The MYC 5’ upstream region facilitates autophagic adaptations in response to PI3K inhibition

The internal ribosome entry site (IRES) is considered one of the mechanisms in which colorectal cancer cells can utilize to survive the stresses of the tumor microenvironment and to escape cell death. The proto- oncogene MYC is upregulated in 70% of human neoplasia and is considered a key driver of colorectal cancer (CRC). An IRES sequence was discovered in its 5’ UTR, presenting the possibility of tumoral cells utilizing this mechanism to sustain cell growth under stressful conditions. However, controversy exists surrounding the technical approaches used to validate this sequence. We investigated the involvement of the MYC IRES in a diverse range of cellular stresses using a CRIPS-Cas9 approach to excise the sequence from CRC cell lines. Our results demonstrate that the MYC IRES does not facilitate cap-independent translation. Rather, it appears to function as transcriptional enhancer responsive to PI3K signaling mediated through the MAP-kinase pathway. CRC cells adapt to PI3K inhibition by increasing the autophagic flux, however, those cells lacking the IRES fail to do so, and demonstrate antiproliferative sensitivity. This sensitivity is also found in Burkitt Lymphoma (BL) cells that exhibit a MYC translocation dissociating the IRES from the coding sequence. In conclusion, we find that the MYC IRES functions not to facilitate internal ribosome entry, but to enhance MYC transcription in response to PI3K inhibition in order to promote cancer cell survival though increased autophagic flux.