Secreted Phosphoprotein 24 is a Biomarker of Mineral Metabolism

The 24 kD form of secreted phosphoprotein (SPP-24), a cytokine-binding bone matrix protein with various truncated C-terminal products, is primarily synthesized by the liver. SPP-24 shares homology with fetuin-A, a potent vascular and soft tissue calcification inhibitor and SPP-24 is one component of calciprotein particles (CPPs), a circulating fetuin-mineral complex. The limited molecular evidence to date suggests that SPP-24 may also function as an inhibitor of bone formation and ectopic vascular calcification, potentially through bone morphogenic protein 2 (BMP-2) and Wnt-signaling mediated actions. The C-terminal products of SPP-24 bind to BMP-2 and attenuate BMP-2-induced bone formation. The aim of this study was to assess circulating SPP-24 in relation to kidney function and in concert with markers of mineral metabolism in humans. SPP-24 was measured in the serum of total of 192 subjects using ELISA-based measurements. Subjects were participants of one of two cohorts: (1) mGFR Cohort (n = 80) was participants of a study of measured GFR (mGFR) using inulin urinary clearance, recruited mostly from a chronic kidney disease clinic with low-range kidney function (eGFR 38.7 ± 25.0 mL/min/1.73 m²) and (2) CaMOS Cohort (n = 112) was a subset of randomly selected, community-dwelling participants of year 10 of the Canadian Multicentre Osteoporosis Study with eGFR in the normal range of 75.0 ± 15.9 mL/min/1.73 m². In the combined cohort, the mean SPP-24 was 167.7 ± 101.1 ng/mL (range 33.4-633.6 ng/mL). The mean age was 66.5 ± 11.3, 57.1% female and mean eGFR (CKD-EPI) was 59.9 ± 27.0 mL/min/1.73 m² (range 8-122 mL/min/1.73 m²). There was a strong inverse correlation between SPP-24 and eGFR (R = - 0.58, p < 0.001) that remained after adjustment for age. Following adjustment for age, eGFR, and sex, SPP-24 was significantly associated with phosphate (R = - 0.199), PTH (R = 0.298), and the Wnt-signaling inhibitor Dickkopf-related protein 1 (R = - 0.156). The results of this study indicate that SPP-24 is significantly altered by kidney function and is the first human data linking levels of SPP-24 to other biomarkers involved in mineral metabolism. Whether there is a role for circulating SPP-24 in bone formation and ectopic mineralization requires further study.

The modified amino sugar N-Butyryl Glucosamine fed to ovariectomized rats preserves bone mineral through increased early mineralization, but does not affect body composition

Background:  The toxicities of pharmaceuticals for chronic arthritis and osteoporosis should be of concern to consumers. This partially accounts for the popularity of consumption of the amino sugar glucosamine, in-spite of controversy about its efficacy. We chemically synthesized N-butyryl glucosamine (GlcNBu), which we discovered protected bone and cartilage in an inflammatory arthritis rat model when used as a feed supplement. GlcNBu can also be potentially synthesized biochemically, since we recently demonstrated that human acetyl-CoA: glucosamine-6-phosphate N-acetyltransferase 1 has a relaxed donor specificity and transfers acyl groups of up to four carbons in length, i.e. the butyryl moiety. Oral GlcNBu had no detectable toxicity and also protected against bone loss in ovariectomized (OVX) rats as a model for osteoporosis. However, we demonstrated this only for bones excised at 6 months. Thus, the current study aims to determine when bone mineralization is maximized during daily GlcNBu supplementation, in both OVΧ and Sham-OVX rats, in addition to the relationship of bone mineralization to body composition.Methods: Female Sprague-Dawley rats were randomized into 4 groups, containing 8 rats each. The groups consisted of OVX or Sham-OVX rats whose diets were supplemented with either 200 mg/kg/day of GlcNBu or an equimolar amount of glucose. We performed sequential bone density and body composition measurements, by dual-energy X-ray absorptiometry in the live, anesthetised rats, over a 6-month experimental period, starting at the age of 8 weeks. Results were analyzed by descriptive statistics and 2-way ANOVA. Results: The major increases in the mineral content and density of the spine and the femur in GlcNBu-supplemented rats occurred early, from the baseline to week 8.  Ovariectomy resulted in a number of significant differences in body composition, while feeding GlcNBu had no significant effects on body composition. The significant effects of ovariectomy on body composition initially appeared at 8 weeks, while the GlcNBu effects on increased bone mineral initially appeared at 2 weeks. An interaction between OVX and GlcNBu was seen only at 16 weeks for the bone mineral density of the femoral head.Conclusions: Supplementation of the diet by GlcNBu in both OVX and Sham-OVX rats increases bone mineral as early as 2 weeks. Ovariectomy but not GlcNBu supplementation had a significant effect on body composition. The effect of GlcNBu occurs independently of changes in body composition, probably as a direct effect of stimulation of bone matrix synthesis which continues to be mineralized.  This work represents an important step in the development and commercialization of GlcNBu for the prevention and treatment osteoporosis, where there is now an increasing demand for safe, long term agents.Key words: osteoporosis, ovariectomy, N-butyryl glucosamine, bone, mineralization, body composition, dual-energy X-ray absorptiometry

N-acylated glucosamines for bone and joint disorders: effects of N-butyryl glucosamine on ovariectomized rat bone

The benefit of glucosamine (GlcN) in bone and joint disorders remains controversial. N-acetylation and other N-acylations of GlcN alter its biological properties fundamentally. We have shown previously that N-butyryl glucosamine (GlcNBu) preserved strikingly the subchondral bone structure in a destructive arthritis rat model. Here, we examine whether GlcNBu preserves bone in the ovariectomized (OVX) rat, a model for postmenopausal osteoporosis. Rats were randomized into 4 groups: group 1, sham OVX glucose (Glc) fed; group 2, sham OVX GlcNBu fed; group 3, OVX Glc fed; and group 4, OVX GlcNBu fed. A single, oral, 200-mg/kg dose of GlcNBu or Glc was administered daily for 6 months. Bone mineral content (BMC) and bone mineral density, and biomechanical properties of the femurs and spines were determined by standardized techniques. Two-way analysis of variance with a Bonferroni post hoc test was used for statistical analysis. Ovariectomy in group 3 resulted either in significant or highly significant effects in a number of the tests. For spinal BMCs the interaction between GlcNBu and OVX was significant. For the femurs, this interaction was also seen in energy to failure, and ultimate displacement and ultimate strain tests. In general, ovariectomy was necessary to show significant preventive effects of GlcNBu on mineral content and some biomechanical properties. We conclude that GlcNBu feeding in the OVX rat preserves bone mineral and some biomechanical properties. Translationally, GlcNBu can be positioned between nutriceuticals and pharmaceuticals for the prevention and treatment of osteoporosis. Advantages include low production costs and a favorable safety profile.

Modulation of articular chondrocyte proliferation and anionic glycoconjugate synthesis by glucosamine (GlcN), N-acetyl GlcN (GlcNAc) GlcN sulfate salt (GlcN.S) and covalent glucosamine sulfates (GlcN-SO4)

OBJECTIVES

To investigate, in chondrocyte cultures under conditions for maximizing responses in proliferation and proteoglycan (PG) synthesis, the effects of glucosamine hydrochloride (GlcN.HCl) and glucosamine sulfate (GlcN.S) salts, N-acetyl glucosamine (GlcNAc), and covalently substituted GlcN-X,Y,Z(SO(4))(n) (general formula).

METHODS

Bovine articular chondrocytes (BAC) were studied under anchorage-independent (AI, alginate beads) and anchorage-dependent (AD, plastic surface) conditions. Differentiation markers were evaluated (e.g., cartilage-specific (V+C)(-) fibronectin). Varying concentrations of GlcN.HCl, GlcN.S, GlcNAc and GlcN sulfated at positions -2, -3, -6, (-2,3), (-3,6) and (-3,4,6), were tested. Cell proliferation, DNA synthesis and [(35)S]-sulfate incorporation into newly synthesized PG were determined.

RESULTS

Increasing GlcN.HCl or GlcN.S concentrations gave decreasing net PG synthesis. Compounds showed more pronounced effects in AD cultures (expressing the V(-)C(+) fibronectin isoform) compared to AI cultures ((V+C)(-) isoform). Addition of GlcN.HCl or GlcN.S gave a concentration-dependent decrease in BAC proliferation, partially prevented by glucose (Glc). GlcNAc was not inhibitory. Addition of GlcN-2-SO(4) or GlcN-2,6-diSO(4) did not affect proliferation or DNA synthesis. The other GlcN-sulfates gave varying inhibitory effects, which for GlcN-3-SO(4) were reversed by inosine.

CONCLUSIONS

The free amino group of GlcN seems responsible for inhibition of chondrocyte proliferation and PG synthesis. These effects were greater under higher concentrations of GlcN in AD vs AI conditions. GlcN.HCl behaves similarly to GlcN.S, but differential effects with GlcN-X,Y,Z(SO(4))(n) isomers were observed. Acetylation or sulfation of the GlcN amino group reverses or partially reverses, respectively, anti-proliferative effects of GlcN. Sulfation of GlcN, at positions 3 and 6 results in complex effects on AC proliferation and PG synthesis.

N-acylation of glucosamine modulates chondrocyte growth, proteoglycan synthesis, and gene expression

OBJECTIVE

To examine the effects of glucosamine (GlcN) and some N-acylated (GlcNAcyl) derivatives on the proliferation and proteoglycan (PG) synthesis of bovine articular chondrocyte (BAC); and to expand these results to human articular chondrocytes (HAC) and study the modulation of gene regulation by these compounds.

METHODS

The compounds tested were: glucose (Glc), GlcN.HCl, N-acetyl GlcN (GlcNAc), and N-butyryl GlcN, (GlcNBu). GlcNBu was synthesized from GlcN and butyric anhydride. For the chondrocyte cultures, both anchorage-dependent (AD) and an anchorage-independent (AI) system (alginate beads) were evaluated. Following the various additions, BAC were assessed for total cell number, DNA, or total PG synthesis at different times. Utilizing similar conditions, human cDNA microarrays were performed for the HAC after harvesting total RNA.

RESULTS

For AD cultures, the addition of GlcN.HCl (0.1-5.0 mM) to BAC or HAC cultures inhibited cell proliferation and total PG synthesis in a dose-dependent manner. For AI cultures, the inhibitory effects of GlcN.HCl on cell proliferation were less prominent, and PG synthesis increased slightly more for the GlcNAcyl than the GlcN additions. In the AD system, the addition of GlcNAc did not result in the inhibitory effect of GlcN.HCl, while GlcNBu addition resulted in an increase in BAC proliferation and PG synthesis that could not be explained by the Bu moiety alone. For the HAC, additions of 0.1 mM GlcNBu resulted in upregulation of a large number of genes, with only a few downregulated, while GlcN addition resulted in no upregulation and one downregulated gene, for preset stringency criteria.

CONCLUSION

Addition of GlcNBu to BAC or HAC to AD cultures generally stimulated cell proliferation and PG synthesis, while addition of GlcN resulted in inhibition of these indicators. The inhibitory effects of the GlcN molecule appear to be related to the unsubstituted amino group. Additions of GlcNBu, but not GlcN, to HAC resulted in upregulation in the expression of a large number of genes.

Molecular cloning and expression of a human phosphodiesterase 4C

A cDNA coding for a human phosphodiesterase 4C (PDE4C2) was isolated from the mRNA prepared from the glioblastoma cell line, U87. The cDNA contained an ORF of 1818 bp corresponding to a 605 amino acid polypeptide. The sequence differed at the 5' end from the human PDE4C previously reported (Engels, P. et al, 1995 FEBs Letters 358, 305-310) indicating that it represents a novel splice variant of the human PDE4C gene. Evidence was also obtained for a third 5' splice variant. The PDE4C2 cDNA was transfected into both COS 1 cells and yeast cells, and shown to direct the expression of an 80 kD polypeptide by Western blotting using a PDE4C specific antiserum. The activity of cell lysates was typical of PDE4 being specific for cAMP and inhibitable by the selective inhibitor, rolipram. However, the Km for cAMP of the enzyme produced in COS cells was 0.6 microM compared to 2.6 microM for the yeast 4C activity. In addition the COS cell PDE4 activity was much more sensitive to R rolipram than the yeast PDE4 enzyme (IC50 of 23 nM compared to 1648 nM). This difference in rolipram sensitivity was associated with the detection of a high affinity [3H] R rolipram binding site on the COS cell 4C enzyme but not on the yeast expressed enzyme. The results indicate that the enzyme can adopt more than one active conformation, which are distinguished by their interaction with rolipram.

Detection of metal binding to bovine inositol monophosphatase by changes in the near and far ultraviolet regions of the CD spectrum

Mg2+ ions, essential for the catalytic activity of mammalian inositol monophosphatase, increase the ellipticity in the near-ultraviolet region of the CD spectrum of the enzyme. These spectral changes are not affected by the additional presence of substrate and are reversed if EDTA is added to the solution of enzyme and metal ions. Titration of the spectral perturbation at 275 nm shows that this binding occurs with a dissociation constant (Kd) around 275 microM, 292 microM and 302 microM for the wild-type, [Gln217]inositol monophosphatase and [Phe219]inositol monophosphatase enzymes respectively. The source of the spectroscopic change at 275 nm is not Trp219. The addition of Mg2+ also causes a decrease in ellipticity over most of the far-ultraviolet region of the spectrum (between 205-240 nm). The Kd values describing the binding of Mg2+ ions are 3.9 mM, 6.8 mM and 29.1 mM for the wild-type, [Gln217]inositol monophosphatase and [Phe219]inositol monophosphatase enzymes, respectively, each showing an approximate 12% change in ellipticity. In the additional presence of 10 mM Pi, there is a fourfold increase in the affinity of wild-type enzyme for Mg2+. It is concluded that CD spectral changes at wavelengths around 275 nm are indicative of metal ions interacting with a high-affinity metal-binding site (site 1). The spectral changes around 225 nm are associated with interactions at a lower-affinity site normally occupied by the Mg2+ ion which is reflected by the Km value for this metal ion. Other metal ions such as Ca2+ and Tb3+ (but not Mn2+ or Zn2+) also perturb the CD spectrum of the enzyme in both regions of the spectrum. The amplitudes of these signal changes are greater for Mg2+ or Tb3+ (25%) ions than for Ca2+ (8.5%), although two Ca2+-binding sites with Kd values of 20 microM and 100 microM have been identified. The uncompetitive inhibitor Li+ causes little change in the near-ultraviolet spectrum in the absence or presence of either substrate or Pi. However, in contrast to other metal ions, Li+ ions elicit a 10% increase in ellipticity at 220 nm with a Kd of 0.8 mM.

Reversible denaturation of myo-inositol monophosphatase. The stability of the metal-binding loop

The unfolding of bovine brain myo-inositol monophosphatase by guanidine. HCl (Gdn. HCl) has been investigated. The recovery of circular dichroism, emission spectra, and catalytic activity after dilution of Gdn.HCl-treated samples indicate that the overall process is reversible. The steepness of the spectroscopic changes between 3 M and 5 M Gdn.HCl, and the lack of any discernible plateau suggest that unfolding of the protein is a cooperative process. The sensitized luminescence of bound Tb(III) was used as a probe of conformational changes of the metal-binding loop. Denaturation of the enzyme by Gdn.HCl does not abolish sensitized luminescence. A 50% decrease in sensitized luminescence was observed in 5 M Gdn.HCl. Under this set of experimental conditions, the protein binds terbium with an association constant of 1 x 10(6)M-1. It is suggested that a residual structure of denatured myo-inositol monophosphatase is responsible for the binding of terbium ions. The kinetics of unfolding and refolding as a function of Gdn.HCl concentration were monitored by protein fluorescence in a stopped-flow instrument. The monophosphatase unfolded in a single kinetic phase with rate constants in the range 80-65 s-1 at 25 degrees C. The refolding kinetics fit monoexponential functions with rate constants in the range 120-65 s-1 depending on the Gdn.HCl concentration. Substantial refolding of the protein occurs within the dead time of mixing.