Characterization of insulin-like growth factor I-stimulated tyrosine kinase activity associated with the beta-subunit of type I insulin-like growth factor receptors of rat liver cells

Nuclear localization and actions of the insulin-like growth factor 1 (IGF-1) system components: Transcriptional regulation and DNA damage response

Insulin-like growth factor (IGF) system stimulates growth, proliferation, and regulates differentiation of cells in a tissue-specific manner. It is composed of two insulin-like growth factors (IGF-1 and IGF-2), six insulin-like growth factor-binding proteins (IGFBPs), and two insulin-like growth factor receptors (IGF-1R and IGF-2R). IGF actions take place mostly through the activation of the plasma membrane-bound IGF-Rs by the circulating ligands (IGFs) released from the IGFBPs that stabilize their levels in the serum. This review focuses on the IGF-1 part of the system. The IGF-1 gene, which is expressed mainly in the liver as well as in other tissues, comprises six alternatively spliced exons that code for three protein isoforms (pro-IGF-1A, pro-IGF-1B, and pro-IGF-1C), which are processed to mature IGF-1 and E-peptides. The IGF-1R undergoes autophosphorylation, resulting in a signaling cascade involving numerous cytoplasmic proteins such as AKT and MAPKs, which regulate the expression of target genes. However, a more complex picture of the axis has recently emerged with all its components being translocated to the nuclear compartment. IGF-1R takes part in the regulation of gene expression by forming transcription complexes, modifying the activity of chromatin remodeling proteins, and participating in DNA damage tolerance mechanisms. Four IGFBPs contain a nuclear localization signal (NLS), which targets them to the nucleus, where they regulate gene expression (IGFBP-2, IGFBP-3, IGFBP-5, IGFBP-6) and DNA damage repair (IGFBP-3 and IGFBP-6). Last but not least, the IGF-1B isoform has been reported to be localized in the nuclear compartment. However, no specific molecular actions have been assigned to the nuclear pro-IGF-1B or its derivative EB peptide. Therefore, further studies are needed to shed light on their nuclear activity. These recently uncovered nuclear actions of different components of the IGF-1 axis are relevant in cancer cell biology and are discussed in this review.

Skeletal effects of growth hormone and insulin-like growth factor-I therapy

The growth hormone/insulin-like growth factor (GH/IGF) axis is critically important for the regulation of bone formation, and deficiencies in this system have been shown to contribute to the development of osteoporosis and other diseases of low bone mass. The GH/IGF axis is regulated by a complex set of hormonal and local factors which can act to regulate this system at the level of the ligands, receptors, IGF binding proteins (IGFBPs), or IGFBP proteases. A combination of in vitro studies, transgenic animal models, and clinical human investigations has provided ample evidence of the importance of the endocrine and local actions of both GH and IGF-I, the two major components of the GH/IGF axis, in skeletal growth and maintenance. GH- and IGF-based therapies provide a useful avenue of approach for the prevention and treatment of diseases such as osteoporosis.

Molecular and functional characterizations of the association and interactions between nucleophosmin-anaplastic lymphoma kinase and type I insulin-like growth factor receptor

Nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) is aberrantly expressed in a subset of T cell lymphoma that commonly affects children and young adults. NPM-ALK possesses significant oncogenic potential that was previously documented using in vitro and in vivo experimental models. The exact mechanisms by which NPM-ALK induces its effects are poorly understood. We have recently demonstrated that NPM-ALK is physically associated with type I insulin-like growth factor receptor (IGF-IR). A positive feedback loop appears to exist between NPM-ALK and IGF-IR through which these two kinases interact to potentiate their effects. We have also found that a single mutation of the Tyr(644) or Tyr(664) residue of the C terminus of NPM-ALK to phenylalanine decreases significantly, but does not completely abolish, the association between NPM-ALK and IGF-IR. The purpose of this study was to determine whether the dual mutation of Tyr(644) and Tyr(664) abrogates the association and interactions between NPM-ALK and IGF-IR. We also examined the impact of this dual mutation on the oncogenic potential of NPM-ALK. Our results show that NPM-ALK(Y644,664F) completely lacks association with IGF-IR. Importantly, we found that the dual mutation of Tyr(644) and Tyr(664) diminishes the oncogenic effects of NPM-ALK, including its ability to induce anchorage-independent colony formation and to sustain cellular transformation, proliferation, and migration. Furthermore, the association between NPM-ALK and IGF-IR through Tyr(644) and Tyr(664) appears to contribute to maintaining the stability of NPM-ALK protein. Our results provide novel insights into the mechanisms by which NPM-ALK induces its oncogenic effects through interactions with IGF-IR in this aggressive lymphoma.

Autocrine and Paracrine Actions of IGF-I Signaling in Skeletal Development

Insulin-like growth factor-I (IGF-I) regulates cell growth, survival, and differentiation by acting on the IGF-I receptor, (IGF-IR)-a tyrosine kinase receptor, which elicits diverse intracellular signaling responses. All skeletal cells express IGF-I and IGF-IR. Recent studies using tissue/cell-specific gene knockout mouse models and cell culture techniques have clearly demonstrated that locally produced IGF-I is more critical than the systemic IGF-I in supporting embryonic and postnatal skeletal development and bone remodeling. Local IGF-I/IGF-IR signaling promotes the growth, survival and differentiation of chondrocytes and osteoblasts, directly and indirectly, by altering other autocrine/paracrine signaling pathways in cartilage and bone, and by enhancing interactions among these skeletal cells through hormonal and physical means. Moreover, local IGF-I/IGF-IR signaling is critical for the anabolic bone actions of growth hormone and parathyroid hormone. Herein, we review evidence supporting the actions of local IGF-I/IGF-IR in the above aspects of skeletal development and remodeling.

The IGF system

The insulin-like growth factor (IGF) system plays essential role in the regulation of cell growth, proliferation and survival and affects nearly every organ system in the body. IGF-I, which has a high structural similarity to insulin, exerts growth-promoting effects, influences glucose metabolism and has neuroprotective and cardioprotective effects, partly because of its cell-proliferative and antiapoptotic properties. Aberrations in the IGF system may associate with various pathological conditions, including cancer. Insulin and its synthetic analogs are known to possess IGF-IR binding affinity, and concern has been raised about their mitogenic potential in humans. The present review summarizes the main aspects of the IGF system biology and the interactions among IGF-I, insulin, insulin analogs and their receptors.

Modulation of response to radiation of human lung cancer cells following insulin-like growth factor 1 receptor inactivation

Targeted disruption of the insulin-like growth factor 1 receptor (IGF-1R) restricts proliferation of tumor cells and enhances their in vitro radiosensitivity. However, there is little information regarding the effect of IGF-1R expression and function on the lung cancer response to radiotherapy. In this study, we evaluated the cell surface expression of IGF-1R and the antitumoral effect of IGF-1R blockade in combination with irradiation in 6 non-small cell lung cancer (NSCLC) cell lines. All cell lines showed specific IGF-1 binding with an affinity ranging from 0.95x10(-9) to 2.3x10(-9) M, which was evaluated by competitive binding assay. The amount of binding sites ranged from 118 to 377 fmol/mg protein. In one cell line (U1810), the combined treatment led to synergistic cell death and was associated with an accumulation of cells in the G2 phase. IGF-1R activation was able to obstruct serum starvation/radiation-induced cell death in U1810 cell line. Additive interactions were found for four cell lines (A549, H157, H23 and H125) whereas only subadditive effects were observed in U1752 cell line. Our results indicate that the IGF-1R is present on NSCLC cells and thereby its involvement in the modulation of radiosensitivity in lung cancer cells.

Distribution of insulin-like growth factor-I mRNA in the mandibular condyle and rib cartilage of the rat during growth

This study makes a molecular biological comparison of primary and secondary cartilage at an early phase of postnatal development. The distribution of insulin-like growth factor-I (IGF-I) mRNA expression in the mandibular condyle and rib cartilage of 1-28-day-old rats was examined after in situ hybridisation using an oligo probe cocktail for IGF-I mRNA. In the condyle, expression was localised to a narrow strip under the articular layer where the cells are undifferentiated. Essentially, no differences were found in IGF-I synthesis within three samples from the same age group or between different age groups. In rib cartilage, IGF-I mRNA was localised within the germinative, proliferative and early hypertrophic cell layers in 1-28-day-old rats. Again, there were no differences in expression among animals of the same age or as a function of age. This pattern of IGF-I mRNA expression indicates that IGF-I synthesis during growth of the mandibular condylar cartilage is different from that of costal cartilage. The findings shed light on the problem of overgrowth often associated with the use of costochondral grafts to replace defective mandibular condyles.

The somatomedin hypothesis: 2001

Since the original somatomedin hypothesis was conceived, a number of important discoveries have allowed investigators to modify the concept. Originally somatic growth was thought to be controlled by pituitary GH and mediated by circulating insulin-like growth factor-I (IGF-I, somatomedin C) expressed exclusively by the liver. With the discovery that IGF-I is produced by most, if not all, tissues, the role of autocrine/paracrine IGF-I vs. the circulating form has been hotly debated. Recent experiments using transgenic and gene-deletion technologies have attempted to answer these questions. In the liverspecific igf-1 gene-deleted mouse model, postnatal growth and development are normal despite the marked reduction in circulating IGF-I and IGF-binding protein levels; free IGF-I levels are normal. Thus, the normal postnatal growth and development in these animals may be due to normal free IGF-I levels (from as yet unidentified sources), although the role of autocrine/paracrine IGF-I has yet to be determined.

Modulation of a calcium-sensitive nonspecific cation channel by closely associated protein kinase and phosphatase activities

Regulation of nonspecific cation channels often underlies neuronal bursting and other prolonged changes in neuronal activity. In bag cell neurons of Aplysia, it recently has been suggested that an intracellular messenger-induced increase in the activity of a nonspecific cation channel may underlie the onset of a 30-min period of spontaneous action potentials referred to as the "afterdischarge. " In patch clamp studies of the channel, we show that the open probability of the channel can be increased by an average of 10. 7-fold by application of ATP to the cytoplasmic side of patches. Duration histograms indicate that the increase is primarily a result of a reduction in the duration and percentage of channel closures described by the slowest time constant. The increase in open probability was not observed using 5'-adenylylimidodiphosphate, a nonhydrolyzable ATP analog, and was blocked in the presence of H7 or the more specific calcium/phospholipid-dependent protein kinase C (PKC) inhibitor peptide(19-36). Because the increase in activity observed in response to ATP occurred without application of protein kinase, our results indicate that a kinase endogenous to excised patches mediates the effect. The effect of ATP could be reversed by exogenously applied protein phosphatase 1 or by a microcystin-sensitive phosphatase also endogenous to excised patches. These results, together with work demonstrating the presence of a protein tyrosine phosphatase in these patches, suggest that the cation channel is part of a regulatory complex including at least three enzymes. This complex may act as a molecular switch to activate the cation channel and, thereby, trigger the afterdischarge.

Structure and function of the insulin-like growth factor I receptor

Insulin-like growth factors I and II (IGF-I, IGF-II) were originally identified as potent mitogens and as the mediators of growth hormone action. Besides being mitogenic, however, these polypeptide growth factors play a crucial role in cell survival, and contribute to transformation and to maintenance of the malignant phenotype. Here we will discuss signaling by the IGFs, focusing specifically on the structure and function of the IGF-I receptor and the domains of this receptor responsible for distinct IGF functions: mitogenesis, transformation, and protection from apoptosis. We will also compare the structural domains of the related but functionally distinct receptor for insulin.

Effects of growth hormone and insulin-like growth factor-I singly and in combination on in vivo capacity of urea synthesis, gene expression of urea cycle enzymes, and organ nitrogen contents in rats

Improvement of nitrogen balance is desirable in patients with acute or chronic illness. Both growth hormone (GH) and insulin-like growth factor-I (IGF-I) are promising anabolic agents, and their combined administration has been shown to reverse catabolism more efficiently than each of the peptides alone. This is believed to be mediated primarily through increased peripheral protein synthesis, whereas little attention has focused on a possible participation of amino acid metabolism in the liver. Four groups of rats were given: 1) placebo; 2) GH (200 micrograms/d); 3) IGF-I (300 micrograms/d); and 4) both GH and IGF-I. After 3 days, the maximum capacity of urea-nitrogen synthesis was determined by saturating infusion of alanine (n = 8 in each group), together with measurements of liver messenger RNA (mRNA) levels for urea cycle enzymes (n = 5 in each group) and N-contents of muscles, heart, and kidney. Basal plasma alpha-amino acid concentrations were similar in all groups. The capacity of urea-N synthesis [mumol/(min x 100 g body weight)] was reduced in a stepwise manner (placebo: 8.25 +/- 1.2; GH treatment: 6.52 +/- 0.8; IGF-I treatment: 5.5 +/- 0.6; and GH/IGF-I: 4.22 +/- 1.6 [P < .001 by ANOVA]), each step being lower than the former. Serum IGF-I increased stepwise from placebo (699 +/- 40 to 1,579 +/- 96 micrograms/L in the combined GH/IGF-I group), and was correlated negatively with the capacity of urea-nitrogen synthesis (P < .01). mRNA levels for urea cycle enzymes in the liver decreased after GH and IGF-I treatment, and the effect was more pronounced after the combined treatment in which the rate-limiting enzyme, argininosuccinate synthetase, was halved. Nitrogen contents of organs increased after both GH and IGF-I treatment, and even more so after the combination treatment, reaching an increase of 30% (P < .05). Data suggest that GH and IGF-I singly and, even more so in combination, additively inhibit urea synthesis. This is supposed to favor protein buildup in organs. We speculate that this inhibitory effect on the capacity of urea synthesis is caused by a decreased translation rate of the urea cycle enzymes caused by GH and IGF-I's down-regulatory effect on urea cycle enzyme gene transcription. The findings may indicate a novel mechanism of the protein anabolic action of GH and IGF-I.

Two primitive neuroectodermal tumor cell lines require an activated insulin-like growth factor I receptor for growth in vitro

OBJECTIVE

To determine the expression of the insulin-like growth factors (IGFs) and the IGF-I receptor in primitive neuroectodermal tumor cell lines and to assess the importance of these proteins in the growth of cell lines in vitro.

METHODS

Ribonucleic acid blotting and reverse transcriptase-polymerase chain reaction were used for detection of IGF and IGF-I expression. Ribonucleic acid blotting was used for detection of up-regulation of c-fos in the presence of exogenous growth factor. Immunoprecipitation was used to demonstrate autophosphorylation of the receptor in the presence of exogenous growth factor. Ligand binding analysis was used to determine the binding affinity of the receptor and the number of receptors per cell. Growth of curves in the presence of monoclonal antibody that blocks binding of ligand to receptor was measured to determine the requirement for an activated receptor during growth.

RESULTS

Expression of IGF-II was identified in one cell line. No expression of IGF-I was seen in any cell line. Expression of IGF-I receptor was detected in all three cell lines. Immunoprecipitation experiments demonstrated autophosphorylation of the receptor after addition of IGF-I to growing cells. Ligand binding analysis revealed 9.2 x 10(4) and 4 x 10(4) receptors per cell in the Daoy and PFSK cell lines, respectively. Addition of either IGF alone or in combination to serum-starved cells was not able to restore growth of the cell lines. A blocking monoclonal antireceptor antibody decreased growth of Daoy and PFSK cells in a dose-dependent fashion. Complete arrest of growth occurred at 1 microgram/ml antibody in both cell lines.

CONCLUSION

The IGF-I receptor is expressed by primitive neuroectodermal tumor cell lines in vitro. An activated receptor is important for cell proliferation in vitro. Additional work will establish the importance of these findings for tumors in vivo.

Expression of insulin-like growth factor (IGF)-I receptors, IGF-II/cation-independent mannose 6-phosphate receptors (CI-MPRs), and cation-dependent MPRs in polarized human intestinal Caco-2 cells

We have analyzed the surface distribution and functional expression of the insulin-like growth factor I (IGF-I) receptor and the IGF-II/cation-independent mannose 6-phosphate (IGF-II/CI-MPR) in the polarized human colon adenocarcinoma cell line, Caco 2. Domain-selective biotinylation of the apical and basolateral surfaces of Caco-2 cells grown on filter supports revealed a 3-4-fold enrichment of these receptors on basolateral membranes. In addition, the biotinylation studies revealed the presence of the cation-dependent MPR on both membrane surfaces, with a 3.4-fold enrichment on basolateral membranes. Binding of 125I-IGF-I at 4 degrees C confirmed similar higher levels of expression of the IGF-I receptor at the basolateral surface than at the apical surface. Cell surface-specific binding of the iodinated lysosomal enzyme beta-glucuronidase was detected at 4 degrees C on both plasma membrane domains. However, significant uptake of beta-glucuronidase at 37 degrees C was observed only from the basolateral surface. These results indicate that the MPRs and the IGF-I receptor are expressed in a polarized fashion in Caco-2 cells and that the IGF-II/CI-MPR present on apical membranes, unlike the IGF-II/CI-MPR expressed on the basolateral surface, is not functional in endocytosing lysosomal enzymes.

Identification of tyrosine 489 in the carboxy terminus of the Tpr-Met oncoprotein as a major site of autophosphorylation

The Met receptor tyrosine kinase is the receptor for hepatocyte growth factor/scatter factor. HGF/SF is a multifunctional cytokine that can stimulate proliferation, motility, and morphogenesis in epithelial and endothelial cells. Oncogenic activation of the Met receptor occurs through a genomic rearrangement that generates a hybrid protein in which tpr sequences are directly fused amino terminal to the met receptor kinase domain. The resultant Tpr-Met hybrid protein possesses tyrosine kinase activity, is constitutively phosphorylated on tyrosine residues in vivo, and transforms fibroblasts in culture. We have identified two tyrosine residues within the catalytic domain of the Tpr-Met oncoprotein (Y365, Y366) and Met receptor (Y1234, Y1235) that are phosphorylated and essential for both the catalytic and biological activity of the oncoprotein and receptor. However, a detailed analysis of phosphorylation in these proteins has not been undertaken. In order to determine the sites of tyrosine phosphorylation in the Tpr-Met oncoprotein, in vitro mutagenesis, phosphopeptide mapping, and dephosphorylation protection assays were performed. Here we identify that a single tyrosine (Y489) in the carboxy terminus of the Tpr-Met oncoprotein is highly phosphorylated and is essential for biological activity. In contrast, additional tyrosines (Y482, Y498) located in the carboxy terminus are not phosphorylated at detectable levels and are not essential for the biological activity of the oncoprotein.

Substrate specificities of the insulin and insulin-like growth factor 1 receptor tyrosine kinase catalytic domains

To compare the substrate specificities of the insulin and insulin-like growth factor 1 (IGF-1) receptor tyrosine kinases, the catalytic domains of the enzymes have been expressed in Escherichia coli as fusion proteins. The purified proteins have kinase activity, demonstrating that the catalytic domain of IGF-1 receptor, like that of insulin receptor, is active independent of its ligand-binding and transmembrane domains. The specificities of the two enzymes for the divalent cations Mg2+ and Mn2+ are indistinguishable. A series of peptides has been prepared that reproduces the major phosphorylation sites of insulin receptor substrate-1, a common substrate for the two receptor tyrosine kinases in vivo. Insulin and IGF-1 receptors show distinct preferences for these peptides; whereas insulin receptor prefers peptides based on Tyr-987 or Tyr-727 of insulin receptor substrate-1, the IGF-1 receptor preferentially recognizes the Tyr-895 site. The latter site, when phosphorylated, is a binding site for the SH2 domain-containing adapter protein Grb2. The ability of the two receptor tyrosine kinases to be phosphorylated and activated by v-Src has also been examined. The catalytic activity of IGF-1 receptor is stimulated approximately 3.4-fold by treatment with purified v-Src, while insulin receptor shows very little effect of Src phosphorylation under these conditions. This observation is relevant to recent findings of IGF-1 receptor activation in Src-transformed cells, and may represent one method by which Src amplifies its mitogenic signal. Collectively the data suggest that the catalytic domains of the two receptor kinases possess inherently different substrate specificities and signaling potentials.

Stimulatory effect of human insulin on erythroid progenitors (CFU-E and BFU-E) in human CD34+ separated bone marrow cells and the relationship between insulin and erythropoietin

Erythropoietin is known to be effective for the treatment of anemia in chronic renal failure, but the efficacy of erythropoietin for anemia in other diseases is not so great. Insulin exerts a growth promoting activity in various kinds of cells. In the present study, the effects of insulin on erythroid progenitors (colony forming units-erythroid, CFU-E; and burst forming units-erythroid, BFU-E) in human bone marrow were examined at various concentrations of recombinant human erythropoietin (rh-Epo) to clarify the relationship between erythropoietin and insulin. Human insulin stimulated the formation of CFU-E and BFU-E in the presence of three concentrations (0.25, 5, and 100 U/ml) of rh-Epo. Stimulatory effects of human insulin on CFU-E and BFU-E were also observed in the nonphagocytic and nonadherent bone marrow fraction (NP-NA fraction) and in the NP-NA and T cell-depleted fraction at each concentration of rh-Epo. Human insulin further stimulated the CFU-E and BFU-E growth in CD34+ separated bone marrow cells. These results indicate that the enhancing effect of human insulin on erythroid progenitors is not mediated through monocytes and macrophages or T cells, suggesting a direct action on erythroid progenitors.

Insulin-like growth factor-I (IGF-I) dependent phosphorylation of the IGF-I receptor in MG-63 cells

Insulin-like growth factor I (IGF-I) stimulates multiplication of the human osteosarcoma cell line, MG-63. by acting through the IGF-I receptor. We have characterized IGF-I stimulated phosphorylation of the IGF-I receptor in this cell line. Serum starved MG-63 cells were metabolically labeled with [32P]orthophosphoric acid and the cells were treated with IGF-I. Phosphotyrosine containing proteins were immunoprecipitated from the cell lysates with antiphosphotyrosine-Agarose and eluted with phenyl phosphate. Further immunoprecipitation with IGF-I receptor monoclonal antibodies (alpha IR-3, 18E9) and analysis by sodium dodecylsulfate polyacrylamide gel electrophoresis and autoradiography demonstrated IGF-I dependent autophosphorylation of the IGF-I receptor. Phosphoamino acid analysis of the IGF-I receptor beta subunit and the observation that antiphosphotyrosine-Agarose did not immunoprecipitate [35S]methionine-labeled receptor from unstimulated cells, demonstrated that in the absence of IGF-I, the receptor was not phosphorylated on tyrosine residues. Western blotting of cell lysates with a monoclonal phosphotyrosine antibody did not identify the IGF-I receptor or pp185 but demonstrated IGF-I dependent phosphorylation on tyrosine residues in three other proteins, p110, p70 and p40.

Effects of insulin and insulin-like growth factors on neurofilament mRNA and tubulin mRNA content in human neuroblastoma SH-SY5Y cells

Insulin-like growth factors (IGFs) are implicated in the development of the vertebrate neural circuitry, and increase neurite growth in vitro and in vivo. The construction of the cytoskeleton is necessary for growth of axons and dendrites, and the neurofilament (NF) 68 kDa and 170 kDa proteins assemble to help form major fibrillar elements of the neurite cytoskeleton. We report that physiological concentrations of insulin, IGF-I or IGF-II increased the contents of 68 kDa NF, 170 kDa NF, alpha-tubulin, and beta-tubulin mRNAs, relative to total RNA, in cultured human neuroblastoma SH-SY5Y cells. In contrast, the relative contents of histone 3.3 mRNA, and poly(A)+ RNA were not increased. Ligand concentrations which increased NF mRNAs were very similar to those which increased neurite outgrowth. Although each gene was evidently independently regulated, the 68 kDa NF, 170 kDa NF, alpha-tubulin, and beta-tubulin mRNAs were nevertheless all transiently elevated over approximately the same time interval in response to insulin. These data, when considered together with studies by others with nerve growth factor, show that the 68 kDa and 170 kDa NF mRNAs are elevated in a biochemical pathway activated in common during neurite outgrowth directed by insulin, IGF-I, IGF-II, and nerve growth factor.

A monoclonal antibody to the T-cell receptor increases IGF-I receptor content in normal T-lymphocytes: comparison with phytohemagglutinin

The biological effects of the IGFs are mediated through interaction with specific cell surface receptors. It has been previously reported that mitogenic activation of T-lymphocytes by phytohemagglutinin (PHA) is associated with increased IGF-I receptor content. However, the mechanisms which regulate IGF-I receptor expression during T-lymphocyte activation are unknown. To explore further the regulation of IGF-I receptor expression in T-cells, we investigated IGF-I receptor content and mRNA abundance in T-lymphocytes after stimulation either by PHA or OKT-3, the latter being a monoclonal antibody directed against the CD-3 antigen of the T-cell receptor. IGF-I binding in T-cells demonstrated increased IGF-I receptor content after stimulation by both PHA and OKT-3. Peak binding was induced after 72 h of treatment with PHA and 48 h of treatment with OKT-3. Affinity cross-linking of 125I-IGF-I to T-cell membranes demonstrated a single approximately 130 kDa band which was increased after treatment with PHA or OKT-3. This band was inhibited by the addition of alpha-IR3, a monoclonal antibody to the IGF-I receptor. Both PHA and OKT-3 increased IGF-I receptor mRNA abundance with peak increases at 20 h and 60 h, respectively. Parallel increases in IGF-I receptor and beta-actin mRNA abundance were observed, consistent with previous studies demonstrating increased actin gene expression after T-cell activation. Thus, the increase in IGF-I receptor mRNA abundance markedly preceded the increase in IGF-I receptor content after PHA stimulation, but the increase in IGF-I receptor mRNA abundance followed the increase in IGF-I receptor content after OKT-3.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between heterologous receptor tyrosine kinases. Hormone-stimulated insulin receptors activate unoccupied IGF-I receptors

To determine whether heterologous receptor tyrosine kinases interact with each other we have investigated the ability of insulin receptors to transphosphorylate and transactivate IGF-I receptors. Using partially purified receptors we show that hormone-stimulated insulin receptors induced a 40% increase in IGF-I receptor phosphorylation. Remarkably, this transphosphorylation of IGF-I receptors by insulin receptors resulted in a 2.5-fold augmentation of the IGF-I receptor tyrosine kinase activity for substrates. Our findings demonstrate that transphosphorylation with transactivation can occur between insulin and IGF-I receptors. We would like to propose that such a phenomenon participates in the insulin-induced pleiotropic program by mediating the growth promoting effects of the hormone.

Nucleoside triphosphates are required to open the CFTR chloride channel

The CFTR Cl- channel contains two predicted nucleotide-binding domains (NBD1 and NBD2); therefore, we examined the effect of ATP on channel activity. Once phosphorylated by cAMP-dependent protein kinase (PKA), channels required cytosolic ATP to open. Activation occurred by a PKA-independent mechanism. ATP gamma S substituted for ATP in PKA phosphorylation, but it did not open the channel. Several hydrolyzable nucleotides (ATP greater than GTP greater than ITP approximately UTP greater than CTP) reversibly activated phosphorylated channels, but nonhydrolyzable analogs and Mg(2+)-free ATP did not. Studies of CFTR mutants indicated that ATP controls channel activity independent of the R domain and suggested that hydrolysis of ATP by NBD1 may be sufficient for channel opening. The finding that nucleoside triphosphates regulate CFTR begins to explain why CF-associated mutations in the NBDs block Cl- channel function.

Human teratocarcinoma cells express functional insulin-like growth factor I receptors

Using iodinated insulin-like growth factors (IGFs) we have detected receptors for IGF-I at the cell surface of the clonally derived human embryonal carcinoma cell line Tera 2 clone 13. Affinity crosslinking of IGFs to Tera 2 clone 13-derived membrane preparations revealed the presence of proteins with features of both type-I and type-II IGF receptors. Treatment of Tera 2 clone 13 cells with retinoic acid to induce differentiation results in an increased number of cell surface receptors, apparently without altering the ratio of type-I and type-II receptors. In addition, Tera 2 clone 13 IGF-I receptors catalyze (auto)phosphorylation at tyrosine upon IGF-I and insulin binding. These findings suggest that type-I IGF receptors might be involved in mediating the effects of IGFs and insulin upon the proliferation of Tera 2 clone 13 cells.

Characterization of insulin-like growth factor I and epidermal growth factor receptors in meningioma

Receptors for insulin-like growth factor I (IGF-I) and epidermal growth factor (EGF) were localized and characterized in eight samples of human meningioma (four fibrous, two meningothelial, and two angioblastic types), using quantitative autoradiographic techniques. Effects of both growth factors on deoxyribonucleic acid (DNA) synthesis in the cultured meningioma cells were examined. High numbers of specific binding sites for both IGF-I and EGF were homogeneously present in tissue sections derived from fibrous and meningothelial types of meningiomas, whereas binding sites for these growth factors were not detectable in adjacent leptomeninges. While relatively large numbers of IGF-I binding sites were located in the wall of the intratumoral vasculature, the number of binding sites in the stromal component was lower in angioblastic-type meningiomas, including a low number of EGF binding sites detected only in the stromal portion. Scatchard analysis revealed the presence of a single class of high-affinity binding sites for both IGF-I and EGF in the meningiomas examined (dissociation constant (Kd) = 0.6 to 2.9 nM, and the maximum number of binding sites (Bmax) = 16 to 80 fmol/mg for IGF-I; and Kd = 0.6 to 4.0 nM, Bmax = 3 to 39 fmol/mg for EGF). Both growth factors increased the synthesis of DNA, in a dose-dependent manner, as measured by 3H-thymidine incorporation. The combination of IGF-I and EGF synergistically stimulated the synthesis of DNA, and the effects seen with 10% fetal bovine serum could be reproduced at a concentration of 10(-10) M. These observations can be interpreted to mean that both IGF-I and EGF may be involved in the growth modulation of meningiomas, possibly through paracrine or autocrine mechanisms.

Cytochrome P450 in livers of diabetic rats: regulation by growth hormone and insulin

The effects of pituitary and pancreatic hormones on the change in hepatic cytochrome P450s were studied in alloxan- or streptozotocin-induced male rats. In two major sex-specific forms, P450-male and P450(6 beta-1), the former was decreased in chronic (5 week) diabetes to only less than one-third of controls and the latter was also reduced in early (1 week) diabetes. In contrast, a main phenobarbital-inducible form, P450b, was enhanced 25- to 30-fold in these diabetic rats. 3-Methylcholanthrene-inducible P448H was also elevated 3-fold in alloxan-induced diabetes. These changes in hepatic contents of P450-male, P450-6 beta-1, and P450b, which are under the regulation of pituitary growth hormone, associated well with the reported results of time-dependent changes in growth hormone levels in diabetes (G.S. Tannenbaum (1981) Endocrinology 108, 76-82), suggesting that the change in growth hormone level is a factor responsible for alterations in hepatic cytochrome P450s. Normalizing effects of insulin on these forms were also studied. Treatment of diabetic rats with insulin reversed the decreased amounts of both P450-male protein and mRNA. Insulin also normalized hepatic contents of P450b, P4506 beta-1, and P448H. However, the treatment of hypophysectomized rats with insulin had no effect, and treatment of diabetic rats with growth hormone or a suppressing agent of somatostatin, cysteamine, showed trivial effects on P450-male and P450b. These results suggest that insulin does not act directly as a substitute of growth hormone, but exerts its effect indirectly through the normalization of a growth hormone-mediated process(es) in diabetic rats.

The insulin receptor: structure and function

Promising progress in understanding the molecular basis of insulin action has been achieved by demonstrating that the insulin receptor is an insulin-sensitive tyrosine kinase. Here we discuss the structure of this receptor kinase and compare it with receptors for related growth factors. We review the known modes to regulate the receptor kinase activity, either through its autophosphorylation (on tyrosine residues) or through its phosphorylation by other kinases (on serine and threonine residues). We discuss the role of the receptor kinase activity in hormone signal transduction in light of results indicating a reduced kinase activity in insulin-resistant states. Finally, studies to identify natural substrates for the insulin receptor kinase are presented. The possible physiological role of these phosphorylated substrates in mediating insulin action is evaluated.

Numerous proteins phosphorylated on tyrosine and enhanced tyrosine kinase activities in vanadate-treated NIH 3T3 fibroblasts

A monoclonal antibody that can immunoprecipitate proteins containing phosphotyrosine has been isolated and characterized. To identify proteins that can act as substrates for tyrosine kinases in intact cells, extracts of phosphate-labeled NIH cells that had been treated with the phosphotyrosyl phosphatase inhibitor, vanadate, were precipitated with the antibody, and the immunoprecipitates were analyzed by two-dimensional gel electrophoresis. Numerous proteins were specifically precipitated from vanadate-treated NIH 3T3 cells by the antibody. The high level of phosphotyrosine detected in vanadate-treated cells is presumably primarily due to phosphatase inhibition, but approx. 2-fold increased tyrosine kinase activities were also detected in extracts of the cells after treatment with vanadate. The enhanced tyrosine kinase activity may contribute to the generation of the transformed phenotype seen in response to treatment with vanadate.

Characterization of insulin-like growth factor I (IGF-I) receptors of human breast cancer cells

Studies of binding of IGF-I to a plasma-membrane-enriched subcellular fraction prepared from MCF-7 human breast cancer cells reveal the presence of 0.2 pmols specific binding sites for this mitogen per mg membrane protein, with an equilibrium affinity constant of 1.45 nM-1. Competition studies with insulin, IGF-II, and an anti-IGF-I receptor antibody are consistent with the presence of specific IGF-I receptors, and SDS-PAGE showed binding to a 130 kDa subunit identical to that of receptors from human placenta. In addition, we show that IGF-I is more potent than estradiol and comparable to EGF in stimulating in vitro proliferation of MCF-7 cells, and that IGF-I-stimulated proliferation of these cells is inhibited by a blocking monoclonal antibody against the IGF-I receptor. These results demonstrate that IGF-I is an important mitogen for MCF-7 cells and that the mitogenic effect is mediated by specific IGF-I receptors.

Interleukin 1 and the glomerular mesangium. III. IL-1-dependent stimulation of mesangial cell protein kinase activity

Interleukin 1 (IL-1) exerts a number of biologic actions upon cultured glomerular mesangial cells (MC). These include stimulation of cellular proliferation and induction of prostaglandin and type IV collagenase secretion. It was determined that this activity, as with other polypeptide growth factors, was associated with the activation of specific MC plasma membrane protein kinases. Plasma membranes from cycling MC were incubated with purified IL-1 and (32P) ATP in the absence of calcium and cyclic nucleotides. Macrophage IL-1 stimulated the rapid phosphorylation of several plasma membrane proteins, the most significant of which were 52-55 kd, 46 kd, and 20 kd in size. Macrophage IL-1 induced specific membrane phosphorylation in concentrations as low as 1.5 x 10(-12) M, an effect obtained with equivalent concentrations of purified MC IL-1. The 46 kd phosphoprotein, which was the most prominent, was alkali-resistant and contained phosphotyrosine when examined by phosphoamino acid analysis. The 52-55 kd and 20 kd phosphoproteins were alkali-labile and contained phosphoserine. The 46 kd phosphoprotein was the major phosphoprotein recovered from Con A-Sepharose and IL-1 affinity columns. Induction of plasma membrane-associated protein kinase activity may represent one mechanism whereby IL-1 initiates mesangial cellular activation.

Insulin and insulin-like growth factor 1 receptors during postnatal development of rat brain

The binding of insulin and insulin-like growth factor 1 (IGF1) to high-affinity sites in the brain of rats aged 2-37 days was studied. Specific binding of insulin and IGF1 was assessed using tracer concentrations of 125I-insulin or 125I-IGF1. Sites for insulin and IGF1 were distinguished in these conditions as shown by competition experiments. The Kd were 3.6 nM (insulin) and 2.0 nM (IGF1). These values did not change significantly over the age range studied. The numbers of high-affinity binding sites for insulin and IGF1 were similar in adult animals. IGF1 binding was higher than the insulin binding in 2-day-old animals. The binding capacity for both insulin and IGF1 decreased from birth to age 15 and days remained stable thereafter. Tyrosine kinase activity, which is associated with these receptors, was measured using the artificial substrate poly (Glu, Tyr). It decreased over the first 15 days of life and remained stable thereafter. Autophosphorylation of the receptors confirmed this result. This decrease appears to be due to changes in the numbers of the two types of receptors, and is probably a reflection mainly of the variation in the number of IGF1 receptors. Similar results for insulin and IGF1 binding as well as tyrosine kinase activity were obtained with hypothyroid rats.

Insulin-like growth factor I receptors on mouse neuroblastoma cells. Two beta subunits are derived from differences in glycosylation

We have characterized receptors for the insulin-like growth factor (IGF-I) on the mouse neuroblastoma cell line N18 as well as NG108, the hybrid cell line of N18 and rat glioma (C6). In this cell-free system, IGF-I and insulin stimulated the phosphorylation of 95-kDa and 105-kDa proteins. Using appropriate antibodies we were able to demonstrate that the IGF-I receptor beta subunit has two subtypes of 95 kDa and 105 kDa. On the other hand, insulin receptor beta subunit is a separate single 95-kDa protein. Enzymatic digestion of IGF-I receptor beta subunit subtypes by glycopeptidase F resulted in similar molecular masses (84 kDa and 86 kDa) on SDS-PAGE, which suggests that the difference in molecular masses between two subtypes is attributable to the differences in N-linked complex-type carbohydrate chains on the extracellular domain of beta subunits. This conclusion is further supported by peptides of similar molecular mass following staphylococcal V8 protease digestion. Analysis of IGF-I receptor beta subunit subtypes in these cells may provide insights into the mechanism of action of IGF-I on neural tissues.

Functional properties of an isolated alpha beta heterodimeric human placenta insulin-like growth factor 1 receptor complex

Treatment of human placenta membranes at pH 8.5 in the presence of 2.0 mM dithiothreitol (DTT) for 5 min, followed by the simultaneous removal of the DTT and pH adjustment to pH 7.6, resulted in the formation of a functional alpha beta heterodimeric insulin-like growth factor 1 (IGF-1) receptor complex from the native alpha 2 beta 2 heterotetrameric disulfide-linked state. The membrane-bound alpha beta heterodimeric complex displayed similar curvilinear 125I-IGF-1 equilibrium binding compared to the alpha 2 beta 2 heterotetrameric complex. Triton X-100 solubilization of the alkaline pH and DTT-pretreated placenta membranes, followed by Bio-Gel A-1.5m gel filtration chromatography, was found to effectively separate the alpha 2 beta 2 heterotetrameric and alpha beta heterodimeric IGF-1 receptor species, 125I-IGF-1 binding to both the isolated alpha 2 beta 2 heterotetrameric and alpha beta heterodimeric complexes demonstrated a marked straightening of the Scatchard plots, compared to the placenta membrane-bound IGF-1 receptors, with a 2-fold increase in the high-affinity binding component. Similar to the membrane-bound IGF-1 receptor species, the 125I-IGF-1 binding properties between the alpha 2 beta 2 heterotetrameric and alpha beta heterodimeric complexes were not significantly different. IGF-1 stimulation of IGF-1 receptor autophosphorylation indicated that the ligand-dependent activation of alpha beta heterodimeric protein kinase activity occurred concomitant with the reassociation into a covalent alpha 2 beta 2 heterotetrameric state.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure of the receptor for insulin-like growth factor II: the puzzle amplified

The insulin-like growth factor II (IGF-II) is a polypeptide hormone with structural homologies to insulin and insulin-like growth factor I (IGF-I). In contrast to these other hormones, the in vivo function of IGF-II is not known. Although IGF-II can stimulate a broad range of biological responses in isolated cells, these responses have usually been found to be mediated by the insulin and IGF-I receptors. Recently, the receptor for IGF-II was found to also be the receptor for mannose-6-phosphate. Since this latter receptor has been implicated in targeting of lysosomal enzymes, the question is now raised of whether the same protein can also mediate metabolic responses to IGF-II.

Rapid isolation of plasmalemma from cultured A431 cells: characterization of epidermal growth factor receptors

A rapid method for the purification of plasma membrane from a relatively small number of A431 cells is described. The method is a simple, two-step differential centrifugation in the presence of Ca2+ that requires a total centrifugation time of 7 min. The membrane preparations contained a high level of epidermal growth factor (EGF) receptor activity demonstrated by both the quantity of specific ligand binding and the amount of EGF-dependent phosphorylation of the receptor and an exogenous substrate. EGF-dependent autophosphorylation identified the EGF receptor in the purified membranes as an undegraded 170-kDa protein.

Specificity of tyrosine protein kinases of the structurally related receptors for insulin and insulin-like growth factor I: Tyr-containing synthetic polymers as specific inhibitors or substrates

The receptors for insulin and insulin-like growth factor (IGF) I are structurally similar transmembrane proteins. Ligand binding to the extracellular domain of the receptor stimulates its cytoplasmic tyrosine protein kinase which phosphorylates its own beta subunit as well as exogenous substrates. It is believed, from several lines of evidence, that tyrosine-specific protein kinases are mediating some or all of the actions of insulin (or IGF-I). In order to gain insights into the substrate specificity of the structurally related insulin and IGF-I receptor kinases, we have studied the action of highly purified receptors isolated from human placental membranes. Present studies using selected tyrosine-containing polymers have revealed: (i) Polymers such as (Y,A,E)n and (Y-A-E)n inhibit beta subunit autophosphorylation and exogenous substrate phosphorylation by autophosphorylated receptors. (ii) Insulin receptor kinase is at least 10 times more sensitive to these inhibitors than IGF-I receptor kinase. (iii) (Y-A-E)n is approximately 8 times more potent an inhibitor than (Y,A,E)n toward both receptors. (iv) While (E4,Y1)n and (E6,A3,Y1)n are good substrates for both receptor kinases, the ratio of phosphate incorporation into the former to the latter is characteristically high (approximately 4) for the IGF-I receptor and low (approximately 1) for the insulin receptor. These results imply that the substrate specificity and enzymatic action of these two receptor kinases are distinct.