**3.7. Tumor-promoting inflammation and microRNAs**

**3.5. Antiapoptosis and microRNAs**

42 Neurooncology - Newer Developments

suppressing the expression of ITGB8 target (61).

dependent kinase inhibitors p27 and p57 (100).

present in GBM and affects the prognosis of patients.

**3.6. Genome instability and microRNAs**

increased acetylation of histone H3.

Among the tumor-associated miRNAs, miR-21 is frequently overexpressed in various types of tumors, including GBMs (7, 9), and plays a critical role in cell death and apoptosis. Knock‐ down of miR-21 in cultured GBM cells triggered activation of caspases and led to increased apoptotic cell death (8). miR-21 knockdown disrupts glioma growth *in vivo* and displays synergistic cytotoxicity when combined with the agent tumor necrosis factor-related apoptosis (s-TRAIL), leading to an increase in caspase activity (95). In particular, it was reported that the downregulation of miR-21 in GBM-derived cell lines resulted in an increased expression of a specific target, PDCD4 (programmed cell death 4), a known tumor suppressor gene, with a consequent decrease in proliferation and increase in apoptosis (96). Furthermore, miR-21 regulated a network of p53, TGF-b, and mitochondrial apoptosis tumor suppressor genes in GBM cells, targeting HNRPK (heterogeneous nuclear ribonucleoprotein K), TP53BP2 (tumor

protein p53-binding protein 2), and p63, a member of the p53 family of genes (97).

miR-92a and miR-92b, upregulated in glioma, target BCL2L11 (98) and NLK (99), respectively. Their inhibition promotes tumor-suppressive phenotypes through the induction of apoptosis. In addition, miR-93 was found to be upregulated in glioma specimens and resulted in an enhancement in cell survival, promoting sphere formation, and augmenting tumor growth by

miR-221 and 222 were extensively investigated in glioma. These miRNAs were found to be upregulated in this cancer, and both were reported to target the cell growth-suppressive cyclin-

Genomic instability is defined as a high frequency of mutations within the genome, including changes in nucleic acid sequences, chromosomal rearrangements, or aneuploidy. The stability of the human genome is maintained by multiple mechanisms such as the cell cycle checkpoint, DNA damage response, and mitotic separation machinery. Defects in the DNA damage response could cause genomic DNA mutations, deletions, insertions, or gross chromosomal gains and losses upon cell division and subsequently lead to cancer. Genomic instability is

Several members of the miR-106b-25 cluster and its paralog miR-17-92 cluster were associated with DNA replication and mitosis. Overexpression of miR-106b-5p in glioma tumor cells significantly promoted cell proliferation, suggesting a role of this miRNA in cell cycle regula‐ tion (101). A mechanistic study revealed that two target genes, retinoblastoma-like 1 (RBL1) and RBL2, were involved in miR-106b-5p's regulation of cell proliferation (101). RBL proteins were involved in genomic instability, coinciding with decreased DNA methylation and

p27, one of the cyclin-dependent kinase inhibitor, inhibits cell cycle progression at the G1 phase by blocking the activation of cyclin-CDK complexes (102). The downregulation of p27 expression by miRNAs could abrogate the cell cycle checkpoints and could increase DNA damages in GBM cells. The regulatory effect of miR-221/222 on p27 and the subsequent effect

NF-kB is a transcription factor with pleiotropic activity owing to its central roles in inflamma‐ tory processes. A critical regulator of NF-kB activation is the IkB kinase (IKK-β) complex (110). Ectopic expression of miR-218, which is downregulated in GBM, reduced NF-kB activity, whereas inhibition of miR-218 enhanced the transcriptional activity of NF-kB (78). miR-218 could inactivate NF-kB signaling by directly targeting the 3′-UTR of the IKK-β (78). miR-21 was revealed as another regulator involved in the NF-kB-signaling pathway in GBM. Li et al. confirmed LRRFIP1 (leucine-rich repeat interacting protein 1) as a direct target of miR-21 (111). LRRFIP1 is a transcriptional repressor that preferentially binds to the GC-rich consensus sequence and regulates expression of TNF, EGFR, and PDGFA signaling. miR-21 contributes to drug resistance through the depression of LRRFIP1 expression, leading to the reduction of cytotoxicity of chemotherapeutic drugs through the activation of the NF-kB pathway (111).

miR-146a is known as miRNA associated with the innate immune response to microbial infection (112). Promotor analysis revealed that miR-146a is a NF-kB-dependent miRNAs (112). miR-146a has complementary sequences in the mRNA of the TNF receptor-associated factor 6 (TRAF6) and IL-1 receptor-associated kinase 1 (IRAK1), key adaptor molecules downstream of toll-like and cytokine receptors (113). miR-146a overexpression enhanced apoptosis and suppressed NF-kB activation in TMZ-treated GBM cells (114). Increased expression of miR-146a was observed in glioneuronal lesions (115), and miR-146a expression in human glial cells was strongly induced by IL-1β (115). miR-146a expression by transfection in astrocytes regulated many mRNA expression levels, including those of IRAK-1, IRAK-2, and TRAF-6. In addition, the expression of IL-6 and COX-2 was suppressed by miR-146a (115).

Recent evidence suggests that the signal transducer and activator of transcription (STAT) family proteins play a crucial role in selectively inducing and maintaining a procarcinogenic inflammatory microenvironment, both upon the initiation of malignant transformation and during cancer progression (116). Upon upregulating miR-124, a miRNA downregulated in GBM, specifically in glioma cancer stem cells (gCSC), the STAT3 pathway was inhibited, and miR-124 reversed gCSC-mediated immunosuppression of T cell proliferation and induction of regulatory T cells (Treg) (117). Systemic administration of miR-124-transfected T cell transfers exerted potent antiglioma therapeutic effects in murine models of GBM (117).

Human glioma immune activation is potently elicited by a cytokine combination. Cytokines such as IL-1β and TNFα induce the expression of miR-155 and miR-155\*, the miRNAs crucial in immunity and inflammation-induced oncogenesis, and this expression is dose dependently suppressed by IRF3. Importantly, IRF3 also inhibits glioma proliferation, migration, and invasion (118).
