Kamlesh Asotra

Calcification in atherosclerosis: Bone biology and chronic inflammation at the arterial crossroads

Dystrophic or ectopic mineral deposition occurs in many pathologic conditions, including atherosclerosis. Calcium mineral deposits that frequently accompany atherosclerosis are readily quantifiable radiographically, serve as a surrogate marker for the disease, and predict a higher risk of myocardial infarction and death. Accelerating research interest has been propelled by a clear need to understand how plaque structure, composition, and stability lead to devastating cardiovascular events. In atherosclerotic plaque, accumulating evidence is consistent with the notion that calcification involves the participation of arterial osteoblasts and osteoclasts. Here we summarize current models of intimal arterial plaque calcification and highlight intriguing questions that require further investigation. Because atherosclerosis is a chronic vascular inflammation, we propose that arterial plaque calcification is best conceptualized as a convergence of bone biology with vascular inflammatory pathobiology.

Increased Expression of Membrane Type 3-Matrix Metalloproteinase in Human Atherosclerotic Plaque Role of Activated Macrophages and Inflammatory Cytokines

Background—Matrix metalloproteinases (MMPs) are thought to play a prominent role in atherogenesis and destabilization of plaque. Pericellularly localized membrane-type (MT)-MMPs activate secreted MMPs. We investigated the hypothesis that MT3-MMP is expressed in human atherosclerotic plaques and is regulated by locally produced inflammatory cytokines and oxidized low-density lipoprotein (Ox-LDL). Methods and Results—Expression and cellular localization of MT3-MMP in normal and atherosclerotic human coronary arteries were examined using specific antibodies. Abundant MT3-MMP expression was noted in medial smooth muscle cells (SMCs) of normal arteries. In atherosclerotic arteries, MT3-MMP expression was observed within complex plaques and colocalized with SMCs and macrophages (M&phgr;). Cultured human monocyte-derived M&phgr; constitutively expressed MT3-MMP mRNA and proteolytically active protein, as demonstrated by mRNA analyses, immunoblotting, and gelatin zymography, respectively. Ox-LDL, tumor necrosis factor-&agr;, or macrophage colony-stimulating factor caused dose- and time-dependent increases in steady-state levels of MT3-MMP mRNA in cultured M&phgr;. This correlated with a 2- to 4-fold increase in levels of MT3-MMP immunoreactive protein and enzymatic activity in M&phgr; membranes. Confocal microscopy and flow cytometry confirmed induction and spatial distribution of MT3-MMP protein from intracellular domains to the M&phgr; plasma membrane by Ox-LDL, tumor necrosis factor-&agr;, or macrophage colony-stimulating factor. Conclusions—MT3-MMP is expressed by SMCs and M&phgr; in human atherosclerotic plaques. Proinflammatory molecules cause a progressive increase in the expression of MT3-MMP in cultured M&phgr;. Our results suggest a mechanism by which inflammatory molecules could promote M&phgr;-mediated degradation of extracellular matrix and thereby contribute to plaque destabilization.

Rationale for the role of osteoclast-like cells in arterial calcification

Atherosclerotic arteries frequently become calcified, and these calcium deposits are associated with a high risk of adverse clinical events. Descriptive studies suggest calcification is an organized and regulated process with many similarities to osteogenesis, yet the mechanism and its relationship to atherosclerosis remain largely unknown. In bone development and homeostasis, mineral deposition by osteoblasts and mineral resorption by osteoclasts are delicately balanced such that there is no overall gain or loss in bone mass. We hypothesize that there exists in arteries a mechanism that similarly balances mineral deposition with resorption. We propose that the cellular mediators of arterial mineral resorption are osteoclast‐like cells (OLCs) derived from hematopoietic precursors of the mononuclear phagocytic lineage. In arterial microenvironments, mononuclear precursors are induced to differentiate toward OLCs by macrophage‐colony stimulating factor and receptor activator of NF‐κB ligand, both of which are necessary and sufficient for osteoclastogenesis and mineral resorption in bone. OLCs may participate in normal mineral homeostasis within the arterial wall or, alternatively, may be recruited to specific sites within developing plaque. Net calcium deposition occurs as a result of focal perturbation of the balance between the activity of osteoblast‐like cells and OLCs. Our proposed mechanism thus views arterial mineral deposition not so much as an active pathological process, but as a localized failure of protective mechanisms that actively oppose mineral deposition within the disordered metabolic milieu of developing atherosclerotic plaque.—Doherty, T. M., Uzui, H., Fitzpatrick, L. A., Tripathi, P. V., Dunstan, C. R., Asotra, K., Rajavashisth, T. B. Rationale for the role of osteoclast‐like cells in arterial calcification. FASEB J. 16, 577–582 (2002)

DNA repair gene O6-methylguanine-DNA methyltransferase: Promoter hypermethylation associated with decreased expression and G:C to A:T mutations of p53 in brain tumors

The DNA repair enzyme O6‐methylguanine‐DNA methyltransferase (MGMT) removes alkylating adducts from the O6 position of guanine and protects cells from cytotoxic and mutagenic effects. Expression of MGMT is decreased in some cancers, which may be the result of methylation of CpG islands of both the promoter and coding regions of the gene. We studied the methylation status of the MGMT promoter in a very large collection of brain tumors (85) using methylation‐specific polymerase chain reaction (PCR). Aberrant methylation occurred in 48% of 85 human brain tumor samples. Quantitative real‐time PCR showed that expression of MGMT mRNA levels was significantly decreased (P < 0.001) in those brain tumors that had methylation of the promoter region of their MGMT gene. MGMT can prevent G to A mutations by removing alkyl groups from the O6 position of guanine. We found a significantly increased frequency of G:C to A:T mutations of the p53 gene in brain tumors having a methylated MGMT promoter compared with those having an unmethylated MGMT promoter (P < 0.05), and all the non–CpG dinucleotide G:C to A:T mutations of p53 were in samples with a methylated MGMT promoter.

Therapeutic developments in matrix metalloproteinase inhibition

A family of zinc-dependent endopeptidases called matrix metalloproteinases (MMPs) have the capacity to degrade all elements of the extracellular matrix (ECM) and are required for homeostatic maintenance of the ECM. However, interest in MMPs predominantly arises from the accumulating evidence implicating that dysregulated MMP expression plays a role in mediating or accompanying a diverse array of pathologies. These include tumour invasion and metastasis and inflammatory diseases characterised by excessive tissue destruction, such as arthritis, periodontal disease, atherosclerosis, plaque rupture, arterial aneurysms, postmyocardial infarction, ventricular remodelling and cardiac rupture. Several patents representing therapeutic drugs and strategies to treat the associated conditions have been claimed, some resulting in clinical drug trials. This review will: i) summarise the current status of our understanding of MMPs and how they participate in normal and functional ECM degradation; ii) review therapeutic efforts to favourably alter the balance between MMP proteolysis and ECM sythesis; and iii) critically evaluate recent studies that have importantly advanced our understanding of the complexities of MMP function and propose areas where future efforts to develop therapeutic strategies might be most beneficial and productive.

Correlation between bradykinin-induced blood-tumor barrier permeability and B2 receptor expression in experimental brain tumors.

Abstract Localization of B2 receptors in brain tumor cells and microvessel endothelial cells of the brain tumors was investigated to study the differential sensitivity of brain tumors to bradykinin. The present study shows that B2 receptor expression levels vary in cultured RG2, C6 and 9L glioma cells as well as in the intracerebral tumors established with these cell lines in rats. The double immunohistochemical data indicate that B2 receptors are localized to tumor cells and not to the tumor capillaries. Immunostaining and Western blot analysis for B2 receptor showed that the B2 receptor expression was in the order C6>RG2>9L. The permeability studies on RG2, C6 and 9L tumors in rats showed that C6 tumor had the highest increase (178%) in Ki (unidirectional transport across blood–brain barrier (BBB)/blood–tumor barrier (BTB)), while 9L tumor had the least increase of Ki (35%) over the control group, following intracarotid infusion of bradykinin. We found a positive correlation (r = 0.965, p<0.001) between B2 receptor levels and bradykinin-induced increase in BTB permeability. We conclude that B2 receptors are localized to tumor cells and not to normal or tumor capillary endothelial cells. C6 tumor with highest B2 receptor expression was most responsive to bradykinin, while RG2 and 9L tumors with lower B2 receptor expression level were less sensitive to bradykinin with regard to BTB permeability.

Modified immunoregulation associated with interferon-γ treatment of rat glioma

Abstract Little is known about modulation by cytokines of major histocompatibility complex (MHC) antigen expression on intracranial tumors in vivo. The ability of cytokines to up-regulate MHC class-1 (MHC-1) antigen expression was investigated first in vitro using three rat glioma cell lines. Immunohistochemistry showed that incubation with recombinant rat interferonγ (rrIFNγ) increased MHC-1 antigen expression in RG2, C6, and 9L cell lines. Flow cytometric analysis revealed different baseline levels of MHC-1 antigen expression in each line (RG2 lowest, C6 highest), and that these levels increased in all lines after stimulation with 100 U ml–1 or more of rrlFNγ. The antitumor effect of rrIFNγ in vivo was evaluated by assessing survival of rats with implanted intracerebralRG2 gliomas after intracarotid infusion of rrIFNγ. A high dose of rrIFNγ (2.4×105 U kg–1) significantly increased the survival, compared to control (p<0.02). Intracarotid pre-treatment with the bradykinin analogue RMP-7 did not further increase survival. Immunohistochemical staining of tumor sections after in vivo rrIFNγ infusion showed no clear increase in MHC-1 antigen expression on tumor cells but increased staining for ED2 antigen within tumor tissue, presumably from perivascular cells with MHC class-2 antigen.