Adipocytes
5
Nucleus
HSL
LPL
LPL
TG
Adipocyte
Lipid
droplet
(TG)
Capillary
TG-rich
lipoprotein
LPL
Fed state – High insulin
Nucleus
HSL
LPL
Adipocyte
Lipid
droplet
(TG)
Capillary
cAMP
+
Catecholamines
FA
FA
FA
Albumin
Fasted state – Low insulin
Fig. 1
Lipid storage and mobilization in adipocytes during fed and fasted states. See text for
details. Flux of dietary lipid from circulating lipoprotein particles to the adipocyte lipid droplet
is shown in green. Flux of stored lipid from adipocyte back into circulation shown in blue. TG,
triglyceride; FA, fatty acid; HSL, hormone-sensitive lipase; LPL, lipoprotein lipase. (See color
plate p. xxii).
adipocytes directly. Additionally, fatty acids
can enter adipocytes via transport proteins,
such as CD36 and FATP1. The flow
of free fatty acids is governed by its
concentration gradient. In the case of
adipocytes, free fatty acid (FFA) that enter
cells quickly encounter proteins that bind
FFA, such as members of the fatty acid
binding protein (FABP) family involved
in cytoplasmic transport, or enzymes
involved in metabolic conversion into
triglyceride, such as long chain acyl-
CoA synthase, thereby maintaining a
signi±cant concentration gradient.
2.2
Mobilization and Release of Stored Lipid
into the Circulation
Triglycerides are typically stored within the
adipocyte as a single lipid droplet. Recent
work indicates that the lipid droplet is an
extension of the endoplasmic reticulum
surrounded by a single phospholipid layer.
Closely associated with the lipid droplet
are ‘‘coat proteins’’, especially perilipin
isoforms and caveolin 2. Perilipin is the
most abundant target for phosphorylation
by cyclic AMP-dependent protein kinase
(PK) in fat cells, and plays a key role
in the retention and mobilization of
energy from lipid droplets. The energy
contained in the droplet is liberated when
the triglyceride is hydrolyzed into free
fatty acids and glycerol. The rate-limiting
step in this process is the activity of
hormone-sensitive lipase (HSL) at the
surface of the lipid droplet. Although
several protein kinases
can influence
lipolytic rate, the most signi±cant of these
is cyclic AMP-dependent protein kinase
(PKA). Overall, the rate of lipolysis is
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