Resonance assignment and secondary structure determination and stability of the recombinant human uteroglobin with heteronuclear multidimensional NMR

Uteroglobin: a steroid-inducible immunomodulatory protein that founded the Secretoglobin superfamily

Blastokinin or uteroglobin (UG) is a steroid-inducible, evolutionarily conserved, secreted protein that has been extensively studied from the standpoint of its structure and molecular biology. However, the physiological function(s) of UG still remains elusive. Isolated from the uterus of rabbits during early pregnancy, UG is the founding member of a growing superfamily of proteins called Secretoglobin (Scgb). Numerous studies demonstrated that UG is a multifunctional protein with antiinflammatory/ immunomodulatory properties. It inhibits soluble phospholipase A(2) activity and binds and perhaps sequesters hydrophobic ligands such as progesterone, retinols, polychlorinated biphenyls, phospholipids, and prostaglandins. In addition to its antiinflammatory activities, UG manifests antichemotactic, antiallergic, antitumorigenic, and embryonic growth-stimulatory activities. The tissue-specific expression of the UG gene is regulated by several steroid hormones, although a nonsteroid hormone, prolactin, further augments its expression in the uterus. The mucosal epithelia of virtually all organs that communicate with the external environment express UG, and it is present in the blood, urine, and other body fluids. Although the physiological functions of this protein are still under investigation, a single nucleotide polymorphism in the UG gene appears to be associated with several inflammatory/autoimmune diseases. Investigations with UG-knockout mice revealed that the absence of this protein leads to phenotypes that suggest its critical homeostatic role(s) against oxidative damage, inflammation, autoimmunity, and cancer. Recent studies on UG-binding proteins (receptors) provide further insight into the multifunctional nature of this protein. Based on its antiinflammatory and antiallergic properties, UG is a potential drug target.

Coupe du Roi Bisection of Proteins. Spontaneous Tetramerization of Two Peptides That Span the Sequence of the Rabbit Uteroglobin Monomer

The study of dividing objects into isometric segments has yielded novel approaches to the synthesis of high-symmetry organic compounds. Reported herein is the first application of this concept to a protein, rabbit uteroglobin (UG). Bisection of UG into two identical homochiral segments led to the design of the heterodimeric 70mer peptide alpha(1,2)-S-S-alpha(3,4) that spans the sequence of the native UG monomer. The ability of this compound to form a globular 140mer tetramer consisting of two noncovalently bound heterodimers was assessed by ultracentrifugation at sedimentation equilibrium and by fluorescent spectroscopy. On the other hand, the monomeric peptides alpha(1,2)-SH and alpha(3,4)-SH were shown to selectively form the alpha(1,2)-S-S-alpha(3,4) heterodimer via spontaneous air oxidation in phosphate buffer at neutral pH.

Lys 43 and Asp 46 in alpha-helix 3 of uteroglobin are essential for its phospholipase A2 inhibitory activity

Uteroglobin (UG) is an anti-inflammatory, secreted protein with soluble phospholipase A2 (sPLA2)-inhibitory activity. However, the mechanism by which UG inhibits sPLA2 activity is unknown. UG is a homodimer in which each of the 70-amino acid subunits forms four alpha-helices. We previously reported that sPLA2-inhibitory activity of UG may reside in a segment of alpha-helix 3 that is exposed to the solvent. In addition, it has been suggested that UG may inhibit sPLA2 activity by binding and sequestering Ca++, essential for sPLA2 activation. By site-specific mutation, we demonstrate here that Lys 43 Glu, Asp 46 Lys or a combination of the two mutations in the full-length, recombinant human UG (rhUG) abrogates its sPLA2-inhibitory activity. We demonstrate further that recombinant UG does not bind Ca++ although when it is expressed with histidine-tag (H-tag) it is capable of binding Ca++. Taken together our results show that: (i) Lys 43 and Asp 46 in rhUG are critical residues for the sPLA2-inhibitory activity of UG and (ii) Ca++-sequestration by rhUG is not likely to be one of the mechanisms responsible for its sPLA2-inhibitory activity.

Crystal structure analysis of recombinant human uteroglobin and molecular modeling of ligand binding

Uteroglobin, a steroid-inducible, cytokine-like, secreted protein with immunomodulatory properties, has been reported to bind progesterone, polychlorinated biphenyls (PCB), and retinol. Structural studies may delineate whether binding of ligands is a likely physiological function of human uteroglobin (hUG). We report a refined crystal structure of uncomplexed recombinant hUG (rhUG) at 2.5-A resolution and the results of our molecular modeling studies of ligand binding to the central hydrophobic cavity of rhUG. The crystal structure of rhUG is very similar to that of reported crystal structures of uteroglobins. Using molecular modeling techniques, the three ligands--PCB, progesterone, and retinol--were docked into the hydrophobic cavity of the dimer structure of rhUG. We undocked the progesterone ligand by pulling the ligand from the cavity into the solvent. From our modeling and undocking studies of progesterone, it is clear that these types of hydrophobic ligands could slip into the cavity between helix-3 and helix-3' of the dimer instead of between helix-1 and helix-4 of the monomer, as proposed earlier. Our results suggest that at least one of the physiological functions of UG is to bind to hydrophobic ligands, such as progesterone and retinol.

The uteroglobin fold

Uteroglobin (UTG) forms a fascinating homodimeric structure that binds small- to medium-sized ligands through an internal hydrophobic cavity, located at the interface between the two monomers. Previous studies have shown that UTG fold is not limited to the UTG/CC10 family, whose sequence/structure relationships are highlighted here, but can be extended to the cap domain of Xanthobacter autotrophicus haloalkane dehalogenase. We show here that UTG fold is adopted by several other cap domains within the alpha/beta hydrolase family, making it a well-suited "geode" structure allowing it to sequester various hydrophobic molecules. Additionally, some data about a new crystal form of oxidized rabbit UTG are presented, completing previous structural studies, as well as results from molecular dynamics, suggesting an alternative way for the ligand to reach the internal cavity.

Antiflammins. Bioactive peptides derived from uteroglobin

Uteroglobin/Clara cell 10-kDa protein (UG/CC10) is a hormonally regulated small secretory protein that has a variety of in vitro and in vivo pharmacological effects. These include a potent anti-inflammatory activity and inhibitory effects on neutrophil migration, thrombin-induced platelet aggregation, in vitro chemoinvasion, as well as "tumor suppressor"-like effects and other properties. Several mechanisms of action have been proposed for these effects. Pharmacological properties suggest that UG itself or substances derived from it may be used as experimental drugs for several indications. The group of oligopeptides collectively known as "antiflammins" (AFs) were originally described in 1988. Their design was derived from the region of highest sequence similarity between UG and another group of proteins with anti-inflammatory properties, the lipocortins or annexins. Nanomolar concentrations of these peptides can reproduce several of the pharmacological activities of UG, including its in vivo anti-inflammatory effects and inhibition of platelet aggregation. The AFs have been safely and effectively used to suppress inflammation and fibrosis in several animal models. Progress in clarifying the mechanism of action of the AFs may facilitate the structure-based design of a novel class of potent anti-inflammatory, antichemotactic drugs.

Uteroglobin (UG) suppresses extracellular matrix invasion by normal and cancer cells that express the high affinity UG-binding proteins

Uteroglobin (UG) is a steroid-inducible, multifunctional, secreted protein with antiinflammatory and antichemotactic properties. Recently, we have reported a high affinity UG-binding protein (putative receptor), on several cell types, with an apparent molecular mass of 190 kDa (Kundu, G. C., Mantile, G., Miele, L., Cordella-Miele, E., and Mukherjee, A. B. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 2915-2919). Since UG is a homodimer in which the 70 amino acid subunits are connected by two disulfide bonds, we sought to determine whether UG monomers also interact with the 190-kDa UG-binding protein and if so, whether it has the same biological activity as the dimer. Surprisingly, we discovered that in addition to the 190-kDa species, another protein, with an apparent molecular mass of 49 kDa, binds reduced UG with high affinity and specificity. Both 49- and 190-kDa proteins are readily detectable on nontransformed NIH 3T3 and some murine cancer cells (e. g. mastocytoma, sarcoma, and lymphoma), while lacking on others (e.g. fibrosarcoma). Most interestingly, pretreatment of the cells, which express the binding proteins, with reduced UG dramatically suppresses extracellular matrix (ECM) invasion, when such treatment had no effect on fibrosarcoma cells that lack the UG-binding proteins. Tissue-specific expression studies confirmed that while both 190- and 49-kDa UG-binding proteins are present in bovine heart, spleen, and the liver, only the 190-kDa protein is detectable in the trachea and in the lung. Neither the 190-kDa nor the 49-kDa protein was detectable in the aorta. Purification of these binding proteins from bovine spleen by UG-affinity chromatography and analysis by SDS-polyacrylamide gel electrophoresis followed by silver staining identified two protein bands with apparent molecular masses of 40 and 180 kDa, respectively. Treatment of the NIH 3T3 cells with specific cytokines (i.e. interleukin-6) and other agonists (i.e. lipopolysaccharide) caused a substantially increased level of 125I-UG binding but the same cells, when treated with platelet-derived growth factor, tumor necrosis factor-alpha, interferon-gamma, and phorbol 12-myristate 13-acetate, did not alter the UG binding. Taken together, these findings raise the possibility that UG, through its binding proteins, plays critical roles in the regulation of cellular motility and ECM invasion.