The tumor-suppressive and potential therapeutic functions of miR-34a in epithelial carcinomas

Abstract

Introduction: Many RNA species have been identified as important players in the development of chronic diseases including cancer. Certain classes of regulatory RNAs such as microRNAs (miRNAs) have been investigated in such detail that bona fide tumor suppressive and oncogenic miRNAs have been identified. Because of this, there has been a major effort to therapeutically target these small RNAs. One in particular, a liposomal formulation of miR-34a (MRX34), has entered Phase I trials.

Areas covered: This review aims to summarize miRNA biology, its regulation within normal versus disease states and how it can be targeted therapeutically, with a particular emphasis on miR-34a. Understanding the complexity of a single miRNA will aid in the development of future RNA-based therapeutics for a broader range of chronic diseases.

Expert opinion: The potential of miRNAs to be developed into anti-cancer therapeutics has become an increasingly important area of research. miR-34a is a tumor suppressive miRNA across many tumor types through its ability to inhibit cellular proliferation, invasion and tumor sphere formation. miR-34a also shows promise within certain in vivo solid tumor models. Finally, as miR-34a moves into clinical trials it will be important to determine if it can further sensitize tumors to certain chemotherapeutic agents.

Keywords: Biomarker; breast cancer; cancer; lung; miR-34; microRNA; therapy; treatment resistance; tumor suppressor.

Figure 1. The TP53 Feedback Loop That Controls miR-34a Expression

Transcriptional regulation of miR-34a primarily occurs through the activity of TP53. DNA damage results in the activation of the ATM kinases and subsequent phosphorylation of TP53. Additionally, activation of oncogenes can promote replicative or genotoxic stress, resulting in the repression of the TP53-inhibitor MDM2 by way of ARF activation. While TP53 can transcriptionally downregulate genes important for growth and survival, it transcriptionally activates miRNAs such as miR-34a. The increased levels of miR-34a re-enforces the TP53 response by posttranscriptionally targeting genes important in cell cycle control. Furthermore, a TP53–miR-34 ‘feed-forward’ mechanism is established whereby bona fide repressors of TP53 including SIRT1, HDMX, and E2F3, are targeted and downregulated by miR-34a. Collective regulation of multiple genes by miR-34a is responsible for growth arrest and cell death in response to cell stress and DNA damage. ATM, ataxia telangiectasia mutated; ATR, ataxia telangiectasia and Rad3 related; CDK, cyclin-dependent kinase; CHK, checkpoint kinase.

Figure 2. The Regulation and Function of miR-34a During Tissue Differentiation

(A) In mouse embryonic stem cells (mESCs) in the canonical differentiation model of leukemia inhibitory factor (LIF) withdrawal and retinoic acid (RA) addition, the absence of miR-34a results in high levels of BCL2 thereby reducing the frequency of apoptosis. Given mESCs are TP53-WT the loss of miR-34a presumably abrogates the pro-apoptotic effects of TP53 under states of differentiation. (B) Additional transcriptional networks that can promote miR-34a expression during the differentiation of K562 cells. Blue: TPA-induced megakaryocytic differentiation of K562 cells activates unknown factors that bind to a promoter region distal to the canonical TP53 binding site (grey box, E1’). Enhanced miR-34a levels result in the loss of CDK4, CDK6, and c-MYB target genes. Orange: PMA-induced megakaryocytic differentiation of K562 cells. The activation of the MAPK pathway, subsequently promotes c-JUN and c-FOS binding to an AP-1 binding site in a promoter region proximal to the canonical TP53 binding site (grey box, AP-1). miR-34a targets MEK1, establishing a negative feedback mechanism to stall cell proliferation and allow for megakaryocytic differentiation. Red: K562 cells provided with exogenous TP53-WT, where TP53 activates miR-34a transcription. Green: K562 cells treated with RA or stably expressing C/EBPα results in high levels of miR-34a, which stimulates granulopoiesis via targeting of E2F3. C/EBPα binds near the proximal promoter (grey box, E2’). TPA, tetradecanoyl phorbol acetate; PMA, phorbol 12-myristate 13-acetate; ; sites of transcription initiation; TBP, TATA-binding protein; TFIID, transcription factor II D, yellow box within pri-miR-34a indicates location of pre-miR-34a.

Figure 3. The Dysregulation and Suppressive Functions of miR-34a During Tumorigenesis

(A) Numerous mechanisms are utilized during tumorigenesis to reduce the levels and activity of miR-34a. CpG islands found upstream of the pri-miR-34a gene are recognized by the enzyme DNA methyltransferase (DNMT1/3), which converts them to Me-CpG and ultimately results in the inactivation of the miR-34a promoter. In malignancies such as neuroblastomas and those with MYCN-amplification, chr:1p36.2 , on which miR-34a resides, is commonly deleted. Additionally, tumors harboring a chr:17p deletion or TP53 mutation, loose the transcriptional TP53-miR-34a feed forward loop described above. Epithelial-to-mesenchymal-transition (EMT) promoting transcription factors such as ZEB and SNAIL can negatively regulate miR-34a. Signaling pathways activated by external cues such as hypoxia and NOTCH signaling also feedback on miR-34a and inhibit its expression. (B) Additional miR-34a targets control many cellular processes including growth, survival, stemness, and potentially immune responses[103]. When miR-34a is lost, these target genes become aberrantly expressed and support tumorigenesis. Also highlighted is the unique interplay of hormones, which can alter miR-34a levels during tumorigenesis.