miR-125a—Does the difference lie in the isoform?

MicroRNAs (miRs) are small noncoding RNAs that modulate expression of proteins derived from complementary target mRNA transcripts by repressing their translation or directing mRNA degradation, and are considered the fine-tuning executers of cellular pathways. A large number of miRNAs have been implicated in carcinogenic processes, displaying an aberrant and variable level of expression. miR-125a has been involved in a variety of cellular pathways within cancer cells; various mechanisms of action have been attributed to its 3p and 5p isoforms. The well-studied miR-125a-5p, an isoform derived from the 5′ arm of pre-miR-125a, was shown to be down-regulated in several types of malignancies, and its potential to serve as a therapeutic target is under examination. miR-125a-3p, an isoform derived from the 3′ arm of pre-miR-125a, was first described by L. Jiang et, al.¹ who suggested it serves as a tumor suppressor miR, based on its ability to reduce invasion and migration of lung cancer cells.¹ The study illustrated an inverse correlation between hsa-miR-125a-3p expression and the pathological stage or the occurrence of lymph node metastasis in human tissue samples. Further studies supported this observation, though the roles or targets of miR-125a-3p were not clearly elucidated yet. We had recently studied the role of miR-125a-3p in HEK cells² and in prostate cancer cells³ and demonstrated that miR-125a-3p mediates cellular pathways that account for motility and migration of prostate cancer cells. We showed an inverse correlation between expression patterns of miR-125a-3p in cancerous tissues and Gleason score.

Studies focusing on miR-125a-3p isoform, performed by others and us, raise the question of whether the low levels of miR-125a-3p reflect epiphenomenon of cancer or play a role in carcinogenesis. By functional assays using confocal-live cell imaging, we demonstrated that cells overexpressing miR-125a-3p exhibited decreased migration capability as well as decreased mean track-speed and track-displacement, both indicators of cell migration.³ These data, along with data derived from other studies, shed light on the involvement of miR-125a-3p in cancer pathogenesis. In light of our results, the postulated mechanism of action involves modulation of expression of FAK and paxillin, 2 focal adhesion proteins, by miR-125a-3p and an impairment of the dynamic interplay between the actin cytoskeleton and cell adhesion sites, thus causing reduced cell motility.³

The formerly common approach had supported that only one mature isoform of pre-miR remains while the complementary isoform is degraded. Recent evidence lends credence to concomitance of both isoforms, with a delicate balance between them. Isoforms 3p or 5p appear to have different evolutionarily conserved seed sequences which carry differential impact on distinct cellular targets and pathways. These data carry significant implication for further research of the unique correlation between the 2 isoforms in both physiological processes and carcinogenesis. A transcriptional repression of miR-125a by an EGFR-related transcription factor⁴ has been documented in ovarian cancer cells, though the expression level of each of the isoforms was not evaluated. Another approach of miRs regulation has been suggested recently throughout the cleaving of pre-miR into the mature isoform. Several studies showed that the process of converting pre-miR125a to mature miR-125a is disrupted by a single nucleotide polymorphism (SNP). This mechanism had been described in breast cancer⁵ and esophageal squamous cell carcinoma,⁶ Yet, in this study, as in previous studies, the expression level of either isoform was not determined.⁶

In non-small cell lung cancer both 3p and 5p isoforms were shown to be downregulated; functional in-vitro assays demonstrated an inverse correlation - whereas isoform 3p decreased the invasiveness and migratory capability of lung cancer cells,¹ isoform 5p increased both processes. This had been an exceptional study because its experimental platform included manipulation of the cells with only one specific isoform, while proposing a differential role for the 5p isoform, whereas in other studies cells were manipulated with pre-miR-125a, which is capable of maturing into 2 isoforms.

We suggest that the intricate regulatory mechanism carried out by miR-125a does not rely only upon the expression level of each isoform but also upon the delicate balance between the isoforms.

Future studies should comprise a broader evaluation of the interplay in the ratio of the 3p/5p isoforms. Further consideration should be given to whether the experimental manipulation of gene expression should be performed by transfecting the cells with pre-miRNA or with a specific isoform, which may yield different types of regulation.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.