Site-directed mutagenesis of the N-linked glycosylation site in platelet-derived growth factor B-chain results in diminished intracellular retention.

Pulse-chase analysis of human platelet-derived growth factor (PDGF) B-chain was conducted in stably transfected Chinese hamster ovary cells to determine precisely the kinetics of processing, intracellular trafficking and secretion. Newly synthesized 31 kDa monomers of the B-chain (p31) dimerized rapidly via disulfide bonds to a p54 species (t1/2 < 30 min). The p54 dimer was processed to a group of intracellular, cell surface (suramin-releasable) and secreted forms whose rates of appearance and disappearance from the cell were measured over a 48 h period. The newly synthesized p31 species was quantitatively converted to p27 by treatment with endoglycosidase H, consistent with efficient N-glycosylation at a site in the N-terminal propeptide region (Asn63-Met64-Thr65). Interruption of B-chain glycosylation by oligodeoxynucleotide-directed mutagenesis resulted in a significant increase in suramin-releasable forms at the cell surface (p34-38) and a concomitant decrease in accumulation of an intracellular p24 species. The glycosylation-defective mutant exhibited slight increases in receptor binding and mitogenic activity. Our results suggest that N-linked glycosylation of the B-chain is not important for formation of mitogenically active protein, but that it plays a role in early intracellular sorting and proteolytic processing events.

Isolation of a 110 000 molecular weight protein in the purification of the nuclear chick intestinal 1,25-dihydroxyvitamin D-3 receptor.

A single protein band of molecular weight 110 000 has been obtained after sodium dodecyl sulfate polyacrylamide gel electrophoresis of purified 1,25-dihydroxyvitamin D-3 (1,25-(OH)2D-3) receptor from crude nuclear extracts of chick intestinal mucosa, prepared in the presence of the protease inhibitors phenylmethylsulfonyl fluoride and epsilon-aminocaproic acid. The nuclear extract was subjected to a six-step purification scheme, involving polymin P and ammonium sulfate fractionation, DNA-cellulose affinity chromatography, Sephacryl S-200 gel filtration, blue dextran-Sepharose and a final DNA-cellulose chromatographic step. The receptor was obtained in about 1% yield and was purified approx. 3700-fold from the nuclear extract, as assessed by specific activity. Single peaks were observed with 3H-1,25-(OH)2D-3-labeled crude nuclear extracts on Sephacryl S-200 gel filtration (Stokes' radius = 35.5 A) and sucrose density gradient centrifugation (3.5 S). Although the identity of the Mr 110 000 protein will remain inconclusive until methods for further characterization are available, it may represent evidence for a higher molecular weight form of the 1,25-(OH)2D-3 receptor than that observed previously.

The effect of epidermal growth factor receptor (EGFR) expression on in vivo growth of rat C6 glioma cells.

The discovery of EGFR gene amplification in glioblastoma multiforme has prompted interest in experimental therapies to target the receptor on brain tumor cells. To develop an animal model for in vivo study of such strategies, we transfected C6 glioma cells with a plasmid containing the neomycin resistance gene and the human EGFR gene under the control of the glucocorticoid-inducible MMTV promoter. Following selection with G418, individual clones that expressed EGFR at high levels were selected. Kinetics of EGF binding fit a dual site model indicating the presence of both high (KA = 2.5 x 10(9) M-1) and low (KA = 3.3 x 10(7) M-1) affinity receptors. To assess growth in vivo, graded numbers of either wild-type or transfected cells were implanted into the brains of CD Fischer 344 rats. No differences in survival were observed between groups of animals injected with either wild-type or transfected cells at inocula of 10(3) or 10(4) respectively. In addition, one-third of animals (7/21) challenged with 10(5) or 10(6) transfected cells survived > 50 days compared to 0% of animals (0/12) challenged with 10(5) or 10(6) wild-type cells. Such an effect suggests greater immunogenicity of transfected cells, but only at the larger inocula. Since C6 glioma cells will grow in both outbred and inbred strains, our model should have a number of applications including the in vivo study of EGFR targeting for glioma therapy.

Disruption of N-linked glycosylation of bovine luteinizing hormone beta-subunit by site-directed mutagenesis dramatically increases its intracellular stability but does not affect biological activity of the secreted heterodimer.

The single site for N-linked glycosylation of the beta-subunit of bovine LH (LH beta) was disrupted by oligonucleotide-directed mutagenesis to assess its potential roles in the biosynthesis, transport, and hormonal activity of the LH alpha/beta heterodimer. Pulsechase studies performed with stably transfected Chinese hamster ovary cells that expressed both alpha-subunit (fully glycosylated) and nonglycosylated LH beta revealed that turnover, transport, and secretion of newly synthesized, nonglycosylated LH beta were effectively blocked over a 22-h span. Free nonglycosylated LH beta, like free wild-type LH beta, was sequestered inside the cell; therefore, the intracellular retention of uncombined LH beta-subunit is not due to a signal located within the N-glycan moiety. Nevertheless, an older pool of unlabeled, nonglycosylated LH beta-subunit was available for combination with newly synthesized alpha-subunit, as verified by immunoprecipitation of radiolabeled alpha-subunit from cell lysates and culture medium with anti-LH beta-antiserum. This heterodimer displayed normal kinetics of secretion (t 1/2 = 2.4 h) as compared to fully glycosylated LH (t 1/2 = 2.1 h). The wild-type and mutant forms of LH were also purified from culture supernatants of the two cell lines, and were compared for their relative abilities to stimulate progesterone secretion in cultured rat Leydig cells. Both proteins displayed similar potency (ED50 = 32 vs. 41 ng/ml, respectively) and maximal stimulation of progesterone release Pmax = 2.7 vs 2.5 micrograms/ml), indicating that N-linked glycosylation of the LH beta-subunit does not play a significant role in LH signal transduction. Collectively, these results indicate that N-linked glycosylation is important for intracellular degradation of free LH beta, but is not essential for either its assembly with alpha-subunit or the transport and secretion of biologically active heterodimer.

Interferon-gamma induces polymeric immunoglobulin receptor mRNA in human intestinal epithelial cells by a protein synthesis dependent mechanism.

Transport of secretory IgA into external fluids is mediated by the polymeric immunoglobulin receptor (pIgR) on the surface of mucosal epithelial cells. We studied the mechanism by which interferon-gamma (IFN-gamma) induces pIgR expression in HT-29.74 cells, a subclone of the HT-29 cell line selected for high concns of pIgR. Here we report the isolation of genomic DNA and cDNA clones encoding human pIgR and development of a sensitive ribonuclease protection assay for pIgR mRNA. This assay was used to determine if induction of pIgR by IFN-gamma is mediated by accumulation of pIgR mRNA. After an initial lag of 12 hr, pIgR mRNA increased seven-fold in response to IFN-gamma, reaching a plateau at 24 hr. Concentrations of pIgR protein also increased seven-fold, but the increase was delayed until 48 hr following stimulation with IFN-gamma. Cycloheximide treatment abolished the IFN-gamma induced increase in pIgR mRNA, indicating that induction of pIgR mRNA by IFN-gamma requires de novo protein synthesis. These results suggest that induction of pIgR expression by IFN-gamma involves an increase in steady-state concns of pIgR mRNA via a protein synthesis dependent mechanism.

Transcriptional control of the platelet-derived growth factor subunit genes.

Platelet-derived growth factor (PDGF) is a potent mitogen consisting of heterodimers of two distinct but homologous polypeptide chains, denoted A and B. PDGF-like homodimers of the A- and B-chains have been isolated, as well as two distinct receptor types (alpha and beta), which discriminate among the PDGF isoforms. The PDGF A- and B-chains are encoded by distinct genes located on human chromosomes 7 and 22, respectively. Although PDGF has been implicated as an important participant in development, tissue repair, and numerous pathologic states including tumorigenesis, atherosclerosis and inflammation, the mechanisms which determine the rate of its synthesis are only beginning to be understood. Basal expression of the PDGF A- and B-chain genes has been characterized in a number of cell types and is directed in part by elements in the respective proximal promoter-regulatory regions of the two genes. In addition, the first intron of PDGF-B has been shown to contain both positive and negative regulatory elements. Transcription of the PDGF subunit genes is inducible by a wide variety of mitogenic growth factors, cytokines and other agonists. These agents produce a rapid increase in steady-state concentrations of PDGF A- and B-chain mRNAs, peaking within 4-8 h of stimulation. The inductive effects of protein kinase C-activating phorbol 12-myristate 13-acetate (PMA), thrombin and transforming growth factor-beta (TGF-beta) are mediated through increases in the transcription rates of both genes. In addition, cAMP blocks the increases in transcription of the B-chain gene induced by thrombin and TGF-beta. Studies have demonstrated the importance of sequences immediately upstream of the B-chain transcription start site for induction in response to PMA-initiated megakaryocyte differentiation, an effect which is dependent on protein synthesis. However, cis-acting elements which mediate more rapid transcriptional induction seen in endothelial cells and astrocytes have yet to be identified in the proximal 5'-flanking sequences of either the A- or B-chain genes, suggesting that such events may be mediated by elements located outside of this region.

Effect of dietary calcium stress on plasma vitamin D3 metabolites in the egg-laying Japanese quail.

Experiments were performed to ascertain whether circulating levels of 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 varied in a cyclic manner during the 24-hr ovulatory period in Japanese quail hens and to assess the effects of dietary calcium restriction upon levels of the plasma vitamin D3 metabolites during that period. Birds fed a marginally-deficient calcium diet (1.7%) had significantly elevated plasma 1,25-dihydroxyvitamin D3 (.71 +/- .11 ng/ml, mean +/- SEM) compared with those fed high (3.5%) calcium (.35 +/- .04; P less than .01), although these values did not reveal any significant cyclic variation or correlation with known diurnal pulses in circulating 17 beta-estradiol. The magnitude of the plasma 1,25-dihydroxyvitamin D3 response to dietary calcium stress in some individuals was as great as 2.2 ng/ml, which are certainly among the highest values reported in any avian or mammalian species to date. These observations suggest that the total circulating level of plasma 1,25-dihydroxyvitamin D3 is not important in the direct cyclic regulation of laying hen calcium metabolism. The levels of free plasma hormone, target cell membrane permeability of the hormone, or regulation of cytosolic/nuclear 1,25-dihydroxyvitamin D3 receptor activity are potential aternate levels of the control of vitamin D3 mediated calcium metabolism in the quail hen.

Toxicity of a vitamin D steroid to laying hens.

Two experiments were conducted with 56-week-old or 104-week-old Leghorn hens to determine if feeding vitamin D steroids in excess of requirement levels caused any marked affects on eggshell quality. In the first experiment caged hens had reduced feed consumption, egg shell quality, and egg production as early as 6 weeks after initially consuming a basal diet supplemented with 6.8 micrograms 1 alpha-hydroxycholecalciferol (1 alpha-OH-D3)/kg. The second experiment confirmed the previous results and showed that extensive weight loss occurred with continued feeding of 10 or 15 micrograms 1 alpha-OH-D3/kg diet. No adverse affects were observed in either experiment when the level of 1 alpha-OH-D3 supplementation was 5.0 micrograms/kg diet or less. No toxic effects were observed when the hormone precursor 25-OH-D3 was supplemented to diets at 6 or 12 micrograms/kg. It is suggested that the pathological effects observed are related to the potent calcium homeostatic properties of 1 alpha-OH-D3 that at elevated levels may cause aberrations in circulating calcium.

c-Abl and Arg are activated in human primary melanomas, promote melanoma cell invasion via distinct pathways, and drive metastatic progression.

Despite 35 years of clinical trials, there is little improvement in 1-year survival rates for patients with metastatic melanoma, and the disease is essentially untreatable if not cured surgically. The paucity of chemotherapeutic agents that are effective for treating metastatic melanoma indicates a dire need to develop new therapies. Here, we found a previously unrecognized role for c-Abl and Arg in melanoma progression. We demonstrate that the kinase activities of c-Abl and Arg are elevated in primary melanomas (60%), in a subset of benign nevi (33%) and in some human melanoma cell lines. Using siRNA and pharmacological approaches, we show that c-Abl/Arg activation is functionally relevant because it is requiredfor melanoma cell proliferation, survival and invasion. Significantly, we identify the mechanism by which activated c-Abl promotes melanoma invasion by showing that it transcriptionally upregulates matrix metalloproteinase-1 (MMP-1), and using rescue approaches we demonstrate that c-Abl promotes invasion through a STAT3 → MMP-1 pathway. Additionally, we show that c-Abl and Arg are not merely redundant, as active Arg drives invasion in a STAT3-independent manner, and upregulates MMP-3 and MT1-MMP, in addition to MMP-1. Most importantly, c-Abl and Arg not only promote in vitro processes important for melanoma progression, but also promote metastasis in vivo, as inhibition of c-Abl/Arg kinase activity with the c-Abl/Arg inhibitor, nilotinib, dramatically inhibits metastasis in a mouse model. Taken together, these data identify c-Abl and Arg as critical, novel, drug targets in metastatic melanoma, and indicate that nilotinib may be useful in preventing metastasis in patients with melanomas harboring active c-Abl and Arg.

PDGF-A promoter and enhancer elements provide efficient and selective antineoplastic gene therapy in multiple cancer types.

Development of antineoplastic gene therapies is impaired by a paucity of transcription control elements with efficient, cancer cell-specific activity. We investigated the utility of promoter (AChP) and 5'-distal enhancer (ACE66) elements from the platelet-derived growth factor-A (PDGF-A) gene, which are hyperactive in many human cancers. Efficacy of these elements was tested in multiple tumor cell lines, both in cell culture and as tumor explants in athymic nude mice. Plasmid and viral vectors were constructed with the AChP promoter alone or in fusion with three copies of the ACE66 enhancer for expression of the prototype suicide gene, thymidine kinase (TK). ACE/AChP and AChP cassettes elicited ganciclovir (GCV)-induced cytotoxicity in multiple tumor cell lines. The ACE enhancer element also exhibited synergism with placental and liver-specific promoter elements. An adenovirus containing the AChP-TK cassette produced striking increases in GCV sensitivity in cultured tumor cell lines, as well as GCV-induced regression of U87 MG glioblastoma explants in vivo. TK expression was distributed throughout tumors receiving the therapeutic virus, whereas TdT-mediated dUTP nick end labeling (TUNEL) analysis revealed numerous regions undergoing apoptosis. Vascularization and reticulin fiber networks were less pronounced in virus-GCV-treated tumors, suggesting that both primary and stromal cell types may have been targeted. These studies provide proof-of-principle for utility of the PDGF-A promoter and ACE66 enhancer in antineoplastic gene therapy for a diverse group of human cancers.

Expression of the genes encoding bovine LH in a line of Chinese hamster ovary cells.

Synthesis of biologically active LH is complex, due in part to its heterodimeric subunit structure and to the numerous post-translation modifications of each subunit. Through the use of mammalian expression vectors we have been able to introduce the bovine alpha subunit and LH-beta genes into a Chinese hamster ovary cell line deficient in dihydrofolate reductase. The bovine genes are actively expressed and the Chinese hamster ovary cells secrete biologically active LH. The expression vector containing the bovine alpha subunit gene also contains a modified mouse gene encoding dihydrofolate reductase, permitting the use of methotrexate to amplify selectively the bovine alpha subunit gene after its integration into the genome of the Chinese hamster cells. This provides a novel means for assessing the importance of alpha subunit concentration with respect to assembly of the heterodimer. In addition, methotrexate selection leads to the over-production of LH (10 micrograms/10(6) cells/24 h). Finally, because the bovine LH produced in the Chinese hamster ovary cells is glycosylated, this transfection system can be used in conjunction with in-vitro mutagenesis to determine whether site-specific changes in glycosylation have an effect on subunit assembly and biological activity. This transfection approach therefore offers multiple avenues to explore further the molecular mechanisms underlying the complex biosynthetic pathway of bovine LH.

Methotrexate-induced amplification of the bovine lutropin genes in Chinese hamster ovary cells. Relative concentration of the alpha and beta subunits determines the extent of heterodimer assembly.

Methotrexate-induced gene amplification increased the expression of biologically active bovine luteinizing hormone (bLH) approximately 11-fold after stable transfection of a line of Chinese hamster ovary cells with genes encoding dihydrofolate reductase and the alpha and beta subunits of bLH. Subsequent analysis of the bovine genes revealed that while the alpha gene was amplified in response to methotrexate selection, the LH beta subunit gene remained unaffected. This effect was probably due to the linkage of the alpha subunit gene with the dihydrofolate reductase gene, the selectable and methotrexate-sensitive marker in the plasmid construct. Prior to methotrexate selection, the concentration of LH beta mRNA and the rate of LH beta synthesis exceeded that of alpha subunit mRNA and protein. Stepwise selection with methotrexate led to a progressive increase in the synthesis and secretion of biologically active bLH. Enhanced production of bLH correlated directly with similar increases in both the steady-state level of alpha subunit mRNA and the relative synthesis rate of alpha subunit protein. Despite progressive changes in alpha subunit concentration, formation of the alpha/beta heterodimer was always incomplete, even when the concentration of alpha subunit exceeded that of LH beta. Cumulatively, these results are consistent with a model in which the extent of steady-state combination of the subunits is determined by the mutual affinity and concentration of both subunits within the lumen of the secretory pathway. This stands in contrast to the long held view that the extent of glycoprotein hormone assembly is limited by the concentration of the beta subunit.

Repression of platelet-derived growth factor A-chain gene transcription by an upstream silencer element. Participation by sequence-specific single-stranded DNA-binding proteins.

Platelet-derived growth factor A-chain is a potent mitogen expressed in a restricted number of normal and transformed cells. Transient transfection and deletion analysis in BSC-1 (African green monkey, renal epithelial) cells revealed that the -1680 to -1374 region of the A-chain gene repressed homologous and heterologous promoter activities by 60-80%. An S1 nuclease-hypersensitive region (5'SHS) was identified within this region (-1418 to -1388) that exhibited transcriptional silencer activity in BSC-1 and a variety of human tumor cell lines (U87, HepG2, and HeLa). Electrophoretic mobility shift assays conducted with 5'SHS oligodeoxynucleotide probes revealed several binding protein complexes that displayed unique preferences for binding to sense, antisense, and double-stranded forms of the element. Southwestern blot analysis revealed that the antisense strand of 5'SHS binds to nuclear proteins of molecular mass 97, 87, 44, and 17 kDa, whereas the double-stranded form of 5'SHS is recognized by a 70-kDa factor. Mutations within 5'SHS element indicated the necessity of a central 5'-GGGGAGGGGG-3' motif for protein binding and silencer function, while nucleotides flanking both sides of the motif were also critical for repression. These results support a model in which silencer function of 5'SHS is mediated by antisense strand binding proteins, possibly by stabilizing single-stranded DNA conformations required for interaction with enhancer sequences in the proximal promoter region of the A-chain gene.

Human NM23/nucleoside diphosphate kinase regulates gene expression through DNA binding to nuclease-hypersensitive transcriptional elements.

NM23-H2/NDP kinase B has been identified as a sequence-specific DNA-binding protein with affinity for a nuclease-hypersensitive element of the c-MYC gene promoter (Postel et al., 1993). The ability of Nm23-H2 to activate c-MYC transcription in vitro and in vivo via the same element demonstrates the biological significance of this interaction. Mutational analyses have identified Arg34, Asn69 and Lys135 as critical for DNA binding, but not required for the NDP kinase reaction. However, the catalytically important His118 residue is dispensible for sequence-specific DNA binding, suggesting that sequence-specific DNA recognition and phosphoryl transfer are independent properties. Nm23-H2 also has an activity that cleaves DNA site-specifically, involving a covalent protein-DNA complex. In a DNA sequence-dependent manner, Nm23-H2 recognizes additional target genes for activation, including myeloperoxidase, CD11b, and CCR5, all involved in myeloid-specific differentiation. Moreover, both NM23-H1 and Nm23-H2 bind to nuclease hypersensitive elements in the platelet-derived growth factor PDGF-A gene promoter sequence-specifically, correlating with either positive or negative transcriptional regulation. These data support a model in which NM23/NDP kinase modulates gene expression through DNA binding and subsequent structural transactions.

Identification of a cell type-specific enhancer in the distal 5'-region of the platelet-derived growth factor A-chain gene.

Transient transfection analysis of DNA subfragments from the distal 5'-flanking region of the human platelet-derived growth factor A-chain gene (-18.3 to -1.8 kilobase pairs (kb)) revealed enhancer and silencer elements that contribute significantly to transcriptional regulation. Two adjacent regions (-8.2 to -7.5 kb and -7.5 to -7.0 kb) enhanced transcription of both A-chain and heterologous thymidine kinase promoters, whereas repression was observed in two other nearby regions (-9.9 to -8.2 kb and -7.0 to -5. 9 kb). The -7.5 to -7.0-kb fragment, or J, was the strongest enhancer, and its activity was localized to a 66-base pair element (A-chain cell type-specific enhancer (ACE 66)). ACE66 activity was highly cell type-specific, with greatest activity seen in choriocarcinoma cell lines (4-10-fold enhancement). Progressive 5'- and 3'-deletions of the ACE66 revealed distribution of activity across the element, with nucleotides 1-33 being critical for function. Electrophoretic mobility shift assays revealed cell type-specific patterns of high affinity protein binding to the element. Ethylation interference footprinting of JEG-3 extract localized guanine contacts on nucleotides 1-18 of both strands of the ACE element, whereas more extensive contacts were made with the phosphate backbone (nucleotides 1-32). The ACE66 element is a potent transcriptional regulator in placental cells and represents a valuable model of long distance regulation in a growth factor gene.

A dominant-negative mutant of the platelet-derived growth factor A-chain increases survival of hamsters implanted intracerebrally with the highly invasive CxT24-neo3 glioblastoma cell.

Evidence is accumulating to suggest a role for PDGF in stimulating malignant growth in astrocytoma, although it has been obtained using model systems (growth in 2-dimensional cell culture, athymic nude mice) that do not assess the complex interactions of these tumors with normal brain tissue. In the current study, the highly invasive hamster glioblastoma cell line CxT24-neo3 was used as a model to study the role of platelet-derived growth factor (PDGF) in mediating malignant growth both in vitro and in vivo when implanted directly into the right lateral ventricle of the brain. Co-expression of PDGF B-chain mRNA and PDGF alpha-receptors was detected in these cells, indicating potential for autocrine activation of their growth. CxT24-neo3 cells transfected with wild-type and receptor binding-deficient forms of the PDGF A- and B-chains displayed alterations in their abilities to grow as three-dimensional spheroids, with overexpression of wild-type B-chain resulting in increased spheroid formation, but a decreased rate of spheroid growth. Influence of these PDGF polypeptides on tumor invasion and survival time in vivo was evaluated following implantation of these spheroids in the brain. While all hamsters implanted with control spheroids died within 21 d (average 17 d), those implanted with cells expressing the receptor binding-deficient A-chain survived for much greater periods of time (average 80 d). Modest increases in survival were also seen in cells stably expressing wild-type A-chain (25 d) and mutant B-chain (26 d) proteins. The present study suggests an important role of PDGF in mediating the malignant growth of the CxT24-neo3 cell line in cerebral cortex, possibly via paracrine interactions with normal cortical cell types (i.e., glia, neurons).

Characterization of the cAMP responsive elements from the genes for the alpha-subunit of glycoprotein hormones and phosphoenolpyruvate carboxykinase (GTP). Conserved features of nuclear protein binding between tissues and species.

Cyclic AMP responsive elements (CRE) have been identified in several genes, including those encoding the alpha-subunit of glycoprotein hormones and the cytosolic form of phosphoenolpyruvate carboxykinase (PEPCK) from the rat. Common to the CRE in these genes is the palindromic sequence T(G/T)ACGTCA. Based upon the strong conservation of this element, we hypothesize that the CRE functions by binding a protein that has been conserved across tissue and species lines. Scatchard analysis of gel mobility shift assays indicate that a nuclear protein in extracts prepared from rat liver and from a human choriocarcinoma cell line binds with high affinity to the cAMP responsive element from either gene (Kd approximately 10(-10) M). In order to identify the critical nucleotides within the CRE from these two genes, a series of oligodeoxynucleotides containing systematic mutations was synthesized and tested for protein binding and transcriptional function. Mutations within the palindromic core of either CRE resulted in a marked loss of binding to the nuclear proteins. Sequences outside the 8-base pair element were less important for nuclear protein binding to the PEPCK CRE and were not important for the alpha-subunit CRE. The relative binding, as determined by gel shift assays, correlated with the ability to confer cAMP responsive transcription to a viral promoter in transfected choriocarcinoma cells. DNase I protection assays suggest that binding of the nuclear factor from rat liver to the PEPCK CRE is more efficient when the core sequence is present in the intact PEPCK promoter regulatory region as compared to the isolated CRE oligodeoxynucleotide. Collectively, these results indicate that the nuclear factors necessary for cAMP induction of transcription of the alpha-subunit and PEPCK genes are conserved between tissues and species. In addition to the conserved features of these cis- and trans-active elements, nonconserved sequences and other elements of the promoter regulatory region influence the affinity of the protein-DNA interaction.

Expression of biologically active bovine luteinizing hormone in Chinese hamster ovary cells.

Biologically active bovine luteinizing hormone (LH) has been obtained through expression of the alpha- and LH beta-subunit genes in stably transformed clones of DUXB11, a Chinese hamster ovary cell line deficient in dihydrofolate reductase (DHFR). Expression of alpha-and LH beta-subunit mRNAs of the expected sizes (approximately 910 and 770 nucleotides, respectively) were revealed by blot analysis after electrophoresis of total cellular RNA. Furthermore, presence or absence of the gonadotropin mRNAs in several clonal lines was directly correlated with the appearance of one or both bovine LH subunits in the culture medium. Media from three clones secreting significant immunoreactive levels of both subunits also stimulated the release of progesterone in ovine luteal cells, suggesting that the secreted LH was assembled into a biologically active and glycosylated dimer. Immunoprecipitation and NaDodSO4/PAGE of [35S]methionine-labeled proteins secreted from one of the clones, CHODLH20, further confirmed the presence of an alpha/beta dimer with apparent subunit molecular weights of 20,500 and 16,000, only slightly higher than those of pituitary alpha and LH beta subunits.

Platelet-derived growth factor A-chain gene transcription is mediated by positive and negative regulatory regions in the promoter.

Platelet-derived growth factor (PDGF) is a disulphide-linked heterodimer of two polypeptide chains, the A and B chains, which are encoded by genes on separate chromosomes. The A-chain gene is transcribed in a number of transformed and non-transformed cell lines and is inducible by a wide variety of growth factors, cytokines and other mitogenic agonists. To localize DNA elements that mediate basal transcription in the promoter regulatory region of the A-chain gene, we have employed 5'-endpoint deletion mutagenesis and transient expression analysis in the renal epithelial cell line BSC-1 (African green monkey). Studies conducted in this cell line, which expresses high concentrations of PDGF A-chain mRNA, reveal a positive regulatory element (PRE) in a GC-rich stretch of the A-chain promoter between -82 and -40, relative to the transcription start site. Two discrete regions of the promoter were identified as negative regulatory elements (NREs), located between -1029 and -880 (NRE1) and between -1800 and -1029 (NRE2). The -1800 to -812 region, which contains both NREs, functions as a potent NRE when relocated in either orientation adjacent to the herpes simplex virus thymidine kinase promoter, reducing transcription activity by 60% in the positive orientation and 85% in the negative orientation. Comparison of BSC-1 cells and Saos-2 cells (human osteogenic sarcoma), which do not express significant quantities of PDGF A-chain mRNA or protein, indicates that basal transcription of the gene is determined by enhancer activity mediated by the GC-rich region rather than through de-repression of the upstream NREs. Electrophoretic gel mobility shift assays reveal a complex pattern of nuclear protein binding to the GC-rich PRE (-73 to -46). Competition studies conducted with mutant oligonucleotides that alternately disrupt consensus binding sites for Sp-1 or Egr-1 demonstrate a requirement for the presence of an Sp1-like core sequence (GGCGGG) but not Egr-1/Krox-24 [GCG(G/T)-GGGCG] for the formation of specific DNA-protein complexes. Our observations suggest that basal transcription of the A-chain gene in renal epithelial cells is achieved through active enhancement, mediated by a GC-rich PRE and nuclear proteins that bind to Sp-1-like consensus DNA sequences.

Loop III region of platelet-derived growth factor (PDGF) B-chain mediates binding to PDGF receptors and heparin.

Site-directed mutagenesis of the platelet-derived growth factor (PDGF) B-chain was conducted to determine the importance of cationic amino acid residues (Arg160-Lys161-Lys162; RKK) located within the loop III region in mediating the biological and cell-association properties of the molecule. Binding to both PDGF alpha-and beta-receptors was inhibited by the conversion of all three cationic residues into anionic glutamates (RKK-->EEE), whereas an RKK-->SSS mutant also exhibited a modest loss in affinity for beta-receptors. Replacements with serine at either Arg160 (RKK-->SKK) or at all three positions (RKK-->SSS) had little effect on binding to alpha-receptors. Replacements with either glutamic or serine residues at any of the three positions also resulted in significant inhibition of heparin-binding activity. Furthermore, the RKK-->EEE mutant exhibited decreased association with the cell surface and accumulated in the culture medium as 29-32 kDa forms. Stable transfection of U87 astrocytoma cells with RKK-->EEE mutants of either the A-chain or the B-chain inhibited malignant growth in athymic nude mice. Despite altered receptor-binding activities, each of the loop III mutants retained full mitogenic activity when applied to cultured Swiss 3T3 cells. CD spectrophotometric analysis of the RKK-->EEE mutant revealed a secondary structure indistinguishable from the wild type, with a high degree of beta-sheet structure and random coil content (50% and 43% respectively). These findings indicate an important role of the Arg160-Lys161-Lys162 sequence in mediating the biological and cell-associative activities of the PDGF-BB homodimer, and reveal that the mitogenic activity of PDGF-BB is insufficient to mediate its full oncogenic properties.