Activation of transforming potential of the human insulin receptor gene

Insulin--carcinogen or mitogen? Preclinical and clinical evidence from prostate, breast, pancreatic, and colorectal cancer research

Diabetes mellitus is a chronic disease that affects > 23.6 million Americans, and occurs when the body is unable to produce or becomes resistant to endogenous insulin. This alteration of insulin's action reduces adequate utilization of glucose transporter type 4 (GLUT4) receptors, which are responsible for cellular glucose uptake. Thus, exogenous administration of human insulin and insulin analogs is an important modality used to reduce morbidity and mortality in both type 1 and type 2 diabetes. According to 2007 estimates, 27% of all patients with diabetes use some form of insulin therapy. The increasing utilization of insulin has become a cause for concern because findings from several observational trials have suggested an association with an increased risk of developing cancer. To help elucidate the potential interplay between insulin use and cancer, we searched PubMed and MEDLINE to identify articles that assessed the carcinogenic and/or mitogenic potential of diabetes treatments, focusing on insulin specifically. Data from our review suggest that insulin analogs, particularly insulin glargine, may play more of a mitogenic than a carcinogenic role in association with different types of cancer, suggesting an amplified rate of existing tumor growth in the presence of insulin analogs. Evidence for insulin-induced mitogenicity appears to be most prevalent in prostate, breast, pancreatic, and colorectal cancers. In conclusion, the positive effects of insulin therapy on reducing morbidity and mortality in diabetes greatly outweigh the risks at this time. However, clinicians must be diligent in both screening for new cancers in patients receiving insulin and in monitoring for tumor growth or maintenance of remission in patients with existing cancers.

Deletion of the ectodomain unleashes the transforming, invasive, and tumorigenic potential of the MET oncogene

The c-MET proto-oncogene, encoding the p190 hepatocyte growth factor tyrosine kinase receptor, can acquire oncogenic potential by multiple mechanisms, such as gene rearrangement, amplification and overexpression, point mutation, and ectopic expression, all resulting in its constitutive activation. Hepatocyte growth factor receptor truncated forms are generated by post-translational cleavage: p140 and p130 lack the kinase domain and are inactive. Their C-terminal remnant fragments are generally undetectable in normal cells, but a membrane-associated truncated form is recognized by anti-C-terminus antibodies in some human tumors, suggesting that a hepatocyte growth factor receptor lacking the ectodomain, but retaining the transmembrane and intracellular domains (Met-EC-), could acquire oncogenic properties. Herein we show that NIH-3T3 cells transduced with MET-EC- expressed a membrane-associated constitutively tyrosine-phosphorylated 60-kDa protein and, similarly to NIH-3T3 cells expressing the cytosolic oncoprotein Tpr-Met, showed activated extracellular regulated kinase 1/2 mitogen-activated protein kinase and Akt downstream transducers. Compared to control NIH-3T3 cells, NIH-3T3-Met-EC- cells grew faster and showed anchorage-independent growth and invasive properties in all aspects similar to cells expressing the transforming TPR-MET. Nude female mice injected subcutaneously with NIH-3T3-Met-EC- cells developed visible tumors, displaying the typical morphology of carcinomas with polygonal cells, in contrast to sarcomas with spindle-shaped cells induced by the injection of NIH-3T3-Tpr-Met cells. It is suggested that the different subcellular localization of the oncoproteins, more than differences in signal transduction, could be responsible for the tumor phenotype. All together, these data show that deletion of the ectodomain activates the hepatocyte growth factor receptor and its downstream signaling pathways, unleashing its transforming, invasive, and tumorigenic potential.

The mas oncogene as a neural peptide receptor: expression, regulation and mechanism of action

The human mas oncogene, which renders transfected NIH/3T3 cells tumorigenic, was identified as a subtype of angiotensin receptor by transient expression in Xenopus oocytes and stable expression in the mammalian neuronal cell line, NG115-401L. The mas receptor preferentially recognizes angiotensin III, and is expressed at high levels in brain. The mas/angiotensin receptor functions through the breakdown of inositol lipids and can drive DNA synthesis, unlike another inositol-linked peptide receptor, that for bradykinin. Comparative analysis of several early biochemical events elicited by either angiotensin or bradykinin stimulation of mas-transfected cells has not indicated a specific difference correlated with mitogenic activity. In particular, the inositol lipid kinase, phosphatidylinositol-3-kinase, thought to be involved in the mitogenic mechanism of platelet-derived growth factor receptors, is unaffected by activation of mas. These results have shown that a proto-oncogene encodes a neural peptide receptor, indicating that peptide receptors may be involved in differentiation and proliferation processes, as are other identified proto-oncogenes.

Soluble Axl is generated by ADAM10-dependent cleavage and associates with Gas6 in mouse serum

Axl receptor tyrosine kinase exists as a transmembrane protein and as a soluble molecule. We show that constitutive and phorbol 12-myristate 13-acetate-induced generation of soluble Axl (sAxl) involves the activity of disintegrin-like metalloproteinase 10 (ADAM10). Spontaneous and inducible Axl cleavage was inhibited by the broad-spectrum metalloproteinase inhibitor GM6001 and by hydroxamate GW280264X, which is capable of blocking ADAM10 and ADAM17. Furthermore, murine fibroblasts deficient in ADAM10 expression exhibited a significant reduction in constitutive and inducible Axl shedding, whereas reconstitution of ADAM10 restored sAxl production, suggesting that ADAM10-mediated proteolysis constitutes a major mechanism for sAxl generation in mice. Partially overlapping 14-amino-acid stretch deletions in the membrane-proximal region of Axl dramatically affected sAxl generation, indicating that these regions are involved in regulating the access of the protease to the cleavage site. Importantly, relatively high circulating levels of sAxl are present in mouse sera in a heterocomplex with Axl ligand Gas6. Conversely, two other family members, Tyro3 and Mer, were not detected in mouse sera and conditioned medium. sAxl is constitutively released by murine primary cells such as dendritic and transformed cell lines. Upon immobilization, sAxl promoted cell migration and induced the phosphorylation of Axl and phosphatidylinositol 3-kinase. Thus, ADAM10-mediated generation of sAxl might play an important role in diverse biological processes.

Differential requirements of the MAP kinase and PI3 kinase signaling pathways in Src- versus insulin and IGF-1 receptors-induced growth and transformation of rat intestinal epithelial cells

There have been few studies on the specific signaling pathways involved in the transformation of epithelial cells by oncogenic protein tyrosine kinases. Here we investigate the requirement of MAP (MAPK) and phosphatidylinositol 3- (PI3K) kinases in the transformation of rat intestinal epithelial (RIE) cells by oncogenic forms of insulin receptor (gag-IR), insulin-like growth factor-1 receptor (gag-IGFR), and v-Src. MAPK is not significantly activated in cells transformed by gag-IR and gag-IGFR but is activated in v-Src transformed cells. Treatment with PD98059, a MEK inhibitor, at concentrations where MAPK activity was reduced below the basal level showed that MAPK is partially required for the monolayer growth of parental and transformed RIE cells. However, MAPK is not essential for the focus forming ability of the three oncogene-transformed cells. It is also not necessary for the colony forming ability of gag-IR- and gag-IGFR-, but is partially required for v-Src-transformed cells. PI3K is significantly activated in all three oncogene transformed RIE cells. LY294002, a PI3K inhibitor, potently inhibited monolayer growth of all three oncogene-transformed cells. However, at concentrations of LY294002 where activated forms of Akt, a downstream component of the PI3K pathway, were undetectable, colony and focus forming abilities of the v-Src-RIE cells were only slightly affected whereas those of gag-IR/IGFR-RIE cells were greatly inhibited. These results were confirmed using a different pharmacological inhibitor, wortmannin, and a dominant negative form of PI3K, Ap85. Similarly, rapamycin, known to inhibit p70S6 kinase, a downstream component of the PI3K-Akt pathway, also inhibited gag-IR/IGFR-induced, but not v-Src-induced, focus and colony formation. We conclude that the MAPK and PI3K signaling pathways are differentially required for transformation of RIE cells by oncogenic IR and IGFR versus Src and the pattern of requirements is different from that of fibroblast transformation.

Quantitation of IGF-I, IGF-II, and multiple insulin receptor family member messenger RNAs during embryonic development in rainbow trout

The IGF system has been shown to be important for normal embryonic growth in mice. Characterization of the IGF system in lower vertebrates is still in progress. To gain a greater understanding of the IGF system during embryonic development in teleosts, a competitive reverse transcription-polymerase chain reaction (RT-PCR) assay was developed and used to quantitate the levels of IGF-I and IGF-II mRNAs from rainbow trout embryos isolated from a staged series. The absolute number of molecules of IGF-I mRNA/microg total RNA was significantly lower than the absolute number of molecules of IGF-II mRNA/microg total RNA both during early and late embryonic development. The recent identification of multiple IGF type I receptor (rtIGFR) and insulin receptor (rtIR) cDNAs in rainbow trout has provided us with a tool to investigate the expression of these mRNAs. A relative quantitative RT-PCR assay was used to determine the steady state levels of two forms of rtIGFR and three forms of rtIR mRNAs in rainbow trout embryos. The relative levels of rtIGFR mRNAs were greater in embryos compared to adult tissues while the relative levels of rtIR mRNAs were generally lower. In a RT-PCR based assay, a differential ability to detect rtIGFR and rtIR mRNAs was shown, suggesting developmental regulation of polyadenylation. Our results suggest that IGF-II mRNA is the predominant IGF expressed in rainbow trout embryos. Our characterization of IGF ligand and receptor mRNA levels in rainbow trout embryos suggests that a functional IGF system exists during embryonic development in teleosts.

Activation of the insulin-like growth factor type 1 receptor by deletion of amino acids 870-905

We have created a deletion mutant of the insulin-like growth factor type 1 receptor (IGF-1 R) which lacks the 36 amino acids (aa) immediately N-terminal to the transmembrane domain (Delta870-905 IGF-1 R). This region has been reported to have a negative effect on the transforming potential of an avian sarcoma virus gag-IGF-1 R fusion protein. We have sought to determine whether this region plays a similar role in the intact IGF-1 R. Analysis of the tyrosine kinase activity of the Delta870-905 IGF-1 R shows that the mutant receptor is autophosphorylated without IGF-1 stimulation, indicating that the tyrosine kinase domain is constitutively active. In addition, processing of the receptor is decreased, resulting in accumulation of a high molecular weight proreceptor containing both alpha and beta-subunits. A well-characterized substrate of the IGF-1 R, IRS-1, is constitutively phosphorylated by the Delta870-905 IGF-1 R and phosphoinositide (PI) 3-kinase activity, which is normally activated by the phosphorylation of IRS-1 following IGF-1 stimulation, is increased even in the absence of IGF-1. A second intracellular signal pathway normally activated by IGF-1, the MAP kinase pathway, showed no increase in activity in the absence of IGF-1. The Delta870-905 IGF-1 R promoted cell proliferation only in the presence of IGF-1. We conclude that this deletion increases the basal activity of the IGF-1 receptor tyrosine kinase and activates PI 3-kinase, but is unable to stimulate MAP kinase in the absence of ligand. These results confirm those seen in the gag-IGF-1 R fusion protein and indicate that aa 870-905 exert a negative effect on the tyrosine kinase domain of the beta-subunit of the IGF-1 R.

Constitutive proteolysis of the ErbB-4 receptor tyrosine kinase by a unique, sequential mechanism

The heregulin receptor tyrosine kinase ErbB-4 is constitutively cleaved, in the presence or absence of ligand, by an exofacial proteolytic activity producing a membrane-anchored cytoplasmic domain fragment of 80 kD. Based on selective sensitivity to inhibitors, the proteolytic activity is identified as that of a metalloprotease. The 80-kD product is tyrosine phosphorylated and retains tyrosine kinase activity. Importantly, the levels of this fragment are controlled by proteasome function. When proteasome activity is inhibited for 6 h, the kinase-active 80-kD ErbB-4 fragment accumulates to a level equivalent to 60% of the initial amount of native ErbB-4 (approximately 10(6) receptors per cell). Hence, proteasome activity is essential to prevent the accumulation of a significant level of ligand-independent, active ErbB-4 tyrosine kinase generated by metalloprotease activity. Proteasome activity, however, does not act on the native ErbB-4 receptor before the metalloprotease-mediated cleavage, as no ErbB-4 fragments accumulate when metalloprotease activity is blocked. Although no ubiquitination of the native ErbB-4 is detected, the 80-kD fragment is polyubiquitinated. The data, therefore, describe a unique pathway for the processing of growth factor receptors, which involves the sequential function of an exofacial metalloprotease and the cytoplasmic proteasome.

Insulin receptor what role in breast cancer?

It is commonly believed that the insulin receptor mainly mediates the metabolic effects of insulin, whereas the closely related IGF-I receptor is considered a major factor for the regulation of cell proliferation. Experimental and epidemiological evidence indicates, however, that insulin and insulin receptors may play an important role in breast cancer. This article reviews evidence indicating that (a) insulin receptors are overexpressed in human breast cancer, (b) insulin stimulates growth in breast cancer cells, (c) cells transfected with human insulin receptor may acquire a ligand-dependent transformed phenotype, and (d) breast cancer is associated with insulin resistance and hyperinsulinemia. These findings may open new possibilities in breast cancer prevention, prognosis assessment, and therapy. (Trends Endocrinol Metab 1997; 8:306-312). (c) 1997, Elsevier Science Inc.

Transforming activity of retroviral genomes encoding Gag-Axl fusion proteins

Retroviral genomes encoding a portion of the Moloney murine leukemia virus Gag protein fused to portions of the murine axl cDNA were constructed so as to mimic naturally occurring transforming viruses. Virus MA1 retained 5 amino acids of the extracellular domain and the complete transmembrane and intracellular domains of Axl; virus MA2 retained only the intracellular Axl sequences beginning 33 amino acids downstream of the transmembrane region. Although both viruses could transform NIH 3T3 cells, they induced different morphological changes. MA1 transformants became elongated and assumed a cross-hatched pattern, while MA2 transformants were round and very refractile and grew to high density. Gag-Axl and Glyco-Gag-Axl proteins were detected in both types of transformed cells and were predominantly localized to the cytoplasmic compartment. When cell-free v-axl virus supernatants were introduced into wild-type BALB/c neonates, Rag-2-deficient mice, or c-myc transgenic mice, they did not cause tumors in a 3-month period. However, MA2-transformed NIH 3T3 cells, but not MA1 or control cells, could establish sarcomas by subcutaneous or intraperitoneal injection into BALB/c neonates. These results show that the transforming potential of the axl gene can be activated by truncation of the extracellular domain of the receptor and fusion of the remaining sequence to the gag gene.

Cloning and Expression of Chicken Protein Tyrosine Phosphatase Gamma

A 5,403 bp cDNA encoding chicken protein tyrosine phosphatase gamma (PTPgamma) was isolated and sequenced. The predicted open reading frame of 1,422 amino acids (aa) includes 742 aa of extracellular (EC) domain, 26 aa of transmembrane (TM) domain and 634 aa of intracellular domain. The chicken PTPgamma has a 86.7% aa identity to its human homolog and contains the carbonic anhydrase-like domain and fibronectin type III homologous regions in the EC domain, as well as the tandem linked catalytic sequences in the cytoplasmic domain. However, the chicken PTPgamma lacks 29 aa immediate downstream of the putative TM domain in comparison with its human counterpart. Northern analysis revealed the presence of two transcripts of 6.3 and 9.5 kb in various tissues. The cytoplasmic domain of the PTPgamma could be expressed as an enzymatically active form in SF9 insect cells. PTPgamma could also be expressed in normal and rsc-transformed NIH3T3 and Rat 1 cells as a gag-PTP fusion protein, but no detectable effects on growth and colony formation of these cells were observed. Copyright 1996 S. Karger AG, Basel

Biological electron transfer

Many oxidoreductases are constructed from (a) local sites of strongly coupled substrate-redox cofactor partners participating in exchange of electron pairs, (b) electron pair/single electron transducing redox centers, and (c) nonadiabatic, long-distance, single-electron tunneling between weakly coupled redox centers. The latter is the subject of an expanding experimental program that seeks to manipulate, test, and apply the parameters of theory. New results from the photosynthetic reaction center protein confirm that the electronic-tunneling medium appears relatively homogeneous, with any variances evident having no impact on function, and that control of intraprotein rates and directional specificity rests on a combination of distance, free energy, and reorganization energy. Interprotein electron transfer between cytochrome c and the reaction center and in lactate dehydrogenase, a typical oxidoreductase from yeast, are examined. Rates of interprotein electron transfer appear to follow intraprotein guidelines with the added essential provision of binding forces to bring the cofactors of the reacting proteins into proximity.

The transforming receptor tyrosine kinase, Axl, is post-translationally regulated by proteolytic cleavage

Several receptor tyrosine kinases generate soluble ligand binding domains either by differential splicing resulting in a truncated RNA transcript, or by proteolytic cleavage. Although the exact role in vivo of these soluble extracellular domains is unclear, proteolysis may function to down-regulate the receptor, and soluble extracellular domains (ECD) may compete with the intact receptor binding to ligand. Axl is a member of a new class of receptor tyrosine kinases characterized by an ECD resembling cell adhesion molecules and unique sequences in the kinase domain. In addition, Axl is transforming in both fibroblast and hematopoietic cells, and appears to be involved in mesenchymal development. We now find that Axl is post-translationally processed by cleavage in a 14 amino acid region immediately NH2-terminal to the transmembrane domain resulting in a soluble ECD and a membrane bound kinase domain. The sequence of this putative cleavage site shares no homology with recognition sites of known proteases. Characterization of this proteolytic processing shows that it does not require protein synthesis or transport but is augmented by phorbol ester treatment. Since the cleavage of Axl enhances turnover of the kinase on the cell surface, we suggest that proteolytic processing down-regulates Axl kinase activity.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

Ala-->Gly mutation in the putative catalytic loop confers temperature sensitivity on Ros, insulin receptor, and insulin-like growth factor I receptor protein-tyrosine kinases

Temperature-sensitive mutations in the avian sarcoma virus UR2 oncogene ros, encoding a receptor protein-tyrosine kinase (PTK), were identified. The Ala385-->Gly change mapping within the highly conserved RDLAARN motif in the Ros kinase domain was responsible for the temperature-sensitive phenotype. Based on the sequence homology of all known protein kinases and the crystalline structure of the cAMP-dependent protein kinase, this conserved region probably represents the PTK catalytic loop. The same mutation when introduced into the human insulin and insulin-like growth factor I receptors made these PTKs temperature sensitive in both biological function and kinase activity. Our results support the presumed catalytic role of this highly conserved sequence in PTKs. Due to its highly conserved nature, we predict that the same mutation would probably confer temperature sensitivity on other PTKs.

Distinctive effects of the carboxyl-terminal sequence of the insulin-like growth factor I receptor on its signaling functions

We have shown previously that the extracellular sequences of the human insulin receptor (IR) and the insulin-like growth factor I receptor (IGFR) have an inhibitory effect on protein tyrosine kinase (PTK) activity and on the biological functions of their respective Gag-receptor fusion proteins. To study the role of IGFR carboxyl sequence in modulation of the Gag-IGFR PTK and biological activities, five mutants, CM1, CM2, CM3, CM4, and CM5, containing carboxyl deletions of 17, 27, 47, 67, and 88 amino acids (aa), respectively, were constructed from the parental virus UIGFR encoding the Gag-IGFR. Deletion of up to 27 aa had little effect on the cell-transforming and PTK activities of UIGFR. Deletions of 47 aa in CM3 abolished PTK and transforming activities. Surprisingly, a further deletion of 20 aa in CM4 beyond that in CM3 reactivated the kinase and transforming activities. CM5, containing a deletion of 20 aa beyond that in CM4, had only marginal transforming and PTK activities. We conclude that deletion of the carboxyl region of the Gag-IGFR inactivates, instead of activating as in the case with Gag-IR, its transforming activity and the amino acid sequence 1250 to 1310 is essential for PTK and transforming activities. Analysis of the ability of the full-length IGFR and its mutant receptors described above to associate with phosphatidylinositol 3 kinase indicated that the association required PTK activity and tyrosine phosphorylation of the receptors and correlated well with their transforming activities. The carboxyl 88 aa are not essential for the association.

Modulating effects of the extracellular sequence of the human insulinlike growth factor I receptor on its transforming and tumorigenic potential

We reported previously that an N-terminally truncated insulinlike growth factor I receptor (IGFR) fused to avian sarcoma virus UR2 gag p19 had a greater transforming potential than did the native IGFR, but it failed to cause tumors in vivo. To investigate whether the 36 amino acids (aa) of the IGFR extracellular (EC) sequence in the gag-IGFR fusion protein encoded by the retrovirus UIGFR have a modulatory effect on the biological and biochemical properties of the protein, four mutants, NM1, NM2, NM3, and NM4 of the EC sequence were constructed. NM1 lacks the entire 36 aa residues; NM2 lacks the N-terminal 16 aa residues (aa 870 to 885), including two potential N-linked glycosylation sites of the EC sequence; NM3 contains a deletion of the C-terminal 20 aa residues (aa 886 to 905) of the EC sequence; and NM4 contains N-to-Q substitutions at both N-linked glycosylation sites. NM1 was the strongest of the four mutants in promoting anchorage-independent growth of transfected chicken embryo fibroblasts, while NM2 and NM4 had weaker transforming potential than did the original UIGFR virus. Only NM1 and NM3 were able to induce sarcomas in chickens. The four NM mutant-transformed cells expressed the expected proteins with comparable steady-state levels. The in vitro tyrosine kinase activity of P53NM1 was about fourfold higher than that of the parental P57-75UIGFR, whereas NM2 and NM4 proteins exhibited four- to fivefold-lower kinase activities. Despite lacking the IGFR EC sequence, P53NM1 formed covalent dimers similar to those formed by the parental P57-75UIGFR. Increased phosphatidylinositol (PI) 3-kinase activity was found to be associated with the mutant IGFR proteins. Among NM4 proteins. Elevated tyrosine phosphorylation of cellular proteins of 35, 120, 140, 160, and 170 kDa was detected in all mutant IGFR-transformed cells. We conclude that the EC 36-aa sequence of IGFR in the gag-IGFR fusion protein exerts intricate modulatory effects on the protein's transforming and tumorigenic potential. The 20 aa residues immediately upstream of the transmembrane domain have an inhibitory effect on the tumorigenic potential of gag-IGFR, whereas N-linked glycosylation within the EC sequence appears to have a positive effect on the transforming potential of UIGFR. Increased in vitro kinase activity and, to a lesser extent, in vivo tyrosine phosphorylation as well as the elevated association of PI 3-kinase activity with IGFR proteins seem to be correlated with the transforming potential of IGFR mutant proteins.

Transforming properties and substrate specificities of the protein tyrosine kinase oncogenes ros and src and their recombinants

To determine the sequences of the oncogenes src (encoded by Rous sarcoma virus [RSV]) and ros (encoded by UR2) that are responsible for causing different transformation phenotypes and to correlate those sequences with differences in substrate recognition, we constructed recombinants of the two transforming protein tyrosine kinases (PTKs) and studied their biological and biochemical properties. A recombinant with a 5' end from src and a 3' end from ros, called SRC x ROS, transformed chicken embryo fibroblasts (CEF) to a spindle shape morphology, mimicking that of UR2. Neither of the two reverse constructs, ROS x SRC I and ROS x SRC II, could transform CEF. However, a transforming variant of ROS x SRC II appeared during passages of the transfected cells and was called ROS x SRC (R). ROS x SRC (R) contains a 16-amino-acid deletion that includes the 3' half of the transmembrane domain of ros. Unlike RSV, ROS x SRC (R) also transformed CEF to an elongated shape similar to that of UR2. We conclude that distinct phenotypic changes of RSV- and UR2-infected cells do not depend solely on the kinase domains of their oncogenes. We next examined cellular proteins phosphorylated by the tyrosine kinases of UR2, RSV, and their recombinants as well as a number of other avian sarcoma viruses including Fujinami sarcoma virus Y73, and some ros-derived variants. Our results indicate that the UR2-encoded receptorlike PTK P68gag-ros and its derivatives have a very restricted substrate specificity in comparison with the nonreceptor PTKs encoded by the rest of the avian sarcoma viruses. Data from ros and src recombinants indicate that sequences both inside and outside the catalytic domains of ros and src exert a significant effect on the substrate specificity of the two recombinant proteins. Phosphorylation of most of the proteins in the 100- to 200-kDa range correlated with the presence of the 5' src domain, including the SH2 region, but not with the kinase domain in the recombinants. This corroborates the conclusion given above that the kinase domain of src or ros per se is not sufficient to dictate the transforming morphology of these two oncogenes. High-level tyrosyl phosphorylation of most of the prominent substrates of src is not sufficient to cause a round-shape transformation morphology.

Enhancement of transforming potential of human insulinlike growth factor 1 receptor by N-terminal truncation and fusion to avian sarcoma virus UR2 gag sequence

The human insulinlike growth factor 1 (hIGF-1) receptor (hIGFR) is a transmembrane protein tyrosine kinase (PTK) molecule which shares high sequence homology in the PTK domain with the insulin receptor and, to a lesser degree, the ros transforming protein of avian sarcoma virus UR2. To assess the transforming potential of hIGFR, we introduced the intact and altered hIGFR into chicken embryo fibroblasts (CEF). The full-length hIGFR cDNA (fIGFR) was cloned into a UR2 retroviral vector, replacing the original oncogene v-ros. fIGFR was able to promote the growth of CEF in soft agar and cause morphological alteration in the absence of added hIGF-1 to medium containing 11% calf and 1% chicken serum. The transforming ability of hIGFR was not further increased in the presence of 10 nM exogenous hIGF-1. The 180-kDa protein precursor of hIGFR was synthesized and processed into alpha and beta subunits. The overexpressed hIGFR in CEF bound hIGF-1 with high affinity (Kd = 5.4 x 10(-9) M) and responded to ligand stimulation with increased tyrosine autophosphorylation. The cDNA sequence coding for part of the beta subunit of hIGFR, including 36 amino acids of the extracellular domain and the entire transmembrane and cytoplasmic domains, was fused to the 5' portion of the gag gene in the UR2 vector to form an avian retrovirus. The resulting virus, named UIGFR, was able to induce morphological transformation and promote colony formation of CEF with a stronger potency than did fIGFR. The UIGFR genome encodes a membrane-associated, glycosylated gag-IGFR fusion protein. The specific tyrosine phosphorylation of the mature form of the fusion protein, P75, is sixfold higher in vitro and threefold higher in vivo than that of the native IGFR beta subunit, P95. In conclusion, overexpression of the native or an altered hIGFR can induce transformation of CEF with the gag-IGFR fusion protein possessing enhanced transforming potential, which is consistent with its increased in vitro and in vivo tyrosine phosphorylation.

Molecular basis of the activation of the tumorigenic potential of Gag-insulin receptor chimeras

A previous study showed that the human insulin receptor (IR) could be activated by insertion of a 3' portion of the cDNA encoding the beta subunit into a retrovirus genome to form a Gag-IR fusion protein. While capable of transforming cells in culture, this IR cDNA-containing virus, called UIR, was not able to induce tumors in animals. Subsequently, we isolated a spontaneous sarcomagenic variant called UIR19t from the parental UIR. UIR19t was molecularly cloned, sequenced, and found to harbor two mutations. A 44-amino acid deletion immediately upstream from the transmembrane domain of the Gag-IR fusion protein removes all the extracellular sequence of the IR remaining in the original UIR construct. In addition, a single nucleotide deletion at the 3' end results in truncation and replacement of the carboxyl-terminal 12 amino acids by 4 new amino acids. The specific kinase activity of UIR19t is 4- to 5-fold higher than that of the parental UIR. However, no new cellular substrates were detected in UIR19t-transformed cells as compared to UIR cells. Viruses containing either the 5' or the 3' deletion mutation were constructed and assessed for their biological function. Our data indicate that the 5' deletion alone is sufficient to confer tumorigenic ability. We conclude that sequence immediately upstream from the transmembrane domain imposes a negative effect on the transforming and tumorigenic potential of the Gag-IR fusion protein.

Two point mutations in the transmembrane domain of P68gag-ros inactive its transforming activity and cause a delay in membrane association

The transforming protein of the avian sarcoma virus UR2 is a 68-kDa transmembrane tyrosine protein kinase. We examined the relationship between membrane localization and transforming activity of P68 by changing Val-168-Val-169 in its hydrophobic domain into Asp-168-Glu-169. The resulting transmembrane (TM) mutant (P68TM) lost transforming activity toward chicken embryo fibroblasts (CEF). We found that the mutant protein was expressed and rapidly degraded into a smaller form which was still membrane associated and kinase active. The instability of the TM mutant protein is a phenomenon only manifested in CEF, because the same mutant protein was expressed with efficiency and stability similar to those of the wild-type protein in a transient expression system in COS cells. However, there are several differences between the wild-type and the TM mutant proteins in COS cells. The wild-type protein is more heavily phosphorylated and associated with membrane fractions in a cotranslational manner. It is enzymatically active when recovered from membrane fractions. The TM mutant protein is less phosphorylated, more labile toward protease degradation, and delayed in membrane association, with a lag period of 30 min or longer, and has little kinase activity when recovered from membrane fractions. Most of the kinase-active TM mutant protein was found in the cytosol fractions. Despite the delay, most of the TM protein in COS cells was found to be membrane associated, and its orientation on the cell surface was similar to that of the wild-type protein. It is probable that loss of the CEF-transforming activity of the TM mutant protein is due to its susceptibility to protease degradation resulting from improper membrane association of the newly synthesized product. The differences in the kinetics of membrane association and the distribution of kinase activity in COS cells might not be directly applicable in explaining the inability of the TM mutant to transform CEF but are intriguing as regards protein biosynthesis and translocation. The difference between CEF and COS cells implies that different factors or pathways are involved in the biosynthesis and processing of the TM mutant protein in these two cellular environments. Changes of P68TM in the kinetics of membrane association indicate that the transmembrane domain of ros, besides functioning as a membrane anchor, also plays a role in directing initial membrane association.

Role of gag sequence in the biochemical properties and transforming activity of the avian sarcoma virus UR2-encoded gag-ros fusion protein

The transforming protein P68gag-ros of avian sarcoma virus UR2 is a transmembrane tyrosine protein kinase molecule with the gag portion protruding extracellularly. To investigate the role of the gag moiety in the biochemical properties and biological functions of the P68gag-ros fusion protein, retroviruses containing the ros coding sequence of UR2 were constructed and analyzed. The gag-free ros protein was expressed from one of the mutant retroviruses at a level 10 to 50% of that of the wild-type UR2. However, the gag-free ros-containing viruses were not able to either transform chicken embryo fibroblasts or induce tumors in chickens. The specific tyrosine protein kinase activity of gag-free ros protein is about 10- to 20-fold reduced as judged by in vitro autophosphorylation. The gag-free ros protein is still capable of associating with membrane fractions including the plasma membrane, indicating that sequences essential for recognition and binding membranes must be located within ros. Upon passages of the gag-free mutants, transforming and tumorigenic variants occasionally emerged. The variants were found to have regained the gag sequence fused to the 5' end of the ros, apparently via recombination with the helper virus or through intramolecular recombination between ros and upstream gag sequences in the same virus construct. All three variants analyzed code for gag-ros fusion protein larger than 68 kDa. The gag-ros recombination junction of one of the transforming variants was sequenced and found to consist of a p19-p10-p27-ros fusion sequence. We conclude that the gag sequence is essential for the transforming activity of P68gag-ros but is not important for its membrane association.

Enhancement of tyrosine kinase activity of the Drosophila epidermal growth factor receptor homolog by alterations of the transmembrane domain

The Drosophila epidermal growth factor receptor homolog (DER) displays sequence similarity to both the epidermal growth factor (EGF) receptor and the neu vertebrate proteins. We have examined the possibility of deregulating the tyrosine kinase activity of DER by introducing structural changes which mimic the oncogenic alterations in the vertebrate counterparts. Substitution of valine by glutamic acid in the transmembrane domain, in a position analogous to the oncogenic mutation in the rat neu gene, elevated the in vivo kinase activity of DER in Drosophila Schneider cells sevenfold. A chimera containing the oncogenic neu extracellular and transmembrane domains and the DER kinase region, also showed a threefold elevated activity relative to a similar chimera with normal neu sequences. Double truncation of DER in the extracellular and cytoplasmic domains, mimicking the deletions in the v-erbB oncogene, did not however result in stimulation of in vivo kinase activity. The chimeric constructs were also expressed in monkey COS cells, and similar results were obtained. The ability to enhance the DER kinase activity by a specific structural modification of the transmembrane domain demonstrates the universality of this activation mechanism and strengthens the notion that this domain is intimately involved in signal transduction. These results also support the inclusion of DER within the tyrosine-kinase receptor family.

Overexpression of the human insulinlike growth factor I receptor promotes ligand-dependent neoplastic transformation

The human insulinlike growth factor I receptor was overexpressed in NIH 3T3 cells as well as human and rat primary fibroblast strains. The NIH 3T3 cells displayed a ligand-dependent, highly transformed phenotype. When exposed to insulinlike growth factor I or supraphysiologic levels of insulin, NIH 3T3 cells that expressed high levels of receptors formed aggregates in tissue culture dishes, colonies in soft agar, and tumors in nude mice. Expression of 1 million receptors per cell, a 40-fold increase above the base-line level, was required for anchorage-independent growth. Primary fibroblasts that expressed high levels of receptors displayed a ligand-dependent change in morphology and an increase in saturation density but did not acquire a fully transformed phenotype. The results demonstrate that when amplified, this ubiquitous growth factor receptor behaves like an oncogenic protein and is capable of promoting neoplastic growth in vivo.

Overexpression of the human insulinlike growth factor I receptor promotes ligand-dependent neoplastic transformation

The human insulinlike growth factor I receptor was overexpressed in NIH 3T3 cells as well as human and rat primary fibroblast strains. The NIH 3T3 cells displayed a ligand-dependent, highly transformed phenotype. When exposed to insulinlike growth factor I or supraphysiologic levels of insulin, NIH 3T3 cells that expressed high levels of receptors formed aggregates in tissue culture dishes, colonies in soft agar, and tumors in nude mice. Expression of 1 million receptors per cell, a 40-fold increase above the base-line level, was required for anchorage-independent growth. Primary fibroblasts that expressed high levels of receptors displayed a ligand-dependent change in morphology and an increase in saturation density but did not acquire a fully transformed phenotype. The results demonstrate that when amplified, this ubiquitous growth factor receptor behaves like an oncogenic protein and is capable of promoting neoplastic growth in vivo.

Proteolytic generation of constitutive tyrosine kinase activity of the human insulin receptor

Structural modification induced by partial digestion with trypsin has been shown to stimulate the tyrosine kinase activity of the insulin receptor both in solution and in intact cells [Tamura, Fujita-Yamaguchi & Larner (1983) J. Biol. Chem. 258, 14749-14752; Goren, White & Kahn (1987) Biochemistry 26, 2374-2382; Leef & Larner (1987) J. Biol. Chem. 262, 14837-14842]. Furthermore, experiments involving deletion of sequences encoding the extracellular domain of the insulin receptor suggest that it may function as a protooncogene in fibroblasts [Wang et al., (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 5725-5729]. To further understand the structural requirements that generate this activity, the major activated fragments generated in solution following trypsin digestion have been characterized here, one of which is shown to have a similar amino acid sequence to a transforming protein. Furthermore, treatment with trypsin of intact Chinese hamster ovary cells that overexpress the human insulin receptor stimulates both autophosphorylation of the receptor and 2-deoxyglucose uptake into the cells, but does not enhance receptor internalization. Unlike digestion in solution, no proteolysis or loss of activity of the activated insulin receptor beta-subunit could be detected using intact cells, even at high trypsin concentrations, despite the existence of extracellular sites that are readily cleaved by trypsin in the solubilized receptor. These studies provide further detail of a mechanism used during trypsinization of cells in culture which mimics activation of the insulin receptor and contributes to stimulation of growth.