Glioma Overview - Signaling Pathway. Diagnostics Marker. Targeted Therapy and Clinical Trials.

An Introduction to Glioma

Brain tumors have high morbidity and mortality due to their locally invasive growth. Most neoplastic brain lesions are metastases that originate from cancers outside the brain, which are 5-10 times more than a primary brain tumor. Glioma is one of the most common primary brain tumors, about 30% of brain tumors are gliomas, accounting for 80% of the malignant brain tumor. This heterogeneous tumor is the main cause of death in brain tumors and is considered to arise from neuroglial stem cells in the brain, including astrocytes, oligodendrocytes, and ependymal cells. Hence, gliomas are histologically classified into astrocytomas, oligodendrogliomas, mixed oligoastrocytic gliomas, and ependymomas. Further classification goes according to the basis of location, characteristic differentiation patterns, and features of anaplasia. Malignancy grades of tumors are assigned from WHO grade I to IV, with WHO grade I representing the least malignancy. Common gliomas in adults include infiltrative astrocytomas of various grades (diffuse astrocytoma (WHO II)), anaplastic astrocytoma (WHO III), glioblastoma (WHO IV), oligodendrogliomas, and the mixed oligoastrocytomas.

Fig.1 Glioma signaling pathway

1 Main Signaling Pathways in Glioma Therapy

1.1 EGFR signaling cascade

The epidermal growth factor receptor (EGFR) gene encodes a receptor tyrosine kinase (RTK) known as ErbB1/her1, which is the prototype of the ErbB family of RTKs. The transmembrane EGFR mainly consists of three domains including an extracellular ligand binding and dimerization arm with the N-terminal domain, a hydrophobic transmembrane domain, and the C-terminal domain with the intracellular tyrosine kinase domain. On binding of ligand to the extracellular domain, the receptor dimerizes and leads to autophosphorylation or cross phosphorylation of the cytosolic tail, which subsequently activates the downstream kinases and finally transcription factors. Activation of the EGFR regulates the expression of various adaptors and molecules including mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK), Janus kinase/signal transducers, and activators of transcription (JAK-STAT) pathways, etc., and finally participate in the regulation of cell proliferation, differentiation, and migration.

1.2 Wnt signaling cascade

The Wnt signaling pathway is involved in both physiological and pathophysiological processes and regulates diverse cellular functions including morphogenesis, differentiation, and proliferation. Aberrant activation of the canonical Wnt pathway (β-catenin dependent) is a leading cause of several human cancers including glioma. Initiation of the Wnt/β-catenin pathway occurs by the interaction between frizzled receptor and low-density lipoprotein receptor-related protein (LRP) caused by the accumulation of Wnt ligands. Then the β-catenin accumulates in the cytoplasm and translocate into the nucleus to activate the transcription of the target gene through binding with the T-cell transcription factor (TCF)/ lymphoid enhancer factor (LEF), which regulates transcription of genes involved in cellular proliferation, differentiation, survival, and apoptosis. While Axin/APC/GSK3β/β-catenin would form a degradation complex in the absence of Wnt, leading to phosphorylation of β-catenin in the cytoplasm. And then TCF/LEF proteins recruit transcriptional co-repressors in the nucleus for the repression of target genes.

1.3 Hippo signaling cascade

The Hippo signaling pathway is involved in the regulation of various cell processes, such as cell proliferation, apoptosis, survival, migration, and differentiation. MST1/2 phosphorylates LAST1/2 and MOB1 for the activation of the Hippo pathway, and then the downstream components, YAP1 and TAZ are activated and accumulate in the cytoplasmic, leading to their sequestration and proteasome-mediated degradation in the cytoplasm. When the pathway is inactive, the dephosphorylated YAP1 and TAZ would translocate to the nucleus and bind to TEAD1-4, which induces transcriptional activity for cell proliferation and differentiation. Due to the complex origin and pathogenesis of glioma, it is of great significance to better understand the specific pathogenesis of glioma for the development of targeted therapy. YAP1, which is found up-regulated in glioma, could be a potential molecular target for the treatment of glioma.

1.4 Akt signaling cascade

The synthesized Akt is phosphorylated by the mammalian target of rapamycin complex 2 (mTORC2) or protected by the Heat Shock Protein 90 (HSP90) to prevent it from being degraded. Activation of Akt initiates from a series of phosphorylation processes when cells are exposed to the signaling molecules. Tyrosine kinase receptors, integrins, cytokine receptors, G-protein-coupled receptors, and other receptors could induce the production of PIP3 by PI3K, which is essential for the translocation of Akt to the plasma membrane. PI3K is frequently highly activated by RTKs under the stimulation of cytokines and growth factors in cancer cells. After recruitment to the plasma membrane, Akt is modified to allow phosphorylation on Thr308 and/or the Ser473 site for full activation.

2 Glioma diagnosis

2.1 Molecular Markers for Glioma

Conventional diagnosis of glioblastoma generally includes clinical presentation, MRI, and histopathological analysis. Compared with the generic classification of glioma grade provided by histopathological imaging, the molecular diagnosis could help with developing patient-specific therapies that target different pathways. For example, the most common aberrations involved in the progression of glioma include IDH mutations, EGFR amplification, P53 mutation, and abnormal signaling pathway involving RTK, Akt, PI3K, and Ras. A great number of studies are focused on developing agents targeting EGFR and its mutants, EGFR III, which can facilitate both molecular diagnosis and target therapies for glioma.

Mutations in metabolic gene IDH were first found by the genomic evaluation of glioblastoma (GBM), in which the amino acid 132 mutated from arginine to histidine in approximately 20% of the tumors analyzed. Further studies showed that the R132 mutation are found in 80-90% of grade II or grade III gliomas, as well as astrocytic and oligodendroglial subtypes. EGFR amplification is a common alteration of GBM and is observed in 40-50% of the patients. About a half of the EGFR mutated GBM express EGFR III, of which gene includes contains a deletion in exons 2-7, leading to the expression of a constitutively active protein that further dysregulates the EGFR pathway. Although the frequency of EGFR amplification in glioma is high, tyrosine kinase inhibitors (TKIs) that are effective in other cancers showed little response, which is probably due to the intratumoral heterogeneity of GBM.

2.2 Protein Markers for Glioma

Proteins exist in body fluids such as serum, CSF, and urine, which could be potential markers for the diagnosis and treatment response assessment of glioma. P53 is one of the most studied tumor suppressor proteins and is associated with almost every cancer including glioma. Alterations in the p53 pathway could promote the progression of low-grade glioma to high-grade. Direct mutations of the p53 gene are frequently detected in secondary GBMs. However, due to the complex functions that the p53 pathway has in cellular responses such as cell apoptosis, differentiation, and DNA damage response, the prognostic and predictive response value of the protein still needs to be further studied. PI3K is responsible for the synthesize of PIP3, which activates the downstream proteins PKB/Akt. Activation of the PI3K pathway is regulated by the EGFR and other stimulants. Although mutations of PI3K are detected in less than 15% of GMB, almost all GBMs show increased activity in the pathway. PTEN is a negative regulator of PI3K, which indirectly activates mTOR1/2 and participates in cell survival, proliferation, and migration. Mutations in PTEN are found in approximately 40% of GBM.

3 Targeted Therapy for Glioma

3.1 Glioma therapy for EGFR pathway

Strategies for targeting the EGFR pathway consist of anti-EGFR monoclonal antibodies and tyrosine kinase inhibitors (TKIs). There are currently three generations of TKIs approved for clinical use. The first generation of TKIs inhibits the EGFR by competitive binding with ATP, which include erlotinib, gefitinib, lapatinib, and vandetanib. Subsequent generations of TKIs are aimed to deal with drug resistance. Second-generation TKIs could inhibit four ERBB receptors irreversibly (afatinib, dacomitinib, tesevatinib), while the third-generation is designed specifically targeting the T790M resistance mutation. Osimertinib was the first third-generation TKI.

The monoclonal antibodies could bind to the extracellular component of the EGFR and block the binding of ligands. Cetuximab was the first monoclonal antibody to be announced with convincing preclinical data. It is a chimeric immunoglobulin G (IgG) antibody that can bind to the external domain of EGFR and lead to the internalization and degradation of EGFR. While panitumumab, the fully humanized antibody was developed with a lower risk of hypersensitivity reaction compared with cetuximab. Inhibition of angiogenesis is another strategy that plays an important role in tumor initiation, growth, and metastasis.

Table 1 Clinical trials of EGFR TKIs Afatinib

Table 2 Clinical trials of EGFR TKIs lapatinib

Table 3 Clinical trials of EGFR TKIs Osimertinib

Table 4 Clinical trials of EGFR mAB Cetuximab

3.2 Glioma therapy for Wnt pathway

Various small molecules targeting Wnt have been used to regulate the pathway, such as SFRP1, HBP1, Dkk1, and LGK974. SFRP1 is an important Wnt antagonist for the treatment of glioma malignancy that is related to miR 27a. Knockdown of HBP1 could abolish the proliferation inhibition of miR 155 and activate Wnt genes. Recent studies showed that the novel Wnt antagonist ICG 001 and agonist AZD2858 could inhibit proliferation, invasion, and survival of glioma effectively.

3.3 Glioma therapy for Hippo pathway

Verteporfin (VP), a porphyrin derivative, was identified as the inhibitor of YAP. The binding of VP and YAP can lead to a conformational change, which blocks the formation of the TEAD-YAP complex. Furthermore, VP can also up-regulate the level of 14-3-3s and retain YAP/TAZ in the cytoplasm.

3.4 Glioma therapy for Akt pathway

As the research investigating the impact of the PI3K/Akt pathway in glioma progress, therapeutic agents that target components of the pathway have been tested in clinical studies. Temsirolimus (mTOR inhibitor) showed a significantly longer median time in 36% of treated patients. Enzastaurin (PI3K/PKCb inhibitor) and everolimus (mTOR inhibitor) are well tolerated when administrated to newly diagnosed GBM patients. The combination of temsirolimus with bevacizumab and everolimus with temozolomide also showed safety when administered to glioma patients.

Table 5 Clinical trials of Akt pathway targeting glioma

Features

• An Introduction to Glioma
• Main Signaling Pathways in Glioma Therapy
• EGFR signaling cascade
• Wnt signaling cascade
• Hippo signaling cascade
• Akt signaling cascade
• Glioma Diagnosis
• Molecular Markers for Glioma
• Protein Markers for Glioma
• Targeted Therapy for Glioma
• Glioma for EGFR pathway
• Glioma for Wnt pathway
• Glioma for Hippo pathway
• Glioma for Akt pathway