Abstract 1474: IDO1 signaling supports inflammatory neovascularization

We have identified a distinct role for the immunoregulatory, tryptophan-catabolizing enzyme IDO1 (indoleamine 2,3-dioxygenase) in supporting neovascularization through its positioning as a key regulatory node between the inflammatory cytokines IFNγ (interferon γ) and IL6 (interleukin 6). IFNγ is a primary inducer of IDO1, but is also a key mediator of immune-based tumor suppression, which some studies have associated with its anti-angiogenic activity. Conversely, genetic studies in mice have clearly established a tumor-promoting role for IDO1, suggesting that it may interact with IFNγ in a negative feedback capacity. Targeted disruption of the Ido1 gene in mice resulted in enhanced resistance to lung tumor and metastasis development. This corresponded with attenuated induction of the pro-angiogenic cytokine IL6, which, when provided through ectopic expression, was able to restore pulmonary metastasis susceptibility to Ido1-/- mice. These initial findings suggested that IDO1 might contribute to cancer promotion by countering the anti-neovascular effect of IFNγ, possibly through IDO1-potentiated elevation of IL6. Consistent with this hypothesis, mice lacking either IDO1 or IL6 exhibited pulmonary metastasis resistance that was dependent on IFNγ and coupled with reduced tumor neovascularization. Neovascularization in a mouse oxygen-induced retinopathy model was likewise affected by IDO1 or IL6 loss, separating this process from the contextual complexity of the tumor microenvironment. Investigation into the cellular and molecular basis for the observed biology revealed that, within the heterogeneous expanse of Gr1+ MDSCs (myeloid-derived suppressor cells), the ability to independently elicit neovascularization in vivo was restricted to a highly autofluorescent population of CD11blo cells. This included a discrete, IDO1-expressing subpopulation, designated IDVCs (IDO1-dependent vascularizing cells), that, in an IDO1-dependent fashion, dominantly determined whether neovascularization was sustained. Mechanistically, the induction of IDO1 in IDVCs was found to provide a negative feedback constraint on the anti-angiogenic effect of host IFNγ through GCN2 (general control nonderepressible 2)-mediated activation of the integrated stress response and potentiation of IL6 production within these cells. These findings reveal fundamental molecular and cellular insights into how IDO1 interfaces with the inflammatory milieu to support neovascularization.

Citation Format: Souvik Dey, Arpita Mondal, James B. DuHadaway, Erika Sutanto-Ward, Lisa Laury-Kleintop, Sunil Thomas, George C. Prendergast, Laura Mandik-Nayak, Alexander J. Muller. IDO1 signaling supports inflammatory neovascularization [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1474.

Reduction of the expression of the late-onset Alzheimer's disease (AD) risk-factor BIN1 does not affect amyloid pathology in an AD mouse model

Alzheimer's disease (AD) is pathologically characterized by the deposition of the β-amyloid (Aβ) peptide in senile plaques in the brain, leading to neuronal dysfunction and eventual decline in cognitive function. Genome-wide association studies have identified the bridging integrator 1 (BIN1) gene within the second most significant susceptibility locus for late-onset AD. BIN1 is a member of the amphiphysin family of proteins and has reported roles in the generation of membrane curvature and endocytosis. Endocytic dysfunction is a pathological feature of AD, and endocytosis of the amyloid precursor protein is an important step in its subsequent cleavage by β-secretase (BACE1). In vitro evidence implicates BIN1 in endosomal sorting of BACE1 and Aβ generation in neurons, but a role for BIN1 in this process in vivo is yet to be described. Here, using biochemical and immunohistochemistry analyses we report that a 50% global reduction of BIN1 protein levels resulting from a single Bin1 allele deletion in mice does not change BACE1 levels or localization in vivo, nor does this reduction alter the production of endogenous murine Aβ in nontransgenic mice. Furthermore, we found that reduction of BIN1 levels in the 5XFAD mouse model of amyloidosis does not alter Aβ deposition nor behavioral deficits associated with cerebral amyloid burden. Finally, a conditional BIN1 knockout in excitatory neurons did not alter BACE1, APP, C-terminal fragments derived from BACE1 cleavage of APP, or endogenous Aβ levels. These results indicate that BIN1 function does not regulate Aβ generation in vivo.

Stromal cyclin D1 promotes heterotypic immune signaling and breast cancer growth

The cyclin D1 gene encodes the regulatory subunit of a holoenzyme that drives cell autonomous cell cycle progression and proliferation. Herein we show cyclin D1 abundance is increased >30-fold in the stromal fibroblasts of patients with invasive breast cancer, associated with poor outcome. Cyclin D1 transformed hTERT human fibroblast to a cancer-associated fibroblast phenotype. Stromal fibroblast expression of cyclin D1 (cyclin D1^Stroma) in vivo, enhanced breast epithelial cancer tumor growth, restrained apoptosis, and increased autophagy. Cyclin D1^Stroma had profound effects on the breast tumor microenvironment increasing the recruitment of F4/80⁺ and CD11b⁺ macrophages and increasing angiogenesis. Cyclin D1^Stroma induced secretion of factors that promoted expansion of stem cells (breast stem-like cells, embryonic stem cells and bone marrow derived stem cells). Cyclin D1^Stroma resulted in increased secretion of proinflammatory cytokines (CCL2, CCL7, CCL11, CXCL1, CXCL5, CXCL9, CXCL12), CSF (CSF1, GM-CSF1) and osteopontin (OPN) (30-fold). OPN was induced by cyclin D1 in fibroblasts, breast epithelial cells and in the murine transgenic mammary gland and OPN was sufficient to induce stem cell expansion. These results demonstrate that cyclin D1^Stroma drives tumor microenvironment heterocellular signaling, promoting several key hallmarks of cancer.

Specific in situ detection of murine indoleamine 2, 3-dioxygenase

Indoleamine 2,3-dioxygenase-1 (IDO1) catabolizes the essential amino acid tryptophan, acting as a modifier of inflammation and immune tolerance. Recent work has implicated IDO1 in many human diseases, including in cancer, chronic infection, autoimmune disorders, and neurodegenerative disease, stimulating a major surge in preclinical and clinical studies of its pathogenic functions. In the mouse, IDO1 is expressed widely but in situ detection of the enzyme in murine tissues has been unreliable due to the lack of specific antibodies that do not also react with tissues from animals that are genetically deficient in IDO1. Such probes are crucial to establish cellular mechanisms since IDO1 appears to act in different cell types depending on disease context, but reliable probes have been elusive in the field. In this report, we address this issue with the development of IDO1 monoclonal antibody 4B7 which specifically recognizes the murine enzyme in tissue sections, offering a reliable tool for immunohistology in preclinical disease models.

Targeting RhoB as a novel approach for the treatment of Rheumatoid Arthritis (THER6P.856)

Rheumatoid arthritis (RA) is an autoimmune disease characterized by high titers of autoantibodies and increased levels of proinflammatory cytokines leading to chronic inflammation of the distal joints. Current biologic therapies, such as the B cell depleting antibody rituximab, have uncovered the important role that autoreactive B cells play in the disease process. Unfortunately, neither immunosuppressive nor biologic therapies specifically target autoreactive B cells. Here, we present data identifying a novel molecular target and newly developed therapeutic that may specifically target these cells. The small GTPase RhoB is a stress response protein that mediates tyrosine kinase receptor stimulated proliferation and migration. While much of the work on RhoB has focused on its function in cancer, we suggest a new role for this GTPase in autoimmune disease. Using the K/BxN model of RA, we show that treatment with an anti-RhoB antibody lowers autoantibody production and attenuates joint swelling. Mice genetically deficient in RhoB exhibit reduced arthritis, providing genetic confirmation of RhoB’s role in mediating autoimmunity. Interestingly, both RhoB deficient mice and mice treated with anti-RhoB Ig mounted productive antibody responses to immunization with model antigens, suggesting that RhoB may have a specific role in the autoreactive B cell. Our data suggest that RhoB antibody therapy could be a new effective therapeutic to treat antibody-mediated autoimmune diseases.

Indoleamine 2,3-dioxygenase pathways of pathogenic inflammation and immune escape in cancer

Genetic and pharmacological studies of indoleamine 2,3-dioxygenase (IDO) have established this tryptophan catabolic enzyme as a central driver of malignant development and progression. IDO acts in tumor, stromal and immune cells to support pathogenic inflammatory processes that engender immune tolerance to tumor antigens. The multifaceted effects of IDO activation in cancer include the suppression of T and NK cells, the generation and activation of T regulatory cells and myeloid-derived suppressor cells, and the promotion of tumor angiogenesis. Mechanistic investigations have defined the aryl hydrocarbon receptor, the master metabolic regulator mTORC1 and the stress kinase Gcn2 as key effector signaling elements for IDO, which also exerts a non-catalytic role in TGF-β signaling. Small-molecule inhibitors of IDO exhibit anticancer activity and cooperate with immunotherapy, radiotherapy or chemotherapy to trigger rapid regression of aggressive tumors otherwise resistant to treatment. Notably, the dramatic antitumor activity of certain targeted therapeutics such as imatinib (Gleevec) in gastrointestinal stromal tumors has been traced in part to IDO downregulation. Further, antitumor responses to immune checkpoint inhibitors can be heightened safely by a clinical lead inhibitor of the IDO pathway that relieves IDO-mediated suppression of mTORC1 in T cells. In this personal perspective on IDO as a nodal mediator of pathogenic inflammation and immune escape in cancer, we provide a conceptual foundation for the clinical development of IDO inhibitors as a novel class of immunomodulators with broad application in the treatment of advanced human cancer.

Meglumine exerts protective effects against features of metabolic syndrome and type II diabetes

Metabolic syndrome, diabetes and diabetes complications pose a growing medical challenge worldwide, accentuating the need of safe and effective strategies for their clinical management. Here we present preclinical evidence that the sorbitol derivative meglumine (N-methyl-D-glucamine) can safely protect against several features of metabolic syndrome and diabetes, as well as elicit enhancement in muscle stamina. Meglumine is a compound routinely used as an approved excipient to improve drug absorption that has not been ascribed any direct biological effects in vivo. Normal mice (SV129) administered 18 mM meglumine orally for six weeks did not display any gastrointestinal or other observable adverse effects, but had a marked effect on enhancing muscle stamina and at longer times in limiting weight gain. In the established KK.Cg-Ay/J model of non-insulin dependent diabetes, oral administration of meglumine significantly improved glycemic control and significantly lowered levels of plasma and liver triglycerides. Compared to untreated control animals, meglumine reduced apparent diabetic nephropathy. Sorbitol can improve blood glucose uptake by liver and muscle in a manner associated with upregulation of the AMPK-related enzyme SNARK, but with undesirable gastrointestinal side effects not seen with meglumine. In murine myoblasts, we found that meglumine increased steady-state SNARK levels in a dose-dependent manner more potently than sorbitol. Taken together, these findings provide support for the clinical evaluation of meglumine as a low-cost, safe supplement offering the potential to improve muscle function, limit metabolic syndrome and reduce diabetic complications.

The SOCS3-independent expression of IDO2 supports the homeostatic generation of T regulatory cells by human dendritic cells

Dendritic cells (DCs) are professional APCs that have a role in the initiation of adaptive immune responses and tolerance. Among the tolerogenic mechanisms, the expression of the enzyme IDO1 represents an effective tool to generate T regulatory cells. In humans, different DC subsets express IDO1, but less is known about the IDO1-related enzyme IDO2. In this study, we found a different pattern of expression and regulation between IDO1 and IDO2 in human circulating DCs. At the protein level, IDO1 is expressed only in circulating myeloid DCs (mDCs) and is modulated by PGE2, whereas IDO2 is expressed in both mDCs and plasmacytoid DCs and is not modulated by PGE2. In healthy subjects, IDO1 expression requires the presence of PGE2 and needs continuous transcription and translation, whereas IDO2 expression is constitutive, independent from suppressor of cytokine signaling 3 activity. Conversely, in patients suffering from inflammatory arthritis, circulating DCs express both IDO1 and IDO2. At the functional level, both mDCs and plasmacytoid DCs generate T regulatory cells through an IDO1/IDO2-dependent mechanism. We conclude that, in humans, whereas IDO1 provides an additional mechanism of tolerance induced by proinflammatory mediators, IDO2 is stably expressed in steady-state conditions and may contribute to the homeostatic tolerogenic capacity of DCs.

Abstract 295: IDO drives tumor-promoting, pathogenic inflammation in lung

First identified as a mediator of acquired immune tolerance of the ‘foreign’ fetus from maternal immunity, the tryptophan-catabolizing enzyme IDO (indoleamine 2,3-dioxygenase) has since been implicated in tumor escape from the host immune system. Insight into the intricate role of IDO in the classical DMBA/TPA skin carcinogenesis model suggested that inflammatory tumor environments can induce IDO production resulting in de novo tumor development. In the genetically deficient model of IDO, mice show resistance to tumor formation. This provided a basis for our current studies exploring the importance of IDO in the microenvironment of the lung. To this end, we have investigated both primary tumor formation and metastatic disease in the lungs of IDO-deficient mice using the KRAS-induced lung adenocarcinoma and the metastatic 4T1 breast cancer models. Elevation of the inflammatory cytokine IL6 was associated with tumor outgrowth in the lungs in both models but was greatly attenuated with the loss of IDO, consistent with the in vitro demonstration that IDO activity markedly potentiates IL6 production. MDSCs (myeloid derived suppressor cells) exhibited reduced T cell suppressive activity when isolated from tumor-bearing, IDO-deficient animals that could be rescued by ectopic production of IL6 in the tumor. IL6 production could likewise reverse the pulmonary metastasis resistance exhibited by IDO-deficient mice. Interestingly, while there is a clear role of the immune system in lung tumor and metastatic outgrowth, IDO-deficient mice appear to have reduced vascularization in the lung which may partly contribute to reduced tumor formation. Together, these findings genetically validate IDO as a therapeutic target in the settings of pulmonary cancer and metastasis and establish the importance of IDO as a driver of IL6 production and MDSC function. Furthermore, the correlation of IDO to angiogenesis may be a new insight into the role of this enzyme in cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 295. doi:1538-7445.AM2012-295

Celecoxib combined with atorvastatin prevents progression of atherosclerosis

BACKGROUND

Increased expression of cyclooxygenase (COX-2) contributes to atherosclerosis. Recent studies suggest that COX-2 inhibitors prevent early plaque development but their effects on established lesions are less clear, while the statins promote plaque stability. The purpose of this study is to investigate whether administering a combination of a COX-2 inhibitor with a statin drug alters plaque progression in apo E-/- mice.

MATERIALS AND METHODS

Apo E-/- mice were fed a Western diet from 6 to 26 wk of age. At 26 wk, the Western diets supplemented with atorvastatin, celecoxib, or atorvastatin plus celecoxib were given for an additional 12 wk.

RESULTS

When the mice were 38 wk of age, the total area occupied by the atherosclerotic lesion was 53% less in the mice fed the combination of atorvastatin + celecoxib P ≤ 0.05) than that of the apo E-/- mice fed the Western diet alone, atorvastatin alone, or celecoxib alone. The decreased extent of atherosclerosis observed in the apo E-/- mice fed the combination of drugs was associated with reduced levels of prostaglandin (PG) E(2,) decreased protein expression of metalloproteinase (MMP)-9, macrophage chemotactic protein (MCP-1), and COX 2, and decreased staining for MMP-9, F4-80 (a marker for macrophages), and vascular cell adhesion molecule (VCAM).

CONCLUSION

This study indicates that using statins with a COX-2 inhibitor reduced the extent of atherosclerosis and inflammatory/cell adhesion molecule levels in the apo E-/- mouse model.

The select cyclooxygenase-2 inhibitor celecoxib reduced the extent of atherosclerosis in apo E-/- mice

Many investigators have suggested that immune activation may trigger the atherosclerotic process. The benefits of aspirin in preventing myocardial infarction have been attributed, in part, to its anti-inflammatory effects. Several reports have documented that cyclooxygenase (COX)-2 is up-regulated in human and mouse atherosclerotic lesions. To clarify the role of COX-2 in atherosclerosis, we conducted a study to test whether the COX-2 inhibitor, celecoxib, prevents the development and progression of the atherosclerotic process. We have used the apo E-/- mouse, a relevant animal model of atherosclerosis that develops fibrofatty lesions similar to human atherosclerosis. Treatment of 4-wk old apo E-/- mice with a standard rodent no. 5020 diet supplemented with 900 ppm of celecoxib for 16 wk led to an 81% reduction in lesion size. The mean lesion area per section (mean +/- SD) of proximal aorta from the apo E-/- mice fed the diet with celecoxib (33,991 +/- 7863 microm2, P < 0.001) was significantly less than that of the untreated apo E-/- mice (183,401 +/- 36,212 microm2). There were no lesions detected in the C57B1/6 mice. Immunohistochemistry of the ileum revealed that there was 80% reduction in staining for intercellular adhesion molecule and 60% reduction in staining for vascular cell adhesion molecule in the celecoxib treated mice. The protective effect of celecoxib was not maintained when the mice were switched after feeding the celecoxib-supplemented diet to the control 5020 diet for an additional 10 wk. These findings demonstrate that selective inhibition of the COX-2 enzyme with celecoxib prevented the development of atherosclerotic lesions in the proximal aortas from apo E-/- mice. One of the possible mechanisms is reduction in expression of the endothelial cell adhesion cell molecules intercellular adhesion molecule and vascular cell adhesion molecule, which plays a key role in the recruitment of inflammatory cells during the early stages of atherogenesis.

Novel tryptophan catabolic enzyme IDO2 is the preferred biochemical target of the antitumor indoleamine 2,3-dioxygenase inhibitory compound D-1-methyl-tryptophan

Small-molecule inhibitors of indoleamine 2,3-dioxygenase (IDO) are currently being translated to clinic for evaluation as cancer therapeutics. One issue related to trials of the clinical lead inhibitor, D-1-methyl-tryptophan (D-1MT), concerns the extent of its biochemical specificity for IDO. Here, we report the discovery of a novel IDO-related tryptophan catabolic enzyme termed IDO2 that is preferentially inhibited by D-1MT. IDO2 is not as widely expressed as IDO but like its relative is also expressed in antigen-presenting dendritic cells where tryptophan catabolism drives immune tolerance. We identified two common genetic polymorphisms in the human gene encoding IDO2 that ablate its enzymatic activity. Like IDO, IDO2 catabolizes tryptophan, triggers phosphorylation of the translation initiation factor eIF2alpha, and (reported here for the first time) mobilizes translation of LIP, an inhibitory isoform of the immune regulatory transcription factor NF-IL6. Tryptophan restoration switches off this signaling pathway when activated by IDO, but not IDO2, arguing that IDO2 has a distinct signaling role. Our findings have implications for understanding the evolution of tumoral immune tolerance and for interpreting preclinical and clinical responses to D-1MT or other IDO inhibitors being developed to treat cancer, chronic infection, and other diseases.

B23 is a downstream target of polyamine-modulated CK2

Our previous studies have shown that the overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, increases the enzymatic activity of the polyamine-responsive enzyme casein kinase 2 (CK2). Because CK2 is known to preferentially associate with the nuclear matrix in response to other trophic stimuli, we investigated the effects of ODC overexpression on CK2 localisation and on the CK2-mediated phosphorylation of a known CK2 substrate, the nucleolar phosphoprotein B23. Immunofluorescence analysis of CK2 and B23 in primary keratinocytes revealed that ODC overexpression resulted in the colocalisation of CK2 with B23 at the nucleolar borders. ODC overexpression also increased CK2 kinase activity 2-fold at the nuclear matrix, a response which could be abrogated by treatment of K6/ODC transgenic keratinocytes with the ODC inhibitor alpha-difluoromethylornithine (DFMO). Levels of B23 protein were also elevated in ODC-overexpressing cells compared to normal cells or transgenic cells treated with DFMO. This increase in protein level was neither due to an increase in steady-state mRNA levels, nor was it due to increased stability of B23 protein. Phosphorylation of B23 was also increased in ODC-overexpressing cells, and this increased phosphorylation could be blocked by treatment of the cells with the CK2 kinase inhibitors apigenin or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). These data suggest that B23 may be a downstream effector of polyamines via phosphorylation by the protein kinase CK2.

Suprabasal induction of ornithine decarboxylase in adult mouse skin is sufficient to activate keratinocytes

To study the effects of de novo induction of ornithine decarboxylase (ODC) activity in adult, quiescent skin, we generated transgenic mice in which the suprabasal expression of an inducible form of the ODC protein fused to a modified estrogen receptor ligand-binding domain (ODCER) is driven by an involucrin promoter. After topical treatment with the inducing agent 4-hydroxytamoxifen (4OHT), ODC activity and putrescine levels were dramatically increased in the epidermis but not in the dermis of transgenic mice. 4OHT treatment stimulated both proliferation as measured by bromodeoxyuridine incorporation in basal epidermal cells and differentiation shown by increased expression of differentiation markers. Furthermore, induction of ODC activity did not rescue primary epidermal keratinocyte cultures isolated from ODCER2 mice from a calcium-triggered DNA synthesis block, as measured by [3H]thymidine incorporation. In vivo induction of epidermal ODC enzyme activity significantly stimulated the vascularization of ODCER transgenic skin. Increased expression of interleukin-1beta and keratin 6, markers of keratinocyte activation seen in wound healing, was also observed in 4OHT-treated transgenic skin. These results suggest that de novo suprabasal induction of ODC activity in adult mouse skin activates keratinocytes and stimulates vascularization in the dermal layer in a manner similar to skin undergoing wound healing.

The smooth muscle cell membrane during atherogenesis: a potential target for amlodipine in atheroprotection

BACKGROUND

Atherosclerotic disease has been present in the human population apparently from the beginning of time. However, it has only been in the 20th century that improvements in the control of infectious diseases have allowed the average life span to increase to the point where atherosclerosis has been able to affect the general population. By the middle of the 20th century, atherosclerosis had reached epidemic levels, and it is currently pandemic and increasing worldwide. Despite its growing significance to health care, we still know relatively little about the cellular basis for plaque genesis in the vessel wall. Current thinking holds that atherosclerosis is caused by an unchecked chronic inflammatory process involving the cells of the arterial wall and their interaction with LDL and various inflammatory cells. Considerable evidence suggests that the principal insults underlying atherogenesis are serum dyslipidemias and oxidative stress mediated primarily by oxidized LDL. However, just how these insults alter the cell biology of vascular cells and lead to the atherosclerotic phenotype is still under intense investigation. Moreover, recent clinical trials have provided evidence that certain classes of drugs, including newer calcium channel blockers (CCBs), can remodel the arterial smooth muscle cell (SMC) membrane and inhibit the progression of atherosclerotic disease.

METHODS

This review summarizes our current thinking on atherogenesis in the arterial SMC and considers recent developments regarding alterations in the SMC membrane during the very early period of atherogenesis. We also discuss how certain CCBs might operate to produce atheroprotection.

RESULTS

The SMC membrane becomes enriched in unesterified cholesterol soon after the development of serum hypercholesterolemia. With excess membrane cholesterol, the membrane becomes thicker and develops distinct cholesterol domains. These alterations in the membrane increase the permeability of SMC to calcium and induce a variety of alterations in SMC function that contribute to cellular atherogenic processes during plaque genesis. Amlodipine, a third-generation CCB, markedly inhibits the progression of lesions. The explanation of this novel action may lie in the effects of this drug on various potential cellular targets.

CONCLUSIONS

Evidence is accumulating that excess membrane cholesterol may contribute to the cellular defects responsible for the transformation of the SMC to the atherosclerotic phenotype. Amlodipine, which has membrane-remodeling properties, is emerging as an important atheroprotective drug.

Atheroprotection with amlodipine: cells to lesions and the PREVENT trial. Prospective Randomized Evaluation of the Vascular Effects of Norvasc Trial

Oxidized lipid and calcium regulatory abnormalities appear to play important roles in early atherogenesis secondary to cholesterol enrichment of the cell membrane in endothelial and arterial smooth muscle cells (SMCs). However, the link between the two is poorly understood. The findings reviewed here demonstrate that amlodipine has membrane-modifying and antioxidant actions at the cell membrane level in addition to its classical calcium channel blocking properties. These multiple pharmacologic actions may explain the cellular mechanisms of the atheroprotective effects of amlodipine in spontaneous atherogenesis and in accelerated atherosclerotic syndromes. Recent animal model studies have demonstrated that amlodipine inhibits the progression of atherosclerotic lesions and protects against restenosis after angioplasty. Amlodipine inhibits the cholesterol-induced increase in calcium permeability in SMCs, and has been shown to repair abnormalities in SMC membrane structure. Recent data have also demonstrated that amlodipine has a marked antioxidant action in membrane bilayers enriched with polyunsaturated fatty acids. However, these findings have been in animal models only; the efficacy of amlodipine in atheroprotection in humans cannot be predicted. The PREVENT trial has therefore been launched to examine the atheroprotective potential of amlodipine in spontaneous lesion development in humans with ischemic heart disease and in the prevention of restenosis after angioplasty.

Cholesterol, calcium and atherosclerosis: is there a role for calcium channel blockers in atheroprotection?

It is well known that the atherogenic dyslipidemias of either elevated serum LDL or reduced HDL levels correlate with the degree and severity of atherosclerosis. However, how this leads to atherogenesis is poorly understood. A role for cellular oxidative stress mediated by oxidized LDL has gained widespread acceptance, but this pathway is unlikely to be the sole atherogenic signal. Recent evidence obtained from arterial smooth muscle cells (SMC) and endothelial cells (EC) is consistent with another pathway that may explain, in part, the early alterations contributing to the initiation of cellular atherogenic modifications. This pathway involves enrichment of the cell plasma membrane with cholesterol. In SMC, in vitro (cell culture) and in vivo (cholesterol feeding) experiments demonstrate that cholesterol enrichment of the SMC membrane occurs rapidly and is associated with an increase in membrane bilayer width, calcium permeability, and cell proliferation. Removal of excess membrane cholesterol with human HDL restores these alterations, suggesting that this membrane structural 'defect' mediates these changes in cell function. In vitro, the increased calcium permeability is inhibitable by calcium channel blockers (CCBs), but in vivo, a calcium 'leak' pathway develops that is virtually uninhibitable. It is not surprising that the literature on the application of CCBs for atheroprotection is not wholly convincing. However, with the advent of the new third generation of CCBs, new hope arises. One of the first CCBs of this generation is amlodipine (Norvasc), a charged dihydropyridine that has a remarkable pharmacologic profile. First, it is markedly lipophilic allowing it to partition readily into cell membranes. Second, in the membrane it has the ability to re-order, or restore, the 'swollen' membrane bilayer back to normal in atherosclerotic SMC. Third, it has potent antioxidant properties. Fourth, it appears to inhibit the expression of a variety of genes implicated in atherogenesis. Fifth, it is a CCB. Amlodipine has demonstrated atheroprotection in both rabbit and subhuman primate models of this disease. We propose that cellular alterations induced by enrichment of the cell membrane with cholesterol, which appears to modulate SMC to the atherosclerotic phenotype, are inhibitable by amlodipine through a combination of its varied pharmacologic properties. The potential for atheroprotection with amlodipine is currently being investigated in a human trial (PREVENT trial) and the results of this trial will determine the relevance of the preclinical findings to humans.