p19^ARF and Ras^V12 Offer Opposing Regulation of DHX33 Translation To Dictate Tumor Cell Fate

Abstract

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.

ACKNOWLEDGMENTS

We thank the members of the Weber laboratory for their advice and technical assistance. Lian-Fai Yee provided Arf-null ear fibroblasts. The Genome Institute and Children's Discovery Institute at Washington University provided lentiviral RNAi library constructs.

A.J.S. was supported by Komen for the Cure. This study was supported by National Institutes of Health grant CA120436 and by an Era of Hope Scholar Award in Breast Cancer Research (BC007304) to J.D.W.