Elevation of serum cholesterol and increased fatty streaking in egg yolk:lard fed castrated miniature pigs

Minipig: Advantages and Disadvantages as a Model in Toxicity Testing

The choice of animal models for toxicity testing is usually based on convenience rather than validity for application in human beings. Attention must be paid to the similarities of the test system with those of humans so that extrapolations can be made with some margin of safety. Miniature swine offer many advantages in terms of being phylogenetically closer to humans and having many physiological similarities. The neonatal piglet is an excellent model for studies of perinatal toxicology and pediatrics because of its physiological resemblance to the human infant. The size of mature miniature swine, short reproductive cycle, ease of breeding, high prolificacy, large litter size, diversity of gene pool, early maturity, and adaptability to the experimental situation are advantages that do not exist in other species. All in all, the miniature swine is as yet an unappreciated and underutilized species for experimental purposes.

Effect of testosterone deficiency on cholesterol metabolism in pigs fed a high-fat and high-cholesterol diet

Background

Testosterone deficiency is associated with increased serum cholesterol levels. However, how testosterone deficiency precisely affects cholesterol metabolism remains unclear. Therefore, in the current study, we examined the effect of testosterone deficiency on cholesterol metabolism and liver gene expression in pigs fed a high-fat and high-cholesterol (HFC) diet.

Methods

Sexually mature male miniature pigs (6–7 months old) were randomly divided into 3 groups as follows: intact male pigs fed an HFC diet (IM + HFC), castrated male pigs fed an HFC diet (CM + HFC), and castrated pigs with testosterone replacement fed an HFC diet (CM + HFC + T). Serum testosterone levels and lipid profiles were measured, and gene expression levels associated with hepatic cholesterol metabolism were determined. Furthermore, total hepatic cholesterol contents and the activities of enzymes mediating hepatic cholesterol metabolism were measured.

Results

Serum testosterone levels were significantly decreased in CM + HFC pigs, and testosterone replacement attenuated castration-induced testosterone deficiency. Castration significantly increased the serum levels of total cholesterol, low-density lipoprotein cholesterol and triglycerides, as well as hepatic lipid contents in pigs fed an HFC diet. Compared with IM + HFC and CM + HFC + T pigs, low-density lipoprotein receptor (LDLR) mRNA expression and protein levels were significantly decreased in the livers of CM + HFC pigs. In contrast, we found that compared with IM + HFC pigs, hepatic proprotein convertase subtilisin/kexin type 9 (PCSK9) mRNA and serum PCSK9 protein levels were significantly increased in CM + HFC pigs. Moreover, testosterone treatment reversed the increase in PCSK9 expression in CM + HFC pigs. However, neither castration nor testosterone replacement affected the expression of the other hepatic genes that were tested.

Conclusions

This study demonstrated that castration-induced testosterone deficiency caused severe hypercholesterolemia in pigs fed an HFC diet; furthermore, these effects could be reversed by testosterone replacement therapy. Altered hepatic PCSK9 and LDLR expression, resulting in reduced LDL-cholesterol clearance, may contribute to the increased serum cholesterol levels induced by testosterone deficiency and an HFC diet. These results deepen our understanding of the underlying molecular mechanisms that mediate the effects of testosterone deficiency on cholesterol metabolism.

Transcriptomic analysis of hepatic responses to testosterone deficiency in miniature pigs fed a high-cholesterol diet

Background

Recent studies have indicated that low serum testosterone levels are associated with increased risk of developing hepatic steatosis; however, the mechanisms mediating this phenomenon have not been fully elucidated. To gain insight into the role of testosterone in modulating hepatic steatosis, we investigated the effects of testosterone on the development of hepatic steatosis in pigs fed a high-fat and high-cholesterol (HFC) diet and profiled hepatic gene expression by RNA-Seq in HFC-fed intact male pigs (IM), castrated male pigs (CM), and castrated male pigs with testosterone replacement (CMT).

Results

Serum testosterone levels were significantly decreased in CM pigs, and testosterone replacement attenuated castration-induced testosterone deficiency. CM pigs showed increased liver injury accompanied by increased hepatocellular steatosis, inflammation, and elevated serum alanine aminotransferase levels compared with IM pigs. Moreover, serum levels of total cholesterol, low-density lipoprotein cholesterol, and triglycerides were markedly increased in CM pigs. Testosterone replacement decreased serum and hepatic lipid levels and improved liver injury in CM pigs. Compared to IM and CMT pigs, CM pigs had lower serum levels of superoxide dismutase but higher levels of malondialdehyde. Gene expression analysis revealed that upregulated genes in the livers of CM pigs were mainly enriched for genes mediating immune and inflammatory responses, oxidative stress, and apoptosis. Surprisingly, the downregulated genes mainly included those that regulate metabolism-related processes, including fatty acid oxidation, steroid biosynthesis, cholesterol and bile acid metabolism, and glucose metabolism. KEGG analysis showed that metabolic pathways, fatty acid degradation, pyruvate metabolism, the tricarboxylic acid cycle, and the nuclear factor-kappaB signaling pathway were the major pathways altered in CM pigs.

Conclusions

This study demonstrated that testosterone deficiency aggravated hypercholesterolemia and hepatic steatosis in pigs fed an HFC diet and that these effects could be reversed by testosterone replacement therapy. Impaired metabolic processes, enhanced immune and inflammatory responses, oxidative stress, and apoptosis may contribute to the increased hepatic steatosis induced by testosterone deficiency and an HFC diet. These results deepened our understanding of the molecular mechanisms of testosterone deficiency-induced hepatic steatosis and provided a foundation for future investigations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1283-0) contains supplementary material, which is available to authorized users.

Dietary fat and cholesterol induced modification of minipig lipoprotein fluidity and composition

1. Miniature swine were fed a low (2.7%) fat control stock diet alone or supplemented with either 20% lard plus 1% cholesterol or 20% lard alone for periods of up to 6 months. 2. Cholesterol feeding reduced VLDL fluidity drastically and LDL fluidity minimally but had no effect on HDL fluidity. 3. Lard feeding had no effect on lipoprotein fluidity. 4. The rigid VLDL produced by cholesterol feeding was enriched in cholesterol and phospholipid contents, similar to beta-VLDL. 5. Plasma cholesterol concentrations were increased by 1.5 to 5-fold in pigs fed stock diets supplemented with 20% lard, with or without added cholesterol, but plasma triacylglycerol concentrations were not affected by either diet modification. 6. Diet effects were complete within 4 weeks with no further changes for periods up to 6 months. 7. Regression of the induced hypercholesterolemia was also accomplished within one month of removing cholesterol from the diet.

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.

Flow cytometric quantification of cholesteryl ester-containing "foam" cells. II. Analysis of aortas from cholesterol-fed swine

We have examined foam cell accumulation in abdominal and thoracic aortic segments of swine with experimentally induced atherosclerosis. Young (less than 1-year-old) male swine were divided into four groups that were fed control, lard (20%), lard (20%) plus cholesterol (1%), and regression (3 months cholesterol-lard followed by 3 months control) diets for 6 months. Aortas were removed from animals and enzymatically dissociated. Foam cells were detected by specifically staining their cholesteryl ester inclusions with the fluorescent dye filipin. Flow cytometry was used to quantify the number of foam cells in each aortic cell suspension. Cholesterol-lard feeding increased serum cholesterol levels 6-fold and induced a substantial increase in both abdominal and thoracic foam cell densities. Lesion development in cholesterol-lard-fed animals, as assessed by macroscopic evaluation of aortas, correlated with foam cell accumulation as determined by flow cytometry. Accumulation of foam cells was extremely variable from animal to animal and did correlate significantly with serum cholesterol but not with serum triglyceride levels. Interestingly, although serum cholesterol levels increased 1.5-fold in swine fed the lard diet, foam cells did not increase significantly as compared to control animals. Animals placed on a control diet following 3 months of the cholesterol-lard diet showed a trend toward lower foam cell densities as compared to animals fed the cholesterol-lard diet for the entire 6 months. Flow cytometric analysis of filipin-stained aortic foam cells provides a new means to evaluate atherosclerotic lesion development.

Flow cytometric quantification of cholesteryl ester-containing "foam" cells. I. Analysis of aortas from normolipidemic swine

We have quantified using flow cytometry foam cells of aortas from normolipidemic swine varying in age from 6 months to 12 years. These swine were maintained throughout their lives on a low-fat, cholesterol-free diet. Intimal-medial tissues removed from the swine aortas were enzymatically dissociated to prepare Formalin-fixed cell suspensions. Foam cells were labeled by specific staining of their intracellular cholesteryl ester using the fluorescent dye filipin. This was carried out by first removing cellular unesterified cholesterol with ethanol, then enzymatically hydrolyzing cellular cholesteryl ester, and finally staining with filipin the unesterified cholesterol derived from hydrolysis of cholesteryl ester. Results of flow cytometric analysis indicated that thoracic foam cell densities of female swine became more variable and tended to increase with age. It appeared that there were two subgroups of female swine with either high or low levels of thoracic foam cells. A similar finding was not observed for abdominal foam cells in female swine nor for thoracic or abdominal foam cells in the smaller number of older male swine. Abdominal foam cell densities in younger males, however, also appeared to be comprised of low and high foam cell groups. Foam cell densities did not correlate with serum cholesterol or triglyceride levels. However, a genetic basis for some of the variability in foam cell densities among these animals was suggested by the observation that females with high thoracic foam cell densities had greater commonality among ancestors than did females with low thoracic foam cell densities.

Influence of age and sex on composition and lipid fluidity in miniature swine plasma lipoproteins

Age- and sex-related differences were observed in the plasma cholesterol level, the plasma concentrations of certain lipoprotein components, and the HDL lipid phase fluidity in miniature swine from post-weaning (6 weeks) through puberty (6 months), maturity (2-6 years), and old age (10-12 years). Age effects were more dominant in the males, with VLDL protein; LDL protein, triacylglycerol, and phospholipid; and HDL triacylglycerol, phospholipid, cholesterol, and polyunsaturated fatty acids showing statistically significant negative correlations with age. These effects were not observed in females. HDL cholesterol was positively correlated with age in females. Total plasma cholesterol decreased with age in males only, but plasma triacylglycerol was not influenced by age in either sex. Higher concentrations of all lipoprotein lipids were observed in the female minipigs regardless of age. HDL lipids became less fluid with age in the males alone suggesting a physical chemical basis for the lower incidence of heart disease among females. The more fluid HDL circulating in the female may be more capable of mobilizing peripheral tissue cholesterol for catabolism thus protecting her from developing atherosclerotic lesions.