In cancer, undifferentiated stem cells could help tumors such as glioblastoma become more aggressive than other forms of the disease. Certain groups of genes are thought to help cells along the path to maturity, leaving their youthful "stemness" behind. The common microRNA suppression in cancer cells reflects their undifferentiated status. Previous studies have suggested that the widespread loss of microRNAs in cancer stem-like cells (CSCs) enhanced their "stemness" and that the restoration of premalignant microRNA landscape favors more differentiated and a less neoplastic milieu. Therefore, the mechanism of the initial loss of microRNAs is in the research crosshairs now, with a focus on the disruption of microRNA maturation. However, the mechanisms of cancer cell type-specific alterations of microRNAome are still unclear.
In a new study, researchers from Brigham and Women's Hospital in the United States pinpointed the pivotal changes in an enzyme called Dicer, which creates a cascade of effects on this microRNAome. They identified the primary actors circ2082 (a circular RNA) and RBM3 (an RNA-binding protein), which form a complex with DICER to stay in the nucleus of glioblastoma cells, thereby destroying the microRNAome in the cytoplasm. The related research results were recently published in Science Advances with the title of "The nuclear DICER–circular RNA complex drives the deregulation of the glioblastoma cell microRNAome".
Figure 1. DICER is retained in the nucleus of GSCs and form a complex with RBM3 protein. (Bronisz A, et al. 2020)
CircRNAs originate from linear transcripts by various mechanisms, and their hallmarks are a unique splice junction site, which mediates circularization, leading to a covalently closed "head-to-tail" structure, along with the lack of 5'-3' polarity and polyadenylated tail, as well as a low probability for encoding protein. CircRNAs have been shown to act as microRNA sponges, but their function remains largely unknown because only a handful contain microRNA target sites. More and more circRNAs are being implicated in numerous cancers; however, the functional relevance of the vast majority is yet to be found.
Researchers have established that DICER, an enzyme responsible for microRNA maturation, predominantly localizes in the nucleus of CSCs, while in neural progenitor/stem cells (NPCs) and other nonmalignant cells, it is localized in the cytosol. Therefore, they wanted to know what mechanism was responsible for this phenomenon and whether this change in localization was accountable for the observed defect in microRNA processing. Follow-up scrutiny of nuclear DICER interactome revealed the presence of RNA-protein complex formed by DICER with RNA binding motif protein 3 (RBM3) and circRNA denoted as circ2082. Circ2082 originates from the well-established noncoding oncogenic lncRNA MALAT1 that promotes protumorigenic traits in numerous solid tumors, including glioblastoma. The disruption of the found complex by circ2082 antisense oligonucleotide (ASO) resulted in the cytosolic release of DICER, the resumption of microRNA processing leading to the restoration of premalignant microRNAome, and, thus mitigated tumorigenicity in vitro and in vivo.
The researchers used patient-derived glioblastoma cells, which made their results genetically and phenotypically similar to those of real patients. They implanted these tumor cells into the brains of laboratory mice and studied them in vivo and in vitro. The microRNA expression level of these mice changed their survival rate. If circ2082 was knocked down, the nuclear DICER complex of circ2082, DICER, and RBM3 was disrupted, more microRNAs were present in the cytoplasm, and the survival outcome was far greater. In these mice whose tumors had circ2082 knocked down, death by tumorigenesis never occurred, while their non-knockdown counterparts all experienced death due to tumorigenesis.
On the basis of the past discoveries on the association between global microRNAs down-regulation and cancer growth, researchers propose that loss of microRNAome by circ2082-dependent DICER nuclear retention determines CSC molecular identity and their tumorigenic potential. Therefore, the targeting of circ2082 reverses cell fate in CSCs through cytoplasmic relocalization of DICER that mediates microRNAome correction.