YAP1 and PRDM14 converge to promote cell survival and tumorigenesis.

The transcriptional co-activator YAP1 oncogene is the downstream effector of the Hippo pathway, which regulates tissue homeostasis, organ size, regeneration, and tumorigenesis. Multiple cancers are dependent on sustained expression of YAP1 for cell proliferation, survival, and tumorigenesis, but the molecular basis of this oncogene dependency is not well understood. To identify genes that can functionally substitute for YAP1, we performed a genome-scale genetic rescue screen in YAP1-dependent colon cancer cells expressing an inducible YAP1-specific shRNA. We found that the transcription factor PRDM14 rescued cell proliferation and tumorigenesis upon YAP1 suppression in YAP1-dependent cells, xenografts, and colon cancer organoids. YAP1 and PRDM14 individually activated the transcription of calmodulin 2 (CALM2) and a glucose transporter SLC2A1 upon YAP1 suppression, and CALM2 or SLC2A1 expression was required for the rescue of YAP1 suppression. Together, these findings implicate PRDM14-mediated transcriptional upregulation of CALM2 and SLC2A1 as key components of oncogenic YAP1 signaling and dependency.

Targeting p130Cas- and microtubule-dependent MYC regulation sensitizes pancreatic cancer to ERK MAPK inhibition.

To identify therapeutic targets for KRAS mutant pancreatic cancer, we conduct a druggable genome small interfering RNA (siRNA) screen and determine that suppression of BCAR1 sensitizes pancreatic cancer cells to ERK inhibition. Integrative analysis of genome-scale CRISPR-Cas9 screens also identify BCAR1 as a top synthetic lethal interactor with mutant KRAS. BCAR1 encodes the SRC substrate p130Cas. We determine that SRC-inhibitor-mediated suppression of p130Cas phosphorylation impairs MYC transcription through a DOCK1-RAC1-ß-catenin-dependent mechanism. Additionally, genetic suppression of TUBB3, encoding the ßIII-tubulin subunit of microtubules, or pharmacological inhibition of microtubule function decreases levels of MYC protein in a calpain-dependent manner and potently sensitizes pancreatic cancer cells to ERK inhibition. Accordingly, the combination of a dual SRC/tubulin inhibitor with an ERK inhibitor cooperates to reduce MYC protein and synergistically suppress the growth of KRAS mutant pancreatic cancer. Thus, we demonstrate that mechanistically diverse combinations with ERK inhibition suppress MYC to impair pancreatic cancer proliferation.

Discovery of a selective inhibitor of doublecortin like kinase 1.

Doublecortin like kinase 1 (DCLK1) is an understudied kinase that is upregulated in a wide range of cancers, including pancreatic ductal adenocarcinoma (PDAC). However, little is known about its potential as a therapeutic target. We used chemoproteomic profiling and structure-based design to develop a selective, in vivo-compatible chemical probe of the DCLK1 kinase domain, DCLK1-IN-1. We demonstrate activity of DCLK1-IN-1 against clinically relevant patient-derived PDAC organoid models and use a combination of RNA-sequencing, proteomics and phosphoproteomics analysis to reveal that DCLK1 inhibition modulates proteins and pathways associated with cell motility in this context. DCLK1-IN-1 will serve as a versatile tool to investigate DCLK1 biology and establish its role in cancer.

Synthetic Lethal Interaction of SHOC2 Depletion with MEK Inhibition in RAS-Driven Cancers.

The mitogen-activated protein kinase (MAPK) pathway is a critical effector of oncogenic RAS signaling, and MAPK pathway inhibition may be an effective combination treatment strategy. We performed genome-scale loss-of-function CRISPR-Cas9 screens in the presence of a MEK1/2 inhibitor (MEKi) in KRAS-mutant pancreatic and lung cancer cell lines and identified genes that cooperate with MEK inhibition. While we observed heterogeneity in genetic modifiers of MEKi sensitivity across cell lines, several recurrent classes of synthetic lethal vulnerabilities emerged at the pathway level. Multiple members of receptor tyrosine kinase (RTK)-RAS-MAPK pathways scored as sensitizers to MEKi. In particular, we demonstrate that knockout, suppression, or degradation of SHOC2, a positive regulator of MAPK signaling, specifically cooperated with MEK inhibition to impair proliferation in RAS-driven cancer cells. The depletion of SHOC2 disrupted survival pathways triggered by feedback RTK signaling in response to MEK inhibition. Thus, these findings nominate SHOC2 as a potential target for combination therapy.

ATXN1L, CIC, and ETS Transcription Factors Modulate Sensitivity to MAPK Pathway Inhibition.

Intrinsic resistance and RTK-RAS-MAPK pathway reactivation has limited the effectiveness of MEK and RAF inhibitors (MAPKi) in RAS- and RAF-mutant cancers. To identify genes that modulate sensitivity to MAPKi, we performed genome-scale CRISPR-Cas9 loss-of-function screens in two KRAS mutant pancreatic cancer cell lines treated with the MEK1/2 inhibitor trametinib. Loss of CIC, a transcriptional repressor of ETV1, ETV4, and ETV5, promoted survival in the setting of MAPKi in cancer cells derived from several lineages. ATXN1L deletion, which reduces CIC protein, or ectopic expression of ETV1, ETV4, or ETV5 also modulated sensitivity to trametinib. ATXN1L expression inversely correlates with response to MAPKi inhibition in clinical studies. These observations identify the ATXN1L-CIC-ETS transcription factor axis as a mediator of resistance to MAPKi.

Sox2 Suppresses Gastric Tumorigenesis in Mice.

Sox2 expression marks gastric stem and progenitor cells, raising important questions regarding the genes regulated by Sox2 and the role of Sox2 itself during stomach homeostasis and disease. By using ChIP-seq analysis, we have found that the majority of Sox2 targets in gastric epithelial cells are tissue specific and related to functions such as endoderm development, Wnt signaling, and gastric cancer. Unexpectedly, we found that Sox2 itself is dispensable for gastric stem cell and epithelial self-renewal, yet Sox2(+) cells are highly susceptible to tumorigenesis in an Apc/Wnt-driven mouse model. Moreover, Sox2 loss enhances, rather than impairs, tumor formation in Apc-deficient gastric cells in vivo and in vitro by inducing Tcf/Lef-dependent transcription and upregulating intestinal metaplasia-associated genes, providing a mechanistic basis for the observed phenotype. Together, these data identify Sox2 as a context-dependent tumor suppressor protein that is dispensable for normal tissue regeneration but restrains stomach adenoma formation through modulation of Wnt-responsive and intestinal genes.

SOX15 governs transcription in human stratified epithelia and a subset of esophageal adenocarcinomas.

BACKGROUND & AIMS: Intestinal metaplasia (Barrett's esophagus, BE) is the principal risk factor for esophageal adenocarcinoma (EAC). Study of the basis for BE has centered on intestinal factors, but loss of esophageal identity likely also reflects absence of key squamous-cell factors. As few determinants of stratified epithelial cell-specific gene expression are characterized, it is important to identify the necessary transcription factors. METHODS: We tested regional expression of mRNAs for all putative DNA-binding proteins in the mouse digestive tract and verified esophagus-specific factors in human tissues and cell lines. Integration of diverse data defined a human squamous esophagus-specific transcriptome. We used chromatin immunoprecipitation (ChIP-seq) to locate transcription factor binding sites, computational approaches to profile transcripts in cancer datasets, and immunohistochemistry to reveal protein expression. RESULTS: The transcription factor SOX15 is restricted to esophageal and other murine and human stratified epithelia. SOX15 mRNA levels are attenuated in BE and its depletion in human esophageal cells reduced esophageal transcripts significantly and specifically. SOX15 binding is highly enriched near esophagus-expressed genes, indicating direct transcriptional control. SOX15 and hundreds of genes co-expressed in squamous cells are reactivated in up to 30% of EAC specimens. Genes normally confined to the esophagus or intestine appear in different cells within the same malignant glands. CONCLUSIONS: These data identify a novel transcriptional regulator of stratified epithelial cells and a subtype of EAC with bi-lineage gene expression. Broad activation of squamous-cell genes may shed light on whether EACs arise in the native stratified epithelium or in ectopic columnar cells.

Cbl ubiquitination of p85 is essential for Epo-induced EpoR endocytosis.

Erythropoietin (Epo) binding to the Epo receptor (EpoR) elicits downstream signaling that is essential for red blood cell production. One important negative regulatory mechanism to terminate Epo signaling is Epo-induced EpoR endocytosis and degradation. Defects in this mechanism play a key role in the overproduction of erythrocytes in primary familial and congenital polycythemia (PFCP). Here we have identified a novel mechanism mediating Epo-dependent EpoR internalization. Epo induces Cbl-dependent ubiquitination of the p85 regulatory subunit of PI3K, which binds to phosphotyrosines on EpoR. Ubiquitination allows p85 to interact with the endocytic protein epsin-1, thereby driving EpoR endocytosis. Knockdown of Cbl, expression of its dominant negative forms, or expression of an epsin-1 mutant devoid of ubiquitin-interacting motifs all compromise Epo-induced EpoR internalization. Mutated EpoRs mimicking those from PFCP patients cannot bind p85, co-localize with epsin-1, or internalize on Epo stimulation and exhibit Epo hypersensitivity. Similarly, knockdown of Cbl also causes Epo hypersensitivity in primary erythroid progenitors. Restoring p85 binding to PFCP receptors rescues Epo-induced epsin-1 co-localization and EpoR internalization and normalizes Epo hypersensitivity. Our results uncover a novel Cbl/p85/epsin-1 pathway in EpoR endocytosis and show that defects in this pathway contribute to excessive Epo signaling and erythroid hyperproliferation in PFCP.

Ubiquitination regulates the internalization, endolysosomal sorting, and signaling of the erythropoietin receptor.

Ubiquitination is a common mechanism of down-regulation of mitogenic receptors. Here, we show that ubiquitination of the erythropoietin receptor (EpoR) at Lys(256) is necessary and sufficient for efficient Epo-induced receptor internalization, whereas ubiquitination at Lys(428) promotes trafficking of activated receptors to the lysosomes for degradation. Interestingly, EpoR that cannot be ubiquitinated has reduced mitogenic activities and ability to stimulate the STAT5, Ras/MAPK, and PI3K/AKT signaling pathways. We therefore propose that ubiquitination of the EpoR critically controls both receptor down-regulation and downstream signaling.

Differentiation-specific histone modifications reveal dynamic chromatin interactions and partners for the intestinal transcription factor CDX2.

VIDEO ABSTRACT: Cell differentiation requires remodeling of tissue-specific gene loci and activities of key transcriptional regulators, which are recognized for their dominant control over cellular programs. Using epigenomic methods, we characterized enhancer elements specifically modified in differentiating intestinal epithelial cells and found enrichment of transcription factor-binding motifs corresponding to CDX2, a critical regulator of the intestine. Directed investigation revealed surprising lability in CDX2 occupancy of the genome, with redistribution from hundreds of sites occupied only in proliferating cells to thousands of new sites in differentiated cells. Knockout mice confirmed distinct Cdx2 requirements in dividing and mature adult intestinal cells, including responsibility for the active enhancer configuration associated with maturity. Dynamic CDX2 occupancy corresponds with condition-specific gene expression and, importantly, to differential co-occupancy with other tissue-restricted transcription factors, such as GATA6 and HNF4A. These results reveal dynamic, context-specific functions and mechanisms of a prominent transcriptional regulator within a cell lineage.

TCF4 and CDX2, major transcription factors for intestinal function, converge on the same cis-regulatory regions.

Surprisingly few pathways signal between cells, raising questions about mechanisms for tissue-specific responses. In particular, Wnt ligands signal in many mammalian tissues, including the intestinal epithelium, where constitutive signaling causes cancer. Genome-wide analysis of DNA cis-regulatory regions bound by the intestine-restricted transcription factor CDX2 in colonic cells uncovered highly significant overrepresentation of sequences that bind TCF4, a transcriptional effector of intestinal Wnt signaling. Chromatin immunoprecipitation confirmed TCF4 occupancy at most such sites and co-occupancy of CDX2 and TCF4 across short distances. A region spanning the single nucleotide polymorphism rs6983267, which lies within a MYC enhancer and confers colorectal cancer risk in humans, represented one of many co-occupied sites. Co-occupancy correlated with intestine-specific gene expression and CDX2 loss reduced TCF4 binding. These results implicate CDX2 in directing TCF4 binding in intestinal cells. Co-occupancy of regulatory regions by signal-effector and tissue-restricted transcription factors may represent a general mechanism for ubiquitous signaling pathways to achieve tissue-specific outcomes.

The membrane-proximal region of the thrombopoietin receptor confers its high surface expression by JAK2-dependent and -independent mechanisms.

Janus tyrosine kinase 2 (JAK2) is essential for signaling by the thrombopoietin (TpoR) and erythropoietin (EpoR) receptors. In the absence of JAK2 most EpoR molecules are retained in the endoplasmic reticulum in an Endo H-sensitive form. In contrast, we show that in the absence of JAK2 a large fraction of the TpoR is processed to the mature Endo H-resistant form and reaches the cell surface. By studying chimeras of the TpoR and EpoR we show that high surface expression of the TpoR is entirely conferred by the membrane-proximal region of the intracellular domain that includes the juxtamembrane, Box 1, and Box 2 regions. The TpoR intracellular domain shows similar effects on receptor endocytosis rate as that of the EpoR, but does stabilize the mature receptor isoform from degradation. Co-expression of JAK2 further stabilizes mature TpoR and thus further increases its surface expression. This JAK2 effect depends on the Box 1 region, the only JAK2 interacting site in the TpoR. By contrast, EpoR requires Box 1 as well as the flanking 20 residues on the C-terminal side for JAK2 interaction and JAK2-dependent surface expression. Our study suggests that whereas cell surface expression of type I cytokine receptors requires their cognate JAKs, the mechanisms governing receptor-JAK interactions differ among receptors interacting with the same JAK protein.

The proteasomal ATPase complex is required for stress-induced transcription in yeast.

Sug1 and Sug2 are two of six ATPases in the 19S regulatory particle of the 26S proteasome. We have shown previously that these proteins play a non-proteolytic role in the transcription of the GAL genes in yeast. In this study, we probe the requirement for these factors in stress-induced transcription in yeast. It is known that proteasomal proteolysis is not required for these events. Indeed, proteasome inhibitors strongly stimulate expression of these stress response genes. However, shifting strains carrying temperature-sensitive alleles of SUG1 and SUG2 to the restrictive temperature strongly inhibited the expression of HSP26, HSP104 and GAD1 in response to heat shock or treatment with menadione bisulfate. Furthermore, chromatin immunoprecipitation analysis revealed the recruitment of Sug1, Sug2 and Cim5 (another of the ATPases), but not 20S proteasome core proteins, to the promoters of these genes. These data show that the non-proteolytic requirement for the proteasomal ATPases extends beyond the GAL genes in yeast and includes at least the heat and oxidative stress-responsive genes.

Human T-cell lymphotropic virus type I-infected cells extravasate through the endothelial barrier by a local angiogenesis-like mechanism.

Extravasation of tumor cells through the endothelial barrier is a critical step in cancer metastasis. Human T-cell lymphotropic virus type I (HTLV-I)-associated adult T-cell leukemia/lymphoma (ATL) is an aggressive disease characterized by visceral invasion. We show that ATL and HTLV-I-associated myelopathy patients exhibit high plasma levels of functional vascular endothelial growth factor and basic fibroblast growth factor. The viral oncoprotein Tax transactivates the promoter of the gap-junction protein connexin-43 and enhances gap-junction-mediated heterocellular communication with endothelial cells. The interaction of HTLV-I-transformed cells with endothelial cells induces the gelatinase activity of matrix metalloproteinase (MMP)-2 and MMP-9 in endothelial cells and down-regulates the tissue inhibitor of MMP. This leads to subendothelial basement membrane degradation followed by endothelial cell retraction, allowing neoplastic lymphocyte extravasation. We propose a model that offers a mechanistic explanation for extravasation of HTLV-I-infected cells: after specific adhesion to endothelia of target organs, tumor cells induce a local and transient angiogenesis-like mechanism through paracrine stimulation and direct cell-cell communication with endothelial cells. This culminates in a breach of the endothelial barrier function, allowing cancer cell invasion. This local and transient angiogenesis-like sequence that may facilitate visceral invasion in ATL represents a potential target for ATL therapy.