Low density lipoprotein receptors on epithelial cell (Madin-Darby canine kidney) monolayers. Asymmetric distribution correlates with functional difference

Low density lipoprotein (LDL) receptors are present on both the apical and basal surfaces of confluent monolayers of Madin-Darby canine kidney (MDCK) epithelial cells grown on gelatin-coated polycarbonate filters. Although there is only a single species of receptor protein present, as shown by immunoblotting, the receptors on the two surfaces were found to behave differently. LDL receptors on the basal surface show all of the characteristics of the LDL receptor described in fibroblasts in that their number is dependent upon the sterol (or LDL) content of the medium; however, regulation is only affected by LDL in the medium in contact with the basal side. In contrast, the apical surface LDL receptors are not regulated by the presence of LDL in the media on either the apical or basal surface. LDL particles can be transported across the monolayer in a temperature-dependent and -specific manner from the apical to the basal sides of the cell, but not in the opposite direction. The binding of 125I-LDL to both surfaces can be effectively inhibited not only by unlabeled LDL and very low density lipoprotein, but also by an antibody directed against the LDL receptor. The data suggest that the LDL receptors on the two aspects of the cell surface are biochemically identical, but differ in function. Thus, the basal surface receptor is involved in the control of cell cholesterol homeostasis, while that on the apical surface is responsible for the transport of LDL to the basal side.     

Transcytosis of epidermal growth factor. The epidermal growth factor receptor mediates uptake but not transcytosis

Madin-Darby canine kidney (MDCK) cells polarize and generate distinct apical and basolateral membrane domains when grown on permeable filter supports. Under these conditions, they transcytose fluid-phase markers. Recently, receptor-mediated transcytosis of epidermal growth factor (EGF) across MDCK cells has been reported (Maratos-Flier, E., Kao, C.-Y. Y., Verdin, E. M., and King, G. L. (1987) J. Cell Biol. 105, 1595-1601). We examined the role of the EGF receptor in this process. Transcytosis of EGF occurred only in the basolateral-to-apical direction, was time-dependent, and inhibited by the addition of unlabeled EGF in a concentration-dependent manner. In contrast to previous work, we found that only about 5% of basolaterally bound EGF was transported to the apical chamber. The half-time of transport was 90 min. A mutant cell line of MDCK, MDCKII-RCAr, was used to study the expression of the EGF receptor. Cell surface glycoproteins of these mutant cells can be efficiently labeled with [3H]galactose by exogalactosylation. The EGF receptor was found to be expressed only on the basolateral surface. Addition of EGF to the basolateral medium resulted in rapid internalization and degradation of the receptor. Testing directly for transcytosis of basolateral glycoproteins, we detected several proteins transported across the cell. The EGF receptor, however, was not among this group of proteins. Taking these results together, we suggest the following model. Internalization of EGF on the basolateral surface is mediated by the EGF receptor. EGF dissociates from the receptor in an endocytic compartment. A fraction of the EGF is then diverted nonselectively to the transcytotic pathway, as found for other fluid-phase markers previously (Bomsel, M., Prydz, K., Parton, R. G., Gruenberg, J., and Simons, K. (1989) J. Cell Biol. 109, 3243-3258.     

Inhibition of insulin and epidermal growth factor (EGF) receptor autophosphorylation by a human polyclonal IgG

The immunoglobulin G of a polyclonal antiserum (pIgG) from a patient with insulin resistance and hypoglycemia was tested for its ability to inhibit insulin binding and to affect the autophosphorylation of partially-purified insulin receptors extracted from rat liver membranes. pIgG, when added 4 hr prior to insulin, inhibited subsequent insulin binding by 50% at 30 micrograms added protein; however, insulin previously bound to the receptor could not be displaced by a 4 hr subsequent exposure of up to 70 micrograms pIgG. pIgG, independent of its effect on insulin binding, inhibited both basal and insulin-stimulated autophosphorylation of the insulin receptor in a dose-dependent manner with a half maximal effect at 3.3 to 7 micrograms protein. Furthermore, pIgG also reduced basal autophosphorylation of the EGF receptor. The effect of pIgG to inhibit basal autophosphorylation of insulin and EGF receptors, together with its ability to reduce autophosphorylation of insulin receptors fully occupied by insulin, imply that the effect of pIgG on receptor autophosphorylation is largely independent of its effect on ligand binding. Moreover, these findings suggest that pIgG may inhibit autophosphorylation by acting on domains which are similar in the insulin and EGF receptors.     

Polarized entry of canine parvovirus in an epithelial cell line.

The binding and uptake of canine parvovirus (CPV) in polarized epithelial cells were investigated by growing the cells on a permeable support and inoculating with the virus either from the apical or basolateral surface. Binding of radiolabeled CPV occurred preferentially on the basolateral surface. In contrast, when a similar experiment was carried out on nonpolarized A72 cells, virus binding occurred regardless of the direction of virus input. Binding appeared to be specific for CPV and could not be competitively inhibited by either bovine or porcine parvovirus. Analysis of the binding data revealed a high-affinity receptor (10(5) per cell) for CPV on the basolateral surfaces of MDCK cells (Kd, 29 pM). In indirect immunofluorescence studies, virus entered only from the basolateral surfaces of MDCK cells. These results provide evidence for a functional CPV-specific receptor that is expressed only on the basolateral surfaces of polarized epithelial cells, a result that has interesting consequences for viral pathogenesis.     

Functional expression of intrinsic factor-cobalamin receptor by renal proximal tubular epithelial cells.

Previous studies from our laboratory (Seetharam, B., Levine, J. S., Ramasamy, M., and Alpers, D. H. (1988) J. Biol. Chem. 263, 4443-4449; Fyfe, J. C., Ramanujam, K. S., Ramaswamy, K., Patterson, D. F., and Seetharam, B. (1991) J. Biol. Chem. 266, 4489-4494) have identified and isolated a 230-kDa receptor from rat and canine kidney which binds with high affinity [57Co]cyanocobalamin (Cbl) complexed to gastric intrinsic factor (IF). Although these studies have identified a renal receptor which binds intrinsic factor-cobalamin (IFCR), it is not known whether the binding is specific for IF-Cbl and whether renal cells internalize [57Co]Cbl bound to IF and transport [57Co]Cbl across the cell. Using a variety of renal cells, our results show that IF-[57Co]Cbl binding activity is detected in proximal tubular-derived epithelial cells from opossum (OK) and porcine kidney (LLC-PK1) but not in distal tubular-derived cells from canine kidney cells (MDCK). Metabolic labeling studies with Tran 35S-label confirmed the presence of a 230-kDa IFCR in OK and LLC-PK1 cells. Cell surface labeling and binding studies demonstrated that IFCR is targeted to the apical membrane. This apical expression of IFCR in OK cells is inhibited by the microtubule-disruptive drugs, colchicine and nocodazole. Opossum kidney cells when grown on culture inserts are polarized and transport [57Co]Cbl only when bound to IF and not to other Cbl binders. Furthermore, the transport of [57Co]Cbl occurred unidirectionally from the apical to the basolateral surface. Treatment of cells with colchicine or nocodazole inhibited the surface binding of IF-[57Co]Cbl as well as the transcytosis of [57Co]Cbl by 70-75%. IFCR retained intracellualarly by incubation of cells with colchicine or nocodazole is degraded by leupeptin-sensitive proteases. Based on these results, we suggest that proximal tubular-derived epithelial cells transport [57Co]Cbl bound to IF in a saturable way via receptor-mediated endocytosis.     

Apical Enrichment of Human EGF Precursor in Madin-Darby Canine Kidney Cells Involves Preferential Basolateral Ectodomain Cleavage Sensitive to a Metalloprotease Inhibitor

EGF precursor (proEGF) is a member of the family of membrane-anchored EGF-like growth factors that bind with high affinity to the epidermal growth factor receptor (EGFR). In contrast to human transforming growth factor-α precursor (proTGFα), which is sorted basolaterally in Madin-Darby canine kidney (MDCK) cells (Dempsey, P., and R. Coffey, 1994. J. Biol. Chem. 269:16878–16889), we now demonstrate that human proEGF overexpressed in MDCK cells is found predominantly at the apical membrane domain under steady-state conditions. Nascent proEGF (185 kD) is not sorted but is delivered equally to the apical and basolateral membranes, where it is proteolytically cleaved within its ectodomain to release a soluble 170-kD EGF form into the medium. Unlike the fate of TGFα in MDCK cells, the soluble 170-kD EGF species accumulates in the medium, does not interact with the EGFR, and is not processed to the mature 6-kD peptide. We show that the rate of ectodomain cleavage of 185-kD proEGF is fourfold greater at the basolateral surface than at the apical surface and is sensitive to a metalloprotease inhibitor, batimastat. Batimastat dramatically inhibited the release of soluble 170-kD EGF into the apical and basal medium by 7 and 60%, respectively, and caused a concordant increase in the expression of 185-kD proEGF at the apical and basolateral cell surfaces of 150 and 280%, respectively. We propose that preferential ectodomain cleavage at the basolateral surface contributes to apical domain localization of 185-kD proEGF in MDCK cells, and this provides a novel mechanism to achieve a polarized distribution of cell surface membrane proteins under steady-state conditions. In addition, differences in disposition of EGF and TGFα in polarized epithelial cells offer a new conceptual framework to consider the actions of these polypeptide growth factors.     

Caco-2 cell line: a system for studying intestinal iron transport across epithelial cell monolayers.

Iron transport across polarized intestinal epithelium was studied by using Caco-2 cells grown in bicameral chambers. When cells were grown under conditions of low, normal, or high iron concentration not only was the iron content of the cells markedly altered but the low iron cells exhibited a nearly 2-fold increase in transepithelial electrical resistance (TEER). 59Fe uptake from the apical surface into cells and transport into the basal chamber was affected both by the valency of the iron and the iron status of the cells. Uptake from 59Fe(II)-ascorbate was about 600 pmol 59Fe/h per mg protein, increased about 2-fold in low iron cells, and was about 13-200-fold greater than uptakes from 59Fe(III) chelated to nitrilotriacetic acid, BSA, or citrate. Transport into the basal chamber from 59Fe(II)-ascorbate was 3.7 +/- 1.7 pmol/h per cm2 for Fe-deficient cells vs. 0.72 +/- 0.1 pmol/h per cm2 for normal-Fe cells and from 59Fe(III)-BSA 1.1 +/- 0.2 pmol/h per cm2 vs. 0.3 +/- 0.03 pmol/h per cm2 for deficient vs. normal iron cells, respectively. The greater transport of iron both from Fe(II) and in iron deficient cells supports the use of the Caco-2 cells as a model for iron transport.     

Urokinase-Type Plasminogen Activator Receptor Is Internalized by Different Mechanisms in Polarized and Nonpolarized Madin–Darby Canine Kidney Epithelial Cells

Accumulated data indicate that endocytosis of the glycosylphosphatidyl-inositol-anchored protein urokinase plasminogen activator receptor (uPAR) depends on binding of the ligand uPA:plasminogen activator inhibitor-1 (PAI-1) and subsequent interaction with internalization receptors of the low-density lipoprotein receptor family, which are internalized through clathrin-coated pits. This interaction is inhibited by receptor-associated protein (RAP). We show that uPAR with bound uPA:PAI-1 is capable of entering cells in a clathrin-independent process. First, HeLa^K44A cells expressing mutant dynamin efficiently internalized uPA:PAI-1 under conditions in which transferrin endocytosis was blocked. Second, in polarized Madin–Darby canine kidney (MDCK) cells, which expressed human uPAR apically, the low basal rate of uPAR ligand endocytosis, which could not be inhibited by RAP, was increased by forskolin or phorbol ester (phorbol 12-myristate 13-acetate), which selectively up-regulate clathrin-independent endocytosis from the apical domain of epithelial cells. Third, in subconfluent nonpolarized MDCK cells, endocytosis of uPA:PAI-1 was only decreased marginally by RAP. At the ultrastructural level uPAR was largely excluded from clathrin-coated pits in these cells and localized in invaginated caveolae only in the presence of cross-linking antibodies. Interestingly, a larger fraction of uPAR in nonpolarized relative to polarized MDCK cells was insoluble in Triton X-100 at 0°C, and by surface labeling with biotin we also show that internalized uPAR was mainly detergent insoluble, suggesting a correlation between association with detergent-resistant membrane microdomains and higher degree of clathrin-independent endocytosis. Furthermore, by cryoimmunogold labeling we show that 5–10% of internalized uPAR in nonpolarized, but not polarized, MDCK cells is targeted to lysosomes by a mechanism that is regulated by ligand occupancy.     

Stimulated release of arachidonate and prostaglandins is vectorial in MDCK epithelial cells.

The receptor mediated activation of phospholipase A2 by appropriate ligands results in the synthesis and release of eicosanoids, a class of potent bioregulatory molecules. Madin-Darby canine kidney cells (MDCK) are polarized epithelial cells, with structurally and functionally distinct plasma membrane domains separated by tight junctions. Using MDCK cells grown in dual sided chambers, we show in this report, that a) the receptor mediated release of prostaglandins and arachidonate into the extracellular medium is predominantly unidirectional, b) the direction of release is agonist specific, and c) the magnitude of the response due to a given agonist is cell-domain specific. These characteristics, if operative in vivo, would contribute towards the optimal function of trans-cellular metabolism of eicosanoids already demonstrated.     

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.     

Receptor remodeling and regulation in the action of epidermal growth factor

Epidermal growth factor (EGF) initiates a wide variety of events when added to responsive cultured cells. These range from early events requiring only brief exposure to EGF, e.g., stimulation of transport of amino acids or ions, to later events such as commitment of cells to a round of DNA synthesis, a process requiring 6 h or more of continuous exposure to hormone. EGF binding is followed first by phosphorylation of EGF receptors, which can be detected in purified membranes and permeabilized cells, and then by internalization and proteolytic processing of receptors in lysosomes. Native 160,000-dalton EGF receptors contain a site that is not exposed on the cell surface and is highly sensitive to cleavage by an endogenous protease, which yields a 145,000-dalton receptor fragment that retains phosphate acceptor activity. Cleavage of receptor at a trypsin-sensitive site, also not exposed to the cell surface, yields a 115,000-dalton fragment that binds EGF, but contains no phosphorylated species. The data indicate that the phosphate acceptor sites on EGF receptors are localized on a 45,000-dalton cytosolic region.     

Modulation of the mitogenic response of an epidermal growth factor-dependent keratinocyte cell line by dexamethasone, insulin, and transforming growth factor-beta

The stimulation of DNA synthesis by epidermal growth factor (EGF) has been studied for a cell line having properties useful for investigating the mechanism of action of EGF in epithelial cell populations. These studies employ a mouse keratinocyte cell line (MK), isolated by Weissman and Aaronson (1983), which is stringently dependent on exogenous EGF for growth in serum containing medium. The studies reported here characterize the compliment of EGR receptors present on the surface of MK cells and demonstrate the regulatory influence of other hormones on the capacity of EGF to stimulate DNA synthesis. Up-regulated MK cells contain approximately 22,000 EGF receptors per cell, but when the cells are grown in the presence of EGF the receptor number is reduced to about 4,000. It is estimated that only a small number of high-affinity receptors (less than 500) are required for EGF-dependent cell proliferation. In contrast to its action in fibroblastic cells, dexamethasone is a strong inhibitor of EGF-stimulated DNA synthesis of MK cells. Insulin at high concentrations, or insulin-like growth factors I or II (IGF-I, IGF-II) at physiological concentrations, synergistically enhance the EGF response. Interestingly, insulin or IGF-I or II are also able to reverse most of the dexamethasone inhibition of DNA synthesis. Transforming growth factor-beta (TGF-beta) inhibits, in reversible manner, the EGF stimulation of DNA synthesis and this inhibition is not overcome by insulin. TGF-beta receptors have been measured in MK cells and Scatchard analysis indicates approximately 20,000 receptors per cell. None of the modulatory hormones (insulin, dexamethasone, TGF-beta) significantly altered 125I-EGF binding characteristics in MK cells, suggesting a point of action distal to 125I-EGF binding.     

Receptors for epidermal growth factor and insulin-like growth factor-I on preimplantation trophoderm of the pig.

125I-labelled epidermal growth factor (125I-EGF) and 125I-labelled insulin-like growth factor-I (125I-IGF-I) bound to trophoderm cells from pig blastocysts obtained on days 15-19 of pregnancy. Specific binding was detected on freshly isolated cell suspensions and on cells cultured for several days. The binding of 125I-EGF was inhibited by increasing concentrations of EGF, but not by various other growth factors and hormones. Chemical cross-linking of 125I-EGF to its receptors using disuccinimidyl suberate (DSS) revealed a radiolabelled band of relative molecular mass 160,000, similar to that identified as the EGF receptor in other cell types. The binding of 125I-IGF-I was inhibited by both IGF-I and insulin, indicating that the receptors were either type I IGF receptors or insulin receptors. Cross-linking of 125I-IGF-I to serum-free supernatants from trophoderm cultures showed that the cells secreted an IGF-binding protein, giving a complex of relative molecular mass about 45,000. The presence of receptors for EGF and IGF/insulin suggests that these factors could be involved in regulating the growth and development of the early blastocyst.     

Stimulated release of arachidonate and prostaglandins is vectorial in MDCK epithelial cells

The receptor mediated activation of phospholipase A₂ by appropriate ligands results in the synthesis and release of eicosanoids, a class of potent bioregulatory molecules. Madin-Darby canine kidney cells (MDCK) are polarized epithelial cells, with structurally and functionally distinct plasma membrane domains separated by tight junctions. Using MDCK cells grown in dual sided chambers, we show in this report, that a) the receptor mediated release of prostaglandins and arachidonate into the extracellular medium is predominantly unidirectional, b) the direction of release is agonist specific, and c) the magnitude of the response due to a given agonist is cell-domain specific. These characteristics, if operative in vivo, would contribute towards the optimal function of trans-cellular metabolism of eicosanoids already demonstrated.     

Transcellular processing of disulfide- and thioether-linked peroxidase--polylysine conjugates in cultured MDCK epithelial cells

Horseradish peroxidase (HRP) was conjugated to nondegradable polycationic poly(D-lysine) (PDL) through either a thioether (HRP-S-PDL) or a disulfide (HRP-SS-PDL) linkage. The binding and transcytosis of these conjugates was studied in Madin-Darby canine kidney (MDCK) cell monolayers grown on 3-microns microporous polycarbonate filters. Conjugation of HRP to PDL with both linkages markedly increased the binding of this protein onto the cell monolayers. However, an enhancement of the transcellular transport of HRP in both apical-to-basal and basal-to-apical directions was observed only in HRP-SS-PDL, but not in HRP-S-PDL. HRP-SS-PDL transport was inhibited by colchicine and by 4 degrees C incubation. The transport of 14C-sucrose was not affected by the presence of conjugates. These results indicate that the transport of the conjugate across the cell monolayers was due to a transcellular process rather than to any leakage of the cell junction caused by polycations. The disulfide linkage between HRP and PDL was cleaved rapidly at the basal and, to a lesser extent, at the apical surface of the cell. Neuraminidase treatment decreased the binding of the conjugates onto the cell surface, but did not decrease the transcellular transport, suggesting that not all surface-bound conjugates were available for transcytosis. These results demonstrate that disulfide linkages can be cleaved during transcytosis in MDCK cells. The cleavage, however, occurs mostly at the binding site on the cell surface, which may prevent the cellular uptake of the intact conjugate.     

Cytoplasmic domains determine signal specificity, cellular routing characteristics and influence ligand binding of epidermal growth factor and insulin receptors.

The cell surface receptors for insulin and epidermal growth factor (EGF) both employ a tyrosine-specific protein kinase activity to fulfil their distinct biological roles. To identify the structural domains responsible for various receptor activities, we have generated chimeric receptor polypeptides consisting of major EGF and insulin receptor structural domains and examined their biochemical properties and cellular signalling activities. The EGF-insulin receptor hybrids are properly synthesized and transported to the cell surface, where they form binding competent structures that are defined by the origin of their extracellular domains. While their ligand binding affinities are altered, we find that these chimeric receptors are fully functional in transmitting signals across the plasma membrane and into the cell. Thus, EGF receptor and insulin receptor cytoplasmic domain signalling capabilities are independent of their new heterotetrameric or monomeric environments respectively. Furthermore, the cytoplasmic domains carry the structural determinants that define kinase specificity, mitogenic and transforming potential, and receptor routing.     

Degradation of insulin receptors by hepatoma cells: insulin-induced down-regulation results from an increase in the rate of basal receptor degradation

The degradation of insulin receptors was studied in cultured Zajdela hepatoma cells (ZHC). Receptor distribution within the cell was evaluated by estimating: i) surface receptor level on entire cells, ii) total cell receptors solubilized by Triton from cell membranes and iii) intracellular receptors solubilized from cells whose surface receptors had been inactivated with trypsin. In the absence of insulin, 80-90% of the insulin binding sites were located on the cell surface. When insulin was added, a rapid decrease of surface receptors was observed. After 2 h, their level was reduced nearly by half; this reduction was accounted for by an actual receptor loss from the cell without an increase in the intracellular pool. These results indicate that insulin enhanced the rate of receptor degradation within the cell. Basal receptor inactivation was studied by using tunicamycin which inhibits new receptor synthesis. The surface receptor number was decreased with a half-life of 7 h, while the level of internal sites remained unchanged. Both basal and insulin-activated receptor degradation were markedly slowed down by chloroquine or dansylcadaverine, indicating the importance of endocytic pathways in this process. Similarly, when de novo protein glycosylation was inhibited for 24 h by tunicamycin, both basal and insulin-activated receptor inactivation were precluded.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ligand-induced activation of A431 cell epidermal growth factor receptors occurs primarily by an autocrine pathway that acts upon receptors on the surface rather than intracellularly

A431 cells express high numbers of epidermal growth factor (EGF) receptors and produce a ligand for these receptors, transforming growth factor-alpha (TGF-alpha). We have obtained evidence that the EGF receptors on these cells may be activated through an "autocrine" pathway by ligand and have investigated whether activation of phosphorylation of the receptor by the endogenously produced TGF-alpha occurs intracellularly or at the cell surface. When A431 cells were cultured under serum-free conditions, in the absence of exogenous ligand, EGF receptors were found to have a basal level of phosphorylation. When cells were labeled by culturing with 32Pi in the continuous presence of monoclonal antibodies that block binding of TGF-alpha to the EGF receptor, phosphorylation decreased to 30 +/- 10% of the basal level. This reduction could not be accounted for by the decrease in receptor content attributable to down-regulation and catabolism of EGF receptors that resulted from the binding of anti-receptor monoclonal antibodies. The reduction in receptor phosphorylation mediated by antibody was accompanied by the accumulation of increased levels of secreted TGF-alpha species in the culture medium. We also pulse-labeled A431 cells for 15 min with [35S]cysteine and immunoprecipitated the cell lysate with anti-phosphotyrosine antibody after various chase periods. Tyrosine-phosphorylated EGF receptor became detectable after 40 min of chase and reached a maximum after 4-6 h; these times are in agreement with the intervals required for EGF receptors to reach the cell surface after synthesis and then to achieve maximal expression. In addition, only the 170-kDa, mature EGF receptor species, and not the 160-kDa intracellular precursor, was immunoprecipitated with the anti-phosphotyrosine antibody. The results of these pulse-chase experiments and the finding that anti-receptor monoclonal antibody can block receptor phosphorylation suggest that activation of EGF receptors can result from the binding of an endogenous ligand (presumably TGF-alpha), which occurs at the cell surface and not during receptor biosynthesis and intracellular processing.