A novel role for long-chain acyl-CoA synthetase-4 (ACSL4) in adipocyte biology.
Abstract: The rate
of adult obesity in the United States is approximately 35%. This is a condition that
predisposes to the development of type 2 diabetes mellitus (T2DM). In order to
ameliorate the remarkable prevalence of obesity and T2DM, it is necessary to develop a
greater understanding of the pathways that lead to obesity. While regulation of fatty
acid (FA) metabolism is central to adip... read moreocyte dysfunction due to diet-induced obesity,
evidence is limited on the pathways that direct the incorporation of FA into
phospholipids (PL), neutral lipids like triacylglycerol (TAG), and into a variety of
bioactive lipid mediators. Of interest, the long-chain acyl-CoA synthetases (ACSL)
family of enzymes catalyze the addition of a coenzyme-A (CoA) group to FA to form fatty
acyl-CoAs and are hypothesized to direct fatty acyl-CoAs to distinct metabolic fates.
Specifically, ACSL4 has been hypothesized to modulate the metabolic fates of
polyunsaturated FA (PUFA), including arachidonic acid (AA). Expression of ACSL4 is less
abundant in adipocytes than other ACSLs, but its enzymatic activity is directly
inhibited by the antidiabetic drugs thiazolidinediones (TZDs) through
PPAR-ɣ-independent mechanisms. However, to date, there are no data reporting the
in vivo actions of ACSL4. The purpose of these experiments was to determine the role of
ACSL4 in regulating adipocyte metabolism and its subsequent effects on obesity. We
generated a novel mouse model utilizing LoxP-Cre technology to specifically ablate ACSL4
in adipocytes in order to characterize both in vivo and cellular mechanisms of ACSL4's
effects, particularly in the context of diet-induced obesity (DIO). We fed a high fat
diet (HFD) to mice with adipocyte-specific ablation of ACSL4 (Ad-KO) and demonstrated
that they were protected against DIO and its associated metabolic dysfunctions.
Adipocytes from Ad-KO mice fed the HFD had reduced incorporation of AA into PL and free
fatty acid pools and production of lipid peroxidation products, like 4-hydroxynonenal
(4-HNE). Consistent with the known actions of 4-HNE, adipocytes from Ad-KO mice had
reduced p53 activation and increased adipocyte oxygen consumption. This work presents
the first in vivo elucidation of ACSL4's actions. Interestingly, we also observed that
Ad-KO mice fed a HFD had significantly reduced levels of serum total cholesterol.
Therefore, we explored potential mechanisms that protect Ad-KO mice against high
circulating cholesterol on a HFD. Results demonstrated that Ad-KO mice had lower serum
low-density lipoprotein (LDL) and high-density lipoprotein (HDL), and they accumulated
more cholesterol in their adipose tissue. Gene expression data in isolated adipocytes
suggested that these actions were mediated through a non-sterol-dependent upregulation
of low-density lipoprotein receptor (LDLR), which takes up lipoproteins from circulation
into adipose tissue and dramatically downregulates sterol regulatory element-binding
protein 2 (SREBP2) and de novo cholesterol synthesis gene expression in adipocytes. This
study is the first data to link ACSL4 to adipocyte cholesterol homeostasis. Overall,
results from these experiments (1) provided a link between diet-induced obesity, adipose
tissue inflammation, and adipocyte dysfunction; (2) identified ACSL4 as a regulator of
adipocyte metabolism and obesity through its regulation of AA and linoleic acid (LA) in
vivo; and (3) described a novel relationship between ACSL4 and LDL receptor in DIO
adipocyte cholesterol metabolism. This work has identified adipocyte ACSL4 as a novel
target to prevent adipocyte dysfunction in the context of
Thesis (Ph.D.)--Tufts University, 2016.
Submitted to the Dept. of Biochemical and Molecular Nutrition.
Advisor: Andrew Greenberg.
Committee: Stefania Lamon-Fava, and Nirupa Matthan.
Keywords: Nutrition, Cellular biology, and Physiology.read less