RESEARCH HIGHLIGHTS Nature Reviews Cancer | AOP, published online 4 April 2014; doi:10.1038/nrc3727
MICROENVIRONMENT
Tumour-promoting tissue mechanics this work provides a link between oncogenic miRNA induction, tumour mechanics and known tumour suppressors
Tissue mechanics have been impli‑ cated in tumour progression but the underlying molecular mechanisms have been unclear. Now, Mouw et al. show that increased extracellular matrix (ECM) stiffness induces expression of the microRNA miR‑18a in breast tissue, and this miRNA in turn targets the tumour suppressor PTEN. Microarray profiling of miRNAs in non-malignant human mam‑ mary epithelial cells (hMECs) grown on soft or stiff polyacryl amide gels indicated that the miR‑17‑92 cluster was upregulated with one miRNA in the cluster, miR‑18a, consistently showing significantly increased expression in stiffer tissue. miR‑18a expression
Paul Bolotov / Alamy
was also decreased in mice when mammary tumours were treated with lysyl oxidase (LOX) inhibitors, which decrease tissue stiffness. The authors then identified and confirmed that miR‑18a binds to and inhibits both the tumour supp ressor PTEN and homeobox A9 (HOXA9). Furthermore, reduced levels of PTEN and HOXA9 proteins were correlated with greater tissue stiffness, and either inhibiting stiffen‑ ing or knocking down miR‑18a in cells grown on a stiff gel rescued the expression of both proteins. In addition, the level of phosphorylated AKT was increased when PTEN expression was lost, which indicates that the PI3K arm of PTEN signalling was affected. What is the relationship between PTEN and HOXA9? Using short hairpin RNA knockdown of HOXA9 and reporter assays, the authors found that HOXA9 binds directly to the PTEN promoter and thereby regulates its expression. This relation‑ ship is regulated by tissue stiffness, as ectopic expression of HOXA9 rescued PTEN expression in hMECs that were grown on a stiff gel. What else might be involved in the tissue stiffness signalling ‘circuit’? The authors found that the expres‑ sion of MYC, which was previously shown to stimulate the miR‑17‑92 cluster, was increased in stiffer tissue and that inhibition of MYC reduced miR‑18a levels. MYC is downstream of β-catenin signalling, and the authors found that nuclear (active) β-catenin was increased in hMECs that were grown on a stiff gel. In addition, proteins that positively reg‑ ulate β-catenin nuclear localization were upregulated, and proteins that negatively regulate active β-catenin were downregulated in stiff tissue.
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This was further verified in vivo, using a conditional β1 integrinmutant mouse that recapitulates tension-dependent integrin cluster‑ ing specifically in the mammary gland, which results in stiffer mammary tissue. Consistent with the in vitro results, MYC, nuclear β-catenin and miR‑18a were upregu‑ lated, and HOXA9 and PTEN were downregulated. Furthermore, analysis of this signalling circuit in patient samples showed that the level of miR‑18a was higher in breast cancer samples compared with non-malignant breast samples and, specifically, that the expression of miR‑18a was significantly increased in luminal B-type breast cancer samples, relative to luminal A-type cancers. Using atomic force microscopy, the authors measured the stiffness of these tumour samples and, similar to the mouse and in vitro experiments, they found that increased stiffness was correlated with miR‑18a expression and decreased PTEN and HOXA9 expression. Using publically available gene expression sets, they further noted that patients with the highest expression of miR‑18a at diagnosis had a shorter time to relapse. Taken together, this work provides a link between oncogenic miRNA induction, tumour mechanics and known tumour suppressors. Moreover, by determining the signalling circuit that underlies increased tissue stiff‑ ness, it provides a proxy for the meas‑ urement of stiffness in a tumour that is predictive of patient outcome. Isabel Lokody ORIGINAL RESEARCH PAPER Mouw, J. K. et al. Tissue mechanics modulate microRNA-dependent PTEN expression to regulate malignant progression. Nature Med. http://dx.doi. org/10.1038/nm.3497 (2014)
VOLUME 14 | MAY 2014 © 2014 Macmillan Publishers Limited. All rights reserved
MICROENVIRONMENT
Tumour-promoting tissue mechanics this work provides a link between oncogenic miRNA induction, tumour mechanics and known tumour suppressors
Tissue mechanics have been impli‑ cated in tumour progression but the underlying molecular mechanisms have been unclear. Now, Mouw et al. show that increased extracellular matrix (ECM) stiffness induces expression of the microRNA miR‑18a in breast tissue, and this miRNA in turn targets the tumour suppressor PTEN. Microarray profiling of miRNAs in non-malignant human mam‑ mary epithelial cells (hMECs) grown on soft or stiff polyacryl amide gels indicated that the miR‑17‑92 cluster was upregulated with one miRNA in the cluster, miR‑18a, consistently showing significantly increased expression in stiffer tissue. miR‑18a expression
Paul Bolotov / Alamy
was also decreased in mice when mammary tumours were treated with lysyl oxidase (LOX) inhibitors, which decrease tissue stiffness. The authors then identified and confirmed that miR‑18a binds to and inhibits both the tumour supp ressor PTEN and homeobox A9 (HOXA9). Furthermore, reduced levels of PTEN and HOXA9 proteins were correlated with greater tissue stiffness, and either inhibiting stiffen‑ ing or knocking down miR‑18a in cells grown on a stiff gel rescued the expression of both proteins. In addition, the level of phosphorylated AKT was increased when PTEN expression was lost, which indicates that the PI3K arm of PTEN signalling was affected. What is the relationship between PTEN and HOXA9? Using short hairpin RNA knockdown of HOXA9 and reporter assays, the authors found that HOXA9 binds directly to the PTEN promoter and thereby regulates its expression. This relation‑ ship is regulated by tissue stiffness, as ectopic expression of HOXA9 rescued PTEN expression in hMECs that were grown on a stiff gel. What else might be involved in the tissue stiffness signalling ‘circuit’? The authors found that the expres‑ sion of MYC, which was previously shown to stimulate the miR‑17‑92 cluster, was increased in stiffer tissue and that inhibition of MYC reduced miR‑18a levels. MYC is downstream of β-catenin signalling, and the authors found that nuclear (active) β-catenin was increased in hMECs that were grown on a stiff gel. In addition, proteins that positively reg‑ ulate β-catenin nuclear localization were upregulated, and proteins that negatively regulate active β-catenin were downregulated in stiff tissue.
NATURE REVIEWS | CANCER
This was further verified in vivo, using a conditional β1 integrinmutant mouse that recapitulates tension-dependent integrin cluster‑ ing specifically in the mammary gland, which results in stiffer mammary tissue. Consistent with the in vitro results, MYC, nuclear β-catenin and miR‑18a were upregu‑ lated, and HOXA9 and PTEN were downregulated. Furthermore, analysis of this signalling circuit in patient samples showed that the level of miR‑18a was higher in breast cancer samples compared with non-malignant breast samples and, specifically, that the expression of miR‑18a was significantly increased in luminal B-type breast cancer samples, relative to luminal A-type cancers. Using atomic force microscopy, the authors measured the stiffness of these tumour samples and, similar to the mouse and in vitro experiments, they found that increased stiffness was correlated with miR‑18a expression and decreased PTEN and HOXA9 expression. Using publically available gene expression sets, they further noted that patients with the highest expression of miR‑18a at diagnosis had a shorter time to relapse. Taken together, this work provides a link between oncogenic miRNA induction, tumour mechanics and known tumour suppressors. Moreover, by determining the signalling circuit that underlies increased tissue stiff‑ ness, it provides a proxy for the meas‑ urement of stiffness in a tumour that is predictive of patient outcome. Isabel Lokody ORIGINAL RESEARCH PAPER Mouw, J. K. et al. Tissue mechanics modulate microRNA-dependent PTEN expression to regulate malignant progression. Nature Med. http://dx.doi. org/10.1038/nm.3497 (2014)
VOLUME 14 | MAY 2014 © 2014 Macmillan Publishers Limited. All rights reserved
