A possible antiatherogenic role for phosphocitrate through modulation of low density lipoprotein uptake and degradation in aortic smooth muscle cells

Phosphocitrate is potentially a disease-modifying drug for noncrystal-associated osteoarthritis

Phosphocitrate (PC), a calcification inhibitor, inhibits the development of crystal-associated osteoarthritis (OA) in Hartley guinea pigs. However, the molecular mechanisms underlying its disease-modifying effect remain elusive. This study sought to test the hypothesis that PC has calcium crystal-independent biological activities which are, at least in part, responsible for its disease-modifying activity. We found that PC inhibited the proliferation of OA fibroblast-like synoviocytes in the absence of calcium crystals. Consistent with its effect on cell proliferation, PC downregulated the expression of numerous genes classified in cell proliferation. PC also downregulated the expression of many genes classified in angiogenesis and inflammatory response including prostaglandin-endoperoxide synthase 2, interleukin-1 receptor, type I, and chemokine (C-C motif) ligand 2. In contrast, PC upregulated the expression of many genes classified in musculoskeletal tissue development, including aggrecan, type I collagen, and insulin-like growth factor binding protein 5. These findings suggest that PC is not only a promising disease-modifying drug for crystal-associated OA but also for noncrystal-associated OA.

Inhibition of calcium pyrophosphate dihydrate crystal formation in articular cartilage vesicles and cartilage by phosphocitrate

Articular cartilage vesicles (ACV), isolated by differential centrifugation of adult hyaline articular cartilage collagenase digests, mineralized in the presence of calcium and ATP. Mineral analysis by microscopy, chemical analysis, energy-dispersive analysis, and infrared spectroscopy revealed crystals resembling calcium pyrophosphate dihydrate (CPPD). Adult articular cartilage also underwent ATP-dependent mineralization, supporting the contention that vesicles in situ fostered adult articular cartilage mineralization. Phosphocitrate (PC) is a recognized in vitro inhibitor of hydroxyapatite and calcium oxalate monohydrate crystal formation, but it is not known whether PC can similarly restrict CPPD crystal development. In the present study we examine the effect of PC, citrate, and n-sulfo-2-amino-tricarballylate (SAT, a PC analogue) on the ATP-induced CPPD crystal formation in both ACV and articular cartilage models. Only PC (10-1000 microM) blocked both the ATP-dependent and -independent mineralization in ACV in a dose-dependent fashion. At 1 mM, SAT and citrate blocked the ATP-independent mineralization. Similarly, only PC blocked both the ATP- and non-ATP-dependent mineralization in native articular cartilage slices. PC, SAT, and citrate had no effect on ACV nucleoside triphosphate pyrophosphohydrolase activity, suggesting that none of these agents blocked mineralization through the inhibition of nucleoside triphosphate pyrophosphohydrolase activity, which generates inorganic pyrophosphate from ATP.

Liposome and multiple emulsion formulations augment the anticalcifying efficacy of phosphocitrate in a cutaneous calcergy model

The anticalcifying agent phosphocitrate was incorporated into phosphatidylcholine/cholesterol liposomes by reverse-phase evaporation. The compound was entrapped to the extent of 11.6% (mol mol-1 of lipid) and the liposomes exhibited prolonged retention of the compound when incubated with rat plasma. Phosphocitrate's ionic contribution in solution adversely influenced the encapsulation efficiency but improvements were made through ion-pairing with the quaternary ammonium detergent centrimide, or with the inclusion of stearylamine in the lipid phase. The liposomal dose that could be practically administered in-vivo was restricted to 2.5 mg phosphocitrate kg-1 day-1. The formulation of a multiple emulsion preparation of phosphocitrate, however, offered an alternative delivery mode permitting infrequent dosing to be successfully investigated. In a rat calcergy model, both vehicles effectively reduced the formation of induced subcutaneous calcified plaques at doses for which the phosphocitrate salt alone was inactive. The current formulations demonstrate that the therapeutic efficacy of phosphocitrate can be markedly improved through an appropriately designed drug delivery system, signalling a new approach for the future therapeutic application of this compound.

High-performance liquid chromatographic and mass spectrometric identification of phosphocitrate synthesized by human kidney homogenate

Two high-performance liquid chromatographic techniques for the analysis of phosphocitrate, a metabolite synthesized by the cell that is thought to play a role in a number of physiological as well as pathological events, have been developed. The mass spectra of the isolated compound obtained under fast-atom bombardment and collisionally activated decomposition mass-analysed ion kinetic energy conditions are also reported.

Inhibition of basic calcium phosphate crystal-induced mitogenesis by phosphocitrate

Basic calcium phosphate crystals control the traverse of cells from the Go/G1 to S-phase of the cell cycle and initiate proliferation by rendering fibroblasts competent to respond to insulin-like growth factors in plasma. Simultaneous addition of phosphocitrate [a powerful inhibitor of hydroxyapatite crystallization] to cells exposed to basic calcium phosphate crystals caused a dose-dependent inhibition of crystal-induced DNA synthesis and c-fos transcription. This inhibition was specific for crystal-induced mitogenesis, since similar concentrations of phosphocitrate had no effects on either PDGF or 10% calf serum-induced thymidine incorporation and c-fos transcription.

Evidence in vitro for an enzymatic synthesis of phosphocitrate

A biological synthesis of phosphocitrate is described from precursors, citrate and adenosinetriphosphate reacting in the presence of rat liver homogenate. Identity of the newly formed product was examined by enzymatic digestion of reactions mixtures, HPLC chromatography and 1H-NMR spectra. Authenticity of product was established by comparison to chemically synthesized phosphocitrate. Recognition of the existence of a biologically synthetic pathway adds credence to the known presence of phosphocitrate in mitochondria and a postulated role to control calcium phosphate deposition in that organelle.

Protection of crystal-induced polymorphonuclear leukocyte membranolysis by phosphocitrate

The protection afforded by phosphocitrate, a phosphorylated polycarboxylic acid, against crystal-induced membrane damage to polymorphonuclear leukocytes was studied in vitro. Membranolysis was assessed by nitro blue tetrazolium salt reduction, lactate dehydrogenase release, and scanning electron microscopy. Phosphocitrate protected strongly against hydroxyapatite crystal-induced damage, an action attributable to crystal surface binding of phosphocitrate rather than to the membrane. The ability of phosphocitrate to prevent hydroxyapatite crystallization, together with its membrane protective effect against preformed crystals, would suggest that the compound might have a useful future role against crystal-induced arthropathies.