Progesterone action through aggregation of a receptor on the sperm plasma membrane

Rapid steroid effects, reported in several cell types, have pointed out the possibility of non-genomic mechanisms of action, presumably on cell surface receptors. Here we analyzed the effects of antibody-mediated aggregation of a novel type of progesterone receptor on the plasma membrane of human sperm cells. We report that aggregation of hormone-receptor complexes induces Ca2+ influx and a Ca(2+)-dependent exocytotic event in this system. These data suggest a possible mechanism for rapid steroid-induced events. Further research is warranted to examined if a similar mechanism is involved in rapid steroid effects in other cell types.

Isolation and characterization of a T lymphocyte mutant defective in the protein kinase C signal transduction pathway

The phorbol ester TPA is a potent protein kinase C (PKC) activator and a cofactor in the activation of the human Jurkat leukemic T cell line. We have studied the implication of the PKC signaling pathway in the process of T cell activation by generating TPA resistant mutants of Jurkat. These mutants were obtained by recovery of cells that survived a growth arrest induced by TPA. Several cellular phenomena dependent on TPA were dramatically altered in the mutated cells. The mutants were unable to form homoaggregates upon TPA stimulation. Moreover, they did not produce interleukin-2 after activation through engagement of the T cell receptor, in the presence of TPA. These results suggest that the PKC signaling pathway activated by TPA is defective in these cells. In an attempt to define and locate the defect present in the mutants, we have analysed the biochemical properties of PKC, the cellular receptor of TPA. The increase in kinase activity and the translocation of the enzyme to the plasma membrane after stimulation by TPA appeared to be normal in the mutants. We hypothesize that a metabolic step, critical for the completion of T cell activation, distinct from protein kinase C, is impaired in the mutant cells.

A phosphatidic acid-sensitive intracellular pool of calcium is released by anti-CD3 in Jurkat T cells

Jurkat T cells, loaded with the fluorescent calcium probe Indo 1, responded to exogenous phosphatidic acid (PA) by transiently increasing their cytosolic Ca2+ concentration. This effect was dose-dependent, remained unmodified when external Ca2+ was chelated with EGTA, and was totally inhibited when cells were first exposed to CD3 monoclonal antibodies, indicating that it was solely due to the release of an intracellular pool, which is also mobilized during a stimulation via the CD3 T-cell receptor (TcR) molecular complex. CD3- and phytohaemagglutinin (PHA)-stimulated Jurkat cells also produced PA, the dose-responses and kinetics of which were consistent with those of calcium release. Moreover, diacylglycerol (DAG) kinase inhibitors abrogated PA production and lowered calcium release by CD3-stimulated cells. PA did not induce any apparent increase in inositol triphosphates (IP3), nor did it modify the increase entailed by activation of the CD3 pathway, pointing out that IP3 can be supplemented in mobilizing calcium from intracellular stores. Conversely, a first exposure to PA only partially inhibited the CD3- or ionomycin-induced internal release of calcium, suggesting either a rapid restoration of the PA-sensitive stores, or a contribution of other mediators, such as IP3, in the CD3 activation pathway.

Pertussis toxin-induced mitogenesis in human T lymphocytes

Pertussis toxin (PT) has previously been shown to affect a wide variety of immune responses and to cause lymphocyte proliferation. We have investigated the biochemical basis for the mitogenic activity of PT by using human peripheral blood lymphocytes. PT was found to induce a rapid rise in cytosolic free calcium concentration and an alkalinization of the cytosol through the Na+/H+ antiporter. The toxin was also found to induce expression of IL-2-receptor on CD3+ cells and to stimulate IL-2 production. PT induced proliferation of both CD4+ and CD8+ T cells in the presence (but not in the absence) of accessory cells. PT also stimulated IL-1 production by monocytes but neither IL-1, IL-6 alone nor a combination of the two lymphokines could replace accessory cells suggesting that cell:cell contact is required. Low doses of PT induced ADP-ribosylation of G proteins but this treatment did not affect significantly PHA-induced [Ca2+]i increase and IL-2-induced DNA synthesis suggesting that the substrates of the ADP-ribosyltransferase activity of PT are not involved in the signalling pathways leading to DNA replication.

Pertussis toxin-sensitive G-proteins are not involved in activation of T-lymphocytes

Multiple effects of pertussis toxin (PT) on Jurkat T-cells can be distinguished on the basis of their dose-response and their kinetics. High concentrations of PT deliver to cells an activating signal resulting in a rapid rise in [Ca2+]i followed by IL-2 synthesis. This activation is accompanied (within 2 h) by a down-regulation of the CD3/TCR complex from the cell surface. Cells then become refractory towards stimulation by CD3 mAb or PHA. All these effects, referred to as 'mitogenic effects', present the same dose-response curves with an EC50 of 0.5 micrograms/ml. Short term effects (PT-induced Ca2+ movements, down-regulation of CD3/TCR complex and inhibition of PHA and CD3-induced Ca2+ signal) are observed under conditions where no PT-induced ADP-ribosylation can be detected. In contrast, ADP-ribosylation of the 40,000 alpha-subunit of G-proteins requires a sustained (18 h) incubation of intact cells in the presence of low concentration (EC50 = 0.3 ng/ml) of PT. Dose-response curves for PT-dependent ADP-ribosylation and mitogenic effects are separated by three orders of magnitude. Covalent modification of G-protein has no effect on CD3-induced increase in [Ca2+]i and IL-2 synthesis induced by a combination of phorbol ester and either CD3 mAb, PHA or calcium ionophore. These data indicate that transduction of the mitogenic signal does not involve a PT-sensitive G-protein. Furthermore, inhibition of mitogenic signals following PT treatment results from a PT-induced activation leading to a down-regulation of the CD3/T cell receptor complex.

Induction of helper T cells by pertussis toxin during in vivo priming to insulin

We have studied the effect of pertussis toxin (PT) on in vivo priming of T lymphocytes to insulin. Mice were immunized with bovine insulin in complete Freund's adjuvant and antigen-specific DNA synthesis was measured in lymphoid cell suspensions from lymph nodes and spleens. Insulin-specific response was greatly enhanced both in spleen and lymph nodes if mice were given PT at the time of immunization. Mice given PT presented 3 times more cells in spleen and 4 times less in lymph nodes. However, the major antigen-specific response was still observed in lymph nodes. PT had a strong mitogenic effect in vitro on lymph node cells but a weak effect on spleen cells indicating that the adjuvant activity of PT involves other effects besides the mitogenic activity.

A chymotryptic-type serine protease is required for IL-2 production by Jurkat T cells

Interleukin-2 (IL-2) production by activated Jurkat T cells was markedly delayed when these cells were treated with low concentrations of the chymotryptic-type protease inhibitor N-alpha-p-tosyl-L-phenylalanine chloromethylketone (TPCK). This increased lag time observed in the presence of TPCK directly correlates with the interaction of the inhibitor with a unique 42,000 molecular weight (MW) serine protease, which can be labelled with [3H]DFP, and was not due to an intracellular accumulation of a non-mature form of IL-2 nor to a non-specific inhibition of overall protein synthesis. The results presented in this report indicate that a 42,000 MW chymotryptic-like serine protease is required for IL-2 production by activated Jurkat T cells.

Inhibition of phosphatidylserine synthesis induced by triggering CD2 or the CD3-TCR complex in a human T cell line. Relationships with G proteins and receptors modulation

Activation of Jurkat T cells with phytohemagglutinin, CD3 or CD2 mAbs results in a marked inhibition of phosphatidylserine (PS) synthesis. Monitoring PS synthesis in T cells shows that: (i) after modulation of CD3 molecules the cells become refractory to further treatment with CD3 mAbs as well as to a further challenge with CD2 mAbs; and (ii) treatment of T cells with fluoride ions and cholera toxin, two known effectors of guanosine triphosphate-binding proteins, also resulted in a strong inhibition of the synthesis of this phospholipid. The inhibition of PS synthesis thus appears to be regulated similarly to the other activation events, suggesting that transmembrane signalling mechanisms leading to PS inhibition are the same as those previously proposed for increasing phosphatidylinositides turnover and subsequent rise in the intracellular calcium concn in lymphocytes.

Phospholipid metabolism and T cell activation: receptor triggering is associated with the inhibition of phosphatidylserine synthesis

Activation of Jurkat T lymphocytes to produce IL2 is accompanied by a strong inhibition of phosphatidylserine (PS) synthesis. This inhibition was obtained either with the mitogenic lectin PHA, anti-CD3 monoclonal antibodies (mAb), anti-CD2 mAb or anti-Ti mAb. Bypassing membrane receptor signalling, by using a Ca2+ ionophore or a protein phosphatase inhibitor, sodium ortho-vanadate, also results in a marked inhibition of PS synthesis. Activators of phospholipid -Ca2+ dependent protein kinase C (PKC) did not significantly modify PS synthesis, suggesting that the observed changes only involve the transduction of the first activation signal. PS being a necessary cofactor for PKC, our results strongly suggest that the inhibition of PS synthesis induced by receptor triggering exerts a feed back control on PKC therefore leading to a transient activation of the enzyme upon full lymphocyte activation.

Modulation of T cell activation by differential regulation of the phosphorylation of two cytosolic proteins. Implication of both Ca2+ and cyclic AMP-dependent protein kinases

Interleukin 2 production by activated Jurkat T cells is markedly decreased by prostaglandin E2 (PGE2). The target of PGE2 action has been investigated in the present study. Among the biochemical events occurring after CD3.TCR triggering by anti-CD3 monoclonal antibody, phosphorylation of two cytosolic proteins, pp21 and pp23, was strongly inhibited by PGE2, forskolin, and 8-bromo-cAMP, whereas anti-CD3 monoclonal antibody-induced CD3.TCR modulation and Ca2+ influx were not affected. The inhibition of both pp21 and pp23 phosphorylation and interleukin 2 synthesis by PGE2 can be largely reversed by the cAMP-dependent protein kinase inhibitor, N-[2-(methylamino)-ethyl-1]-5-isoquinoline sulfonamide. Together with the demonstration of a cAMP-dependent protein kinase activity in Jurkat T cells, these results are consistent with the participation of the cAMP-dependent protein kinase mediating the inhibitory action of PGE2, probably through the inhibition of pp21 and pp23 phosphorylation. Thus, it appears that the modulation of the phosphorylation of these cytosolic proteins represents an essential step in the regulation of T lymphocyte activation.

Binding of antigen to Ia molecules on intact antigen presenting cells demonstrated by photoaffinity labeling

We used a photoaffinity labeling technique to investigate whether a molecular interaction occurs between antigen and Ia molecules on antigen presenting cells (APC) in the absence of T lymphocytes. M.12.4.1 B lymphoma cells (Iad), which are able to present bovine insulin to Iad lymph node primed T cells, were given radioiodinated bovine insulin derivatized with the photoreactive group (2-nitro-4-azidophenylacetyl) at Lys 29 of the B chain of the insulin molecule. Processing of insulin was allowed by incubating the APC with antigen for increasing periods of time at 37 degrees C or 4 degrees C. The covalent coupling of the processed photoreactive antigen to any neighboring cellular protein was thereafter induced by u.v. irradiation. Immunoprecipitation of membrane proteins by monoclonal antibodies showed that under these conditions, the alpha and beta subunits of the Ia molecules were selectively photolabeled. Labeling was time- and temp-dependent as was the internalization of insulin. The apparent mol. wts of the antigen-Ia molecule complexes were not significantly different from that of native Ia molecules radioiodinated by surface labeling, indicating that only a small fragment of the antigen was covalently coupled to Ia molecules. Similar experiments performed with human B lymphoma cells (526 cells) gave similar results. These observations therefore indicate: (1) that Ia molecules expressed by intact APC are able to bind antigens in the absence of T lymphocyte antigen receptor; and (2) that this association, at least for insulin, requires uptake and a proteolytic fragmentation of the antigen by the APC.

Phosphorylation of two cytosolic proteins. An early event of T-cell activation

T lymphocytes can be activated to proliferate by triggering the T-cell antigen-receptor complex (CD3-Ti) with anti-CD3 (Cluster of Differentiation 3) monoclonal antibody (mAb) or with the mitogenic lectin phytohaemagglutinin A (PHA). We have investigated the relationship between lymphocyte activation and protein phosphorylation in the human leukaemic T-cell line Jurkat. Incubation of 32P-labelled Jurkat cells with anti-CD3 mAb or PHA induced the phosphorylation of two cytosolic proteins that migrate with apparent Mr values of 21,000 (pp21) and 23,000 (pp23) and pI values of 5.1 and 5.0 respectively. Peptide mapping of the two proteins produced the same phosphopeptides pattern, suggesting that pp21 and pp23 are closely related. The phosphorylation of pp21 and pp23 induced by anti-CD3 mAb appeared to be transient, since it was already detected 2 min after the addition of the mAb, reached a maximum at 10 min and recovered its basal level after 1 h. Phosphorylation of pp21 and pp23 could also be elicited by the Ca2+ ionophore A23187 and sodium orthovanadate (Na3VO4), two agents that bypass the T-cell-receptor complex and produced an increase in cytosolic Ca2+ concentration. In addition, we found that vanadate, like the Ca2+ ionophore, induced the secretion of interleukin-2 (IL-2) when used in combination with a submitogenic concentration of the phorbol ester 12-O-tetradecanoylphorbol 13-acetate. These results show that the Ca2+-dependent phosphorylation of pp21 and pp23 represents an early event in the process of signal transduction through the CD3-Ti receptor complex.

Chymotryptic-type protease inhibitors block the increase in Ca2+ and Il-2 production in activated Jurkat T cells

Several chymotryptic-type protease inhibitors were found to inhibit both anti-CD3 mAb- and PHA-induced rise in Ca2+ and IL-2 production in Jurkat T cells. The magnitude of inhibition was a function of the effectors used to stimulate Ca2+ entry and depended on the concentration of the inhibitors. Neither tryptic-type protease inhibitors nor an elastase substrate prevented anti-CD3 mAb- or PHA-induced Ca2+ rise in Jurkat cells. The inhibitory effect of N-alpha-p-tosyl-L-phenylalanine chloromethyl-ketone on anti-CD3 mAb- and PHA-induced rise in Ca2+ resulted from a rapid increase in Ca2+ efflux. The inhibitors which were effective on Ca2+ mobilization also inhibited IL-2 production initiated by an anti-CD3 mAb in the presence of 12-O-tetradecanoylphorbol-13-acetate, and to a lesser extent by PHA or the calcium ionophore A23187. No inhibition of IL-2 production was observed when tryptic-type protease inhibitors or the elastase inhibitor were used. In addition, membrane preparations from Jurkat cells were found to hydrolyze the chymotryptic substrate Suc-Ala-Ala-Phe-paranitroaniline, an effect markedly inhibited by N-alpha-p-tosyl-L-phenylalanine chloromethylketone. Moreover, this inhibitor protected one potential endogenous substrate (Mr 38 kDa) from proteolysis. Taken together, these observations show that chymotryptic-type protease inhibitors block the responses generated by the binding of anti-CD3 mAb to Jurkat cells, and suggest that a chymotryptic-like membrane protease contributes to T cell activation.

Chymotryptic-type protease inhibitors block the increase in Ca2+ and Il-2 production in activated Jurkat T cells

Several chymotryptic-type protease inhibitors were found to inhibit both anti-CD3 mAb- and PHA-induced rise in Ca2+ and IL-2 production in Jurkat T cells. The magnitude of inhibition was a function of the effectors used to stimulate Ca2+ entry and depended on the concentration of the inhibitors. Neither tryptic-type protease inhibitors nor an elastase substrate prevented anti-CD3 mAb- or PHA-induced Ca2+ rise in Jurkat cells. The inhibitory effect of N-alpha-p-tosyl-L-phenylalanine chloromethyl-ketone on anti-CD3 mAb- and PHA-induced rise in Ca2+ resulted from a rapid increase in Ca2+ efflux. The inhibitors which were effective on Ca2+ mobilization also inhibited IL-2 production initiated by an anti-CD3 mAb in the presence of 12-O-tetradecanoylphorbol-13-acetate, and to a lesser extent by PHA or the calcium ionophore A23187. No inhibition of IL-2 production was observed when tryptic-type protease inhibitors or the elastase inhibitor were used. In addition, membrane preparations from Jurkat cells were found to hydrolyze the chymotryptic substrate Suc-Ala-Ala-Phe-paranitroaniline, an effect markedly inhibited by N-alpha-p-tosyl-L-phenylalanine chloromethylketone. Moreover, this inhibitor protected one potential endogenous substrate (Mr 38 kDa) from proteolysis. Taken together, these observations show that chymotryptic-type protease inhibitors block the responses generated by the binding of anti-CD3 mAb to Jurkat cells, and suggest that a chymotryptic-like membrane protease contributes to T cell activation.

The conformation of the CD3 complex on T lymphocytes is modified by calcium ions

While calcium ions are known to play a prominent role in signal transduction in the activation of T lymphocytes, its mechanism of action, target and function have not been elucidated. One crucial event in the calcium-dependent process is the activation of the CD3 complex, and this, too, is not understood. While studying the release of Ca2+ from intracellular stores by several monoclonal antibodies (mAb) against CD3, we found that one of them, mAb 141, was ineffective unless free Ca2+ was present in the external medium. By flow cytometric analysis of the binding of this mAb to Jurkat T cells and peripheral blood lymphocytes we showed that 141 does not recognize the CD3 complex when external Ca2+ is chelated by EGTA. The binding was restored by addition of Ca2+ but not Mg2+. Finally at least one subunit of the CD3 complex displayed a modified electrophoretic migration rate when immunoprecipitated by Leu-4 in the absence of external free Ca2+. These results suggest that the conformation of the CD3 complex depends on Ca2+, the epitope recognized by 141 being concealed at low Ca2+ concentration.

A chymotryptic-type protease inhibitor decreases interleukin 2 synthesis and induces prostaglandin production in Jurkat T cells

TPCK (N-alpha-p-tosyl-L-phenylalanine chloromethylketone), a potent inhibitor of chymotryptic-type serine proteases, was found to decrease IL2 synthesis in Jurkat T cells. Conversely, the tryptic-type protease inhibitor, TLCK (N-alpha-p-tosyl-lysine chloromethylketone), which structurally is very similar to TPCK, had no effect on IL2 synthesis. Prostaglandin synthesis, a process that is known to reduce IL2 production in T cells, was increased by TPCK but not by TLCK, suggesting that this process could be, at least in part, responsible for the inhibition of IL2 production. Our results imply that a chymotryptic-type serine protease plays an active role in the regulation of IL2 synthesis and thus in the whole process of T-lymphocyte activation.

Activation of human T cells is associated with tyrosine phosphorylation of several cellular proteins

Human T lymphocytes are activated to proliferate after triggering the T Cell Antigen Receptor Complex. CD3-Ti, with either antigen, mitogenic lectins or monoclonal antibodies against its different subunits. Stimulation of Jurkat leukemic human T cells with anti-CD3 or anti-Ti monoclonal antibodies was found to induce, within 1 min, an increase in the phosphorylation of a set of cellular proteins that can be precipitated with anti-phosphotyrosine antibodies. Seven phosphotyrosine-containing proteins were separated with respective mol. wt of 21, 25, 38, 55, 70, 80 and 110 kDa, among which the 38 kDa species is predominant. Moreover, incubation of Jurkat T cells with sodium orthovanadate, a potent inhibitor of phosphotyrosine protein-phosphatases, was found to potentiate the effects of anti-CD3 mAb on tyrosine phosphorylation. In addition vanadate also induced IL-2 secretion in Jurkat cells when associated with the phorbol ester TPA, further demonstrating the importance of these phosphorylation reactions in the process of T cell activation. Our results therefore allow us to identify several protein substrates of a tyrosine kinase activity, whose stimulation appears to be an early event in human T cell activation through the antigen receptor pathway.

Phosphorylation of class I histocompatibility antigens in human B lymphocytes. Regulation by phorbol esters and insulin

Phosphorylation of membrane proteins is one of the earliest steps in cell activation induced by growth-promoting agents. Since MHC (major histocompatibility complex) class I molecules are known to contain phosphorylation sites in their C-terminal intracellular domain, we have studied the regulation of HLA (human leucocyte antigen) phosphorylation in intact cells by two mitogens, namely TPA (12-O-tetradecanoylphorbol 13-acetate), a phorbol ester, and insulin, which are thought to exert their mitogenic effects through the stimulation of different protein kinases (protein kinase C and a tyrosine kinase respectively). Human B lymphoblastoid cells (526 cell line) were pulsed with [32P]Pi to label the intracellular ATP pool. Cells were then stimulated for 10 min with TPA, insulin, cyclic AMP or EGF (epidermal growth factor). The reaction was stopped by cell lysis in the presence of kinase and phosphatase inhibitors, and class I HLA antigens were immunoprecipitated with monoclonal antibodies. Analysis of labelled proteins by gel electrophoresis and autoradiography revealed that TPA increased the phosphorylation of the 45 kDa class I heavy chain by 5-7-fold, and insulin increased it by 2-3-fold. Cyclic AMP and EGF had no stimulatory effect. Analysis of immunoprecipitated HLA molecules by two-dimensional gel electrophoresis showed that TPA and insulin stimulated the incorporation of 32P into different 45 kDa molecular species, suggesting that different sites were phosphorylated by two agents. Moreover, incubation of purified class I MHC antigens with partially purified insulin-receptor tyrosine kinase and [gamma-32P]ATP revealed that class I antigens could also be phosphorylated in vitro by this tyrosine kinase. Altogether, these results therefore confirm that insulin receptors and HLA class I molecules are not only structurally [Fehlmann, Peyron, Samson, Van Obberghen, Brandenburg & Brossette (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 8634-8637] but also functionally associated in the membranes of intact cells.

Regulation of glucose carrier activity by AlCl3 and phospholipase C in fat-cells

Recently it was speculated that activation of GTP-binding proteins and of phospholipase is involved in the transmission of a signal from the insulin-receptor kinase to effector systems in the cell. To confirm this hypothesis, we have tested the effect of AlCl3, which has been recently used as an experimental tool to activate GTP-binding proteins, on glucose transport in fat-cells. We found that AlCl3 has a partial insulin-like effect on glucose transport activity (3-O-methylglucose uptake, expressed as % of equilibrium value per 4 s: basal 9.6 +/- 2, AlCl3 29.6 +/- 4, insulin 74.0 +/- 3). The AlCl3 effect is totally blocked by pertussis toxin, whereas the insulin effect was not altered. The effect starts at [AlCl3] greater than 1 fM and reaches its maximum at 0.1 nM. Addition of phospholipase C (PLC; 50 munits/ml) also stimulated glucose transport (maximal 53.0 +/- 5%). Both substances acted faster than insulin itself (maximal values within 1 min for PLC, 2 min for AlCl3 and 5-10 min for insulin). Using the cytochalasin-B-binding assay to determine the effects of AlCl3 and PLC on the distribution of glucose carrier sites in subcellular fractions, we found that their glucose-transport-stimulating effect does not occur through an increase in glucose carrier sites in the plasma-membrane fraction. When PLC was combined with the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate), which increases glucose carrier sites in the plasma membrane, an additive effect on glucose transport was found [PLC (50 munits/ml), 53.0 +/- 5%, TPA (1 nM), 17.3 +/- 2%; PLC + TPA, 68.0 +/- 3%].

IN CONCLUSION

(1) the data show that AlCl3, probably through activation of a pertussis-toxin-inhibitable G protein, and PLC are able to modulate the intrinsic glucose carrier activity; (2) as pertussis toxin did not modify the effect of insulin, it seems unlikely that the insulin signal on glucose transport involves activation of this specific G protein.

Detection of human IL-1 alpha and IL-1 beta at the subpicomolar level by colorimetric sandwich enzyme immunoassay

Two separate convenient sandwich enzyme immunoassay methods were developed for measuring the production of the monokines interleukin-1 alpha (IL-1 alpha) and interleukin-1 beta (IL-1 beta) from peripheral blood mononuclear cells. Polyclonal antisera raised against the recombinant proteins and selected on the basis of their ability to neutralize IL-1-induced IL-2 secretion were used for coating microtiter plates or preparing peroxidase-Fab' conjugates. Both techniques were able to accurately and specifically detect monokines from various sources in the sub-picomolar range and were not influenced by compounds currently used for cell activation. A high molecular weight form of IL-1 beta was demonstrated under certain conditions and the two enzyme immunoassays were successfully applied to the detection of IL-1 alpha and IL-1 beta present in cell supernatants following stimulation with mitogenic or chemical agents.

IL-1 signaling for IL-2 production in T cells involves a rise in phosphatidylserine synthesis

Activation of Jurkat T cells with anti-TCR, anti-CD3, anti-CD2, or PHA is accompanied by a strong inhibition of phosphatidylserine (PS) synthesis. The inhibition of the synthesis of this phospholipid could be partially reversed by IL-1. In Jurkat cells, IL-1 did not activate phosphodiesterases as demonstrated by the lack of change of inositol triphosphate and diacylglycerol levels as well as the lack of change in cytosolic Ca2+ concentration. Furthermore, IL-1 did not modify the intracellular level of cGMP and cAMP, suggesting that the observed rise of PS synthesis could play the role of mediator IL-1 action. As PS is a necessary cofactor for the activation of protein kinase C, our results suggest strongly that IL-1 modulate protein kinase C activity in the activated lymphocyte through its action on PS synthesis.

IL-1 signaling for IL-2 production in T cells involves a rise in phosphatidylserine synthesis

Activation of Jurkat T cells with anti-TCR, anti-CD3, anti-CD2, or PHA is accompanied by a strong inhibition of phosphatidylserine (PS) synthesis. The inhibition of the synthesis of this phospholipid could be partially reversed by IL-1. In Jurkat cells, IL-1 did not activate phosphodiesterases as demonstrated by the lack of change of inositol triphosphate and diacylglycerol levels as well as the lack of change in cytosolic Ca2+ concentration. Furthermore, IL-1 did not modify the intracellular level of cGMP and cAMP, suggesting that the observed rise of PS synthesis could play the role of mediator IL-1 action. As PS is a necessary cofactor for the activation of protein kinase C, our results suggest strongly that IL-1 modulate protein kinase C activity in the activated lymphocyte through its action on PS synthesis.

Monoclonal antibody internalization and degradation during modulation of the CD3/T-cell receptor complex

Although it is well known that the CD3/T-cell receptor (TCR) complex modulates from the surface of T cells upon exposure to monoclonal antibodies (mAb) directed against it, the fate of bound mAb has not been yet elucidated. We therefore perform direct binding experiments of 125I-labeled mAb against CD3 or TCR to investigate their fate in Jurkat T cells. We demonstrated that all mAb were progressively internalized and degraded in Jurkat T cells and that this degradation was inhibited by chloroquine, an inhibitor of lysosomal degradation enzymes. The sequestration of anti-CD3 mAb in acid compartments was furthermore shown using cytofluorometry. All together our results show that antibodies against CD3 or against TCR follow the same endocytic pathway.

T cell activation: distinct pathways involve phosphorylation of different cellular proteins

The murine T cell clone D10.G4.1 can be induced to proliferate by monoclonal antibodies (mAb) to the T cell receptor (TcR) or to Thy-1 molecules. When cells were stimulated by anti-TcR mAb, a group of 4 proteins (19-25 kDa) was specifically phosphorylated. This effect was completely mimicked by the Ca2+ ionophore A23187, whereas only two of these proteins (19 kDa and 25 kDa) were phosphorylated after cell exposure to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate. By contrast, anti-Thy-1 mAb had no effect on the phosphorylation of these proteins, but induced specifically the phosphorylation of a protein of 32 kDa. These results therefore demonstrate that distinct activating pathways in T cells involve the phosphorylation of different proteins, suggesting that the stimulation of protein kinases in T lymphocytes is an early event in cell activation.

Inhibition and activation of interleukin 2 synthesis by direct modification of guanosine triphosphate-binding proteins

To investigate whether guanosine triphosphate-binding proteins (G proteins) are involved in T cell activation, tests were made of the effect of pertussis toxin, cholera toxin, guanosine 5'-(3-O-thio)-triphosphate, and fluoride ions on interleukin 2 (IL-2) synthesis in Jurkat cells. It was found: 1) that pertussis toxin interferes with the first pathway of T cell activation insofar as it can substitute for phytohemagglutinin or monoclonal antibodies directed against the CD3 surface proteins, suggesting that a G protein serves as transducer for signals via the T cell receptor-CD3 complex; and 2) that fluoride ions induce the release of diacylglycerol (DAG) from [3H] arachidonic acid or [3H]oleic acid-prelabeled cells. In [3H]inositol or 32P-prelabeled cells, the increase in DAG production was also found to be accompanied by a 280% increase of intracellular inositol phosphate (IP), without significant modification of IP2 and IP3. These results suggest that a G protein controls the activity of a phospholipase C in Jurkat cells that upon stimulation releases DAG but not IP3. Inasmuch as DAG, like the phorbol ester tetradecanoyl phorbol acetate, activates protein kinase C, it suggests that a G protein is also involved in the transduction of the second signal for lymphocyte activation. Fluoride ions were found to be as effective as tetradecanoyl phorbol acetate to stimulate IL-2 synthesis in Jurkat cells when used in combination with phytohemagglutinin. Finally, cholera toxin and guanosine 5'-(3-O-thio)-triphosphate were found to increase intracellular cyclic adenosine triphosphate and to inhibit IL-2 synthesis. All together these results suggest that several G proteins are involved in the transduction of the two signals necessary for T cell activation as well as in the negative regulation of IL-2 synthesis.

Inhibition and activation of interleukin 2 synthesis by direct modification of guanosine triphosphate-binding proteins

To investigate whether guanosine triphosphate-binding proteins (G proteins) are involved in T cell activation, tests were made of the effect of pertussis toxin, cholera toxin, guanosine 5'-(3-O-thio)-triphosphate, and fluoride ions on interleukin 2 (IL-2) synthesis in Jurkat cells. It was found: 1) that pertussis toxin interferes with the first pathway of T cell activation insofar as it can substitute for phytohemagglutinin or monoclonal antibodies directed against the CD3 surface proteins, suggesting that a G protein serves as transducer for signals via the T cell receptor-CD3 complex; and 2) that fluoride ions induce the release of diacylglycerol (DAG) from [3H] arachidonic acid or [3H]oleic acid-prelabeled cells. In [3H]inositol or 32P-prelabeled cells, the increase in DAG production was also found to be accompanied by a 280% increase of intracellular inositol phosphate (IP), without significant modification of IP2 and IP3. These results suggest that a G protein controls the activity of a phospholipase C in Jurkat cells that upon stimulation releases DAG but not IP3. Inasmuch as DAG, like the phorbol ester tetradecanoyl phorbol acetate, activates protein kinase C, it suggests that a G protein is also involved in the transduction of the second signal for lymphocyte activation. Fluoride ions were found to be as effective as tetradecanoyl phorbol acetate to stimulate IL-2 synthesis in Jurkat cells when used in combination with phytohemagglutinin. Finally, cholera toxin and guanosine 5'-(3-O-thio)-triphosphate were found to increase intracellular cyclic adenosine triphosphate and to inhibit IL-2 synthesis. All together these results suggest that several G proteins are involved in the transduction of the two signals necessary for T cell activation as well as in the negative regulation of IL-2 synthesis.

Monoclonal antibodies (GB16, GB18, GB19, GB22) raised against human placental microvilli recognize the transferrin receptor

GB16, GB18, GB19 and GB22 are mouse monoclonal antibodies produced against full-term human placental microvilli. These antibodies reacted predominantly with the apical surface of the syncytiotrophoblast from first-trimester and full-term placentae, and also reacted with several cell lines derived from non-haematopoietic tissues by immunofluorescence. The radioiodinated BeWo (choriocarcinoma) cell surface proteins were immunoprecipitated with these four antibodies and analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The results demonstrated that the immunoprecipitates migrated at 180 and 90 kilodaltons under non-reducing and reducing conditions, respectively. Using enzyme-linked immunosorbent assay, the antigens recognized by GB16, GB18, GB19, and GB22 were able to bind human transferrin. Immunoabsorption studies showed that these four antibodies bound to the same molecule as OKT9, an antibody to the transferrin receptor. Moreover, the reactivities of these antibodies with HL-60 (promyelocytic leukaemia) cells diminished following dimethyl sulphoxide-induced differentiation by flow cytometric analysis. These data indicate that these four antibodies recognize the transferrin receptor. By competition assay, GB18 bound to an epitope different from those recognized by GB16, GB19 and GB22. In addition, GB22 displayed significant growth inhibition against the activated lymphocytes and Daudi cells, but not against HL-60 or Jurkat cells. These data suggest that these four monoclonal anti-transferrin receptor antibodies will provide additional means to investigate the physiological roles played by the transferrin receptor.

Regulation of interleukin 2 synthesis by cAMP in human T cells

T cell activation requires two initial signals that first lead to the expression of interleukin 2 (IL 2) receptors and the initiation of IL 2 synthesis and then to T cell proliferation. Jurkat T lymphoma cells have been shown to be a good model for studying IL 2 synthesis because these cells also require two signals for activation. The first signal can be provided by the lectin phytohaemagglutinin (PHA), and the second one by the phorbol ester, 12-o-tetradecanoylphorbol 13-acetate (TPA). The regulation of IL 2 synthesis in Jurkat cells, however, is unclear, and the present study deals with the role of cAMP on IL 2 synthesis. In Jurkat cells, IL 2 synthesis appears to be highly regulated by the activity of adenylate cyclase. This was demonstrated by using different means to increase intracellular cAMP level, namely by using permeant cAMP analogs, using the activator of adenylate cyclase, forskolin, using the activator of the alpha subunit of the stimulatory GTP binding protein cholera toxin, and using inhibitors of phosphodiesterase. In addition, prostaglandins E1 and E2 were shown to bind specifically to Jurkat cells, to induce a rise in intracellular cAMP level, and to markedly decrease IL 2 synthesis. All together, these results suggest that in T lymphocytes, the prostaglandin E2 receptor is linked to adenylate cyclase through a GTP binding protein and regulates the production of IL 2 by controlling the intracellular cAMP level.

Regulation of interleukin 2 synthesis by cAMP in human T cells

T cell activation requires two initial signals that first lead to the expression of interleukin 2 (IL 2) receptors and the initiation of IL 2 synthesis and then to T cell proliferation. Jurkat T lymphoma cells have been shown to be a good model for studying IL 2 synthesis because these cells also require two signals for activation. The first signal can be provided by the lectin phytohaemagglutinin (PHA), and the second one by the phorbol ester, 12-o-tetradecanoylphorbol 13-acetate (TPA). The regulation of IL 2 synthesis in Jurkat cells, however, is unclear, and the present study deals with the role of cAMP on IL 2 synthesis. In Jurkat cells, IL 2 synthesis appears to be highly regulated by the activity of adenylate cyclase. This was demonstrated by using different means to increase intracellular cAMP level, namely by using permeant cAMP analogs, using the activator of adenylate cyclase, forskolin, using the activator of the alpha subunit of the stimulatory GTP binding protein cholera toxin, and using inhibitors of phosphodiesterase. In addition, prostaglandins E1 and E2 were shown to bind specifically to Jurkat cells, to induce a rise in intracellular cAMP level, and to markedly decrease IL 2 synthesis. All together, these results suggest that in T lymphocytes, the prostaglandin E2 receptor is linked to adenylate cyclase through a GTP binding protein and regulates the production of IL 2 by controlling the intracellular cAMP level.

Effect of alpha-fetoprotein and indomethacin on arachidonic acid metabolism in P388D1 macrophages: role of leukotrienes

Alpha-fetoprotein (AFP), is able to modify arachidonic acid metabolism in P388D1 macrophage-like cells. AFP has been shown to induce mobilization of arachidonic acid and consequently an enhanced production of leukotrienes. The action of AFP on the cyclooxygenase pathway appears, however, more complex, as this protein simultaneously decreases the total amount of cyclooxygenase products and raises the production of PGE2, 6-keto-PGF1 alpha and to a lesser extent of thromboxane B2. On the other hand, indomethacin abolishes the synthesis of PGE2 but has no effect on 6-keto-PGF1 alpha and thromboxane B2. Indomethacin used in combination with AFP partly loses its inhibiting effect on PGE2 synthesis and paradoxically leads to a "superactivation" of P388D1 cells demonstrated by a very high arachidonic acid mobilization, an enhancement of both leukotriene synthesis and cyclooxygenase activity. Evidence for a binding of indomethacin to AFP was obtained that partly explains these results. In addition, it is shown that the effects of AFP and AFP+Indomethacin on cyclooxygenase activity might be explained by the endogenous synthesis of leukotrienes.

Prostaglandin synthesis in human T cells: its partial inhibition by lectins and anti-CD3 antibodies as a possible step in T cell activation

The human leukemic T cell line Jurkat was used to study arachidonic acid (AA) metabolism. We demonstrated that Jurkat cells are able to convert AA into prostaglandins (PG) and thromboxanes. The presence of tritiated 6-keto-PGF1 alpha, PGE2, PGA2 (B2), and thromboxane B2 in the culture medium was shown either by thin-layer chromatography after a 4-hr incubation period of [3H]AA-prelabeled Jurkat cells or by using specific radioimmuno assays. PG synthesis was inhibited by both indomethacin and niflumic acid, two cyclooxygenase inhibitors. AA metabolism through the cyclooxygenase pathway was followed during T cell activation. T cells were activated by lectins or anti-CD3 monoclonal antibodies (mAb) to trigger the T3-Ti complex and by 12-0-tetradecanoylphorbol 13-acetate (TPA) to mimic IL 1-dependent pathways. Our results show that lectins and anti-CD3 mAb both reduce the amount of PG released by the cells, whereas TPA did not. We confirmed that a combination of TPA and lectins or TPA and anti-CD3 mAb is necessary to obtain full activation of Jurkat cells if this event is monitored by using measurement of IL 2 synthesis. In addition, lectins and anti-CD3 mAb can be replaced by the cyclooxygenase inhibitors indomethacin or niflumic acid. Indeed, a combination of TPA and one of these two drugs induced maximal IL 2 synthesis. These results thus suggest that a reduction in PG synthesis might be a prerequisite to allow the cascade of events involved in T cell activation.

Prostaglandin synthesis in human T cells: its partial inhibition by lectins and anti-CD3 antibodies as a possible step in T cell activation

The human leukemic T cell line Jurkat was used to study arachidonic acid (AA) metabolism. We demonstrated that Jurkat cells are able to convert AA into prostaglandins (PG) and thromboxanes. The presence of tritiated 6-keto-PGF1 alpha, PGE2, PGA2 (B2), and thromboxane B2 in the culture medium was shown either by thin-layer chromatography after a 4-hr incubation period of [3H]AA-prelabeled Jurkat cells or by using specific radioimmuno assays. PG synthesis was inhibited by both indomethacin and niflumic acid, two cyclooxygenase inhibitors. AA metabolism through the cyclooxygenase pathway was followed during T cell activation. T cells were activated by lectins or anti-CD3 monoclonal antibodies (mAb) to trigger the T3-Ti complex and by 12-0-tetradecanoylphorbol 13-acetate (TPA) to mimic IL 1-dependent pathways. Our results show that lectins and anti-CD3 mAb both reduce the amount of PG released by the cells, whereas TPA did not. We confirmed that a combination of TPA and lectins or TPA and anti-CD3 mAb is necessary to obtain full activation of Jurkat cells if this event is monitored by using measurement of IL 2 synthesis. In addition, lectins and anti-CD3 mAb can be replaced by the cyclooxygenase inhibitors indomethacin or niflumic acid. Indeed, a combination of TPA and one of these two drugs induced maximal IL 2 synthesis. These results thus suggest that a reduction in PG synthesis might be a prerequisite to allow the cascade of events involved in T cell activation.

Human interleukin 2. Detection at the picomolar level by sandwich enzyme immunoassay

Human interleukin 2 was detected at the pM level by a simple sequential sandwich enzyme immunoassay. The lymphokine to be assayed was first extracted from supernatants of mitogen-activated Jurkat leukemic T cells or peripheral blood lymphocytes using anti-recombinant interleukin 2 rabbit IgG insolubilized onto polystyrene microtiter plates and was revealed by an anti-interleukin 2 Fab' fragment conjugated to peroxidase. The whole method could be performed within 8 h and allowed the measurement of interleukin 2 irrespective of its degree of glycosylation. Among the currently used mitogens, only ConA at a concentration above 10 micrograms/ml interfered with the assay. The method was carefully compared to the reference bioassay and was found to be only 3-5 times less sensitive.

Monoclonal antibody GB36 raised against human trophoblast recognizes a novel epithelial antigen

Monoclonal antibody GB36, which was raised against human term syncytiotrophoblastic microvilli, was found to recognize a novel epithelial antigen. The antibody immunoprecipitated several membrane proteins from BeWo (choriocarcinoma) and HT-29 (colon adenocarcinoma) cells. Under non-reducing conditions, four peptides of 180, 155, 135 and 130 kDa were revealed by SDS-PAGE analysis. Three peptides of slightly higher molecular weights were revealed under reducing conditions; these three peptides had identical pI (6.2) as shown by two-dimensional gel electrophoresis. By immunofluorescence, GB36 reacted with villous cytotrophoblasts and cytotrophoblasts of chorion laeve, as well as with the basal surface of syncytiotrophoblast and amniotic epithelium. Extravillous trophoblasts of cytotrophoblastic shell in the basal plate and the cytotrophoblastic cell island were non-reactive. It is suggested that the antigen of GB36 may play a role in the polarity of epithelial cells and the adhesion of epithelial cells to extracellular matrix.

Internalization of insulin receptors and HLA antigens in human hepatoma cells

Human HepG2 hepatoma cells express a high number of insulin receptors. Growing cells exhibit 70% of their insulin receptors on the plasma membrane. Moreover, cell-surface insulin receptors form molecular complexes with class I major histocompatibility antigens, as determined by co-immunoprecipitation of the receptors by anti-class I monoclonal antibodies. On exposure to saturating concentrations of insulin, the hormone is rapidly internalized into a Pronase-resistant compartment. Internalization of insulin is accompanied by a rapid (t1/2 = 2-3 min) redistribution of insulin receptors from the cell surface to an intracellular compartment. On removal of insulin from the medium, functional receptors recycle back to the plasma membrane, where they can bind insulin again. With chronic exposure of HepG2 cells to insulin, the initial redistribution of receptors is followed by a slow (t1/2 = 9 h) down-regulation of the receptors. Finally, notwithstanding their interaction at the cell surface, insulin receptors and class I major histocompatibility antigens are internalized at different rates and with independent regulation.

Effect of alpha-fetoprotein on arachidonic acid metabolism in a human T-cell line

Alpha-fetoprotein (AFP), an onco-fetal protein which binds specifically polyunsaturated fatty acids, has been shown to modulate arachidonic acid metabolism in a human T-cell line. In Jurkat lymphoma cells, AFP decreases the synthesis of prostaglandins and increases the release of arachidonic acid and leukotrienes. These effects of AFP were observed both in normal and in activated cells. These results suggest that the T lymphocyte directly participates to the immunosuppressive action of AFP through the synthesis of leukotrienes.

Cross-linking of insulin receptors to MHC antigens in human B lymphocytes: evidence for selective molecular interactions

Molecular interactions between insulin receptors and MHC antigens were investigated in human B cells. Two B lymphoblastoid cell lines, IM-9 and 526, chosen for their high insulin binding capacity, were found to express 15,000 and 25,000 insulin receptors per cell, respectively. Insulin receptors were labeled with a 125I-photoreactive insulin analogue, and all other surface proteins by lactoperoxidase-catalyzed radioiodination. Neighbor proteins were cross-linked with a cleavable homobifunctional reagent dithio-bis-(succinimidyl propionate) (DSP) and solubilized before immunoprecipitation by anti-HLA monoclonal antibodies. Gel analysis of the precipitated proteins showed that 90% of insulin receptors precipitable by anti-insulin receptor antibodies were precipitated by anti-class I antibodies (anti-heavy chain and anti-beta 2-microglobulin) after cross-linking with 2 mM DSP. In neither IM-9- nor 526 cells could HLA antigens be precipitated by anti-insulin receptor antibodies, suggesting that the concentration of class I antigens largely exceeds the concentration of insulin receptors at the cell surface. In 526 lymphocytes, class I MHC antigens were also found to adjoin class II antigens, since both molecules could be coprecipitated with anti-HLA A, B, C and with anti-HLA-DR antibodies after chemical cross-linking. Down-regulation of insulin receptors by chronic exposure of IM-9 cells to insulin did not affect the amount of MHC molecules present on the cell surface, and conversely, class I MHC molecules were internalized in 526 cells irrespective of the presence of insulin. These results thus show that insulin receptors and MHC antigens form multimolecular complexes in the plasma membrane of cultured human B cells. These interactions, which do not appear to influence the regulation of these proteins on the cell surface, may be involved in the mechanism of hormone signaling.

Internalization and recycling of 125I-photoreactive insulin-receptor complexes in hepatocytes in primary culture

When the insulin receptor is tagged with a 125I-photoreactive insulin analogue that can be covalently coupled to it by UV irradiation, the fate of this labeled receptor can be followed both morphologically and biochemically. In the present study we have applied this tool to trace the pathway followed by 125I-photoreactive insulin-receptor complex in hepatocytes in primary culture. As determined by quantitative electron microscopic autoradiography, the internalized labeled material first associates with clear vesicles, second is found in multivesicular bodies, third associates with dense bodies and fourth returns to the cell surface via clear vesicles. This recycling process is inhibited by lysosomotropic agents, i.e. NH4Cl or chloroquine. These data confirm, in another cell system, our previous observations carried out in freshly isolated rat hepatocytes and demonstrate the feasibility and complementarity of both freshly isolated hepatocytes and hepatocytes in primary culture to study internalization and recycling of the insulin receptor.

Cross-linking of insulin receptors to MHC antigens in human B lymphocytes: evidence for selective molecular interactions

Molecular interactions between insulin receptors and MHC antigens were investigated in human B cells. Two B lymphoblastoid cell lines, IM-9 and 526, chosen for their high insulin binding capacity, were found to express 15,000 and 25,000 insulin receptors per cell, respectively. Insulin receptors were labeled with a 125I-photoreactive insulin analogue, and all other surface proteins by lactoperoxidase-catalyzed radioiodination. Neighbor proteins were cross-linked with a cleavable homobifunctional reagent dithio-bis-(succinimidyl propionate) (DSP) and solubilized before immunoprecipitation by anti-HLA monoclonal antibodies. Gel analysis of the precipitated proteins showed that 90% of insulin receptors precipitable by anti-insulin receptor antibodies were precipitated by anti-class I antibodies (anti-heavy chain and anti-beta 2-microglobulin) after cross-linking with 2 mM DSP. In neither IM-9- nor 526 cells could HLA antigens be precipitated by anti-insulin receptor antibodies, suggesting that the concentration of class I antigens largely exceeds the concentration of insulin receptors at the cell surface. In 526 lymphocytes, class I MHC antigens were also found to adjoin class II antigens, since both molecules could be coprecipitated with anti-HLA A, B, C and with anti-HLA-DR antibodies after chemical cross-linking. Down-regulation of insulin receptors by chronic exposure of IM-9 cells to insulin did not affect the amount of MHC molecules present on the cell surface, and conversely, class I MHC molecules were internalized in 526 cells irrespective of the presence of insulin. These results thus show that insulin receptors and MHC antigens form multimolecular complexes in the plasma membrane of cultured human B cells. These interactions, which do not appear to influence the regulation of these proteins on the cell surface, may be involved in the mechanism of hormone signaling.

Alpha-fetoprotein stimulates leukotriene synthesis in P388D1 macrophages

Alpha-fetoprotein (AFP) is able to bind specifically polyunsaturated fatty acids, especially arachidonic acid, the major precursor for prostaglandin and leukotriene synthesis. In P388D1 macrophages, AFP was found to reduce prostaglandin synthesis. This reduced synthesis was counter-balanced by a higher release of unmetabolized arachidonic acid and an enhanced production of leukotrienes. The same results were obtained with unactivated and activated cells irrespective of the activator used: lipopolysaccharide, Ca2+ ionophore A23187, phorbol myristate acetate, interferon-gamma, silica, or zymozan particles. The stimulation of leukotriene synthesis by AFP in macrophages thus appears to be a possible mechanism for the in vitro immunosuppressive effects of this oncofetal protein.

Intracellular pathway followed by the insulin receptor covalently coupled to 125I-photoreactive insulin during internalization and recycling

After it interacts with a specific receptor on the cell surface, insulin is internalized in its target cell by an adsorptive endocytotic process and eventually degraded in lysosomes. It was also recently shown that the initial surface interaction between the hormone and its receptor is followed by an internalization of the receptor, which later is recycled back to the cell surface. In the present study the insulin receptor was tagged with a 125I-photoreactive insulin analogue that can be covalently coupled to the insulin receptor by ultraviolet irradiation. Using this tool we could trace by quantitative electron microscope autoradiography the intracellular pathway followed by this labeled receptor. The quantitative analysis of the intracellular distribution of the labeled material as a function of incubation time at 37 degrees C supports the following sequence of events: association first with clear vesicles, second with multivesicular bodies, third with dense bodies, and fourth, a return to the cell surface via clear vesicles. This insulin receptor recycling process is inhibited by monensin but unaffected by cycloheximide.

Two types of receptor for insulin-like growth factors in mammalian brain

Two types of receptor for insulin-like growth factors (IGFs) have been identified on adult rat and human brain plasma membranes by competitive binding assay, affinity labelling, receptor phosphorylation and interaction with antibodies to insulin receptors. The type I IGF receptor consists of two species of subunits: alpha-subunits (mol. wt. approximately 115 000), which bind IGF I and IGF II with almost equal affinity and beta-subunits (mol. wt. approximately 94 000), the phosphorylation of which is stimulated by IGFs. The alpha-subunits of type I IGF receptors in brain and other tissues differ significantly (mol. wt. approximately 115 000 versus 130 000), whereas the beta-subunits are identical (mol. wt. approximately 94 000). The type II IGF receptor in brain is a monomer (mol. wt. approximately 250 000) like that in other tissues. Two antibodies to insulin receptors, B2 and B9, interact with type I but not with type II IGF receptors. B2 is more potent than B9 in inhibiting IGF binding and in immunoprecipitating type I IGF receptors, in contrast to their almost equal effects on insulin receptors. This pattern is characteristic for IGF receptors in other cells. The presence of two types of IGF receptor in mammalian brain suggests a physiological role of IGFs in regulation of nerve cell function and growth. Since IGF II, but not IGF I, is present in human brain, we propose that IGF II interacts with both types of IGF receptor to induce its biological actions.

Molecular association between major histocompatibility complex class I antigens and insulin receptors in mouse liver membranes

Molecular association between major histocompatibility complex (MHC) antigens and cellular proteins are thought to be involved in various immunological and nonimmunological functions of MHC antigens, including hormone signaling. The existence of physical interactions between insulin receptors and MHC class I antigens was investigated in liver plasma membranes from congenic H-2k mice. Insulin receptors were specifically labeled with a 125I-labeled photoreactive insulin analogue, and cellular proteins were solubilized and incubated with various monoclonal antibodies. Immunoprecipitates were analyzed by polyacrylamide gel electrophoresis followed by autoradiography. Antibodies reacting with distinct epitopes on H-2k class I antigens were all able to precipitate up to 25% of the labeled insulin receptors in H-2k mouse liver membranes, whereas no insulin receptors were precipitated in H-2b mouse liver membranes. Sequential immunoprecipitations showed that insulin receptors and H-2 antigens were coprecipitated and that no cross-reactivity occurred. The specificity of the interaction between insulin receptors and H-2 antigens was demonstrated after double labeling of membrane proteins by photoreactive insulin and lactoperoxidase-catalyzed iodination. These results thus show that, in mouse liver membranes, insulin receptors are physically associated to class I antigens of the MHC.

The insulin receptor kinase

The insulin receptor appears as a tetrameric glycoprotein consisting of two Mr 130,000 subunits (alpha), and two Mr 95,000 subunits (beta) in a disulfide-linked complex. Insulin bound to its specific cell surface receptors in its target cells leads to a complex array of molecular events resulting in insulin effects. It is now generally believed that protein phosphorylation-dephosphorylation reactions represent an important mechanism by which a variety of extracellular stimuli regulate cellular functions. Insulin mediates such reactions, but it is not known whether these are the biochemical link between the binding of insulin to its receptor and its final cellular effects. In search of initial post-binding events which might play a role in insulin action, we looked for phosphorylation of insulin receptors. We show that the insulin receptor displays two functional domains, an insulin binding alpha-subunit, and an insulin responsive protein kinase contained in the beta-subunit. We envisage the insulin receptor as an integrated system for transmembrane signal transmission in which hormone binding to the alpha-subunit leads to activation of the beta-subunit via conformational changes.