Insulin and Insulin-like Growth Factor I (IGF I) Stimulate Phosphorylation of a Mr 175,000 Cytoskeleton-associated Protein in Intact FRTL5 Cells

Thyroid cancer and the immune system: a model for effective immune surveillance

Differentiated thyroid cancers, including papillary and follicular variants, are a useful model with which to examine interactions between cancer and the immune system. Differentiated thyroid cancers are detected in only 20,000 individuals annually in the USA, but thyroid microcarcinomas (< 1 cm in diameter) are far more common. This suggests that the immune system might restrain the growth of these microcarcinomas. On the clinical level, patients with lymphocytes that infiltrate into papillary thyroid cancer have improved survival, supporting the notion that immune system activation might improve this. Together, these observations suggest that the growth and distant spread of thyroid carcinoma are suppressed by mechanisms of immune surveillance, possibly involving lymphocytes, macrophages and their secreted products. In this review, we examine the general hypothesis of immune surveillance and the data pertaining to the roles of lymphocytes, dendritic cells and cytokines in the immune response against thyroid cancers.

IGF-I: a mitogen also involved in differentiation processes in mammalian cells

The main source of insulin-like growth factor I (IGF-I) postnatally is the liver, under growth hormone stimulation, although IGF-I is already present in embryonic tissues and in fetal serum, when its expression is independent of growth hormone. The extracellular alpha-subunit of the IGF-I receptor (IGF-IR) contains an IGF-I binding domain, and the beta-subunit possesses tyrosine kinase activity, which is greatly enhanced when IGF-I binds to the alpha-subunit and leads to its autophosphorylation. Insulin receptor substrate 1 (IRS-1) is the most well characterized cellular substrate for IGF-I, containing at least 20 potential tyrosine phosphorylation sites. The tyrosine phosphorylated form of IRS-1 acts as a docking protein by associating SH2-containing proteins including the p85 regulatory subunit of phosphatidylinositol-3-kinase (P13-kinase), the protein tyrosine phosphatase SH-PTP2, the SH2- and SH3-containing adaptor protein Nck and the growth factor receptor-bound protein-2 (Grb2/Sem5) protein. Grb2 is found associated with mSOS, a GTP/GDP exchange factor involved in converting the inactive Ras-GDP to the active Ras-GTP. The p85 regulatory subunit of PI3-kinase can be also a direct in vitro substrate of the IGF-IR. Although IRS-1 is the major substrate of the IGF-IR, there is another early phosphotyrosine substrate termed SHC, which also activates Ras via Grb2-mSos complex. Activation of p21-Ras induces a serine/threonine kinase cascade leading to the activation of MAP-kinases. The importance of IGF-I as a mitogen throughout development has been clearly demonstrated in IGF-I and IGF-IR knockout mouse studies and also in transgenic mice over-expressing IGF-I. IGF-I is a mitogen in many cell types in culture such as T lymphocytes, chondrocytes or osteoblasts and it is considered to be a progression factor in mouse fibroblasts. IGF-I is also involved in muscle, neurons and adipogenic differentiation of mesenchymal cells. However, IGF-I induces proliferation and differentiation in fetal brown adipocytes, suggesting that both cellular processes are not necessarily mutually exclusive in fetal cells.

Induction of apoptosis in MDA-231 cells by protein synthesis inhibitors is suppressed by multiple agents

In the present study we investigated the ability of several diverse agents to inhibit MDA-231 cell death induced by two different protein synthesis inhibitors, cycloheximide (CHX) and ricin. Cell death was evaluated by several techniques: trypan blue staining, determination of the released lactic dehydrogenase, transmission electron microscopy, and DNA fragmentation. Results from DNA gel electrophoresis and electron microscopy suggest a mechanism of death by apoptosis which terminates in necrosis. Approximately 60% of cell death was induced either by a continuous exposure to 30 micrograms/ml CHX for 48 hr or by a 1-hr exposure to 250 pg/ml ricin followed by a subsequent incubation of 48 hr in the absence of the drug. Cell survival, in the protein synthesis-inhibited cells, was enhanced by the following diverse agents: the growth factors EGF (20 ng/ml) and IGF-1 (20 ng/ml), the protein kinase C activator 12-0-tetradecanoyl-phorbol-13-acetate (5 ng/ml), the protein kinase A activator 8-bromoadenosine 3':5'-cyclic monophosphate (650 micrograms/ml), the nuclease inhibitor aurintricarboxylic acid (100 micrograms/ml), and fetal bovine serum (5%). The survival agents that stimulated protein synthesis in the control untreated cells had no effect on the CHX-inhibited protein synthesis, which indicates that new protein synthesis is not required for cell survival. The same survival agents attenuated the continuous decrease in protein synthesis in the ricin-exposed cells; therefore, the involvement of new protein synthesis in the survival mechanism could not be excluded. The protein kinase C inhibitor staurosporine blocked, in a dose-dependent manner, the survival effect of 12-0-tetradecanoyl-phorbol-13-acetate and EGF, but not that of aurintricarboxyclic acid or fetal bovine serum, in the protein synthesis-inhibited cells. These results provide evidence for several distinctive pathways, the activation of which inhibits MDA-231 cell death induced by protein synthesis inhibitors. Some of these pathways involved activation of protein kinases, probably protein kinase C.

Multiple pathways are involved in protection of MCF-7 cells against death due to protein synthesis inhibition

Previously we have shown that IGF-I protected MCF-7 cells against death induced by the protein synthesis inhibitor cycloheximide (CHX). In the present study we investigated the ability of protein kinase C activator 12-0-tetradecanoyl-phorbol-13-acetate (TPA), the protein kinase A activator 8-bromoadenosine 3'5'-cyclic monophosphate (Br-cAMP), and the enzyme inhibitor aurintricarboxylic acid (ATA) to protect MCF-7 cells against death, due to a continuous presence of CHX. Cell death was evaluated after 48 h of incubation by several techniques (trypan blue staining, release of lactic dehydrogenase, cellular ATP content, transmission electron microscopy, and DNA fragmentation). Apoptosis which terminates in necrosis, characterized this mode of cell death. TPA and ATA at optimal concentrations of 40 ng/ml and 100 micrograms/ml, respectively, reduced cell death to the control level (without CHX), while Br-cAMP at an optimal concentration of 650 micrograms/ml reduced cell death only partially. IGF-1, TPA, and ATA, which stimulated protein synthesis in the control MCF-7 cells, had no effect on protein synthesis in the CHX-treated cells, indicating that the survival effect is not due to new protein synthesis. The protein kinase C inhibitor staurosporine blocked the survival effect of TPA and IGF-1 in a dose-dependent manner, however did not affect the survival effect of ATA. The tyrosine kinase inhibitor genistein blocked the survival effect of IGF-1, but not that of TPA and ATA. Our results provide evidence for several distinctive pathways, the activation of which protects MCF-7 cells against death, due to protein synthesis inhibition.

Interleukin-1 modulates phosphorylation of proteins in human osteoblastic cells

Interleukin-1 (IL-1) is a potent bone resorbing cytokine with diverse biological effects. We previously reported that IL-1 inhibits PDGF-AA-induced biological activities including PDGF-AA-induced tyrosyl phosphorylation. In the present studies, we first investigated and compared the tyrosyl phosphorylation pattern induced by EGF, IGF-1, PDGF-AA, and bFGF in human osteoblastic cells. We then examined the effect of IL-1 on the tyrosyl phosphoproteins induced by each ligand. Immunoblot analyses show that EGF, IGF-1, and PDGF-AA each elicit a different pattern of tyrosyl phosphorylated proteins in normal human osteoblastic cells. IL-1 beta inhibits PDGF-AA induced autophosphorylation by down-regulation of the PDGF-alpha receptor, as demonstrated by immunoprecipitation experiments. For other ligand-induced tyrosyl phosphoproteins, IL-1 beta reduced the intensity of EGF-induced pp55,000, and IGF-1 induced pp185,000 and pp175,000. These experiments indicate that IL-1 inhibits phosphorylation of specific proteins induced by growth factors. By using inhibitors of secondary message pathways, we determined that the inhibitory effect of IL-1 beta on PDGF-AA receptor binding and receptor tyrosyl autophosphorylation was not dependent on protein kinase A, protein kinase C, or the formation of prostaglandins. These data suggest the existence of an alternative pathway that may participate in IL-1 beta signaling.

Epidermal growth factor, phorbol esters, and aurintricarboxylic acid are survival factors for MDA-231 cells exposed to adriamycin

The ability of epidermal growth factor (EGF), insulin-like growth factor-1 (IGF-1), insulin, 12-O-tetradecanoylphorbol-13-acetate (TPA), and aurintricarboxylic acid (ATA) to protect the human breast cancer cell line MDA-231 from death induced by the anticancer drug adriamycin was investigated. Cell death was induced in the MDA-231 cells either by a short-time exposure to a high dose of adriamycin (2 micrograms.ml-1.1h-1) and further culturing in the absence of the drug, or by continuous exposure to a low dose of adriamycin (0.3 micrograms/ml). Cell death was evaluated after 48 h of incubation by several techniques (trypan blue dye exclusion, lactic dehydrogenase activity, cellular ATP content, transmission electron microscopy, and DNA fragmentation). EGF, TPA, and ATA, each at an optimal concentration of 20 ng/ml, 5 ng/ml, and 100 micrograms/ml respectively, substantially enhanced survival of cells exposed either to a high or low dose of adriamycin. Neither IGF-1 nor insulin, each at concentrations of 20 ng/ml, had an effect on cell survival. The three survival factors enhanced protein synthesis in the untreated cells and attenuated the continuous decrease in protein synthesis in the adriamycin-treated cells. Moreover, the three survival factors protected the MDA-231 cells from death in the absence of protein synthesis (cycloheximide 30 micrograms/ml). These results suggest that EGF, TPA, and ATA promote survival of adriamycin pretreated cells by at least two mechanisms: enhancement of protein synthesis and by a protein synthesis independent process, probably a posttranslational modification effect.

Epidermal growth factor and insulin-like growth factor-1 protect MDA-231 cells from death induced by actinomycin D: the involvement of growth factors in drug resistance

In the present study, we investigated the ability of epidermal growth factor (EGF), insulin-like growth factor-1 (IGF-1), and insulin to protect the human breast cancer cell line MDA-231 from death induced by the antitumor drug actinomycin D (ACT-D). ACT-D is an inhibitor of RNA and protein synthesis, and its cytotoxicity may result due to continuous depletion in some vital protein molecules. Cell death was induced in the MDA-231 cells by either continuous exposure to a low dose of ACT-D (0.2 microgram/ml), or by a short-time exposure to a high dose of ACT-D (2 micrograms/ml) and further culturing in the absence of the drug. Cell death was evaluated by the trypan blue dye exclusion test, the release of lactic dehydrogenase into the culture medium, and the depletion in the cellular ATP content. EGF and IGF-1, each at an optimal concentration of 20 ng/ml, enhanced substantially survival of cells exposed either to a low or a high dose of ACT-D. The combination of EGF (10 ng/ml) and IGF-1 (10 ng/ml) had an additive survival effect, which proposes that each of the growth factors enhanced survival by a distinct pathway. Insulin up to 40 ng/ml had no effect on cell survival. Pretreatment of the cells for 1 to 5 h with EGF and IGF-1 protected cells from the cytotoxic effect of ACT-D. Exposure of the cells to 2 micrograms/ml of ACT-D for 1 h resulted in a drastic inhibition in uridine incorporation and only in a slight inhibition in leucine incorporation.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma membrane p180, which insulin receptor phosphorylates in vivo, is not a tyrosine kinase

The earliest substrates to the transmembrane insulin receptor tyrosine kinase, that would function in insulin signalling, are likely to be associated with the plasma membrane. Rat liver plasma membrane 180,000 M(r) protein (p180) is a substrate to the insulin receptor in vitro [Goren et al. (1990) Cellular Signalling 2, 537-555]. The question as to whether p180 is a substrate in vivo was addressed. Half ml 0.9% NaCl or 500 micrograms insulin was injected into rat livers. Purified plasma membrane glycoproteins from the livers were assayed for in vitro phosphorylation reaction products and endogenous tyrosine-phosphorylated proteins. Membranes from insulin-injected rat livers contained phosphorylated p180 and phosphorylated insulin receptor beta-subunit, whereas saline-injected rat liver membranes contained neither. These data suggested that p180 is an in vivo substrate to the insulin receptor. In vitro p180 is tyrosine-phosphorylated in the absence of insulin. p180, therefore, may be the epidermal growth factor (EGF) receptor or another tyrosine kinase that could be part of a phosphorylation cascade initiated by insulin. Two different experiments suggested that p180 is not the EGF receptor: (i) two-dimensional gel electrophoresis (first dimension--non-equilibrium pH-gradient gel electrophoresis) indicated that p180 is a more basic glycoprotein than EGF receptor; and (ii) based on reverse-phase high pressure liquid chromatography, the tryptic-phosphopeptides of carboxymethyl-Sepharose-purified phosphorylated-p180 were different from those of A431 cell phosphorylated-EGF receptor. Similarly, two different experiments demonstrated that p180 is not a tyrosine kinase: (i) gel-permeation chromatography separated the insulin receptor from p180 and only insulin receptor was autophosphorylated in vitro; and (ii) membrane proteins not bound to immobilized ATP contained p180. Thus, p180 can associate with the insulin receptor and be phosphorylated in vitro and in vivo; however, p180 does not function in an insulin receptor-mediated phosphorylation cascade.

Epidermal growth factor and insulin-like growth factor-1 preserve cell viability in the absence of protein synthesis

Prolonged exposure of cells to the potent protein synthesis inhibitor cycloheximide (CHX) terminates in cell death. In the present study we investigated the effect of epidermal growth factor (EGF), insulin-like growth factor-1 (IGF-1), and insulin on cell death induced by CHX in the human cancerous cell lines MDA-231 and MCF-7 (breast), KB (oral epidermoid), HEP-2 (larynx epidermoid), and SW-480 (colon), and correlated this effect to the inhibition rate of protein synthesis. Cell death was evaluated by measuring either dead cells by trypan blue dye exclusion test or by the release of lactic dehydrogenase into the culture medium. CHX was shown to induce cell death in a concentration (1 to 60 micrograms/ml) and time (24 to 72 h)-dependent manner in each of the five cell lines. EGF at physiologic concentrations (2 to 40 ng/ml) reduced cell death close to control level (without CHX) in the cell lines HEP-2, KB, MDA-231, and SW-480, but had almost no effect on cell death in the MCF-7 cells. IGF-1 at physiologic concentrations (2 to 40 ng/ml) reduced cell death nearly to control level in the MCF-7 cells, but had only a partial effect in the other four cell lines. Insulin at supraphysiologic concentration (10,000 ng/ml) mimicked the effect of IGF-1 in each of the cell lines. CHX at concentrations that induced about 60% cell death, inhibited about 90% of protein synthesis as measured by [3H]leucine incorporation. Protein synthesis remained inhibited although cell viability was preserved by EGF or IGF-1. These results indicated that the mechanism by which EGF or IGF-1 preserve cell viability does not require new protein synthesis and may be mediated via a posttranslational modification effect.

Insulinlike growth factor-1 inhibits cell death induced by cycloheximide in MCF-7 cells: a model system for analyzing control of cell death

Prolonged exposure of cells to the potent protein synthesis inhibitor cycloheximide terminates in cell death. In the present study we investigated the effect of insulinlike growth factor-1, insulin, and epidermal growth factor on cell death induced by cycloheximide in the confluent MCF-7 cells, and correlated this effect to the inhibition rate of protein synthesis. Cell death was evaluated by measuring either dead cells by the trypan blue dye exclusion test or by the release of lactic dehydrogenase into the culture medium. After 48 h incubation, cycloheximide (10 to 50 micrograms/ml) was shown to induce cell death in a concentration-dependent manner. Insulinlike growth factor-1, at physiologic concentrations (0.2 to 5 ng/ml), reduced this cell death. Insulin at supraphysiologic concentrations (1 to 10 micrograms/ml) mimicked the effect of insulinlike growth factor-1, whereas epidermal growth factor (10 to 50 ng/ml) had no effect. More than 90% of protein synthesis measured by [3H]leucine incorporation was inhibited by 10 to 50 micrograms/ml cycloheximide. Insulinlike growth factor-1 and insulin at the concentrations that reduced cell death to control level, had no effect on the protein synthesis inhibition rate induced by cycloheximide. These results indicate that inhibition of cell death by insulinlike growth factor-1 does not depend on protein synthesis and may be mediated via a posttranslational modification effect.

Insulin-like growth factor I stimulates transcription of the c-jun proto-oncogene in Balb/C 3T3 cells

Treatment of quiescent Balb/c 3T3 cells with insulin-like growth factor I (IGF I) resulted in the stimulation of proto-oncogene c-jun transcription. Cells exposed to cycloheximide and IGF I together showed super-induction of c-jun transcripts. Nuclear run-off assay revealed that IGF I up-regulated c-jun only while cycloheximide was present. The stability of c-jun mRNA was markedly increased in the cells treated with IGF I. These results suggest that IGF I controls the expression of c-jun by increasing the transcriptional activity and stabilizing the existing transcripts.

Phosphotyrosine-containing proteins in bovine chromaffin cells: effects of insulin-like growth factor I (IGF-I)

1. Antiphosphotyrosine antibodies were used to detect phosphotyrosine-containing proteins in immunoblots of bovine chromaffin cell proteins. 2. Unstimulated cells exhibited two major phosphotyrosine-containing proteins, which had Mr's of 121,000 and 70,000. Insulin-like growth factor I (IGF-I) had little effect on the phosphotyrosine content of these two proteins but greatly increased the phosphotyrosine content of three other proteins of Mr 185,000, 170,000, and 96,000. These proteins were found predominantly in the particulate fraction of cell homogenates. 3. The effects of the IGF-I were time and concentration dependent, with maximal increases in phosphorylation occurring after 1 min of treatment with 10 nM IGF-I. Na3VO4, an inhibitor of phosphotyrosine phosphatases, potentiated the effects of IGF-I. 4. Thus, the IGF-I receptor appears to function as an IGF-I-activated protein tyrosine kinase in chromaffin cells. The tyrosine kinase activity of the IGF-I receptor presumably mediates the effects of IGF-I on chromaffin cell function.

Reverse phase chromatography of trypsin digests of a plasma membrane and a cytoplasmic insulin receptor substrate

The 180,000 molecular weight protein from [32P]phosphorylated wheat germ agglutinin-purified rat liver plasma membranes was digested with trypsin. NIH 3T3 HIR 3.5 cells were [32P]phosphate-labelled in the presence of 10(-7) M insulin, and the 185,000 molecular weight cytoplasmic protein was digested with trypsin. Digests were applied to a C18-mu Bondapak column, eluted with acetonitrile gradients, and radioactivity in the eluate was monitored. The chromatogram for the cytoplasmic protein was similar but not identical to chromatograms of trypsin digests of insulin receptor substrates from other cultured cells. Thirteen and seven phosphopeptides were obtained from the plasma membrane and cytoplasmic substrate, respectively. One phosphopeptide from the two digests eluted at the same acetonitrile concentration; however, dissimilarity in elution profiles and dissimilarity in relative yields of individual phosphopeptides, suggest that the primary structures of tyrosine phosphorylation sites in the two insulin receptor substrates are different.

Initial cholinergic differentiation in embryonic chick retina is responsive to insulin and cell-cell interactions

Previous work [Kyriakis et al., Proc. Natl. Acad. Sci. U.S.A., 84 (1987) 7463-7467] had shown that insulin, when added during a window of binding from embryonic days 9-11, stimulates the normal developmental increase in choline acetyltransferase (ChAT) activity (a marker for cholinergic differentiation) in cultured embryonic chick retinal neurons. Here, we investigated the effect of insulin and IGF 1 on embryonic chick retinal neurons at the stage of development (embryonic day 6) when ChAT activity is first expressed. We investigated insulin peptide effects in retinal tissue developing in vitro as well as in cultures of retinal cells. We show that insulin also stimulated the initial embryonic increase in ChAT activity but had no stimulatory effect on glutamic acid decarboxylase activity (a marker for GABAergic differentiation), an enzyme whose activity also increases developmentally in the same retinal neurons. In fact, insulin inhibited the expression of GAD activity in the retina. The insulin-mediated increase in ChAT activity was independent of normal cell-cell interactions but could not replace them. Insulin also stimulated choline uptake but only after a two day delay, suggesting that the normal program for cholinergic differentiation in the chick retina was induced by insulin. IGF 1 did not have any effect on either cholinergic or GABAergic differentiation. We conclude that cholinergic differentiation in chick embryo retinal neurons is dependent on both insulin- and cell contact-mediated signals.

In vitro, insulin receptor catalyses phosphorylation of clathrin heavy chain and a plasma membrane 180,000 molecular weight protein

Insulin receptor mutation studies indicate that the receptor tyrosine kinase activity is necessary for receptor endocytosis, and several insulin receptor-containing tissues have a plasma membrane-associated protein (Mr congruent to 180,000, p180) whose tyrosine phosphorylation is receptor catalysed. Since clathrin heavy chain (Mr congruent to 180,000 in dodecyl sulphate gel electrophoresis) is a major component of coated vesicles, the latter functioning in receptor endocytosis, we investigated whether insulin receptors can catalyse clathrin phosphorylation and whether p180 is clathrin. Bovine brain triskelion or coated vesicles and 32P-ATP were added to prephosphorylated insulin receptor preparations (wheat germ agglutinin-purified human placenta membrane proteins). Antiphosphotyrosine immunoprecipitated a phosphorylated 180,000 molecular weight protein. Insulin (10(-7) M) increased the rate of phosphorylation. Monoclonal anti-clathrin antibody immunoprecipitated the phosphorylated 180,000 molecular weight protein, whereas monoclonal anti-insulin receptor antibodies (alpha-IR1, MA10) immunoprecipitated both insulin receptors and the phosphorylated 180,000 molecular weight protein. In the absence of added clathrin, anticlathrin immunoprecipitated no proteins, and alpha-IR1 immunoprecipitated only the insulin receptor. Density gradient (glycerol 7.5-30%, w/v) centrifugation separated human placenta microsomal membrane proteins into endosomal, plasma membrane, cytoplasmic and coated vesicle fractions. Antiphosphotyrosine immunoprecipitated phosphorylated-microsomal proteins that centrifugated into endosomal and plasma membrane fractions. Addition of glycerol gradient fractions to a prephosphorylated insulin receptor preparation, however, gave a tyrosine-phosphorylated 180,000 molecular weight protein when cytoplasmic and coated vesicle fractions were added. Taken together these results suggest: (1) that, in vitro, human placenta insulin receptors can phosphorylate bovine brain and human placenta clathrin heavy chain; (2) that both assembled and unassembled clathrin can be phosphorylated; and (3) that p180, the plasma membrane-associated insulin receptor substrate, is not clathrin heavy chain.

A 180,000 molecular weight glycoprotein substrate of the insulin receptor tyrosine kinase is present in human placenta and in rat liver, muscle, heart and brain plasma membrane preparations

Cell signalling for insulin may include insulin receptor tyrosine kinase catalysing the phosphorylation of one or more cell proteins. Since temporally the insulin receptor will encounter plasma membrane proteins first, we have studied the in vitro phosphorylation of purified plasma membrane preparations. Two proteins were immunoprecipitated with anti-phosphotyrosine antibody from rat liver, muscle, heart and brain membranes and from human placenta membranes: the insulin receptor (detected as a phosphorylated-beta-subunit) and a 180,000 molecular weight protein (pp180). pp180 is a monomeric glycoprotein that in the absence of dithiothreitol migrated in denaturing gels like a 150,000 molecular weight protein. pp180 was a substrate for the insulin receptor: (i) receptor and pp180 phosphorylation followed a similar insulin dose-response, although fold-stimulation of autophosphorylation was greater; and (ii) removal of insulin receptors with monoclonal antibodies prevented subsequent pp180 phosphorylation. Insulin-activated receptors increased the extent, but not the rate, of pp180 phosphorylation; the increased phosphate was incorporated into tyrosine and appeared to do so in three or four of pp180's 12 tryptic phosphopeptides. Some data suggest that pp180 is the same protein in each of the tested tissues. The occurrence of pp180, an insulin receptor substrate, in plasma membranes of several insulin responsive tissues suggests that it has a role in insulin signalling.