Regulation of EGFR signalling by palmitoylation and its role in tumorigenesis.

The epidermal growth factor receptor (EGFR) is an essential driver of oncogenic signalling, and EGFR inhibitors are some of the earliest examples of successful targeted therapies in multiple types of cancer. The tractability of EGFR as a therapeutic target is overshadowed by the inevitable drug resistance that develops. Overcoming resistance mechanisms requires a deeper understanding of EGFR regulation in cancer cells. In this review, we discuss our recent discovery that the palmitoyltransferase DHHC20 palmitoylates EGFR on the C-terminal domain and plays a critical role in signal regulation during oncogenesis. Inhibiting DHHC20 expression or mutating the palmitoylation site on EGFR alters the EGF-induced signalling kinetics from a transient signal to a sustained signal. The change in signalling is accompanied by a decrease in cell proliferation in multiple human cancer cell lines. Our in vivo studies demonstrate that ablating the gene Zdhhc20 by CRISPR/Cas9-mediated inhibition in a mouse model of oncogenic Kras-driven lung adenocarcinoma potently inhibits tumorigenesis. The negative effect on tumorigenesis is mediated by EGFR since the expression of a palmitoylation-resistant mutant form of EGFR also inhibits Kras-driven lung adenocarcinoma. Finally, reducing EGFR palmitoylation increases the sensitivity of multiple cancer cell lines to existing inhibitors of EGFR and downstream signalling effector pathways. We will discuss the implications of these effects and strategies for targeting these new vulnerabilities.

Blocking EGFR palmitoylation suppresses PI3K signaling and mutant KRAS lung tumorigenesis.

Non-small cell lung cancer (NSCLC) is often characterized by mutually exclusive mutations in the epidermal growth factor receptor (EGFR) or the guanosine triphosphatase KRAS. We hypothesized that blocking EGFR palmitoylation, previously shown to inhibit EGFR activity, might alter downstream signaling in the KRAS-mutant setting. Here, we found that blocking EGFR palmitoylation, by either knocking down the palmitoyltransferase DHHC20 or expressing a palmitoylation-resistant EGFR mutant, reduced activation of the kinase PI3K, the abundance of the transcription factor MYC, and the proliferation of cells in culture, as well as reduced tumor growth in a mouse model of KRAS-mutant lung adenocarcinoma. Knocking down DHHC20 reduced the growth of existing tumors derived from human KRAS-mutant lung cancer cells and increased the sensitivity of these cells to a PI3K inhibitor. Palmitoylated EGFR interacted with the PI3K regulatory subunit PIK3R1 (p85) and increased the recruitment of the PI3K heterodimer to the plasma membrane. Alternatively, blocking palmitoylation increased the association of EGFR with the MAPK adaptor Grb2 and decreased that with p85. This binary switching between MAPK and PI3K signaling, modulated by EGFR palmitoylation, was only observed in the presence of oncogenic KRAS. These findings suggest a mechanism whereby oncogenic KRAS saturates signaling through unpalmitoylated EGFR, reducing formation of the PI3K signaling complex. Future development of DHHC20 inhibitors to reduce EGFR-PI3K signaling could be beneficial to patients with KRAS-mutant tumors.

Wnt5a signaling induced phosphorylation increases APT1 activity and promotes melanoma metastatic behavior.

Wnt5a has been implicated in melanoma progression and metastasis, although the exact downstream signaling events that contribute to melanoma metastasis are poorly understood. Wnt5a signaling results in acyl protein thioesterase 1 (APT1) mediated depalmitoylation of pro-metastatic cell adhesion molecules CD44 and MCAM, resulting in increased melanoma invasion. The mechanistic details that underlie Wnt5a-mediated regulation of APT1 activity and cellular function remain unknown. Here, we show Wnt5a signaling regulates APT1 activity through induction of APT1 phosphorylation and we further investigate the functional role of APT1 phosphorylation on its depalmitoylating activity. We found phosphorylation increased APT1 depalmitoylating activity and reduced APT1 dimerization. We further determined APT1 phosphorylation increases melanoma invasion in vitro, and also correlated with increased tumor grade and metastasis. Our results further establish APT1 as an important regulator of melanoma invasion and metastatic behavior. Inhibition of APT1 may represent a novel way to treat Wnt5a driven cancers.

The depalmitoylase APT1 directs the asymmetric partitioning of Notch and Wnt signaling during cell division.

Asymmetric cell division results in two distinctly fated daughter cells. A molecular hallmark of asymmetric division is the unequal partitioning of cell fate determinants. We have previously established that growth factor signaling promotes protein depalmitoylation to foster polarized protein localization, which, in turn, drives migration and metastasis. We report protein palmitoylation as a key mechanism for the asymmetric partitioning of the cell fate determinants Numb and ß-catenin through the activity of the depalmitoylating enzyme APT1. Using point mutations, we showed that specific palmitoylated residues on Numb were required for its asymmetric localization. By live-cell imaging, we showed that reciprocal interactions between APT1 and the Rho family GTPase CDC42 promoted the asymmetric localization of Numb and ß-catenin to the plasma membrane. This, in turn, restricted Notch- or Wnt-responsive transcriptional activity to one daughter cell. Moreover, we showed that altering APT1 abundance changed the transcriptional signatures of MDA-MB-231 triple receptor-negative breast cancer cells, similar to changes in Notch and ß-catenin-mediated Wnt signaling. We also showed that loss of APT1 depleted a specific subpopulation of tumorigenic cells in colony formation assays. Together, our findings suggest that APT1-mediated depalmitoylation is a major mechanism of asymmetric cell division that maintains Notch- and Wnt-associated protein dynamics, gene expression, and cellular functions.

Induced sensitivity to EGFR inhibitors is mediated by palmitoylated cysteine 1025 of EGFR and requires oncogenic Kras.

Currently, there are no effective therapeutic strategies targeting Kras driven cancers, and therefore, identifying new targeted therapies and overcoming drug resistance have become paramount for effective long-term cancer therapy. We have found that reducing expression of the palmitoyl transferase DHHC20 increases cell death induced by the EGFR inhibitor gefitinib in Kras and EGFR mutant cell lines, but not MCF7 cells harboring wildtype Kras. We show that the increased gefitinib sensitivity in cancer cells induced by DHHC20 inhibition is mediated directly through loss of palmitoylation on a previously identified cysteine residue in the C-terminal tail of EGFR. We utilized an EGFR point mutant in which the palmitoylated cysteine 1025 is mutated to alanine (EGFRC1025A), that results in receptor activation. Expression of the EGFR mutant alone in NIH3T3 cells does not increase sensitivity to gefitinib-induced cell death. However, when EGFRC1025A is expressed in cells expressing activated KrasG12V, EGFR inhibitor induced cell death is increased. Surprisingly, lung cancer cells harboring the EGFR inhibitor resistant mutation, T790M, become sensitive to EGFR inhibitor treatment when DHHC20 is inhibited. Finally, the small molecule, 2-bromopalmitate, which has been shown to inhibit palmitoyl transferases, acts synergistically with gefitinib to induce cell death in the gefitinib resistant cell line NCI-H1975.

Wnt-Dependent Control of Cell Polarity in Cultured Cells.

The secreted ligand Wnt5a regulates cell polarity and polarized cell movement during development by signaling through the poorly defined noncanonical Wnt pathway. Cell polarity regulates most aspects of cell behavior including the organization of apical/basolateral membrane domains of epithelial cells, polarized cell divisions along a directional plane, and front rear polarity during cell migration. These characteristics of cell polarity allow coordinated cell movements required for tissue formation and organogenesis during embryonic development. Genetic model organisms have been used to identify multiple signaling pathways including Wnt5a that are required to establish cell polarity and regulate polarized cell behavior. However, the downstream signaling events that regulate these complex cellular processes are still poorly understood. The methods below describe assays to study Wnt5a-induced cell polarity in cultured cells, which may facilitate our understanding of these complex signaling pathways.

Inhibition of DHHC20-Mediated EGFR Palmitoylation Creates a Dependence on EGFR Signaling.

Inappropriate activation of the receptor tyrosine kinase EGFR contributes to a variety of human malignancies. Here we show a mechanism to induce vulnerability to an existing first line treatment for EGFR-driven cancers. We find that inhibiting the palmitoyltransferase DHHC20 creates a dependence on EGFR signaling for cancer cell survival. The loss of palmitoylation increases sustained EGFR signal activation and sensitizes cells to EGFR tyrosine kinase inhibition. Our work shows that the reversible modification of EGFR with palmitate "pins" the unstructured C-terminal tail to the plasma membrane, impeding EGFR activation. We identify by mass spectrometry palmitoylated cysteine residues within the C-terminal tail where mutation of the cysteine residues to alanine is sufficient to activate EGFR signaling promoting cell migration and transformation. Our results reveal that the targeting of a peripheral modulator of EGFR signaling, DHHC20, causes a loss of signal regulation and susceptibility to EGFR inhibitor-induced cell death.

Menin localization in cell membrane compartment.

Menin is encoded by the MEN1 gene, which is mutated in an inherited human syndrome, multiple endocrine neoplasia type 1(MEN1). Menin is primarily nuclear protein, acting as a tumor suppressor in endocrine organs, but as an oncogenic factor in the mixed lineage leukemia, in a tissue-specific manner. Recently, the crystal structures of menin with different binding partners reveal menin as a key scaffold protein that functionally interacts with various partners to regulate gene transcription in the nucleus. However, outside the nucleus, menin also regulates multiple signaling pathways that traverse the cell surface membrane. The precise nature regarding to how menin associates with the membrane fraction is poorly understood. Here we show that a small fraction of menin associates with the cell membrane fraction likely via serine palmitoylation. Moreover, the majority of the membrane-associated menin may reside inside membrane vesicles, as menin is protected from trypsin-mediated proteolysis, but disruption of the membrane fraction using detergent abolishes the detection. Consistently, cellular staining for menin also reveals the distribution of menin in the cell membrane and the punctate-like cell organelles. Our findings suggest that part of intracellular menin associates with the cell membrane peripherally as well as resides within the membrane vesicles.

Spontaneous Cell Competition in Immortalized Mammalian Cell Lines.

Cell competition is a form of cell-cell interaction by which cells compare relative levels of fitness, resulting in the active elimination of less-fit cells, "losers," by more-fit cells, "winners." Here, we show that in three routinely-used mammalian cell lines - U2OS, 3T3, and MDCK cells - sub-clones arise stochastically that exhibit context-dependent competitive behavior. Specifically, cell death is elicited when winner and loser sub-clones are cultured together but not alone. Cell competition and elimination in these cell lines is caspase-dependent and requires cell-cell contact but does not require de novo RNA synthesis. Moreover, we show that the phenomenon involves differences in cellular metabolism. Hence, our study demonstrates that cell competition is a common feature of immortalized mammalian cells in vitro and implicates cellular metabolism as a mechanism by which cells sense relative levels of "fitness."

Protein Depalmitoylation Is Induced by Wnt5a and Promotes Polarized Cell Behavior.

Wnt5a signaling regulates polarized cell behavior, but the downstream signaling events that promote cell polarity are not well understood. Our results show that Wnt5a promotes depalmitoylation of the melanoma cell adhesion molecule (MCAM) at cysteine 590. Mutation of Cys-590 to glycine is sufficient to polarize MCAM localization, similar to what is observed with Wnt5a stimulation. Inhibition of the depalmitoylating enzyme APT1 blocks Wnt5a-induced depalmitoylation, asymmetric MCAM localization, and cell invasion. Directly altering expression of the basal protein palmitoylation machinery is sufficient to promote cell invasion. Additionally, cancer mutations in palmitoyltransferases decrease MCAM palmitoylation and have impaired ability to suppress cell invasion. Our results provide evidence that Wnt5a induces protein depalmitoylation, which promotes polarized protein localization and cell invasion.

A quantitative comparison of human HT-1080 fibrosarcoma cells and primary human dermal fibroblasts identifies a 3D migration mechanism with properties unique to the transformed phenotype.

Here, we describe an engineering approach to quantitatively compare migration, morphologies, and adhesion for tumorigenic human fibrosarcoma cells (HT-1080s) and primary human dermal fibroblasts (hDFs) with the aim of identifying distinguishing properties of the transformed phenotype. Relative adhesiveness was quantified using self-assembled monolayer (SAM) arrays and proteolytic 3-dimensional (3D) migration was investigated using matrix metalloproteinase (MMP)-degradable poly(ethylene glycol) (PEG) hydrogels ("synthetic extracellular matrix" or "synthetic ECM"). In synthetic ECM, hDFs were characterized by vinculin-containing features on the tips of protrusions, multipolar morphologies, and organized actomyosin filaments. In contrast, HT-1080s were characterized by diffuse vinculin expression, pronounced ß1-integrin on the tips of protrusions, a cortically-organized F-actin cytoskeleton, and quantitatively more rounded morphologies, decreased adhesiveness, and increased directional motility compared to hDFs. Further, HT-1080s were characterized by contractility-dependent motility, pronounced blebbing, and cortical contraction waves or constriction rings, while quantified 3D motility was similar in matrices with a wide range of biochemical and biophysical properties (including collagen) despite substantial morphological changes. While HT-1080s were distinct from hDFs for each of the 2D and 3D properties investigated, several features were similar to WM239a melanoma cells, including rounded, proteolytic migration modes, cortical F-actin organization, and prominent uropod-like structures enriched with ß1-integrin, F-actin, and melanoma cell adhesion molecule (MCAM/CD146/MUC18). Importantly, many of the features observed for HT-1080s were analogous to cellular changes induced by transformation, including cell rounding, a disorganized F-actin cytoskeleton, altered organization of focal adhesion proteins, and a weakly adherent phenotype. Based on our results, we propose that HT-1080s migrate in synthetic ECM with functional properties that are a direct consequence of their transformed phenotype.

Wnt5a directs polarized calcium gradients by recruiting cortical endoplasmic reticulum to the cell trailing edge.

Wnt5a directs the assembly of the Wnt-receptor-actin-myosin-polarity (WRAMP) structure, which integrates cell-adhesion receptors with F-actin and myosin to form a microfilament array associated with multivesicular bodies (MVBs). The WRAMP structure is polarized to the cell posterior, where it directs tail-end membrane retraction, driving forward translocation of the cell body. Here we define constituents of the WRAMP proteome, including regulators of microfilament and microtubule dynamics, protein interactions, and enzymatic activity. IQGAP1, a scaffold for F-actin nucleation and crosslinking, is necessary for WRAMP structure formation, potentially bridging microfilaments and MVBs. Vesicle coat proteins, including coatomer-I subunits, localize to and are required for the WRAMP structure. Electron microscopy and live imaging demonstrate movement of the ER to the WRAMP structure and plasma membrane, followed by elevation of intracellular Ca2+. Thus, Wnt5a controls directional movement by recruiting cortical ER to mobilize a rear-directed, localized Ca2+ signal, activating actomyosin contraction and adhesion disassembly for membrane retraction.

DNMT1 is regulated by ATP-citrate lyase and maintains methylation patterns during adipocyte differentiation.

During adipocyte differentiation, significant epigenomic changes occur in association with the implementation of the adipogenic program. We have previously shown that histone acetylation increases during differentiation in a manner dependent on acetyl coenzyme A (acetyl-CoA) production by the enzyme ATP-citrate lyase (ACL). Whether ACL regulates nuclear targets in addition to histones during differentiation is not clear. In this study, we report that DNA methyltransferase 1 (DNMT1) levels in adipocytes are controlled in part by ACL and that silencing of DNMT1 can accelerate adipocyte differentiation. DNMT1 gene expression is induced early in 3T3-L1 adipocyte differentiation during mitotic clonal expansion and is critical for maintenance of DNA and histone H3K9 methylation patterns during this period. In the absence of DNMT1, adipocyte-specific gene expression and lipid accumulation occur precociously. Later in differentiation, DNMT1 levels decline in an ACL-dependent manner. ACL-mediated suppression of DNMT1 occurs at least in part by promoting expression of microRNA 148a (miR-148a), which represses DNMT1. Ectopic expression of miR-148a accelerates differentiation under standard conditions and can partially rescue a hypermethylation-mediated differentiation block. The data suggest a role for DNMT1 in modulating the timing of differentiation and describe a novel ACL-miR-148a-dependent mechanism for regulating DNMT1 during adipogenesis.

Identification and characterization of a potent activator of p53-independent cellular senescence via a small-molecule screen for modifiers of the integrated stress response.

The Integrated Stress Response (ISR) is a signaling program that enables cellular adaptation to stressful conditions like hypoxia and nutrient deprivation in the tumor microenvironment. An important effector of the ISR is activating transcription factor 4 (ATF4), a transcription factor that regulates genes involved in redox homeostasis and amino acid metabolism and transport. Because both inhibition and overactivation of the ISR can induce tumor cell death, modulators of ATF4 expression could prove to be clinically useful. In this study, chemical libraries were screened for modulators of ATF4 expression. We identified one compound, E235 (N-(1-benzyl-piperidin-4-yl)-2-(4-fluoro-phenyl)-benzo[d]imidazo[2,1-b]thiazole-7-carboxamide), that activated the ISR and dose-dependently increased levels of ATF4 in transformed cells. A dose-dependent decrease in viability was observed in several mouse and human tumor cell lines, and knockdown of ATF4 significantly increased the antiproliferative effects of E235. Interestingly, low µM doses of E235 induced senescence in many cell types, including HT1080 human fibrosarcoma and B16F10 mouse melanoma cells. E235-mediated induction of senescence was not dependent on p21 or p53; however, p21 conferred protection against the growth inhibitory effects of E235. Treatment with E235 resulted in an increase in cells arrested at the G2/M phase with a concurrent decrease in S-phase cells. E235 also activated DNA damage response signaling, resulting in increased levels of Ser15-phosphorylated p53, γ-H2AX, and phosphorylated checkpoint kinase 2 (Chk2), although E235 does not appear to cause physical DNA damage. Induction of γ-H2AX was abrogated in ATF4 knockdown cells. Together, these results suggest that modulation of the ISR pathway with the small molecule E235 could be a promising antitumor strategy.

PERK utilizes intrinsic lipid kinase activity to generate phosphatidic acid, mediate Akt activation, and promote adipocyte differentiation.

The endoplasmic reticulum (ER) resident PKR-like kinase (PERK) is necessary for Akt activation in response to ER stress. We demonstrate that PERK harbors intrinsic lipid kinase, favoring diacylglycerol (DAG) as a substrate and generating phosphatidic acid (PA). This activity of PERK correlates with activation of mTOR and phosphorylation of Akt on Ser473. PERK lipid kinase activity is regulated in a phosphatidylinositol 3-kinase (PI3K) p85α-dependent manner. Moreover, PERK activity is essential during adipocyte differentiation. Because PA and Akt regulate many cellular functions, including cellular survival, proliferation, migratory responses, and metabolic adaptation, our findings suggest that PERK has a more extensive role in insulin signaling, insulin resistance, obesity, and tumorigenesis than previously thought.

A mediator of Rho-dependent invasion moonlights as a methionine salvage enzyme.

RhoA controls changes in cell morphology and invasion associated with cancer phenotypes. Cell lines derived from melanoma tumors at varying stages revealed that RhoA is selectively activated in cells of metastatic origin. We describe a functional proteomics strategy to identify proteins regulated by RhoA and report a previously uncharacterized human protein, named "mediator of RhoA-dependent invasion (MRDI)," that is induced in metastatic cells by constitutive RhoA activation and promotes cell invasion. In human melanomas, MRDI localization correlated with stage, showing nuclear localization in nevi and early stage tumors and cytoplasmic localization with plasma membrane accentuation in late stage tumors. Consistent with its role in promoting cell invasion, MRDI localized to cell protrusions and leading edge membranes in cultured cells and was required for cell motility, tyrosine phosphorylation of focal adhesion kinase, and modulation of actin stress fibers. Unexpectedly MRDI had enzymatic function as an isomerase that converts the S-adenosylmethionine catabolite 5-methylribose 1-phosphate into 5-methylribulose 1-phosphate. The enzymatic function of MRDI was required for methionine salvage from S-adenosylmethionine but distinct from its function in cell invasion. Thus, mechanisms used by signal transduction pathways to control cell movement have evolved from proteins with ancient function in amino acid metabolism.

Functional proteomics identifies targets of phosphorylation by B-Raf signaling in melanoma.

Melanoma and other cancers harbor oncogenic mutations in the protein kinase B-Raf, which leads to constitutive activation and dysregulation of MAP kinase signaling. In order to elucidate molecular determinants responsible for B-Raf control of cancer phenotypes, we present a method for phosphoprotein profiling, using negative ionization mass spectrometry to detect phosphopeptides based on their fragment ion signature caused by release of PO(3)(-). The method provides an alternative strategy for phosphoproteomics, circumventing affinity enrichment of phosphopeptides and isotopic labeling of samples. Ninety phosphorylation events were regulated by oncogenic B-Raf signaling, based on their responses to treating melanoma cells with MKK1/2 inhibitor. Regulated phosphoproteins included known signaling effectors and cytoskeletal regulators. We investigated MINERVA/FAM129B, a target belonging to a protein family with unknown category and function, and established the importance of this protein and its MAP kinase-dependent phosphorylation in controlling melanoma cell invasion into three-dimensional collagen matrix.

C. elegans pur alpha, an activator of end-1, synergizes with the Wnt pathway to specify endoderm.

The endoderm of C. elegans arises entirely from a single progenitor cell, the E blastomere, whose identity is specified by GATA type transcription factors, including END-1. In response to an inductive interaction mediated through Wnt/MAP kinase signaling pathways, POP-1, a Lef/Tcf-type transcription factor, restricts end-1 transcription to the posterior daughter of the mesendoderm progenitor (EMS cell), resulting in activation of endoderm differentiation by the SKN-1 and MED-1/2 transcription factors. We purified a factor from semi-synchronized early embryos that binds to an end-1 cis regulatory region critical for its endoderm-specific expression. Mass spectrometry identified this protein, PLP-1, as a C. elegans orthologue of the vertebrate pur alpha transcription factor. Expression of end-1 is attenuated in embryos depleted for PLP-1. While removal of plp-1 activity alone does not prevent endoderm development, it strongly enhances the loss of endoderm in mutants defective for the Wnt pathway. In contrast, loss of PLP-1 function does not synergize with mutants in the endoderm-inducing MAPK pathway. Moreover, nuclear localization of PLP-1 during interphase requires components of the MAPK pathway, suggesting that PLP-1 is influenced by MAPK signaling. PLP-1 is transiently asymmetrically distributed during cell divisions, with higher levels in the chromatin of the future posterior daughter of EMS and other dividing cells shortly after mitosis compared to that in their sisters. These findings imply that PLP-1 acts as a transcriptional activator of end-1 expression that may be modulated by MAPK signaling to promote endoderm development.

Wnt5a control of cell polarity and directional movement by polarized redistribution of adhesion receptors.

Mechanisms by which Wnt pathways integrate the organization of receptors, organelles, and cytoskeletal proteins to confer cell polarity and directional cell movement are incompletely understood. We show that acute responses to Wnt5a involve recruitment of actin, myosin IIB, Frizzled 3, and melanoma cell adhesion molecule into an intracellular structure in a melanoma cell line. In the presence of a chemokine gradient, this Wnt-mediated receptor-actin-myosin polarity (W-RAMP) structure accumulates asymmetrically at the cell periphery, where it triggers membrane contractility and nuclear movement in the direction of membrane retraction. The process requires endosome trafficking, is associated with multivesicular bodies, and is regulated by Wnt5a through the small guanosine triphosphatases Rab4 and RhoB. Thus, cell-autonomous mechanisms allow Wnt5a to control cell orientation, polarity, and directional movement in response to positional cues from chemokine gradients.

Mapping protein post-translational modifications with mass spectrometry.

Post-translational modifications of proteins control many biological processes, and examining their diversity is critical for understanding mechanisms of cell regulation. Mass spectrometry is a fundamental tool for detecting and mapping covalent modifications and quantifying their changes. Modern approaches have made large-scale experiments possible, screening complex mixtures of proteins for alterations in chemical modifications. By profiling protein chemistries, biologists can gain deeper insight into biological control. The aim of this review is introduce biologists to current strategies in mass spectrometry-based proteomics that are used to characterize protein post-translational modifications, noting strengths and shortcomings of various approaches.