.

Cool-1 knockdown also affected various measures of tumour initiating cell function, such as the vast reduction multicellular spheroid formation ability, reduction in Notch-1, β-catenin and Sox2 (critical regulators of TIC function and initiation in GBMs) and the reduction in CD133 and CD15 (mark glioma cells with tumor initiation capacity). Interestingly, knock down of c-Cbl shRNA in Cool-1 knockdown GBM27 cells restored CD15 expression, and restored neurosphere formation ability. Most importantly, Cool-1 knockdown reduced the tumour forming capabilities of GBM cells after intracranial transplant into the striatum of NOD/SCID mice, an effect which was reversed after c-Cbl knock down.

In this study Stevens et al have uncovered a potentially important therapeutic avenue for the treatment of deadly cancer. Importantly, while targeting the initiating cells of GBM, the therapy seems not to overtly affect the “normal” progenitor versions of these cells.

References

  1. Chen HY, Yang YM, Stevens BM, et al. Inhibition of redox/Fyn/c-Cbl pathway function by Cdc42 controls tumour initiation capacity and tamoxifen sensitivity in basal-like breast cancer cells. EMBO Mol Med 2013;5:723-736.
  2. Li Z, Dong T, Proschel C, et al. Chemically diverse toxicants converge on Fyn and c-Cbl to disrupt precursor cell function. PLoS Biol 2007;5:e35.
  3. Stevens BM, Folts CJ, Cui W, et al. Cool-1 mediated inhibition of c-Cbl modulates multiple critical properties of glioblastomas, including the ability to generate tumors in vivo. Stem Cells 2014;
  4. Smith J, Ladi E, Mayer-Proschel M, et al. Redox state is a central modulator of the balance between self-renewal and differentiation in a dividing glial precursor cell. Proc Natl Acad Sci U S A 2000;97:10032-10037.