The DNA Damage Transducer RNF8 Facilitates Cancer Chemoresistance and Progression through Twist Activation.

Twist has been shown to cause treatment failure, cancer progression, and cancer-related death. However, strategies that directly target Twist are not yet conceivable. Here we reveal that K63-linked ubiquitination is a crucial regulatory mechanism for Twist activation. Through an E3 ligase screen and biochemical studies, we unexpectedly identified that RNF8 functions as a direct Twist activator by triggering K63-linked ubiquitination of Twist. RNF8-promoted Twist ubiquitination is required for Twist localization to the nucleus for subsequent EMT and CSC functions, thereby conferring chemoresistance. Our histological analyses showed that RNF8 expression is upregulated and correlated with disease progression, EMT features, and poor patient survival in breast cancer. Moreover, RNF8 regulates cancer cell migration and invasion and cancer metastasis, recapitulating the effect of Twist. Together, our findings reveal a previously unrecognized tumor-promoting function of RNF8 and provide evidence that targeting RNF8 is an appealing strategy to tackle tumor aggressiveness and treatment resistance.

MIG-6 is essential for promoting glucose metabolic reprogramming and tumor growth in triple-negative breast cancer.

Treatment of triple-negative breast cancer (TNBC) remains challenging due to a lack of effective targeted therapies. Dysregulated glucose uptake and metabolism are essential for TNBC growth. Identifying the molecular drivers and mechanisms underlying the metabolic vulnerability of TNBC is key to exploiting dysregulated cancer metabolism for therapeutic applications. Mitogen-inducible gene-6 (MIG-6) has long been thought of as a feedback inhibitor that targets activated EGFR and suppresses the growth of tumors driven by constitutive activated mutant EGFR. Here, our bioinformatics and histological analyses uncover that MIG-6 is upregulated in TNBC and that MIG-6 upregulation is positively correlated with poorer clinical outcomes in TNBC. Metabolic arrays and functional assays reveal that MIG-6 drives glucose metabolism reprogramming toward glycolysis. Mechanistically, MIG-6 recruits HAUSP deubiquitinase for stabilizing HIF1α protein expression and the subsequent upregulation of GLUT1 and other HIF1α-regulated glycolytic genes, substantiating the comprehensive regulation of MIG-6 in glucose metabolism. Moreover, our mouse studies demonstrate that MIG-6 regulates GLUT1 expression in tumors and subsequent tumor growth in vivo. Collectively, this work reveals that MIG-6 is a novel prognosis biomarker, metabolism regulator, and molecular driver of TNBC.

IKKα activation of NOTCH links tumorigenesis via FOXA2 suppression.

Proinflammatory cytokine TNFα plays critical roles in promoting malignant cell proliferation, angiogenesis, and tumor metastasis in many cancers. However, the mechanism of TNFα-mediated tumor development remains unclear. Here, we show that IKKα, an important downstream kinase of TNFα, interacts with and phosphorylates FOXA2 at S107/S111, thereby suppressing FOXA2 transactivation activity and leading to decreased NUMB expression, and further activates the downstream NOTCH pathway and promotes cell proliferation and tumorigenesis. Moreover, we found that levels of IKKα, pFOXA2 (S107/111), and activated NOTCH1 were significantly higher in hepatocellular carcinoma tumors than in normal liver tissues and that pFOXA2 (S107/111) expression was positively correlated with IKKα and activated NOTCH1 expression in tumor tissues. Therefore, dysregulation of NUMB-mediated suppression of NOTCH1 by TNFα/IKKα-associated FOXA2 inhibition likely contributes to inflammation-mediated cancer pathogenesis. Here, we report a TNFα/IKKα/FOXA2/NUMB/NOTCH1 pathway that is critical for inflammation-mediated tumorigenesis and may provide a target for clinical intervention in human cancer.

Membrane-bound trafficking regulates nuclear transport of integral epidermal growth factor receptor (EGFR) and ErbB-2.

Nuclear localization of multiple receptor-tyrosine kinases (RTKs), such as EGF receptor (EGFR), ErbB-2, FGF receptor (FGFR), and many others, has been reported by several groups. We previously showed that cell surface EGFR is trafficked to the nucleus through a retrograde pathway from the Golgi to the endoplasmic reticulum (ER) and that EGFR is then translocated to the inner nuclear membrane (INM) through the INTERNET (integral trafficking from the ER to the nuclear envelope transport) pathway. However, the nuclear trafficking mechanisms of other membrane RTKs, apart from EGFR, remain unclear. The purpose of this study was to compare the nuclear transport of EGFR family proteins with that of FGFR-1. Interestingly, we found that digitonin permeabilization, which selectively releases soluble nuclear transporters from the cytoplasm and has been shown to inhibit nuclear transport of FGFR-1, had no effects on EGFR nuclear transport, raising the possibility that EGFR and FGFR-1 use different pathways to be translocated into the nucleus. Using the subnuclear fractionation assay, we further demonstrated that biotinylated cell surface ErbB-2, but not FGFR-1, is targeted to the INM, associating with Sec61ß in the INM, similar to the nuclear trafficking of EGFR. Thus, ErbB-2, but not FGFR-1, shows a similar trafficking pathway to EGFR for translocation to the nucleus, indicating that at least two different pathways of nuclear transport exist for cell surface receptors. This finding provides a new direction for investigating the trafficking mechanisms of various nuclear RTKs.

Nuclear EGFR suppresses ribonuclease activity of polynucleotide phosphorylase through DNAPK-mediated phosphorylation at serine 776.

Nuclear existence of epidermal growth factor receptor (EGFR) has been documented for more than two decades. Resistance of cancer to radiotherapy is frequently correlated with elevated EGFR expression, activity, and nuclear translocation. However, the role of nuclear EGFR (nEGFR) in radioresistance of cancers remains elusive. In the current study, we identified a novel nEGFR-associated protein, polynucleotide phosphorylase (PNPase), which possesses 3' to 5' exoribonuclease activity toward c-MYC mRNA. Knockdown of PNPase increased radioresistance. Inactivation or knock-down of EGFR enhanced PNPase-mediated c-MYC mRNA degradation in breast cancer cells, and also increased its radiosensitivity. Interestingly, the association of nEGFR with PNPase and DNA-dependent protein kinase (DNAPK) increased significantly in breast cancer cells after exposure to ionizing radiation (IR). We also demonstrated that DNAPK phosphorylates PNPase at Ser-776, which is critical for its ribonuclease activity. The phospho-mimetic S776D mutant of PNPase impaired its ribonuclease activity whereas the nonphosphorylatable S776A mutant effectively degraded c-MYC mRNA. Here, we uncovered a novel role of nEGFR in radioresistance, and that is, upon ionizing radiation, nEGFR inactivates the ribonuclease activity of PNPase toward c-MYC mRNA through DNAPK-mediated Ser-776 phosphorylation, leading to increase of c-MYC mRNA, which contributes to radioresistance of cancer cells.

The translocon Sec61beta localized in the inner nuclear membrane transports membrane-embedded EGF receptor to the nucleus.

Accumulating evidence indicates that endocytosis plays an essential role in the nuclear transport of the ErbB family members, such as epidermal growth factor receptor (EGFR) and ErbB-2. Nevertheless, how full-length receptors embedded in the endosomal membrane pass through the nuclear pore complexes and function as non-membrane-bound receptors in the nucleus remains unclear. Here we show that upon EGF treatment, the biotinylated cell surface EGFR is trafficked to the inner nuclear membrane (INM) through the nuclear pore complexes, remaining in a membrane-bound environment. We further find that importin ß regulates EGFR nuclear transport to the INM in addition to the nucleus/nucleoplasm. Unexpectedly, the well known endoplasmic reticulum associated translocon Sec61ß is found to reside in the INM and associate with EGFR. Knocking down Sec61ß expression reduces EGFR level in the nucleoplasm portion and accumulates it in the INM portion. Thus, the Sec61ß translocon plays an unrecognized role in the release of the membrane-anchored EGFR from the lipid bilayer of the INM to the nucleus. The newly identified Sec61ß function provides an alternative pathway for nuclear transport that can be utilized by membrane-embedded proteins such as full-length EGFR.

COPI-mediated retrograde trafficking from the Golgi to the ER regulates EGFR nuclear transport.

Emerging evidence indicates that cell surface receptors, such as the entire epidermal growth factor receptor (EGFR) family, have been shown to localize in the nucleus. A retrograde route from the Golgi to the endoplasmic reticulum (ER) is postulated to be involved in the EGFR trafficking to the nucleus; however, the molecular mechanism in this proposed model remains unexplored. Here, we demonstrate that membrane-embedded vesicular trafficking is involved in the nuclear transport of EGFR. Confocal immunofluorescence reveals that in response to EGF, a portion of EGFR redistributes to the Golgi and the ER, where its NH(2)-terminus resides within the lumen of Golgi/ER and COOH-terminus is exposed to the cytoplasm. Blockage of the Golgi-to-ER retrograde trafficking by brefeldin A or dominant mutants of the small GTPase ADP-ribosylation factor, which both resulted in the disassembly of the coat protein complex I (COPI) coat to the Golgi, inhibit EGFR transport to the ER and the nucleus. We further find that EGF-dependent nuclear transport of EGFR is regulated by retrograde trafficking from the Golgi to the ER involving an association of EGFR with gamma-COP, one of the subunits of the COPI coatomer. Our findings experimentally provide a comprehensive pathway that nuclear transport of EGFR is regulated by COPI-mediated vesicular trafficking from the Golgi to the ER, and may serve as a general mechanism in regulating the nuclear transport of other cell surface receptors.

HectH9 hijacks glucose metabolism to fuel tumor growth.

Our study uncovered that HectH9 drives glycolysis and tumor development by K63-linked ubiquitination of Hexokinase 2 (HK2). This mechanism is critical for HK2 localization to mitochondria for activating HK2's functions in glycolysis promotion and apoptosis inhibition, suggesting that targeting HectH9 is a new strategy to tackle metabolism-addicted tumors.

Inhibition of USP2 eliminates cancer stem cells and enhances TNBC responsiveness to chemotherapy.

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer that harbors enriched cancer stem cell (CSC) populations in tumors. Conventional chemotherapy is a standard treatment for TNBC, but it spares the CSC populations, which cause tumor recurrence and progression. Therefore, identification of the core molecular pathway that controls CSC activity and expansion is essential for developing effective therapeutics for TNBC. In this study, we identify that USP2 deubiquitinating enzyme is upregulated in CSCs and is a novel regulator of CSCs. Genetic and pharmacological targeting of USP2 substantially inhibits the self-renewal, expansion and chemoresistance of CSCs. We show that USP2 maintains the CSC population by activating self-renewing factor Bmi1 and epithelial-mesenchymal transition through Twist upregulation. Mechanistically, USP2 promotes Twist stabilization by removing ß-TrCP-mediated ubiquitination of Twist. Animal studies indicate that pharmacological inhibition of USP2 suppresses tumor progression and sensitizes tumor responses to chemotherapy in TNBC. Furthermore, the histological analyses reveal a positive correlation between USP2 upregulation and lymph node metastasis. Our findings together demonstrate a previously unrecognized role of USP2 in mediating Twist activation and CSC enrichment, suggesting that targeting USP2 is a novel therapeutic strategy to tackle TNBC.

Non-proteolytic ubiquitination of Hexokinase 2 by HectH9 controls tumor metabolism and cancer stem cell expansion.

Enormous efforts have been made to target metabolic dependencies of cancer cells for developing new therapies. However, the therapeutic efficacy of glycolysis inhibitors is limited due to their inability to elicit cell death. Hexokinase 2 (HK2), via its mitochondrial localization, functions as a central nexus integrating glycolysis activation and apoptosis resilience. Here we identify that K63-linked ubiquitination by HectH9 regulates the mitochondrial localization and function of HK2. Through stable isotope tracer approach and functional metabolic analyses, we show that HectH9 deficiency impedes tumor glucose metabolism and growth by HK2 inhibition. The HectH9/HK2 pathway regulates cancer stem cell (CSC) expansion and CSC-associated chemoresistance. Histological analyses show that HectH9 expression is upregulated and correlated with disease progression in prostate cancer. This work uncovers that HectH9 is a novel regulator of HK2 and cancer metabolism. Targeting HectH9 represents an effective strategy to achieve long-term tumor remission by concomitantly disrupting glycolysis and inducing apoptosis.

Structural basis for the structure-activity relationships of peroxisome proliferator-activated receptor agonists.

Type 2 diabetes has rapidly reached an epidemic proportion becoming a major threat to global public health. PPAR agonists have emerged as a leading class of oral antidiabetic drugs. We report a structure biology analysis of novel indole-based PPAR agonists to explain the structure-activity relationships and present a critical analysis of reasons for change in selectivity with change in the orientation of the same scaffolds. The results would be helpful in designing novel PPAR agonists.

Two-faced activity of RNF8: What "twists" it from a genome guardian to a cancer facilitator?

The RING finger protein 8 (RNF8)-induced ubiquitination signaling cascade promotes DNA repair and maintains genomic stability. Our study reveals an unexpected action of RNF8 in promoting cancer metastasis, cancer stem cell formation, and chemoresistance through the regulation of TWIST lysine 63 (K63)-linked ubiquitination, suggesting that RNF8 may serve as a new cancer prognosis marker and therapeutic target.

Tyrosine 370 phosphorylation of ATM positively regulates DNA damage response.

Ataxia telangiectasia mutated (ATM) mediates DNA damage response by controling irradiation-induced foci formation, cell cycle checkpoint, and apoptosis. However, how upstream signaling regulates ATM is not completely understood. Here, we show that upon irradiation stimulation, ATM associates with and is phosphorylated by epidermal growth factor receptor (EGFR) at Tyr370 (Y370) at the site of DNA double-strand breaks. Depletion of endogenous EGFR impairs ATM-mediated foci formation, homologous recombination, and DNA repair. Moreover, pretreatment with an EGFR kinase inhibitor, gefitinib, blocks EGFR and ATM association, hinders CHK2 activation and subsequent foci formation, and increases radiosensitivity. Thus, we reveal a critical mechanism by which EGFR directly regulates ATM activation in DNA damage response, and our results suggest that the status of ATM Y370 phosphorylation has the potential to serve as a biomarker to stratify patients for either radiotherapy alone or in combination with EGFR inhibition.

PRMT1-mediated methylation of the EGF receptor regulates signaling and cetuximab response.

Posttranslational modifications to the intracellular domain of the EGFR are known to regulate EGFR functions; however, modifications to the extracellular domain and their effects remain relatively unexplored. Here, we determined that methylation at R198 and R200 of the EGFR extracellular domain by protein arginine methyltransferase 1 (PRMT1) enhances binding to EGF and subsequent receptor dimerization and signaling activation. In a mouse orthotopic colorectal cancer xenograft model, expression of a methylation-defective EGFR reduced tumor growth. Moreover, increased EGFR methylation sustained signaling activation and cell proliferation in the presence of the therapeutic EGFR monoclonal antibody cetuximab. In colorectal cancer patients, EGFR methylation level also correlated with a higher recurrence rate after cetuximab treatment and reduced overall survival. Together, these data indicate that R198/R200 methylation of the EGFR plays an important role in regulating EGFR functionality and resistance to cetuximab treatment.

mMAPS: a flow-proteometric technique to analyze protein-protein interactions in individual signaling complexes.

Signal transduction is a dynamic process that regulates cellular functions through multiple types of biomolecular interactions, such as the interactions between proteins and between proteins and nucleic acids. However, the techniques currently available for identifying protein-protein or protein-nucleic acid complexes typically provide information about the overall population of signaling complexes in a sample instead of information about the individual signaling complexes therein. We developed a technique called "microchannel for multiparameter analysis of proteins in a single complex" (mMAPS) that simultaneously detected individual target proteins either singly or in a multicomponent complex in cell or tissue lysates. We detected the target proteins labeled with fluorophores by flow proteometry, which provided quantified data in the form of multidimensional fluorescence plots. Using mMAPS, we quantified individual complexes of epidermal growth factor (EGF) with its receptor EGFR, EGFR with signal transducer and activator of transcription 3 (STAT3), and STAT3 with the acetylase p300 and DNA in lysates from cultured cells with and without treatment with EGF, as well as in lysates from tumor xenograft tissue. Consistent with the ability of this method to reveal the dynamics of signaling protein interactions, we observed that cells treated with EGF induced the interaction of EGF with EGFR and the autophosphorylation of EGFR, but this interaction decreased with longer treatment time. Thus, we expect that this technique may reveal new aspects of molecular interaction dynamics.

EGFR modulates DNA synthesis and repair through Tyr phosphorylation of histone H4.

Posttranslational modifications of histones play fundamental roles in many biological functions. Specifically, histone H4-K20 methylation is critical for DNA synthesis and repair. However, little is known about how these functions are regulated by the upstream stimuli. Here, we identify a tyrosine phosphorylation site at Y72 of histone H4, which facilitates recruitment of histone methyltransferases (HMTases), SET8 and SUV4-20H, to enhance its K20 methylation, thereby promoting DNA synthesis and repair. Phosphorylation-defective histone H4 mutant is deficient in K20 methylation, leading to reduced DNA synthesis, delayed cell cycle progression, and decreased DNA repair ability. Disrupting the interaction between epidermal growth factor receptor (EGFR) and histone H4 by Y72 peptide significantly reduced tumor growth. Furthermore, EGFR expression clinically correlates with histone H4-Y72 phosphorylation, H4-K20 monomethylation, and the Ki-67 proliferation marker. These findings uncover a mechanism by which EGFR transduces signal to chromatin to regulate DNA synthesis and repair.

Dual targeting of tumor angiogenesis and chemotherapy by endostatin-cytosine deaminase-uracil phosphoribosyltransferase.

Several antiangiogenic drugs targeting VEGF/VEGF receptor (VEGFR) that were approved by the Food and Drug Administration for many cancer types, including colorectal and lung cancer, can effectively reduce tumor growth. However, targeting the VEGF signaling pathway will probably influence the normal function of endothelial cells in maintaining homeostasis and can cause unwanted adverse effects. Indeed, emerging experimental evidence suggests that VEGF-targeting therapy induced less tumor cell-specific cytotoxicity, allowing residual cells to become more resistant and eventually develop a more malignant phenotype. We report an antitumor therapeutic EndoCD fusion protein developed by linking endostatin (Endo) to cytosine deaminase and uracil phosphoribosyltransferase (CD). Specifically, Endo possesses tumor antiangiogenesis activity that targets tumor endothelial cells, followed by CD, which converts the nontoxic prodrug 5-fluorocytosine (5-FC) to the cytotoxic antitumor drug 5-fluorouracil (5-FU) in the local tumor area. Moreover, selective targeting of tumor sites allows an increasing local intratumoral concentration of 5-FU, thus providing high levels of cytotoxic activity. We showed that treatment with EndoCD plus 5-FC, compared with bevacizumab plus 5-FU treatment, significantly increased the 5-FU concentration around tumor sites and suppressed tumor growth and metastasis in human breast and colorectal orthotropic animal models. In addition, in contrast to treatment with bevacizumab/5-FU, EndoCD/5-FC did not induce cardiotoxicity leading to heart failure in mice after long-term treatment. Our results showed that, compared with currently used antiangiogenic drugs, EndoCD possesses potent anticancer activity with virtually no toxic effects and does not increase tumor invasion or metastasis. Together, these findings suggest that EndoCD/5-FC could become an alternative option for future antiangiogenesis therapy.

Crosstalk between Arg 1175 methylation and Tyr 1173 phosphorylation negatively modulates EGFR-mediated ERK activation.

Epidermal growth factor receptor (EGFR) can undergo post-translational modifications, including phosphorylation, glycosylation and ubiquitylation, leading to diverse physiological consequences and modulation of its biological activity. There is increasing evidence that methylation may parallel other post-translational modifications in the regulation of various biological processes. It is still not known, however, whether EGFR is regulated by this post-translational event. Here, we show that EGFR Arg 1175 is methylated by an arginine methyltransferase, PRMT5. Arg 1175 methylation positively modulates EGF-induced EGFR trans-autophosphorylation at Tyr 1173, which governs ERK activation. Abolishment of Arg 1175 methylation enhances EGF-stimulated ERK activation by reducing SHP1 recruitment to EGFR, resulting in augmented cell proliferation, migration and invasion of EGFR-expressing cells. Therefore, we propose a model in which the regulatory crosstalk between PRMT5-mediated Arg 1175 methylation and EGF-induced Tyr 1173 phosphorylation attenuates EGFR-mediated ERK activation.

ARD1 stabilization of TSC2 suppresses tumorigenesis through the mTOR signaling pathway.

Mammalian target of rapamycin (mTOR) regulates various cellular functions, including tumorigenesis, and is inhibited by the tuberous sclerosis 1 (TSC1)-TSC2 complex. Here, we demonstrate that arrest-defective protein 1 (ARD1) physically interacts with, acetylates, and stabilizes TSC2, thereby repressing mTOR activity. The inhibition of mTOR by ARD1 inhibits cell proliferation and increases autophagy, thereby inhibiting tumorigenicity. Correlation between ARD1 and TSC2 abundance was apparent in multiple tumor types. Moreover, evaluation of loss of heterozygosity at Xq28 revealed allelic loss in 31% of tested breast cancer cell lines and tumor samples. Together, our findings suggest that ARD1 functions as an inhibitor of the mTOR pathway and that dysregulation of the ARD1-TSC2-mTOR axis may contribute to cancer development.

Investigation of the dimer interface and substrate specificity of prolyl dipeptidase DPP8.

DPP8 belongs to the family of prolyl dipeptidases, which are capable of cleaving the peptide bond after a penultimate proline residue. Unlike DPP-IV, a drug target for type II diabetes, no information is available on the crystal structure of DPP8, the regulation of its enzymatic activity, or its substrate specificity. In this study, using analytical ultracentrifugation and native gel electrophoresis, we show that the DPP8 protein is predominantly dimeric when purified or in the cell extracts. Four conserved residues in the C-terminal loop of DPP8 (Phe(822), Val(833), Tyr(844), and His(859)), corresponding to those located at the dimer interface of DPP-IV, were individually mutated to Ala. Surprisingly, unlike DPP-IV, these single-site mutations abolished the enzymatic activity of DPP8 without disrupting its quaternary structure, indicating that dimerization itself is not sufficient for the optimal enzymatic activity of DPP8. Moreover, these mutations not only decreased k(cat), as did the corresponding DPP-IV mutations, but also dramatically increased K(m). We further show that the K(m) effect is independent of the substrate assayed. Finally, we identified the distinctive and strict substrate selectivity of DPP8 for hydrophobic or basic residues at the P2 site, which is in sharp contrast to the much less discriminative substrate specificity of DPP-IV. Our study has identified the residues absolutely required for the optimal activity of DPP8 and its unique substrate specificity. This study extends the functional importance of the C-terminal loop to the whole family of prolyl dipeptidases.