The emerging role of miR‐19 in glioma

Abstract Glioma has been regarded as the most common, highly proliferative and invasive brain tumour. Advances in research of miRNAs in glioma are toward further understanding of the pathogenesis of glioma. MiR‐19, a member of miR‐17~92 cluster, was reported to play an oncogenic role in tumourigenesis. Here we review the identified data about the effect of miR‐19 on proliferation, apoptosis, migration and invasion of glioma cells, the target genes regulated by miR‐19, and correlation of miR‐19 with the sensitivity of glioma cells to chemotherapy and radiotherapy. It is concluded that miR‐19 plays an important role in the pathogenesis of glioma and can be a potential target for gene therapy of glioma.


| INTRODUCTION
MicroRNAs, a class of endogenous non-coding RNA with [18][19][20][21][22][23][24][25] nucleotides, repress the expression of corresponding genes by binding to the 3′-UTR region of target genes at post-transcriptional level. 1,2 MiRNAs have been identified to participate in a variety of cell biological processes such as proliferation, apoptosis, migration and invasion. 3,4 During the development of cancer, miRNAs are dysregulated and play oncogenic or tumour suppressive role by enhancing or suppressing proliferation, invasion of tumour cells. 5,6 Thus, miRNA deregulation is one of the key mechanisms in glioma pathogenesis. The relevant miRs can be used as new targets of glioma therapy and provide clues for diagnosis. This review will discuss the role of miR-19 in glioma cell proliferation, apoptosis and migration and its effect on chemotherapy and radiotherapy of glioma.
MiR-19 is a member of miR-17-92 cluster, this cluster participates not only in the development of heart and lung, 7,8 but also in ageing and cancer. 9 The target genes of miR-17-92 cluster have been experimentally identified so far including: STAT3, Map-k14 10 and Rb2/p130. 11 MiR-17-92 cluster plays an important role in tumourigenesis of thyroid cancer, leukaemia and lymphoma. [12][13][14] The expression of miR-17-92 cluster is up-regulated in glioma tissues. MiR-17-92 cluster inhibition decreases cell proliferation and induces apoptosis in glioblastoma spheroids culture by upregulating the expression of CDKN1A (cyclin-dependent kinase inhibitor 1A), E2F1 and PTEN. 15 MiR-17-92 cluster is regarded as the first miRNA cluster with oncogenic potential, 15 23 (Figure 1).

| MIR-19 IN GLIOMA
The expression of miR-19 is up-regulated in glioma. In 75 archival paraffin-embedded glioma specimens with different grades of malignancy and five normal control, miR-19 is significantly up-regulated in glioma tissues and positively correlates with the tumour grade. 24 Increased expression level of miR-19 is also detected in glioma cell lines. 25 MiR-19 is confirmed to participate in the process of glioma recurrence. A research report has demonstrated that miR-19 is progression-associated up-regulation in patients who has been operated as WHO grade II originally and spontaneously progress to WHO grade IV secondary glioblastoma. 26 This indicates that miR-19 plays an important role in glioma progression. MiR-19 is also regarded as prognostic biomarker of glioma, high expression of miR-19 in patient's serum is associated with poor survival. 27 MiR-19 exerts its effect on biological characters of tumour cells through regulation on its target genes. It has been identified to regulate hundred of target genes in TargetScan and Pictar database, among them there are a few target genes have been experimentally confirmed such as PTEN, 24 which play a significant role in glioma pathogenesis and progression. The effect of miR-19 on glioma cell proliferation, apoptosis and migration and the impact of miR-19 on chemotherapy and radiation therapy of glioma will be discussed separately as follows.

| MiR-19 and apoptosis
Studies demonstrate that miR-19 inhibits apoptosis of glioma cells.
Anti-miR-19 (antisense oligonucleotide of miR-19) is introduced to knock down miR-19 expression, apoptosis is induced in glioma cells. 28 We have confirmed PTEN as the target gene of miR-19 experimentally. 24 PTEN plays a significant role as a tumour suppressor gene that induces glioma cell apoptosis and as a negative regulator of PI3K/AKT pathway, whereas PI3K/AKT inhibits apoptosis through repressing JNK and p38, or promoting FoxM1 expression in glioma. 29, 30 Wang also reported that miR-19 was suppressed by resveratrol in glioma and induced apoptosis, PTEN up-regulation and F I G U R E 1 Sketch of miR-17-92 cluster and miR-19 PI3K/AKT repression. 31 It has been reported that p53 is up-regulated when miR-19 is inhibited in glioma cells. 31 p53 is a key proapoptosis gene. p53 induces p53-dependent apoptosis through enhanced expression of transcription targets including STAG1, PUMA and PERP. [32][33][34] MiR-19 expression has been identified up-regulated in glioma as oncomiR, 24 apoptosis inducing proteins such as PTEN (directly suppressed by miR-19) and p53 (negative regulated by miR- 19) are suppressed, so miR-19 reduces apoptosis to promote tumour cell survival. It has been reported that miR-19 also inhibit apoptosis in SH-SY5Y human neuroblastoma cells, transfection of miR-19 inhibitor leads to induction of apoptosis and increases expression of apoptosis-related proteins including PTEN, p53, Bax and caspase-3, decreases the expression of Bcl-2. 35

| MiR-19 and cell proliferation
Inhibition of miR-19 by anti-miR-19 results in diminished proliferation of glioma cell in vitro. 28 MiR-19 depresses growth of glioma cells by negative regulation of PTEN, which can inhibit glioma cell proliferation by suppressing PI3k/AKT pathway. 36 It also has been reported that miR-19 promotes glioma progression by directly suppressing PPARα (the peroxisome proliferator-activated receptor α, PPARα). 37

| MiR-19 and cell migration
It has been demonstrated that knocking down miR-19 suppresses migration of glioma cell, 28  inhibitor, the expression of its target genes will be regulated accordingly, so it is predictable that multiple genes therapeutic effect can be achieved if novel anti-oncomiR-19 measures are adopted.

| MiR-19 and radiotherapy and chemotherapy sensitivity of glioma
In order to improve the prognosis of patients, malignant glioma patients are given radiotherapy and chemotherapy simultaneously after surgical resection. However, resistance to radiotherapy and chemotherapy turn out to be the important source of glioma recurrence.
MiR-19 has been reported to exert effects on drug resistance in diverse tumour chemotherapy. In breast cancer, miR-19 expression is up-regulated in MDR (multidrug resistance) cell lines com- Leung has shown that miR-19 is significantly up-regulated in breast cancer cell MDA-MB-361 cells after exposed to radiation, indicating that miR-19 appears to be radiation-associated miRNA in breast cancer. 61 Knocking down miR-19 in SiHa cervical cancer cells, the radiotherapy sensitivity of SiHa cells is increased and cell proliferation is inhibited. 62 Above studies demonstrate that miR-19 is associated with radiosensitivity of tumour cells. However, research on the relationship between miR-19 and radiotherapy in glioma is limited. Chaudhry has reported that miR-19 is up-regulated in both irradiated glioma cell line M059J which is deficient in DNA-PK (DNA-dependent protein kinase) and glioma cell line M059K with normal DNA-PK activity, indicating that miR-19 paritcipates in the reaction of different types of glioma cell lines to ionizing radiation. 63 Some of the target genes of miR-19 are related to the radiosensitivity of glioma cells. As previously reported, PTEN overexpression leads to an increase in sensitivity to ionizing radiation in glioma cells, 60 LRIG1 enhances the sensitivity of radiotherapy in glioma cells by suppressing EGFR/AKT pathway. 64 However, RhoB induces F I G U R E 2 The role of miR-19 in proliferation, migration, apoptosis and treatment of glioma radioresistance in glioma cells. 65 It has been identified that CtIP is the target gene of miR-19 and CtIP plays a role in the DNA end resection and homologous recombination in response to DNA damage. 66 Then we can infer that miR-19 aberrant expression down-regulating CtIP suppresses the repair of the most hazardous type of lesion, DNA double-strand breaks induced by ionizing radiation, and may be helpful to enhance the effect of radiotherapy. Certainly, this assumption will be contradictory to the anti-miR-19 therapy for its oncogenic potential in glioma and the relationship between miR-19 and radiation response of glioma needs to be investigated further.

| CON CLUSION AND PERSPECTIVE
To date, studies on miR-19 have been demonstrated to reduce apoptosis, promote proliferation and migration of glioma cells. Besides, it is associated with sensitivity of chemotherapy and radiotherapy of glioma cells. Intensive studies of the mechanism by which miR-19 regulates proliferation and invasion of glioma cells will be helpful for providing a noval therapeutic target for glioma treatment. Standard care of glioma patients with combining surgery, chemotherapy and radiation therapy does not improve the survival rate of patients. Targeting oncomiRs seems to be the new trends in the development of miRNA therapeutic strategies. Thus, miR-19 can be a new target in glioma treatment, or might regulate the effect of chemotherapy and radiotherapy by adjusting the sensitivity of glioma cells, which will provide a new clue for glioma therapy (Figure 2). Further study on miR-19 will demonstrate its more roles in the pathogenesis of glioma, biomarker of prognosis and glioma treatment.

ACKNOWLEDGEMENT
This work was supported by the National Natural Science Foundation of China (Grant Nos. 81101915).

CONF LICT OF I NTEREST
The authors declare that we have no conflict of interest.