Melanoma is an aggressive and treatment-resistant malignancy. Delineation of melanoma resistance mechanisms is of cardinal importance. RNA editing by ADAR enzymes results in significant biological effects. Decreased expression level of ADARs was demonstrated in many types of tumors, including melanoma.
Here, we study the role of ADAR1 in regulating melanoma immune resistance.
We show that ADAR1-knockdown cells were more resistant than control to tumor infiltrating lymphocytes (TILs), as evident by decreased killing. Conditioned medium, transwell assays, and inhibition of protein synthesis suggest a contact-dependent mechanism. miR-222, which targets ICAM1, a protein required to generate tumor-specific CTL responses, was up-regulated in ADAR1-knockdown cells while ICAM1 expression was reduced. Forced expression of miR-222 down-regulated ICAM1 and conferred immune resistance. Blocking of ICAM1 with mAb proved its role in immune interactions. Importantly, overexpression of ADAR1 exhibited an opposite phenotype in all of the above experiments. Analysis of pri- and mature miR-222 expression suggested that ADAR1 affects the biogenesis of miR-222 at the nuclear level. Transfection of ADAR1 constructs lacking the catalytic domain yielded similar results to those obtained with the full protein, suggesting an RNA-editing independent function. Taken together, these are the first mechanistic evidence for a pivotal role of ADAR1 in cancer immune resistance.
Finally, we tested miR-222 expression in melanoma specimens derived from patients treated with Ipilimumab, an anti-CTLA-4 monoclonal antibody that potentiates an immune response. miR-222 expression was higher (4.5-fold) in patients who had no clinical benefit as compared to patients that responded. This observation is in line with the described immune protective effect of miR-222, and places miR-222 as a reliable biomarker for the prediction of response to Ipilimumab.
These results provide not only novel insights on melanoma immune resistance, but also pave the way to the development of innovative personalized tools to enable optimal drug selection and treatment.
