MAF1 is a chronic repressor of RNA polymerase III transcription in the mouse

Abstract

Maf1 −/− mice are lean, obesity-resistant and metabolically inefficient. Their increased energy expenditure is thought to be driven by a futile RNA cycle that reprograms metabolism to meet an increased demand for nucleotides stemming from the deregulation of RNA polymerase (pol) III transcription. Metabolic changes consistent with this model have been reported in both fasted and refed mice, however the impact of the fasting-refeeding-cycle on pol III function has not been examined. Here we show that changes in pol III occupancy in the liver of fasted versus refed wildtype mice are largely confined to low and intermediate occupancy genes; high occupancy genes are unchanged. However, in Maf1 −/− mice, pol III occupancy of the vast majority of active loci in liver and the levels of specific precursor tRNAs in this tissue and other organs are higher than wild-type in both fasted and refed conditions. Thus, MAF1 functions as a chronic repressor of active pol III loci and can modulate transcription under different conditions. Our findings support the futile RNA cycle hypothesis, elaborate the mechanism of pol III repression by MAF1 and demonstrate a modest effect of MAF1 on global translation via reduced mRNA levels and translation efficiencies for several ribosomal proteins. The synthesis of the translational apparatus is energetically costly and, consequently, subject to tight regulation (see 1 and references therein). For example, the activities of both pol I and III are controlled in response to nutrient availability 2-6. In the case of pol III, this control is exerted largely by the MAF1 protein, which constitutes its main repressor 7. In yeast, under favorable growth conditions, phosphorylation of Maf1 in a TOR complex 1-(TORC1) and protein kinase A-dependent manner inhibits its interaction with pol III and results in its nuclear exclusion 4,5. Upon nutrient deprivation or stresses like DNA damage, TORC1 is inactivated, Maf1 becomes dephosphorylated and shuttles into the nucleus where it represses pol III transcription. In mammalian cells, mTORC1 directly phosphorylates MAF1 on three residues (S60, S68, and S75) 8. Following serum deprivation, MAF1 undergoes dephosphorylation, increasingly targets pol III genes as determined by DamIP-seq, and represses pol III transcription 9,10. Mice lacking Maf1 are viable and fertile. They are slightly smaller and leaner than their wild-type (WT) counterparts, and they are resistant to diet-induced obesity and non-alcoholic fatty liver disease 11. This is due in part to reduced food intake but also to reduced metabolic efficiency. Maf1 −/− mice display increased energy expenditure throughout the diurnal cycle even though they are not physically more active than WT mice 11. Studies with fasted mice revealed elevated pol III transcription and precursor tRNA levels in the liver and numerous others tissues (~ 3 to 9 fold changes in precursor tRNAs) but no change in total tRNA or mature tRNA levels. These and other findings led to the proposal of a futile RNA cycle to account for t...

Alteration of the Premature tRNA Landscape by Gammaherpesvirus Infection

“…When active, Maf1 directly targets the Brf1 subunit of TFIIIB and RNAPIII to inhibit transcription. In addition, in proliferating and differentiated mammalian cells, Maf1 chronically represses RNAPIII transcription to balance RNA synthesis with cellular demand ( 43 , 44 ). Thus, if the mechanism underlying MHV68 induction of pre-tRNAs involved antagonizing Maf1-dependent repression, we reasoned that cells lacking Maf1 should no longer display a difference in pre-tRNA levels between uninfected and infected cells.…”

POLR3-Related Leukodystrophy: Exploring Potential Therapeutic Approaches

“…Finally, functional studies of POLR3A mutations associated with POLR3-HLD have demonstrated transcriptional defects when introduced in both yeast and human cells (Choquet et al, 2019a ; Moir et al, 2020 ). Further research on molecular pathways and other POLR3 interactors will be valuable in determining whether suppression of upstream POLR3 inhibitors, such as MAF1 (Reina et al, 2006 ; Johnson et al, 2007 ; Bonhoure et al, 2020 ; Vorländer et al, 2020 ), are appropriate for future treatments. It is also possible that a small molecule screening approach could identify drugs for the treatment of specific molecular mechanisms, such as upregulation of complex assembly cofactors or signaling molecules for nuclear import of POLR3 (Cloutier and Coulombe, 2010 ; Lesniewska and Boguta, 2017 ; Willis and Moir, 2018 ).…”

Maf1 suppression of ATF5-dependent mitochondrial unfolded protein response contributes to rapamycin-induced radio-sensitivity in lung cancer cell line A549

“…Maf1 is an mTORC1 effector that has significant roles in cancer biology [ 20 – 24 ]. Maf1 is phosphorylated by mTORC1 at certain Serine and Threonine to regulate RNA Polymerase III-dependent transcription of tRNAs, microRNAs and other small nuclear RNAs [ 7 , 25 , 26 ]. Inhibiting Maf1 expression can induce cancerous transformation in hepatocellular cells and Maf1 overexpression can suppress tumor growth in vivo [ 21 , 23 ].…”