Regulation of Monocyte/Macrophage ATP-Binding Cassette Transporters and Cholesterol Efflux by Elevated Glucose and Free Fatty Acid Concentrations.
with diseases of poor glycemic control, including pre-diabetes and type 2 diabetes
mellitus, are at increased risk of developing atherosclerosis, the major form of
cardiovascular disease (CVD). Atherosclerotic lesion formation is promoted by
monocyte/macrophage recruitment into the arterial wall and subsequent cholesterol
accumulation in macrophages. However, the relatio... read morenship between elevated circulating
glucose and free fatty acids (FFA) concentrations, frequently associated with poor
glycemic control, and macrophage cholesterol accumulation is yet to be fully elucidated.
We hypothesized that factors which regulate monocyte/macrophage cholesterol efflux,
particularly ATP-binding cassette (ABC) transporter expression will be down-regulated in
response to elevated glucose and FFA conditions in vitro and in vivo, mediated by
liver-x-receptor (LXR)-α and sterol regulatory element binding protein (SREBP)-1c.
The goal of this thesis was to determine the effect of elevated glucose and FFA
concentrations on factors mediating macrophage cholesterol homeostasis using two
different systems: human monocytes and cultured murine bone marrow-derived macrophage
cells (BMDM). Human monocytes were isolated from individuals in the fasting state and
after an oral glucose challenge to determine the relationship between elevated blood
glucose concentration and changes in monocyte ABC-transporter (ABCA1 and ABCG1) mRNA
expression. Additionally, murine BMDM were used as a model system to determine the
effect of elevated glucose and FFA concentrations on factors governing macrophage
intracellular cholesterol accumulation and cellular cholesterol efflux through
ABC-transporters and scavenger receptor (SR)-B1. LXR-α and SREBP-1a, -1c and -2
were also targeted for their potential role as transcriptional mediators of transporters
involved in macrophage cholesterol efflux, specifically with respect to glucose and FFA
concentrations. In humans a glucose challenge increased leukocyte ABCA1 and ABCG1 mRNA
expression but had no significant effect on monocyte ABCA1 or ABCG1 mRNA expression. We
also determined that leukocyte ABC-transporter mRNA expression was significantly higher
than PBMC or monocytes in vivo. However, in an in vitro macrophage model, BMDM,
cholesterol efflux was suppressed after exposure to elevated glucose concentrations, a
FFA mixture (linoleic acid [18:2], palmitic acid [16:0], and oleic acid [18:1]) as well
as 18:2 alone, but not 16:0 alone. Elevated glucose concentrations did not have a
significant effect on ABC-transporter mRNA or protein expression in oxidized low density
lipoprotein (oxLDL)-stimulated BMDM. In contrast, ABC-transporter and SREBP-1c mRNA
expression and ABCA1 protein expression was suppressed by 18:2. Neither LXR-α or
SREBP-1 protein expression was affected by 18:2, indicating that changes in expression
of these transcription factors are not mediating the effect. The results of this
research will add to our understanding of the relationship between glucose and FFA
concentrations, and monocyte/macrophage cholesterol homeostasis, in vivo and in vitro.
These data will advance efforts to gain perspective on novel mechanisms by which
diseases of poor glycemic control accelerate the development of
Thesis (Ph.D.)--Tufts University, 2013.
Submitted to the Dept. of Biochemical and Molecular Nutrition.
Advisor: Alice Lichtenstein.
Committee: Andrew Greenberg, Stefania Lamon-Fava, Nirupa Matthan, and Martin Obin.
Keywords: Nutrition, Biochemistry, and Molecular biology.read less