Lipid accumulation in muscular foci in White Carneau and Show Racer pigeon aortas

The pigeon (Columba livia) model of spontaneous atherosclerosis

Multiple animal models have been employed to study human atherosclerosis, the principal cause of mortality in the United States. Each model has individual advantages related to specific pathologies. Initiation, the earliest disease phase, is best modeled by the White Carneau (WC-As) pigeon. Atherosclerosis develops spontaneously in the WC-As without either external manipulation or known risk factors. Furthermore, susceptibility is caused by a single gene defect inherited in an autosomal recessive manner. The Show Racer (SR-Ar) pigeon is resistant to atherosclerosis. Breed differences in the biochemistry and metabolism of celiac foci cells have been described. For example, WC-As have lower oxidative metabolism but higher amounts of chondroitin-6-sulfate and nonesterified fatty acids compared with SR-Ar. Gene expression in aortic smooth muscle cells was compared between breeds using representational difference analysis and microarray analysis. Energy metabolism and cellular phenotype were the chief gene expression differences. Glycolysis and synthetic cell types were related to the WC-As but oxidative metabolism and contractile cell types were related to the SR-Ar. Rosiglitazone, a PPARγ agonist, blocked RNA binding motif (RBMS1) expression in WC-As cells. The drug may act through the c-myc oncogene as RBMS1 is a c-myc target. Proteomic tests of aortic smooth muscle cells supported greater glycosylation in the WC-As and a transforming growth factor β effect in SR-Ar. Unoxidized fatty acids build up in WC-As cells because of their metabolic deficiency, ultimately preventing the contractile phenotype in these cells. The single gene responsible for the disease is likely regulatory in nature.

Rosiglitazone modulates pigeon atherosclerotic lipid accumulation and gene expression in vitro

Atherosclerosis is a major contributor to the overall United States mortality rate, primarily in the form of heart attacks and stroke. Unlike the human disease, which is believed to be multifactorial, pigeon atherosclerosis is due to a single gene autosomal recessive trait. The White Carneau (WC-As) strain develops atherosclerotic plaques without the presence of known environmental risk factors such as diet and classic predictors such as blood pressure or blood cholesterol levels. With similar parameters, the Show Racer (SR-Ar) is resistant to plaque development. Thiazolidinediones, including rosiglitazone, activate the peroxisome proliferator-activated receptor gamma (PPARγ) raising cellular sensitivity to insulin. The effect of rosiglitazone was evaluated in aortic smooth muscle cells (SMC) from these 2 pigeon breeds. Primary SMC cultures were prepared from WC-As and SR-Ar squabs. Cell monolayers, which achieved confluence in 7 d, were treated with 0 or 4 µM rosiglitazone for 24 h. Cellular lipid accumulation was evaluated by oil red O staining. Control WC-As cells had significantly higher vacuole scores and lipid content than did the SR-Ar control cells. Rosiglitazone treatment decreased WC-As lipid vacuoles significantly compared with the control cells. On the other hand, lipid vacuoles in the treated and untreated SR-Ar cells did not differ significantly. The effect of rosiglitazone on WC-As SMC gene expression was compared with control SMC using representational difference analysis. Significant transcript increases were found for caveolin and RNA binding motif in the control cells compared with the rosiglitazone-treated cells as well as cytochrome p450 family 17 subfamily A polypeptide 1 (CYP171A) in the rosiglitazone-treated cells compared with the control cells. Although rosiglitazone was selected for these experiments because of its role as a PPARγ agonist, it appears that the drug also tempers c-myc expression, as genes related to this second transcription factor were differentially expressed. Both PPARγ and c-myc appear to affect WC-As SMC gene expression, which may relate to disease development, progression, or both.

Atherosclerosis-susceptible and atherosclerosis-resistant pigeon aortic cells express different genes in vivo

Spontaneous atherosclerosis in the White Carneau (WC-As) pigeon is inherited as a single gene disorder, and its progression closely mirrors the human disease. Representational difference analysis and microarray were used to identify genes that were differentially expressed between the susceptible WC-As and resistant Show Racer (SR-Ar) aortic tissue. The RNA extracted from 1-d-old squab aortas was used to make cDNA for each experiment. Fifty-six unique genes were found using representational difference analysis, with 25 exclusively expressed in the WC-As, 15 exclusive to the SR-Ar, and 16 nonexclusive genes having copy number variation between breeds. Caveolin and β-actin were expressed in the WC-As, whereas the proteasome maturation protein and the transcription complex CCR4-NOT were exclusive to the SR-Ar. Microarray analysis revealed 48 genes with differential expression. Vascular endothelial growth factor and p53 binding protein were among the 17 genes upregulated in the WC-As. Thirty-one genes were upregulated in the SR-Ar including the transforming growth factor-β signaling factor SMAD2 and heat shock protein 90. Genes representing several biochemical pathways were distinctly different between breeds. The most striking divergences were in cytoskeletal remodeling, proteasome activity, cellular respiration, and immune response. Actin cytoskeletal remodeling appears to be one of the first differences between susceptible and resistant breeds, lending support to the smooth muscle cell phenotypic reversion hypothesis of human atherogenesis.

Oxygen tension within the arterial wall: relationship to altered bioenergetic metabolism and lipid accumulation

Oxygen tension (pO2) was measured in upper thoracic arteries and in muscular foci at the celiac bifurcation from atherosclerosis-susceptible White Carneau and atherosclerosis-resistant Show Racer pigeons at 6, 12, and 24 weeks of age. At each site the pO2 was measured within adventitial (outer), medial (middle), and subendothelial (inner) zones with polarographic oxygen microelectrodes while the animals were under dissociative anesthesia. The atherosclerosis-resistant Show Racer pigeons exhibited no significant age or site differences in pO2 levels. The medial pO2 was approximately 25 mm Hg, while the adventitial and subendothelial zones had pO2 levels of approximately 30 mm Hg. The pO2 profiles through the various zones of the arterial wall of 6-week-old atherosclerosis-susceptible White Carneau pigeons were similar to those of the Show Racers. However, significantly lower oxygen tension was found in the subendothelial zone from 12- and 24-week White Carneau celiac bifurcations (27 and 21 mm Hg) than in corresponding zones in Show Racer sites. Also, the medial zone in 24-week White Carneau celiac foci exhibited a significantly lower pO2 (20 mm Hg) than the medial zone of younger White Carneau. When correlated with previously described biochemical and morphological changes in White Carneau pigeons, the findings presented herein indicate that focal decreases in oxygen availability within arterial tissue occur after the onset of spontaneous atherogenesis, and probably result from decreased diffusion of oxygen into the arterial wall.

Sex mediated lipid metabolism in human aortic smooth muscle cells

When human aortic smooth muscle cells in culture were treated with pharmacological doses of estrogen and testosterone for 48 hrs, the rate of cholesterol synthesis measured both by acetate incorporation and the 3, hydroxy 3-methylglutaryl Co enzyme A reductase (HMG-CoA) activity declined significantly as compared to control. However, the rate of cholesterol esterification increased by 132% and 45% in response to testosterone and estrogen respectively. Also, acetate incorporation into fatty acids and fatty acid synthetase enzyme activity increased by hormonal treatment but remained in the free form especially by estrogen. Testosterone treatment resulted in more esterification (p less than .025) of fatty acid than estrogen treatment. Incubation with hormones for 48 hrs resulted in enhanced uptake of 14C-labeled cholesterol along with increased accumulation of cellular cholesterol. Increased synthesis of phospholipid and triglyceride by estrogen may be responsible for excretion of cellular sterol and fat. These results indicate that sex-hormones have an important effect on the regulation of lipid metabolism in human aortic cells.

Heparitin sulfates (heparan sulfates) of atherosclerosis-susceptible and atherosclerosis-resistant pigeon aortas

Electrophoretic mobilities of heparitin sulfates (HS) from the thoracic aorta and the celiac bifurcation of atherosclerosis-susceptible White Carneau and atherosclerosis-resistant Show Racer pigeons at four ages were examined. Atherosclerosis-susceptible White Carneau aortas contain, in addition to their normal HS component, a variant type of HS (HS') in pre, early- and moderately-atherosclerotic celiac foci which is not found in the thoracic regions of either breed or in Show Racer celiac foci at any age. These findings are interpreted to suggest a role of HS' in the pathogenesis of atherosclerosis and to propose an explanation for atherosclerotic susceptibility and resistance in pigeons.

Inherited depression of arterial lipoamide dehydrogenase activity associated with susceptibility to atherosclerosis in pigeons

The activity of lipoamide dehydorgenase (E.C.1.6.4.3) was measured in arterial homogenates from very young pigeons (5-8 weeks old) known to differ in their susceptibility to atherosclerosis. The activity of the arterial enzyme was significantly lower in the atherosclerosis-susceptible White Carneau pigeons than it was in the atherosclerosis-resistant Show Racer pigeons. Lipoamide dehydrogenase is a component of the pyruvate dehydrogenase and alpha-ketoglutarate multienzyme complexes. The first complex catalyzes the conversion of pyruvate to oxaloacetate via acetyl-CoA, and this reaction represents a crucial link between glycolysis and the Krebs cycle. The second complex is essential for the oxidative breakdown of carbohydrates, fats, and amino acids via the Krebs cycle. Reduced activity of these complexes, resulting from low activity of lipoamide dehydrogenase, favors reduction of pyruvate to lactate and a shift to glycolysis. This situation is in accord with other results obtained in avian and human arteries which appear to indicate a higher rate of glycolysis in atherosclerosis-susceptible and atherosclerotic arteries. It appears that the increased dependence of the White Carneau arteries on glycolysis, suggested by the reduced lipoamide dehydrogenase activity, facilitates the development of atherosclerosis in this pigeon strain.