An Interplay between Senescence, Apoptosis and Autophagy in Glioblastoma Multiforme-Role in Pathogenesis and Therapeutic Perspective

Abstract

Autophagy, cellular senescence, programmed cell death and necrosis are key responses of a cell facing a stress. These effects are partly interconnected, but regulation of their mutual interactions is not completely clear. That regulation seems to be especially important in cancer cells, which have their own program of development and demand more nutrition and energy than normal cells. Glioblastoma multiforme (GBM) belongs to the most aggressive and most difficult to cure cancers, so studies on its pathogenesis and new therapeutic strategies are justified. Using an animal model, it was shown that autophagy is required for GBM development. Temozolomide (TMZ) is the key drug in GBM chemotherapy and it was reported to induce senescence, autophagy and apoptosis in GBM. In some GBM cells, TMZ induces small toxicity despite its significant concentration and GBM cells can be intrinsically resistant to apoptosis. Resveratrol, a natural compound, was shown to potentiate anticancer effect of TMZ in GBM cells through the abrogation G2-arrest and mitotic catastrophe resulting in senescence of GBM cells. Autophagy is the key player in TMZ resistance in GBM. TMZ can induce apoptosis due to selective inhibition of autophagy, in which autophagic vehicles accumulate as their fusion with lysosomes is blocked. Modulation of autophagic action of TMZ with autophagy inhibitors can result in opposite outcomes, depending on the step targeted in autophagic flux. Studies on relationships between senescence, autophagy and apoptosis can open new therapeutic perspectives in GBM.

Keywords: DNA damage response; apoptosis; autophagy; glioblastoma; senescence; temozolomide.

Figure 1

Primary and secondary glioblastoma multiforme (GBM)—origin and genetic changes. EGF/EGFR, epidermal growth factor/receptor; MDM2, mouse double minute 2 homolog; CDKN2A, cyclin dependent kinase inhibitor 2A; RB, retinoblastoma; TP53, tumor protein p53; PTEN, phosphatase and tensin homolog; NF1, neurofibromin 1; PDGFRα, platelet derived growth factor receptor alpha; IDH1/2, isocitrate dehydrogenase (NADP(+)) 1, cytosolic/2, mitochondrial; LOH, loss of heterozygosity; ch., chromosome(s).

Figure 2

Differentiation of neural stem cells and cancer transformation. Neuronal stem cells can produce normal (green blocks) or cancer (red blocks) cells of various potential. Mutations occurring during differentiation of neural stem cell (NSCs) can contribute to tumor formation. NSCs can differentiate into neural/glial progenitor cell, but their transformation can lead to the formation of tumor-initiating cells (TICs). Neural progenitors undergo differentiation into neuronal cells, but glial progenitors differentiate into astrocytes. Genetic aberrations in glial progenitor cell can lead to tumor progenitor stem-like cells, but oligodendrocytes and astrocytes are also potential candidates involved in glioblastoma formation. TICs are also committed to the tumor formation via its differentiation into brain tumor progenitor-like cells.

Figure 3

A senescent cell displaying senescence-associated phenotype (SASP). In a senescent state a cell is irreversibly stopped at the G1/S or G2/M checkpoint and shows different morphology than its normal counterpart. It displays increased activity of senescence-associated-β-galactosidase (SA-β-gal), releases various soluble factor, enhanced extent of DNA damage and chromosomal aberrations collectively determining SASP.

Figure 4

Autophagy is initiated by a series of events leading to the nucleation of the phagophore, a structure sequestering material to be degraded (cargo) with the involvement of many proteins including a palette of the ATG (autophagy related) proteins 7, 2 variants of LC3B (microtubule associated protein 1 light chain 3 beta). Maturation of phagophore results in the formation of a double-membrane vehicle (autophagosome) encircling the cargo. Autophagosome fuses with lysosome into autolysosme, in which degradation of the cargo occurs. Degraded material can be recycled providing new products to be used by the cell. Many other proteins regulating autophagy are not presented here.

Figure 5

Mechanism of action of temolozamide (TMZ). TMZ methylates guanine (G) paired with cytosine (C) in the genomic DNA at the O⁶ position, yielding O⁶-methyl guanine (MeO⁶G). The methyl group can be removed from G by O⁶-methylguanine-DNA methyltransferase (MGMT), otherwise MeO⁶G can be paired with thymine (T) by DNA polymerase (DNA pol) in the next replication round, giving the MeO⁶G:T mismatch, which can be processed by mismatch repair (MMR) system. When these DNA repair mechanisms cannot cope with all MeO⁶Gs and their consequences, the cell is arrested at the cell cycle checkpoint, G1/S or G2/M, to have more time for repair. If this fails, the cell can adopt senescence or be directed on a programmed death pathway, usually apoptosis. Autophagy is another effect, which can be induced by TMZ and it can interact with senescence and apoptosis (more details in the main text). Extremely high TMZ concentrations can induce necrosis due to general toxicity of this drug. MeO⁶G is not the only DNA damage induced by TMZ.

Figure 6

Relationships between autophagy and apoptosis in cancer cells. Autophagy can protect against apoptotic death, but, when too extensive, can lead to self-destruction of a cancer cell. On the other hand, inhibition of autophagy can result in apoptosis activation and cell death, but, when a cancer cell is intrinsically resistant to apoptosis, accumulation of toxic waste not cleared by autophagy can increase genomic instability and result in a more aggressive cancer.