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Organ growth control in the absence of Hippo signaling

ABSTRACT: Organs of all animals reliably grow to a specific size and it is widely thought that such developmental growth control is instructed by the Hippo signaling pathway. This is based on studies in flies and mice showing that deregulation of Hippo signaling causes dramatic tissue overgrowth of various organs. Here we use two major developmental paradigms and provide evidence that removing the key transcriptional downstream effectors of the Hippo pathway does not abolish organ growth control. Specifically, eliminating the function of the Hippo downstream co-activators Yki as well as Yap/Taz that complex with TEAD family transcription factors did not interfere with organ growth control in the developing mouse liver or Drosophila eye. Rather, mutant cells were able to proliferate appropriately during development, properly exit the cell cycle, and form organs of normal size. Remarkably, a complementing whole transcriptome analysis of mutant Drosophila eye imaginal discs corroborated these genetic findings. It showed that loss of the single TEAD transcription factor Scalloped (Sd) fully reverted the effect of overexpression of hyperactivated Yki back to a wildtype gene expression profile. In contrast, Sd/Yap/Taz proved to be crucial for cell survival in injury response models, namely X-ray irradiation of flies and toxic liver injury in mice. Therefore, we propose that Hippo signaling is dispensable for instructing normal organ growth and instead suggest that the classical Hippo mutant overgrowth phenotypes represent a de facto Yap/Taz/Yki gain-of-function situation that does not reflect an endogenous role in growth control. Instead, we hypothesize that the potency of Yap/Taz/Yki to drive tissue overgrowth draws from their broad and primary function in injury responses.

ORGANISM(S): Drosophila melanogaster

PROVIDER: GSE96887 | GEO | 2025/02/28

REPOSITORIES: GEO

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Project description:Regulation of organ size is important for development and tissue homeostasis. In Drosophila, Hippo signaling controls organ size by regulating the activity of a TEAD transcription factor, Scalloped, through modulation of its coactivator protein Yki. The role of mammalian Tead proteins in growth regulation, however, remains unknown. Here we examined the role of mouse Tead proteins in growth regulation. In NIH3T3 cells, cell density and Hippo signaling regulated the activity of Tead proteins by modulating nuclear localization of a Yki homologue, Yap, and the resulting change in Tead activity altered cell proliferation. Tead2-VP16 mimicked Yap overexpression, including increased cell proliferation, reduced cell death, promotion of EMT, lack of cell contact inhibition, and promotion of tumor formation. Growth promoting activities of various Yap mutants correlated with their Tead-coactivator activities. Tead2-VP16 and Yap regulated largely overlapping sets of genes. However, only a few of the Tead/Yapregulated genes in NIH3T3 cells were affected in Tead1-/-;Tead2-/- or Yap-/- embryos. Most of the previously identified Yap-regulated genes were not affected in NIH3T3 cells or mutant mice. In embryos, levels of nuclear Yap and Tead1 varied depending on cell types. Strong nuclear accumulation of Yap and Tead1 were seen in myocardium, correlating with requirements of Tead1 for proliferation. However, their distribution did not always correlate with proliferation. Taken together, mammalian Tead proteins regulate cell proliferation and contact inhibition as a transcriptional mediator of Hippo signaling, but the mechanisms by which Tead/Yap regulate cell proliferation differ depending on cell types, and Tead, Yap and Hippo signaling may play multiple roles in mouse embryos. We used microarrays to know the gene expression profiles regurated by Tead2-VP16, Tead2-EnR, Yap, and cell density in NIH3T3 cells. Keywords: Cell density, genetic modification Tead2-VP16-, Tead2-EnR-, Yap- and control vector-expressing cells were cultured at low or high density for RNA extraction and hybridization on Affymetrix microarrays.

Project description:Uncontrolled Transforming growth factor-beta (TGFβ) signaling promotes aggressive metastatic properties in late-stage breast cancers. However, how TGFβ-mediated cues are directed to induce late-stage tumorigenic events is poorly understood, particularly given that TGFβ has clear tumor suppressing activity in other contexts. Here we demonstrate that the transcriptional regulators TAZ and YAP (TAZ/YAP), key effectors of the Hippo pathway, are necessary to promote and maintain TGFβ-induced tumorigenic phenotypes in breast cancer cells. Interactions between TAZ/YAP, TGFβ-activated SMAD2/3, and TEAD transcription factors reveal convergent roles for these factors in the nucleus. Genome-wide expression analyses indicate that TAZ/YAP, TEADs and TGFβ-induced signals coordinate a specific pro-tumorigenic transcriptional program. Importantly, genes cooperatively regulated by TAZ/YAP, TEAD, and TGFβ, such as the novel targets NEGR1 and UCA1, are necessary for maintaining tumorigenic activity in metastatic breast cancer cells. Nuclear TAZ/YAP also cooperate with TGFβ signaling to promote phenotypic and transcriptional changes in non-tumorigenic cells to overcome TGFβ repressive effects. Our work thus identifies crosstalk between nuclear TAZ/YAP and TGFβ signaling in breast cancer cells, revealing novel insight into late-stage disease-driving mechanisms. Expression profiling was conducted following the repression of the transcriptional regulators TAZ and YAP (TAZ/YAP), the TEAD family of transcription factors (TEAD1/2/3/4), or the TGFb signaling pathway (with SB-431542, an inhibitor of the TBRI recpeptor) in human MDA-MB-231-LM2 breast cancer cells treated with TGFβ1. Human MDA-MB-231-LM2-4 breast cancer cells were transfected with control siRNA, or siRNAs targeting TAZ/YAP or all four TEADs and were treated 24 hours later with 500pM TGFβ1 or 5mM SB-431542 for an additional 24 hours. Total RNA was isolated and twelve microarrays in total were performed, with each condition carried out three times on separate days. The Boston University Microarray Core generated the data using the Affymetrix Human Gene 1.0 St Array.

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Organ growth control in the absence of Hippo signaling

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