Phosphorylation of interleukin-6 at serine54: an early event in the secretory pathway in human fibroblasts

Sildenafil Reduces Expression and Release of IL-6 and IL-8 Induced by Reactive Oxygen Species in Systemic Sclerosis Fibroblasts

Oxidative stress linked to vascular damage plays an important role in the pathogenesis of systemic sclerosis (SSc). Indeed, vascular damage at nailfold capillaroscopy in patients with Raynaud's Phenomenon (RP) is a major risk factor for the development of SSc together with the presence of specific autoantiobodies. Here, we investigated the effects of the phosphodiesterase type 5 inhibitor (PDE5i) sildenafil, currently used in the management of RP, in modulating the proinflammatory response of dermal fibroblasts to oxidative stress in vitro. Human fibroblasts isolated from SSc patients and healthy controls were exposed to exogenous reactive oxygen species (ROS) (100 µM H2O2), in the presence or absence of sildenafil (1 µM). Treatment with sildenafil significantly reduced dermal fibroblast gene expression and cellular release of IL-6, known to play a central role in the pathogenesis of tissue damage in SSc and IL-8, directly induced by ROS. This reduction was associated with suppression of STAT3-, ERK-, NF-κB-, and PKB/AKT-dependent pathways. Our findings support the notion that the employment of PDE5i in the management of RP may be explored for its efficacy in modulating the oxidative stress-induced proinflammatory activation of dermal fibroblasts in vivo and may ultimately aid in the prevention of tissue damage caused by SSc.

Anthocyanins and their physiologically relevant metabolites alter the expression of IL-6 and VCAM-1 in CD40L and oxidized LDL challenged vascular endothelial cells

SCOPE

In vitro and in vivo studies suggest that dietary anthocyanins modulate cardiovascular disease risk; however, given anthocyanins extensive metabolism, it is likely that their degradation products and conjugated metabolites are responsible for this reported bioactivity.

METHODS AND RESULTS

Human vascular endothelial cells were stimulated with either oxidized LDL (oxLDL) or cluster of differentiation 40 ligand (CD40L) and cotreated with cyanidin-3-glucoside and 11 of its recently identified metabolites, at 0.1, 1, and 10 μM concentrations. Protein and gene expression of IL-6 and VCAM-1 was quantified by ELISA and RT-qPCR. In oxLDL-stimulated cells the parent anthocyanin had no effect on IL-6 production, whereas numerous anthocyanin metabolites significantly reduced IL-6 protein levels; phase II conjugates of protocatechuic acid produced the greatest effects (>75% reduction, p ≤ 0.05). In CD40L-stimulated cells the anthocyanin and its phase II metabolites reduced IL-6 protein production, where protocatechuic acid-4-sulfate induced the greatest reduction (>96% reduction, p ≤ 0.03). Similarly, the anthocyanin and its metabolites reduced VCAM-1 protein production, with ferulic acid producing the greatest effect (>65% reduction, p ≤ 0.04).

CONCLUSION

These novel data provide evidence to suggest that anthocyanin metabolites are bioactive at physiologically relevant concentrations and have the potential to modulate cardiovascular disease progression by altering the expression of inflammatory mediators.

Secreted protein kinases

Protein kinases constitute one of the largest gene families and control many aspects of cellular life. In retrospect, the first indication for their existence was reported 130 years ago when the secreted protein, casein, was shown to contain phosphate. Despite its identification as the first phosphoprotein, the responsible kinase has remained obscure. This conundrum was solved with the discovery of a novel family of atypical protein kinases that are secreted and appear to phosphorylate numerous extracellular proteins, including casein. Fam20C, the archetypical member, phosphorylates secreted proteins within Ser-x-Glu/pSer motifs. This discovery has solved a 130-year-old mystery and has shed light on several human disorders of biomineralization.

Characterization of the post-translational modification of recombinant human BMP-15 mature protein

Bone morphogenetic protein-15 (BMP-15) is an oocyte-secreted factor critical for the regulation of ovarian physiology. When recombinant human BMP-15 (rhBMP-15) produced in human embryonic kidney 293 cells was subjected to SDS-PAGE analysis, two mature protein forms corresponding to 16 kDa (P16) and 17 kDa (P17) were observed. Despite the physiological relevance and critical function of BMP-15 in female reproduction, little is known about the structure of rhBMP-15. Here, we have analyzed the structure of the rhBMP-15 mature proteins (P16 and P17) using state-of-the-art proteomics technology. Our findings are as follows: (1) the N-terminal amino acid of P16 and P17 is pyroglutamic acid; (2) the Ser residue at the sixth position of P16 is phosphorylated; (3) P17 is O-glycosylated at Thr10; and (4) the C-terminal amino acid of P16 and P17 is truncated. These findings are the first knowledge of the structure of rhBMP-15 mature protein toward understanding the molecular basis of BMP-15 function and could provide an important contribution to the rapidly progressing research area involving oocyte-specific growth factors in modulation of female fertility.

Paradigm-shifters: phosphorylated prolactin and short prolactin receptors

Since the discovery of physiologically-regulated prolactin (PRL) phosphorylation, one focus of the laboratory has been an examination of the different functions of the unmodified and phosphorylated hormone. In the mammary gland, unmodified PRL promotes growth activities, whereas phosphorylated or pseudophosphorylated PRL antagonizes this while also being a superior agonist for changes that favor differentiation. Phosphorylated PRL also increases expression of the short forms of the PRL receptor. These short forms of the receptor have functions beyond the accepted dominant negative and in mammary epithelial cells are capable of generating an intracellular signal leading to increased tight junction formation and beta-casein expression.

Phosphorylation of bone morphogenetic protein-15 and growth and differentiation factor-9 plays a critical role in determining agonistic or antagonistic functions

Two highly homologous oocyte-secreted growth factors, bone morphogenetic protein (BMP)-15 and growth and differentiation factor (GDF)-9, are known to control folliculogenesis and ovulation through direct effects on granulosa cells in the developing follicles. Although much is known about the expression and biology of these proteins, the impact of posttranslational modifications of BMP-15 and GDF-9 is unknown. Here, we report that: 1) recombinant human (rh) BMP-15 and rhGDF-9 are phosphorylated; 2) the phosphorylation is essential for bioactivity; and 3) the dephosphorylated forms of rhBMP-15 and rhGDF-9 can abolish the bioactivity of rhBMP-15, rhGDF-9, and rhBMP-7, but not rh activin A. These results indicate that the phosphorylation state of rhBMP-15 and rhGDF-9 is a determinant of their agonistic and antagonistic activities.

GRP94 hyperglycosylation and phosphorylation in Sf21 cells

GRP94 is an inducible resident endoplasmic reticulum/sarcoplasmic reticulum (ER/SR) glycoprotein that functions as a protein chaperone and Ca(2+) regulator. GRP94 has been reported to be a substrate for protein kinase CK2 in vitro, although its phosphorylation in intact cells remains unreported. In Sf21 insect cells, overexpression of canine GRP94 led to the appearance of a multiplet of three or more molecular-mass isoforms which was reduced to a single mobility form following treatment of cells with tunicamycin, suggesting stable accumulations of consecutively modified protein. Metabolic labeling of Sf21 cells with (32)P(i) led to a constitutive phosphorylation of GRP94 which, based upon phosphopeptide mapping, occurred specifically on CK2-sensitive sites. Among the GRP94 multiplet, however, only the lowest mobility form of GRP94 was phosphorylated, even though in vitro phosphorylation of GRP94 by CK2 led to phosphorylation of all glycosylated forms. The (32)P(i) incorporation into GRP94 indicated a slow turnover of phosphate incorporation that was unaffected by inhibition of biosynthesis, resulting in a steady-state level of phospho-GRP94 on CK2 sites. These data support a role for protein kinase CK2 in the cell biology for GRP94 and other resident ER/SR proteins that may occur in ER compartments.

Phosphorylation of gastrin-related peptides: physiological casein kinase like enzyme in Golgi membranes from bovine adrenal chromaffin cells and GH3 cells

Prohormones such as the gastrin precursor can be phosphorylated at Ser residues, on passage along the secretory pathway. The phosphorylation site occurs in a sequence (-Ser-Ala-Glu-) that suggests these peptides are substrates for physiological casein kinase, but the presence of this enzyme in endocrine cells is unknown. We have examined the specificity of Golgi membrane kinases from lactating rat mammary gland, bovine adrenal medulla and the GH3 cell line, for phosphorylation of progastrin fragments and analogues. The kinetics of phosphorylation of peptides with the native sequence, -Arg-Arg-Ser-Ala-Glu- were similar to those of tryptic cleavage fragments (Ser-Ala-Glu-) in both mammary and endocrine cell preparations. The product of in vitro phosphorylation was chromatographically indistinguishable from native peptide. Peptides with the sequence Ser-Ala-Ala (i.e., substitution of Glu to Ala) were not phosphorylated. We conclude that a physiological casein kinase like enzyme can act on both the gastrin precursor and its COOH-terminal cleavage product, and occurs in the Golgi complex of both mammary gland and peptide-producing endocrine cells.

Influence of interleukin-6 (IL-6) autoantibodies on IL-6 binding to cellular receptors

Neutralizing autoantibodies to interleukin-6 (aAb-IL-6) have been reported in healthy individuals, in patients with autoimmune diseases, and in pharmaceutically prepared pooled IgG (IVIg). We investigated the ability of aAb-IL-6 derived from IVIg to interfere with IL-6 binding to the undifferentiated monocytic cell line U-937. High-affinity aAb-IL-6, primarily of the IgG1 subclass, constituted approximately 1:10(6) of the total IgG in IVIg preparations. IL-6 binding to cellular receptors was strongly inhibited by one class of aAb-IL-6. These antibodies recognized epitope(s) on IL-6 essential for the binding of IL-6 to the alpha subunit of the IL-6 receptor (IL-6R). Another class of aAb-IL-6 recognized epitope(s) on IL-6, which is not essential for the binding to IL-6R but nevertheless important for the formation of high-affinity cellular IL-6 binding. These antibodies presumably interfered with the association of IL-6 receptor beta chains (gp130) with IL-6/IL-6R complexes, implicating that small IL-6/aAb-IL-6 immune complexes bound saturably (low affinity/high capacity) to cellular IL-6 receptors. There was no detectable binding of IL-6 through aAb-IL-6 and Fc receptors on U-937, and IVIg had no direct IL-6 receptor antagonizing activity. Dissociation kinetics of IL-6/aAb-IL-6 complexes at 37 degrees C revealed that IL-6 was liberated from 75% of the aAb-IL-6 with a half-time (t/2) approximately 4 h but bound almost irreversibly to the remaining aAb-IL-6 (t/2 > 20 h). Cellular IL-6 uptake and degradation was suppressed by aAb-IL-6. Taken together, the data suggest that loss of immunologic tolerance against IL-6 might be a novel physiological mechanism by which IL-6 activities are effectively attenuated. Finally, binding of IL-6 in complex with IgG1 aAb-IL-6 on cells expressing IL-6 receptors implicates that such cells could be targets of antibody-dependent immunological reactions, including cytotoxic reactions.

Conserved phosphorylation of serines in the Ser-X-Glu/Ser(P) sequences of the vitamin K-dependent matrix Gla protein from shark, lamb, rat, cow, and human

The present studies demonstrate that matrix Gla protein (MGP), a 10-kDa vitamin K-dependent protein, is phosphorylated at 3 serine residues near its N-terminus. Phosphoserine was identified at residues 3, 6, and 9 of bovine, human, rat, and lamb MGP by N-terminal protein sequencing. All 3 modified serines are in tandemly repeated Ser-X-Glu sequences. Two of the serines phosphorylated in shark MGP, residues 2 and 5, also have glutamate residues in the n + 2 position in tandemly repeated Ser-X-Glu sequences, whereas the third, shark residue 3, would acquire an acidic phosphoserine in the n + 2 position upon phosphorylation of serine 5. The recognition motif found for MGP phosphorylation, Ser-X-Glu/Ser(P), has been seen previously in milk caseins, salivary proteins, and a number of regulatory peptides. A review of the literature has revealed an intriguing dichotomy in the extent of serine phosphorylation among secreted proteins that are phosphorylated at Ser-X-Glu/Ser(P) sequences. Those phosphoproteins secreted into milk or saliva are fully phosphorylated at each target serine, whereas phosphoproteins secreted into the extracellular environment of cells are partially phosphorylated at target serine residues, as we show here for MGP and others have shown for regulatory peptides and the insulin-like growth factor binding protein 1. We propose that the extent of serine phosphorylation regulates the activity of proteins secreted into the extracellular environment of cells, and that partial phosphorylation can therefore be explained by the need to ensure that the phosphoprotein be poised to gain or lose activity with regulated changes in phosphorylation status.(ABSTRACT TRUNCATED AT 250 WORDS)

Cells transfected with human interleukin 6 cDNA acquire binding sites for the hepatitis B virus envelope protein

Earlier studies revealed that human interleukin 6 (IL-6) contains recognition sites for the hepatitis B virus (HBV) envelope (env) protein, and that IL-6 and anti-IL-6 antibodies, respectively, inhibited the interaction of cells expressing a receptor for HBV with the preS(21-47) segment of the HBV env protein, encompassing the complementary attachment site for IL-6. This suggested that IL-6 mediates HBV-cell interactions. We report that: (a) Chinese hamster ovary cells transfected with human IL-6 cDNA and Spodoptera frugiperda ovarian insect cells infected with recombinant baculovirus carrying human IL-6 cDNA expressed receptors for the preS(21-47) region of the HBV env protein, indicating that expression of IL-6 on the surface of cells is sufficient to endow them with receptors for HBV. (b) Among peptides covering the entire sequence of human IL-6 and the corresponding antipeptide antibodies, the peptide IL-6[35-66] and anti-IL-6[35-66] most effectively inhibited the interaction between human hepatoma HepG2 cells and the preS(21-47) ligand, suggesting that this region of the human IL-6 sequence encompasses a binding site for the HBV env protein. (c) Studies with replacement set peptides from the preS(21-47) sequence indicated that residues 21-25, 28, 31, 33-35, 39, and 43-45 can be replaced by alanine (serine) residues, while all the other residues are essential for maintaining the cell receptor/IL-6 binding activity. Further delineation of complementary sites on IL-6 and on the HBV env protein may contribute to the design of compounds inhibiting HBV replication.