**8. MicroRNAs with oncogenic potential: antiapoptotic and proliferative functions**

### **8.1. MiR-9**

The gene that codifies miR-9 is located in the genome of three different regions: miR-9-1 is located in the 1q22 chromosome, miR-9-2 in 5q14.3 and miR-9-3 in 15q26.1. This miRNA is expressed almost exclusively in the brain and it is a neurogenetic mediator. In the fetal brain, it is highly expressed, compared with that of an adult [49]. Nass et al. studied the expression of several miRNAs in primary brain tumours and metastatic brain tumours through microarrangements and qRT-PCR, and they observed that miR-9/9\* were mainly overexpressed in primary brain tumours, by comparison with metastatic brain tumours, and they concluded that it is possible to distinguish between both types of tumours with a high degree of reliability [50]. Up until now, it has only been described as one of its targets for the REST transcription factor.

### **8.2. MiR15b**

Located in chromosome 3q25.33, Xia et al. identified a panel of miRNAs expressed differentially in glioma tissue. One of the significantly deregulated miRNAs was miR-15b. Afterwards, they identified their potential targets being CCNE1 (protein related with the transition of the cell cycle of G1/s) as one of them. The levels of expression of RNAm of CCNE1 in the cell lines after transfection with exogenous miR-15b were analysed, as the anti-senses of miR-15b, and they observed that the levels remained without changes. Nevertheless, protein levels of CCNE1 were significantly reduced after the transfection with exogenous miR-15b and they were increased after transfecting the antisense of miR-15b. These results suggest that CCNE1 is a potential target. The overexpression of this miRNA causes arrest in the cell cycle in its G0/G1 phase, whereas its inhibition results in a reduction of the cell population in G0/G1 and therefore also represents an increase in phase S (**Figure 5**) [40].

high-grade astrocytomas, because they observed a greater expression in high-grade tumours and a lower survival rate compared with low-grade astrocytomas [43]. It is evident that the overexpression of miR-21 in astrocytomas results in the activation of multiple oncogenic routes [57]. Many other studies have confirmed the over-expression of this miR in the four grades of astrocytomas and in other tumours of the SNC as oligoastrocytoma, oligodendroglioma and medulloblastomas, having a greater expression in the multi-shaped glioblastoma (**Figure 6**) [26, 38, 58].

**Figure 5.** Signalization routes influenced by miR-21 in glioblastoma cells. MiR-21 regulates apoptosis, cell cycle and

Astrocytomas and miRNAs: Are They Useful? http://dx.doi.org/10.5772/intechopen.77419 65

MiR-221/222 are located in chromosome Xp11.3 and they are over-expressed in astrocytomas, their expression is co-regulated and they have the same specificity of targets because the region considered as "origin" or "seed" region is the same in both cases [7]. Ciafrè et al., through microarrangements of expression and Northern blot, analysed nine samples of patients with GBM and 10 cell strands of glioma and identified miR-221 as one of the miR-NAs with greatest overexpression in comparison with values obtained in normal brains and samples of healthy tissue that were close to the tumour [38]. Gillies et al. 2007 described p27kip1 as a direct target of miR-221/miR-222. P27kip1 is a protein that regulates the cell cycle, its function is inhibiting the cyclin-depending kinase (CDK) in such a manner that there is an arrest in the cell cycle in the phase G1, avoiding cell proliferation [59] (**Figure 5**). Medina et al. studied the participation of several microRNAs in the regulation of the cell cycle and observed that the expression of miR-221 and miR-222 was increased in human quiescent cells which are stimulated for proliferation. They predicted and proved two targets: p27 and p57; both suppress the cell growth because they inhibit cyclin-dependent kinases. The over-expression of these miRs is closely linked to the control of the cell cycle, which assures

**8.4. MiR-221/222**

translation [52].

### **8.3. MiR-21**

The gene which codifies for the miR-21 is located in chromosome 17q23.1. The overexpression of this miRNA was described for the first time in the GBM and afterwards in other types of solid tumours [31, 44]. Chan et al. studied the expression of miR-21 in patients with GBM and in cell lines of gliomas and observed that, in tissues, the expression of miR-21 was increased five to 100 times in comparison with non-neoplastic brain tissues. There are several important targets which contribute to its anti-apoptotic and proliferative actions, such as some molecules involved in the suppressor tumour routes for p53, TGF-β (β-transforming growth factor) and a mitochondrial apoptotic route [52–54]. In a recent study, developed with 124 samples of astrocytomas of high and low grade, it was found that miR-21 is more sensible to predict the clinical development of

**Figure 5.** Signalization routes influenced by miR-21 in glioblastoma cells. MiR-21 regulates apoptosis, cell cycle and translation [52].

high-grade astrocytomas, because they observed a greater expression in high-grade tumours and a lower survival rate compared with low-grade astrocytomas [43]. It is evident that the overexpression of miR-21 in astrocytomas results in the activation of multiple oncogenic routes [57]. Many other studies have confirmed the over-expression of this miR in the four grades of astrocytomas and in other tumours of the SNC as oligoastrocytoma, oligodendroglioma and medulloblastomas, having a greater expression in the multi-shaped glioblastoma (**Figure 6**) [26, 38, 58].

### **8.4. MiR-221/222**

analysed in this study and which are considered in the study as having oncogenic potential are miR-15b, miR-21 and miR-221 and the tumour suppressors are miR-124, miR-128, miR-137 and

The gene that codifies miR-9 is located in the genome of three different regions: miR-9-1 is located in the 1q22 chromosome, miR-9-2 in 5q14.3 and miR-9-3 in 15q26.1. This miRNA is expressed almost exclusively in the brain and it is a neurogenetic mediator. In the fetal brain, it is highly expressed, compared with that of an adult [49]. Nass et al. studied the expression of several miRNAs in primary brain tumours and metastatic brain tumours through microarrangements and qRT-PCR, and they observed that miR-9/9\* were mainly overexpressed in primary brain tumours, by comparison with metastatic brain tumours, and they concluded that it is possible to distinguish between both types of tumours with a high degree of reliability [50]. Up until now, it has only been described as one of its targets for the REST transcription factor.

Located in chromosome 3q25.33, Xia et al. identified a panel of miRNAs expressed differentially in glioma tissue. One of the significantly deregulated miRNAs was miR-15b. Afterwards, they identified their potential targets being CCNE1 (protein related with the transition of the cell cycle of G1/s) as one of them. The levels of expression of RNAm of CCNE1 in the cell lines after transfection with exogenous miR-15b were analysed, as the anti-senses of miR-15b, and they observed that the levels remained without changes. Nevertheless, protein levels of CCNE1 were significantly reduced after the transfection with exogenous miR-15b and they were increased after transfecting the antisense of miR-15b. These results suggest that CCNE1 is a potential target. The overexpression of this miRNA causes arrest in the cell cycle in its G0/G1 phase, whereas its inhibition results in a reduction of the cell population in G0/G1 and

The gene which codifies for the miR-21 is located in chromosome 17q23.1. The overexpression of this miRNA was described for the first time in the GBM and afterwards in other types of solid tumours [31, 44]. Chan et al. studied the expression of miR-21 in patients with GBM and in cell lines of gliomas and observed that, in tissues, the expression of miR-21 was increased five to 100 times in comparison with non-neoplastic brain tissues. There are several important targets which contribute to its anti-apoptotic and proliferative actions, such as some molecules involved in the suppressor tumour routes for p53, TGF-β (β-transforming growth factor) and a mitochondrial apoptotic route [52–54]. In a recent study, developed with 124 samples of astrocytomas of high and low grade, it was found that miR-21 is more sensible to predict the clinical development of

miR-221. Next, each one of them is described [5, 23, 45–48].

64 Glioma - Contemporary Diagnostic and Therapeutic Approaches

therefore also represents an increase in phase S (**Figure 5**) [40].

**proliferative functions**

**8.1. MiR-9**

**8.2. MiR15b**

**8.3. MiR-21**

**8. MicroRNAs with oncogenic potential: antiapoptotic and** 

MiR-221/222 are located in chromosome Xp11.3 and they are over-expressed in astrocytomas, their expression is co-regulated and they have the same specificity of targets because the region considered as "origin" or "seed" region is the same in both cases [7]. Ciafrè et al., through microarrangements of expression and Northern blot, analysed nine samples of patients with GBM and 10 cell strands of glioma and identified miR-221 as one of the miR-NAs with greatest overexpression in comparison with values obtained in normal brains and samples of healthy tissue that were close to the tumour [38]. Gillies et al. 2007 described p27kip1 as a direct target of miR-221/miR-222. P27kip1 is a protein that regulates the cell cycle, its function is inhibiting the cyclin-depending kinase (CDK) in such a manner that there is an arrest in the cell cycle in the phase G1, avoiding cell proliferation [59] (**Figure 5**). Medina et al. studied the participation of several microRNAs in the regulation of the cell cycle and observed that the expression of miR-221 and miR-222 was increased in human quiescent cells which are stimulated for proliferation. They predicted and proved two targets: p27 and p57; both suppress the cell growth because they inhibit cyclin-dependent kinases. The over-expression of these miRs is closely linked to the control of the cell cycle, which assures

20q13.33. It is the most profuse brain-specific miRNA; during neural differentiation, it expresses itself mainly in neurons [49]. It is considered a tumour suppressor weakly expressed in anaplastic astrocytomas and GBM, in relation with the non-neoplastic brain tissue. In this regard, Silber et al. studied the expression of several miRNAs during the differentiation of adult neural stem cells, and it was observed that miR-124 increased its expression eight times, instead of what happens in high-grade tumours, where their expression is less. In this same study, they also determined that miR-124 may induce differentiation and inhibit the proliferation of glioblastoma stem cells when inhibiting CDK6 (cyclin 6, dependent on kinases) which,

Astrocytomas and miRNAs: Are They Useful? http://dx.doi.org/10.5772/intechopen.77419 67

MiR-128-1 is located in chromosome 2q21.3 and miR-128-2 in 3p22.3. It is an miRNA specific to the brain, where it finds itself enriched. On the other hand, in gliomas and glioma cell strands, its expression is lowered [25, 38, 42]. Zhang et al. studied the expression of miR-128 in astrocytomas GII, GIII and GIV and in cell strands, and they observed that it lowers itself progressively as the grade of the tumour increases. Its tumour-suppressing characteristics were evidenced when transfecting miR-128 in glioma cell strands, observing an inhibition in cell proliferation [25]. Godlewski et al. proved the low expression of miR-128 in gliomas and in cell strands and focused in finding a target that was related with cell differentiation and self-renewal. MiR-128 makes up for an important biological target against the 'tumour stem cells' which are characteristic and part of the origin of the glioma

as a goal, promotes the progress of the cell cycle (**Figure 8**) [45, 54].

**9.2. MiR-128**

(**Figure 9**) [42].

**Figure 8.** MiR-128 has Bmi and E2F as its main targets.

**Figure 6.** The miR-221 oncogene promotes the progression of the cell cycle because it inhibits the translation of the tumour suppressor p27kip1, whose reduction causes the expression of CDK and, with it, the progress of the cell cycle [52].

**Figure 7.** MiR-124 and miR-137 has CDK6 as target, CDK6 which is a regulator of the cell cycle and differentiation. PTBP1 is also one of its targets and is related with alternative 'splicing'.

the survival of the cell by a coordinated competence between the entrance in phase S and signalization routes of the growth factor that stimulates the cell proliferation [55]. The high expression of miR-221 in high-grade astrocytomas and cell strands, and they strongly imply that it is a candidate to becoming a specific tumour marker (**Figure 7**) [59].
