GRIM-1, a Novel Growth Suppressor, Inhibits rRNA Maturation by Suppressing Small Nucleolar RNAs

Shreeram C Nallar,Padmaja Gade,Limei Lin,Dhananjaya V Kalvakolanu,Varsha Srivastava,Hafiz Ahmed,Edward R Hofmann,Sekhar P Reddy,Dhyan Chandra

doi:10.1371/journal.pone.0024082

Shreeram C Nallar, Padmaja Gade + Show 7 more

Open Access

https://doi.org/10.1371/journal.pone.0024082

Copy DOI

Abstract

We have recently isolated novel IFN-inducible gene, Gene associated with Retinoid-Interferon-induced Mortality-1 (GRIM-1), using a genetic technique. Moderate ectopic expression of GRIM-1 caused growth inhibition and sensitized cells to retinoic acid (RA)/IFN-induced cell death while high expression caused apoptosis. GRIM-1 depletion, using RNAi, conferred a growth advantage. Three protein isoforms (1α, 1β and 1γ) with identical C-termini are produced from GRIM-1 mRNA. We show that GRIM-1 isoforms interact with NAF1 and DKC1, two essential proteins required for box H/ACA sno/sca RNP biogenesis and suppresses box H/ACA RNA levels in mammalian cells by delocalizing NAF1. Suppression of these small RNAs manifests as inefficient rRNA maturation and growth suppression. Interestingly, yeast Shq1p also caused growth suppression in mammalian cells. Consistent with its growth-suppressive property, GRIM-1 expression is lost in a number of human primary prostate tumors. Our observations support a recent study that GRIM-1 might act as a co-tumor suppressor in the prostate.

Highlights

The IFN family of cytokines suppress abnormal cell growth by two mechanisms: 1) directly activating growth arrest/apoptosis in the neoplastic cells [1] and 2) shaping the immune response against neoplastic cells [2]
We depleted Gene associated with Retinoid-Interferon-induced Mortality-1 (GRIM-1) using lentiviral vectors coding for a GRIM-1-specific shRNA, and monitored growth in the presence of retinoic acid (RA)/IFN
Cells expressing GRIM-1-specific shRNA continued to grow in presence of RA/IFN

Summary

Introduction

The IFN family of cytokines suppress abnormal cell growth by two mechanisms: 1) directly activating growth arrest/apoptosis in the neoplastic cells [1] and 2) shaping the immune response against neoplastic cells [2]. Among the known direct effects of IFNs on mRNA are - 1) degradation by 29,59A-dependent RNaseL [3]; 2) inhibition of cap-dependent translation initiation by PKR via phosphorylation of eIF-2a; 3) sequestration of eIF3 by p56 [4] and 4) mTOR-dependent phosphorylation of cap-binding factor eIF4E-BP1 [5]. We and others have shown that combination of IFNs with cell-differentiating agents such as retinoic acid (RA) effectively suppresses tumor growth in clinical and experimental models [1]. To understand the molecular mechanisms that regulate these processes, we employed a genome-wide knockdown method and isolated GRIMs [6,7]. One such novel gene product GRIM-1, suppressed cell growth [8]; and exhibited similarity (,53%) to a yeast protein, Shq1p. We describe a mechanism by GRIM-1 that controls mature rRNA levels and cell growth

Methods

Results

Conclusion