The Hippo pathway is a conserved signaling cascade that modulates tissue growth. Although its core elements are well defined, factors modulating Hippo transcriptional outputs remain elusive. Here we show that elements of the steroid-responsive ecdysone (Ec) pathway modulate Hippo transcriptional effects in imaginal disc cells. The Ec receptor coactivator Taiman (Tai) interacts with the Hippo transcriptional coactivator Yorkie (Yki) and promotes expression of canonical Yki-responsive genes. Tai enhances Yki-driven growth while Tai loss, or a form of Tai unable to bind Yki, suppresses Yki-driven tissue growth. This growth suppression is not correlated with impaired induction of canonical Hippo-responsive genes but with suppression of a distinct pro-growth program of Yki-induced/Tai-dependent genes, including the germline stem cell factors nanos and piwi. These data reveal Hippo/Ec pathway crosstalk in the form a Yki-Tai complex that collaboratively induces germline genes as part of a transcriptional program that is normally repressed in developing somatic epithelia.

Melanin is the main skin and hair pigment found throughout the animal kingdom. It is responsible for protecting animals from damaging solar radiation and plays a role in all aspects of animal coloration (Hill, 1992). Mammalian pathways of melanin biosynthesis have been extensively studied (d'Ischia et al., 2015;Ito & Wakamatsu, 2008;Sugumaran & Barek, 2016). The two major types of melanin include brown-to-black eumelanin and yellow-to-red pheomelanin, both arising from tyrosine and its hydroxylated product, dopa. Tyrosinase

In hepatocellular carcinomas (HCCs), the levels of histone H3 dimethylation at lysine 9 (H3K9me2) and its corresponding histone methyltransferase G9a are significantly elevated. Recently, G9a was reported to form a complex with the H3K9 methylation effector protein CDYL, but little is known about the expression of CDYL in HCC patients. The human CDYL gene produces two transcripts, a long form (CDYLa) and a short form (CDYLb), but it is unclear whether the protein products have different functions. The aim of this study was to investigate the distinctions between CDYLa and CDYLb and their expression levels in HCC tissues. We first examined binding abilities of the different CDYL forms with methylated H3 peptides by a pull-down assay. Human CDYLb (h-CDYLb) specifically recognized H3Kc9me2 and H3Kc9me3 modifications, whereas human CDYLa (h-CDYLa) did not interact with any methylated H3 peptides. Similarly, mouse CDYLb (m-CDYLb) specifically bound with di- and tri-methylated H3Kc9 peptides, while mouse CDYLa (m-CDYLa) lacked that ability. Affinity purification also was used to identify the distinct composition of the h-CDYLa or h-CDYLb protein complex. h-CDYLb was found in a multiprotein complex with G9a and GLP, while the h-CDYLa complex did not contain these two enzymes. Consistent with the protein complex composition, h-CDYLb and G9a were both upregulated in HCC tissues, compared with adjacent non-cancerous liver tissues. Furthermore, the positive correlation between expression levels of h-CDYLb and G9a was statistically significant. In contrast, h-CDYLa showed no enrichment in HCC tissues. These findings suggest that h-CDYLb and G9a are cooperatively involved in HCC.

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Dysregulation of the Hippo pathway results in organ overgrowth and contributes to carcinogenesis in humans. Although the Hippo pathway has been intensively studied for its regulation of cell proliferation and organ growth, its role beyond growth control remains elusive. To explore possible novel functions of the Hippo pathway, we carried out an affinity purification-mass spectrometry (AP-MS) analysis in Drosophila embryos and cultured cells, using transcriptional coactivator Yorkie (Yki, YAP1 homolog) as bait. Prominent groups of identified interactors included all of the core pathway components and known accessory regulators, as well as subunits of RNA polymerase II and components of the Trithorax-related (Trr) complex. Among putative novel interactors we found Bonus (Bon), the Drosophila homolog of the mammalian transcriptional intermediary factor-1/tripartite motif containing (TIF1/TRIM24/TRIM33) family of proteins. Protein interaction between Bon and Yki requires the WW domains in Yki and PPxY motifs in Bon. Interestingly, Bon overexpression leads to formation of epidermal extensions (trichomes) on the surface of adult eyes. This phenotype requires Yki and Scalloped (Sd, TEAD homolog), as well as Bon-Yki interaction. To obtain a mechanistic understanding of this apparent switch towards an epidermal cell fate, we searched for novel genes that may be jointly regulated by Bon and Yki, and identified several candidates based on published ChIP-seq and RNA-seq analyses. These putative target genes are not canonical Hippo pathway effectors, but include factors involved in early eye specification as well as genes controlled by steroid hormone signaling. Loss or gain of function of a subset of these genes modified the trichome phenotype, suggesting that they are under the control of Bon and Yki and are involved in establishing a choice between a “generic” epidermal cell fate and eye field specification. In summary, our study reveals a novel function of the Hippo pathway in the regulation of epidermal cell differentiation. This regulation occurs through the interaction between Yki and Bon as well as their joint transcriptional control of nonclassical target genes. This work broadens our understanding of the Hippo pathway beyond its role in growth control. Citation Format: Heya Zhao, Can Zhang, Kenneth Moberg, Alexey Veraksa. Joint control of epidermal cell fate by Yorkie and Bonus [abstract]. In: Proceedings of the AACR Special Conference on the Hippo Pathway: Signaling, Cancer, and Beyond; 2019 May 8-11; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(8_Suppl):Abstract nr B30.

The Hippo pathway controls organ growth, however its role in cell fate determination and the underlying mechanism is not well understood. Here, we uncover the function of the Hippo pathway in developmental cell fate decisions in the Drosophila eye, exerted through the interaction of Yorkie (Yki) with a transcriptional regulator Bonus (Bon). Activation of either Bon or Yki is sufficient to promote epidermal fate at the expense of eye fate through the recruitment of multiple transcriptional and post-transcriptional regulators. Transcriptome analysis reveals that Bon and Yki jointly upregulate epidermal differentiation genes and downregulate Notch target genes that modulate the eye-to-epidermal fate switch. The Hippo pathway and Bon also control the early eye-antennal specification, with activated Yki and Bon suppressing eye fate and promoting antennal fate. Our work has revealed that the Hippo pathway and Bon control cell fate decisions during Drosophila eye development at multiple levels.