Date Published: April 18, 2013
Publisher: Public Library of Science
Author(s): Justine Marsolier, Sandra Pineau, Souhila Medjkane, Martine Perichon, Qinyan Yin, Erik Flemington, Matthew D. Weitzman, Jonathan B. Weitzman, Elisabetta Ullu.
The intracellular parasite Theileria is the only eukaryote known to transform its mammalian host cells. We investigated the host mechanisms involved in parasite-induced transformation phenotypes. Tumour progression is a multistep process, yet ‘oncogene addiction’ implies that cancer cell growth and survival can be impaired by inactivating a single gene, offering a rationale for targeted molecular therapies. Furthermore, feedback loops often act as key regulatory hubs in tumorigenesis. We searched for microRNAs involved in addiction to regulatory loops in leukocytes infected with Theileria parasites. We show that Theileria transformation involves induction of the host bovine oncomiR miR-155, via the c-Jun transcription factor and AP-1 activity. We identified a novel miR-155 target, DET1, an evolutionarily-conserved factor involved in c-Jun ubiquitination. We show that miR-155 expression led to repression of DET1 protein, causing stabilization of c-Jun and driving the promoter activity of the BIC transcript containing miR-155. This positive feedback loop is critical to maintain the growth and survival of Theileria-infected leukocytes; transformation is reversed by inhibiting AP-1 activity or miR-155 expression. This is the first demonstration that Theileria parasites induce the expression of host non-coding RNAs and highlights the importance of a novel feedback loop in maintaining the proliferative phenotypes induced upon parasite infection. Hence, parasite infection drives epigenetic rewiring of the regulatory circuitry of host leukocytes, placing miR-155 at the crossroads between infection, regulatory circuits and transformation.
Both infection and cancer have been extensively linked to the induction of microRNAs (miRs) which can exert diverse effects on cellular phenotypes by targeting many genes , . microRNAs (miRNAs) are a class of small non-coding RNAs, 22 nt in length, that modulate post-transcriptional gene expression . It is likely that miRNAs play critical roles in fine-tuning the host response to infection and inflammation , . OncomiRs are miRNAs that are upregulated in tumours and which have oncogenic functions depending on the genes they target , . However, It has been relatively difficult to identify essential miR pathways in infection and critical OncomiR target genes in tumorigenesis , . ‘Oncogene addiction’ is an emerging concept which suggests that underlying the multistep nature of tumour progression, cancer cell growth and survival can often be impaired by targeting a single oncogene pathway, thereby offering a promise for the development of targeted molecular therapies , , .
Theileria-induced transformation offers an attractive experimental model, as it appears that infection of host leukocytes is accompanied by a rewiring of the cellular circuitry , , . The identification of molecular players that play key roles in maintaining proliferative phenotypes could be relevant for identifying effective therapeutic strategies to reverse transformation. Thus, oncogenic pathways in Theileria-infected cells may highlight examples of oncogene addiction for future studies. We have extended this hypothesis to investigate microRNA pathways and identified molecular targets that create an addictive regulatory loop. This is the first study to show that Theileria manipulates host gene expression via microRNAs. This observation underlines the increasing importance being given to non-coding RNAs in the regulation of gene expression, inflammatory response and tumour cell phenotypes , , . miRNA networks are affected by several parasites of the apicomplexa phylum (e.g. Toxoplasma, Cryptosporidium or Eimeria). Some of these may be related to the infection process and initial inflammatory responses, while others may be relevant to long-term features of host-parasite interactions. C. parvum infection of epithelial cells was also shown to induce a range of host miRNAs which are regulated by NFκB-dependent transcription . However, there does not seem to be any significant overlap with the miRNA network regulated by Theileria. Similarly, T. gondii was shown to induce transcriptional regulation of a distinct set of host miRNAs, whereas the related Neospora caninum parasite did not . Future studies might reveal common and distinct pathways related to miRNA induction by parasites across the apicomplexa phylum. miR-155 induction does seem to be a common feature in several inflammatory and tumorigenic scenarios. For example, Helicobacter pylori infection, which is associated with gastric adenocarcinoma, also induces miR-155 expression in T cells, but via Foxp3 . We show here that activated AP-1 transcription factors in parasitized transformed cells drives the transcription of the BIC gene, leading to increased miR-155 expression in both artificially infected bovine B cells and in naturally-infected bovine Thei macrophages. We provide evidence that miR-155 targets the DET1 protein, which leads to accumulation of the c-Jun protein and increased transcription of the miR-155-encoding BIC gene (Figure 7D). This feedback loop is critical for maintaining the transformed phenotype, as inhibiting any node in the loop reverses the transformed phenotypes (growth in soft agar and cell survival) of parasitized cells. Thus, our study has provided the molecular events in a miR-155 loop that links infection and transformation.