Modulation of the insulin growth factor II/mannose 6-phosphate receptor in microvascular endothelial cells by phorbol ester via protein kinase C

Phosphorylation of hormone receptors by protein kinase C (PKC) may be involved in the regulation of receptor recycling. We have studied the recycling and the phosphorylation state of the insulin growth factor (IGF) II/mannose 6-phosphate (Man-6-P) receptor in microvascular endothelial cells from rat adipose tissue. Scatchard analysis showed these cells have over 2 x 10(6) receptors/cell with an affinity constant of 1 x 10(9) M-1. In the presence of phorbol myristate acetate (PMA), an activator of PKC and analog of diacylglycerol, IGF-II receptor number increased in the plasma membrane by 60% without changes in the binding affinity. This increase in cell surface receptor number was confirmed by affinity cross-linking and 125I-surface labeling studies, occurred with a half-time of 20 min, and was reversible upon withdrawal of PMA. The redistribution of IGF-II/Man-6-P receptors was not due to an inhibition of internalization which was in fact stimulated by PMA. The effect of PMA on IGF-II receptor recycling correlated with its stimulation of PKC activity. Furthermore, after down-regulation of cellular PKC levels by preincubation with PMA, PMA was unable to activate residual PKC activity in the membranous pool or increase IGF-II receptor number at the cell surface. The phosphorylation state of the IGF-II/Man-6-P receptor was determined by 32P labeling of intact cells and immunoprecipitation with anti-receptor antibodies. In the basal state, the receptor was phosphorylated only on serine residues which was increased by 75% after treatment with PMA. In contrast, IGF-II decreased receptor phosphorylation and plasma membrane binding in a parallel and dose-dependent manner. Thus, PKC-stimulated serine phosphorylation of IGF-II/Man-6-P receptor may promote the translocation of the receptor to the cell surface, whereas IGF-II-stimulated dephosphorylation of the receptor may lead to a decrease in the number of cell surface receptors. These data suggest a role for PKC-mediated serine phosphorylation in the regulation of intracellular trafficking of receptors in endothelial cells.

The insulin receptor with phenylalanine replacing tyrosine-1146 provides evidence for separate signals regulating cellular metabolism and growth

We have studied the function of a mutant insulin receptor (IR) molecule in which Tyr-1146, one of the first autophosphorylation sites in the beta subunit, was replaced with phenylalanine (IRF1146). Autophosphorylation of the partially purified IRF1146 was reduced 60-70% when compared to the wild-type IR but was still stimulated by insulin. The phosphotransferase activity of the dephospho form of both the IR and IRF1146 toward exogenous substrates was stimulated 3- to 4-fold by insulin. However, the wild-type IR was activated 12-fold by autophosphorylation, whereas the IRF1146 was activated only 2-fold. When the IRF1146 was expressed in Chinese hamster ovary (CHO) cells, insulin binding was normal, whereas autophosphorylation was reduced 80% when compared to cells expressing the wild-type IR. Endogenous substrates of the insulin receptor kinase were not detected during insulin stimulation of CHO cells expressing the IRF1146. Moreover, the IRF1146 did not internalize insulin rapidly or stimulate DNA synthesis in the presence of insulin. In contrast, both the IR and IRF1146 stimulated glycogen synthase equally in CHO cells. These data suggest that activation of the IR tyrosine kinase can be resolved into two components: the first is dependent on insulin binding and the second is dependent on the subsequent insulin-stimulated autophosphorylation cascade. Thus, at least two signal transduction pathways diverging from the IR are implicated in the mechanism of insulin action.

Binding kinetics of mutated insulin receptors in transfected cells grown in suspension culture: application to the Tyr----Phe 960 insulin receptor mutant

Site-directed mutagenesis of the insulin receptor cDNA is now widely used to elucidate the role of various domains and residues of the receptor, particularly in order to examine the functional importance of the beta chain-associated tyrosine kinase. However, little has been done to correlate the functional repercussions of such mutations with alterations in the complex insulin binding kinetics. This is due in part to the difficulty of conducting large scale experiments using transfected cells on culture dishes. In an effort to overcome this problem, we have developed a method for culturing Chinese hamster ovary (CHO) cells in suspension culture, which provides a large number of cells and obviates the need for enzymatic or mechanical detachment of cells. The feasibility of this approach is demonstrated in a detailed study of the kinetics of insulin binding to the Tyr----Phe 960 insulin receptor mutant.

Hepatic phosphotyrosine phosphatase activity and its alterations in diabetic rats

Phosphotyrosine phosphatase (PTPase) activity in rat liver was measured using a phosphopeptide substrate containing sequence identity to the major site of insulin receptor autophosphorylation. PTPase activity was detected in both cytosolic and particulate fractions of rat liver and produced linear dephosphorylation over a 15-min time course. In rats made insulin-deficient diabetic by streptozotocin treatment (STZ), cytosolic PTPase activity increased to 180% of the control values after 2 d of diabetes and remained elevated at 30 d (P less than 0.02). Gel filtration on Sephadex-75 revealed a single peak of activity in the cytosol in both control and diabetic animals and confirmed the increased levels. In BB diabetic rats, another model of insulin deficiency, the PTPase activity in the cytosolic fraction was increased to approximately 230% of control values. PTPase activity in the particulate fraction of liver was also increased by 30 and 80% after 2 and 8 d of STZ diabetes, respectively. However, this increase was not sustained and after 30 d of STZ diabetes, PTPase activity associated with the particulate fraction in the BB diabetic rat was reduced to approximately 70% of the control levels. Treatment of STZ diabetic rats with subcutaneous insulin or vanadate in their drinking water for 3 d reduced tyrosine PTPase activity in the particulate, but not in the cytosolic fraction. This was associated with a change in blood glucose toward normal. These data indicate insulin deficient diabetes is accompanied by significant changes in hepatic PTPase activity. Since tyrosine phosphorylation plays a central role in the cellular action of insulin receptor, an increase in PTPase activity may be an important factor in the altered insulin action associated with these diabetic states.

Tyrosine phosphorylation of the insulin receptor is not required for receptor internalization: studies in 2,4-dinitrophenol-treated cells

The relation between insulin-stimulated autophosphorylation of the insulin receptor and internalization of the receptor was studied in Fao rat hepatoma cells. Treatment of Fao cells with 2,4-dinitrophenol for 45 min depleted cellular ATP by 80% and equally inhibited insulin-stimulated receptor autophosphorylation, as determined by immunoprecipitation of surface-iodinated or [32P]phosphate-labeled cells with anti-phosphotyrosine antibody. In contrast, internalization of the insulin receptor and internalization and degradation of 125I-labeled insulin by 2,4-dinitrophenol-treated cells were normal. These data show that autophosphorylation of the insulin receptor is not required for the receptor-mediated internalization of insulin in Fao cells and suggest that insulin receptor recycling is independent of autophosphorylation.

Tyrosine phosphorylation of the insulin receptor during insulin-stimulated internalization in rat hepatoma cells

We have studied the phosphorylation state of the insulin receptor during receptor-mediated endocytosis in the well-differentiated rat hepatoma cell line Fao. Insulin induced the rapid internalization of surface-iodinated insulin receptors into a trypsin-resistant compartment, with a 3-fold increase in the internalization rate over that seen in the absence of insulin. Within 20 min of insulin stimulation, 30-35% of surface receptors were located inside the cell. This redistribution was half-maximal by 10.5 min. Similar results were obtained when the loss of surface receptors was measured by 125I-insulin binding. Tyrosyl phosphorylation of internalized insulin receptors was measured by immunoprecipitation with antiphosphotyrosine antibody. Immediately after insulin stimulation, 70-80% of internalized receptors were tyrosine phosphorylated. Internalized receptors persisted in a phosphorylated state after the dissociation of insulin but were dephosphorylated prior to their return to the plasma membrane. After 45-60 min of insulin stimulation, the tyrosine phosphorylation of the internal receptor pool decreased by 45%, whereas the phosphorylation of surface receptors was unchanged. These data suggest that insulin induces the internalization of phosphorylated insulin receptors into the cell and that the phosphorylation state of the internal receptor pool may be regulated by insulin.

Mutation of the insulin receptor at tyrosine 960 inhibits signal transmission but does not affect its tyrosine kinase activity

Tyrosyl phosphorylation is implicated in the mechanism of insulin action. Mutation of the beta-subunit of the insulin receptor by substitution of tyrosyl residue 960 with phenylalanine had no effect on insulin-stimulated autophosphorylation or phosphotransferase activity of the purified receptor. However, unlike the normal receptor, this mutant was not biologically active in Chinese hamster ovary cells. Furthermore, insulin-stimulated tyrosyl phosphorylation of at least one endogenous substrate (pp185) was increased significantly in cells expressing the normal receptor but was barely detected in cells expressing the mutant. Therefore, beta-subunit autophosphorylation was not sufficient for the insulin response, and a region of the insulin receptor around Tyr-960 may facilitate phosphorylation of cellular substrates required for transmission of the insulin signal.

Reconstitution of a phospholipid flippase from rat liver microsomes

The endoplasmic reticulum is the principal site of synthesis and initial incorporation of membrane lipids in eukaryotic cells; the enzymes of glycerolipid biosynthesis are exclusively located on its cytoplasmic surface. To maintain a phospholipid bilayer in this organelle, newly synthesized phospholipids must be translocated to the lumenal surface. Consistent with this are measurements indicating that movement of phospholipids across microsomal membranes is rapid, with a half-time less than 5 min (refs 3 and 4). Rapid movement of phospholipids has also been detected across the plasma membrane of Bacillus megaterium, another site of de novo lipid biosynthesis. The rapid transmembrane movement of phosphatidylcholine has not been detected, however, in vesicles prepared from microsomal lipids. These latter data suggest involvement in the endoplasmic reticulum of a phospholipid-translocating protein, as was first proposed by Bretscher who called it 'flippase'. Here we report reconstitution of a phospholipid flippase from rat liver microsomes into lipid vesicles.

p21 ras proteins and guanine nucleotides modulate the phosphorylation of 36- and 17-kilodalton mitochondria-associated proteins

We have found that, when isolated rat liver mitochondria are incubated with [gamma-32P]ATP, there is phosphorylation of 36- and 17-kDa proteins. These proteins together with their protein kinase(s) are released as a complex by incubation of the isolated rat liver mitochondria at 20 degrees C for 30 min with 10 mM glucose 6-phosphate, 0.5 mM inositol phosphate, or 0.01 mM inositol triphosphate. Phosphorylation of the 36- and 17-kDa proteins in this soluble protein fraction is modulated by p21 proteins encoded by ras oncogenes and synthesized in Escherichia coli via recombinant DNA methods. A normal p21 ras protein stimulates phosphorylation of the 36-kDa protein and inhibits phosphorylation of the 17-kDa protein, whereas two transforming p21 ras proteins inhibit phosphorylation of both the 36- and 17-kDa proteins. Although GDP and 5'-guanylyl imidodiphosphate also influence the phosphorylation of these proteins, we present evidence that the effects of p21 ras protein are not simply due to their bound GDP. This novel system may be useful for further studies on the biochemical functions of the p21 ras proteins.

Covalent linkage of ribonuclease S-peptide to microinjected proteins causes their intracellular degradation to be enhanced during serum withdrawal

The amino-terminal 20 amino acids are required for microinjected ribonuclease A (RNase A) to be taken up by lysosomes and degraded at an enhanced rate during serum withdrawal. We used water-soluble carbodiimides to covalently attach the RNase S-peptide (residues 1-20) to [3H]RNase S-protein (residues 21-124) at unspecified locations. We then measured catabolism of the [3H]S-protein-S-peptide conjugate after its microinjection into human diploid fibroblasts. The attached S-peptide caused the degradation of S-protein to be enhanced 2-fold in the absence of serum. Control experiments showed that degradation of [3H]RNase S-protein remained unresponsive to serum after conjugation with the inactive fragment, RNase S-peptide (residues 1-10). Covalent attachment of RNase S-peptide had a similar effect on the catabolism of two other proteins. Degradation rates of microinjected 125I-labeled lysozyme and 125I-labeled insulin A chain are normally unresponsive to serum withdrawal. However, breakdown rates of microinjected 125I-labeled lysozyme-S-peptide and 125I-labeled insulin A chain-S-peptide conjugates were increased 2-fold during serum deprivation. We suggest that RNase S-peptide acts as a "single sequence" that directs cytosolic proteins to lysosomes through a pathway that is activated by deprivation conditions.

Regulation of catabolism of microinjected ribonuclease A. Identification of residues 7-11 as the essential pentapeptide

We have identified a pentapeptide region of microinjected ribonuclease A that is required for enhanced degradation of this protein during serum withdrawal. We introduced reductively methylated [3H]ribonuclease A, [3H]ribonuclease S-protein (residues 21-124), and [3H]ribonuclease S-peptide (residues 1-20) into the cytosol of human fibroblasts by red cell-mediated microinjection and osmotic lysis of pinosomes. The degradative rates of ribonuclease A and ribonuclease S-peptide are increased 2-fold upon withdrawal of serum, while catabolism of ribonuclease S-protein is not regulated in this manner. Certain fragments of ribonuclease S-peptide are also degraded in a serum-dependent fashion (residues 1-14 and 4-13), while other fragments are not (residues 1-10 and 2-8). [3H]Ribonuclease S-peptide is cleaved into two smaller radioactive peptides during loading into red cell ghosts. We tentatively identified the larger fragment as residues 7-11 based on its molecular weight determined by Sephadex chromatography in the presence of 8 M urea combined with sequential Edman degradation to identify the position of radioactive lysines. The smaller peptide fragment appears to be the amino-terminal dipeptide, Lys-Glu, and/or residues 7-8, Lys-Phe. After microinjection into fibroblasts, the pentapeptide is degraded at an enhanced rate in the absence of serum, while degradation of the dipeptide is not affected. We confirmed that residues 7-11 constitute the larger hydrolysis product of S-peptide by synthesizing this pentapeptide and radiolabeling it by reductive methylation. It migrated at the expected position after Sephadex chromatography in 8 M urea and was further hydrolyzed only slightly during loading into red cells. Finally, degradation of this pentapeptide after injection into fibroblasts was enhanced 2-fold upon serum withdrawal. These results, combined with our other recent studies (McElligott, M. A., Miao, P., and Dice, J. F. (1985) J. Biol. Chem. 260, 11986-11993), suggest that the pentapeptide, Lys-Phe-Glu-Arg-Gln, targets microinjected ribonuclease A to lysosomes for enhanced degradation during serum deprivation.

Protein phosphorylation in isolated mitochondria and the effects of protein kinase C

When isolated intact rat liver mitochondria are incubated with [gamma-32P]ATP the major phosphorylated proteins are those of 47 and 36 kDa. Phosphorylation of the 47 kDa protein, but not of the 36 kDa protein, is inhibited by carboxyatractyloside, an inhibitor of mitochondrial ATP uptake, while phosphorylation of the 36 kDa protein is inhibited by various uncouplers and an inhibitor of mitochondrial respiration. Addition of purified protein kinase C to the isolated mitochondria leads to the phosphorylation of 69, 37 and 17 kDa proteins. As with other substrates for protein kinase C, phosphorylation of these proteins is dependent on Ca2+ and markedly stimulated by various tumor promoters.

Proteins encoded by ras oncogenes stimulate or inhibit phosphorylation of specific mitochondrial membrane proteins

We have examined the effects of a series of purified p21 proteins encoded by ras oncogenes and synthesized in E. coli via recombinant DNA methods, on the phosphorylation of proteins associated with isolated rat liver mitochondria. We find that these proteins markedly enhance the phosphorylation of a 36KD protein and inhibit phosphorylation of a 17KD protein. The phosphorylated residues on the 36KD protein are hydrolyzed by mild acid, suggesting that they involve phosphoamide bonds. These results suggest that p21 proteins may play a role in vivo by altering the phosphorylation of certain proteins.

Regulation of catabolism of microinjected ribonuclease A requires the amino-terminal 20 amino acids

RNase A introduced into the cytoplasm of IMR-90 human diploid fibroblasts by erythrocyte-mediated microinjection is degraded with a half-life of approximately equal to 75 hr in the presence of fetal bovine serum. In response to serum deprivation the degradative rate of microinjected RNase A is enhanced 2-fold. RNase S protein (amino acids 21-124) is degraded with a half-life similar to that of RNase A in the presence of serum, but its catabolism is not increased during serum withdrawal. Reconstitution of RNase S protein with RNase S peptide (amino acids 1-20) restored full enzymatic activity to the S protein as well as the ability of fibroblasts to increase its catabolism during serum deprivation. Finally, RNase S peptide microinjected alone shows the full 2-fold increase in degradative rate during serum withdrawal. These results show that recognition of RNase A for enhanced breakdown during serum deprivation is based on some feature of its amino-terminal 20 amino acids. Furthermore, our results indicate that the enhanced protein catabolism during serum deprivation can be highly selective.

Interaction of benzo(a)pyrene and its dihydrodiol-epoxide derivative with nuclear and mitochondrial DNA in C3H10T 1/2 cell cultures

To analyze the distribution of radioactive carcinogens and [3H]thymidine between nuclear DNA (nDNA) and mitochondrial DNA (mtDNA), we have developed a simple and rapid method for the separation of nDNA and mtDNA using gel electrophoresis of cell lysates. Using this method, we found that, when C3H10T 1/2 cells are exposed to either 0.5 microM [3H]-(+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene ([3H]BPDE) or 1 microM [3H]benzo(a)pyrene, the mtDNA contains a major fraction of the total adducts formed with cellular DNAs. Deoxynucleoside adducts formed between benzo(a)pyrene and mtDNA in intact C3H10T 1/2 cells or between BPDE and isolated rat liver mtDNA were analyzed by high-performance liquid chromatography, and were found to be much more heterogeneous than those present in nDNA of C3H10T 1/2 cells. The extensive modification of mtDNA in BPDE in C3H10T 1/2 cells is associated with preferential inhibition of the incorporation of [3H]thymidine into mtDNA, when compared to incorporation of [3H]thymidine into nDNA. To analyze the factors responsible for the extensive modification of mtDNA by BPDE, we investigated the role of a lipid phase utilizing liposome:DNA complexes as a model system. We found that the liposomes protect BPDE from spontaneous hydrolysis and enhance the extent of DNA modification at low DNA concentrations. These findings extent previous evidence suggesting that the mitochondria may be important cellular targets in the process of chemical carcinogenesis.

Mechanism of cholesterol exchange between phospholipid vesicles

The kinetics of cholesterol exchange between two populations of small unilamellar vesicles has been investigated. There is no change in the initial rate of this exchange process over a 100-fold change in the acceptor vesicle concentration at a constant donor concentration. These results are not consistent with a collision-dependent exchange mechanism. In support of transfer via the aqueous phase, the inclusion of a negatively charged lipid into the vesicles did not affect the exchange rate. Evidence for a water-soluble pool of cholesterol that had partitioned ut of the vesicle was obtained. Finally, cholesterol exchange was observed when donor and acceptor membranes were separated by a barrier through which neither could pass. These data together support our contention that the exchange of cholesterol between these vesicles involves a water-soluble intermediate.

The rapid transbilayer movement of thiocholesterol in small unilamellar phospholipid vesicles

Cholesterol is a major component of biological membranes, yet there is very little information concerning its distribution across the membrane. Recent experiments in our laboratory, using cholesterol oxidase, have demonstrated that cholesterol can undergo a rapid transbilayer movement in lecithin-cholesterol vesicles in a half-time of 1 min or less at 37 degrees C. In order to support this conclusion, we have sought other approaches to the measurement of this process. We now report our finding that the transbilayer movement of thiocholesterol in phospholipid vesicles occurs in a half-time of 1 min or less at 20 degrees C.

Transmembrane movement of cholesterol in small unilamellar vesicles detected by cholesterol oxidase

Greater than 90% of the cholesterol in small unilamellar vesicles composed of egg lecithin and cholesterol (molar ratio 1:0.7) was oxidized by a cholesterol oxidase from Brevibacterium sp., with a single time constant and a half-time of 1 min at 37 degrees C. The enzyme preparation used was at least 95% pure and possessed no detectable phospholipase C activity. Since cholesterol is present in both halves of the bilayer, it was concluded that transmembrane movement of cholesterol in these vesicles occurs with a half-time of 1 min or less at 37 degrees C.

Mitochondrial DNA is a major cellular target for a dihydrodiol-epoxide derivative of benzo[a]pyrene

When mammalian cell cultures are exposed for 2 hours to (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, a mutagenic and carcinogenic derivative of benzo[a]pyrene, the extent of covalent modificationof mitochondrial DNA is 40 to 90 times greater than that of nuclear DNA. Evidence is presented that this reflects the lipophilic character of the derivative and the very high ratio of lipid to DNA in mitochondria. These results suggest that mitochondrial DNA may be an important cellular target of chemical carcinogens.

The rapid transmembrane movement of cholesterol in small unilamellar vesicles

The exchange of cholesterol between two populations of small unilamellar vesicles has been investigated using a new system. Uniformly sized egg lecithin-cholesterol vesicles containing [3H]cholesterol and the glycolipid N-palmitoyl-DL-dihydrolactocerebroside were used as donors, whereas similar vesicles containing unlabelled cholesterol and no glycolipid were used as cholesterol acceptors. The two populations of vesicles were separated with the castor bean lectin Ricinus communis. It was found that greater than 90% of the cholesterol in the donor vesicle could be exchanged with a single time constant, the half-time for the completion of this exchange process being 1.5 h at 37 degrees C. Therefore, the rate of transmembrane movement or flip-flop of cholesterol in these vesicles must be at least as fast as the intermembrane exchange process. Similar results were obtained using hemoglobin-free human erythrocyte ghosts as the acceptor membrane. If the molecular-sieve chromatography step used to fractionate the vesicles was omitted, a non-exchangeable pool of cholesterol was detected which was shown not to be due to the presence of multilamellar vesicles.