Phosphorus transport pathways to streams in tile-drained agricultural watersheds

Agriculture is a major nonpoint source of phosphorus (P) in the Midwest, but how surface runoff and tile drainage interact to affect temporal concentrations and fluxes of both dissolved and particulate P remains unclear. Our objective was to determine the dominant form of P in streams (dissolved or particulate) and identify the mode of transport of this P from fields to streams in tile-drained agricultural watersheds. We measured dissolved reactive P (DRP) and total P (TP) concentrations and loads in stream and tile water in the upper reaches of three watersheds in east-central Illinois (Embarras River, Lake Fork of the Kaskaskia River, and Big Ditch of the Sangamon River). For all 16 water year by watershed combinations examined, annual flow-weighted mean TP concentrations were >0.1 mg L(-1), and seven water year by watershed combinations exceeded 0.2 mg L(-1). Concentrations of DRP and particulate P (PP) increased with stream discharge; however, particulate P was the dominant form during overland runoff events, which greatly affected annual TP loads. Concentrations of DRP and PP in tiles increased with discharge, indicating tiles were a source of P to streams. Across watersheds, the greatest DRP concentrations (as high as 1.25 mg L(-1)) were associated with a precipitation event that followed widespread application of P fertilizer on frozen soils. Although eliminating this practice would reduce the potential for overland runoff of P, soil erosion and tile drainage would continue to be important transport pathways of P to streams in east-central Illinois.

Estimated historical and current nitrogen balances for Illinois

The Midwest has large riverine exports of nitrogen (N), with the largest flux per unit area to the Mississippi River system coming from Iowa and Illinois. We used historic and current data to estimate N inputs, outputs, and transformations for Illinois where human activity (principally agriculture and associated landscape drainage) have had a dominant impact. Presently, approximately 800,000 Mg of N is added each year as fertilizer and another 420,000 Mg is biologically fixed, primarily by soybean (Glycine max L. Merr.). These annual inputs are greater than exports in grain, which results in surplus N throughout the landscape. Rivers within the state export approximately 50% of this surplus N, mostly as nitrate, and the remainder appears to be denitrified or temporarily incorporated into the soil organic matter pool. The magnitude of N losses for 1880, 1910, 1950, and 1990 are compared. Initial cultivation of the prairies released large quantities of N (approximately 500,000 Mg N year(-1)), and resulted in riverine N transport during the late 19th century that appears to have been on the same order of magnitude as contemporary N losses. Riverine flux was estimated to have been at a minimum in about 1950, due to diminished net mineralization and low fertilizer inputs. Residual fertilizer N from corn (Zea mays L.), biological N fixed by soybean, short-circuiting of soil water through artificial drainage, and decreased cropping-system diversity appear to be the primary sources for current N export.

Defective cleavage of membrane bound TGFalpha leads to enhanced activation of the EGF receptor in malignant cells

Transforming growth factor alpha (TGFalpha) is widely expressed in malignant as well as normal cells and is involved in regulating cell growth and differentiation. Although processing of TGFalpha has been extensively studied in normal cells, there is little information regarding TGFalpha cleavage in malignant cells. Therefore, we compared the processing of TGFalpha in two human colon carcinoma cell lines. We found that there was a defective cleavage pattern for the TGFalpha precursor resulting in retention of partially processed TGFalpha on the cell surface of both the HCT116a2alphaS3 and CBS4alphaS2 cell lines. This raised the possibility that signaling from the resulting defective cleavage species could differ from that of soluble TGFalpha. The membrane-associated TGFalpha induced higher phosphorylation of EGFR on the cell surface of adjacent cells than equivalent levels of mature TGFalpha. The interaction of membrane bound TGFalpha precursor with the EGFR caused a slower internalization of activated EGFR relative to the internalization of the soluble TGFalpha/EGFR complexes. In addition, the tethered TGFalpha was resistant to the ability of protein-tyrosine phosphatases (PTPs) to reduce EGFR tyrosine phosphorylation, also contributing to higher activation of EGFR. The enhanced activation of EGFR by the tethered form of TGFalpha was reflected by higher activation of Grb2, SHC and Erk downstream mediators of EGF receptor signaling. The higher activation of EGFR by membrane tethered TGFalpha indicates that defective TGFalpha processing provides a mechanism whereby malignant cells can obtain a growth advantage over normal cells.

Role of carbohydrate structures in the binding of beta1-latency-associated peptide to ligands

Transforming growth factor beta1 (TGF-beta1) is a potent growth differentiation and morphogenesis factor. The amino-terminal 248 amino acid pro region of TGF-beta1, the beta1-latency-associated peptide (beta1-LAP), is noncovalently associated with TGF-beta1 in an inactive complex. Previous studies suggested that deglycosylated beta1-LAP can not form this latent complex with TGF-beta1. To study the role of the carbohydrate structures of beta1-LAP in its biological functions, we expressed simian beta1-LAP in Escherichia coli with a 10 histidine residue tag on the N-terminus. This polypeptide was solubilized from inclusion bodies with 6 M guanidine hydrochloride and purified by metal chelate affinity chromatography. Purified beta1-LAP was refolded to its dimeric form using a chaotrope-mediated folding procedure. The dimeric beta1-LAP forms 90 kDa complexes with TGF-beta1, TGF-beta2, and TGF-beta3, and reverses the inhibitory activity of TGF-beta1 on Mv1Lu cells. Solid phase binding assays demonstrate that refolded beta1-LAP binds to heparin and thrombospondin 1. FET cell adhesion promoted by refolded beta1-LAP was blocked by an RGD peptide. Purified beta1-LAP produced in Chinese hamster ovary cells, deglycosylated with N-glycosidase F, forms a 80-90 kDa complex with mature TGF-beta1. The carbohydrate structures of beta1-LAP are not required for binding to ligands or for its biological activity.

Differential control of expression of type I and type II receptors of transforming growth factor-beta in colon carcinoma cells

We investigated TGF-beta response and the expression of TGF-beta receptors in clones of MOSER colon carcinoma cells (designated MOSER II and MOSER III-10) as a function of their growth state. TGF-beta 1 response as assessed by induction of fibronectin expression was higher (3.0-fold) in exponentially growing cells than in quiescent cells. The expression of type I receptor (RI) mRNA was greater (2.5 to 3.0-fold) in exponentially growing cells than in quiescent cells. In contrast, the expression of type II receptor (RII) mRNA was marginally increased in quiescent cells relative to exponential cells. Nuclear run-off assays, and actinomycin-D treatment indicated that the increased expression of RI mRNA in exponentially growing cells was primarily due to an increase in transcription, while a marginal increase in mRNA level for RII in quiescent cells was primarily due to an increase in mRNA stability. Affinity cross-linking with 125I-labeled TGF-beta 1, showed that the exponentially growing cells displayed greater amounts of 125I TGF-beta 1 binding to RI and RII than quiescent cells, indicating that increased cell surface expression of receptors was correlated with increased response in the exponential growth state. Immunoblot analysis also indicated greater amounts of RI protein in exponential compared to quiescent cells; however, no difference in RII protein was observed in the two growth states. These data indicate that expression of the receptors responsible for TGF-beta signal transduction are differentially controlled.

Structural characterization of the latent complex between transforming growth factor beta 1 and beta 1-latency-associated peptide

The formation of a non-covalent complex between mature transforming growth factor beta 1 (TGF-beta 1) and its pro region, the beta 1-latency-associated peptide (beta 1-LAP), is important in regulating the activity of this multipotent growth factor. We have overexpressed simian beta 1-LAP in Chinese hamster ovary (CHO) cells to produce a cell line which secretes beta 1-LAP into the culture medium at > 1 mg/l, thus enabling structural studies of complex formation between beta 1-LAP and TGF-beta 1. The simian beta 1-LAP expressed in CHO cells reversed the growth inhibitory effect of exogenous TGF-beta 1 on Mv1Lu (mink lung epithelial) cells and was able to form a cross-linked complex with 125I-TGF-beta 1. Simian beta 1-LAP was purified to homogeneity by a combination of ammonium sulphate precipitation, gel filtration, dye ligand chromatography and anion-exchange chromatography, with a yield of 15%. The purified protein had an apparent molecular mass of 114 kDa as determined by SDS/PAGE, which is greater than that determined for the transient expression of simian beta 1-LAP in COS-1 and for the simian precursor of TGF-beta 1 (pro-TGF-beta 1) in CHO cells, this major difference being due to more extensive glycosylation of beta 1-LAP expressed by this CHO clone. Far-UV CD spectroscopy of simian beta 1-LAP indicates a mostly beta-sheet structure, with extensive structural rearrangements occurring upon formation of the latent complex between TGF-beta 1 and beta 1-LAP.

Mutations within subdomain II of the extracellular region of epidermal growth factor receptor selectively alter TGF alpha binding

The interactions of epidermal growth factor (EGF) and transforming growth factor alpha (TGF alpha) with the epidermal growth factor receptor (EGFR) were examined by insertion mutagenesis of the receptor. Seventeen insertions were made throughout a construct containing only the extracellular domain. This truncated receptor (sEGFR) was secreted and had a dissociation constant similar to that of the full-length solubilized receptor. Receptors with insertions within subdomain III were not secreted. Two receptors with insertions at positions 291 and 474, which border subdomain III, have significantly decreased binding to both EGF and TGF alpha relative to wild type. This confirms previous work demonstrating that subdomain III forms the primary binding site for EGF and TGF alpha. Four of the mutants within subdomain II had a decreased binding to TGF alpha relative to wild type, but had wild type binding to EGF. These results suggest that a region within subdomain II may selectively regulate the binding of TGF alpha. Two receptors which contained insertions within subdomains II and IV, approximately equidistant from the center of subdomain III, bound twofold more ligand molecules than wild type receptor, with an affinity similar to that of wild type receptor. These findings suggest that insertion at these positions allows the access of more than one ligand molecule to the binding site.

Demonstration that mutation of the type II transforming growth factor beta receptor inactivates its tumor suppressor activity in replication error-positive colon carcinoma cells

Escape from negative growth regulation by transforming growth factor beta (TGF-beta) as a result of the loss of TGF-beta type II receptor (RII) expression has been found to be associated with the replication error (RER) colorectal cancer genotype, which is characteristic of hereditary nonpolyposis colorectal cancers. The RER-positive HCT 116 colon carcinoma cell line was examined for RII mutations. A 1-base deletion was found within a sequence of 10 repeating adenines (nucleotides 709-718), which resulted in a frameshift mutation. Although it is reasonable to predict that the loss of RII function would be an important determinant of malignancy, the large number of potential mutations in cells of this phenotype raises the possibility that an RII mutation may not be a key event in the tumorigenic phenotype of these cells. One way to test directly the importance of RII mutations in determining the malignant phenotype would be to restore its expression. If restoration of expression leads to diminished tumorigenicity, it would indicate that RII mutation is an important determinant of malignancy in the RER phenotype. To determine whether restoration of RII would lead to reversal of malignancy in RER colon cancers, an RII expression vector was transfected into the HCT 116 cell line. RII stable clones showed mRNA and protein expression of transfected RII. The fibronectin mRNA level was increased by exogenous TGF-beta 1 treatment in a dose-dependent manner in RII-positive clones, whereas the control cells remained insensitive. The RII transfectants showed reduced clonogenicity in both monolayer culture and soft agarose. They were growth arrested at a lower saturation density than control cells. TGF-beta 1-neutralizing antibody stimulated the proliferation of RII-transfected but not control cells, indicating that the alterations in the growth parameters of the transfected cells were due to the acquisition of autocrine-negative activity. Tumorigenicity in athymic mice was reduced and delayed in RII transfectants. These results indicate that reconstitution of TGF-beta autocrine activity by reexpression of RII can reverse malignancy in RER colon cancers, thus verifying that the malignancy of hereditary nonpolyposis colorectal cancer can be directly associated with the loss of RII expression.

Processing of transforming growth factor beta 1 precursor by human furin convertase

Proteolytic processing of the transforming growth factor beta precursor (pro-TGF beta) is an essential step in the formation of the biologically active TGF beta homodimeric protein (TGF beta). The 361-amino-acid precursor pro-TGF beta 1 has within its primary structure the R-H-R-R processing signal found in many constitutively secreted precursor proteins and potentially recognized by members of the mammalian convertase family of endoproteases. To determine whether cleavage of pro-TGF beta 1 can be achieved by the furin convertase in vitro, purified precursor was incubated in the presence of a truncated/secreted form of the enzyme. Immunoblots showed that the 55-kDa pro-TGF beta 1 was converted into the 44 and 12.5 kDa bands corresponding to the pro-region and the mature monomer, respectively. Treatment of pro-TGF beta 1 with furin resulted in a 5-fold increase in the production of biologically active TGF beta 1. Furthermore, when expressed in the furin-deficient LoVo cells, no processing of pro-TGF beta 1 was observed. In contrast, efficient processing was observed when pro-TGF beta was coexpressed with the furin convertase. Collectively, these results provide evidence that in our experimental systems the TGF beta 1 precursor is efficiently and correctly processed by human furin thus permitting release of the biologically active peptide.

Expression of transforming growth factor beta type II receptor leads to reduced malignancy in human breast cancer MCF-7 cells

The role of transforming growth factor (TGF) beta type II receptor in reversing the malignant phenotype of human breast cancer MCF-7 cells was examined. MCF-7 cells were insensitive to TGF beta 1 and expressed undetectable levels of cell surface TGF beta type I receptor (RI) and type II receptor (RII) by cross-linking with 125I-TGF beta 1. Stable transfection of a RII expression vector yielded 3 transfectants with varying levels of exogenous RII mRNA and protein levels. Expression of RII also increased TGF beta 1 binding to RI in all 3 clones. Proliferation of RII-positive clones was inhibited by exogenous TGF beta 1 in a dose-dependent manner, whereas the control clones remained TGF beta-insensitive. The RII transfectants were growth arrested in monolayer culture at saturation densities which were 41-66% of that of the Neo controls. They also showed reduced clonogenicity in soft-agarose. Tumorigenicity in ovariectomized, estrogen-supplemented nude mice was delayed in transfectants with low RII levels. Transfectants expressing high levels of RII showed a large reduction in tumorigenicity as well as a longer delay in tumor formation. Tumor growth was associated with loss of exogenous RII expression in transfectants. The results indicate that when systems for TGF beta signal transduction are intact, reconstitution of the TGF beta receptor system can lead to reversion of malignancy in cells lacking RII.

Differential stereospecificities and affinities of folate receptor isoforms for folate compounds and antifolates

Two membrane folate receptor (MFR) isoforms are present in human tissues i.e. MFR-1 (e.g. placenta) and MFR-2 (e.g. placenta, KB cells, CaCo-2 cells). MFR-1 was expressed in COS-1 cells and the resulting protein had the same polypeptide molecular weight as the native protein. The affinities of (6S) and (6R) diastereoisomers of N5-methyltetrahydrofolate, N5-formyltetrahydrofolate, and 5,10-dideazatetrahydrofolate as well as folic acid and methotrexate to MFR-1, MFR-2 and placental MFR (MFR-1 plus MFR-2) were determined in terms of the Ki values for their competitive inhibition of the binding of [3H]folic acid to these proteins. The results indicated a striking difference in the stereospecificity of MFR-1 and MFR-2 for reduced folate coenzymes; MFR-2 preferentially bound to the physiological (6S) diastereoisomers and MFR-1 bound preferentially to the unphysiological (6R) diastereoisomers, while dideazatetrahydrofolate did not show significant stereospecificity for MFR-1. Furthermore, MFR-2 displayed significantly (2- to 100-fold) greater affinities for all the compounds tested compared to MFR-1. Purified placental MFR, a natural source of MFR-1 which contains variable amounts of MFR-2, showed intermediate Ki values for the compounds tested compared with MFR-1 and MFR-2 and stereospecificities similar to MFR-1. These observations demonstrate striking differences in the ligand binding sites of MFR-1 and MFR-2 which could potentially be exploited in the design of MFR isoform specific antifolates.

Transforming growth factor-beta 1 inhibits aldosterone and stimulates adrenal renin in cultured bovine zona glomerulosa cells

Transforming growth factors-beta (TFG beta s) are multifunctional peptides that affect proliferation, differentiation, and many other functions in a variety of cell types. In this study we examined the effect of TGF beta 1 on aldosterone and adrenal renin production using cultured bovine adrenal zona glomerulosa cells. Collagenase-dispersed zona glomerulosa cells were incubated in PFMR-4 medium containing 10% fetal calf serum for 72 h, and the medium was replaced with serum-free medium for the next 24 h. The cells during this 24-h period were exposed to TGF beta 1, ACTH, and (Bu)2cAMP (dbcAMP). It was observed that TGF beta 1 at 1 nM 1) inhibited basal aldosterone secretion from 680.0 +/- 40.0 to 270.0 +/- 10.0 pg/10(6) cells.h, 2) inhibited ACTH- and dbcAMP-stimulated aldosterone production, 3) increased levels of active renin in the cells from 17.8 +/- 2.5 to 70.7 +/- 4.4 pg angiotensin-I/10(6) cells.h and prorenin from 270.0 +/- 5.0 to 970.0 +/- 90 pg angiotensin-I/10(6) cells.h, 4) stimulated prorenin in the medium synergistically in combination with ACTH and dbcAMP, and 5) had no significant effect on basal cAMP production, but significantly inhibited the ACTH-stimulated production of cAMP. These observations show that TGF beta 1 is a potent inhibitor of basal and ACTH- and cAMP-stimulated aldosterone production and inhibits ACTH-stimulated cAMP production. Contrary to its effect on aldosterone, TGF beta 1 stimulates the synthesis and release of adrenal renin and prorenin. TGF beta 1 may act as an autocrine or paracrine regulator of aldosterone production.

Characterization of the binding of transforming growth factor-beta 1, -beta 2, and -beta 3 to recombinant beta 1-latency-associated peptide

Preprotransforming growth factor-beta 1 (TGF beta 1) is a 390-amino acid precursor polypeptide that undergoes a number of processing steps to yield mature TGF beta 1 (amino acid residues 279-390) and a pro portion (residues 30-278) termed beta 1-latency-associated peptide (beta 1LAP). The dimeric form of beta 1LAP has been shown to associate noncovalently with the mature growth factor, resulting in inactivation of biological activity. To further characterize this interaction, the mature TGF beta 1 was radioiodinated and used to determine dissociation constants. A cross-linking method using the bifunctional covalent cross-linker bis-(sulfosuccinimidyl)suberate was found to be the best approach for measuring the amount of bound growth factor. The efficiency of cross-linking was constant within each experiment and varied between 45-55%. Saturation plots and their associated Scatchard analyses indicate apparent Kd values between 1.1-1.8 nM. Competition of TGF beta 1 binding to beta 1LAP by TGF beta 2 and TGF beta 3 (two closely related growth factors) revealed that the latter also bind beta 1LAP tightly, with apparent Kd values of 1.9 and 0.4 nM, respectively.

Demonstration of relaxin precursors in pregnant rat ovaries with antisera against bacterially expressed rat prorelaxin

The existence of rat 18-kilodalton (kDa) prorelaxin, which has been postulated from the coding sequence of cloned cDNA and the results of cell-free translation studies, has been directly demonstrated in rat ovaries with antibodies against bacterially expressed rat prorelaxin. The peptide expressed in E. coli from a rat prorelaxin cDNA construct was comprised of the B- and A-chains of relaxin and a 105-amino acid connecting region. Immunoreactive bands of 18 and 16.5 kDa were shown in ovaries from day 20 pregnant rats. Partial amino acid sequence analysis of both peptides revealed that they had identical N-terminal sequences, corresponding to rat prorelaxin. Both 18- and 16.5-kDa bands were present only from midpregnancy until near term, when they declined sharply. These changes in the concentration of 18-kDa prorelaxin match changes in preprorelaxin mRNA levels, suggesting that relaxin synthesis is regulated at the transcriptional level and not by protein processing. Prorelaxin was transiently secreted by COS-1 cells transfected with preprorelaxin cDNA. Treatment of culture medium with trypsin resulted in the appearance of material corresponding in size to mature relaxin. Thus, correctly folded prorelaxin appears to be a suitable precursor for relaxin. The combined concentrations of 18- and 16.5-kDa peptides in ovaries on day 20 of pregnancy were considerably more than 30 times greater than that of relaxin, however, suggesting that prorelaxin might also be more than a precursor per se.

Identification and analysis of discrete functional domains in the pro region of pre-pro-transforming growth factor beta 1

A series of site-specific insertion and deletion mutants was prepared in the pro domain of transforming growth factor beta 1 (TGF beta 1) encoded by simian TGF beta 1 cDNA. These mutants were transiently expressed in COS-1 cells and the ability of each to be properly processed, folded correctly, and secreted was determined by immunoblot analysis of cells and culture supernatants. Insertions in regions corresponding to amino acid residues 50, 154, and 170 blocked secretion; culture supernatants from COS-1 cells showed no immunologically reactive proteins, whereas intact cells contained high levels of the mutant polypeptides. Insertions in the middle portion of the pro domain at residues 81, 85, and 144 affected disulfide maturation of the mature TGF beta 1. An insertion at residue 110, on the other hand, appeared to destabilize the mature TGF beta 1 polypeptide, resulting in degraded growth factor. Relatively small (10 amino acids) to large (125 amino acids) deletion mutations in the pro domain of TGF beta 1, when expressed as the full-length pre-pro-TGF beta 1, appeared to block secretion. By contrast, if the pro domain (designated beta 1-latency-associated peptide [beta 1-LAP]) was expressed independently, deletion mutants in the region 40-110 were readily secreted by the COS-1 cells, whereas deletions in residues 110-210 either destabilized the structure of the protein or blocked its intracellular transport. Cross-linking assays employing radioiodinated TGF beta 1 and biological assays indicate that residues 50-85 of beta 1-LAP are required for association with mature TGF beta 1.

The pro domain of pre-pro-transforming growth factor beta 1 when independently expressed is a functional binding protein for the mature growth factor

Transforming growth factor beta 1 (TGF-beta 1) is proteolytically derived from the carboxyl terminus of a 390 amino acid precursor molecule termed pre-pro-TGF-beta 1. Previous studies have suggested that the pro piece of pre-pro-TGF-beta 1 may play an important role in the formation of an inactive, latent complex. These latent forms are thought to be important in the regulation of TGF-beta 1 activity. To understand this latent complex in more detail, we have expressed the pro domain of pre-pro-TGF-beta 1 in tissue culture cells independent of the mature growth factor. A stop codon was genetically engineered into the cDNA of pre-pro-TGF-beta 1 by changing the Arg-278 codon from CGA to the STOP codon TGA. The resulting protein is truncated just prior to the amino-terminal Ala residue of the mature growth factor. Transient expression studies and immunoblotting indicate that this pro piece is readily made and secreted by the COS-1 cells; the major form of the expressed pro piece, when analyzed by SDS-polyacrylamide gel electrophoresis, behaves as a disulfide-linked dimer (Mr 80,000). Bioassays, using mink lung indicator cells, reveal that the pro domain forms an inactive complex with exogenously added mature TGF-beta 1. Treatment of this complex with heat or acid results in the release of active TGF-beta 1, indicating an in vitro structure similar to natural, latent TGF-beta 1 complexes. The pro piece from TGF-beta 1 was also found to form latent structures with two closely related family members, TGF-beta 1.2 and TGF-beta 2.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of activation of latent recombinant transforming growth factor beta 1 by plasmin

Medium conditioned by Chinese hamster ovary (CHO) cells transfected with the simian pre-pro-TGF beta 1 cDNA contains high levels of latent TGF beta 1. The amino-terminal region of the TGF beta 1 precursor is secreted and can be detected in the conditioned medium by immunoblotting using peptide antibodies specific for amino-terminal peptides. Chemical cross-linking of CHO-conditioned medium using bis-(sulfosuccinimidyl)-suberate (BS3) followed by immunoblot analyses indicates that latent recombinant TGF beta 1 contains both the cleaved amino-terminal glycopeptide and mature TGF beta 1 polypeptide in a noncovalent association and that this association confers latency. The data presented here do not support the involvement of a unique TGF beta binding protein(s) in latent recombinant TGF beta 1. Plasmin treatment of CHO-conditioned medium resulted in the appearance of TGF beta competing activity. In addition, immunoblot analysis of plasmin-treated CHO-conditioned medium indicates that the amino-terminal glycopeptide is partially degraded and that mature TGF beta 1 is released. Thus, activation of latent TGF beta 1 may occur by proteolytic nicking within the amino-terminal glycopeptide thereby causing a disruption of tertiary structure and noncovalent bonds, which results in the release of active, mature TGF beta 1. Acid activation of latent TGF beta, in comparison, appears to be due to dissociation of the amino-terminal glycopeptide from the mature polypeptide.

Localization of cells containing transforming growth factor-alpha precursor immunoreactivity in the basal ganglia of the adult rat brain

Transforming growth factor-alpha-like immunoreactivity (TGF-alpha-ir) was visualized in the adult rat forebrain using three antisera directed against carboxyterminal sequences in the TGF-alpha precursor. Using immunoperoxidase and immunofluorescence techniques with all three antisera, TGF-alpha-ir was found to be present in a subpopulation of astrocytes in the forebrain. Striatal and pallidal regions of the basal ganglia were studied in detail. In the striatum, there was an uneven distribution of astrocytes containing TGF-alpha-ir, with the greatest number in the dorsal medial third of the caudate-putamen and the overlying corpus callosum/external capsule. In addition, the region of the caudate-putamen bordering the globus pallidus contained numerous clusters of TGF-alpha-ir astrocytes. The globus pallidus itself contained numerous and more evenly distributed TGF-alpha-ir astrocytes. Other pallidal structures--including the ventral pallidum, entopeduncular nucleus, and substantia nigra pars reticulata--contained moderate numbers of TGF-alpha-ir astrocytes. These results suggest that TGF-alpha precursor is present and, perhaps, synthesized in astrocytes. A related growth factor, epidermal growth factor (EGF), has also been reported to be present in pallidal regions of rat brain. Therefore, the TGF-alpha/EGF family of trophic factors may play a role in the function of the central nervous system.

Transforming growth factor beta 1: importance of glycosylation and acidic proteases for processing and secretion

The role of glycosylation of the transforming growth factor-beta 1 (TGF-beta 1) precursor was investigated by treating a transfected Chinese hamster ovary (CHO) cell line expressing high levels of recombinant TGF-beta 1 (TGF-beta 3-2000 cells) with a series of glycosylational inhibitors. Tunicamycin, a nucleoside antibiotic which prevents the formation of the dolichol intermediate necessary for oligosaccharide addition of the nascent polypeptide chain, appeared to block secretory exit and led to an increase in the cellular associated, nonglycosylated pro-TGF-beta 1 form. 1-Deoxymannojirimycin and swainsonine, inhibitors of the mannosidases I and II, respectively, blocked complete glycoprotein processing of the TGF-beta 1 precursor as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by sensitivity to glycosidases. However, the abnormal TGF-beta 1 polypeptides containing the altered carbohydrate side chains were secreted readily by the CHO cells. In contrast, inhibitors of the glucosidases at the first step in glycoprotein remodeling, 1-deoxynojirimycin and castanospermine, markedly inhibited secretion of the TGF-beta 1 polypeptides from transfected CHO cells. In all cases, these inhibitors did not appear to affect proteolytic processing of the TGF-beta 1 polypeptides. Furthermore, inhibitor treatment did not affect mannose-6-phosphorylation of the TGF-beta 1 polypeptides. These results suggest that glycosylation and early stage remodeling of oligosaccharide side chains are necessary for secretion of TGF-beta 1. Treatment of the transfected CHO cells with weak bases (NH4Cl and chloroquine), or a monovalent ionophore (monensin), prevented proteolytic processing of the TGF-beta 1 precursor indicating that cleavage occurs by proteases in an acidic cellular compartment.

Identification of mannose 6-phosphate in two asparagine-linked sugar chains of recombinant transforming growth factor-beta 1 precursor

Recombinant transforming growth factor-beta 1 (TGF-beta 1) precursor produced and secreted by a clone of Chinese hamster ovary cells was found to be glycosylated and phosphorylated. Treatment of 32P-labeled precursor protein with N-glycanase indicated that phosphate was incorporated into asparagine-linked complex carbohydrate moieties. Fractionation of 32P-labeled glycopeptides followed by amino acid sequence analysis indicated that greater than 95% of the label was incorporated into two out of three glycosylation sites at Asn-82 and Asn-136 of the TGF-beta 1 precursor. Two-dimensional electrophoretic analysis of acid hydrolyzed precursor protein and precursor protein-derived glycopeptides indicated that 32P was incorporated as mannose 6-phosphate. Binding studies with the purified receptor for mannose 6-phosphate indicated that the TGF-beta 1 precursor could bind to this receptor and the binding was specifically inhibited with mannose 6-phosphate.

Molecular events in the processing of recombinant type 1 pre-pro-transforming growth factor beta to the mature polypeptide

Recently, the simian type 1 transforming growth factor beta (TGF-beta 1) cDNA was expressed at high levels in Chinese hamster ovary (CHO) cells by dihydrofolate reductase-induced gene amplification (L.E. Gentry, N.R. Webb, G.J. Lim, A.M. Brunner, J.E. Ranchalis, D.R. Twardzik, M.N. Lioubin, H. Marquardt, and A.F. Purchio, Mol. Cell. Biol. 7:3418-3427, 1987). We have now purified and characterized the recombinant proteins released by these cells. Analyses of the precursor proteins by amino acid sequencing identified potentially important proteolytic processing sites. Signal peptide cleavage occurs at the Gly-29-Leu-30 peptide bond of pre-pro-TGF-beta 1, yielding pro-TGF-beta 1 (30 to 390). In addition, proteolytic processing of the precursor to yield mature TGF-beta 1 occurs at the dibasic cleavage site immediately preceding Ala-279, indicating that CHO cells possess the appropriate processing enzyme. Greater than 95% of the biological activity detected in the conditioned medium of the CHO transfectant was due to mature, properly processed growth factor. Highly purified recombinant TGF-beta 1 had the same specific biological activity as natural TGF-beta 1. The concentration of TGF-beta 1 required for half-maximal inhibition of Mv1Lu mink lung epithelial cell growth was approximately 1 to 2 pM. Purified precursor inhibited mink lung cell proliferation at 50 to 60 pM concentrations. The purified precursor preparation was shown to consist of pro-TGF-beta 1 (30 to 390), the pro region of the precursor (30 to 278), and mature TGF-beta 1 (279 to 390) interlinked by at least one disulfide bond with the pro portion of the precursor. These recombinant forms of TGF-beta1 should prove useful for further structural and functional studies.

Recombinant type 1 transforming growth factor beta precursor produced in Chinese hamster ovary cells is glycosylated and phosphorylated

Analyses of cDNA clones coding for simian type 1 transforming growth factor beta (TGF-beta 1) suggest that there are three potential sites for N-linked glycosylation located in the amino terminus of the precursor region. Analysis of [3H]glucosamine-labeled serum-free supernatants from a line of Chinese hamster ovary cells which secrete high levels of recombinant TGF-beta 1 indicate that the TGF-beta 1 precursor, but not the mature form, is glycosylated. Digestion with neuraminidase resulted in a shift in migration of the two TGF-beta 1 precursor bands, which suggests that they contain sialic acid residues. Endoglycosidase H had no noticeable effect. Treatment with N-glycanase produced two faster-migrating sharp bands, the largest of which had a molecular weight of 39 kilodaltons. TGF-beta 1-specific transcripts produced by SP6 polymerase programmed the synthesis of a 42-kilodalton polypeptide which, we suggest, is the unmodified protein backbone of the precursor. Labeling with 32Pi showed that the TGF-beta 1 precursor was phosphorylated in the amino portion of the molecule.

Characterization of vaccinia virus growth factor biosynthetic pathway with an antipeptide antiserum

A synthetic peptide derived from vaccinia virus growth factor (VGF) was used as an immunogen to prepare antiserum able to immunoprecipitate native VGF from both vaccinia virus-infected cell lysate and cell-free medium. Pulse-chase, tunicamycin treatment, and carbohydrate trimming experiments revealed that VGF is synthesized as a 19-kilodalton (kDa) precursor which is rapidly modified to a high-mannose-type 22-kDa protein. This cell-associated form is further processed into a 25-kDa polypeptide which, after proteolytic cleavage, releases the mature VGF into the medium as a 22-kDa glycoprotein.

Structural requirements of diacylglycerols for binding and activating phospholipid-dependent, Ca++-sensitive protein kinase

Phospholipid-dependent, Ca++-sensitive protein kinase (protein kinase C) is activated by phorbol esters and diacylglycerols. A series of diacylglycerols was synthesized with different substituents at positions 1 and 2 in order to expand known structure-activity relationships for these compounds with respect to binding and activating purified protein kinase C. Compounds were synthesized with saturated and unsaturated long chain fatty acyl groups at position 1 and acetyl, butyryl, or hexanoyl groups at position 2. Binding to protein kinase C correlated well with in-vitro activation of the enzyme. These diacylglycerols activated protein kinase C in an intact cellular system causing the phosphorylation of pp60c-src. This indicates that the length of the fatty acyl group at C2 is critical and that the existence of unsaturation in the fatty acyl group at C1 is not essential.

Type 1 transforming growth factor beta: amplified expression and secretion of mature and precursor polypeptides in Chinese hamster ovary cells

Recombinant type 1 transforming growth factor beta (TGF-beta) was expressed to high levels in CHO cells by using dihydrofolate reductase (dhfr) gene amplification. The expression plasmid was derived from the pSV2 vectors and contained, in tandem, the simian TGF-beta and mouse dhfr cDNAs. Transcription of both cDNAs was controlled by the simian virus 40 early promoter. Stepwise selection of transfected CHO cells in increasing concentrations of methotrexate yielded cell lines that expressed amplified TGF-beta nucleic acid sequences. The expression plasmid DNA was amplified greater than 35-fold in one of the methotrexate-selected transfectants. The major proteins secreted by these cells consisted of latent TGF-beta and TGF-beta precursor polypeptides, as judged by immunoblots by using site-specific anti-peptide antibodies derived from various regions of the TGF-beta precursor. Levels of recombinant TGF-beta protein secreted by these cells approached 30 micrograms/24 h per 10(7) cells and required prior acidification for optimal activity; nonacidified supernatants were approximately 1% as active as acidified material. Antibodies directed toward sequences present in the mature growth factor readily identified a proteolytically processed recombinant TGF-beta which, on sodium dodecyl sulfate-polyacrylamide gels, comigrated with highly purified natural TGF-beta. In addition to mature recombinant TGF-beta, site-specific antibodies demonstrated the existence of larger TGF-beta precursor polypeptides. The availability of biologically active recombinant type 1 TGF-beta and precursor forms should provide a means to examine the structure, function, and potential in vivo therapeutic use of this growth factor.

Expression and characterization of transforming growth factor alpha precursor protein in transfected mammalian cells

Analysis of a cDNA clone derived from retrovirus-transformed rat fibroblasts has recently suggested that the mature 50-amino-acid form of transforming growth factor alpha (TGF alpha) is derived from a 159-amino-acid transmembrane precursor by proteolytic cleavage. To understand the processing of the TGF alpha precursor molecule in more detail, we have expressed this protein in baby hamster kidney (BHK) fibroblasts under control of the metal-ion-inducible metallothionein promoter and characterized the expressed precursor with site-specific antipeptide antibodies. One of the BHK transfectants, termed 5:2, expressed the TGF alpha mRNA in a cadmium- and zinc-inducible manner. The TGF alpha precursor protein was detected by immunoprecipitation analysis of radiolabeled cell cultures. In the induced 5:2 cells, a polypeptide of Mr 13,000 to 17,000 was readily identified by peptide antisera made to three different regions of the TGF alpha precursor protein. No such protein species were observed in BHK cells treated with cadmium and zinc or in uninduced 5:2 cells. However, two cell lines known to produce TGF alpha naturally, Leydig testicular tumor cells and Snyder-Theilan feline sarcoma virus-transformed Fisher rat embryo fibroblasts, possessed detectable levels of immunologically related Mr 13,000 to 17,000 proteins. Cell fractionation studies indicate that the Mr 13,000 to 17,000 species expressed in induced 5:2 cells is membrane associated, consistent with predictions based on the cDNA sequence of the TGF alpha precursor. Media conditioned by induced 5:2 cells contained epidermal growth factor receptor-competing activity, which, upon size fractionation, was similar in size to the mature processed form of TGF alpha. These data show that these nontransformed BHK cells possess the ability to process the TGF alpha precursor molecule into its native form.

Stimulation of T cells through the CD3/T-cell receptor complex: role of cytoplasmic calcium, protein kinase C translocation, and phosphorylation of pp60c-src in the activation pathway

Stimulation of T cells or the Jurkat T-cell line with soluble antibodies to the CD3/T-cell receptor complex causes mobilization of cytoplasmic Ca2+, which is blocked by pertussis toxin but not by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, and translocation of protein kinase C activity from the cytoplasm to the membrane. Such stimulation also causes phosphorylation of pp60c-src at an amino-terminal serine residue. These activities are consistent with induction of phosphatidylinositol metabolism after antibody binding. Anti-CD3 stimulation with antibody in solution, however, does not cause Jurkat cells to release interleukin 2 and blocks rather than induces proliferation of T cells. Induction of interleukin 2 production by Jurkat cells and proliferation by normal T cells requires anti-CD3 stimulation with antibody on a solid support, such as Sepharose beads or a plastic dish. Thus, we examined phosphorylation of pp60c-src after stimulation of Jurkat cells with anti-CD3 in solution or on solid phase. Both of these caused serine phosphorylation of pp60c-src that was indistinguishable even after 4 h of stimulation. These results indicate that the mode of anti-CD3 stimulation (in solution or on solid phase) controls a cellular function that modifies the consequences of signal transduction through phosphatidylinositol turnover.

Isolation and characterization of a putative endogenous benzodiazepineoid (endozepine) from bovine and human brain

A protein termed endozepine (EP) which inhibits the binding of benzodiazepines to synaptosomal membranes (Ki approximately 5 microM) has been purified to electrophoretic homogeneity from bovine and human brain using acidic ethanol/chloroform extraction, Bio-Sil TSK-250 gel permeation chromatography, and reverse-phase high performance liquid chromatographies. Bovine and human EP are single-chain polypeptides and have molecular weights of approximately 10,000. Both proteins are very hydrophilic and contain an abundance of lysine, glutamic, and aspartic residues. Antisera prepared against bovine EP have been used to develop a sensitive radioimmunoassay for the detection of EP in tissue and body fluids. EP immunoreactivity is widely distributed in mammalian tissues, body fluids, and various cell lines. Substantial variation in the concentrations of EP is observed in different regions of the brain.

Characterization of site-specific antibodies to the erbB gene product and EGF receptor: inhibition of tyrosine kinase activity

Site-specific antibodies were generated against the erbB protein and epidermal growth factor (EGF) receptor by immunizing rabbits with a synthetic peptide corresponding to amino acid residues 285-296 of the predicted AEV-H erbB protein sequence. This peptide region lies within the tyrosine kinase domain of erbB and EGF receptor. Antibodies directed against this region readily identified native and denatured forms of the erbB gene product and EGF receptor as demonstrated by immuneprecipitation and immunoblot analysis. The anti-peptide antibody immuneprecipitated a functional EGF binding receptor molecule. Scatchard analysis demonstrated a KD for 125I-labeled EGF binding of 40 nM, a value consistent with that of detergent solubilized EGF receptor. Immuneprecipitates, though able to bind EGF, were unable to transfer phosphate from gamma-labeled ATP in a standard phosphorylation reaction. In detergent solubilized extracts of crude A431 microsomes, the anti-peptide antibody inhibited in a dose dependent manner the autophosphorylation of EGF receptor as well as receptor mediated phosphorylation of exogenously added substrates. In addition, this anti-peptide antibody reduced the overall level of tyrosine kinase activity present in microsomes prepared from AEV-transformed erythroblasts. This site-specific antisera should be useful for understanding the role of EGF receptor and erbB tyrosine kinase activity and their link with cell proliferation.

Novel serine phosphorylation of pp60c-src in intact cells after tumor promoter treatment

Treatment of normal cells with the tumor promoters 12-O-tetradecanoylphorbol-13-acetate and mezerein results in increased phosphorylation of pp60c-src. Two-dimensional tryptic phosphopeptide analysis of partial V8 protease fragments indicated that this phosphorylation takes place on a serine residue which lies within the amino-terminal 18 kilodaltons of pp60c-src and represents the major phosphorylation site following tumor promoter treatment. Untreated cells exhibited a low but detectable level of phosphorylation at this serine residue. The significance of these results with respect to the phosphoregulation of pp60c-src as well as tumor promotion is discussed.

Site-specific increased phosphorylation of pp60v-src after treatment of RSV-transformed cells with a tumor promoter

When vole cells that had been transformed by Rous sarcoma virus were treated with the tumor-promoting phorbol ester 12-O-tetradecanoyl-13-acetate (TPA), specific phosphorylation of pp60v-src was increased. Partial V8 protease mapping indicated that the increased phosphorylation occurred exclusively on serine residues located in the amino terminus of the molecule. Treatment of cells with dimethyl sulfoxide or 4 alpha-phorbol-12,13-didecanoate did not elicit this response. Two-dimensional tryptic phosphopeptide mapping of pp60v-src immunoprecipitated from untreated and TPA-treated cells indicated that a specific tryptic amino-terminal peptide was hyperphosphorylated.

Identification of the tyrosine protein kinase from LSTRA cells by use of site-specific antibodies

The lymphoma cell line LSTRA contains an elevated level of tyrosine protein kinase activity. It has been suggested that this elevated level of activity is due to the presence of a phosphoprotein with a molecular weight of 56,000 (pp56, formerly referred to as a 58,000-dalton protein). This paper describes the preparation of antibodies against pp56 through the use of a synthetic peptide that contains the sequence around the site of tyrosine phosphorylation in pp56, which is identical to the phosphorylation site in pp60src. These antipeptide antibodies specifically immunoprecipitated 32P-labeled pp56 from detergent extracts of LSTRA cells. In immunoblotting experiments, pp56 was the major antigen detected in the particulate fraction from LSTRA cells by the antipeptide antibodies. The antibodies were also used to show that the level of pp56 is greatly elevated in LSTRA cells. Incubation of the detergent extract of the particulate fraction from LSTRA cells with the antipeptide antibodies resulted in inhibition of most of the LSTRA cell tyrosine protein kinase activity. These results indicate that pp56 is the tyrosine protein kinase whose activity is elevated in LSTRA cells. This enzyme may be a member of the large family of protein kinases involved in the regulation of cell growth.

Common features of the yes and src gene products defined by peptide-specific antibodies

Anti-peptide antibodies generated against a hydrophilic domain of pp60src comprising amino acid residues 498 through 512 were shown to be cross-reactive with the corresponding region in the yes transforming proteins encoded by Yamaguchi 73 and Esh sarcoma viruses. This cross-reactivity was demonstrated by immunoblot and immunoprecipitation analyses, and the identity of the proteins was verified by partial proteolytic mapping. By utilizing a combination of immunofluorescence and interference-reflection microscopy, these cross-reactive anti-peptide antibodies were shown to produce an immunofluorescence staining pattern in Yamaguchi 73 and Esh sarcoma virus-transformed chicken embryo fibroblasts remarkably similar to that pp60src in Rous sarcoma virus-infected chicken cells. Like the src gene products, the yes transformation-specific polyproteins were found to be concentrated within adhesion plaque structures and needle-like interdigitating cell-cell junctions. This analogous subcellular distribution suggests that these onc proteins are functionally related and may share common intracellular targets.

Antibodies to a defined region of pp60src neutralize the tyrosine-specific kinase activity

Site-specific antibodies to pp60src, the transforming protein of Rous sarcoma virus (RSV), have been prepared by immunizing rabbits with a chemically synthesized pentadecapeptide corresponding to residues 498-512 (Cys-Trp-Arg-Lys-Asp-Pro-Glu-Glu-Arg-Pro-Thr-Phe-Lys-Tyr-Leu) as deduced from the nucleotide sequence of the Prague C src gene. Antibodies specific for the synthetic peptide were purified from immune sera by affinity chromatography on peptide-bound Sepharose and characterized by a number of immunocytochemical techniques. Immunoprecipitation and Western blot analyses of normal and RSV-transformed cell lines revealed that this peptide antibody identified the authentic viral src gene product. This finding was further supported by indirect immunofluorescence on RSV-transformed rat kidney cells. The anti-peptide antibodies produced dramatic intracellular staining patterns characteristic of the src protein. Although able to immunoprecipitate pp60src, in vitro kinase reactions indicated that, unlike sera from RSV-induced tumor-bearing rabbits, the peptide antibody did not serve as a phosphate acceptor in the immunocomplex. Moreover, immunoprecipitates of pp60src prepared from this site-specific immune reagent were unable to phosphorylate exogenously added casein or the synthetic peptide substrate, Arg-Arg-Leu-Ile-Glu-Asp-Ala-Glu-Tyr-Ala-Ala-Arg-Gly. In contrast, pp60src-containing immunoprecipitates made from an anti-peptide serum specific for the COOH-terminal six amino acids (residues 521-526), a region only eight amino acids removed, readily phosphorylated both substrates. This evidence indicates that an antibody directed against residues 498-512 neutralizes the kinase activity of pp60src and suggests that this region may be functionally necessary for the tyrosine-specific kinase activity of this transforming protein.