Comment on “miR-199b-5p-DDR1-ERK signalling axis suppresses prostate cancer metastasis via inhibiting epithelial-mesenchymal transition”

We have read with great interest the recently published manuscript by Zhao and co-workers,¹ which investigates the clinical impact and functional relevance of the miR-199b/DDR1/ERK signalling axis in prostate cancer (PCa). In this study, miR-199b downregulation was found significantly associated with metastatic disease and shorter overall survival in PCa patients. At the functional level, miR-199b impaired proliferation, migration and invasion in vitro and tumour growth in vivo. Discoidin domain receptor tyrosine kinase 1 (DDR1) was shown as direct target of miR-199b, and also predicted poor outcome in the patient cohort analysed. Moreover, the authors performed rescue experiments and claimed that the miR-199b functions described above are mediated by the negative regulation of DDR1, which also affects ERK activation status thereby controlling epithelial to mesenchymal transition (EMT). From our point of view, this study provides novel interesting findings, but we feel that there are several issues that merit more discussion.

First of all, one cannot be surprised by the fact that luciferase assays performed by Zhao and colleagues showed a positive result, since DDR1 has been previously identified to be a direct target of miR-199b in several previous works in acute myeloid leukaemia, clear cell renal cell carcinoma, colorectal and breast cancers. Regarding the analysis of the clinical impact of miR-199b, the authors failed to show a multivariate analysis to evaluate if miR-199b and/or DRR1 could serve as independent prognostic markers of poor outcome in PCa. Therefore, their prognostic significance in this disease remains to be fully elucidated in forthcoming studies.

Furthermore, the authors claim in their work that miR-199b controls EMT through the negative regulation of DDR1 thereby activating ERK signalling pathway. However, the experiments carried out do not support this conclusion at all. Both miR-199b and DDR1 have previously been reported to regulate EMT through different molecular mechanisms. Therefore, it is obvious that ectopic modulation of their expression, as performed by the authors, would show changes in EMT. To demonstrate that miR-199b regulates EMT mainly through DDR1, it would have been desirable to ectopically modulate miR-199b expression in PCa cells after DDR1 silencing and overexpression. Of note, these experiments would have also shown if this microRNA is able to regulate EMT, and also ERK signalling, in a DDR1-independent manner.

In fact, numerous evidences in the literature would be suggesting that this EMT and ERK regulation by miR-199b could be involving a more complex signalling network that includes more actors than DDR1. Thus, miR-199b has been reported to directly target SET,^2,3 which has been found to be deregulated and play oncogenic roles in PCa.^4,5 SET has been described to promote EMT, decreasing E-cadherin and increasing both N-cadherin and vimentin.⁶ In fact, it has been demonstrated that SET transcriptionally activates N-cadherin via Rac-1/JNK/c-Jun signalling axis.⁷ Moreover, SET activates ERK through its role as a potent endogenous PP2A inhibitor.⁸ Interestingly, miR-199b could be regulating DDR1 also indirectly through the SET/PP2A interplay since it has been reported that AKT, a direct target of PP2A, regulates DDR1 by suppressing miR-199a.⁹ Of note, miR-199a is another microRNA that directly targets DDR1, and that has also been found deregulated in PCa.¹⁰

Another relevant issue is that the work by Zhao and co-workers does not include any experiment to demonstrate that ERK signalling is a key mediator of the EMT regulation induced by miR-199b and DDR1. It would have been easy to at least analyse by western blot if any change in EMT markers is observed after ectopic modulation of miR-199b or DDR1 in PCa cells treated with an ERK inhibitor. Altogether, some of the conclusions raised by Zhao and co-workers should be taken with caution since the experimental results provided do not support them completely. Moreover, it would be very interesting to clarify the potential role of other targets such as SET to fully understand the functional role of miR-199b in PCa.

Author contributions

I.C., A.S. and J.R. conceived the work and drafted the manuscript. F.R. and J.G.F. designed the work and revised the manuscript. All authors approved the final version of the manuscript and agACKreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Ethics approval and consent to participate

Not applicable.

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Data availability

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Competing interests

The authors declare no competing interests.

Funding information

This work was supported by PI18/00382 and PI16/01468 grants from “Instituto de Salud Carlos III FEDER”.