A Harvard Stem Cell Institute (HSCI) collaboration between a molecular chemist who studies microRNAs and a stem cell biologist interested in cell growth has led to new understanding of what goes wrong in several kinds of cancer.
Boston Children’s Hospital scientists Richard Gregory, PhD, and Fernando Camargo, PhD, who is also at Harvard’s Department of Stem Cell and Regenerative Biology, discuss their joint research project, published by Cell, in this video abstract:
Richard Gregory: MicroRNAs are small 22-nucleotide, regulatory RNAs that base pair with complementary messenger RNAs and function to suppress gene expression.
We know of some of the core machinery in a cell that’s responsible for making microRNAs but how this is regulated, this is the important question that my lab aims to address.
The nice thing about this Cell paper is that it potentially can explain two previously reported phenomenon.
The first is that when cells reach confluence, microRNA expression is enhanced in the confluent cells compared to the proliferating cells.
The other observation is that in tumor cells, compared to their normal tissue, then you will see that there is a global suppression of microRNAs.
And so we thought that perhaps these two phenomena may be linked and that’s what led us to look toward the Hippo signaling pathway as a potential regulator of microRNA processing in cells.
Fernando Camargo: One of the pathways that my lab has been working on for several years now, is this pathway, this biochemical cascade, this set of genes known as Hippo.
It basically stops the growth of your tissues once they have reached a predetermined size. And it does so by sequestering this protein called YAP.
YAP is a pretty crucial protein. Basically, when YAP is unleashed the cell basically turns on its proliferative program and it grows and divides and duplicates.
So one of the postdocs that came into my lab to work on Hippo and growth regulation was also interested in RNA biology. Other people had shown that microRNAs were also regulated by cell density, obviously this is one of the critical observations that our postdoc Masaki had, and allowed us to start thinking about how Hippo signaling could be connected to microRNA processing.
Masaki Mori: I tried to clarify the role of microRNA biogenesis in the context of organ growth and tumorigenesis.
We identified p72, an RNA helicase, as a target protein of Hippo signaling pathway to regulate microRNA biogenesis.
Fernando Camargo: At low density, right, less crowded conditions, YAP is nuclear, YAP sequesters, kidnaps basically p72, which is important for microRNA processing.
So that at low density, microRNA production is lower. Whereas at high cell density, when YAP is out of the cytoplasm, when the cells are not growing, then p72 is released and it can bind to the Microprocessor component of the microRNAs and then these microRNAs are being produced more efficiently.
Richard Gregory: In cancer, the Hippo pathway can be perturbed such that cells no longer respond to this cell contact inhibition and YAP is constitutively nuclear and then this would lead to the global suppression of microRNAs that have been described in a wide variety of different tumors.
And so what we think is happening is that there are key microRNA target genes, for example, C-MYC, which is a known oncogene, which can be dysregulated and leading to elevated expression of these key oncogenes.
Masaki Mori: With the laborious help of the lab members we were able to see the suppression of microRNAs induced by constitutive activation of YAP protein. The results recapitulated the findings in human cancers and our observations in vitro.
Richard Gregory: I think this opens up a lot of avenues for future research. For example, would we be able to manipulate this pathway to be able to restore microRNA expression in cancer cells as a potential therapeutic.
Fernando Camargo: Our lab is trying to identify small molecules and other modulators of the Hippo pathway. So, the idea is that we’ll be able to take a drug, a peptide perhaps, and be able to give it to a patient so that we can control, exogenously control the activity of YAP.
Richard Gregory: We’ve been working on this for the last several years, probably about five or six years, and really the breakthrough came with having this joint postdoc with Fernando Camargo and having the interactions between two complementary labs. And so integrating some of the basic machinery that is responsible for microRNA processing and understanding how that can be dynamically regulated in different cellular contexts, I think that that’s an area of active investigation that this is perhaps one of the first examples of how cell signaling pathways can function to regulate microRNA expression.
Fernando Camargo: I think this data also opens up the possibility that it’s not only tumor cells but perhaps it’s highly proliferative cells, perhaps stem cells, perhaps progenitors in a regenerative setting that actually globally repress microRNA production. An area of investigation to pursue is to really try to understand whether microRNA processing is also selectively downregulated in those conditions of regeneration or just very rapid growth and we’re looking into this.
The research was supported by the National Institutes of Health, the Department of Defense, the US National Institute of General Medical Sciences, the Stand Up to Cancer-AACR Initiative, a Simeon Burt Wolbach Fellowship, a Japan Heart Foundation/Bayer Yakuhin Research Grant Abroad.
Cited: Hippo signaling regulates Microprocessor and links cell-density dependent miRNA biogenesis to cancer. Cell. February 27, 2014