P19^ARF and Ras^V12 offer opposing regulation of DHX33 translation to dictate tumor cell fate

DHX33 is a pivotal DEAH-box RNA helicase in the multistep process of RNA polymerase I-directed transcription of the ribosomal DNA locus. We explored the regulation of DHX33 expression by Ras^V12 and ARF to determine DHX33's role in sensing these opposing signals to regulate ribosome biogenesis. In wild-type primary fibroblasts, Ras^V12 infection induced a transient increase in DHX33 protein level, as well as an rRNA transcriptional rate that was eventually suppressed by a delayed activation of the ARF/p53 pathway. DHX33 expression was exclusively controlled at the level of translation. ARF caused a dramatic reduction in polysome-associated DHX33 mRNAs, while Ras^V12 led to a complete shift of existing DHX33 mRNAs to actively translating polysomes. The translation of DHX33 by Ras^V12 was sensitive to inhibitors of phosphatidylinositol 3-kinase, mTOR, and mitogen-activated protein and was pivotal for enhanced rRNA transcription and enhanced overall cellular protein translation. In addition, DHX33 knockdown abolished Ras^V12-induced rRNA transcription and protein translation and prevented both the in vitro and in vivo transforming properties of oncogenic Ras^V12. Our results directly implicate DHX33 as a crucial player in establishing rRNA synthesis rates in the face of Ras^V12 or ARF signals, adjusting ribosome biogenesis to match the appropriate growth or antigrowth signals.