Karonudib has potent anti-tumor effects in preclinical models of B-cell lymphoma.

Chemo-immunotherapy has improved survival in B-cell lymphoma patients, but refractory/relapsed diseases still represent a major challenge, urging for development of new therapeutics. Karonudib (TH1579) was developed to inhibit MTH1, an enzyme preventing oxidized dNTP-incorporation in DNA. MTH1 is highly upregulated in tumor biopsies from patients with diffuse large B-cell lymphoma (DLBCL) and Burkitt lymphoma, hence confirming a rationale for targeting MTH1. Here, we tested the efficacy of karonudib in vitro and in preclinical B-cell lymphoma models. Using a range of B-cell lymphoma cell lines, karonudib strongly reduced viability at concentrations well tolerated by activated normal B cells. In B-cell lymphoma cells, karonudib increased incorporation of 8-oxo-dGTP into DNA, and prominently induced prometaphase arrest and apoptosis due to failure in spindle assembly. MTH1 knockout cell lines were less sensitive to karonudib-induced apoptosis, but were displaying cell cycle arrest phenotype similar to the wild type cells, indicating a dual inhibitory role of the drug. Karonudib was highly potent as single agent in two different lymphoma xenograft models, including an ABC DLBCL patient derived xenograft, leading to prolonged survival and fully controlled tumor growth. Together, our preclinical findings provide a rationale for further clinical testing of karonudib in B-cell lymphoma.

Artesunate shows potent anti-tumor activity in B-cell lymphoma.

BACKGROUND: Although chemo-immunotherapy has led to an improved overall survival for most B-cell lymphoma types, relapsed and refractory disease remains a challenge. The malaria drug artesunate has previously been identified as a growth suppressor in some cancer types and was tested as a new treatment option in B-cell lymphoma. METHODS: We included artesunate in a cancer sensitivity drug screen in B lymphoma cell lines. The preclinical properties of artesunate was tested as single agent in vitro in 18 B-cell lymphoma cell lines representing different histologies and in vivo in an aggressive B-cell lymphoma xenograft model, using NSG mice. Artesunate-treated B lymphoma cell lines were analyzed by functional assays, gene expression profiling, and protein expression to identify the mechanism of action. RESULTS: Drug screening identified artesunate as a highly potent anti-lymphoma drug. Artesunate induced potent growth suppression in most B lymphoma cells with an IC50 comparable to concentrations measured in serum from artesunate-treated malaria patients, while leaving normal B-cells unaffected. Artesunate markedly inhibited highly aggressive tumor growth in a xenograft model. Gene expression analysis identified endoplasmic reticulum (ER) stress and the unfolded protein response as the most affected pathways and artesunate-induced expression of the ER stress markers ATF-4 and DDIT3 was specifically upregulated in malignant B-cells, but not in normal B-cells. In addition, artesunate significantly suppressed the overall cell metabolism, affecting both respiration and glycolysis. CONCLUSIONS: Artesunate demonstrated potent apoptosis-inducing effects across a broad range of B-cell lymphoma cell lines in vitro, and a prominent anti-lymphoma activity in vivo, suggesting it to be a relevant drug for treatment of B-cell lymphoma.

Human c-SRC kinase (CSK) overexpression makes T cells dummy.

Adoptive cell therapy with T-cell receptor (TCR)-engineered T cells represents a powerful method to redirect the immune system against tumours. However, although TCR recognition is restricted to a specific peptide-MHC (pMHC) complex, increasing numbers of reports have shown cross-reactivity and off-target effects with severe consequences for the patients. This demands further development of strategies to validate TCR safety prior to clinical use. We reasoned that the desired TCR signalling depends on correct pMHC recognition on the outside and a restricted clustering on the inside of the cell. Since the majority of the adverse events are due to TCR recognition of the wrong target, we tested if blocking the signalling would affect the binding. By over-expressing the c-SRC kinase (CSK), a negative regulator of LCK, in redirected T cells, we showed that peripheral blood T cells inhibited anti-CD3/anti-CD28-induced phosphorylation of ERK, whereas TCR proximal signalling was not affected. Similarly, overexpression of CSK together with a therapeutic TCR prevented pMHC-induced ERK phosphorylation. Downstream effector functions were also almost completely blocked, including pMHC-induced IL-2 release, degranulation and, most importantly, target cell killing. The lack of effector functions contrasted with the unaffected TCR expression, pMHC recognition, and membrane exchange activity (trogocytosis). Therefore, co-expression of CSK with a therapeutic TCR did not compromise target recognition and binding, but rendered T cells incapable of executing their effector functions. Consequently, we named these redirected T cells "dummy T cells" and propose to use them for safety validation of new TCRs prior to therapy.

BMP-7 induces apoptosis in human germinal center B cells and is influenced by TGF-ß receptor type I ALK5.

Selection and maturation of B cells into plasma cells producing high-affinity antibodies occur in germinal centers (GC). GCs form transiently in secondary lymphoid organs upon antigen challenge, and the GC reaction is a highly regulated process. TGF-ß is a potent negative regulator, but the influence of other family members including bone morphogenetic proteins (BMPs) is less known. Studies of human peripheral blood B lymphocytes showed that BMP-6 suppressed plasmablast differentiation, whereas BMP-7 induced apoptosis. Here, we show that human naïve and GC B cells had a strikingly different receptor expression pattern. GC B cells expressed high levels of BMP type I receptor but low levels of type II receptors, whereas naïve B cells had the opposite pattern. Furthermore, GC B cells had elevated levels of downstream signaling components SMAD1 and SMAD5, but reduced levels of the inhibitory SMAD7. Functional assays of GC B cells revealed that BMP-7 suppressed the viability-promoting effect of CD40L and IL-21, but had no effect on CD40L- and IL-21-induced differentiation into plasmablasts. BMP-7-induced apoptosis was counteracted by a selective TGF-ß type I receptor (ALK4/5/7) inhibitor, but not by a selective BMP receptor type I inhibitor. Furthermore, overexpression of truncated ALK5 in a B-cell line counteracted BMP-7-induced apoptosis, whereas overexpression of truncated ALK4 had no effect. BMP-7 mRNA and protein was readily detected in tonsillar B cells, indicating a physiological relevance of the study. Altogether, we identified BMP-7 as a negative regulator of GC B-cell survival. The effect was counteracted by truncated ALK5, suggesting greater complexity in regulating BMP-7 signaling than previously believed.

Incidence of Data Duplications in a Randomly Selected Pool of Life Science Publications.

Since the solution to many public health problems depends on research, it is critical for the progress and well-being for the patients that we can trust the scientific literature. Misconduct and poor laboratory practice in science threatens the scientific progress, leads to loss of productivity and increased healthcare costs, and endangers lives of patients. Data duplication may represent one of challenges related to these problems. In order to estimate the frequency of data duplication in life science literature, a systematic screen through 120 original scientific articles published in three different cancer related journals [journal impact factor (IF) <5, 5-10 and >20] was completed. The study revealed a surprisingly high proportion of articles containing data duplication. For the IF < 5 and IF > 20 journals, 25% of the articles were found to contain data duplications. The IF 5-10 journal showed a comparable proportion (22.5%). The proportion of articles containing duplicated data was comparable between the three journals and no significant correlation to journal IF was found. The editorial offices representing the journals included in this study and the individual authors of the detected articles were contacted to clarify the individual cases. The editorial offices did not reply and only 1 out of 29 cases were apparently clarified by the authors, although no supporting data was supplied. This study questions the reliability of life science literature, it illustrates that data duplications are widespread and independent of journal impact factor and call for a reform of the current peer review and retraction process of scientific publishing.

The targeting of human and mouse B lymphocytes by dasatinib.

Dasatinib inhibits B-cell receptor-Abelson murine leukemia viral oncogene homologue 1, Src, and other tyrosine kinases. Few studies have addressed the impact of dasatinib on normal blood cells, especially in vivo. Here we show that dasatinib leads to a reduced number of human CD19+ peripheral B cells owing to a strong induction of apoptosis. In contrast, no similar effect on T-cell viability was observed. However, dasatinib induced a comparable broad inhibition of the early events of B- and T-cell receptor signaling. Furthermore, dasatinib was shown to be a more pronounced inhibitor of both basal and B-cell receptor-induced activity of Bruton's tyrosine kinase and PLCγ2 compared with the more specific Bruton's tyrosine kinase inhibitor ibrutinib. Human progenitor B cells from the pre-B stage were sensitive to dasatinib. In an in vivo murine model, dasatinib reduced B-lineage cells in the bone marrow with a marked effect on the pre-B subpopulation. Dasatinib led to a reduced spleen size, with a loss of large immature transitional immunoglobulin M(+)/immunoglobulin D(-) B cells and a reduction in germinal center B cells. Dasatinib caused a marked loss of thymocytes without affecting myeloid lineage cells or hematopoietic progenitors. This study reveals important side effects of dasatinib with specific loss of activated B and thymocyte populations, which may have an impact during long-term treatment.

Magnetic bead-based isolation of exosomes.

Exosomes are here defined as extracellular vesicles (EVs) in the approximate size range of 30-100 nm in diameter, and are observed in most body fluids containing typical exosomal markers such as CD9, CD63, and CD81. Potential subpopulations of exosomes can be captured by targeting these markers using magnetic beads. Magnetic beads are versatile tools for exosome isolation and downstream analysis. Here, we describe the workflow of immuno magnetic isolation and analysis of exosomes by flow cytometry, Western immunoblotting, and electron microscopy.

Expression of B-cell surface antigens in subpopulations of exosomes released from B-cell lymphoma cells.

PURPOSE: Exosomes are small (30- to 100-nm) vesicles secreted by all cell types in culture and found in most body fluids. A mean of 1 mL of blood serum, derived from healthy donors, contains approximately 10(12) exosomes. Depending on the disease, the number of exosomes can fluctuate. Concentration of exosomes in the bloodstream and all other body fluids is extremely high. Several B-cell surface antigens (CD19, CD20, CD22, CD23, CD24, CD37, CD40, and HLA-DR) and the common leukocyte antigen CD45 are interesting in terms of immunotherapy of hematologic malignant neoplasms. The established standard for exosome isolation is ultracentrifugation. However, this method cannot discriminate between exosome subpopulations and other nanovesicles. The main purpose of this study was to characterize CD81(+) and CD63(+) subpopulations of exosomes in terms of these surface markers after release from various types of B-cell lymphoma cell lines using an easy and reliable method of immunomagnetic separation. METHODS: Western blotting, flow cytometry, and electron microscopy were used to compare the total preenriched extracellular vesicle (EV) pool to each fraction of vesicles after specific isolation, using magnetic beads conjugated with antibodies raised against the exosome markers CD63 and CD81. FINDINGS: Magnetic bead-based isolation is a convenient method to study and compare subpopulations of exosomes released from B-cell lymphoma cells. The data indicated that the specifically isolated vesicles differed from the total preenriched EV pool. CD19, CD20, CD24, CD37, and HLA-DR, but not CD22, CD23, CD40, and CD45, are expressed on exosomes from B-cell lymphoma cell lines with large heterogeneity among the different B-cell lymphoma cell lines. Interestingly, these B-cell lymphoma-derived EVs are able to rescue lymphoma cells from rituximab-induced complement-dependent cytotoxicity. IMPLICATIONS: Distribution of exosomes that contain CD19, CD20, CD24, CD37, and HLA-DR may intercept immunotherapy directed against these antigens, which is important to be aware of for optimal treatment. The use of an immunomagnetic separation platform enables easy isolation and characterization of exosome subpopulations for further studies of the exosome biology to understand the potential for therapeutic and diagnostic use.

Role of Smad proteins in resistance to BMP-induced growth inhibition in B-cell lymphoma.

Bone morphogenetic protein (BMP) expression and signaling are altered in a variety of cancers, but the functional impact of these alterations is uncertain. In this study we investigated the impact of expression of multiple BMPs and their signaling pathway components in human B-cell lymphoma. BMP messages, in particular BMP7, were detected in normal and malignant B cells. Addition of exogenous BMPs inhibited DNA synthesis in most lymphoma cell lines examined, but some cell lines were resistant. Tumor specimens from three out of five lymphoma patients were also resistant to BMPs, as determined by no activation of the BMP effectors Smad1/5/8. We have previously shown that BMP-7 potently induced apoptosis in normal B cells, which was in contrast to no or little inhibitory effect of this BMP in the lymphoma cells tested. BMP-resistance mechanisms were investigated by comparing sensitive and resistant cell lines. While BMP receptors are downregulated in many cancers, we documented similar receptor levels in resistant and sensitive lymphoma cells. We found a positive correlation between activation of Smad1/5/8 and inhibition of DNA synthesis. Gene expression analysis of two independent data sets showed that the levels of inhibitory Smads varied across different B-cell lymphoma. Furthermore, stable overexpression of Smad7 in two different BMP-sensitive cell lines with low endogenous levels of SMAD7, rendered them completely resistant to BMPs. This work highlights the role of Smads in determining the sensitivity to BMPs and shows that upregulation of Smad7 in cancer cells is sufficient to escape the negative effects of BMPs.

Effect of cycloheximide on epidermal growth factor receptor trafficking and signaling.

Cycloheximide is the most common protein synthesis inhibitor, and is believed to specifically inhibit the cytoplasmic protein synthesis. Here we demonstrate that cycloheximide induces internalization and redistribution of EGF receptor to early endosomes in HeLa cells independent of receptor tyrosine phosphorylation, but dependent on p38 MAPK activity. Degradation of EGF receptor or its downstream effectors was not observed. EGF-induced activation of ERK1/2 was inhibited upon pre-treatment with cycloheximide, but did not activate JNK. The observed effects of treatment with cycloheximide alone are significant and therefore results involving the use of cycloheximide for inhibition of protein synthesis must be interpreted with caution.

SARA is dispensable for functional TGF-ß signaling.

Smad anchor for receptor activation (SARA or ZFYVE9) has been proposed to mediate transforming growth factor ß (TGF-ß) signaling by direct interaction with the non-activated Smad proteins and the TGF-ß receptors; however, these findings are controversial. We demonstrate no correlation between SARA expression and the levels of TGF-ß-induced phosphorylation of Smads in various B-cell lymphomas. Moreover, knockdown of SARA in HeLa cells did not interfere with TGF-ß-induced Smad activation, Smad nuclear translocation, or induction of TGF-ß target genes. Various R-Smads and TGF-ß receptors did not co-immunoprecipitate with SARA. Collectively, our results demonstrate that SARA is dispensable for functional TGF-ß-mediated signaling.

Bone morphogenetic proteins inhibit CD40L/IL-21-induced Ig production in human B cells: differential effects of BMP-6 and BMP-7.

Bone morphogenetic proteins (BMPs) are members of the TGF-ß superfamily. TGF-ß can affect class switch recombination in human B cells, but whether BMPs also play a role have not been tested. We investigated the functional effects of exogenously added BMPs on CD27(-) naive and CD27(+) memory B cells from healthy donors. BMP-2, -4, -6 and -7 inhibited CD40L/IL-21-induced production of IgM, IgG and IgA. BMP-6 reduced Ig production by 70% in memory B cells and more than 55% in naive B cells, whereas the other BMPs were slightly less potent. We observed a striking difference in functional effects between the structurally similar BMP-6 and BMP-7, as BMP-6 mainly inhibited plasmablast differentiation, and BMP-7 mainly induced apoptosis. In memory B cells, BMP-6 upregulated expression of DNA-binding protein inhibitor genes, but potently inhibited CD40L/IL-21-induced upregulation of the transcription factor XBP1, necessary for the late stages of plasmacytic differentiation. Expression of transcription factors regulating earlier stages (IRF4, PRDM1) was not affected by BMP-6. Taken together, these results show that BMPs are potent suppressors of naive and memory B cells.

TGF-ß-induced growth inhibition in B-cell lymphoma correlates with Smad1/5 signalling and constitutively active p38 MAPK.

BACKGROUND: Cytokines of the transforming growth factor ß (TGF-ß) superfamily exert effects on proliferation, apoptosis and differentiation in various cell types. Cancer cells frequently acquire resistance to the anti-proliferative signals of TGF-ß, which can be due to mutations in proteins of the signalling cascade. We compared the TGF-ß-related signalling properties in B-cell lymphoma cell lines that were sensitive or resistant to TGF-ß-induced anti-proliferative effects. RESULTS: TGF-ß sensitive cell lines expressed higher cell surface levels of the activin receptor-like kinase 5 (Alk-5), a TGF-ß receptor type 1. The expression levels of the other TGF-ß and bone morphogenetic protein receptors were comparable in the different cell lines. TGF-ß-induced phosphorylation of Smad2 was similar in TGF-ß sensitive and resistant cell lines. In contrast, activation of Smad1/5 was restricted to cells that were sensitive to growth inhibition by TGF-ß. Moreover, with activin A we detected limited anti-proliferative effects, strong phosphorylation of Smad2, but no Smad1/5 phosphorylation. Up-regulation of the TGF-ß target genes Id1 and Pai-1 was identified in the TGF-ß sensitive cell lines. Constitutive phosphorylation of MAPK p38 was restricted to the TGF-ß sensitive cell lines. Inhibition of p38 MAPK led to reduced sensitivity to TGF-ß. CONCLUSIONS: We suggest that phosphorylation of Smad1/5 is important for the anti-proliferative effects of TGF-ß in B-cell lymphoma. Alk-5 was highly expressed in the sensitive cell lines, and might be important for signalling through Smad1/5. Our results indicate a role for p38 MAPK in the regulation of TGF-ß-induced anti-proliferative effects.

Epidermal growth factor receptor levels are reduced in mice with targeted disruption of the protein kinase A catalytic subunit.

BACKGROUND: Epidermal Growth Factor Receptor (EGFR) is a key target molecule in current treatment of several neoplastic diseases. Hence, in order to develop and improve current drugs targeting EGFR signalling, an accurate understanding of how this signalling pathway is regulated is required. It has recently been demonstrated that inhibition of cAMP-dependent protein kinase (PKA) induces a ligand-independent internalization of EGFR. Cyclic-AMP-dependent protein kinase consists of a regulatory dimer bound to two catalytic subunits. RESULTS: We have investigated the effect on EGFR levels after ablating the two catalytic subunits, Calpha and Cbeta in two different models. The first model used targeted disruption of either Calpha or Cbeta in mice whereas the second model used Calpha and Cbeta RNA interference in HeLa cells. In both models we observed a significant reduction of EGFR expression at the protein but not mRNA level. CONCLUSION: Our results suggest that PKA may represent a target that when manipulated can maintain EGFR protein levels at the single cell level as well as in intact animals.

Distinct functions of H-Ras and K-Ras in proliferation and survival of primary hepatocytes due to selective activation of ERK and PI3K.

Ras proteins mediate signals both via extracellular signal-regulated kinase 1 and 2 (ERK), and phosphoinositide 3-kinase (PI3K). These signals are key events in cell protection and compensatory cell growth after exposure to cell damaging and pro-apoptotic stimuli, thus maintaining homeostasis. By transfection techniques, we found that both H-Ras and K-Ras were expressed and appeared functionally active in primary hepatocytes. We compared the ability of H-Ras and K-Ras homologues to preferentially activate one of the two pathways, thereby differentially controlling cell survival and growth. We found that ectopic expression of dominant negative (DN) H-RasN17, but not DN K-RasN17, efficiently inhibited both phosphorylation and translocation of ERK to the nuclear compartment, which are prerequisites for cell cycle progression. Furthermore, ectopic expression of constitutive active (CA) H-RasV12, but not CA K-RasV12, potentiated EGF-induced proliferation. We also found that expression of CA mutants of either H-Ras or K-Ras protected hepatocytes from transforming growth factor-beta1 (TGF-beta1)-induced apoptosis. However, H-Ras-induced survival was mediated by ERK/RSK as well as by PI3K, whereas K-Ras-induced survival was mediated by PI3K only. In conclusion, H-Ras and K-Ras had differential functions in proliferation and survival of primary hepatocytes. H-Ras was the major mediator of ERK-induced proliferation and survival, whereas H-Ras and K-Ras both mediated PI3K-induced survival.

The Cbl-b RING finger domain has a limited role in regulating inflammatory cytokine production by IgE-activated mast cells.

The RING finger type E3 ubiquitin ligase, Cbl-b, is abundantly expressed in bone marrow-derived mast cells (BMMCs) and functions as a potent negative regulator of signalling responses from the high-affinity IgE receptor (FcvarepsilonRI). To determine the contribution of Cbl-b E3 ligase activity we generated knockin mice with a loss-of-function mutation in the RING finger domain. We find the mice to be healthy and, unlike equivalent c-Cbl RING finger mutant mice, produce homozygous offspring at the expected frequency. Comparative analyses of BMMCs from Cbl-b knockout and Cbl-b RING finger mutant mice revealed that both showed similarly enhanced FcvarepsilonRI signalling compared to wild-type cells for most parameters examined. A notable exception was a markedly higher level of activation of IkappaB kinase (IKK) in Cbl-b knockout BMMC compared to RING finger mutant-derived cells. In addition BMMCs from the Cbl-b RING finger mutant did not retard FcvarepsilonRI internalization to the extent observed for knockout cells. Most striking however was the finding that RING finger mutant mast cells do not produce the very high levels of TNF-alpha, IL-6, and MCP-1 evident in Cbl-b knockout cultures following FcvarepsilonRI activation. Thus the ability of Cbl-b to function as a negative regulator of FcvarepsilonRI signalling that promotes inflammatory cytokine production is largely independent of the RING finger domain.

Cytoplasmic retention of peroxide-activated ERK provides survival in primary cultures of rat hepatocytes.

Reactive oxygen species (ROS) are implicated in tissue damage causing primary hepatic dysfunction following ischemia/reperfusion injury and during inflammatory liver diseases. A potential role of extracellular signal-regulated kinase (ERK) as a mediator of survival signals during oxidative stress was investigated in primary cultures of hepatocytes exposed to ROS. Hydrogen peroxide (H(2)O(2)) induced a dose-dependent activation of ERK, which was dependent on MEK activation. The ERK activation pattern was transient compared with the ERK activation seen after stimulation with epidermal growth factor (EGF). Nuclear accumulation of ERK was found after EGF stimulation, but not after H(2)O(2) exposure. A slow import/rapid export mechanism was excluded through the use of leptomycin B, an inhibitor of nuclear export sequence-dependent nuclear export. Reduced survival of hepatocytes during ROS exposure was observed when ERK activation was inhibited. Ribosomal S6 kinase (RSK), a cytoplasmic ERK substrate involved in cell survival, was activated and located in the nucleus of H(2)O(2)-exposed hepatocytes. The activation was abolished when ERK was inhibited with U0126. In conclusion, our results indicate that activity of ERK in the cytoplasm is important for survival during oxidative stress in hepatocytes and that RSK is activated downstream of ERK. Supplementary material for this article can be found on the HEPATOLOGY website (http://www.interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Subtype-specific sorting of the ETA endothelin receptor by a novel endocytic recycling signal for G protein-coupled receptors.

We have previously reported that endocytic sorting of ET(A) endothelin receptors to the recycling pathway is dependent on a signal residing in the cytoplasmic carboxyl-terminal region. The aim of the present work was to characterize the carboxyl-terminal recycling motif of the ET(A) receptor. Assay of truncation mutants of the ET(A) receptor with increasing deletions of the carboxyl-terminal tail revealed that amino acids 390 to 406 contained information critical for the ability of the receptor to recycle. This peptide sequence displayed significant sequence similarity to several protein segments confirmed by X-ray crystallography to adopt antiparallel beta-strand structures (beta-finger). One of these segments was the beta-finger motif of neuronal nitric-oxide synthase reported to function as an internal PDZ (postsynaptic density-95/disc-large/zona occludens) domain-binding ligand. Based on these findings, the three-dimensional structure of the recycling motif of ET(A) receptor was predicted to attain a beta-finger conformation acting as an internal PDZ ligand. Site-directed mutagenesis at residues that would be crucial to the structural integrity of the putative beta-finger conformation or PDZ ligand function prevented recycling of the ET(A) receptor. Analysis of more than 300 G protein-coupled receptors (GPCRs) identified 35 different human GPCRs with carboxylterminal sequence patterns that fulfilled the structural criteria of an internal PDZ ligand. Among these are several receptors reported to follow a recycling pathway. In conclusion, recycling of ET(A) receptor is mediated by a motif with the structural characteristics of an internal PDZ ligand. This structural motif may represent a more general principle of endocytic sorting of GPCRs.

Identification of 14-3-3zeta as an EGF receptor interacting protein.

The 14-3-3 proteins are known to interact with a number of proteins involved in the regulation of cell signaling. Here, we describe an association of 14-3-3zeta with the epidermal growth factor receptor (EGFR) that is rapidly induced by EGF. The 1028-EGFR truncated mutant which lacks the cytoplasmic tail from amino acids 1029-1186 identified the binding site for 14-3-3 to be between amino acid 1028 and the receptor carboxyl terminus. Mutational deletion of serine residues 1046, 1047, 1057 and 1142 did not inhibit EGF-induced 14-3-3 association with the receptor. Immunofluorescence microscopy indicated an EGF-induced co-localization of EGFR and HA-14-3-3zeta along the plasma membrane. Our finding adds to the growing complexity of EGF receptor signaling and indicates a role for 14-3-3 proteins in EGF receptor signaling or regulation.

UV-radiation-induced internalization of the epidermal growth factor receptor requires distinct serine and tyrosine residues in the cytoplasmic carboxy-terminal domain.

The mechanism of UV-radiation-induced EGF receptor (EGFR) internalization remains to be established. In the present study, we found UV-radiation-mediated internalization of the EGFR to be dependent on the cytoplasmic carboxy-terminal region. UV radiation was unable to induce internalization of EGFR carboxy-terminal truncation mutants where all or four of the five major autophosphorylation sites were missing (963- and 1028-EGFR, respectively). Mutational removal of serine residues 1046, 1047, 1057 and 1142 within the carboxy-terminal receptor region was also sufficient to abolish UV-radiation-induced internalization of the EGFR. Furthermore, the UV-radiation-induced internalization was abrogated for an EGFR mutated in tyrosine 1045 (Y1045F), the major c-Cbl binding site. However, UV radiation did not induce phosphorylation at tyrosine 1045, in contrast to the prominent phosphorylation induced by EGF. Our results suggest a mechanism for UV-radiation-induced internalization of EGFR involving a conformational change that is dependent on structural elements formed by specific serine and tyrosine residues in the carboxy-terminal domain.