Valproic acid regulates the miR-155/Jarid2 axis by affecting miR-155 promoter methylation in glioma

Abstract

The most frequent primary brain tumor in adults is glioma, yet no effective curative treatments are currently available. Our previous study demonstrated the enhancing effects of JARID2 on glioma sensitivity to TMZ treatment. In this study, miR-155 is predicted to target JARID2. miR-155 is overexpressed in clinical glioma specimens and cell lines. miR-155 overexpression in glioma cells enhances cell viability and represses cell apoptosis. Through targeting, miR-155 inhibits JARID2 expression. miR-155 inhibition inhibits glioma cell viability and enhances cell apoptosis, whereas JARID2 knockdown enhances cell viability and inhibits cell apoptosis; JARID2 knockdown partially reverses miR-155 inhibition effects on glioma phenotypes. miR-155 inhibition reduces but knockdown of JARID2 promotes the tumor formation ability of glioma cells in vivo. Valproic acid (VPA) upregulates JARID2 expression, inhibits glioma cell viability and enhances cell apoptosis. VPA downregulates the expression level of miR-155 by increasing the methylation level of the miR-155 promoter, suggesting that the miR-155/JARID2 axis is implicated in VPA inhibition of glioma cell viability and enhancement of glioma cell apoptosis. This study demonstrates a new mechanism of VPA treatment of gliomas by affecting the miR-155/JARID2 axis, which could be regarded as a new strategy for the prevention and treatment of glioma.

1. Introduction

The most frequent primary brain tumor in adults is glioma, which can develop in either the white or gray brain matter and is likely to have a glial origin from the white brain matter [1,2]. Adults are more likely to develop infiltrative astrocytoma rather than any other type of primary brain tumor. Compared to patients with the two low-grade astrocytomas, those with high-grade astrocytomas have a significantly greater mortality rate [3,4].

The standard first-line chemotherapeutics for recurrent high-grade glioma and other solid neoplasias are alkylating agents such as temozolomide (TMZ) [5,6]. The induction of methyl adducts at multiple DNA bases, such as O6-guanine, N7-guanine, and N3-adenine, leads to the antitumor effects of TMZ [7,8]. Acquired chemotherapeutic resistance is still a major challenge. Moreover, the second chemotherapy cycle is ineffective for more than 90% of recurrent gliomas [9]. Some mechanisms have been shown to exert crucial effects on TMZ chemoresistance [10–12]. Our previous study confirmed the enhancing effects of JARID2 on glioma sensitivity to TMZ treatment. Overexpressing JARID2 in TMZ-treated glioma cells dramatically suppressed cell viability, enhanced cell apoptosis, and elevated the levels of p21, cleaved-PARP, and cleaved-caspase3 [13]. However, because JARID2 expression is markedly downregulated in gliomas, identification of the inhibitory regulator may lead to the development of new therapeutic targets in gliomas.

Recent research has demonstrated that microRNAs (miRNAs) play a crucial role in controlling gene expression in healthy and cancer cells. MiRNAs are endogenous, noncoding, and 20–23 nt in length, targeting mRNAs that may be posttranscriptionally bound by RNAs, which may cause mRNA cleavage, direct translational inhibition, mRNA degradation, or a combination of the two in some instances [14,15]. An individual miRNA can modulate the expression of multiple target RNAs and thereby regulate several gene pathways, subsequently modulating almost all cellular processes, including proliferation, differentiation, apoptosis, and cell growth. MiRNAs may be important as tumor suppressors or oncogenes (oncogenic miRNAs or ‘oncomirs’). MiRNAs are believed to be promising therapeutic targets/tools for cancer therapy and serve as potential biomarkers for human cancers [16,17]. Piwecka et al. [18] identified over 290 miRNAs deregulated in glioma tissues by performing a meta-analysis of miRNA expression profiling studies. Using the extensive data generated by deep sequencing, Moore et al. [19] discovered a complex network of gene expression alterations in the miRNA biogenetic pathway that affect miRNA maturation and are connected to glioma development. Thus, JARID2 downregulation in glioma may be caused by miRNAs that target JARID2. Moreover, valproic acid (VPA), one of the most common histone deacetylase inhibitors (HDACIs), has been detected to directly or synergistically exert inhibitory effects on glioma in vitro and in vivo [20]. For instance, Han et al. [21] reported that VPA enhanced apoptosis by promoting autophagy via Akt/mTOR signaling in glioma. Phospho-VPA suppressed glioblastoma growth in preclinical models through the inhibition of STAT3 phosphorylation [22]. In vivo efficacy experiments showed that combination treatment with MSCs-TK and VPA significantly inhibit