Ligand-independent activation of the EGFR by lipid raft disruption

EGFR-driven lung adenocarcinomas coopt alveolar macrophage metabolism and function to support EGFR signaling and growth

The limited efficacy of currently approved immunotherapies in EGFR-driven lung adenocarcinoma (LUAD) underscores the need to better understand alternative mechanisms governing local immunosuppression to fuel novel therapies. Elevated surfactant and GM-CSF secretion from the transformed epithelium induces tumor-associated alveolar macrophage (TA-AM) proliferation which supports tumor growth by rewiring inflammatory functions and lipid metabolism. TA-AM properties are driven by increased GM-CSF-PPARγ signaling and inhibition of airway GM-CSF or PPARγ in TA-AMs suppresses cholesterol efflux to tumor cells, which impairs EGFR phosphorylation and restrains LUAD progression. In the absence of TA-AM metabolic support, LUAD cells compensate by increasing cholesterol synthesis, and blocking PPARγ in TA-AMs simultaneous with statin therapy further suppresses tumor progression and increases proinflammatory immune responses. These results reveal new therapeutic combinations for immunotherapy resistant EGFR-mutant LUADs and demonstrate how cancer cells can metabolically co-opt TA-AMs through GM-CSF-PPARγ signaling to provide nutrients that promote oncogenic signaling and growth.

Cholesterol, Eukaryotic Lipid Domains, and an Evolutionary Perspective of Transmembrane Signaling

Transmembrane signaling is essential for complex life forms. Communication across a bilayer lipid barrier is elaborately organized to convey precision and to fine-tune strength. Looking back, the steps that it has taken to enable this seemingly mundane errand are breathtaking, and with our survivorship bias, Darwinian. While this review is to discuss eukaryotic membranes in biological functions for coherence and theoretical footing, we are obliged to follow the evolution of the biological membrane through time. Such a visit is necessary for our hypothesis that constraints posited on cellular functions are mainly via the biomembrane, and relaxation thereof in favor of a coordinating membrane environment is the molecular basis for the development of highly specialized cellular activities, among them transmembrane signaling. We discuss the obligatory paths that have led to eukaryotic membrane formation, its intrinsic ability to signal, and how it set up the platform for later integration of protein-based receptor activation.

EGFR + lung adenocarcinomas coopt alveolar macrophage metabolism and function to support EGFR signaling and growth

The limited efficacy of currently approved immunotherapies in EGFR-mutant lung adenocarcinoma (LUAD) underscores the need to better understand mechanisms governing local immunosuppression. Elevated surfactant and GM-CSF secretion from the transformed epithelium induces tumor-associated alveolar macrophages (TA-AM) to proliferate and support tumor growth by rewiring inflammatory functions and lipid metabolism. TA-AM properties are driven by increased GM-CSF-PPARγ signaling and inhibition of airway GM-CSF or PPARγ in TA-AMs suppresses cholesterol efflux to tumor cells, which impairs EGFR phosphorylation and restrains LUAD progression. In the absence of TA-AM metabolic support, LUAD cells compensate by increasing cholesterol synthesis, and blocking PPARγ in TA-AMs simultaneous with statin therapy further suppresses tumor progression and increases T cell effector functions. These results reveal new therapeutic combinations for immunotherapy resistant EGFR-mutant LUADs and demonstrate how such cancer cells can metabolically co-opt TA-AMs through GM-CSF-PPARγ signaling to provide nutrients that promote oncogenic signaling and growth.

Rafting on the Plasma Membrane: Lipid Rafts in Signaling and Disease

The plasma membrane is not a uniform phospholipid bilayer; it has specialized membrane nano- or microdomains called lipid rafts. Lipid rafts are small cholesterol and sphingolipid-rich plasma membrane islands. Although their existence was long debated, their presence in the plasma membrane of living cells is now well accepted with the advent of super-resolution imaging techniques. It is interesting to note that lipid rafts function to compartmentalize receptors and their regulators and substantially modulate cellular signaling. In this review, we will examine the role of lipid rafts and caveolae-lipid raft-like microdomains with a distinct 3D morphology-in cellular signaling. Moreover, we will investigate how raft compartmentalized signaling regulates diverse physiological processes such as proliferation, apoptosis, immune signaling, and development. Also, the deregulation of lipid raft-mediated signaling during tumorigenesis and metastasis will be explored.

A Novel Mechanism Underlying the Inhibitory Effects of Trastuzumab on the Growth of HER2-Positive Breast Cancer Cells

To improve the efficacy of trastuzumab, it is essential to understand its mechanism of action. One of the significant issues that makes it difficult to determine the precise mechanism of trastuzumab action is the formation of various HER receptor dimers in HER2-positive breast cancer cells. So far, studies have focused on the role of HER2-HER3 heterodimers, and little is known regarding EGFR-HER2 heterodimers. Here, we study the role of trastuzumab on the cell signaling and cell proliferation mediated by EGFR-HER2 heterodimers in BT474 and SRBR3 cells. EGF stimulates the formation of both EGFR homodimer and EGFR-HER2 heterodimer. Trastuzumab only binds to HER2, not EGFR. Therefore, any effects of trastuzumab on EGF-induced activation of EGFR, HER2, and downstream signaling proteins, as well as cell proliferation, are through its effects on EGFR-HER2 heterodimers. We show that trastuzumab inhibits EGF-induced cell proliferation and cell cycle progression in BT474 and SKBR3 cells. Interestingly trastuzumab strongly inhibits EGF-induced Akt phosphorylation and slightly inhibits EGF-induced Erk activation, in both BT474 and SKBR3 cells. These data suggest the presence of a novel mechanism that allows trastuzumab to inhibit EGR-induced Akt activation and cell proliferation, without blocking EGF-induced EGFR-HER2 heterodimerization and activation. We show that trastuzumab inhibits EGF-induced lipid raft localization of the EGFR-HER2 heterodimer. Disruption of the lipid raft with MβCD blocks HER2-mediated AKT activation in a similar way to trastuzumab. MβCD and trastuzumab synergically inhibit AKT activation. We conclude that trastuzumab inhibits EGF-induced lipid raft localization of EGFR-HER2 heterodimer, which leads to the inhibition of Akt phosphorylation and cell proliferation, without blocking the formation and phosphorylation of the EGFR-HER2 heterodimer.

Anoctamins and Calcium Signalling: An Obstacle to EGFR Targeted Therapy in Glioblastoma?

Glioblastoma is the most common form of high-grade glioma in adults and has a poor survival rate with very limited treatment options. There have been no significant advancements in glioblastoma treatment in over 30 years. Epidermal growth factor receptor is upregulated in most glioblastoma tumours and, therefore, has been a drug target in recent targeted therapy clinical trials. However, while many inhibitors and antibodies for epidermal growth factor receptor have demonstrated promising anti-tumour effects in preclinical models, they have failed to improve outcomes for glioblastoma patients in clinical trials. This is likely due to the highly plastic nature of glioblastoma tumours, which results in therapeutic resistance. Ion channels are instrumental in the development of many cancers and may regulate cellular plasticity in glioblastoma. This review will explore the potential involvement of a class of calcium-activated chloride channels called anoctamins in brain cancer. We will also discuss the integrated role of calcium channels and anoctamins in regulating calcium-mediated signalling pathways, such as epidermal growth factor signalling, to promote brain cancer cell growth and migration.

Alteration of cholesterol distribution at the plasma membrane of cancer cells: From evidence to pathophysiological implication and promising therapy strategy

Cholesterol-enriched domains are nowadays proposed to contribute to cancer cell proliferation, survival, death and invasion, with important implications in tumor progression. They could therefore represent promising targets for new anticancer treatment. However, although diverse strategies have been developed over the years from directly targeting cholesterol membrane content/distribution to adjusting sterol intake, all approaches present more or less substantial limitations. Those data emphasize the need to optimize current strategies, to develop new specific cholesterol-targeting anticancer drugs and/or to combine them with additional strategies targeting other lipids than cholesterol. Those objectives can only be achieved if we first decipher (i) the mechanisms that govern the formation and deformation of the different types of cholesterol-enriched domains and their interplay in healthy cells; (ii) the mechanisms behind domain deregulation in cancer; (iii) the potential generalization of observations in different types of cancer; and (iv) the specificity of some alterations in cancer vs. non-cancer cells as promising strategy for anticancer therapy. In this review, we will discuss the current knowledge on the homeostasis, roles and membrane distribution of cholesterol in non-tumorigenic cells. We will then integrate documented alterations of cholesterol distribution in domains at the surface of cancer cells and the mechanisms behind their contribution in cancer processes. We shall finally provide an overview on the potential strategies developed to target those cholesterol-enriched domains in cancer therapy.

Membrane lipid rafts are required for AMPA receptor tyrosine phosphorylation

Membrane lipid rafts are sphingolipids and cholesterol-enriched membrane microdomains, which form a center for the interaction or assembly of palmitoylated signaling molecules, including Src family non-receptor type protein tyrosine kinases. Lipid rafts abundantly exist in neurons and function in the maintenance of synapses. Excitatory synaptic strength is largely controlled by the surface expression of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in the mammalian brain. AMPA receptor endocytosis from the synaptic surface is regulated by phosphorylation of the GluA2 subunit at tyrosine 876 by Src family kinases. Here, I revealed that tyrosine phosphorylated GluA2 is concentrated in the lipid rafts fraction. Furthermore, stimulation-induced upregulation of GluA2 tyrosine phosphorylation is disrupted by the treatment of neurons with a cholesterol-depleting compound, filipin III. These results indicate the importance of lipid rafts as enzymatic reactive sites for AMPA receptor tyrosine phosphorylation and subsequent AMPA receptor internalization from the synaptic surface.

EGFR signaling pathway as therapeutic target in human cancers

Epidermal Growth Factor Receptor (EGFR) enacts major roles in the maintenance of epithelial tissues. However, when EGFR signaling is altered, it becomes the grand orchestrator of epithelial transformation, and hence one of the most world-wide studied tyrosine kinase receptors involved in neoplasia, in several tissues. In the last decades, EGFR-targeted therapies shaped the new era of precision-oncology. Despite major advances, the dream of converting solid tumors into a chronic disease is still unfulfilled, and long-term remission eludes us. Studies investigating the function of this protein in solid malignancies have revealed numerous ways how tumor cells dysregulate EGFR function. Starting from preclinical models (cell lines, organoids, murine models) and validating in clinical specimens, EGFR-related oncogenic pathways, mechanisms of resistance, and novel avenues to inhibit tumor growth and metastatic spread enriching the therapeutic portfolios, were identified. Focusing on non-small cell lung cancer (NSCLC), where EGFR mutations are major players in the adenocarcinoma subtype, we will go over the most relevant discoveries that led us to understand EGFR and beyond, and highlight how they revolutionized cancer treatment by expanding the therapeutic arsenal at our disposal.

The Cross-Talk Between EGFR and E-Cadherin

Epidermal growth factor receptor (EGFR) and adhesion protein E-cadherin are major regulators of proliferation and differentiation in epithelial cells. Consistently, defects in both EGFR and E-cadherin-mediated intercellular adhesion are linked to various malignancies. These defects in either are further exacerbated by the reciprocal interactions between the two transmembrane proteins. On the one hand, EGFR can destabilize E-cadherin adhesion by increasing E-cadherin endocytosis, modifying its interactions with cytoskeleton and decreasing its expression, thus promoting tumorigenesis. On the other hand, E-cadherin regulates EGFR localization and tunes its activity. As a result, loss and mutations of E-cadherin promote cancer cell invasion due to uncontrolled activation of EGFR, which displays enhanced surface motility and changes in endocytosis. In this minireview, we discuss the molecular and cellular mechanisms of the cross-talk between E-cadherin and EGFR, highlighting emerging evidence for the role of endocytosis in this feedback, as well as its relevance to tissue morphogenesis, homeostasis and cancer progression.

The epidermal growth factor receptor is a relevant host factor in the early stages of Zika virus life cycle in vitro

Zika virus (ZIKV) is a flavivirus well-known for the epidemic in the Americas in 2015-2016, where microcephaly in newborns and other neurological complications were connected to ZIKV infection. Many aspects of the viral life cycle, including binding and entry into the host cell, are still enigmatic. Based on the observation that CHO cells lack the expression of EGFR and are not permissive for various ZIKV strains, the relevance of EGFR for the viral life cycle was analyzed. Infection of A549 cells by ZIKV leads to a rapid internalization of EGFR that colocalizes with the endosomal marker EEA1. Moreover, the infection by different ZIKV strains is associated with an activation of EGFR and subsequent activation of the MAPK/ERK signaling cascade. However, treatment of the cells with MβCD, which on the one hand leads to an activation of EGFR but on the other hand prevents EGFR internalization, impairs ZIKV infection. Specific inhibition of EGFR or of the RAS-RAF-MEK-ERK signal transduction cascade hinders ZIKV infection by inhibition of ZIKV entry. In accordance to this, knockout of EGFR expression impedes ZIKV entry. In case of an already established infection, inhibition of EGFR or of downstream signaling does not affect viral replication. Taken together, these data demonstrate the relevance of EGFR in the early stages of ZIKV infection and identify EGFR as a target for antiviral strategies. Importance These data deepen the knowledge about the ZIKV infection process and demonstrate the relevance of EGFR for ZIKV entry. In light of the fact that a variety of specific and efficient inhibitors of EGFR and of EGFR-dependent signaling were developed and licensed, repurposing of these substances could be a helpful tool to prevent the spreading of ZIKV infection in an epidemic outbreak.

Regulation of EGFR activation and signaling by lipids on the plasma membrane

Lipids on the plasma membrane are not only components of the membrane biophysical structures but also regulators of receptor functions. Recently, the critical roles of lipid-protein interactions have been intensively highlighted. Epidermal growth factor receptor (EGFR) is one of the most extensively studied receptors exhibiting various lipid interactions, including interactions with phosphatidylcholine, phosphatidylserine, phosphatidylinositol phosphate, cholesterol, gangliosides, and palmitate. Here, we review recent findings on how direct interaction with these lipids regulates EGFR activation and signaling, providing unprecedented insight into the comprehensive roles of various lipids in the control of EGFR functions. Finally, the current limitations in investigating lipid-protein interactions and novel technologies to potentially overcome these limitations are discussed.

UBTD1 regulates ceramide balance and endolysosomal positioning to coordinate EGFR signaling

To adapt in an ever-changing environment, cells must integrate physical and chemical signals and translate them into biological meaningful information through complex signaling pathways. By combining lipidomic and proteomic approaches with functional analysis, we have shown that ubiquitin domain-containing protein 1 (UBTD1) plays a crucial role in both the epidermal growth factor receptor (EGFR) self-phosphorylation and its lysosomal degradation. On the one hand, by modulating the cellular level of ceramides through N-acylsphingosine amidohydrolase 1 (ASAH1) ubiquitination, UBTD1 controls the ligand-independent phosphorylation of EGFR. On the other hand, UBTD1, via the ubiquitination of Sequestosome 1 (SQSTM1/p62) by RNF26 and endolysosome positioning, participates in the lysosomal degradation of EGFR. The coordination of these two ubiquitin-dependent processes contributes to the control of the duration of the EGFR signal. Moreover, we showed that UBTD1 depletion exacerbates EGFR signaling and induces cell proliferation emphasizing a hitherto unknown function of UBTD1 in EGFR-driven human cell proliferation.

Toward a Membrane-Centric Biology

With advancements of modern biophysical tools and superresolution imaging, cell biology is entering a new phase of research with technological power fitting for membrane dynamics analyses. However, our current knowledge base of cellular signaling events is mostly built on a network of protein interactions, which is incompatible with the essential roles of membrane activities in those events. The lack of a theoretical platform is rendering biophysical analyses of membrane biology supplementary to the protein-centric paradigm. We hypothesize a framework of signaling events mediated by lipid dynamics and argue that this is the evolutionarily obligatory developmental path of cellular complexity buildup. In this framework, receptors are the late comers, integrating into the pre-existing membrane based signaling events using their lipid interface as the point of entry. We further suggest that the reason for cell surface receptors to remain silent at the resting state is via the suppression effects of their surrounding lipids. The avoidance of such a suppression, via ligand binding or lipid domain disruption, enables the receptors to autonomously integrate themselves into the preexisting networks of signaling cascades.

The desmosome as a model for lipid raft driven membrane domain organization

Desmosomes are cadherin-based adhesion structures that mechanically couple the intermediate filament cytoskeleton of adjacent cells to confer mechanical stress resistance to tissues. We have recently described desmosomes as mesoscale lipid raft membrane domains that depend on raft dynamics for assembly, function, and disassembly. Lipid raft microdomains are regions of the plasma membrane enriched in sphingolipids and cholesterol. These domains participate in membrane domain heterogeneity, signaling and membrane trafficking. Cellular structures known to be dependent on raft dynamics include the post-synaptic density in neurons, the immunological synapse, and intercellular junctions, including desmosomes. In this review, we discuss the current state of the desmosome field and put forward new hypotheses for the role of lipid rafts in desmosome adhesion, signaling and epidermal homeostasis. Furthermore, we propose that differential lipid raft affinity of intercellular junction proteins is a central driving force in the organization of the epithelial apical junctional complex.

Alcohol protects the CNS by activating HSF1 and inducing the heat shock proteins

Although alcohol abuse and dependence have profound negative health consequences, emerging evidence suggests that exposure to low/moderate concentrations of ethanol protects multiple organs and systems. In the CNS, moderate drinking decreases the risk of dementia and Alzheimer's disease. This neuroprotection correlates with an increased expression of the heat shock proteins (HSPs). Multiple epidemiological studies revealed an inverse association between ethanol intoxication and traumatic brain injury mortality. In this case, ethanol-induced HSPs limit the inflammatory immune response diminishing cell death and improving the neurobehavioural outcome. Ethanol also protects the brain against ischemic injuries via the HSPs. In our laboratory, we demonstrated that ethanol increased the expression of several HSP genes in neurons and astrocytes by activating the transcription factor, heat shock factor 1 (HSF1). HSF1 induces HSPs that target misfolded proteins for refolding or degradation, increasing the survival chances of the cells. These data indicate that ethanol neuroprotection is mediated by the activation HSF1 and the induction of HSPs.

Leader cell PLCγ1 activation during keratinocyte collective migration is induced by EGFR localization and clustering

Re-epithelialization is a critical step in wound healing and results from the collective migration of keratinocytes. Previous work demonstrated that immobilized, but not soluble, epidermal growth factor (EGF) resulted in leader cell-specific activation of phospholipase C gamma 1 (PLCγ1) in HaCaT keratinocytes, and that this PLCγ1 activation was necessary to drive persistent cell migration. To determine the mechanism responsible for wound edge-localized PLCγ1 activation, we examined differences in cell area, cell-cell interactions, and EGF receptor (EGFR) localization between wound edge and bulk cells treated with vehicle, soluble EGF, or immobilized EGF. Our results support a multistep mechanism where EGFR translocation from the lateral membrane to the basolateral/basal membrane allows clustering in response to immobilized EGF. This analysis of factors regulating PLCγ1 activation is a crucial step toward developing therapies or wound dressings capable of modulating this signal and, consequently, cell migration.

Emerging Allosteric Mechanism of EGFR Activation in Physiological and Pathological Contexts

Protein kinases are key players in signal transduction pathways where they are crafted into two functional states. In response to growth factor binding stimulus, epidermal growth factor receptor (EGFR), which is physiologically populated in an autoinhibited inactive state, becomes active. Here, we outline a simple allostery scheme to clarify how an extracellular (ligand-dependent) binding event activates the intracellular EGFR kinase domain via (dimer-dependent) asymmetric dimerization, as well as how pathologically overexpressed EGFR or constitutively active mutants, leads to oncogenic pathway activation. Our underlying allosteric activation mechanism derives from a collection of inactive versus active EGFR structural, biochemical (negatively cooperative ligand binding), and biophysical (weak coupling between extracellular and intracellular kinase dimerization) data. The emerging structural insight reveals that ligand-dependent physiological activation is an outside-in allosteric activation with strong structural coupling across the membrane. In contrast, ligand-independent pathological activation is a weak inside-out activation mediated by intracellular kinase dimerization, which is structurally accommodated by additional extracellular dimers.

Kinase-Inactivated EGFR Is Required for the Survival of Wild-Type EGFR-Expressing Cancer Cells Treated with Tyrosine Kinase Inhibitors

Inhibiting the tyrosine kinase activity of epidermal growth factor receptor (EGFR) using small molecule tyrosine kinase inhibitors (TKIs) is often ineffective in treating cancers harboring wild-type EGFR (wt-EGFR). TKIs are known to cause dimerization of EGFR without altering its expression level. Given the fact that EGFR possesses kinase-independent pro-survival function, the role of TKI-inactivated EGFR in cancer cell survival needs to be addressed. In this study, using wt-EGFR-expressing cancer cells A549 (lung), DU145 (prostate), PC3 (prostate), and MDA-MB-231 (breast), we characterized the TKI-induced dimerization status of EGFR and determined the dependency of cells on kinase-inactivated EGFR for survival. We report that TKI-induced EGFR dimerization is dependent on palmitoylation and independent of its kinase activity, and that mutations of the cysteine residues known to be critical for EGFR’s palmitoylation abolished TKI-induced EGFR dimerization. Furthermore, TKI-induced EGFR dimerization is persistent in TKI-resistant cells, and inhibition of palmitoylation by 2-bromopalmitate, or targeted reduction of the kinase-inactivated EGFR by siRNA or by an EGFR-downregulating peptide, are lethal to TKI-resistant cancer cells. This study suggests that kinase-inactivated EGFR remains to be a viable therapeutic target for wt-EGFR cancers and that inhibiting palmitoylation or downregulating EGFR may overcome TKI resistance.

CD317 Activates EGFR by Regulating Its Association with Lipid Rafts

EGFR regulates various fundamental cellular processes, and its constitutive activation is a common driver for cancer. Anti-EGFR therapies have shown benefit in cancer patients, yet drug resistance almost inevitably develops, emphasizing the need for a better understanding of the mechanisms that govern EGFR activation. Here we report that CD317, a surface molecule with a unique topology, activated EGFR in hepatocellular carcinoma (HCC) cells by regulating its localization on the plasma membrane. CD317 was upregulated in HCC cells, promoting cell-cycle progression and enhancing tumorigenic potential in a manner dependent on EGFR. Mechanistically, CD317 associated with lipid rafts and released EGFR from these ordered membrane domains, facilitating the activation of EGFR and the initiation of downstream signaling pathways, including the Ras-Raf-MEK-ERK and JAK-STAT pathways. Moreover, in HCC mouse models and patient samples, upregulation of CD317 correlated with EGFR activation. These results reveal a previously unrecognized mode of regulation for EGFR and suggest CD317 as an alternative target for treating EGFR-driven malignancies. SIGNIFICANCE: Activation of EGFR by CD317 in hepatocellular carcinoma cells suggests CD317 as an alternative target for treating EGFR-dependent tumors.

Sterol structure dependence of insulin receptor and insulin-like growth factor 1 receptor activation

The plasma membrane is a dynamic environment with a complex composition of lipids, proteins, and cholesterol. Areas enriched in cholesterol and sphingolipids are believed to form lipid rafts, domains of highly ordered lipids. The unique physical properties of these domains have been proposed to influence many cellular processes. Here, we demonstrate that the activation of insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R) depends critically on the structures of membrane sterols. IR and IGF1R autophosphorylation in vivo was inhibited by cholesterol depletion, and autophosphorylation was restored by the replacement with exogenous cholesterol. We next screened a variety of sterols for effects on IR activation. The ability of sterols to support IR autophosphorylation was strongly correlated to the propensity of the sterols to form ordered domains. IR autophosphorylation was fully restored by the incorporation of ergosterol, dihydrocholesterol, 7-dehydrocholesterol, lathosterol, desmosterol, and allocholesterol, partially restored by epicholesterol, and not restored by lanosterol, coprostanol, and 4-cholesten-3-one. These data support the hypothesis that the ability to form ordered domains is sufficient for a sterol to support ligand-induced activation of IR and IGF1R in intact mammalian cells.

Docoxahexaenoic Acid Induces Apoptosis of Pancreatic Cancer Cells by Suppressing Activation of STAT3 and NF-κB

The ω3-polyunsaturated fatty acid docosahexenoic acid (DHA) is known to induce apoptosis of cancer cells. In this study, DHA was shown to reduce viability of pancreatic cancer cells (PANC-1) by inducing DNA fragmentation, activating caspase-3, and increasing the ratio of Bax/Bcl-2. To determine the DHA mechanism of action, the impact of DHA on the activation of the key signaling proteins epidermal growth factor receptor (EGFR), signal transducer and activator of transcription factor 3 (STAT3), nuclear transcription factor-κB (NF-κB), and IκBα in PANC-1 cells was probed. The observed DHA suppression of NF-κB DNA-binding activity was found to result from reduced IκBα phosphorylation. The observed DHA-induced suppression of STAT3 activation was found to be the result of suppressed EGFR activation, which derives from the inhibitory effect of DHA on the integrity of localization of EGFR to cell membrane lipid rafts. Since the activation of STAT3 and NF-κB mediates the expression of survival genes cyclin D1 and survivin, DHA induced apoptosis by suppressing the STAT3/NF-κB-cyclin D1/survivin axis. These results support the proposal that DHA-induced apoptosis of pancreatic cells occurs via disruption of key pro-cell survival signaling pathways. We suggest that the consumption of DHA-enriched foods could decrease the incidence of pancreatic cancer.

Sialylation of EGFR by the ST6Gal-I sialyltransferase promotes EGFR activation and resistance to gefitinib-mediated cell death

BACKGROUND

The ST6Gal-I sialyltransferase is upregulated in numerous cancers, and high expression of this enzyme correlates with poor patient prognosis in various malignancies, including ovarian cancer. Through its sialylation of a select cohort of cell surface receptors, ST6Gal-I modulates cell signaling to promote tumor cell survival. The goal of the present study was to investigate the influence of ST6Gal-I on another important receptor that controls cancer cell behavior, EGFR. Additionally, the effect of ST6Gal-I on cancer cells treated with the common EGFR inhibitor, gefitinib, was evaluated.

RESULTS

Using the OV4 ovarian cancer cell line, which lacks endogenous ST6Gal-I expression, a kinomics assay revealed that cells with forced overexpression of ST6Gal-I exhibited increased global tyrosine kinase activity, a finding confirmed by immunoblotting whole cell lysates with an anti-phosphotyrosine antibody. Interestingly, the kinomics assay suggested that one of the most highly activated tyrosine kinases in ST6Gal-I-overexpressing OV4 cells was EGFR. Based on these findings, additional analyses were performed to investigate the effect of ST6Gal-I on EGFR activation. To this end, we utilized, in addition to OV4 cells, the SKOV3 ovarian cancer cell line, engineered with both ST6Gal-I overexpression and knockdown, as well as the BxPC3 pancreatic cancer cell line with knockdown of ST6Gal-I. In all three cell lines, we determined that EGFR is a substrate of ST6Gal-I, and that the sialylation status of EGFR directly correlates with ST6Gal-I expression. Cells with differential ST6Gal-I expression were subsequently evaluated for EGFR tyrosine phosphorylation. Cells with high ST6Gal-I expression were found to have elevated levels of basal and EGF-induced EGFR activation. Conversely, knockdown of ST6Gal-I greatly attenuated EGFR activation, both basally and post EGF treatment. Finally, to illustrate the functional importance of ST6Gal-I in regulating EGFR-dependent survival, cells were treated with gefitinib, an EGFR inhibitor widely used for cancer therapy. These studies showed that ST6Gal-I promotes resistance to gefitinib-mediated apoptosis, as measured by caspase activity assays.

CONCLUSION

Results herein indicate that ST6Gal-I promotes EGFR activation and protects against gefitinib-mediated cell death. Establishing the tumor-associated ST6Gal-I sialyltransferase as a regulator of EGFR provides novel insight into the role of glycosylation in growth factor signaling and chemoresistance.

c-Src/Cav1-dependent activation of the EGFR by Dsg2

The desmosomal cadherin, desmoglein 2 (Dsg2), is deregulated in a variety of human cancers including those of the skin. When ectopically expressed in the epidermis of transgenic mice, Dsg2 activates multiple mitogenic signaling pathways and increases susceptibility to tumorigenesis. However, the molecular mechanism responsible for Dsg2-mediated cellular signaling is poorly understood. Here we show overexpression as well as co-localization of Dsg2 and EGFR in cutaneous SCCs in vivo. Using HaCaT keratinocytes, knockdown of Dsg2 decreases EGFR expression and abrogates the activation of EGFR, c-Src and Stat3, but not Erk1/2 or Akt, in response to EGF ligand stimulation. To determine whether Dsg2 mediates signaling through lipid microdomains, sucrose density fractionation illustrated that Dsg2 is recruited to and displaces Cav1, EGFR and c-Src from light density lipid raft fractions. STED imaging confirmed that the presence of Dsg2 disperses Cav1 from the cell-cell borders. Perturbation of lipid rafts with the cholesterol-chelating agent MβCD also shifts Cav1, c-Src and EGFR out of the rafts and activates signaling pathways. Functionally, overexpression of Dsg2 in human SCC A431 cells enhances EGFR activation and increases cell proliferation and migration through a c-Src and EGFR dependent manner. In summary, our data suggest that Dsg2 stimulates cell growth and migration by positively regulating EGFR level and signaling through a c-Src and Cav1-dependent mechanism using lipid rafts as signal modulatory platforms.

High-yield production of "difficult-to-express" proteins in a continuous exchange cell-free system based on CHO cell lysates

Cell-free protein synthesis (CFPS) represents a promising technology for efficient protein production targeting especially so called "difficult-to-express" proteins whose synthesis is challenging in conventional in vivo protein production platforms. Chinese hamster ovary (CHO) cells are one of the most prominent and safety approved cell lines for industrial protein production. In this study we demonstrated the ability to produce high yields of various protein types including membrane proteins and single chain variable fragments (scFv) in a continuous exchange cell-free (CECF) system based on CHO cell lysate that contains endogenous microsomal structures. We showed significant improvement of protein yield compared to batch formatted reactions and proved biological activity of synthesized proteins using various analysis technologies. Optimized CECF reaction conditions led to membrane protein yields up to 980 µg/ml, which is the highest protein yield reached in a microsome containing eukaryotic cell-free system presented so far.

High-yield production of "difficult-to-express" proteins in a continuous exchange cell-free system based on CHO cell lysates

Cell-free protein synthesis (CFPS) represents a promising technology for efficient protein production targeting especially so called "difficult-to-express" proteins whose synthesis is challenging in conventional in vivo protein production platforms. Chinese hamster ovary (CHO) cells are one of the most prominent and safety approved cell lines for industrial protein production. In this study we demonstrated the ability to produce high yields of various protein types including membrane proteins and single chain variable fragments (scFv) in a continuous exchange cell-free (CECF) system based on CHO cell lysate that contains endogenous microsomal structures. We showed significant improvement of protein yield compared to batch formatted reactions and proved biological activity of synthesized proteins using various analysis technologies. Optimized CECF reaction conditions led to membrane protein yields up to 980 µg/ml, which is the highest protein yield reached in a microsome containing eukaryotic cell-free system presented so far.

Modelling atopic dermatitis during the morphogenetic process involved in reconstruction of a human epidermis

Most crucial role of epidermis is to maintain efficient barrier between the organism and its environment. This barrier is however perturbed in inflammatory skin conditions like atopic dermatitis (AD), one common chronic disease. This review depicts characteristics of a model intending to reproduce epidermal features of AD in vitro. Firstly, methyl-β-cyclodextrin (MβCD) during reconstruction of epidermis was used to deplete cholesterol from plasma membrane because this condition reproduces characteristics of AD at transcriptomic level in monolayer cultures. Major changes are confirmed after same treatment inside reconstructed human epidermis (RHE). However, since early treatment do not reveal impairment to reconstruct a functional epidermal barrier and given the importance of the Th2 dysregulated immune response in AD, cholesterol-depleted RHE at day 11 of reconstruction were then incubated with three Th2-related cytokines (IL-4, IL-13 and IL-25) previously reported as playing important roles in the development of AD, as well as altering overall function of epidermal barrier. When combining both treatments, essential epidermal features of AD are observed. Indeed, RHE then exhibit spongiosis, disappearing granular layer, alteration of barrier function, as well as dysregulated expression levels for genes involved in AD pathogenesis. Moreover, while trying to identify individual roles for each component used to create AD-like alterations, incubation with IL-4 following cholesterol depletion from plasma membrane was found inducing most of the reported alterations. This model suggests potential for better investigations of epidermal AD features and may be considered for eventual in vitro screening of cosmetics or therapeutic compounds.

The Dipole Potential Modifies the Clustering and Ligand Binding Affinity of ErbB Proteins and Their Signaling Efficiency

Although activation of the ErbB family of receptor tyrosine kinases (ErbB1-4) is driven by oligomerization mediated by intermolecular interactions between the extracellular, the kinase and the transmembrane domains, the transmembrane domain has been largely neglected in this regard. The largest contributor to the intramembrane electric field, the dipole potential, alters the conformation of transmembrane peptides, but its effect on ErbB proteins is unknown. Here, we show by Förster resonance energy transfer (FRET) and number and brightness (N&B) experiments that the epidermal growth factor (EGF)-induced increase in the homoassociation of ErbB1 and ErbB2 and their heteroassociation are augmented by increasing the dipole potential. These effects were even more pronounced for ErbB2 harboring an activating Val → Glu mutation in the transmembrane domain (NeuT). The signaling capacity of ErbB1 and ErbB2 was also correlated with the dipole potential. Since the dipole potential decreased the affinity of EGF to ErbB1, the augmented growth factor-induced effects at an elevated dipole potential were actually induced at lower receptor occupancy. We conclude that the dipole potential plays a permissive role in the clustering of ErbB receptors and that the effects of lipid rafts on ligand binding and receptor signaling can be partially attributed to the dipole potential.

Cell-free synthesis of functional human epidermal growth factor receptor: Investigation of ligand-independent dimerization in Sf21 microsomal membranes using non-canonical amino acids

Cell-free protein synthesis systems represent versatile tools for the synthesis and modification of human membrane proteins. In particular, eukaryotic cell-free systems provide a promising platform for their structural and functional characterization. Here, we present the cell-free synthesis of functional human epidermal growth factor receptor and its vIII deletion mutant in a microsome-containing system derived from cultured Sf21 cells. We provide evidence for embedment of cell-free synthesized receptors into microsomal membranes and asparagine-linked glycosylation. Using the cricket paralysis virus internal ribosome entry site and a repetitive synthesis approach enrichment of receptors inside the microsomal fractions was facilitated thereby providing analytical amounts of functional protein. Receptor tyrosine kinase activation was demonstrated by monitoring receptor phosphorylation. Furthermore, an orthogonal cell-free translation system that provides the site-directed incorporation of p-azido-L-phenylalanine is characterized and applied to investigate receptor dimerization in the absence of a ligand by photo-affinity cross-linking. Finally, incorporated azides are used to generate stable covalently linked receptor dimers by strain-promoted cycloaddition using a novel linker system.

Stearoyl CoA desaturase-1: New insights into a central regulator of cancer metabolism

The processes of cell proliferation, cell death and differentiation involve an intricate array of biochemical and morphological changes that require a finely tuned modulation of metabolic pathways, chiefly among them is fatty acid metabolism. The critical participation of stearoyl CoA desaturase-1 (SCD1), the fatty acyl Δ9-desaturing enzyme that converts saturated fatty acids (SFA) into monounsaturated fatty acids (MUFA), in the mechanisms of replication and survival of mammalian cells, as well as their implication in the biological alterations of cancer have been actively investigated in recent years. This review examines the growing body of evidence that argues for a role of SCD1 as a central regulator of the complex synchronization of metabolic and signaling events that control cellular metabolism, cell cycle progression, survival, differentiation and transformation to cancer.

Reversible cryo-arrest for imaging molecules in living cells at high spatial resolution

The dynamics of molecules in living cells hampers precise imaging of molecular patterns by functional and super-resolution microscopy. We developed a method that circumvents lethal chemical fixation and allows on-stage cryo-arrest for consecutive imaging of molecular patterns within the same living, but arrested, cells. The reversibility of consecutive cryo-arrests was demonstrated by the high survival rate of different cell lines and by intact growth factor signaling that was not perturbed by stress response. Reversible cryo-arrest was applied to study the evolution of ligand-induced receptor tyrosine kinase activation at different scales. The nanoscale clustering of epidermal growth factor receptor (EGFR) in the plasma membrane was assessed by single-molecule localization microscopy, and endosomal microscale activity patterns of ephrin receptor A2 (EphA2) were assessed by fluorescence lifetime imaging microscopy. Reversible cryo-arrest allows the precise determination of molecular patterns while conserving the dynamic capabilities of living cells.

High-yield cell-free synthesis of human EGFR by IRES-mediated protein translation in a continuous exchange cell-free reaction format

Cell-free protein synthesis systems derived from eukaryotic sources often provide comparatively low amounts of several μg per ml of de novo synthesized membrane protein. In order to overcome this, we herein demonstrate the high-yield cell-free synthesis of the human EGFR in a microsome-containing system derived from cultured Sf21 cells. Yields were increased more than 100-fold to more than 285 μg/ml by combination of IRES-mediated protein translation with a continuous exchange cell-free reaction format that allowed for prolonged reaction lifetimes exceeding 24 hours. In addition, an orthogonal cell-free translation system is presented that enabled the site-directed incorporation of p-Azido-L-phenylalanine by amber suppression. Functionality of cell-free synthesized receptor molecules is demonstrated by investigation of autophosphorylation activity in the absence of ligand and interaction with the cell-free synthesized adapter molecule Grb2.

A gut microbial metabolite of ginsenosides, compound K, induces intestinal glucose absorption and Na(+) /glucose cotransporter 1 gene expression through activation of cAMP response element binding protein

SCOPE

The Na(+) /glucose cotransporter 1 (SGLT1) plays a crucial role in glucose uptake in intestinal epithelial cells (IECs), which has been shown essential in ameliorating intestinal inflammation. Ginseng has historically been used to treat inflammatory disorders. Understanding the regulatory mechanism of ginseng-mediated induction of SGLT1 gene expression in human intestinal cells is therefore important.

METHODS AND RESULTS

We demonstrate that ginsenoside compound K (CK) enhances SGLT1-mediated glucose uptake in mice and human intestinal Caco-2 cells. Transient transfection analysis using SGLT1 promoter-luciferase reporters demonstrated that the presence of an essential cAMP response element (CRE) is required for CK-mediated induction of SGLT1 gene expression. The ChIP assays indicated that increased CRE-binding protein (CREB) and CREB-binding protein (CBP) binding to the SGLT1 promoter in CK-treated cells is associated with an activated chromatin state. Our result showed that the increased CREB phosphorylation is directly correlated with SGLT1 expression in IECs. Further studies indicated that the epidermal growth factor receptor (EGFR) signaling pathway is involved in the CK-mediated effect.

CONCLUSION

These findings provide a novel mechanism for the CK-mediated upregulation of SGLT1 expression through EGFR-CREB signaling activation, which could contribute to reducing gut inflammation.

Lung injury and lung cancer caused by cigarette smoke-induced oxidative stress: Molecular mechanisms and therapeutic opportunities involving the ceramide-generating machinery and epidermal growth factor receptor

Chronic obstructive pulmonary disease (COPD) and lung cancer are frequently caused by tobacco smoking. However, these diseases present opposite phenotypes involving redox signaling at the cellular level. While COPD is characterized by excessive airway epithelial cell death and lung injury, lung cancer is caused by uncontrolled epithelial cell proliferation. Notably, epidemiological studies have demonstrated that lung cancer incidence is significantly higher in patients who have preexisting emphysema/lung injury. However, the molecular link and common cell signaling events underlying lung injury diseases and lung cancer are poorly understood. This review focuses on studies of molecular mechanism(s) underlying smoking-related lung injury (COPD) and lung cancer. Specifically, the role of the ceramide-generating machinery during cigarette smoke-induced oxidative stress leading to both apoptosis and proliferation of lung epithelial cells is emphasized. Over recent years, it has been established that ceramide is a sphingolipid playing a major role in lung epithelia structure/function leading to lung injury in chronic pulmonary diseases. However, new and unexpected findings draw attention to its potential role in lung development, cell proliferation, and tumorigenesis. To address this dichotomy in detail, evidence is presented regarding several protein targets, including Src, p38 mitogen-activated protein kinase, and neutral sphingomyelinase 2, the major sphingomyelinase that controls ceramide generation during oxidative stress. Furthermore, their roles are presented not only in apoptosis and lung injury but also in enhancing cell proliferation, lung cancer development, and resistance to epidermal growth factor receptor-targeted therapy for treating lung cancer.

Lipid rafts, KCa/ClCa/Ca2+ channel complexes and EGFR signaling: Novel targets to reduce tumor development by lipids?

Membrane lipid rafts are distinct plasma membrane nanodomains that are enriched with cholesterol, sphingolipids and gangliosides, with occasional presence of saturated fatty acids and phospholipids containing saturated acyl chains. It is well known that they organize receptors (such as Epithelial Growth Factor Receptor), ion channels and their downstream acting molecules to regulate intracellular signaling pathways. Among them are Ca2+ signaling pathways, which are modified in tumor cells and inhibited upon membrane raft disruption. In addition to protein components, lipids from rafts also contribute to the organization and function of Ca2+ signaling microdomains. This article aims to focus on the lipid raft KCa/ClCa/Ca2+ channel complexes that regulate Ca2+ and EGFR signaling in cancer cells, and discusses the potential modification of these complexes by lipids as a novel therapeutic approach in tumor development. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Plasma membranes as heat stress sensors: from lipid-controlled molecular switches to therapeutic applications

The classic heat shock (stress) response (HSR) was originally attributed to protein denaturation. However, heat shock protein (Hsp) induction occurs in many circumstances where no protein denaturation is observed. Recently considerable evidence has been accumulated to the favor of the "Membrane Sensor Hypothesis" which predicts that the level of Hsps can be changed as a result of alterations to the plasma membrane. This is especially pertinent to mild heat shock, such as occurs in fever. In this condition the sensitivity of many transient receptor potential (TRP) channels is particularly notable. Small temperature stresses can modulate TRP gating significantly and this is influenced by lipids. In addition, stress hormones often modify plasma membrane structure and function and thus initiate a cascade of events, which may affect HSR. The major transactivator heat shock factor-1 integrates the signals originating from the plasma membrane and orchestrates the expression of individual heat shock genes. We describe how these observations can be tested at the molecular level, for example, with the use of membrane perturbers and through computational calculations. An important fact which now starts to be addressed is that membranes are not homogeneous nor do all cells react identically. Lipidomics and cell profiling are beginning to address the above two points. Finally, we observe that a deregulated HSR is found in a large number of important diseases where more detailed knowledge of the molecular mechanisms involved may offer timely opportunities for clinical interventions and new, innovative drug treatments. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Key role of lipids in heat stress management

Heat stress is a common and, therefore, an important environmental impact on cells and organisms. While much attention has been paid to severe heat stress, moderate temperature elevations are also important. Here we discuss temperature sensing and how responses to heat stress are not necessarily dependent on denatured proteins. Indeed, it is clear that membrane lipids have a pivotal function. Details of membrane lipid changes and the associated production of signalling metabolites are described and suggestions made as to how the interconnected signalling network could be modified for helpful intervention in disease.

Cholesterol depletion in cell membranes of human airway epithelial cells suppresses MUC5AC gene expression

PURPOSE

If cholesterol in the cell membrane is depleted by treating cells with methyl-β-cyclodextrin (MβCD), the activities of transmembrane receptors are altered in a cell-specific and/or receptor-specific manner. The proinflammatory cytokines, IL-1β is potent inducers of MUC5AC mRNA and protein synthesis in human airway epithelial cells. Cells activated by IL-1β showed increased phosphorylation of extracellular signal regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK). Thus, we investigated the effects of cholesterol depletion on the expression of MUC5AC in human airway epithelial cells and whether these alterations to MUC5AC expression were related to MAPK activity.

MATERIALS AND METHODS

After NCI-H292 cells were pretreated with 1% MβCD before adding IL-1β for 24 hours, MUC5AC mRNA expression was determined by reverse transcription- polymerase chain reaction (RT-PCR) and real time-PCR. Cholesterol depletion by MβCD was measured by modified microenzymatic fluorescence assay and filipin staining. The phosphorylation of IL-1 receptor, ERK and p38 MAPK, was analyzed by western blot.

RESULTS

Cholesterol in the cell membrane was significantly depleted by treatment with MβCD on cells. IL-1β-induced MUC5AC mRNA expression was decreased by MβCD and this decrease occurred IL-1-receptor- specifically. Moreover, we have shown that MβCD suppressed the activation of ERK1/2 and p38 MAPK in cells activated with IL-1β. This result suggests that MβCD-mediated suppression of IL-1β-induced MUC5AC mRNA operated via the ERK- and p38 MAPK-dependent pathway.

CONCLUSION

Cholesterol depletion in NCI-H292 cell membrane may be considered an anti-hypersecretory method since it effectively inhibits mucus secretion of respiratory epithelial cells.

Transforming growth factor-β1 (TGF-β1)-stimulated fibroblast to myofibroblast differentiation is mediated by hyaluronan (HA)-facilitated epidermal growth factor receptor (EGFR) and CD44 co-localization in lipid rafts

Fibroblast to myofibroblast differentiation drives effective wound healing and is largely regulated by the cytokine transforming growth factor-β1 (TGF-β1). Myofibroblasts express α-smooth muscle actin and are present in granulation tissue, where they are responsible for wound contraction. Our previous studies show that fibroblast differentiation in response to TGF-β1 is dependent on and mediated by the linear polysaccharide hyaluronan (HA). Both the HA receptor, CD44, and the epidermal growth factor receptor (EGFR) are involved in this differentiation response. The aim of this study was to understand the mechanisms linking HA-, CD44-, and EGFR-regulated TGF-β1-dependent differentiation. CD44 and EGFR co-localization within membrane-bound lipid rafts was necessary for differentiation, and this triggered downstream mitogen-activated protein kinase (MAPK/ERK) and Ca(2+)/calmodulin kinase II (CaMKII) activation. We also found that ERK phosphorylation was upstream of CaMKII phosphorylation, that ERK activation was necessary for CaMKII signaling, and that both kinases were essential for differentiation. In addition, HA synthase-2 (HAS2) siRNA attenuated both ERK and CaMKII signaling and sequestration of CD44 into lipid rafts, preventing differentiation. In summary, the data suggest that HAS2-dependent production of HA facilitates TGF-β1-dependent fibroblast differentiation through promoting CD44 interaction with EGFR held within membrane-bound lipid rafts. This induces MAPK/ERK, followed by CaMKII activation, leading to differentiation. This pathway is synergistic with the classical TGF-β1-dependent SMAD-signaling pathway and may provide a novel opportunity for intervention in wound healing.

Plant stanols induce intestinal tumor formation by up-regulating Wnt and EGFR signaling in Apc Min mice

The rate of APC mutations in the intestine increases in middle-age. At the same period of life, plant sterol and stanol enriched functional foods are introduced to diet to lower blood cholesterol. This study examined the effect of plant stanol enriched diet on intestinal adenoma formation in the Apc(Min) mouse. Apc(Min) mice were fed 0.8% plant stanol diet or control diet for nine weeks. Cholesterol, plant sterols and plant stanols were analyzed from the caecum content and the intestinal mucosa. Levels of β-catenin, cyclin D1, epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase 1/2 (ERK1/2) were measured from the intestinal mucosa by Western blotting. Gene expression was determined from the intestinal mucosa using Affymetrix and the data were analyzed for enriched categories and pathways. Plant stanols induced adenoma formation in the small intestine, however, the adenoma size was not affected. We saw increased levels of nuclear β-catenin, phosphorylated β-catenin (Ser675 and Ser552), nuclear cyclin D1, total and phosphorylated EGFR and phosphorylated ERK1/2 in the intestinal mucosa after plant stanol feeding. The Affymetrix data demonstrate that several enzymes of cholesterol synthesis pathway were up-regulated, although the cholesterol level in the intestinal mucosa was not altered. We show that plant stanols induce adenoma formation by activating Wnt and EGFR signaling. EGFR signaling seems to have promoted β-catenin phosphorylation and its translocation into the nucleus, where the expression of cyclin D1 was increased. Up-regulated cholesterol synthesis may partly explain the increased EGFR signaling in the plant stanol-fed mice.

Stearoyl-CoA desaturase activity modulates the activation of epidermal growth factor receptor in human lung cancer cells

Stearoyl-CoA desaturase-1 (SCD1), the main enzyme that converts saturated fatty acids into monounsaturated fatty acids, is a key factor in the mechanisms of cancer cell proliferation, survival and tumorigenesis. Evidence indicates that SCD1 activity regulates these events in part by targeting the phosphatidylinositol-3 phosphate kinase/Akt and Ras/extracellular signal-regulated kinase (ERK) pathways, but the molecular mechanisms remain unknown. We now show that in H460 lung cancer cells, the suppression of SCD activity with CVT-11127, a specific small molecule SCD inhibitor, impairs the ligand-induced phosphorylation of epidermal growth factor (EGF) receptor, causing the inactivation of its downstream targets Akt, ERK and mammalian target of rapamycin. Importantly, the mitogenic response to EGF was markedly defective in SCD-depleted cancer cells. The inactivation of EGF receptor (EGFR) promoted by SCD inhibition may be caused by perturbations in the lipid microenvironment surrounding the receptor, since we detected significant alterations in the lateral mobility of plasma lipid microdomains. Finally, incubation of lung cancer cells with SCD blockers potentiated the antigrowth effect of gefitinib, an EGFR inhibitor employed in cancer treatment. Altogether, our data indicate that SCD activity may control cancer cell metabolism, proliferation and survival by modulating the EGFR→Akt/ERK signaling platforms. Our studies also suggest a value for SCD inhibitors as novel pharmacological agents in lung cancer, one of the most common and lethal forms of cancer for which therapeutic options remain very limited.

T-Cadherin is an auxiliary negative regulator of EGFR pathway activity in cutaneous squamous cell carcinoma: impact on cell motility

Genetic and epigenetic studies in different cancers, including cutaneous carcinomas, have implicated T-cadherin (T-cad) as a tumor suppressor. Immunohistochemical and in vitro studies have suggested that T-cad loss promotes incipient invasiveness in cutaneous squamous cell carcinoma (SCC). Molecular mechanisms are unknown. This study found that the main consequence of T-cad silencing in SCC is facilitation of ligand-dependent EGFR activation, whereas T-cad overexpression impedes EGFR activation. Gain- and loss-of-function studies in A431 SCC cells demonstrate T-cad-controlled responsiveness to EGF with respect to pharmacological inhibition of EGFR and to diverse signaling and functional events of the EGFR activation cascade (EGFR phosphorylation, internalization, nuclear translocation, cell retraction/de-adhesion, motility, invasion, integrin β1, and Rho small GTPases such as RhoA, Rac1, and Cdc42 activation). Further, T-cad modulates the EGFR pathway activity by influencing membrane compartmentalization of EGFR; T-cad upregulation promotes retention of EGFR in lipid rafts, whereas T-cad silencing releases EGFR from this compartment, rendering EGFR more accessible to ligand stimulation. This study reveals a mechanism for fine-tuning of EGFR activity in SCC, whereby T-cad represents an auxiliary "negative" regulator of the EGFR pathway, which impacts invasion-associated behavioral responses of SCC to EGF. This action of T-cad in SCC may serve as a paradigm explaining other malignancies displaying concomitant T-cad loss and enhanced EGFR activity.

Duox, Flotillin-2, and Src42A are required to activate or delimit the spread of the transcriptional response to epidermal wounds in Drosophila

The epidermis is the largest organ of the body for most animals, and the first line of defense against invading pathogens. A breach in the epidermal cell layer triggers a variety of localized responses that in favorable circumstances result in the repair of the wound. Many cellular and genetic responses must be limited to epidermal cells that are close to wounds, but how this is regulated is still poorly understood. The order and hierarchy of epidermal wound signaling factors are also still obscure. The Drosophila embryonic epidermis provides an excellent system to study genes that regulate wound healing processes. We have developed a variety of fluorescent reporters that provide a visible readout of wound-dependent transcriptional activation near epidermal wound sites. A large screen for mutants that alter the activity of these wound reporters has identified seven new genes required to activate or delimit wound-induced transcriptional responses to a narrow zone of cells surrounding wound sites. Among the genes required to delimit the spread of wound responses are Drosophila Flotillin-2 and Src42A, both of which are transcriptionally activated around wound sites. Flotillin-2 and constitutively active Src42A are also sufficient, when overexpressed at high levels, to inhibit wound-induced transcription in epidermal cells. One gene required to activate epidermal wound reporters encodes Dual oxidase, an enzyme that produces hydrogen peroxide. We also find that four biochemical treatments (a serine protease, a Src kinase inhibitor, methyl-ß-cyclodextrin, and hydrogen peroxide) are sufficient to globally activate epidermal wound response genes in Drosophila embryos. We explore the epistatic relationships among the factors that induce or delimit the spread of epidermal wound signals. Our results define new genetic functions that interact to instruct only a limited number of cells around puncture wounds to mount a transcriptional response, mediating local repair and regeneration.

An epidermal wound provides signals that initiate a variety of localized responses, some of which act to regenerate and repair the breach in the epidermal barrier. The Drosophila melanogaster embryonic epidermis provides an excellent system to discover new genes that regulate wound-healing processes. Using fluorescent epidermal “wound” reporters that are locally activated around wound sites, we have screened almost 5,000 Drosophila mutants for functions required to activate or delimit wound-induced transcriptional responses to a local zone of epidermal cells. Among the seven new genes required to delimit the spread of wound responses are Flotillin-2 and Src42A. These two genes are also sufficient, when overexpressed at high levels, to inhibit wound-induced transcription in epidermal cells. One new gene required to activate epidermal wound reporters encodes Dual oxidase, an enzyme that produces hydrogen peroxide. We also find that four biochemical treatments (a serine protease, a Src kinase inhibitor, methyl-ß-cyclodextrin, and hydrogen peroxide) are sufficient to globally activate epidermal wound response genes in Drosophila embryos. Our results define new genetic functions, and the interactions among them, which regulate the local transcriptional response to puncture wounds.

Intracellular lipid flux and membrane microdomains as organizing principles in inflammatory cell signaling

Lipid rafts and caveolae play a pivotal role in organization of signaling by TLR4 and several other immune receptors. Beyond the simple cataloguing of signaling events compartmentalized by these membrane microdomains, recent studies have revealed the surprisingly central importance of dynamic remodeling of membrane lipid domains to immune signaling. Simple interventions upon membrane lipid, such as changes in cholesterol loading or crosslinking of raft lipids, are sufficient to induce micrometer-scale reordering of membranes and their protein cargo with consequent signal transduction. In this review, using TLR signaling in the macrophage as a central focus, we discuss emerging evidence that environmental and genetic perturbations of membrane lipid regulate protein signaling, illustrate how homeostatic flow of cholesterol and other lipids through rafts regulates the innate immune response, and highlight recent attempts to harness these insights toward therapeutic development.

EGF receptor exposed to oxidative stress acquires abnormal phosphorylation and aberrant activated conformation that impairs canonical dimerization

Crystallographic studies have offered understanding of how receptor tyrosine kinases from the ErbB family are regulated by their growth factor ligands. A conformational change of the EGFR (ErbB1) was shown to occur upon ligand binding, where a solely ligand-mediated mode of dimerization/activation was documented. However, this dogma of dimerization/activation was revolutionized by the discovery of constitutively active ligand-independent EGFR mutants. In addition, other ligand-independent activation mechanisms may occur. We have shown that oxidative stress (ox-stress), induced by hydrogen peroxide or cigarette smoke, activates EGFR differently than its ligand, EGF, thereby inducing aberrant phosphorylation and impaired trafficking and degradation of EGFR. Here we demonstrate that ox-stress activation of EGFR is ligand-independent, does not induce "classical" receptor dimerization and is not inhibited by the tyrosine kinase inhibitor AG1478. Thus, an unprecedented, apparently activated, state is found for EGFR under ox-stress. Furthermore, this activation mechanism is temperature-dependent, suggesting the simultaneous involvement of membrane structure. We propose that ceramide increase under ox-stress disrupts cholesterol-enriched rafts leading to EGFR re-localization into the rigid, ceramide-enriched rafts. This increase in ceramide also supports EGFR aberrant trafficking to a peri-nuclear region. Therefore, the EGFR unprecedented and activated conformation could be sustained by simultaneous alterations in membrane structure under ox-stress.

Transcriptional profiling after lipid raft disruption in keratinocytes identifies critical mediators of atopic dermatitis pathways

Lipid rafts are cholesterol-rich cell signaling platforms, and their physiological role can be explored by cholesterol depletion. To characterize transcriptional changes ongoing after lipid raft disruption in epidermal keratinocytes, a cell type that synthesizes its cholesterol in situ, we performed whole-genome expression profiling. Microarray results show that over 3,000 genes are differentially regulated. In particular, IL-8, urokinase-like plasminogen activator receptor, and metalloproteinases are highly upregulated after cholesterol extraction. Quantitative reverse transcriptase PCR validation and protein release measurements demonstrate the physiological relevance of microarray data. Major enriched terms and functions, determined by Ingenuity Pathways Analysis, identify cholesterol biosynthesis as a major function, illustrating the specificity of keratinocyte response toward cholesterol depletion. Moreover, the inflammatory skin disorder atopic dermatitis (AD) is identified as the disease most closely associated with the profile of lipid raft-disrupted keratinocytes. This finding is confirmed in skin of AD patients, in whom transcript levels of major lipid raft target genes are similarly regulated in lesional atopic skin, compared with non-lesional and normal skin. Thus, lipid raft disruption evokes typical features of AD, thereby suggesting that lipid raft organization and signaling could be perturbed in atopic keratinocytes.