PACT suppresses PKR activation through dsRNA binding and dimerization, and is a therapeutic target for triple-negative breast cancer

Triple-negative breast cancer (TNBC), the deadliest breast cancer subtype, lacks broadly applicable targeted therapies. Induction of ‘viral mimicry’ through activation of viral double-stranded RNA (dsRNA) sensors has potential therapeutic applications for TNBC and other cancers. Suppressors of dsRNA sensing prevent sensing of endogenous dsRNAs and resulting autoimmunity. Depletion of the suppressor of dsRNA sensing ADAR1 causes activation of dsRNA sensors and cell death in many cancer cell lines. These ADAR1-dependent cells are generally also dependent on the dsRNA-binding protein PACT, which is highly expressed and essential in many TNBC cell lines. While PACT is known as an activator of the dsRNA sensor PKR, overexpression of PACT had no effect on activation of PKR in multiple breast cancer cell lines. Conversely, depletion of PACT in PACT-dependent cell lines caused robust activation of the dsRNA sensor PKR and cell death, in addition to induction of integrated stress response genes and NF-κB targets. These phenotypes were entirely dependent on PKR. Rescue experiments revealed that PACT dimerization and dsRNA binding is required to suppress PKR activation. While depletion of PACT alone in ADAR1/ PACT-independent cell lines had no effect on PKR activation, combined depletion of both PACT and ADAR1 in those cell lines caused robust PKR activation and cell death, supporting a partially redundant role for ADAR1 and PACT in suppression of dsRNA sensing. Taken together, these findings support a vital role for PACT in suppression of PKR activation, and highlight the therapeutic potential of targeting PACT to treat TNBC.