MiR-210: an important player in the pathogenesis of preeclampsia?

Hypoxia is a common feature in solid neoplastic microenvironment and a well-documented factor attributable to therapeutic failure in clinical oncology. Low oxygen availability also plays a central role in the pathogenesis of major ischaemic disorders, such as acute myocardial infarction, stroke and preeclampsia. Most hypoxic responses impacting cellular behaviour are mediated through hypoxia inducible transcription factors (HIFs), composed of alpha (HIF-α) and beta (HIF-β) subunits. For this reason, studies about signalling pathways involved in hypoxic response are mainly concentrated on HIF and its target molecules. Recently, a new class of hypoxia-induced molecules, named hypoxia inducible microRNA (hypoxamirs), is receiving increased attention [1]. Among these miRNAs, miR-210 is unique in that it is robustly and ubiquitously induced by hypoxia in a variety of cell types studied. Several studies have demonstrated the up-regulation of miR-210 in patients with preeclampsia. However, the role of miR-210 in the pathogenesis of the disease is largely unknown. In this issue of the Journal of Cellular and Molecular Medicine, Zhang et al. [2] demonstrate that hypoxia inducible miR-210, regulated by transcriptional factor HIF-1 as well as NF-κB, plays an important role in the molecular mechanism of preeclampsia.

MiR-210 is frequently elevated in primary solid tumours, including head and neck cancer, glioma, melanoma, clear cell renal cell carcinoma and lung, pancreatic and breast cancer [3]. From diagnostic standpoints, it has been found that miR-210 is significantly up-regulated in serum from patients with diffuse large B-cell lymphoma and in plasma of pancreatic cancer patients. In benign disease associated with hypoxia, such as arteriosclerosis obliterans, increased serum levels of miR-210 have also been detected [4]. In recent years, it has been found that miR-210 levels are dramatically increased in the placental tissue derived from patients with preeclampsia. In this issue, Zhang et al. [2] found that miR-210 levels in plasma from preeclampsia patients were significantly higher than those in gestational healthy controls. Furthermore, the expression levels of miR-210 seemed to correlate well with disease severity, suggesting a potential role of circulating miR-210 as novel biomarker for the diagnosis of preeclampsia. However, because of relative small number of patients enrolled, further study in a large scale would be warranted to validate this finding. To verify that the high level of miR-210 could be derived from placental tissues, using cytotrophoblast cells and trophoblast-derived human cells as in vitro model, the authors demonstrated that miR-210 was induced by hypoxia in these cell lines.

It has been demonstrated that miR-210 is clearly involved in diverse biological processes such as differentiation, cell cycle, cell survival and tumour progression [3]. However, depending upon different cellular context, some functions of miR-210 maybe significantly vary. To investigate the role in the pathogenesis of preeclampsia, Zhang et al. [2] cultured the cytotrophoblast cells in normoxia and hypoxia, respectively. Transwell assays showed that ectopic expression of miR-210 inhibited the capability of trophoblast cell migration and invasion in normoxia, with hypoxia treatment yielding the similar results. Furthermore, in hypoxia, blocking miR-210 expression with anti-miR-210 could reverse the inhibitory effects of hypoxia. These findings suggest that hypoxia may inhibit migration and invasion of the cytotrophoblast cells by inducing miR-210 expression. Recently, Fasanaro et al. reported that hypoxia and miR-210 stimulated endothelial cell migration [5]. The discrepancy is not difficult to understand, because in different tissue and cellular context, miR-210 may inactivate different targets genes, which may have opposite functions.

Up to date, at least more than 35 unique transcripts have been identified and validated as direct targets of miR-210. To elucidate the mechanisms involved in inhibitory effects of miR-210 on cell migration and invasion, the authors concentrated on two transcripts which are putative target genes predicated by computational programs: ephrin-A3 (EFNA3) and homeobox-A9 (HOXA9). The predication was confirmed by 3′-UTR luciferase assay. Real-time quantitative RT-PCR and Western blot analysis demonstrated that the expression of EFNA3 and HOXA9 were inhibited through mechanisms involving mRNA degradation or translational repression. The in vivo expression profile of EFNA3 and HOXA9 was also investigated, and a clear down-regulation of these two genes was observed in preeclampsia placenta tissues.

Hypoxia plays a critical role in embryonic development. Increasing evidence has demonstrated that oxygen tension is a powerful signalling molecule that regulates embryonic cell proliferation and differentiation. During human embryonic development, embryonic microenvironment maintains a very low oxygen tension until 11th week of gestation. Even after the cytotrophoblasts traverse, the arterial walls, the local partial pressure of oxygen only increases from ∼2.3% to 7.8%. Why is the expression level of miR-210 only increased in placental tissues from patients with preeclampsia but not in normal placenta? One possible explanation may be that in normal placenta, hypoxia is not severe enough to induce miR-210 expression; whereas, in preeclampsia placenta, shallow cytotrophoblast invasion results in a more hypoxic microenvironment, and miR-210 is induced, leading to a even more hypoxic condition by the way of inducing EFNA3 or HOXA9, which inhibit the migration and invasion of trophoblast cells. Based on this analysis, we can deduce that shallow cytotrophoblast invasion should occur before miR-210 expression; after that, a positive feedback loop is formed between hypoxia and miR-210. Therefore, miR-210 expression may not be a determinant factor responsible for early shallow cytotrophoblast invasion but rather an important secondary event.

It has been demonstrated that miR-210 expression is HIFs dependent. Although there are several other important transcriptional factor binding sites (Oct-4, AP2 and PPARγ) present in the close vicinity of miR-210's HIFs binding site [6], whether these transcriptional factors mediate miR-210 expression has yet to be confirmed. Based on luciferase assay analysis and chromatin immunoprecipitation experiments, Zhang et al. [2] demonstrate that both HIF-1 and NF-κB are involved in the miR-210 regulation and that NF-κB regulates miR-210 expression by directly binding to the miR-210 promoter. NF-κB is a transcriptional factor activated by a variety of factors including inflammatory cytokines, bacterial products, oxidative stress, growth factors and mitogens. Several miRNAs have been reported to be up-regulated by NF-κB in many conditions [7]; however, this study represents the first evidence that, in hypoxia, NF-κB could mediate miR-210 expression. With more hypoxamirs being identified, it is conceivable that some of these miRNAs would be found to be regulated by NF-κB in hypoxia in future studies. In addition, it has been reported that miR-210 could form a feedback loop with HIF-1 during hypoxia [8]; however, it is unknown whether such relationship occurs between miR-210 and NF-κB.

Altogether, Zhang et al.[2] identify a pathway, hypoxia /NF-κB /miR-210/EFNA3 and HOXA9, involved in the pathogenesis of preeclampsia, and this finding provides a new insight into molecular mechanisms for the disease. In addition, they have demonstrated that, for the first time, miR-210 could be regulated by NF-κB during hypoxia, extending our knowledge of the regulation of miR-210.

Conflicts of interest

The authors confirm that there are no conflicts of interest