Ribosome ADP-Ribosylation Controls Protein Synthesis in Ovarian Cancer

The poly(ADP-ribosyl) polymerase (PARP) family of proteins regulates many functions, including transcription, mRNA processing, and DNA repair. Whereas many studies have focused on poly(ADP-ribosylation), especially in the context of PARP inhibition in homologous recombination–deficient tumors, the biological function of mono(ADP-ribosylation) (MARylation) remains poorly understood. Given that PARPs consume NAD⁺ to covalently attach ADP-ribose to substrate proteins and that ovarian tumors upregulate NMNAT2, encoding a cytosolic NAD⁺ synthase, Challa and colleagues investigated the relationship between NAD⁺ synthesis and ADP-ribosylation in ovarian cancer. In human ovarian cancer cells, shRNA-mediated NMNAT2 knockdown reduced cytosolic levels of NAD⁺ and MARylation. Specifically, ribosomal proteins displayed high levels of MARylation, dependent on NAD⁺ and NMNAT2 catalytic activity. Moreover, NMNAT2 activity decreased protein synthesis, supporting the inhibitory role of ribosomal MARylation. In patient samples, MARylation positively correlated with NMNAT2 levels, and high MARylation levels were associated with decreased progression-free survival. An siRNA knockdown screen in ovarian cancer cells highlighted PARP16 as the only mono(ADP-ribosyl) transferase whose knockdown decreased ribosomal MARylation. Enhanced protein synthesis resulting from shRNA-mediated knockdown of NMNAT2 or PARP16 led to the formation of protein aggregates and elevated markers of proteotoxicity-mediated apoptosis in vitro, and knockdown of NMNAT2 or PARP16 in xenograft models of ovarian cancer slowed tumor growth and increased protein synthesis, protein aggregation, and apoptosis in vivo. Polysome RNA sequencing was performed to assess the effects of MARylation on ribosome function, revealing that NMNAT2 or PARP16 depletion increased mRNA loading on polysomes of many transcripts. Analysis of genes with increased loading revealed a probable stem-loop structure in the 3′ untranslated region, which was sufficient to modulate mRNA loading of a reporter construct in response to NMNAT2 or PARP16 depletion. Mechanistically, MARylation of ribosomal protein RPL24 at Glu4 or RPS6 at Glu35 decreased protein synthesis. Together, this work highlights a mechanism by which cancer cells rely on ribosome ADP-ribosylation to regulate translation and prevent protein aggregation.

Challa S, Khulpateea BR, Nandu T, Camacho CV, Ryu KW, Chen H, et al. Ribosome ADP-ribosylation inhibits translation and maintains proteostasis in cancers. Cell 2021 Jul 26 [Epub ahead of print].

Note: Research Watch is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details. For more Research Watch, visit Cancer Discovery online at http://cancerdiscovery.aacrjournals.org/CDNews.