Robert Pasławski
1, Urszula Pasławska
2, Andrzej Szuba
1, Józef Nicpoń
2Swine as a Model of Experimental Atherosclerosis
Świnia jako eksperymentalny model do badań arteriosklerozy
1 Department and Clinic of Internal and Occupational Diseases and Hypertension,
WroclawMedical University, Wrocław, Poland
2 Department of Internal Medicine and Clinic of Diseases of Horses, Dogs and Cats,
Wrocław University of Environmental and Life Sciences, Wrocław, Poland
Abstract
In Western countries, lifestyle changes in the past century are considered to be a major factor in obesity and related diseases such as atherosclerosis and diabetes. To gain a better understanding of the relationship between disorders of lipid metabolism and atherogenesis, many animal species have been tested. The most commonly used are laboratory animals such as mice, rats, rabbits, guinea pigs and pigeons. However, the pig is a considered a very good model of human atherosclerosis, because it is similar to humans in terms of body size and other physiological features, including its tendency to overeat. Even though there is no perfect animal model that completely replicates human atherosclerosis, the pig seems to be a promising subject for exploring the etiopathogenesis of atherosclerosis (Adv Clin Exp Med 2011, 20, 2, 211–215).
Key words: swine model, experimental atherosclerosis.
Streszczenie
Zmiany stylu życia, jakie nastąpiły w ostatnim wieku w państwach kultury zachodniej uważa się za najważniejszy czynnik odpowiedzialny za otyłość i choroby z nią związane, takie jak arterioskleroza i cukrzyca. Aby lepiej zro-zumieć związki między zaburzeniami lipidowymi a rozwojem miażdżycy, wykonywano eksperymenty na wielu gatunkach zwierząt. Najczęściej wykorzystywano zwierzęta laboratoryjne, np. myszy, szczury, króliki, świnki mor-skie i gołębie. Za bardzo dobry jest uważany model świński, ponieważ dorosłe świnie mają podobną do ludzi masę ciała, budowę anatomiczną i właściwości fizjologiczne, np. skłonność do przejadania się. Nie istnieje idealny model zwierzęcy, który wiernie imitowałby rozwój choroby u człowieka, ale wydaje się, że model świński jest najbar-dziej obiecujący w badaniach dotyczących etiopatogenezy zmian miażdżycowych (Adv Clin Exp Med 2011, 20, 2, 211–215).
Słowa kluczowe: model świński, eksperymentalna arterioskleroza.
Adv Clin Exp Med 2011, 20, 2, 211–215 ISSN 1230-025X
REvIEWS
© Copyright by Wroclaw Medical University
In Western countries, lifestyle changes in the past century are considered to be a major factor in obesity and related diseases such as atherosclerosis and diabetes. Excessive consumption of carbohy-drates and fat has a profound effect on the strictly regulated energy balance and thereby provoke met-abolic disorders, resulting in atherosclerosis and cardiovascular diseases. Atherosclerosis is defined as “a chronic immunoinflammatory, fibroprolif-erative disease of large and medium-sized arteries fuelled by lipids” [1–3] and is, as Falk expressed it: “by far the most frequent underlying cause of coronary artery disease, carotid artery disease, and peripheral arterial disease” [4].
As compared with rats, guinea pigs and pigs, the localization of atherogenic lesions in rabbits is less similar to the localization in humans; and rabbits have no spontaneous atherosclerosis. On the other hand, genetically altered rabbits, such as the Wa-tanabe Heritable Hyperlipidemic, Houston RT and St. Thomas’ Hospital strains, develop massive fulmi-nant atherosclerosis, which is similar to the human genetic defect familial hypercholesterolemia [6]. Pigeons have no Apolipoprotein E (ApoE) B48 or chylomicron formation [7].
The pig, however, is a considered a very good model of human atherosclerosis, because it is similar to humans in terms of body size and other physiological features, including its tendency to overeat [8]. Atherosclerosis occurs naturally even on a normal porcine diet and can be exacerbated by modification of the diet [9]. The changes in plasma lipoproteins in response to changes in diet closely resemble those occurring in humans and, like humans, swine transport most cholesterol in low-density lipoprotein (LDL) [8]. As Stender and Zilversmit noted, “It has been well documented that free cholesterol from plasma enters the arte-rial wall in excess of estriefied cholesterol relative to their concentrations in plasma, in both natural and experimental conditions” [10].In contrast to rodents, swine atherosclerosis, like the human ill-ness, progresses to advanced stages. Diet-induced atherosclerosis alone is rarely fatal; it is only si-multaneous erosion or rupture of atherosclerotic plaque superimposed by thrombosis (which oc-curs at advanced stages) that leads to life-threat-ening clinical events such as coronary syndromes and stroke [11].
The maintenance of pigs is expensive and re-quires special facilities that are beyond the capa-bilities of most laboratories. This has led to a ten-dency to conduct research on smaller-size strains of this species: various mini-pig breeds or young (sometimes very young) domestic pigs. Studies on atherosclerosis in pigs have varied markedly in terms of the breed, sex and age of the pigs, the hyperlipidemic diet composition (from 1% to 6% cholesterol) and the feeding schedule. Some ex-amples are presented in Table 1.
In all these experiments, regardless of the composition of the high-fat diet, itwas relatively easy to induce distinct elevation of the plasma cholesterol level; however, the majority of ath-erosclerotic lesions develop only at certain vessels sites – mostly in the distal abdominal aorta [20]. Histological examination of a cross-section of this aortic plaque showed marked intimal thickening containing smooth muscle cells, foam cells, mono-cytes and connective tissue [13, 18].The localiza-tion and features of these plaques are similar to
human atherosclerosis. Reitman et al. speculated that the young age of the mini-pigs used in their study might explain the lack of significant coro-nary artery atherosclerosis they observed [13]. A second possibility is that atherosclerotic pathol-ogies develop in a particular sequence, and shorter research studies allow the formation of changes only in the abdominal part of the aorta. In the 18-month study conducted by Jacobsson et al., mini-pigs fed lipid-enriched food showed fatty streaks not only in the abdominal aorta but also in the thoracic aorta and coronary arteries. Diet cholesterol concentration also affects the exten-sion and exacerbation of atherosclerotic changes, as the results of the research studies presented in Table 1 show. It is thought that the regional differ-ences in the extent of atherosclerotic lesions in the same breed of animal are due to a difference in the susceptibility of specific arterial sites to the devel-opment of atherosclerosis [13]. One disadvantage of the mini-pig is that a mature mini-pig fed ad libitum becomes obese so quickly that it is difficult to conduct control tests [21]. However, domestic pigs are considered unsuitable for chronic studies because of their rapid growth.
Recently genetically modified mice and pigs have been developed to serve as models for study-ing atherosclerosis. Gene deletion technology has allowed the creation of a variety of transgenic ani-mal models. The most popular knock-out (KO) mice provide an excellent opportunity to study gene interactions in atherosclerosis. Successfully created gene knock-out mice include (among others) Apo E-KO mice [22], LDL receptor KO mice [23], he-patic lipase KO mice [24], and mice expressing human Apo-B100 [25] and human cholesterol ester transfer protein [26]. However, it is important to bear in mind thatthese mice are models of athero-genesis, not advanced atherosclerosis, and they do not exhibit the single most important event in hu-mans: plaque rupture leading to vessel occlusion.
“compris-Table 1. Examples of experimental protocols using a swine model Tabela 1. Przykłady protokołów doświadczalnych z użyciem modelu świni
Author
(Autor) Breed of pig (Rasa świni) Age of swine (Wiek świni)
Animal Body weight (Masa ciała zwierzęcia)
Length of study (Czas trwania doświad-czenia)
Diet (Dieta)
at start of research Samochowiec et
al. 1981 [12] domestic cross-breed pigs lack of data 17 kg 6 months standard pig diet + 15% coco-nut oil + 5% cholesterol + 0.5% cholic acid
Reitman et al.
1982 [13] Yucatan minia-ture swine 8–12 weeks 15–20 kg 12 moths conventional pig diet + 1.5% cholesterol + 15% lard
Fuster et al. 1985
[14] crossbred Poland-china and Yorkshire-Hampshire pig
3 months lack of data 6 months modified pig diet (soybean oil meal 190 g/kg, dextrose 10 g/kg, dicalcium phosphate 24 g/kg, vitamins, trace minerals and antibiotics 9 g/kg) + 5 g/kg iodized salt + 20 g/kg choles-terol + 200 g/kg tallow + 10 g/kg hog bile extract
Kim et al. 1989
[15] Yorkshire swine lack of data 11 kg 4 months in g/700 g mash: corn starch 200, corn oil 10, casein (90% protein) 130, alphacel (cellulose type fiber) 64, salt mix 4, vita-min mix 4, butter 90, choles-terol 10
Kobari et al. 1991
[16] Gottingen min-iature swine (vietnamese pig+miniature pig+German Landrace)
7–12 months 25–36 kg 15 months conventional pig diet (11.5% water,15.1% protein, 2.9% fat, 5.2% roughage, 7.4% mineral salts, 57.9% carbohydrate) + 15% beef tallow + 1.5% choles-terol
Jacobsson et al.
1994 [17] Gottingen min-iature swine 10–13 weeks 15 kg 18 months conventional pig diet (carbohy-drates 610 g/kg, protein 148 g/kg, water 130 g/kg, crude fiber 51 g/ /kg, fat 33 g/kg, essential amino-acids 22 g/kg, vitamins and minerals 6 g/kg) + 11.7% freeze-dried egg yolk + 1% cholesterol Holvoet et al.
1998 [18] miniature pig 3 months 28 kg 9 months conventional pig diet + 14% beef tallow + 4% cholesterol + 1% hog bile extract
Johnson et al.
2005 [19] domestic cross-breed pigs lack of data 20–30 kg 2 months conventional pig diet + 6% cho-lesterol
Artinger et al.
ing a mutation in the endogenous ApoE gene or part thereof and/or LDL gene or part thereof, and/or LDL receptor gene, and/or transcriptional and/or translational product or part thereof” [28].
Even though there is no single perfect animal model that completely replicates human athero-sclerosis, pigs seems to be the most similar to
hu-mans in terms of anatomy, pathophysiology and their tendencies for overconsuming, obesity and natural artery wall pathologies. Pig models allow for the observation of all stages of atherosclerosis, including the more advanced stages. Despite some disadvantages swine seems to be a promising sub-ject for exploring atherosclerosis.
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Address for correspondence:
Robert Pasławski
Department and Clinic of Internal and Occupational Diseases and Hypertension WroclawMedical University
Wybrzeże L. Pasteura 4 50-367 Wrocław Poland
Tel.: +48 602 55 99 44
E-mail: [email protected]
Conflict of interest: None declared
