Tyrosine-specific phosphorylation of the epidermal growth factor (EGF) receptor in hormonally stimulated A431 cells is blocked by three chemically distinct classes of tumor promoters. Tumor-promoting esters of the diterpene phorbol (phorbol 12-myristate 13-acetate, P-phorbol 12,13-dibutyrate, and (3-phorbol 12,13-didecanoate), indole alkaloids (teleocidin and lyngbyatoxin A), and polyacetates (aplysiatoxin and debromoaplysiatoxin) all inhibited EGF-stimulated phosphorylation of the receptor. Non-tumor-promoting analogs (,Bphorbol, a-phorbol 12,13-didecanoate, and hydrolyzed teleocidin) had no effect on the levels of receptor phosphorylation.The ED5o values of the inhibitory effect (0.1-3 ng/ml) reflected the relative tumor-promoting abilities of these compounds in vivo. None of the tumor promoters tested significantly de-creased the overall specific binding of 125I-labeled EGF to A431 cells. Scatchard analysis, however, revealed two apparent EGF receptors in this cell type. The dose-responses for tumor-promoter inhibition of EGF receptor tyrosine phosphorylation and high-affinity EGF binding were similar, suggesting that the same initial event is responsible for both effects.This demonstrates a correlation between modulation of EGF receptor binding and phosphorylation of tyrosine by tumor promoters. The data suggest a possible role for protein kinase C, the putative cellular receptor for these tumor promoters, in the mechanism of action.Tumor promoters can modulate the action of epidermal growth factor (EGF) by reducing EGF receptor binding (1-4) and internalization (5, 6) and by potentiating the mitogenic activity of EGF in quiescent cells (7,8). Upon binding to cells, EGF stimulates the tyrosine phosphorylation of its receptors (9, 10) via a receptor-associated kinase activity. The potential regulatory role of tyrosine kinase activity is suggested by its association with the action of other growth factors (insulin and platelet-derived growth factor) (11-13) and its apparent requirement for oncogenic transformation by a number of retroviruses (14,15). This led us to examine whether the effects of tumor promoters on the action of EGF involved changes in receptor phosphorylation.We have found that three chemically distinct classes of tumor promoters [esters of the diterpene phorbol, indole alkaloids (16), and polyacetates (17) 32Pi Labeling and Detergent Extraction of Cells. Low-density cultures (105 cells per 35-mm well) of A431 cells were preincubated with 32p; at 0.5 mCi/ml in phosphate-free DME medium plus dialyzed heat-inactivated fetal calf serum for 3-4 hr at 37°C. Dimethyl sulfoxide (Me2SO; final concentration 1%), tumor promoters, or nonpromoting analogs (dissolved in Me2SO) were added for 10 min. EGF was then added for an additional 40 min. The cultures were placed on ice, the binding media were removed, and the cells were scraped and extracted with Tris-buffered Triton (10 mM Tris'HCl/1% Triton X-100/10 mM NaCl/5 mM EDTA/1 mM phenylmethylsulfonyl fluoride/0.1% bovine serum albumin, pH 8).Phosph...

The objective of these studies was to characterize the macrophage mannose receptor binding and pharmacological properties of carbohydrate remodeled human placental-derived and recombinant β-glucocerebrosidase (pGCR and rGCR, respectively). These are similar but not identical molecules that were developed as enzyme replacement therapies for Gaucher disease. Both undergo oligosaccharide remodeling during purification to expose terminal mannose sugar residues. Competitive binding data indicated carbohydrate remodeling improved targeting to mannose receptors over native enzyme by two orders of magnitude. Mannose receptor dissociation constants (Kd) for pGCR and rGCR were each 13 nmol/L. At 37°C, 95% of the total macrophage binding was mannose receptor specific. In vivo, pGCR and rGCR were cleared from circulation by a saturable pathway. The serum half-life (t1/2) was 3 minutes when less than saturable amounts were injected intravenously (IV) into mice. Twenty minutes postdose, β-glucocerebrosidase activity increased over endogenous levels in all tissues examined. Fifty percent of the injected activity was recovered. Ninety-five percent of recovered activity was in the liver. Parenchymal cells (PC), Kupffer cells (KC), and liver endothelium cells (LEC) were responsible for 75%, 22%, and 3%, respectively, of the hepatocellular uptake of rGCR and for 76%, 11%, and 12%, respectively, of the hepatocellular uptake of pGCR. Both molecules had poor stability in LEC and relatively long terminal half-lives in PC (t1/2 = 2 days) and KC (t1/2 = 3 days).

Phosphorylation of the epidermal growth factor (EGF) receptor following activation of protein kinase C appears to negatively regulate EGF binding and the receptorassociated tyrosine kinase activity. We The epidermal growth factor (EGF) receptor is a tyrosine kinase that is utilized by a variety of cells to mediate cell proliferation. There are several lines of evidence that suggest that activation of the tyrosine kinase of the EGF receptor is essential for triggering a mitogenic response. When this tyrosine kinase activity is abolished by site-directed mutagenesis, EGF fails to stimulate cell division (1). Conversely, there is a strong correlation between constitutive activation of this tyrosine kinase activity and cell transformation. Constitutive activation can result from truncation ofthe EGF receptor or autocrine or paracrine stimulation of the EGF receptor by EGF or transforming growth factor type a.In normal cells, the EGF receptor tyrosine kinase appears to be tightly controlled. Activation not only requires EGF binding but is dependent on the state of phosphorylation of the EGF receptor. Autophosphorylation of tyrosine residues appears to play a role in activation of the receptor-associated tyrosine kinase activity (2). Negative control can occur through increases in serine and/or threonine phosphorylation. For example, treatment with phorbol 12-myristate 13-acetate (PMA)-type tumor promoters causes inhibition of high-affinity EGF receptor binding and inhibition of EGFstimulated tyrosine kinase activity in vivo (3-6). These effects are associated with increased phosphorylation of several serine and threonine residues of the receptor.The actual sites of phosphorylation on the EGF receptor that are responsible for regulating the biological properties of the molecule have not been resolved. One residue, threonine-654, has received much attention because it appears to be uniquely phosphorylated by activators of protein kinase C (7-9). Although there are conflicting reports in the literature, recent site-directed mutagenesis experiments suggest that threonine-654 is required for protein kinase C-dependent inhibition of the EGF receptor kinase but not EGF binding (refs. 10-12; data not shown). Since threonine-654 is neither the only site nor the major site on the EGF receptor that is phosphorylated when cells are treated with tumor promoters or other activators of protein kinase C (7, 9, 13), it is possible that alternative sites of phosphorylation are important in regulating high-affinity EGF receptor binding and/or the ability of EGF to stimulate the EGF receptor tyrosine kinase.One approach for investigating the role of threonine-654 in EGF receptor modulation is to identify pathways other than protein kinase C that alter EGF receptor properties. Several growth factors, such as platelet-derived growth factor (PDGF) (14), factors from v-sis transformed cells (15), acidic fibroblast-derived growth factor (16), and basic fibroblastderived growth factor (unpublished data) have been shown to inhibit EG...

The objective of these studies was to characterize the macrophage mannose receptor binding and pharmacological properties of carbohydrate remodeled human placental-derived and recombinant β-glucocerebrosidase (pGCR and rGCR, respectively). These are similar but not identical molecules that were developed as enzyme replacement therapies for Gaucher disease. Both undergo oligosaccharide remodeling during purification to expose terminal mannose sugar residues. Competitive binding data indicated carbohydrate remodeling improved targeting to mannose receptors over native enzyme by two orders of magnitude. Mannose receptor dissociation constants (Kd) for pGCR and rGCR were each 13 nmol/L. At 37°C, 95% of the total macrophage binding was mannose receptor specific. In vivo, pGCR and rGCR were cleared from circulation by a saturable pathway. The serum half-life (t1/2) was 3 minutes when less than saturable amounts were injected intravenously (IV) into mice. Twenty minutes postdose, β-glucocerebrosidase activity increased over endogenous levels in all tissues examined. Fifty percent of the injected activity was recovered. Ninety-five percent of recovered activity was in the liver. Parenchymal cells (PC), Kupffer cells (KC), and liver endothelium cells (LEC) were responsible for 75%, 22%, and 3%, respectively, of the hepatocellular uptake of rGCR and for 76%, 11%, and 12%, respectively, of the hepatocellular uptake of pGCR. Both molecules had poor stability in LEC and relatively long terminal half-lives in PC (t1/2 = 2 days) and KC (t1/2 = 3 days).

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