The role of RNA processing and regulation in metastatic dormancy

Abstract

Tumor dormancy is a major contributor to the lethality of metastatic disease, especially for cancer patients who develop metastases years-to-decades after initial diagnosis. Indeed, tumor cells can disseminate during early disease stages and persist in new microenvironments at distal sites for months, years, or even decades before initiating metastatic outgrowth. This delay between primary tumor remission and metastatic relapse is known as "dormancy," during which disseminated tumor cells (DTCs) acquire quiescent states in response to intrinsic (i.e., cellular) and extrinsic (i.e., microenvironmental) signals. Maintaining dormancy-associated phenotypes requires DTCs to activate transcriptional, translational, and post-translational mechanisms that engender cellular plasticity. RNA processing is emerging as an essential facet of cellular plasticity, particularly with respect to the initiation, maintenance, and reversal of dormancy-associated phenotypes. Moreover, dysregulated RNA processing, particularly that associated with alternative RNA splicing and expression of noncoding RNAs (ncRNAs), can occur in DTCs to mediate intrinsic and extrinsic metastatic dormancy. Here we review the pathophysiological impact of alternative RNA splicing and ncRNAs in promoting metastatic dormancy and disease recurrence in human cancers.

Keywords: Alternative splicing; Cancer stem cells; Epithelial-mesenchymal transition; Metastatic dormancy; Noncoding RNA; RNA processing; lncRNA; microRNA.

Figure 1.. Dysregulation of splicing factors and alternative splicing in DTC dormancy.

(a) Alternative RNA splicing of a transcript (e.g., gene1) is influenced by splicing regulatory factors Serine/Arginine rich proteins (SRs) and heterogenous nuclear ribonucleoproteins (hnRNPs). SRs promote splicing by recruiting core pre-spliceosome machinery U1 snRNP and splicing factor U2 auxiliary factor (U2AF) to enhance splicing of exon 2 and production of isoform gene1α. Meanwhile, hnRNPs inhibit splicing by binding to exonic splicing silencers (ESS) that prevent spliceosome assembly at the splice site, thereby promoting the production of a distinctly separate isoform, gene1β. (b) Dysregulation of splicing regulatory factors and splicing events can contribute to the acquisition of dormancy-associated phenotypes, including chemotherapy resistance, cancer stem cell behaviors, pro-angiogenic signaling, and cell proliferation. Splicing factors are indicated in bold, italicized, and underlined text, with the corresponding splicing event listed below. For example, increased expression of mRNA isoforms, such as the isoform p38α MAPK, can elicit cell cycle arrest characteristic of dormant DTCs. Figure created with Biorender.com.

Figure 2.. The role of ncRNAs in intrinsic and extrinsic tumor dormancy and metastatic outgrowth.

(a) miRNAs are initially transcribed as primary miRNAs (pri-miRNAs), which are modified to pre-miRNAs by the ribonuclease Drosha and RNA-binding protein DGCR8 (microprocessor complex). Endonuclease Dicer processes pre-mRNAs into their mature forms that are loaded onto Argonaute (Ago) protein to make up the RISC complex that silences target mRNAs. Long noncoding RNAs (lncRNAs) are transcribed, and subsequently can be spliced and polyadenylated before forming structurally complex mature lncRNAs. (b) Intrinsic dormancy can be enhanced by miR-335 and miR-200, which bind to and inhibit expression of SOX4 and mesenchymal transcription factors, respectively. The lncRNA NR2F1 inhibits cell proliferation to promote intrinsic dormancy. (c) Extrinsic dormancy is maintained through an equilibrium between proliferation and apoptosis, which is controlled in part by communication originating within the tumor microenvironment (TME). Both miRNAs and lncRNAs can be packaged into exosomes to communicate with surrounding immune cells (macrophages) and endothelial cells to promote angiogenesis and DTC survival. (d) Metastatic outgrowth and escape from tumor dormancy is mediated by miR-221 and miR-222, which promote proliferation by silencing cell cycle inhibitor p27. LncRNAs such as HOTAIR, BORG, and others (pink box) complex with proteins and act as master regulators of transcription by mediating chromatin structure, thereby promoting phenotypes necessary for metastatic outgrowth. PRC2, polycomb repressive complex 2. Figure created with Biorender.com.