Glioblastoma (GBM) is the most lethal and destructive primary malignant type of brain cancer that has been known for its aggressive growth, propensity to invade adjacent brain tissue, as well as resistance to treatment. Unlike the progress in treatment modalities i.e. surgical resection, radiation therapy and chemotherapy using temozolomide the prognosis for GBM patients is still poor with a median survival of less than 2 years (Mohammed et al. 2022). The rate of mortality of GBM highlights the immediate necessity for a deeper investigation of its molecular mechanisms of development and the introduction of more efficient therapeutic strategies. Besides, GBM is highly diffusive, which complicates the process of complete surgical removal and results in tumor recurrence as well as therapeutic resistance (Goenka et al., 2021). Additionally, the blood–brain barrier limits the access of chemotherapeutic agents to the site of a tumor, making it even more difficult to achieve the target and among other causes, cancer may also remain untreated. Despite these pitfalls, formidable progress in immunotherapy and targeted therapies have demonstrated their utility in improving patients’ prognosis in glioblastoma multiforme.
One of the GBM features is its heterogeneity at cellular and molecular levels which makes it aggressive and resistant to treatment (Jiang et al., 2022). Within the GBM tumors, there is a GSC subpopulation that carries a stem cell-like phenotype that entails self-renewal, multipotency, and the ability to initiate tumor formation (Jiang et al., 2022). GSCs are the prime factors in tumor initiation, further development, and recurrence, as well as in reducing response to standard therapies (Jiang et al., 2022). Hence, attacking GSCs and understanding the molecular systems that control their activity is a focal premise of novel GBM therapy. Besides, the latest studies have illustrated ADARs’ and RNA methylation’s function in preserving mushroom-like properties of GSCs and in GBM’s malignancy evolution (Visvanathan et al, 2019). RNA editing, ADAR expression, and GSC behavior, in the context of the tumor microenvironment, are tightly interconnected with each other. Such understanding can provide new information about the mechanism(s) underlying the GBM heterogeneity and resistance to therapy. This awareness of such complexities allows medicine to switch from general treatments to the application of more specified anti-GSC approaches that can help overcome drug resistance in GBM patients.
RNA editing is a modification process that happens post-transcriptionally and can affect the sequence and structure of RNA molecules and, in turn, gene expression and protein function (Huntley et al., 2016). Adenosine deaminases acting on RNA (ADARs) is a family of enzymes that catalyze RNA editing reactions, prominently adenosine-to-inosine (A-to-I) conversion, which occurs mainly in double-stranded RNA (Anantharaman et al., 2017). The role of two members of the ADAR family, ADAR1 and ADAR2, encoded by the ADARB1 and ADARB2 genes, has been proved to contribute to various cellular functions, covering neuronal development, synaptic transmission or cancer progression (Huntley et al., 2016).
Also, RNA editing by ADARs has been found to be involved in immunomodulation, RNA stability, and splicing alternative, which, once again, property underlines the versatility of this enzyme (Sinigaglia et al., 2019). Dysfunction in RNA (adenosine to inosine) editing has been linked to the causation of a range of human diseases such as neurological disorders, autoimmune diseases, and cancer, which illustrates the significance of understanding the molecular mechanisms associated with this editing process (Costa Cruz & Kawahara, 2021; Kung et al., 2018). The RNA editing activity of the ADAR2 protein, which is encoded by the ADARB1 gene, is highly regulated in the brain and exerts great effects on the glutamate and serotonin receptors (Maroofian et al, 2021). However, the contradiction is that ADAR1 has both catalytic and non-catalytic functions and also is involved in the regulation of immune response and tumorigenesis (Maroofian et al., 2021). Hence, examining the involvement of ADARB1 and ADARB2 in GBM could yield important connotations with regard to the disease mechanisms and the possibility of developing a therapeutic intervention.
In understanding GBM, modifications observed in the patterns of RNA editing suggest a potential role of ADARs in carcinogenesis and tumor development (Huntley et al., 2016). Various aberrant RNA editing events have been demonstrated in GBM and occur in genes that are important for the regulation of cell proliferation, apoptosis, and DNA repair (Huntley et al., 2016). Furthermore, ADAR2 has also been found to govern the editing of GluR-B mRNA, which translates into a gene that encodes the GluR-B subunit of NMDA receptors, targeting synaptic transmission and plasticity in the brain (Maroofian et al., 2021).
The tumor microenvironment with such characteristic modalities as hypoxia and nutrient deprivation serves an important role in tumor progression and therapeutic resistance (Yan et al., 2022). Hypoxia which occurs as a consequence of inadequate delivery of blood and oxygen to tumor tissues is a property of solid tumors, and it is related to tumor aggressiveness, chemoresistance, and poor prognosis (Yan, et al., 2022). Low glucose concentrations, which occur after tumor cells consume the glycogen they need for rapid proliferation, can affect cellular metabolism and gene expression (Yan et al., 2022). The functional interactions between HIFs (hypoxia-inducible factors) and oncogenic signaling pathways are known to participate in the expression of genes related to tumor angiogenesis, migration, and metastasis, and thus lead to the aggressive behavior of GBM (Fakhri et al., 2023). Furthermore, nutrient deprivation can elicit stress responses like the mammalian target of the rapamycin (mTOR) pathway, which promotes cell survival and proliferation under biological stress (Kim & Guan, 2019). Hence, decoding RNA editing through ADAR expression and their impact on GBM hypoxia and glucose deprivation would help to understand the molecular mechanisms underlying the tumor progression and the therapeutic resistance.
This study’s goal is to explore the mRNA expression of the ADARB1 and ADARB2 genes in LN-18 human glioblastoma cell lines cultured in standard conditions, and under hypoxia and low glucose conditions. The prediction is that the hypoxia and the glucose deprivation will change the expression of ADAR genes which will cause changes in the RNA editing patterns contributing to the aggressive phenotypes of GBM. Knowing the ADAR’s role in GBM in those situations sheds novel light on the mechanisms of this lethal disease and can direct research to specific drug targets for therapy. By illuminating the molecular mechanisms involved in the development of GBM as well as therapeutic resistance, this study will make a prospective contribution to the innovation of therapeutic approaches that will result in a better prognosis for GBM patients.
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