Genome-Wide Screen for Context-Dependent Tumor Suppressors Identified Using in Vivo Models for Neoplasia in Drosophila

Mud Loss Restricts Yki-Dependent Hyperplasia in Drosophila Epithelia

Tissue development demands precise control of cell proliferation and organization, which is achieved through multiple conserved signaling pathways and protein complexes in multicellular animals. Epithelia are a ubiquitous tissue type that provide diverse functions including physical protection, barrier formation, chemical exchange, and secretory activity. However, epithelial cells are also a common driver of tumorigenesis; thus, understanding the molecular mechanisms that control their growth dynamics is important in understanding not only developmental mechanisms but also disease. One prominent pathway that regulates epithelial growth is the conserved Hippo/Warts/Yorkie network. Hippo/Warts inactivation, or activating mutations in Yorkie that prevent its phosphorylation (e.g., YkiS168A), drive hyperplastic tissue growth. We recently reported that loss of Mushroom body defect (Mud), a microtubule-associated protein that contributes to mitotic spindle function, restricts YkiS168A-mediated growth in Drosophila imaginal wing disc epithelia. Here we show that Mud loss alters cell cycle progression and triggers apoptosis with accompanying Jun kinase (JNK) activation in YkiS168A-expressing discs. To identify additional molecular insights, we performed RNAseq and differential gene expression profiling. This analysis revealed that Mud knockdown in YkiS168A-expressing discs resulted in a significant downregulation in expression of core basement membrane (BM) and extracellular matrix (ECM) genes, including the type IV collagen gene viking. Furthermore, we found that YkiS168A-expressing discs accumulated increased collagen protein, which was reduced following Mud knockdown. Our results suggest that ECM/BM remodeling can limit untoward growth initiated by an important driver of tumor growth and highlight a potential regulatory link with cytoskeleton-associated genes.

Promoter Proximal Pausing Limits Tumorous Growth Induced by the Yki Transcription Factor in Drosophila

Promoter proximal pausing (PPP) of RNA polymerase II has emerged as a crucial rate-limiting step in the regulation of gene expression. Regulation of PPP is brought about by complexes 7SK snRNP, P-TEFb (Cdk9/cycT), and the negative elongation factor (NELF), which are highly conserved from Drosophila to humans. Here, we show that RNAi-mediated depletion of bin3 or Hexim of the 7SK snRNP complex or depletion of individual components of the NELF complex enhances Yki-driven growth, leading to neoplastic transformation of Drosophila wing imaginal discs. We also show that increased CDK9 expression cooperates with Yki in driving neoplastic growth. Interestingly, overexpression of CDK9 on its own or in the background of depletion of one of the components of 7SK snRNP or the NELF complex necessarily, and specifically, needed Yki overexpression to cause tumorous growth. Genome-wide gene expression analyses suggested that deregulation of protein homeostasis is associated with tumorous growth of wing imaginal discs. As both Fat/Hippo/Yki pathway and PPP are highly conserved, our observations may provide insights into mechanisms of oncogenic function of YAP-the ortholog of Yki in humans.

RhoBTB Proteins Regulate the Hippo Pathway by Antagonizing Ubiquitination of LKB1

The Hippo pathway regulates growth and apoptosis. We identify RhoBTB proteins as novel regulators of Hippo signaling. RhoBTB depletion in the Drosophila wing disc epithelium cooperated with Yki to drive hyperplasia into neoplasia. Depletion of RhoBTB2 caused elevated YAP activity in human cells. RhoBTB2 deficiency resulted in increased colony formation in assays for anchorage-independent growth. We provide evidence that RhoBTBs acts on Hippo signaling through regulation of the kinase LKB1. LKB1 protein levels were reduced upon RhoBTB2 depletion, which correlated with increased LKB1 ubiquitination. Restoring LKB1 levels rescued loss of RhoBTB in Drosophila. Our results suggest that RhoBTB-dependent LKB1 regulation may contribute to its tumor-suppressive function.