Amino-Terminal Charge Affects the Periplasmic Accumulation of Recombinant Heregulin/EGF Hybrids Exported Using theEscherichia coliAlkaline Phosphatase Signal Sequence

AnEscherichia coliexpression system that exploits the bacterial alkaline phosphatase (PhoA) signal sequence to translocate recombinant human epidermal growth factor (hEGF) to the periplasm was used to evaluate how changes in the composition and sequence of amino acids near the PhoA–hEGF junction influence the periplasmic accumulation of recombinant protein. A series of chimeric structural genes was generated byin vitroreplacement of hEGF sequence with analogous segments from the EGF-like domain of human heregulin (HRG), significantly altering the electrostatic character of the amino-terminal region of the mature protein. Quantitation of HRG/EGF protein inE. coliperiplasmic extracts, by RP-HPLC, showed a fourfold decrease after one of two acidic residues located in the amino-terminal region of the mature hEGF, near the PhoA junction, was replaced. An additional threefold decrease was observed when the second acidic residue was replaced with a positively charged lysine. Further extension of the amino-terminal HRG sequence, beyond the first six residues, resulted in net neutralization of a more distant EGF acidic residue with no additional effect on protein yield. The importance of having a negatively charged group in the amino-terminal region of the mature protein was confirmed when insertion of an aspartic acid near the amino-terminus of two poorly expressed hybrid protein sequences resulted in a five- to eightfold increase in their recovery from the periplasm. This study demonstrates the importance of having negatively charged residues near the fusion junction of recombinant proteins expressed inE. coliusing the PhoA signal sequence for protein export.     

Tumor necrosis factor-alpha affects growth hormone secretion by a direct pituitary interaction.

Administration of 50, 250, and 1,250 ng/kg iv of recombinant bovine tumor necrosis factor-alpha (RBTNF) did not affect basal plasma concentrations of growth hormone (GH) or thyroid-stimulating hormone in male calves. However, when administered 30 min before challenge with 1 microgram/kg iv of thyrotropin-releasing hormone (TRH), 250 ng/kg of RBTNF increased the subsequent incremental GH response. At 1,250 ng/kg of RBTNF, GH response to TRH was significantly blunted. For each dose of RBTNF administered, the incremental change in plasma thyroid-stimulating hormone following TRH was not significantly different from control. To examine direct effects of RBTNF on pituitary function, fresh bovine pituitaries were sliced into 1-mm cubes and incubated with 0 or 10(-8), 10(-9), or 10(-10) M RBTNF. Additional cultures were treated with 10(-8) or 10(-9) M GH-releasing factor or 10(-8) M TRH and 0 or 10(-8) M RBTNF. Media GH increased in cultures with 10(-10) M RBTNF and declined linearly as RBTNF concentration increased. RBTNF blocked GH release from GH-releasing factor- and TRH-challenged pituitary slices. Membranes prepared from homogenized bovine pituitaries had specific saturable binding characteristics for monomeric 125I-RBTNF. Membranes treated with 4 M MgCl2 for 10 min and washed free of Mg2+ produced Scatchard plots fit to a two-site model (high affinity site Kd = 6.6 nM), while Scatchards of non-Mg(2+)-treated membranes fit a single site (Kd = 8.9 nM). Polyacrylamide gel electrophoresis separation of 125I-RBTNF cross-linked pituitary membranes showed specific binding of monomeric 125I-RBTNF to protein components ranging in molecular weight from 19,000 to 77,000. The data suggest that RBTNF has modulatory effects on the regulation of GH secretion acting directly at the pituitary through specific receptors.     

Bovine sarcocystosis: How parasites negatively affect growth

Growth, though genetically encoded, is markedly influenced in healthy animals by the interaction of hormonal and nutritional factors. The uptake and use of nutrients by specific tissues is regulated by a priority system that modulates physiological processes. Nutritional, hormonal and immunological consequences of parasitism often lead to partitioning of nutrients away from growth. In this article, Ron Foyer and Ted Elsosser use a bovine sarcocystosis model to show that changes in plasma concentrations of insulin-like growth factor-I (IGF-I), growth hormone (GH) and somotostotin (SSN), as well as the host's immunological response to the parasite via cytokine interactions with the endocrine system, are modulators of perturbed growth.     

Positive regulation of the glucocorticoid receptor in human T-cells sensitive to the cytolytic effects of glucocorticoids

Regulation of glucocorticoid receptor (GR) protein and mRNA were examined in the human leukemic T-cell line CEM-C7. Unlike other cells in which GR regulation has been examined, the growth of these cells is inhibited by glucocorticoids, leading to cell death. Treatment of glucocorticoid-sensitive CEM-C7 cells with 1 microM dexamethasone for 18 h resulted in an increase in both cytoplasmic and nuclear GR protein, as determined by immunoblotting with anti-human GR antisera. Analysis of GR mRNA levels by Northern blotting revealed a corresponding increase in mRNA in steroid-treated cells. An increase in GR mRNA was detectable after as little as 3 h of treatment with dexamethasone, and GR mRNA concentration continued to increase for at least 18 h, well before the onset of growth arrest or cell death. GR mRNA concentration was not altered after dexamethasone treatment of the glucocorticoid-resistant mutant cell line ICR27TK.3, which lacks functional GR. Thus, the increase in GR seen in glucocorticoid-sensitive cells is a GR-mediated response. These results are in sharp contrast to the down-regulation of GR reported in other cells and tissues, and suggest that regulation of the GR by its cognate ligand may be tissue-specific.     

TNF-alpha in smoke inhalation lung injury

Hales, Charles A., T. H. Elsasser, Peter Ocampo, and OlgaEfimova. TNF- in smoke inhalation lung injury.J. Appl. Physiol. 82(5):1433-1437, 1997.Adult respiratory distress syndrome is a majorcause of morbidity in fire victims. Tumor necrosis factor- (TNF-)is edematogenic and has been associated with the etiology of otherforms of adult respiratory distress syndrome. In the sheep lymphfistula model, we measured TNF- after 48 (n = 7) or 128 (n = 3) breaths of cotton smoke andcompared this with sham controls (n = 5) or controls in which left atrial pressure was elevated to 20 mmHg(n = 5) to increase lymph flow in the absence of inflammation. Smoke induced a rise in lymph flow and pulmonary arterial pressure with either no fall in lymph-to-plasma protein ratio (128 breaths) or a modest fall in lymph-to-plasma proteinratio (48 breaths), consistent with a change in microvascular permeability as well as a rise in microvascular pressure.Lymph concentration of TNF- fell in both groups, although lymph flux (concentration × flow) transiently rose in both. In neither case did TNF- flux exceed that induced by left atrial pressure elevation. TNF- was detectable in only one out of five sheep in alveolar lavage. Thus, by utilizing a sensitive and specific radioimmunoassay, we were unable to demonstrate a role for TNF- in smoke-induced microvascular lung injury in sheep.

     

Analysis of glucocorticoid receptor activation by high resolution two-dimensional electrophoresis of affinity-labeled receptor

To determine if activation of the glucocorticoid receptor involves covalent charge modification of the steroid-binding protein, unactivated and activated IM-9 cell glucocorticoid receptors were examined by high resolution two-dimensional gel electrophoresis. As previously reported (Smith, A. C., and Harmon, J. M. (1985) Biochemistry 24, 4946-4951), two-dimensional electrophoresis of immunopurified, [3H]dexamethasone mesylate-labeled, steroid-binding protein from unactivated receptors resolves two 92-kDa isoforms (pI congruent to 5.7 and 6.0-6.5). After activation, the apparent pI of neither isoform was altered, indicating that there had been no covalent charge modification of the steroid-binding protein. Thus, the physicochemical changes observed after activation of the steroid receptor cannot be explained by dephosphorylation or other models which involve covalent charge modification of the steroid-binding protein. This conclusion was consistent with the observation that treatment of immunopurified, affinity-labeled receptors with calf intestine alkaline phosphatase did not alter the apparent pI values or distribution of the steroid-binding protein isoforms. However, chromatography of activated steroid-receptor complexes on DNA-cellulose revealed that only the more basic of the two steroid-binding protein isoforms bound to DNA. Therefore, the charge heterogeneity of the steroid-binding protein may be important in regulating the ability of the steroid-binding protein to interact with DNA.     

Growth and tissue accretion rates of swine expressing an insulin-like growth factor I transgene

The goal of this research was to determine whether directing expression of an insulin-like growth factor I (IGF-I) transgene specifically to striated muscle would alter the growth characteristics in swine. Transgenic pigs were produced with a fusion gene composed of avian skeletal alpha-actin regulatory sequences and a cDNA encoding human IGF-I. Six founder transgenic pigs were mated to nontransgenic pigs to produce 11 litters of G1 transgenic and sibling control progeny. Birth weight, weaning weight, and proportion of pig survival did not differ between transgenic and control pigs. The ADG of pigs as they grew incrementally from 20 to 60 kg, 60 to 90 kg, and 90 to 120 kg, respectively, did not significantly differ between transgenic and control pigs. Efficiency of feed utilization (gain:feed) was also similar for transgenic and control pigs. Plasma IGF-I and porcine growth hormone (pGH) concentrations were determined at 60, 90, and 120 kg body weight. Plasma IGF-I concentrations were 19% higher in transgenic gilts than control gilts and 11.1% higher in transgenic boars than control boars (P=0.0005). Plasma IGF-I concentrations for boars were also higher than for gilts (P=0.0001). At 60, 90, and 120 kg body weight each pig was scanned by dual energy X-ray absorptiometry (DXA) to derive comparative estimates of carcass fat, lean, bone content of the live animal. Control pigs had more fat and less lean tissue than transgenic pigs at each of the scanning periods and the difference became more pronounced as the pigs grew heavier (P<0.005 at each weight). Transgenic pigs also had a slightly lower percentage of bone than control pigs (P<0.05 at each weight). While daily rates of lean tissue accretion did not differ for transgenic and control pigs, daily rates of fat accretion were lower in transgenic pigs than in control pigs (P<0.05). Based on these results we conclude that expression of IGF-I in the skeletal muscles gradually altered body composition as pigs became older but did not have a major affect on growth performance.     

Proteasome subunit Rpn1 binds ubiquitin-like protein domains

The yeast protein Rad23 belongs to a diverse family of proteins that contain an amino-terminal ubiquitin-like (UBL) domain. This domain mediates the binding of Rad23 to proteasomes, which in turn promotes DNA repair and modulates protein degradation, possibly by delivering ubiquitinylated cargo to proteasomes. Here we show that Rad23 binds proteasomes by directly interacting with the base subcomplex of the regulatory particle of the proteasome. A component of the base, Rpn1, specifically recognizes the UBL domain of Rad23 through its leucine-rich-repeat-like (LRR-like) domain. A second UBL protein, Dsk2, competes with Rad23 for proteasome binding, which suggests that the LRR-like domain of Rpn1 may participate in the recognition of several ligands of the proteasome. We propose that the LRR domain of Rpn1 may be positioned in the base to allow the cargo proteins carried by Rad23 to be presented to the proteasomal ATPases for unfolding. We also report that, contrary to expectation, the base subunit Rpn10 does not mediate the binding of UBL proteins to the proteasome in yeast, although it can apparently contribute to the binding of ubiquitin chains by intact proteasomes.