PWP1 Mediates Nutrient-Dependent Growth Control through Nucleolar Regulation of Ribosomal Gene Expression

Abstract

Ribosome biogenesis regulates animal growth and is controlled by nutrient-responsive mTOR signaling. How ribosome biogenesis is regulated during the developmental growth of animals and how nutrient-responsive signaling adjusts ribosome biogenesis in this setting have remained insufficiently understood. We uncover PWP1 as a chromatin-associated regulator of developmental growth with a conserved role in RNA polymerase I (Pol I)-mediated rRNA transcription. We further observed that PWP1 epigenetically maintains the rDNA loci in a transcription-competent state. PWP1 responds to nutrition in Drosophila larvae via mTOR signaling through gene expression and phosphorylation, which controls the nucleolar localization of dPWP1. Our data further imply that dPWP1 acts synergistically with mTOR signaling to regulate the nucleolar localization of TFIIH, a known elongation factor of Pol I. Ribosome biogenesis is often deregulated in cancer, and we demonstrate that high PWP1 levels in human head and neck squamous cell carcinoma tumors are associated with poor prognosis.

PWP1 promotes nutrient-responsive expression of 5S ribosomal RNA

“…S2 " type="url"/> ) in combination with ChIP to analyze Histone 4 lysine 12 (H4K12) acetylation, which is associated with active transcription, as well as H3K9 dimethylation, a repression-associated modification. Similar to the earlier observations on the Pol I target loci ( Liu et al, 2017 ), knockdown of PWP1 reduced the level of H4K12 modification on 5S rDNA loci ( Fig. 4 A), while increasing the level of repressive H3K9 dimethylation ( Fig.…”

“…Our previous study showed that PWP1 acts downstream of nutrient-responsive mTOR signaling in vivo in Drosophila and that PWP1 is essential for the induced expression of the Pol I-dependent rRNAs in response to protein-rich diet ( Liu et al, 2017 ). As the Pol III-dependent gene expression is also dependent on nutrition ( Marshall et al, 2012 ), we wanted to test if Drosophila PWP1 (dPWP1, encoded by the nclb gene) controls the expression of Pol III-dependent 5S rRNA in this setting.…”

“…3 ). PWP1 was earlier shown to regulate the epigenetic status of Pol I target loci ( Liu et al, 2017 ). Therefore, we wanted to analyze if similar regulation occurs in the 5S rDNA genomic region.…”

“…In line with the role in promoting ribosome biogenesis, Drosophila mutant larvae lacking PWP1 show a prominent growth retardation. In addition to the role in Pol I regulation, we observed earlier that pwp1 mutant Drosophila larvae display reduced expression of RNA polymerase III targets, including 5S rRNA ( Liu et al, 2017 ). However, it remained to be tested whether PWP1 controls Pol III-mediated transcription directly and in a nutrient-responsive manner, and whether its role in this setting is conserved in animals.…”

“…We have previously reported that the WD40 domain protein PWP1 contributes to nutrient-dependent control of Pol I. PWP1 is activated by mTOR through phosphorylation and elevated gene expression. PWP1 associates with the rDNA locus, maintains it in a transcription-competent epigenetic state and promotes Pol I-mediated transcription of rRNA in a nutrient-responsive manner ( Liu et al, 2017 ). In line with the role in promoting ribosome biogenesis, Drosophila mutant larvae lacking PWP1 show a prominent growth retardation.…”