Tumour derived microRNA-21 reprograms macrophage responses

Abstract

The role of the oncogenic yet anti-inflammatory microRNA-21 (miR-21) in the tumour microenvironment (TME) is not clearly defined, despite its upregulation in many cancers. Since re-education of macrophages by tumour derived signals is a central step in cancer pathology, we investigated the role of miR-21 in this process. Up-regulation of miR-21 was confirmed in samples from primary liver hepatocellular carcinoma and colorectal liver metastases. Subsequently, it was found that co-culture of naïve human macrophages with the hepatocellular carcinoma derived cell line, HepG2, induces miR-21 expression. This is consistent with a switch from a pro-inflammatory to an immuno-regulatory gene expression profile. Anti-sense technology was used to silence miR-21 expression in macrophages, enhancing the induction of pro-inflammatory mediators such as Tumour necrosis factor (TNF), boosting the capacity to limit growth of tumour cells. These results support a model whereby macrophage miR-21 limits anti-tumour immunity & could be a central point in tumour immune evasion/escape. Interestingly, upregulation of mature miR-21 levels in co-cultured macrophages did not coincide with transcriptional induction of the primary miR-21 transcript and was instead enriched in secreted exosomes from tumour cells, suggesting a novel mechanism employed by tumour cells to re-program macrophages. Another well recognised feature of tumours is their ability to escape the normal metabolic demands of the cell and take on alternative pathways to utilise glucose to support the energy and biosynthetic demands of rapid proliferation. Previous work identified miR-21 as a key regulator of the switch to Warburg metabolism in activated macrophages. This study shows that blocking exosome release leads to an accumulation of miR-21 inside the tumour cell, significantly impairing glycolysis. Furthermore, metabolic stress in the TME impacted miR-21 expression in exosomes and associated up-regulation of miR-21 in macrophages. Therefore, we propose a model whereby, the export of miR-21 has the dual benefit of allowing cancer cells to pursue uninhibited glycolysis avoiding potential autoregulation by miR-21 and driving pro-tumour macrophage responses. These findings reveal a key role for tumour derived miR-21 in the re-programming of tumour associated macrophages facilitating tumour growth and highlights a novel target for improving anti-tumour responses in vivo.