A Non-Tumor Suppressor Role for Basal p19^ARF in Maintaining Nucleolar Structure and Function

Abstract

The nucleolus is the center of ribosome synthesis, with the nucleophosmin (NPM) and p19^ARF proteins antagonizing one another to either promote or inhibit growth. However, basal NPM and ARF proteins form nucleolar complexes whose functions remain unknown. Nucleoli from Arf^−/− cells displayed increased nucleolar area, suggesting that basal ARF might regulate key nucleolar functions. Concordantly, ribosome biogenesis and protein synthesis were dramatically elevated in the absence of Arf, causing these cells to exhibit tremendous gains in protein amounts and increases in cell volume. The transcription of ribosomal DNA (rDNA), the processing of nascent rRNA molecules, and the nuclear export of ribosomes were all increased in the absence of ARF. Similar results were obtained using targeted lentiviral RNA interference of ARF in wild-type MEFs. Postmitotic osteoclasts from Arf-null mice exhibited hyperactivity in vitro and in vivo, demonstrating a physiological function for basal ARF. Moreover, the knockdown of NPM blocked the increases in Arf^−/− ribosome output and osteoclast activity, demonstrating that these gains require NPM. Thus, basal ARF proteins act as a monitor of steady-state ribosome biogenesis and growth independent of their ability to prevent unwarranted hyperproliferation.

ACKNOWLEDGMENTS

We thank Martine Roussel, Charles Sherr, Tyler Jacks, and Gerard Zambetti for plasmids, antibodies, and mice, as well as Michael Benjamin for excellent assistance. We are extremely grateful to Sheila Stewart, Phil Stahl, Arie Perry, and Josh Rubin for insightful discussions.

A.J.A. is a recipient of a grant-in-aid from the Department of Defense Breast Cancer Research Program (BC030793). L.B.M. is supported by the Department of Defense Prostate Cancer Research Program award number W81XWH-04-0909. A.J.S. and M.K. are supported by the Siteman Cancer Biology Pathway. J.D.W. thanks the Pew Charitable Trusts and is a recipient of grants-in-aid from the Susan G. Komen for the Cure and National Institutes of Health (GM066032).