DNA-damage inducible 1 homolog 2 (Ddi2) is a novel protein with a conserved aspartic-type proteolytic active site, known for its function as a protein shuttle that delivers ubiquitylated proteins to proteasomes for degradation. Ddi2 is implicated in the recovery of proteasome activity, after treatment with proteasome inhibitors (an FDA-approved treatment for Multiple myeloma), which can be responsible for resistance. In addition, Ddi2's protease activity has recently been linked to controlling embryonic development by activating the transcription factor NFE2L1. NFE2L1 is processed by Ddi2 during the partial proteasome inhibition induced by proteasome inhibitors. We, therefore, hypothesize that Ddi2 is a novel molecular target for the treatment of cancer.<br/>However, studies that implicated Ddi2 in the recovery of proteasome activity have several deficiencies. Most studies utilized pharmacologically irrelevant continuous treatment with inhibitors. They utilized whether a single CRISPR Knockout (KO) clone or a single siRNA/shRNA. These reagents blocked proteasome inhibitor-induced expression of proteasome genes but the majority of studies did not measure whether they affect the recovery of activity. We decided to rigorously re-evaluate these conclusions using CRISPR clones generated by different gRNA and different siRNA constructs. We confirmed that Ddi2 is involved in the upregulation of proteasome genes. However, we found that cells lacking Ddi2 still exhibit proteasome recovery comparable to the wild-type, suggesting that Ddi2 is not solely responsible for proteasome recovery and an alternative mechanism must play a role. We are currently conducting experiments to elucidate the mechanism.

Revisiting the function of DNA damage inducible 1 Homolog II (Ddi2) in the proteasome recovery for the treatment of cancer

ACS Spring 2022

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