Lee K. Opresko

ErbB-2 amplification inhibits down-regulation and induces constitutive activation of both ErbB-2 and epidermal growth factor receptors.

ErbB-2/HER2 is an important signaling partner for the epidermal growth factor receptor (EGFR). Overexpression of erbB-2 is also associated with poor prognosis in breast cancer. To investigate how erbB-2 amplification affects its interactions with the EGFR, we used a human mammary epithelial cell system in which erbB-2 expression was increased 7–20-fold by gene transfection. We found that amplification of erbB-2 caused constitutive activation of erbB-2 as well as ligand-independent activation of the EGFR. Overexpression of erbB-2 strongly inhibited erbB-2 down-regulation following transactivation by EGFR. Significantly, down-regulation of activated EGFR was also inhibited by erbB-2 amplification, resulting in enhanced ligand-dependent activation of the EGFR. The rate of EGFR endocytosis was not affected by erbB-2 overexpression, but the rate of lysosomal targeting was significantly reduced. In addition, erbB-2 overexpression promoted rapid recycling of activated EGFR back to the cell surface and decreased ligand dissociation from the EGFR. Our data suggest that overexpression of erbB-2 inhibits both its down-regulation and that of the EGFR. The net effect is increased signaling through the EGFR system.

EGF-receptor-mediated mammary epithelial cell migration is driven by sustained ERK signaling from autocrine stimulation.

EGF family ligands are synthesized as membrane-anchored precursors whose proteolytic release yields mature diffusible factors that can activate cell surface receptors in autocrine or paracrine mode. Expression of these ligands is altered in pathological states and in physiological processes, such as development and tissue regeneration. Despite the widely documented biological importance of autocrine EGF signaling, quantitative relationships between protease-mediated ligand release and consequent cell behavior have not been rigorously investigated. We thus explored the relationship between autocrine EGF release rates and cell behavioral responses along with activation of ERK, a key downstream signal, by expressing chimeric ligand precursors and modulating their proteolytic shedding using a metalloprotease inhibitor in human mammary epithelial cells. We found that ERK activation increased monotonically with increasing ligand release rate despite concomitant downregulation of EGF receptor levels. Cell migration speed was directly related to ligand release rate and proportional to steady-state phospho-ERK levels. Moreover, migration speed was significantly greater for autocrine stimulation compared with exogenous stimulation, even at comparable phospho-ERK levels. By contrast, cell proliferation rates were approximately equivalent at all ligand release rates and were similar regardless of whether the ligand was presented endogenously or exogenously. Thus, in our mammary epithelial cell system, migration and proliferation are differentially sensitive to the mode of EGF ligand presentation.

Specific proteolysis regulates fusion between endocytic compartments in Xenopus oocytes

We examined the role of proteolytic ligand modification in endosomal targeting using vitellogenin (VTG) uptake by Xenopus oocytes as a model system. Non-cleavable VTG is internalized, but does not appear in yolk platelets. We identified two inhibitors of VTG processing into the yolk proteins: the ionophore monensin and pepstatin A, a specific inhibitor of cathepsin D. Pepstatin neither affected ligand binding and internalization, nor inhibited the degradation of nonspecifically incorporated proteins, whereas monensin inhibited all of these processes. Inhibiting VTG processing prevented its deposition into yolk platelets by strongly interfering with endosome-yolk platelet fusion. Monensin treatment resulted in morphologically abnormal endosomes, while pepstatin only inhibited VTG cleavage and the subsequent fusion of endosomes with yolk platelets. Since VTG cleavage is initiated prior to its deposition in platelets, we postulate that ligand proteolysis could be necessary for normal endosomal targeting.

Differential signaling and regulation of apical vs. basolateral EGFR in polarized epithelial cells

Overexpression of the epidermal growth factor receptors (EGFR) in polarized kidney epithelial cells caused them to appear in high numbers at both the basolateral and apical cell surfaces. We utilized these cells to look for differences in the regulation and signaling of apical vs. basolateral EGFR. Apical and basolateral EGFR were biologically active and mediated EGF-induced cell proliferation to similar degrees. Receptor downregulation and endocytosis were less efficient at the apical surface, resulting in prolonged EGF-induced tyrosine kinase activity at the apical cell membrane. Tyrosine phosphorylation of EGFR substrates known to mediate cell proliferation, Src-homologous and collagen protein (SHC), extracellularly regulated kinase 1 (ERK1), and ERK2 could be induced similarly by activation of apical or basolateral EGFR. Focal adhesion kinase was tyrosine phosphorylated more by basolateral than by apical EGFR; however, beta-catenin was tyrosine phosphorylated to a much greater degree following the activation of mislocalized apical EGFR. Thus EGFR regulation and EGFR-mediated phosphorylation of certain substrates differ at the apical and basolateral cell membrane domains. This suggests that EGFR mislocalization could result in abnormal signal transduction and aberrant cell behavior.Overexpression of the epidermal growth factor receptors (EGFR) in polarized kidney epithelial cells caused them to appear in high numbers at both the basolateral and apical cell surfaces. We utilized these cells to look for differences in the regulation and signaling of apical vs. basolateral EGFR. Apical and basolateral EGFR were biologically active and mediated EGF-induced cell proliferation to similar degrees. Receptor downregulation and endocytosis were less efficient at the apical surface, resulting in prolonged EGF-induced tyrosine kinase activity at the apical cell membrane. Tyrosine phosphorylation of EGFR substrates known to mediate cell proliferation, Src-homologous and collagen protein (SHC), extracellularly regulated kinase 1 (ERK1), and ERK2 could be induced similarly by activation of apical or basolateral EGFR. Focal adhesion kinase was tyrosine phosphorylated more by basolateral than by apical EGFR; however, β-catenin was tyrosine phosphorylated to a much greater degree following the activation of mislocalized apical EGFR. Thus EGFR regulation and EGFR-mediated phosphorylation of certain substrates differ at the apical and basolateral cell membrane domains. This suggests that EGFR mislocalization could result in abnormal signal transduction and aberrant cell behavior.

Multiple Mechanisms Are Responsible for Transactivation of the Epidermal Growth Factor Receptor in Mammary Epithelial Cells

The number of distinct signaling pathways that can transactivate the epidermal growth factor receptor (EGFR) in a single cell type is unclear. Using a single strain of human mammary epithelial cells, we found that a wide variety of agonists, such as lysophosphatidic acid (LPA), uridine triphosphate, growth hormone, vascular endothelial growth factor, insulin-like growth factor-1 (IGF-1), and tumor necrosis factor-α, require EGFR activity to induce ERK phosphorylation. In contrast, hepatocyte growth factor can stimulate ERK phosphorylation independent of the EGFR. EGFR transactivation also correlated with an increase in cell proliferation and could be inhibited with metalloprotease inhibitors. However, there were significant differences with respect to transactivation kinetics and sensitivity to different inhibitors. In particular, IGF-1 displayed relatively slow transactivation kinetics and was resistant to inhibition by the selective ADAM-17 inhibitor WAY-022 compared with LPA-induced transactivation. Studies using anti-ligand antibodies showed that IGF-1 transactivation required amphiregulin production, whereas LPA was dependent on multiple ligands. Direct measurement of ligand shedding confirmed that LPA treatment stimulated shedding of multiple EGFR ligands, but paradoxically, IGF-1 had little effect on the shedding rate of any ligand, including amphiregulin. Instead, IGF-1 appeared to work by enhancing EGFR activation of Ras in response to constitutively produced amphiregulin. This enhancement of EGFR signaling was independent of both receptor phosphorylation and PI-3-kinase activity, suggestive of a novel mechanism. Our studies demonstrate that within a single cell type, the EGFR autocrine system can couple multiple signaling pathways to ERK activation and that this modulation of EGFR autocrine signaling can be accomplished at multiple regulatory steps.

Chapter 6 Vitellogenin Uptake and in Vitro Culture of Oocytes

Publisher Summary The oocytes of nonmammalian vertebrates are specialized for the endocytic incorporation of proteins. Oocytes grow primarily via the specific internalization of a serum protein, vitellogenin (VTG), which is the precursor to the yolk. VTG is synthesized in the liver under the influence of estrogen, and then secreted into the bloodstream. The circulating levels of VTG in healthy, mature females are approximately 5 mg/ml of serum, and it is internalized exclusively by the ovary. Early oocyte internalization experiments used in vivo labeled Xenopus serum derived from hormonally stimulated (estrogen-treated) animals as a source of VTG. These sera were added to different media at relatively high concentrations (up to 50%), and protein incorporation was measured over an extended period of time (1–25 hours) because of the low specific activity of the in vivo labeled VTG. VTG purification methods were developed in the early 1970s. The first of these used chromatography of vitellogenic serum on TEAE cellulose to obtain a preparation of high purity. The chapter describes various methods: (1) handling of ovarian tissue and oocytes, (2)oocyte culture media, (3) vitellogenin isolation, (4) labeling of vitellogenin and other proteins, and (5) performing protein uptake experiments. .

A comparison of the proteins of yolk platelets and intramitochondrial crystals in the oocytes of the bullfrog

Abstract A step gradient of Ficoll has been used to obtain from the oocytes of Rana catesbeiana a fraction rich in the intramitochondrial crystals generally believed to be a form of yolk. SDS-polyacrylamide gels of the crystal fraction and the yolk platelet fraction have revealed that the intramitochondrial crystals contain neither the lipovitellins nor the phosvitin characteristic of the yolk platelets. It is suggested that use of the term “yolk” in referring to these crystals be discontinued until their role in oogenesis and embryogenesis is clarified.

A Model of Cytokine Shedding Induced by Low Doses of Gamma Radiation

Abstract Miller, J. H., Zheng, F., Jin, S., Opresko, L. K., Wiley, H. S. and Resat, H. A Model of Cytokine Shedding Induced by Low Doses of Gamma Radiation. Radiat. Res. 163, 337– 342 (2005). A model for sustained shedding of epidermal growth factor (EGF) in response to low doses of gamma radiation was developed based on a time delay in the feedback from mitogen-activated protein kinase (MAPK) activation to metalloprotease activity in an autocrine signaling process. We determined the kinetic parameters of our model using the data available for MAPK activation by γ irradiation in the 1–2-Gy dose range and then showed that predictions of the model were consistent with experimental results for the kinetics of EGF shedding into the growth medium after exposure of human mammary epithelial cells to 1–5 cGy of γ radiation in the presence of antibodies that block ligand binding to EGF receptors. The model allowed us to estimate the rate of radiation-induced cytokine release per cell from measurements of EGF concentration in the growth medium and to assess the effectiveness of EGF shedding and subsequent diffusion through the medium as a mechanism for signal transmission between hit cells and bystanders.

An enzymatic method for radiolabeling vertebrate vitellogenin

Phosphoprotein kinases from Xenopus and chicken liver have been purified and these enzymes have been used to label Xenopus vitellogenin, a phosphoprotein, to high specific activity with [gamma-32P]ATP. The enzymes were isolated by (NH4)2SO4 fractionation followed by chromatography on DE-52 cellulose and phosphocellulose. This procedure resulted in greater than 20,000-fold enrichment for the enzymes. Both enzyme preparations were used to selectively label vitellogenin in the serum of estrogen-treated animals. Thus, isolation of the vitellogenin prior to radiolabeling was not necessary. The [32P]vitellogenin labeled in situ was incorporated by oocytes at a rate similar to [32P]vitellogenin labeled in vivo, was translocated to the yolk platelets, and was correctly processed into the yolk proteins.

Quantitative Assessment of Autocrine Cell Loops

Regeneration of functioning tissue essentially involves recapitulating relevant aspects of organogenesis, so that the starting composite of cells, matrix, and molecular factors develops into the desired structure and physiology. A crucial aspect of development is local cell-cell communication; that is, molecular regulatory factors are more typically paracrine and autocrine than endocrine in nature. Autocrine loops were originally thought of predominantly as being involved in pathological behavior, but it is becoming increasingly clear that a large portion of normal physiological behavior-and a tremendous portion of development-is strongly regulated by autocrine factors (1). Thus, continuing progress of the field of tissue engineering will require increased understanding of how autocrine loops operate, so that they can be designed or manipulated systematically. We have made an effort in this direction, and some early experimental and modeling results can be found in the literature (2-5). In this chapter, we describe the methods we have used for creating autocrine cell loops and quantitatively assessing their operation.