Lipid oxidation products have opposite effects on calcifying vascular cell and bone cell differentiation. A possible explanation for the paradox of arterial calcification in osteoporotic patients

Smooth muscle cell phenotypes in atherosclerotic lesions

Recently, there has been a dramatic change in the way we think about the role of vascular smooth muscle cells in atherosclerosis, and it is now generally accepted that a dearth of vascular smooth muscle cells in an atherosclerotic plaque is a detrimental feature of the disease. Indeed, it is now recognized that the phenotypes of vascular smooth muscle cells within a plaque dictate its features, progression and stability. Therefore an understanding of the processes that generate and regulate vascular smooth muscle cell heterogeneity are of critical importance for future therapeutic advancement in the treatment of atherosclerosis.

Inhibition of osteoblast-specific transcription factor Cbfa1 by the cAMP pathway in osteoblastic cells. Ubiquitin/proteasome-dependent regulation

The cAMP pathway, a major intracellular pathway mediating parathyroid hormone signal, regulates osteoblastic function. Parathyroid hormone (through activation of protein kinase A) has also been shown to stimulate ubiquitin/proteasome activity in osteoblasts. Since the osteoblast-specific transcription factor Osf2/Cbfa1 is important for differentiation of osteoblastic cells, we examined the roles of the cAMP and ubiquitin/proteasome pathways in regulation of Cbfa1. In the osteoblastic cell line, MC3T3-E1, continuous treatment with cAMP elevating agents inhibited both osteoblastic differentiation based on alkaline phosphatase assay and DNA binding ability of Cbfa1 based on a gel retardation assay. Cbfa1 inhibition was paralleled by an inhibitory effect of forskolin on Cbfa1-regulated genes. Northern and Western blot analyses suggested that the inhibition of Cbfa1 by forskolin was mainly at the protein level. Pretreatment with proteasome inhibitors prior to forskolin treatment reversed the effect of forskolin. Furthermore, addition of proteasome inhibitors to forskolin-pretreated samples resulted in recovery of Cbfa1 protein levels and accumulation of polyubiquitinated forms of Cbfa1, indicating a role for the proteasome pathway in the degradation of Cbfa1. These results suggest that suppression of osteoblastic function by the cAMP pathway is through proteolytic degradation of Cbfa1 involving a ubiquitin/proteasome-dependent mechanism.

Advanced glycation endproducts accelerate calcification in microvascular pericytes

Vascular calcification in advanced atherosclerosis is frequently associated with diabetes, and is a predictor of future cardiovascular events. To investigate the molecular mechanisms of vascular calcification, we examined whether advanced glycation endproducts (AGE) formed at an accelerated rate under diabetes induce the osteoblastic differentiation of pericytes, a mesenchymal progenitor. First, von Kossa staining demonstrated that AGE significantly increased the number of calcified nodules in a bovine pericyte culture. AGE were also found to induce calcium accumulation in the pericyte monolayer in time- and dose-dependent manners. Second, quantitative reverse transcription-polymerase chain reaction revealed that AGE increased the pericyte levels of mRNAs coding for alkaline phosphatase and osteopontin, the representative markers for early and late osteoblastic differentiation, respectively. Alkaline phosphatase activity was actually enhanced by AGE. The results suggest that AGE have the ability to induce the osteoblatic differentiation of pericytes, which would contribute to the development of vascular calcification in diabetes.

Oxidized phospholipids and isoprostanes in atherosclerosis

Lipid oxidation has been shown to be a prominent feature of atherosclerosis. The presence of isoprostanes and other lipid oxidation products has been clearly demonstrated in lesions. Furthermore, antibodies to oxidized phospholipids have been identified in animal and human models. Recent evidence suggests that oxidized phospholipids may activate all vascular cell types. The identification and isolation of particular oxidative products has provided some important information about the receptors for these active lipids.

osteoprotegerin-deficient mice develop early onset osteoporosis and arterial calcification

Osteoprotegerin (OPG) is a secreted protein that inhibits osteoclast formation. In this study the physiological role of OPG is investigated by generating OPG-deficient mice. Adolescent and adult OPG-/- mice exhibit a decrease in total bone density characterized by severe trabecular and cortical bone porosity, marked thinning of the parietal bones of the skull, and a high incidence of fractures. These findings demonstrate that OPG is a critical regulator of postnatal bone mass. Unexpectedly, OPG-deficient mice also exhibit medial calcification of the aorta and renal arteries, suggesting that regulation of OPG, its signaling pathway, or its ligand(s) may play a role in the long observed association between osteoporosis and vascular calcification.

cAMP stimulates osteoblast-like differentiation of calcifying vascular cells. Potential signaling pathway for vascular calcification

The role of the cAMP signaling pathway in vascular calcification was investigated using calcifying vascular cells (CVC) derived from primary aortic medial cell cultures. We previously showed that CVC have fibroblastic morphology and express several osteoblastic differentiation markers. After confluency, they aggregate into cellular condensations, which later mature into nodules where mineralization is localized. Here, we investigated the effects of cAMP on CVC differentiation because it plays a role in both osteoblastic differentiation and vascular disease. Dibutyryl-cAMP or forskolin treatment of CVC for 3 days induced osteoblast-like "cuboidal" morphology, inhibited proliferation, and enhanced alkaline phosphatase activity, all early markers of osteoblastic differentiation. Isobutylmethylxanthine and cholera toxin had the same effects. Treatment of CVC with pertussis toxin, however, did not induce the morphological change or increase alkaline phosphatase activity, although it inhibited CVC proliferation to a similar extent. cAMP also increased type I procollagen production and gene expression of matrix gamma-carboxyglutamic acid protein, recently shown to play a role in in vivo vascular calcification. cAMP inhibited the expression of osteopontin but did not affect the expression of osteocalcin and core binding factor. Prolonged cAMP treatment enhanced matrix calcium-mineral incorporation but inhibited the condensations resulting in diffuse mineralization throughout the monolayer of cells. Treatment of CVC with a protein kinase A-specific inhibitor, KT5720, inhibited alkaline phosphatase activity and mineralization during spontaneous CVC differentiation. These results suggest that the cAMP pathway promotes in vitro vascular calcification by enhancing osteoblast-like differentiation of CVC.