**3. Role of microRNAs (miRs) in the progression of ovarian cancer and their relation with nerve growth factor**

New targets of NGF and its receptor TRKA include various microRNAs (miRs). Since the 1990s, deregulation of miRs has become important in several pathological processes, including several types of cancer [63]. Currently, miRs could be used as new biomarkers and/or for therapy in various diseases [64]. Particularly in ovarian cancer some miRs are downregulated or upregulated [65], and NGF and its receptor TRKA could be implicated in the deregulation of some miRs.

MicroRNAs are the biggest family of non-coding RNAs; they are ~22-nucleotides (nts) long and regulate mRNAs post-transcriptionally [66]. The first step on miR biogenesis is the synthesis of a long primary miR (pri-miR) by an RNA polymerase II. Then, the pri-miR is cleaved, producing a pre-miR [67] that is transported to the cytoplasm to be enzymatically cleaved in its loop structure, releasing a double-strand miR called duplex [68]. This duplex has two strands, one called "mature" or "guide" miR and the other named "passenger", which is released and degraded [69]. Mature miR has ~22 nts and binds to the three-prime untranslated region (3′-UTR) of a target mRNA in order to regulate protein expression. This regulation depends on miR-mRNA complementary: total complementarity of miR with its mRNA target is a signal to cleave or degrade the mRNA. On the other hand, partial complementarily induces deadenylation of the mRNA target (facilitating its degradation) or inhibition of its translation [70]. In normal cells, microRNAs have an important role maintaining their normal functioning; however, a deregulation in their expression can lead to cellular alterations. Most studies concerning miR roles in pathologies evaluate whether there are changes on miR expression; therefore, miR targets are still being described. Regarding these targets, one miR has several targets, meaning that one miR can be involved in the development of different pathologies.

Cancer development involves miR deregulation. Cancer-related miRs are divided in two groups: oncogenic (oncomiR) and tumor suppressor (oncosuppressor) miRs; oncomirs regulate the mRNA of tumor suppressor genes, while oncosuppressors control the mRNA of oncogenes. Both of these types of miRs are normally in equilibrium; however, during carcinogenesis, they exhibit a deregulation on their expression [71]. One miR can regulate the same mRNA targets in different types of cancer, which makes them an attractive target for the development of new therapies.

Besides their potential as therapeutic targets, currently, miRs' profiles are being described in order to obtain more accurate and reliable biomarkers for cancer development and/or progression [64]; in EOC, several miRs have been found to be upregulated [72].

Interestingly, it has been found that eight miRs could be regulating 89% of the miR-associated genes [73]. Thus, to produce a more accurate clinical diagnosis, it would be beneficial to have miR profiles as biological markers.

EOC development and progression is regulated by several miRs. OncomiRs and tumor suppressor miRs modulate different processes of the hallmarks of cancer, such as proliferation, angiogenesis, migration, invasion, survival and apoptosis, among others (**Table 1** summarizes the most important miRs involved in different cancers, including EOC).

As discussed above, NGF is overexpressed in EOC and it has a significant role in the progression of this disease [35]. Interestingly, studies show that NGF could regulate the expression of some miRs. Most of these studies have been done in PC12 cells: in these cells, NGF stimulation increases the expression of several miRs [74].

Besides, in EOC, an increase of NGF levels induces the expression of c-MYC transcription factor [80], and c-MYC downregulates the miR-23b expression [81]. This miR levels decrease in EOC, and we described that after NGF stimulation, EOC cells diminish miR-23b levels [80]. Therefore, in this cancer, NGF could reduce miR-23b levels through c-Myc. miR-23b targets cell cycle and tumor growth proteins, regulating cyclin-G1 [82] and SP-1 transcription factor

NGF stimulation regulates miRs in these cancers through the upregulation of several miRs, including miR-92a, miR-21

and miR-221/222, while it downregulates other miRs, such as miR-23b and miR-143.

**NGF-related miR Regulation Cancer References** miR-92a ↑ Neuroblastoma [102] miR-21 ↑ Pheocromocitoma [103] miR-221/222 ↑ Pheocromocitoma [77] miR-23b ↓ Ovarian cancer [80] miR-143 ↓ Pheocromocitoma [75, 76]

Signaling Pathways Related to Nerve Growth Factor and miRNAs in Epithelial Ovarian Cancer

http://dx.doi.org/10.5772/intechopen.73804

47

Solid scientific evidences indicate that NGF has important roles in the progression of EOC by promoting the expression or activation of several proteins involved in the different carcinogenic processes, including cell proliferation, angiogenesis and in therapy resistance. For instance, NGF interaction with its TRKA receptor can activate AKT and ERK signaling, promoting cell proliferation and survival. TRKA activation by NGF also increases COX-2 and PGE2 levels, contributing to inflammatory processes, which are important to cancer progression. Besides, NGF can act on the ADAM17 metalloproteinase, which cuts the p75NTR receptor in EOC cells, leaving an intracellular fragment that can activate transcription and that can interact with TRKA, increasing its carcinogenic effects. Furthermore, NGF could modulate the immune response, since it can reduce CRT translocation from the endoplasmic reticulum

Additionally, it is relevant to point out that recent reports describe how NGF regulates the expression of different miRs, which in turn could affect the translation of protein participants of the abovementioned processes. Some examples include miR-143, whose levels are downregulated EOC and correlate with an increase of COX-2 levels. Another miR regulated by NGF is miR-222, which targets the metalloproteinase inhibitor TIMP3, an ADAM17 inhibitor. Furthermore, NGF stimulation reduces miR-23b levels through c-Myc, targeting the cell cycle and tumor growth proteins. Therefore, there is evidence to suggest that NGF-dependent miR regulation could lead to tumor development. Nevertheless, further studies are needed to confirm NGF's role in EOC; therefore, it is important to evaluate new miRs associated with EOC. These findings could result in new biomarkers used for diagnosis or target molecules

to the cell membrane, reducing cancer cells' recognition by immune cells.

that could allow the development of new therapies.

[81], respectively.

**Table 2.** List of miRs regulated by NGF.

**4. Conclusion**

Importantly, in EOC, miR-143 is downregulated [75], which is correlated with an increase of COX-2 levels [76]. As stated in the previous section, NGF increases COX-2 levels [28]. It also decreases the expression of miR-143 in PC12 cells [74]. Therefore, in EOC, the NGF-mediated COX-2 increase could be regulated through miR-143. Another miR regulated by NGF is miR-222 [77], which targets a metalloproteinase inhibitor (TIMP3) [78]. TIMP3 inhibits ADAM17 function [79]; then, NGF could increase miR-222 in order to decrease TIMP3 levels, allowing the ADAM17 activity. Consequently, NGF regulation of miR-143 and miR-222 could be important for EOC development, through the regulation of COX-2 levels and ADAM17 activity, respectively (summarized in **Table 2**).


One miR can be deregulated in different types of cancer; simultaneously, several miRs can be deregulated in one type of cancer. Some examples are described in the table, including oncomiRs and tumor suppressor miRs. miRs can have a dual role. A few of their mRNA targets are also depicted.

**Table 1.** List of miRs and some of their targets de-regulated in cancer.


NGF stimulation regulates miRs in these cancers through the upregulation of several miRs, including miR-92a, miR-21 and miR-221/222, while it downregulates other miRs, such as miR-23b and miR-143.

**Table 2.** List of miRs regulated by NGF.

Besides, in EOC, an increase of NGF levels induces the expression of c-MYC transcription factor [80], and c-MYC downregulates the miR-23b expression [81]. This miR levels decrease in EOC, and we described that after NGF stimulation, EOC cells diminish miR-23b levels [80]. Therefore, in this cancer, NGF could reduce miR-23b levels through c-Myc. miR-23b targets cell cycle and tumor growth proteins, regulating cyclin-G1 [82] and SP-1 transcription factor [81], respectively.

#### **4. Conclusion**

Besides their potential as therapeutic targets, currently, miRs' profiles are being described in order to obtain more accurate and reliable biomarkers for cancer development and/or pro-

Interestingly, it has been found that eight miRs could be regulating 89% of the miR-associated genes [73]. Thus, to produce a more accurate clinical diagnosis, it would be beneficial to have

EOC development and progression is regulated by several miRs. OncomiRs and tumor suppressor miRs modulate different processes of the hallmarks of cancer, such as proliferation, angiogenesis, migration, invasion, survival and apoptosis, among others (**Table 1** summa-

As discussed above, NGF is overexpressed in EOC and it has a significant role in the progression of this disease [35]. Interestingly, studies show that NGF could regulate the expression of some miRs. Most of these studies have been done in PC12 cells: in these cells, NGF stimula-

Importantly, in EOC, miR-143 is downregulated [75], which is correlated with an increase of COX-2 levels [76]. As stated in the previous section, NGF increases COX-2 levels [28]. It also decreases the expression of miR-143 in PC12 cells [74]. Therefore, in EOC, the NGF-mediated COX-2 increase could be regulated through miR-143. Another miR regulated by NGF is miR-222 [77], which targets a metalloproteinase inhibitor (TIMP3) [78]. TIMP3 inhibits ADAM17 function [79]; then, NGF could increase miR-222 in order to decrease TIMP3 levels, allowing the ADAM17 activity. Consequently, NGF regulation of miR-143 and miR-222 could be important for EOC development, through the regulation of COX-2 levels and ADAM17 activ-

**miR Regulation Cancer Targets References**

RAS, HMGA2, cyclin D2,

PTEN, DKK2, PDCD4,

RRAS2, TGFβR2

BIM, E2F1 PTEN [85, 87–89]

ERBB2, P53INP1, HDAC5 [85, 98–100]

[83–86]

[85, 90–93]

[84, 96]

[72, 82, 94, 95]

c-myc

TGFbR2

B-catenin

gression [64]; in EOC, several miRs have been found to be upregulated [72].

rizes the most important miRs involved in different cancers, including EOC).

miR profiles as biological markers.

46 Ovarian Cancer - From Pathogenesis to Treatment

tion increases the expression of several miRs [74].

ity, respectively (summarized in **Table 2**).

miR-125 family

Let-7 family ↓ Lung, hepatocellular, breast and ovarian

miR-17-92 ↑ Myeloma, breast, gastric and colon cancer

dual role. A few of their mRNA targets are also depicted.

**Table 1.** List of miRs and some of their targets de-regulated in cancer.

miR-21 ↑ Oral, colon, breast, glioma, ovarian and cervical cancer

miR-23a/b ↓ Colon, pancreatic and ovarian cancer MAP3K1, Cyclin G1,

miR-122 ↓ Hepatocellular cancer Wnt1, TCF4, Cyclin G1,

↑ Renal cell carcinoma, endometrial and breast cancer

miR-143 ↓ Gastric cancer COX2 [97]

↓ Ovarian cancer SET [101] One miR can be deregulated in different types of cancer; simultaneously, several miRs can be deregulated in one type of cancer. Some examples are described in the table, including oncomiRs and tumor suppressor miRs. miRs can have a Solid scientific evidences indicate that NGF has important roles in the progression of EOC by promoting the expression or activation of several proteins involved in the different carcinogenic processes, including cell proliferation, angiogenesis and in therapy resistance. For instance, NGF interaction with its TRKA receptor can activate AKT and ERK signaling, promoting cell proliferation and survival. TRKA activation by NGF also increases COX-2 and PGE2 levels, contributing to inflammatory processes, which are important to cancer progression. Besides, NGF can act on the ADAM17 metalloproteinase, which cuts the p75NTR receptor in EOC cells, leaving an intracellular fragment that can activate transcription and that can interact with TRKA, increasing its carcinogenic effects. Furthermore, NGF could modulate the immune response, since it can reduce CRT translocation from the endoplasmic reticulum to the cell membrane, reducing cancer cells' recognition by immune cells.

Additionally, it is relevant to point out that recent reports describe how NGF regulates the expression of different miRs, which in turn could affect the translation of protein participants of the abovementioned processes. Some examples include miR-143, whose levels are downregulated EOC and correlate with an increase of COX-2 levels. Another miR regulated by NGF is miR-222, which targets the metalloproteinase inhibitor TIMP3, an ADAM17 inhibitor. Furthermore, NGF stimulation reduces miR-23b levels through c-Myc, targeting the cell cycle and tumor growth proteins. Therefore, there is evidence to suggest that NGF-dependent miR regulation could lead to tumor development. Nevertheless, further studies are needed to confirm NGF's role in EOC; therefore, it is important to evaluate new miRs associated with EOC. These findings could result in new biomarkers used for diagnosis or target molecules that could allow the development of new therapies.
