No abstract

Calcium phosphates are the main minerals in human bone, enamel, atherosclerosis, and dental calculus. Amorphous precursors may play a key role in biomineralization. We studied the formation and transformation of calcium phosphate particles of amorphous phase by stopped-flow spectrophotometry, simultaneous measurements of particle size and solution pH, and high-resolution transmission electron microscopy. Ion pairs and clusters formed in the first few seconds. They then constituted initial amorphous phase containing protonated phosphates and hydrated calcium ions, which was different from that containing Ca 9 (PO 4 ) 6 . Crystalline domains developed at multiple sites inside the primary particles of the amorphous phase. With the consuming of interdomain constituents, these particles partially collapsed, liberating crystallites and inducing rapid precipitation. This study sheds new light on the understanding of crystallization in amorphous phase, as well as the induction period in precipitation kinetics.

Amorphous calcium phosphate often forms as a precursor phase in a solution at sufficiently high supersaturation and pH, and then transforms to the thermodynamically stable hydroxyapatite. The chemical composition, structure, and property of the amorphous phase are dependent on the structure of its composing clusters. Based on the results from the measurements of in situ Ca K-edge X-ray near-edge structure and ex situ X-ray diffraction, as well as the concomitant pH change in the reaction process, here we propose an improved model for the structure of “Posner’s cluster” and identify the three types of reactions that lead to the formation of amorphous calcium phosphate and its subsequent transition to crystalline hydroxyapatite.

Calcium phosphate clusters are present in both aqueous solutions and an amorphous phase during crystallization. It is a challenging task to acquire the structural information on such clusters, owing to their small size, chemical lability, and inaccessibility to most detection techniques. Here, we demonstrate the feasibility of detecting such clusters in situ by synchrotron X-ray absorption near-edge spectroscopy at calcium K-edge, a technique that is sensitive to the short-range order in calcium coordination sphere. At the initial stage of crystallization, the most abundant clusters are detected to be Ca(η) in solution. More reactive clusters engage in the development of an amorphous phase via growing and fusing. The amorphous phase exhibits a dual character in its short-range order: Some of its clusters are similar to hydrated calcium ions, and some others to those in crystalline hydroxyapatite. When the amorphous dissolves, the detected unit is mainly Ca(η 2 -PO 4 3− )(H 2 O) 4 in the released solution clusters. While these findings provide a basis for a better understanding and rational control of calcium phosphate crystallization at molecular level, the experimental technique in assessing wet samples adopted in this work might be applicable to the crystallization studies of other materials as well.

The process of vascular calcification presents several features similar to osteogenesis in which fibronectin (FN) acts as a regulator of osteoblastic differentiation and the ERK signal pathway is involved. In order to find whether FN promotes the osteoblastic differentiation of vascular smooth muscle cells (VSMCs) through the ERK signal pathway, we investigated the effect of FN on the calcification of VSMCs by using an in vitro cell model. VSMCs cultured in plates with FN (0-20 microg/cm2) coating were induced to calcify by 10 mM sodium beta-glycerophosphate (beta-GP). FN exacerbated VSMC calcification in a dose- and time-dependent manner, as indicated by the number of calcifying nodules per slide and the amount of calcium in the deposition. Data from RT-PCR and immunoblotting assay revealed that FN also enhanced the expression of several phenotypic markers of osteoblasts, including alkaline phosphatase (ALP) activity, osteocalcin (OC), and Osf2/Cbfa1, a key transcription factor in osteoblastic differentiation. Furthermore, a specific inhibitor for ERK, PD98059 (10 microM), significantly suppressed the effect of FN on calcification and phenotypic marker expression. These findings seem to suggest that FN enhanced vascular calcification by promoting the osteoblastic differentiation of VSMCs via ERK signal pathway.

Converging lines of evidence suggest that lanthanum tends to deposit in bone. The influence of lanthanum ion (La3+) on osteoblast differentiation and the related mechanism are essential to understanding its effect on bone metabolism. In this study, La3+ treatment enhanced in vitro osteoblast differentiation as evidenced by promoting alkaline phosphatase (ALP) activity, osteocalcin (OC) secretion, and matrix mineralization. The expressions of osteoblast-specific genes of Cbfa-1, osteopontin (OPN), and bone sialoprotein (BSP) were all increased in the presence of La3+, but no change was observed in that of type I collagen (COL-I). Further studies demonstrated that La3+ treatment enhanced phosphorylation of extracellular signal-regulated kinase (ERK). Inhibition of ERK activation by U0126 suppressed the effects of La3+ on osteoblast activity. Moreover, pretreatment of the cells with pertussis toxin (PTx), a Gi protein inhibitor, suppressed the La3+-enhanced ERK phosphorylation and osteoblast differentiation. These findings suggest that La3+ exposure enhances in vitro osteoblast differentiation and the effect depends on ERK phosphorylation via PTx-sensitive Gi protein signaling.

A smart mesoporous silica nanocarrier with intracellular controlled release is fabricated, with folic acid as dual-functional targeting and capping agent. The folate not only improves the efficiency of the nanocarrier internalized by the cancer cells, but also blocks the pores of the mesoporous silica to eliminate premature leakage of the drug. With disulfide bonds as linkers to attach the dual-functional folate within the surface of mesoporous silica, the controlled release can be triggered in the presence of reductant dithiothreitol (DTT) or glutathione (GSH). The cellular internalization via folate-receptor-mediated endocytosis and the intracellular controlled release of highly toxic anticancer drug DOX were demonstrated with an in vitro HeLa cell culture, indicating an efficient cancer-targeted drug delivery.

Rhein, an active ingredient extensively found in plants such as Aloe, Cassitora L., rhubarb and so on, has been used for a long time in China. Pharmacological tests revealed that rhein not only had a strong antibacterial action, but also may be useful in cancer chemotherapy as a biochemical modulator. Its therapeutic action and toxicity is still the subject of considerable research. With microsome incubation assays in vitro and HPLC methods, the inhibition of rat liver CYP1A2, CYP2C9, CYP2D6, CYP2E1 and CYP3A enzymes by rhein were studied kinetically. The results showed the most inhibition of CYP2E1 by rhein (K(i) = 10 microm, mixed); CYP3A and CYP2C9 were also inhibited by rhein, K(i) = 30 microm (mixed) and K(i) = 38 microm (mixed), respectively; rhein revealed some inhibition of CYP1A2 (K(i) = 62 microm, uncompetitive) and CYP2D6 (K(i) = 74 microm, mixed). Drug-drug interactions, especially cytochrome P450 (CYP)-mediated interactions, cause an enhancement or attenuation in the efficacy of co-administered drugs. Inhibition of the five major CYP enzymes observed for rhein suggested that changes in pharmacokinetics of co-administered drugs were likely to occur. Therefore, caution should be paid to the possible drug interaction of medicinal plants containing rhein and CYP substrates.