Roles of C/EBP class bZip proteins in the growth and cell competition of Rp (‘Minute’) mutants in Drosophila

Abstract

Reduced copy number of ribosomal protein (Rp) genes adversely affects both flies and mammals. Xrp1 encodes a reportedly Drosophila-specific AT-hook, bZIP protein responsible for many of the effects including the elimination of Rp mutant cells by competition with wild type cells. Irbp18, an evolutionarily conserved bZIP gene, heterodimerizes with Xrp1 and with another bZip protein, dATF4. We show that Irbp18 is required for the effects of Xrp1, whereas dATF4 does not share the same phenotype, indicating that Xrp1/Irbp18 is the complex active in Rp mutant cells, independently of other complexes that share Irbp18. Xrp1 and Irbp18 transcripts and proteins are upregulated in Rp mutant cells by auto-regulatory expression that depends on the Xrp1 DNA binding domains and is necessary for cell competition. We show that Xrp1 is conserved beyond Drosophila, although under positive selection for rapid evolution, and that at least one human bZip protein can similarly affect Drosophila development.

Cell competition removes segmental aneuploid cells from Drosophila imaginal disc-derived tissues based on ribosomal protein gene dose

“…A stress response pathway that is activated by Rp mutations in Drosophila has recently been described, and is required for Rp point mutated cells to undergo cell competition ( Baillon et al, 2018 ; Kale et al, 2018 ; Lee et al, 2018 ; Ji et al, 2019 ; Blanco et al, 2020 ). In Rp +/- genotypes, RpS12, an essential, eukaryote-specific component of the ribosomal Small Subunit, is required to activate expression of Xrp1, a rapidly evolving AT-hook, bZip domain transcription factor ( Lee et al, 2018 ; Ji et al, 2019 ; Blanco et al, 2020 ). Although rpS12 null mutations are homozygously lethal, a particular point mutation, rpS12 G97D , appears defective only for the cell competition aspect of RpS12 function.…”

“…A key target of RpS12 appears to be the putative transcription factor Xrp1, because Xrp1 protein is barely detected in wild type cells but significantly elevated in Rp +/- wing discs. Xrp1 controls most of the phenotype of Rp +/- cells, including their reduced translation ( Lee et al, 2018 ; Ji et al, 2019 ; Blanco et al, 2020 ). Xrp1 mutants have negligible effect in wild-type backgrounds, and normal lifespan ( Baillon et al, 2018 ; Lee et al, 2018 ; Mallik et al, 2018 ).…”

Mechanisms underlying the cooperation between loss of epithelial polarity and Notch signaling during neoplastic growth in Drosophila

“…Finally, we show that similarly to what was previously described for Ras signaling (Atkins et al, 2016), the cooperation between Notch signaling and polarity loss is controlled by a “tumor transcriptional network” centered around the AP-1/Stat/Yki transcription factors, and including the critical bZIP factors Pdp1 and CEPBG (CG6272/Irbp18). But our analysis uncovered a previously unreported role for the cell competition regulator, and Irbp18 binding partner Xrp1 (Baillon et al, 2018; Blanco et al, 2020; Boulan et al, 2019; Ji et al, 2019), raising the interesting prospect that neoplastic growth could be mediated at least in part, by co-opting cell competition.…”

“…CG6272/Irbp18 also interacts with Xrp1, and the heterodimer has been implicated in DNA repair (Akdemir et al, 2007; Francis et al, 2016). We thus propose that the Irbp18/Xrp1 dimer is activated in NS and is required to facilitate DNA repair and ensure genomic stability to prevent catastrophic genotoxic effect upon the combined cellular stresses of S and replication stress of N. Recently, Xrp1 together with its binding partner CG6272/Irbp18 (but not Atf4) has been shown to mediate a loser status in ribosomal genes deficient cells, through the implementation of a specific transcriptional program (Baillon et al, 2018; Blanco et al, 2020; Ji et al, 2019). This “loser state” promoter role of Xrp1/Irbp18 is however in conflict with our observation that Xrp1 is strongly upregulated in NS overgrowing discs, and that Xrp1 and Irbp18 are required for the overgrowth and invasive capacities of NS neoplastic cells.…”

“…We further provide evidence that this Notch redirection is not mediated by new genomic binding regions for Su(H), but relies on the cooperation with Su(H) of a combination of transcription factors, such as basic leucine zipper (bZIP), whose activity is triggered in response to JNK signaling during polarity loss, extending earlier reports on the cooperation between oncogenic Ras and polarity loss (Atkins et al, 2016; Külshammer et al, 2015; Uhlirova and Bohmann, 2006). Our work highlights in particular the role of the stress response CEBPG homologue CG6272/Irbp18 and of its partner Xrp1, key factors in mediating the loser fate during cell competition (Baillon et al, 2018; Blanco et al, 2020; Ji et al, 2019), suggesting that cellular competition, or parts of the cellular competition program, are co-opted during neoplastic growth.…”