A molecular environment that promotes vascularization around human carcinomas can materialise rapidly, and has been termed the angiogenic switch. Turning this switch toward a proangiogenic state involves an altered interplay between tumor cells and multiple components of the surrounding stroma. The regulatory landscape of these interactions in cervical cancer is now investigated by Huang et al. in this issue of Oncogene, who demonstrate that the microRNA miR-126 is downregulated during cancer progression, particularly in stromal cells. Such a reduction of miR-126 is shown to free at least one target, the proangiogenic adrenomedullin, from repression, enhancing vascular growth especially at the in situ to invasive carcinoma transition. The study implicates the temporal, spatial and progressive nature of tumor–stroma interactions during carcinogenesis, while in turn suggesting therapeutic strategies.
The development of human carcinomas involves an altered interplay with their surrounding stromal cells, but the factors delivered and the changes induced can be difficult to determine experimentally. When the molecular conditions in the stromal microenviroment change sufficiently, endothelial cells are activated from a quiescent state leading to pathological angiogenesis.1 A carcinoma in situ may release molecules such as vascular endothelial growth factor (VEGF) and angiopoietin in response to hypoxia, in turn causing endothelial activation by inducing mitogenic and regulatory changes in their target endothelia.2
Among the downstream regulators induced, microRNAs (miRNAs) have been shown to have regulatory roles in both healthy and malignant blood vessel remodeling, acting either as oncogenes or tumor supressors.3 By binding typically to the 3′ untranslated regions of multiple messenger (mRNAs), miRNAs can direct the degradation or translational repression of many functionally related or unconnected gene targets.4 miR-296, miR-126, miR-132 and miR-92a are all growth factor-induced miRNAs, validated to modulate the angiogenic process,5 yet as suggested by the study discussed herein the actual roles of specific miRNAs may be cell-type and tumor-stage dependent.
In this issue of Oncogene, Huang et al.6 looked at the regulatory interactions between cervical carcinomas, carcinoma-associated fibroblasts (CAFs) and endothelial cells lining the vasculature, and how they changed through space and time. Around cervical cancers the presence of a high density of microvessels displaying chaotic branching patterns has been associated with invasive and high-grade disease,7 while factors controlling these changes remain unclear. Investigating the molecular determinants of these phenomena, the authors focused on miR-126, which has been previously shown to support vascular integrity in developing endothelia.8, 9 Comparing miR-126 levels in healthy and cancerous cervical tissues, miR-126 was found to be significantly lower in the cancerous stromal cells. They hypothesized that carcinoma cells together with stromal fibroblasts could be causing this effect in the endothelium. Indeed, the presence of both carcinomas and fibroblasts reduced miR-126 in human umbilical vein endothelial cells (HUVECs) and increased their tube formation and branch rates. Interestingly, both cancer-derived and normal cervical fibroblasts produced this effect; a comparable result was achieved after co-injecting CaSki cell xenografts into immunocompromised mice, with or without accompanying fibroblasts: the presence of CAFs increased tumor angiogenesis and growth, along with a reduction of mouse-derived miR-126. Performing this co-injection xenograft supplemented with a matrigel mixed miR-126 precursor significantly reduced both angiogenesis and tumor size.
Is miR-126 downregulated to remove a tumor suppressive, antiangiogenic function otherwise present in healthy endothelial cells? To answer this question the authors proceeded to examine a peptide called adrenomedullin (ADM), which they validated as a target of miR-126 by methods that are now remarkably routine in miRNA literature (demonstrating how far we have come in this field). ADM is expressed in a range of endothelial tissues, and has been ascribed proangiogenic, and anti-apoptotic functions.10 ADM was seen as a prime candidate for producing the angiogenic phenotype observed when miR-126 was downregulated, and increased HUVEC tube branching, looping and growth rates in a dose-dependent manner when added to cultures. Importantly they demonstrated that inhibiting miR-126 relieved ADM repression, leading to angiogenesis. If then adding an ADM inhibitor could remove this angiogenic effect, it is conceivable that ADM could have been accounting for the entire angiogenic phenotype observed. In fact, the effect on HUVEC growth was only partially removed, suggesting that other proangiogenic factors could also be targeted by miR-126, meriting further investigation. miR-126 can reportedly target other proangiogenic factors including the growth factors VEGF and angiopoietin, and the adhesion molecule VCAM111 which supports the extravasation of proinflammatory hematologic cells.12 Relief of repression of the growth factors might explain the low miR-126 the authors found in the carcinoma cells. In the co-culture model, which lacked actual circulation, an infiltrate of inflammatory cells could not have contributed. The hunt is on for the additional miR-126 targets aside from ADM that led to the angiogenesis observed.
The existence of an activated stroma that supports carcinogenesis has interesting, if incomplete, experimental support. It is subject to dynamic remodeling, and the infiltration of inflammatory cells and activated fibroblasts into stroma provides numerous growth factors and extracellular matrix components that support tumor development.13 As Huang et al. have shown, interactions between multiple cell types can act together to produce the changes observed in angiogenesis, and careful hypotheses must be devised and tested if these interactions are to be unravelled. In the case of miR-126, we can see that miRNAs can regulate distinct targets in different contexts, either in development,9 within carcinomas11 or in endothelia (this study). Unfortunately this only serves to emphasize the importance of understanding complete tissue interactions, and possibly designing cell-type specific delivery systems if successful therapeutics are to be made from overexpressing ectopic miRNAs, or from miRNA inhibitors. At a minimum, therapeutic strategies in cancer need to consider the stroma, not just the cancer cells, a point this insightful study reminds us of.
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The authors declare no conflict of interest.
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