Determining the in vitro level of foam cell formation in different South African populations
Ralefatane, Maile George
Aim: A critical event in atherosclerosis is the accumulation of oxidised ( ox) LDL-laden foam cells (FCs) derived from macrophages within the arterial wall. Macrophages are transformed to FCs by the uptake of oxLDL by scavenger receptors i.e. MSR-1, CD36 and CD68. Atherosclerosis differs in prevalence across genders (more common in males) and ethnic groups (less common in African than Asian-Indian or European populations). Therefore, the aim of this study was to compare FC formation across genders and ethnicities to determine if gender and ethnic differences in the prevalence of atherosclerotic diseases are mirrored by similar differences in the level of in vitro FC formation. Methods: Forty one subjects were recruited into 2 groups; a younger group (n= 11; 7 African, 4 European) and an older group (n=30; ten from each of the 3 ethnic groups; 15 males and 15 females). Anthropometry and fasting lipid and glucose levels were measured. Monocytes were isolated from whole blood and converted to macrophages and FCs using standard cell culture procedures. The level of FC formation was determined by measuring intra-cellular lipid accumulation (ICLA) using the lipid-specific dye Oil red 0. Macrophage expression of scavenger receptors (MSR-1, CD36, CD68) was measured by quantitative rtPCR. Results: The ICLA in macrophages exposed to oxLDL (measured as a % of ICLA in unexposed cells) was significantly higher in the older (151.1 ± 27.9%) than the younger group (99.9 ± 22.1 %; p=0.0001 ). No differences in macrophage ICLA were observed between ethnic groups (African, 156.8 ± 28.1 %; European, 155.9 ± 57.9%; Indian, 136.8 ± 22.2%; p=0.42) or genders (males, 158.8 ± 48.5%; females; 140.9 ± 25.3%; p=0.24). The expression of all scavenger receptors changed minimally during foam cell formation and did not reflect the gender and ethnic differences in the prevalence of atherosclerosis. Baseline (before addition of oxLDL) macrophage expression levels of CD36, CD68 and MSRl correlated positively with iv serum LDL-C (p<0.05 for MSRl and CD36) and triglyceride levels (p=0.005; CD68 only), and negatively with HDL-C (p=0.003; MSRl only). Conclusions: This study shows that older subjects have a higher level of foam cell formation, which may relate to their higher level of atherosclerosis. Gender and ethnic differences in foam cell formation were not observed; whilst expression levels of scavenger receptors did not mimic the gender and ethnic differences observed in the prevalence of atherosclerosis. This suggests that in vitro foam cell formation does not mirror in vivo formation or that gender and ethnic differences in the prevalence of atherosclerosis are not due to similar differences in foam cell formation. The small effect of oxLDL on scavenger receptor expression suggests that scavenger receptor levels may increase before foam cell formation occurs i.e. during the transformation from monocytes to macrophages. The relationships observed between serum lipid levels and basal scavenger receptor expressions are novel and may reflect an ability of serum lipids to modulate scavenger receptor expression in macrophages even before exposure to oxLDL.
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science in Medicine to the Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, 2018