Biosynthesis of the epidermal growth factor receptor in human epidermoid carcinoma-derived A431 cells

LAMC2 promotes EGFR cell membrane localization and acts as a novel biomarker for tyrosine kinase inhibitors (TKIs) sensitivity in lung cancer

The epidermal growth factor receptor (EGFR) is one of the first and most prominent driver genes known to promote malignant lung cancer. Investigating regulatory mechanisms beyond ligand-receptor binding, phosphorylation, and receptor kinase activation as means of EGFR signaling activation is important for improving EGFR-targeted therapy. Here, we report that Laminin-5γ-2 (LAMC2) retained high oncogenic capacity in lung cancer, silencing LAMC2 inhibited EGFR-induced cell proliferation and tumor growth in vivo. Deletion mutation experiments showed that both the EGF-Lam and LamB regions of LAMC2 are necessary for EGFR receptor binding, and that LAMC2 and EGFR were found to co-localize at the endoplasmic reticulum (ER) membrane. In addition, LAMC2 overexpression enhanced EGFR membrane deposition and promoted EGFR transport from the ER. Moreover, LAMC2 was necessary for preventing EGFR protein degradation via ubiquitination. Lastly, our study showed that high LAMC2 expression is positively associated with response to gefitinib (EGFR tyrosine kinase inhibitor) treatment. Overall, our study revealed a new regulatory mechanism of LAMC2 in promoting EGFR protein expression and stability by facilitating ER transport and preventing protein degradation via ubiquitination. Moreover, LAMC2 may serve as a stratifying biomarker for patients suitable for EGFR-TKI treatment.

Autosomal recessive polycystic kidney disease epithelial cell model reveals multiple basolateral epidermal growth factor receptor sorting pathways

We have discovered that multiple basolateral pathways mediate EGF receptor sorting in renal epithelial cells. The polycystic kidney disease allele in the BPK mouse model, Bicc1, interferes with one specific EGF receptor pathway, causing nonpolar delivery of the receptor without affecting overall cell polarity.

Sorting and maintenance of the EGF receptor on the basolateral surface of renal epithelial cells is perturbed in polycystic kidney disease and apical expression of receptors contributes to severity of disease. The goal of these studies was to understand the molecular basis for EGF receptor missorting using a well-established mouse model for the autosomal recessive form of the disease. We have discovered that multiple basolateral pathways mediate EGF receptor sorting in renal epithelial cells. The polycystic kidney disease allele in this model, Bicc1, interferes with one specific EGF receptor pathway without affecting overall cell polarity. Furthermore one of the pathways is regulated by a latent basolateral sorting signal that restores EGF receptor polarity in cystic renal epithelial cells via passage through a Rab11-positive subapical compartment. These studies give new insights to possible therapies to reconstitute EGF receptor polarity and function in order to curb disease progression. They also indicate for the first time that the Bicc1 gene that is defective in the mouse model used in these studies regulates cargo-specific protein sorting mediated by the epithelial cell specific clathrin adaptor AP-1B.

A novel dileucine lysosomal-sorting-signal mediates intracellular EGF-receptor retention independently of protein ubiquitylation

One of the main goals of this study was to understand the relationship between an epidermal growth factor (EGF) receptor dileucine (LL)-motif (679-LL) required for lysosomal sorting and the protein ubiquitin ligase CBL. We show that receptors containing 679-AA (di-alanine) substitutions that are defective for ligand-induced degradation nevertheless bind CBL and undergo reversible protein ubiquitylation similar to wild-type receptors. We also demonstrate that 679-LL but not CBL is required for EGF receptor downregulation by an endosomal membrane protein encoded by human adenoviruses that uncouples internalization from post-endocytic sorting to lysosomes. 679-LL is necessary for endosomal retention as well as degradation by the adenovirus protein, and is also transferable to reporter molecules. Using NMR spectroscopy, we show that peptides with wild-type 679-LL or mutant 679-AA sequences both exhibit alpha-helical structural propensities but that this structure is not stable in water. A similar analysis carried out in hydrophobic media showed that the alpha-helical structure of the wild-type peptide is stabilized by specific interactions mediated by side-chains in both leucine residues. This structure distinguishes 679-LL from other dileucine-based sorting-signals with obligatory amino-terminal acidic residues that are recognized in the form of an extended beta or beta-like conformation. Taken together, these data show that 679-LL is an alpha-helical stabilizing motif that regulates a predominant step during lysosomal sorting, involving intracellular retention under both sub-saturating and saturating conditions.

Cell surface expression of epidermal growth factor receptor and Her-2 with nuclear expression of Her-4 in primary osteosarcoma

There is controversy over the role of Her-2 in osteosarcoma, with some investigators reporting association between expression and adverse outcome, whereas others point to the lack of gene amplification and membranous expression by immunohistochemistry (IHC) as inconsistent with biological significance. Her-2 normally requires pairing with epidermal growth factor receptor (EGFR), Her-3, or Her-4, but these have been less well studied in osteosarcoma. We evaluated the expression of each of these receptors in osteosarcoma and their potential to contribute to pathogenesis by examining a panel of low-passage primary osteosarcoma cell lines, comparing these with archival tumor specimens. Her-2 immunoreactivity was seen frequently in the diffuse staining pattern described previously. We observed EGFR in all samples by IHC. Her-3 expression was not observed. Her-4 expression was nuclear in distribution in all tumor samples and many cell line samples, consistent with activation and cleavage of the receptor. Quantified expression of Her-2 and EGFR mRNA by quantitative, real-time PCR in cell lines correlated with IHC for Her-2 but not for EGFR. Western blot identified full-length receptors for EGFR and Her-2 in all expected cell lines and showed Her-4 to be predominantly in the p80 form. Flow cytometry identified cell surface Her-2 and EGFR in all lines with receptor expression by IHC. We conclude that the cell surface expression of Her-2 and EGFR and the nuclear localization of the activated p80 fragment of Her-4 suggest that all three may be contributing to osteosarcoma pathogenesis. Therapy directed against this family of receptors may be beneficial for patients with osteosarcoma.

The epidermal growth factor receptor juxtamembrane domain has multiple basolateral plasma membrane localization determinants, including a dominant signal with a polyproline core

The epidermal growth factor (EGF) receptor is located predominantly in the basolateral membrane of polarized epithelia, where it plays a pivotal role during organogenesis and tissue homeostasis. We have shown previously that a 22-amino acid sequence in the EGF receptor juxtamembrane domain contains autonomous sorting information necessary for basolateral localization using the Madin-Darby canine kidney epithelial cell model. The goal of this study was to determine the molecular basis of EGF receptor basolateral membrane expression using site-directed mutagenesis to modify specific residues in this region. We now show that this sequence has two different, functionally redundant basolateral sorting signals with distinct amino acid requirements: one dependent on residues (658)LL(659) conforming to well-characterized leucine-based sorting signals, and a second containing a polyproline core comprising residues Pro(667) and Pro(670) ((667)PXXP(670)). Our data also suggest that Arg(662) contributes to the function of the proline-based signal. (667)PXXP(670) was the dominant signal when both motifs were present and was more effective than (658)LL(659) at overriding strong apical sorting signals located in the same molecule. Site-directed mutations at Arg(662), Pro(667), and Pro(670) were also associated with increased apical expression of full-length EGF receptors, demonstrating for the first time that the juxtamembrane region is necessary for accurate polarized expression of the native molecule.

EGF receptor residues leu(679), leu(680) mediate selective sorting of ligand-receptor complexes in early endosomal compartments

Dileucine-based motifs have been shown to regulate endosomal sorting of a number of membrane proteins. Previously, we have shown that the dileucine motif Leu(679), Leu(680) in the juxtamembrane domain of the human epidermal growth factor receptor is involved in the endosome-to-lysosome transport of ligand-receptor complexes. Substitution of alanine residues for Leu(679), Leu(680) led to a reduction in ligand-induced receptor degradation without affecting internalization. In the current study, we have further characterized ligand-dependent intracellular sorting of EGF receptors containing a L679A, L680A. Immunocytochemical studies reveal that although mutant receptors redistribute from the cell surface to transferrin receptor-positive endocytic vesicles similar to wild-type following ligand stimulation, their accumulation in Lamp-1-positive late endosomes/lysosomes is retarded compared to wild-type. Kinetic analysis of (125)I-EGF trafficking shows that reduced accumulation of internalized mutant receptors in Lamp-1-positive vesicles is due to rapid recycling of ligand-receptor complexes from early endocytic compartments. In addition, the fraction of intracellular (125)I-EGF that is transported to late endocytic compartments in cells with mutant receptors is not as efficiently degraded as it is in cells with wild-type receptors. Furthermore, wild-type receptors in endocytic vesicles isolated by Percoll gradient fractionation are more resistant to in vitro digestion with proteinase K than mutant receptors. We propose that mutant receptors interact inefficiently with lysosomal sorting machinery, leading to their increased recycling. Our results are consistent with a model in which the Leu(679), Leu(680) signal facilitates sequestration of ligand-receptor complexes into internal vesicles of multivesicular endosome-to-lysosome transport intermediates.

Intracellular retention and degradation of the epidermal growth factor receptor, two distinct processes mediated by benzoquinone ansamycins

Epidermal growth factor (EGF) stimulates the growth of various types of cells via its cell surface tyrosine kinase receptor. The EGF receptor (EGF-R) has an oncogenic potential when overexpressed in a wide range of tumor cells. Geldanamycin (GA) and herbimycin (HA), specific inhibitors of the cytosolic chaperone HSP 90 and its endoplasmic reticulum homologue GRP 94, were shown to accelerate degradation of the EGF-R and of its homologue p185(c-)(erbB-2). Here we compared the effects of GA and HA on intracellular degradation and maturation of EGF-R. By using an inhibitor of proteasomal degradation, we learned that GA, but not HA, blocks processing of newly synthesized EGF-R. The effects of GA and HA on receptor degradation are mediated by the cytosolic portion of EGF-R and could be conferred to the erythropoietin receptor (EPO-R), by employing the respective chimera. Neither HA nor GA affected stability of newly synthesized EGF-R lacking the cytosolic domain (Ex EGF-R), but GA caused intracellular retention of this mutant. Taken together, our results imply that GA has two distinct targets of action on the EGF-R, one for promoting its degradation and another for mediating its intracellular retention. Apparently, degradation of the EGF-R mediated by GA or HA requires the presence of the EGF-R cytosolic domain, whereas intracellular retention in the presence of GA is coupled to the extracellular domain of the EGF-R.

Regulation of EGF signaling by cell polarity in MDCK kidney epithelial cells

Although the presence of a dominant basolateral sorting signal ensures that the majority of newly synthesized epidermal growth factor (EGF) receptors are delivered directly to the basolateral surface in polarized epithelial cells, a fraction of the receptors are also delivered to the apical surface. Similar to most basolateral membrane proteins, the EGF receptor has an additional signal(s) that selectively targets molecules lacking a dominant basolateral signal to the apical surface. Although the physiological relevance of signal hierarchy is not known, alternative targeting may occur in different epithelial cell types or during development. The goal of this study, therefore, was to determine the effect of membrane domain location on EGF receptor function, focusing on EGF-induced MAP kinase signaling and DNA synthesis. Whereas ligand responsiveness was restricted to the basolateral domain in Madin-Darby canine kidney (MDCK) cells expressing a normal complement of receptors, apical ligand was effective if apical receptor density was increased by overexpression of an exogenous wild-type human gene. Unexpectedly, cells expressing apically localized, cytoplasmically truncated receptors, which behave as dominant negative mutations in other cell types, were also responsive to apical EGF. The cytoplasmically truncated molecules appear to have at least two effects: first, to increase the local concentration of ligand at the apical cell surface; and second, to facilitate activation of the relatively few native EGF receptors normally located at the apical surface. These results indicate that cell context is a critical determinant of receptor mutant protein phenotype.

A cytosolic domain of the erythropoietin receptor contributes to endoplasmic reticulum-associated degradation

The erythropoietin receptor (EPO-R) is the cellular target for erythropoietin (EPO), the primary hormone that mediates the proliferation of immature erythroblasts and their differentiation into mature erythrocytes. Unusual features of the EPO-R are its short half-life (t(1/2) 1-2 h), its degradation via multiple pathways and the fact that less than 1% of total cellular EPO-R molecules are found on the cell surface. The contribution of EPO-R structural determinants to the regulation of its intracellular metabolism is still unclear. The epidermal growth factor receptor (EGF-R), unlike the EPO-R, is efficiently transported to the cell surface and displays a much longer metabolic half-life. To determine which EPO-R cytosolic domains are involved in intracellular degradation, we studied chimeric receptor molecules constructed of EGF-R extracellular and transmembrane parts, linked to the full length or truncated cytosolic part of the EPO-R. The chimeras were expressed in transiently transfected COS 7 cells and stably expressed in Ba/F3 cells. Our experiments indicate that the cytosolic part of the EPO-R contains determinants that mark it for rapid degradation, in association with the endoplasmic reticulum (ER). This degradation was insensitive to brefeldin A and was inhibited by specific proteasomal inhibitors. A truncated EGF-R/EPO-R chimera containing only 50 amino acids of the EPO-R membrane-proximal cytosolic part was also rapidly degraded suggesting that these 50 amino acids are involved in receptor degradation.

A leucine-based determinant in the epidermal growth factor receptor juxtamembrane domain is required for the efficient transport of ligand-receptor complexes to lysosomes

Ligand binding causes the epidermal growth factor (EGF) receptor to undergo accelerated internalization with eventual degradation in lysosomes. The goal of this study was to investigate the molecular basis of endocytic sorting, focussing on post-internalization events. We have identified a sequence located between amino acid residues 675 and 697, encompassing a dileucine motif at residues 679 and 680, that enhances endosome-to-lysosome transport when conformational restraints in the EGF receptor carboxyl terminus are removed by truncation. The same dileucine motif is also necessary for efficient lysosomal transport of ligand-occupied full-length EGF receptors. A L679A,L680A substitution diminished the degradation of occupied full-length EGF receptors without affecting internalization but had a significant effect on recycling. Rapid recycling of mutant receptors resulted in reduced intracellular retention of occupied EGF receptors and delayed down-regulation of cell surface receptors. We propose that the L679A,L680A substitution acts primarily to impair transport of ligand-receptor complexes through an early endosomal compartment, diverting occupied receptors to a recycling compartment at the expense of incorporation into lysosome transport vesicles. We also found that mutant receptors with truncations at the distal half of tyrosine kinase domain (residues 809-957) were not efficiently delivered to the cell surface but were destroyed in an endoplasmic reticulum-associated degradative pathway.

Transcriptional regulation of the EGF receptor promoter by HPV16 and retinoic acid in human ectocervical epithelial cells

We have previously demonstrated that human papillomavirus 16 (HPV16)-immortalized human ectocervical epithelial cells and cells derived from tumors which express HPV16 oncogenes express high levels of epidermal growth factor receptor (EGFR) compared to normal cervical cells. We have also shown that proliferation of these cells is inhibited by retinoid treatment. We have hypothesized that the retinoid inhibition of cell proliferation may be due to the retinoid-dependent reduction in EGFR level. In this study we examine the regulation of EGFR expression in cervical cells with emphasis on two aspects: (1) the mechanism of retinoid-dependent suppression of EGFR levels in HPV16-positive cells and (2) the mechanism of EGFR upregulation by HPV16. EGFR levels were found to be elevated 5-, 3. 7-, and 1.25-fold in the HPV16-immortalized ECE16-1, ECE16-D1, and ECE16-D2 cells, respectively, compared to normal cervical cells. Treatment of ECE16-1 and ECE16-D1 cells with retinoic acid suppresses proliferation, EGFR level, EGFR mRNA level, and EGFR promoter activity. The reduction in EGFR promoter activity appears to account for the reduction in EGFR protein and mRNA levels. In contrast, retinoic acid does not affect cell growth or EGFR level in ECE16-D2 cells or normal cervical cells. To study the mechanisms regulating EGFR expression in HPV16-positive cells, normal ECE cells were cotransfected with an EGFR promoter reporter plasmid and an expression plasmid encoding the HPV16 E6/E7 open reading frames. In the presence of E6/E7, EGFR promoter activity was increased by 2- to 3-fold, suggesting that the E6/E7 proteins are directly or indirectly responsible for the increased EGFR level and that the EGFR promoter contains the DNA elements necessary to mediate this response. Nevertheless expression of E6/E7 proteins did not confer retinoic acid regulation, as EGFR promoter activity remained elevated in normal cells cotransfected with pHPVE6/E7 and treated with retinoic acid. These results suggest that human papillomavirus and retinoic acid regulate EGFR levels by independent effects on the EGFR promoter.

The cytoplasmic juxtamembrane domain of the epidermal growth factor receptor contains a novel autonomous basolateral sorting determinant

The epidermal growth factor receptor (EGFR) is localized at the basolateral membrane of most epithelial cells in vivo and in cell lines used to study membrane protein sorting. The goal of this study was to define the molecular basis of polar EGFR membrane expression using the Madin-Darby canine kidney cell model. We have identified a 23-amino acid segment located near the cytoplasmic face of the membrane spanning domain (residues Lys-652 to Ala-674) that is necessary and sufficient for targeting EGFRs from the trans-Golgi network directly to the basolateral plasma membrane. Furthermore, the sequence between residues Lys-652 and Ala-674 is sufficient to direct the extracellular domain of an apical membrane protein, decay accelerating factor, to the basolateral membrane. In the absence of this cytoplasmic basolateral sorting signal, information within the extracellular ligand binding domain is sufficient to target EGFRs from the trans-Golgi network directly to the apical plasma membrane. The EGFR basolateral sorting determinant does not have sequence and structural requirements common to most basolateral membrane proteins and does not overlap any of the known EGFR endocytic signals. This 23-residue sequence lies in a predicted amphipathic helical structure, leading us to postulate that hydrophobic and/or electrostatic interactions may be important for activity of this autonomous basolateral sorting determinant.

Cytoplasmic juxtamembrane domain of the human EGF receptor is required for basolateral localization in MDCK cells

Although it is well established that epidermal growth factor receptors (EGFRs) are asymmetrically expressed at the basolateral plasma membrane in polarized epithelial cells, how this process is regulated is not known. The purpose of this study was to address the mechanism of directed EGFR basolateral sorting using the Madin-Darby canine kidney (MDCK) cell model. The first set of experiments established sorting patterns for endogenous canine EGFRs. The polarity of the canine EGFR was not quantitatively affected by differences in electrical resistance exhibited by the MDCK I and MDCK II cell strains. In both cases, greater than 90% of total surface EGFRs was localized to the basolateral surface. Canine EGFRs sort directly to the basolateral membrane from the trans-Golgi network with a half-time of approximately 45 min and have an approximate t1/2 of 12.5 h once reaching the basolateral surface. Human holoreceptors expressed in stably transfected MDCK cells also localize to the basolateral membrane with similar efficiency. To identify EGFR sequences necessary for basolateral sorting, MDCK cells were transfected with cDNAs coding for cytoplasmically truncated human receptor proteins. Human EGFRs truncated at Arg-651 were localized predominantly at the apical surface of filter-grown cells, whereas receptors truncated at Leu-723 were predominantly basolateral. These results suggest that the cytoplasmic juxtamembrane domain contains a positive basolateral sorting determinant. Moreover, the EGFR ectodomain or transmembrane domain may possess a cryptic sequence that specifically interacts with the apical sorting machinery once the dominant basolateral sorting signal is removed. Further elucidation of the precise location of these signals will enhance our basic understanding of regulated plasma membrane sorting, as well as the functional consequences of inappropriate EGFR expression associated with certain pathophysiologic and malignant states.

Epidermal growth factor precursor is present in a variety of human renal cyst fluids

Progressive human renal cystic diseases are characterized by proliferation of the epithelial cells lining the cyst. The kidney synthesizes epidermal growth factor and its presence in renal cyst fluid might contribute to renal epithelial cell proliferation. We screened autosomal dominant polycystic kidney disease, acquired renal cystic disease, the von Hippel-Lindau syndrome, multilocular cystic nephroma, multicystic dysplastic kidney disease and simple cyst fluids for the presence of epidermal growth factor by radioreceptor assay, specific radioimmunoassay, immunoprecipitation and immunoblotting. Multiple epidermal growth factor immunoreactive species of approximately 180 kD. and lower molecular weights were present in almost all cyst fluids examined, suggesting endogenous synthesis and limited proteolysis of epidermal growth factor precursor protein in cyst fluid. Tamm-Horsfall protein was detected by enzyme-linked immunosorbent assay in most cysts (for example 55 of 59 autosomal dominant polycystic kidney disease samples). The majority of simple and autosomal dominant polycystic kidney disease renal cysts contained high sodium ion concentration, epidermal growth factor precursor protein and Tamm-Horsfall protein, characteristic of the early thick ascending limb. Rather than the mere presence of epidermal growth factor in renal cyst fluids, increased sensitivity to epidermal growth factor or other mitogens present in renal cyst fluid may be pathogenic in progressive renal cystic disease.

Cleavage of the epidermal growth factor receptor by a membrane-bound leupeptin-sensitive protease active in nonionic detergent lysates of senescent but not young human diploid fibroblasts

Numerous studies suggest that epidermal growth factor (EGF) signaling is impaired in nonproliferating senescent human diploid fibroblasts downstream of receptor binding. One possible explanation for these results is that senescent cells possess unique enzymatic activities capable of regulating functional levels of the EGF receptor. To test that hypothesis, nonionic detergent lysates of young and senescent cells were compared for proteolytic activity directed towards the EGF receptor, and a protease that cleaves the 170 kDa EGF receptor was identified in lysates from senescent but not young cells. Although studies presented here were carried out with WI-38 cells, our data indicate that other senescent fibroblasts possess a similar activity. The degradation product immunoprecipitated by a monoclonal antibody specific for an EGF receptor exocytosolic epitope had an approximate molecular weight of 100,000. This product was also detected following cell surface labeling with 125I, and by cross-linking 125I-EGF to intact cells with disuccinimidyl suberate. The proteolytic activity in senescent cell lysates was specifically inhibited by leupeptin and did not require divalent cations; it was also inactivated by aprotic solvents such as dimethylsulfoxide (DMSO) or ethylene carbonate. Interestingly, this protease was not active during ligand-induced intracellular processing of the EGF receptor, suggesting that it does not normally function in endocytic or lysosomal compartments. The susceptibility of the protease to inactivation by cell surface trypsinization is consistent with a plasma membrane localization. Since EGF receptor cleavage is not observed unless senescent cells are solubilized with nonionic detergents, it seems likely that the protease is confined to specialized regions of the plasma membrane. Whether or not the EGF receptor is a physiologic target for this protease is unclear. Its expression at the cell surface is nevertheless significant, since it suggests there are mechanisms for regulating membrane-bound proteins, or biologically active peptides in the extracellular space, in senescent cells that are either absent or inactive in young cells.

Adenovirus and protein kinase C have distinct molecular requirements for regulating epidermal growth factor receptor trafficking

The ligand-activated tyrosine kinase receptor for epidermal growth factor (EGF) is down-regulated by an integral membrane protein coded for by the E3 early transcription unit of group C adenoviruses. The E3 protein appears to block recycling of constitutively internalized receptors, causing them instead to traffic to lysosomes where they are degraded. Expression of functional EGF receptors is also regulated by protein kinase C (PKC), which directly phosphorylates the EGF receptor at Thr-654. The goal of this study was to determine potential interactions between PKC and the E3 protein, since membrane-bound PKC activity is elevated by the adenovirus E1A protein. Our results show that although tumor promoters which activate PKC cause a coordinate induction of E3 protein synthesis and EGF receptor degradation, the E3 protein-induced pathway for receptor down-regulation functions independently of PKC and other kinases that are inhibited by staurosporine. This suggests that in contrast to other mechanisms that modulate receptor expression (i.e., ligand and PKC), the E3 protein is not regulated by phosphorylation but is constitutively active. We also report that adenovirus-mediated degradation is the preferred pathway in infected cells stimulated with 12-O-tetradecanoylphorbol-13-acetate (TPA) to induce receptor recycling.

Ligand-induced protein tyrosine kinase activity in living cells coexpressing intact EGF receptors and receptors with an extensive cytosolic deletion

A population of stable NIH 3T3 transfectants with two molecular weight classes of membrane-bound EGF receptors encoded by a human EGF receptor cDNA has been identified and characterized. In addition to intact EGF receptors, these cells also express a molecule with an extensive cytosolic deletion. This deletion includes the ligand-activated intrinsic protein tyrosine kinase catalytic domain. Treatment with EGF caused dimerization of intact and truncated receptors, allowing us to assess protein tyrosine kinase activity in the heterodimer isolated from living cells. In contrast to homodimeric complexes with intact EGF receptor only, heterodimers were deficient in protein tyrosine kinase activity. Moreover, physical association between intact and truncated molecules suppressed receptor auto-phosphorylation by EGF receptor protein tyrosine kinase activated by antibody binding in vitro. Evidence presented here supports the idea that protein tyrosine kinase activation is facilitated by interaction between adjacent receptor molecules with intact catalytic domains. Furthermore, molecules with cytoplasmic deletions that are physically associated with kinase-active EGF receptors appear to behave as dominant negative mutations. The HerC cl cells used in this study were selected with methotrexate to amplify the EGF receptor cDNA, and in that sense may resemble certain tumor-derived cells characterized by overexpressed and rearranged EGF receptor genes.

Evidence for intracellular down-regulation of the epidermal growth factor (EGF) receptor during adenovirus infection by an EGF-independent mechanism

We have reported previously that human group C adenoviruses down-regulate the epidermal growth factor (EGF) receptor (EGF-R) (C. R. Carlin, A. E. Tollefson, H. A. Brady, B. L. Hoffman, and W. S. M. Wold, Cell 57:135-144, 1989). Expression of a 13.7-kDa protein encoded by a gene in the E3 transcription unit is necessary and sufficient for this effect (Carlin et al., Cell, 1989; B. L. Hoffman, A. Ullrich, W. S. M. Wold, and C. R. Carlin, Mol. Cell. Biol. 10:5521-5524, 1990). We show here that EGF-R down-regulation is accelerated in cells which overexpress the receptor when these cells are infected with virus mutants that overproduce the 13.7-kDa protein compared with wild-type virus. This is in contrast to EGF stimulation, for which others have shown that high concentrations of ligand are associated with low rates of receptor internalization in EGF-R-overexpressing cells (D. Kuppuswamy and L. J. Pike, J. Biol. Chem. 264:3357-3363, 1989; H. S. Wiley, J. Cell Biol. 107:801-810, 1988). We also show that the E3 protein is not present in media conditioned by infected cells and that it does not induce secretion of an EGF-like autocrine factor. Moreover, while mature membrane-bound EGF-R is down-regulated, the precursor of the membrane-bound form is not. Adenovirus infection also does not affect receptor-related molecules expressed in the secretory pathway. Interestingly, adenovirus-induced down-regulation is not regulated by concentrations of EGF associated with a slow rate of internalization in A431 cells. This suggests that 13.7-kDa protein expression triggers receptor entry by a novel ligand-independent pathway or, alternatively, that it compensates for a cellular factor that may be rate limiting during EGF-mediated endocytosis.

Inhibition of mouse mammary ductal morphogenesis and down-regulation of the EGF receptor by epidermal growth factor

EGF, initially demonstrated to be a potent mitogen for a variety of cell types, has more recently been shown to inhibit proliferation of several cell lines. Few studies, however, have addressed the effects of EGF on growth and morphogenesis of tissues in vivo, particularly with regard to EGF as a possible inhibitor. We now demonstrate that EGF treatment of vigorously growing mammary ducts, administered directly to the glands by slow release plastic implants, inhibited normal ductal growth. Inhibition was restricted to the region around the implant and untreated glands in the same animal were normal, indicating direct effects of EGF. EGF-treated end buds were smaller and demonstrated reduced levels of DNA synthesis, although remnants of a stem (cap) cell layer persisted. Full inhibition of growth occurred within 3 days of implantation and required extended exposure to EGF, since treatment of 5 hr or less had no effect on ductal growth. At the lower inhibitory doses tested, growth resumed within 8 days, indicating reversibility of inhibition. No lobuloalveolar or hyperplastic response was seen. 125I-EGF autoradiography revealed that ductal growth inhibition was preceded by the disappearance of EGF receptors located in the cap cell layer of the end bud epithelium and in stromal cells adjacent to the buds. These results, in conjunction with our previous evidence demonstrating the growth-stimulatory effect by EGF on nonproliferating mammary ducts, suggest a growth regulatory role for EGF in mouse mammary ductal morphogenesis.

Epidermal growth factor receptor is down-regulated by a 10,400 MW protein encoded by the E3 region of adenovirus

Epidermal growth factor (EGF) binds to specific high affinity receptors (EGF-Rs) and induces endosome-specific internalization and degradation of ligand-receptor complexes in lysosomes. We report here that EGF-R is down-regulated in an analogous manner during early infection of a variety of cell types by group C human adenoviruses. This effect is not a function of viral entry, nor is it due to a nonspecific increase in turnover of membrane proteins. Using a series of virus deletion mutants, the gene responsible for EGF-R down-regulation was mapped to the E3 transcription unit. The E3 gene product, a protein of MW 10,400 (10.4K), induces internalization and degradation of EGF-R, but does not affect synthesis of the EGF-R precursor. The 10.4K protein is not an EGF-like autocrine growth factor, but is similar in sequence to a region in EGF-R at the cytoplasmic face of the transmembrane domain. This suggests that down-regulation of EGF-R during adenovirus infection may occur by a novel mechanism that involves the formation of hetero-oligomers composed of 10.4K and EGF-R.

A monoclonal antibody, R1, and a polyclonal serum, S10, recognize the same molecules: a novel use of DNA transfectants

The monoclonal antibody R1 defines the product of the MIC5 locus which has been localized to the long arm of the human X chromosome in the same region as the fragile site FRAXA. DNA transfectants, selected by R1 on the fluorescence-activated cell sorter (FACS), have been used to demonstrate that a previously described polyclonal antiserum, S10 (Buck & Bodmer, 1976), recognizes the same cell surface molecule as R1. Immunoprecipitation from metabolically and surface-labelled cells has shown that this molecule is a Mr 200 K phosphoprotein which is synthesized as a Mr 180 K precursor and subsequently modified to a Mr 195 K extracellular form. A Mr 150 K molecule is coprecipitated with the Mr 200 K phosphoprotein, but is only detected in lysates of surface-labelled cells, raising the possibility that the 200 K molecule is a cell surface receptor and the 150 K molecule a ligand.

Increased epidermal growth factor receptor in multidrug-resistant human neuroblastoma cells

Multidrug-resistant human neuroblastoma cell lines obtained by selection with vincristine or actinomycin D from two independent clonal lines, SH-SY5Y and MC-IXC, have 3- to 30-fold more cell surface epidermal growth factor (EGF) receptors than the drug-sensitive parental cells as indicated by EGF binding assays and immunoprecipitation, affinity-labeling, and phosphorylation studies. Reversion to drug sensitivity in one line was accompanied by a return to the parental level of EGF receptor. SH-EP cells, a clone derived from the same neuroblastoma cell line as SH-SY5Y but which displays melanocyte rather than neuronal lineage markers, also express significantly more EGF receptor than SH-SY5Y cells. By nucleic acid hybridization analysis with a molecularly cloned probe, increased receptor level in multidrug-resistant cells was shown to be the result of higher levels of EGF receptor mRNA in drug-resistant than in drug-sensitive cells. The increased steady state amount of specific RNA did not result from amplification of receptor-encoding genes. A small difference was observed in the electrophoretic mobility under denaturing conditions of EGF receptor immunoprecipitated from drug-resistant and drug-sensitive cells. Quantitative and qualitative modulation of the EGF receptor might reflect alterations in the transformation and/or differentiation phenotype of the resistant cells or might result from unknown selective pressures associated with the development of multidrug resistance.

Similarities in glycosylation and transport between the secreted and plasma membrane forms of the epidermal growth factor receptor in A-431 cells

We have studied the synthesis and oligosaccharide processing of the 110,000 dalton form of the epidermal growth factor (EGF) receptor that is secreted into the medium of A-431 cells. Its 90,000 dalton precursor is soluble within the lumen of intracellular membrane vesicles shortly after synthesis, indicating that it lacks a membrane anchor. Analysis of labeled glycopeptides reveals that the glycosylation of the 110,000 dalton, secreted receptor is very similar to that of the 170,000 dalton, plasma membrane receptor. Based on Concanavalin A-Sepharose elution profiles of its glycopeptides, the secreted receptor has both complex and high-mannose N-linked oligosaccharides. Also, like the plasma membrane receptor, the secreted receptor contains N-acetylgalactosamine residues in its complex chains. Not only are major features of oligosaccharide processing of the soluble and membrane-bound forms of the receptor similar, but the kinetics of transport to the cell exterior is the same for each. These data indicate that the glycosylation pattern and kinetics of cellular transport of the EGF receptor are determined by factors other than the sequence of its cytoplasmic and transmembrane domains.

Temperature-sensitive Chinese hamster cell mutant with a defect in glycoprotein synthesis: accumulation of the EGF receptor in the endoplasmic reticulum and the role of the glucose-regulated protein GRP78

A temperature-sensitive mutant of Chinese hamster fibroblasts with a defect in glycoprotein synthesis is investigated after transfection and amplification of the gene for the human EGF receptor. We demonstrate that at the nonpermissive temperature a partially glycosylated species of the receptor accumulates in the endoplasmic reticulum. The oligosaccharides present are the high mannose types, since they can be removed completely by treatment with endoglycosidase H. Pulse-chase experiments show that the abnormal species of the receptor cannot be chased to a form that is either resistant to endoglycosidase H, or altered in its mobility on SDS polyacrylamide gels. The abnormal species of the receptor appears within the first hour of a shift to the nonpermissive temperature, and no further changes are observed upon prolonged incubation of cells at 40 degrees C. However, after 3-4 hours immunoprecipitations of the receptor yield another protein, which has properties very similar, if not identical, to the glucose-regulated protein GRP78. The induction of this protein at 40 degrees C can be suppressed completely with an inhibitor of RNA synthesis, without any effect on the glycosylation defect, or on the accumulation of the EGF receptor in the endoplasmic reticulum.

Analysis of the protein glycosylation defect of a temperature-sensitive cell cycle mutant by the use of mutant cells overexpressing the human epidermal growth factor receptor after transfection of the gene

A temperature-sensitive mutant with a defect in glycoprotein synthesis and a cell cycle (G1)-specific arrest at the nonpermissive temperature (Tenner et al., J. Cell. Physiol., 90:145-160, 1977; Tenner and Scheffler, J. Cell. Physiol., 98:251-266, 1979) was investigated further after a human epidermal growth factor (EGF) receptor gene had been transfected and amplified in these cells. While a temperature shift-up lead to an immediate arrest in the biosynthesis of mature EGF receptor and its appearance on the plasma membrane, the observed turnover of the preexisting receptor was too slow to account for the arrest of DNA synthesis in these mutant cells. Tunicamycin could in fact mimic the effect of a temperature shift on the biosynthesis of EGF receptor, but it did not have the same rapid effect on DNA synthesis and cell cycle progression. These mutants have also been shown to induce a set of stress proteins or glucose-regulated proteins, GRPs (Lee et al., J. Cell. Physiol., 129:277-282, 1986). The question is addressed whether the defect in glycoprotein synthesis is the primary defect and a possible cause of the induction of the GRPs, or whether a more basic defect at the level of the endoplasmic reticulum (ER) is responsible for the complex phenotype of the mutant. Our results argue in favor of a primary defect which indirectly affects N-linked glycosylation of proteins, as well as several other functions associated with the ER. We hypothesize that the defect affects the calcium distribution between ER and cytosol, since the calcium ionophore A23187 has an effect similar to that of a temperature shift.

In vitro expression of a 38,000 dalton heparin-binding glycoprotein by morphologically differentiated smooth muscle cells

In vitro, high density monolayer cultures of vascular smooth muscle cells can be induced to form multicellular nodules. The nodular cells appear to be morphologically differentiated smooth muscle cells. Sodium dodecyl sulfate (NaDodSO4)-polyacrylamide gel electrophoresis was used to compare the proteins synthesized and secreted by monolayer and nodular cultures of smooth muscle cells. Although most proteins appeared to be similar, the nodular cultures contained a unique heparin binding protein of Mr = 38,000 (38kD protein) (Millis, A.J.T., Hoyle, M., Reich, E., and Mann, D.M., 1985, J. Biol. Chem., 260:3754-3761). The 38kD protein was glycosylated and its apparent molecular weight was shifted to Mr = 32,500 after synthesis in the presence of tunicamycin or digestion with endoglycosidase F. The production of 38kD protein by nodular cell cultures did not appear to result from the degradation of a high molecular weight precursor in nodular conditioned medium. Further, it was not detected in monolayer cell conditioned medium that had been incubated with nodular cells. Finally, its synthesis was not induced in monolayer cell cultures that had been labeled in nodular cell conditioned medium. The 38kD protein appears to be uniquely associated with nodular cultures of smooth muscle cells.

Human embryonal carcinoma cells express low levels of functional receptor for epidermal growth factor

Using immunological probes, we have detected expression of the receptor for epidermal growth factor (EGF) at the cell surface of the clonally-derived human embryonal carcinoma (EC) cell lines, 2102Ep cl. 4D3 and NTERA-2 cl. D1. By this method, the level of receptor expression by these cells is estimated to be 3- to 5-fold less than for the human diploid fibroblast cell line, WI38, and our results indicate that it is the EC stem cells that display this receptor and not a subpopulation of differentiated cells. The human EC cell receptor binds ligand and catalyses autophosphorylation at tyrosine in a normal fashion. Treatment of NTERA-2 cl. D1 cells with retinoic acid (RA) for 7 days to induce differentiation results in decreased levels of receptor expression, and a subpopulation of differentiated cells possessing a markedly higher level of the EGF receptor was not detected among the cultures exposed to RA for longer periods.