Human paraoxonases (PON1, PON2, and PON3) are lactonases with overlapping and distinct substrate specificities

High-level expression of recombinant human paraoxonase 1 Q in silkworm larvae (Bombyx mori)

Human serum paraoxonase 1 (hPON1) belongs to a family of enzymes that catalyze the hydrolysis of a broad range of esters and lactones. Although the very first identification of hPON1 might have been as a calcium-dependent paraoxonase/arylesterase, PON1 is in fact a lactonase associated with high-density lipoprotein and strongly stimulated by apoA-I. PON1 hydrolyzes various organophosphates, including insecticides and nerve gases. PON1 also plays a key role in prevention of atherosclerosis. Mediation of cholesterol efflux from macrophage is a key in vivo function of PON1. In present study, the hPON1 Q gene was cloned into baculovirus transfer vector pVL1392 and expressed in silkworm expression system. The rhPON1 Q presented two bands with every near molecular weight of about 40 and 43 kDa according to sodium dodecyl sulphate-polyacrylamide gel electrophoresis and Western blotting analysis. The expression level was up to 1,256 mg/L in haemolymph, about 50 times as high as that from BmN cells (24.8 mg/L). After purified by two chromatography steps (DEAE-Sepharose and HiTrap Chelating HP), the purity of rhPON1 Q was up to 90%, and the enzymatic properties are similar to serum hPON1.

Bile acids decrease hepatic paraoxonase 1 expression and plasma high-density lipoprotein levels via FXR-mediated signaling of FGFR4

OBJECTIVE

The purpose of this research was to determine how dietary bile acids repress hepatic expression of paraoxonase 1 (PON1).

METHODS AND RESULTS

C57BL/6 mice and C3H/HeJ mice, having different susceptibilities to atherosclerosis, were fed a chow diet and an atherogenic diet containing taurocholate. Compared with the more atherosclerosis-susceptible C57BL/6 mice, C3H/HeJ mice display resistance to dietary bile acid repression of hepatic PON1 mRNA and decreased high-density lipoprotein cholesterol. Whereas knockout of toll receptor 4 did not affect response to taurocholate, deletion of either FXR or FGFR4 blocked taurocholate repression of PON1 and CYP7A1. FGF19, an activator of FGFR4 expressed in human ileum, decreased expression of both PON1 and CYP7A1 expression by human hepatoma cells. In all of the mice studied, dietary taurocholate increased ileal expression of FGF15, a FXR-inducible murine homologue of human FGF19.

CONCLUSIONS

Hepatic PON1 and CYP7A1 mRNA expression is repressed by bile acids via FXR-mediated induction of FGF15. Thus, the inability of C3H/HeJ mice to display taurocholate repression of PON1 and CYP7A1 mRNAs was not because of a lack of induction of FGF15 but rather signaling events distal to FGF15-FGFR4 association.

Cloning, high level expression of human paraoxonase-3 in Sf9 cells and pharmacological characterization of its product

Human paraoxnase-3 (hPON3) (EC3.1.8.1) is a lipid-associated enzyme with antioxidant activity, and can inhibit the oxidation of low-density lipoprotein (LDL), thereby inhibiting early atherogenic process. In the present study, human PON3 gene was cloned from Human Fetal Liver Marathon-Ready cDNA and expressed in insect cells using baculovirus vector. Twenty-eight milligrams of purified recombinant hPON3 (rhPON3) was obtained from 1L Sf9 cells culture. The Km and Vmax values of rhPON3, with respective to phenylacetate hydrolysis were 7.46+/-4.40 mM and 89+/-10.54 U/mg (n=3). The kinetic parameters of Vmax and Km for dihydrocoumarin hydrolysis by rhPON3 were 698+/-248 U/mg and 0.84+/-0.24 mM (n=3). LDL oxidation assay indicated that rhPON3 could effectively protect LDL against copper-induced oxidation in vitro.

Paraoxonase 1 (PON1) is a more potent antioxidant and stimulant of macrophage cholesterol efflux, when present in HDL than in lipoprotein-deficient serum: relevance to diabetes

The present study analyzed serum paraoxonase 1 (PON1) distribution among HDL and lipoprotein-deficient serum (LPDS) in atherosclerotic patients, and compared PON1 biological functions in these fractions. Serum HDL and LPDS fractions were isolated from control healthy subjects, diabetic and hypercholesterolemic patients. PON1 activities and protein in HDL/LPDS, as well as its ability to protect against lipid peroxidation and to stimulate HDL/LPDS-mediated macrophage cholesterol efflux were measured. In LPDS from controls, PON1 protein and a significant paraoxonase activity were found, whereas arylesterase and lactonase activities were substantially reduced compared to HDL, by 78% and 88%, respectively. In diabetic patients, PON1 protein and paraoxonase activity in HDL were significantly decreased by 2.8- and 1.7-fold, respectively, compared with controls' HDL. In parallel, in these patient's LPDS, PON1 protein and paraoxonase activity were markedly increased by 3.7- and 1.7-fold, respectively, compared with controls' LPDS. PON1 in HDL (but not PON1 in LPDS) significantly decreased AAPH-induced lipid peroxides formation by 33%, and increased macrophage cholesterol efflux by 31%. We conclude that PON1 is less antiatherogenic when present in LPDS than in HDL. The abnormal serum PON1 distribution in diabetic patients, could be responsible for the accelerated atherosclerosis development in these patients.

Human and murine paraoxonase 1 are host modulators of Pseudomonas aeruginosa quorum-sensing

The pathogenic bacterium Pseudomonas aeruginosa uses acyl-HSL quorum-sensing signals to regulate genes controlling virulence and biofilm formation. We found that paraoxonase 1 (PON1), a mammalian lactonase with an unknown natural substrate, hydrolyzed the P. aeruginosa acyl-HSL 3OC12-HSL. In in vitro assays, mouse serum-PON1 was required and sufficient to degrade 3OC12-HSL. Furthermore, PON2 and PON3 also degraded 3OC12-HSL effectively. Serum-PON1 prevented P. aeruginosa quorum-sensing and biofilm formation in vitro by inactivating the quorum-sensing signal. Although 3OC12-HSL production by P. aeruginosa was important for virulence in a mouse sepsis model, Pon1-knock-out mice were paradoxically protected. These mice showed increased levels of PON2 and PON3 mRNA in epithelial tissues suggesting a possible compensatory mechanism. Thus, paraoxonase interruption of bacterial communication represents a novel mechanism to modulate quorum-sensing by bacteria. The consequences for host immunity are yet to be determined.

High Affinity, Stability, and Lactonase Activity of Serum Paraoxonase PON1 Anchored on HDL with ApoA-I

Serum paraoxonase (PON1) is a high-density lipoprotein (HDL)-associated enzyme exhibiting antiatherogenic properties. This study examined the interaction of recombinant PON1 with reconstituted HDL comprised of PC, cholesterol, and various apolipoproteins (apoA-I, -II, and -IV). The affinity, stability, and lactonase activity were strongly correlated, with apoA-I exhibiting the strongest effects, apoA-IV exhibiting weaker yet significant effects, and apoA-II having a negative effect relative to protein-free particles. We found that PON1 binds apoA-I HDL with sub-nanomolar affinities (K(d) << 10(-)(9) M) and slow dissociation rates (t(1/2) > 80 min), while binding affinity for other particles was dramatically lower. A truncated form of PON1 lacking the N-terminal helix maintains considerable binding to apoA-I HDL (K(d) = 1.2 x 10(-)(7) M), validating the structural model which indicates additional parts of the enzyme involved in HDL binding. Kinetic inactivation assays revealed the existence of an equilibrium between two forms of PON1 differing in their stability by a factor of 100. Various lipoproteins and detergent preparations shift this equilibrium toward the more stable conformation. Consistent with its highest affinity, only apoA-I HDL is capable of totally shifting the equilibrium toward the stable form. The paraoxonase and arylesterase activities were stimulated by HDL by 2-5-fold as previously reported, almost independently of the apoliporotein content. In contrast, only apoA-I is capable of stimulating the lactonase activity by <or=20-fold to k(cat)/K(M) values of 10(6)-10(7) M(-)(1) s(-)(1), while apoA-IV and apoA-II have almost no effect. Overall, the results indicate the high stability, selectivity, and catalytic proficiency of PON1 when anchored onto apoA-I HDL, toward lactone substrates, and lipophilic lactones in particular.

Staphylococcal Drp35 is the functional counterpart of the eukaryotic PONs

Drp35 has been identified as a protein that is induced in Staphylococcus aureus in response to exposure to certain antibiotics. Here we demonstrate that Drp35 is a lactonase that does not contribute directly to the resistance to the inducer antibiotics except for bacitracin. The detailed analysis on the expression of Drp35 revealed that in addition to a broad range of antibiotics, agents such as detergents that perturb the membrane integrity could induce its expression. The significance of this characteristic expression is discussed in relation to its activity similarity to the eukaryotic counterparts, paraoxonase family proteins.

Quorum quenching enzyme activity is widely conserved in the sera of mammalian species

Acyl-homoserine lactone (AHL) quorum sensing signals play a key role in synchronizing virulence gene expression in Pseudomonas aeruginosa, which could cause fatal bloodstream infections. We showed that AHL inactivation activity, albeit with variable efficiency, was conserved in the serum samples of all the 6 tested mammalian animals. High-performance liquid chromatography and mass spectrometry analyses revealed that mammalian sera had a lactonase-like enzyme(s), which hydrolyzed the lactone ring of AHL to produce acyl homoserine, with enzyme properties reminiscent of paraoxonases (PONs). We further showed that the animal cell lines expressing three mouse PON genes, respectively, displayed strong AHL degradation activities.