An exploration of mechanisms and effects of oncogenic KRAS on the tumour immune microenvironment

Oncogenic KRAS influences several cancer hallmarks including autonomy from growth promoting signals, enhanced cell survival and altered tumour metabolism. Beyond these, it is now established that oncogenic KRAS can polarise the tumour immune microenvironment, aiding tumour immune evasion. Tristetraprolin (TTP) is an RNA-binding protein, which destabilises its targets through binding to AU-rich elements in their 3'-untranslated regions. It is important in the resolution of macrophage responses to inflammatory stimuli. Additionally, it can bind and destabilise targets in tumour cells. Oncogenic KRAS, through inhibitory phosphorylation of TTP, can prevent it from destablising its targets, resulting in upregulation of oncogenic proteins including programmed death ligand 1. We interrogated the transcriptome-wide effect of TTP overexpression in an immunogenic KRAS-mutant murine colon carcinoma model. Using RNA-seq, we showed that TTP targets are involved in several tumourigenic hallmarks. We discovered that many such targets overlap with those of KRAS, and that this relationship extends across model systems. Co-perturbation of KRAS and TTP in a single model system resulted in modest effect. Next, we used single-cell profiling to understand the effect of mutant-specific onncogenic KRAS inhibition in an orthotopic murine lung carcinoma model. We showed that such inhibition results in compositional change in the repertoire of macrophages with concomitant alteration of gene expression including downregulation of several immunosuppressive transcripts. Furthermore, we showed that tumour cell proliferation is altered, and that the expression of many oncogenic transcripts are reduced. Conversely some oncogenic transcripts, including the emerging clinical target CD47, are upregulated upon KRAS inhibition and may represent potential strategies for combination therapy