Chapter 9 Receptors as Models for the Mechanisms of Membrane Protein Turnover and Dynamics

A local pre-receptor mechanism of hormone stimulus amplification: focus on angiotensin II in resistance blood vessels

BACKGROUND

The in-vivo correlation between vascular tone and the concentration of free angiotensin (Ang) II at the level of the arterioles, under (patho)physiological conditions, is not known.

OBJECTIVE

To examine the in-vivo kinetics of binding of Ang II to Ang II type 1 (AT1) receptors in vascular tissue.

METHODS AND RESULTS

A plane vascular smooth muscle (VSM) sheet containing a single layer of cells, at one side exposed to Ang II, was the starting point for designing a mathematical model based on local receptor density and geometric considerations and on kinetic parameters of Ang II diffusion and Ang II-AT1 receptor complex formation and internalization. Calculations demonstrate that a diffusing Ang II molecule at short distance from the receptor has an almost 100% chance to be actually bound, so that the apparent binding rate constant (per unit of receptor concentration) is greatly augmented. This pre-receptor stimulus amplification (PRESTAMP) mechanism is sustained by AT1 receptor-mediated endocytosis and receptor recycling. On the other hand, PRESTAMP also enhances endocytotic receptor downregulation, and calculations predict that steady-state levels of Ang II above threshold have relatively little additional effect.

CONCLUSION

The results explain why physiological concentrations of free Ang II far below the equilibrium dissociation constant of its reaction with AT1 receptors are sufficient to increase vascular resistance, and why a correlation between blood pressure and the concentration of free Ang II is often difficult to demonstrate.

Disassembly and degradation of photosystem I in an in vitro system are multievent, metal-dependent processes

An in vitro system was created to study the process of membrane protein degradation by using photosystem I (PS1) as a model membrane protein. Purified chloroplast membranes were incubated at 30 degrees C in a defined buffer along with various extracts or reagents to reconstitute the disassembly and degradation of PS1, which was monitored by a variety of techniques that probe the integrity of the PS1 complex: photo-biochemical assays, semi-native gel electrophoresis, low temperature fluorescence spectroscopy, and immunoblots using antibodies against different PS1 subunits. During a typical time course, degradation of PS1 appeared to be a multievent process, with disassembly of the complex preceding proteolysis of the subunits. The first change seen was a rapid (<5 min) decrease in PS1 photochemical activity. This was followed by a diminution of far-red fluorescence emission from the core antenna of PS1 and a slower disassembly of the PS1 chlorophyll-protein core complex, as visualized by semi-native gel electrophoresis. Surprisingly, the latter was not accompanied by a similar rate of proteolysis of the PsaA core subunit. In contrast, addition of soluble proteases caused rapid loss of immuno-detectable PS1 polypeptides and cleavage of the major PS1 polypeptides in interhelical loops. The in vitro degradation process was time- and temperature-dependent but did not require ATP, GTP, or soluble chloroplast proteins. Chelation of divalent cations by EDTA inhibited the later steps of disassembly and proteolysis, and this effect could be reversed by addition of micromolar Zn2+, with Co2+ and Ca2+ providing somewhat lower activity.

Shedding of TNFR1 in regenerative liver can be induced with TNF α and PMA

AIM: Liver regeneration is associated with apoptosis of hepatocytes, which is mediated via tumor necrosis factor receptor 1(TNFR1). The shedding of TNFR1 in liver regeneration and its mechanism to regulate this shedding were investigated.

METHODS: The shedding of TNFR1 in liver regeneration and changes of TNF-α, PMA and plasma membrane purified from hepatocytes on this shedding process were measured with Western blot. Then, the relationship between TNFR1 shedding and apoptosis of hepatocytes induced by TNFα was studied by detecting apoptotic index.

RESULTS: The shedding of TNFR1 began at 4 hours and terminated before 2 months after partial hepatectomy. In culture system, serum from rats at 36 h after partial hepatectomy could also promote this shedding process. With the stimulation of TNF α, PMA or purified plasma membrane from hepatocytes at 36 h after partial hepatectomy or from hepatocytes treated with TNF α for 2 h, membranous TNFR1 was also shed. With the stimulation of both TNF α and plasma membrane from hepatocytes affected with TNF α for 2 h or from hepatocytes at 36 h after partial hepatectomy, apoptotic index of hepatocytes decreased from 21% to 7.52% and 8.45%, respectively. PMA could also reduce apoptotic index to 13.67%. This descent occurred in hepatocytes cultured in serum from rats at 36 h after partial hepatectomy too, but not in serum from rats at 2 months after partial hepatectomy and sham-operated rats.

CONCLUSION: Shedding of TNFR1 may help reduce apoptosis of hepatocytes induced by TNF α. Membrane-anchored metalloprotases could play a role in shedding membranous TNFR1. At the same time, PKC may take part in regulation of this shedding process.

Removal of the membrane-anchoring domain of epidermal growth factor leads to intracrine signaling and disruption of mammary epithelial cell organization

Autocrine EGF-receptor (EGFR) ligands are normally made as membrane-anchored precursors that are proteolytically processed to yield mature, soluble peptides. To explore the function of the membrane-anchoring domain of EGF, we expressed artificial EGF genes either with or without this structure in human mammary epithelial cells (HMEC). These cells require activation of the EGFR for cell proliferation. We found that HMEC expressing high levels of membrane- anchored EGF grew at a maximal rate that was not increased by exogenous EGF, but could be inhibited by anti-EGFR antibodies. In contrast, when cells expressed EGF lacking the membrane-anchoring domain (sEGF), their proliferation rate, growth at clonal densities, and receptor substrate phosphorylation were not affected by anti-EGFR antibodies. The sEGF was found to be colocalized with the EGFR within small cytoplasmic vesicles. It thus appears that removal of the membrane-anchoring domain converts autocrine to intracrine signaling. Significantly, sEGF inhibited the organization of HMEC on Matrigel, suggesting that spatial restriction of EGF access to its receptor is necessary for organization. Our results indicate that an important role of the membrane-anchoring domain of EGFR ligands is to restrict the cellular compartments in which the receptor is activated.

Targetable HPMA copolymer-adriamycin conjugates. Recognition, internalization, and subcellular fate

Recognition, internalization, and subcellular trafficking of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer conjugates containing N-acylated galactosamine (GalN) or monoclonal OV-TL16 antibodies (Ab) have been investigated in human hepatocarcinoma HepG2 and ovarian carcinoma OVCAR-3 cells, respectively. The intrinsic fluorescence of fluorescein or adriamycin (ADR) attached to HPMA copolymers permitted us to follow the subcellular fate of HPMA copolymer conjugates by confocal fluorescence microscopy and fluorescence spectroscopy. The pattern of fluorescence during incubation of HPMA copolymer-ADR-GalN conjugate containing lysosomally degradable tetrapeptide (GFLG) side-chains with HepG2 cells was consistent with conjugate recognition, internalization, localization in lysosomes, followed by the release of ADR from the polymer chains and ultimately diffusion via the cytoplasm into the cell nuclei. A similar pattern was observed in OVCAR-3 cells for Ab targeted HPMA copolymer conjugates. To test our hypothesis that HPMA-copolymer-bound anticancer drugs will be inaccessible to the energy-driven P-glycoprotein efflux pump in multidrug resistant (MDR) cells, we have compared the internalization of the HPMA copolymer-ADR conjugates by sensitive (A2780) and ADR-resistant (A2780/AD) ovarian carcinoma cell lines. Preliminary data on relative retention of ADR in MDR (A2780/AD) cells indicate a higher intracellular ADR concentration after incubation with HPMA copolymer-ADR conjugate when compared to incubation with free (unbound) ADR.

Fast steps in exocytosis and endocytosis studied by capacitance measurements in endocrine cells

The past year has witnessed progress in identifying late steps in exocytosis that are so short-lived as to be difficult to study biochemically. Recent studies have also revealed a novel and surprisingly fast mechanism of endocytosis that may be triggered by a rise in the intracellular concentration of Ca2+ and that retrieves exocytosed membrane in seconds.

Endocytosis and lysosomal targeting of epidermal growth factor receptors are mediated by distinct sequences independent of the tyrosine kinase domain

Ligand-induced internalization of the epidermal growth factor receptor (EGFR) leads to accelerated receptor degradation. Two models have been proposed to explain this. In the first model, induced internalization expands the intracellular pool of receptors, leading to enhanced lysosomal targeting. The second model proposes that activation of intrinsic receptor kinase activity induces inward vesiculation of endosomes, thus interrupting receptor recycling. To test these models, we created EGFR mutants that lack the conserved tyrosine kinase domain, but retain different parts of the distal carboxyl terminus regulatory region. Mutants lacking all distal regulatory sequences underwent slow internalization (0.02 min-1) and turnover (t1/2 approximately 24 h), similar to unoccupied, holo-EGFR. Mutant receptors that lacked the kinase domain, but retained the entire distal regulatory domain, were constitutively internalized and targeted to lysosomes, even in the absence of EGF. The turnover of these receptors (t1/2 approximately 11 h) was similar to that of occupied, kinase-active holo-EGFR (t1/2 approximately 9.5 h). These results show that receptor tyrosine kinase activity is not required for the targeting of EGFR to lysosomes. Receptor mutants which expressed previously identified endocytic sequences underwent rapid internalization. Unexpectedly, enhanced turnover of EGFR mutants required additional sequences located between residues 945 and 991 in the holo-EGFR. Thus, internalization and lysosomal targeting of EGFR are separate processes mediated by distinct sequences. Our results indicate that induced internalization is necessary, but not sufficient, for enhanced EGFR degradation. Instead, down-regulation requires exposure of previously cryptic internalization and lysosomal targeting sequences. Occupied EGFR thus appear to be handled by the endocytic machinery in the same fashion as other constitutively internalized or lysosomally targeted receptors.

Treatment of leukemia with radiolabeled monoclonal antibodies

In contrast to radioimmunotherapy of solid disease, wherein the primary obstacle to success is access of radiolabeled antibody to antigen-positive cells, in the treatment of leukemia delivering a lethal absorbed dose to the isolated cell appears to be the primary obstacle. The isolated cell is defined as one that is exposed only to self-irradiation (from internalized or surface-bound radiolabeled antibody) and to irradiation from free antibody in the blood. It is isolated in the sense that the particulate (beta, electron, alpha) emissions from its nearest neighboring antigen-positive cell do not contribute to its absorbed dose. Disease in the bone marrow and other tissues, since it is confined to a smaller volume, is more easily eradicated because the absorbed dose to a given cell nucleus is enhanced by emissions from adjacent cells (a smaller fraction of the emission energy is 'wasted'). The optimization simulations presented above for the M195 antibody suggest that the optimum dose of antibody that should be administered is that required to yield a concentration within the distribution volume of the antibody that is approximately equal to the concentration of antigen sites as determined by the tumor burden. Although not specifically considered in the modeling example presented above, antibody internalization and catabolism may be expected to play an important role in radioimmunotherapy treatment planning of leukemia. Depending upon the kinetics of internalization and catabolism, the absorbed dose to the red marrow and to antigen-positive cells may be reduced considerably, since catabolism, assuming that it is followed by rapid extrusion of the radioactive label, would decrease the cells' exposure time considerably. The recently demonstrated effectiveness of radioimmunotherapy in certain cases of B-cell lymphoma and in reducing tumor burden in acute myelogenous leukemia suggests that radioimmunotherapy is beginning to fulfill the promise held when it was initially conceived. The long delay in achieving reproducible success has, in large part, been the result of the conceptual simplicity of using agents that specifically 'target' tumor cells and they may thus selectively deliver cytotoxic agents. Emboldened by this apparent simplicity, early trials of radioimmunotherapy failed to consider the many variables involved in its implementation. As has been recently demonstrated using mathematical models of antibody delivery to solid tumor, chief among these may have been the failure to select the appropriate tumor type. By significantly reducing the problems associated with antibody delivery, hematopoietic malignancies offer the optimum conditions for successful radioimmunotherapy. As evinced by the wide range of antibody and radioactivity doses administered in the B-cell lymphoma trials, the case-specific nature of radioimmunotherapy requires an understanding of the relationship between the various input parameters and patient response. The complexity and interrelationship of these parameters precludes an experimental trial-and-error approach to their optimization. A stepwise approach to radioimmunotherapy treatment planning is proposed in which a model of antibody kinetics is developed and validated.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics of insulin internalization and processing in adipocytes: effects of insulin concentration

We have studied the effect of insulin concentration on the kinetics of insulin internalization and efflux in isolated rat adipocytes. To determine internalization rates adipocytes were incubated with 125I-insulin at 37 degrees C; and at frequent, early time points surface-bound and intracellular insulin were quantitated. Surface-bound and intracellular insulin were discriminated by the sensitivity of the former to rapid dissociation by a pH 3.0 buffer at 4 degrees C. From this data the endocytotic (internalization) rate constant (ke) was calculated for six insulin concentrations ranging from 0.3 to 100 ng/ml. Ke was found to decrease in an insulin concentration-dependent manner (P less than .001). Thus, values for ke were 0.121 +/- 0.006 min-1 versus 0.074 +/- 0.011 min-1 at 0.3 ng/ml and 100 ng/ml, respectively. The decrease in ke did not parallel insulin concentration-dependent changes in insulin receptor affinity indicating it was not the result of an inability of low affinity receptors to be internalized. The kinetics of insulin efflux were determined by loading various concentrations of 125I-insulin into the adipocyte interior, washing away surface-bound and extracellular insulin, and then monitoring the subsequent efflux of pre-loaded insulin into medium that contained the same concentration of insulin used in the loading step. The overall rate of efflux was independent of insulin concentration. In summary, these results show that at high insulin concentrations the efficiency of insulin internalization is impaired. In contrast, the rate of insulin efflux is unaffected.

Reassessment of fluid-phase endocytosis and diacytosis in monolayer cultures of human fibroblasts

We have investigated the kinetics of fluid-phase endocytosis and diacytosis in confluent monolayers of human fibroblasts by comparing the behavior of three markers that have been previously used to study this process: [14C]sucrose, 125I-labeled polyvinylpyrrolidone ([125I]PVP), and Lucifer Yellow. Three distinct kinetic compartments were observed with all markers. The first was relatively large (10-60 fl/cell), reached steady state within 15 min at 37 degrees C, and was rapidly lost from monolayers after removing the markers at 37 degrees C but not at 0 degree C. These properties indicate that this compartment is the same as that previously proposed to be the major intracellular compartment involved in diacytosis. However, this compartment is probably extracellular fluid trapped between cells since it is rapidly lost into the medium when the cells are either scraped or enzymatically removed from the culture dishes at 0 degree C. In addition, it very slowly undergoes both filling and emptying at 0 degree C. However, we did observe a second, much smaller, kinetic compartment (approximately 2 fl/cell) undergoing rapid diacytosis that does seem to be intracellular. A third compartment that we observed accumulates markers at a linear rate (10-20 fl cell-1 hr-1) and is not lost from cells even after incubation periods greater than 6 hr. The markers [14C]sucrose and [125I]PVP displayed very similar behavior with respect to all three compartments and yielded nearly linear long-term uptake rates, thus indicating that there is little if any absorbed component in their uptake. However, Lucifer Yellow displayed significantly higher incorporation rates and its uptake rate was strongly nonlinear, indicating its uptake in fibroblasts is predominantly adsorptive. Our observations indicate that the rate of fluid-phase endocytosis in fibroblasts is significantly less than previously reported and that any compartment involved in diacytosis is very small and turns over very rapidly. Significantly, we estimate that the constitutive internalization of clathrin-coated pits is sufficient to account for the majority of fluid-phase endocytosis and thus represents a major mechanism of membrane retrieval in these cells.