Adipocytes
7
Tab. 1
Metabolically active proteins secreted by adipocytes.
Protein/hormone
Physiological effects
Leptin
Appetite, autonomic nervous activity
Adiponectin
Insulin sensitivity, fatty acid oxidation
Resistin
Insulin sensitivity
TNF-
α
Insulin sensitivity, adipocyte differentiation, inflammation
ANG II
Lipogenesis, blood pressure
ASP
Lipogenesis
IGF
Lipogenesis, adipocyte differentiation
Interleukin-6
Lipolysis in adipocytes, inflammation
Kinins
Insulin sensitivity, tissue remodeling
PAI-1
Insulin sensitivity, blood clotting, atherosclerosis
TGF-
β
Lipolysis, angiogenesis
Notes
: ANG: angiotensin; ASP: acylation-stimulating protein; IGF: insulin-like
growth factor; PAI-1: plasminogen activator inhibitor; TNF: tumor necrosis factor;
TGF: transforming growth factor. Table adapted from Schling, P., Loffler, G.
(2002) Cross talk between adipose tissue cells: impact on pathophysiology,
News
Physiol. Sci.
17
, 99–104.
this system requires communication be-
tween adipose tissue and essentially all
the organ systems in the body. In addi-
tion to the hormonal and neural signals
that regulate lipid uptake and storage,
or induce lipolysis and release of fatty
acids into the circulation during a fast
(discussed above), there are signals that
originate in adipose tissue that act to
modify various physiological activities in
tissues and organs throughout the body.
For example, to insure that dietary in-
take is suf±cient to maintain an adequate
level of adiposity, there must be com-
munication between adipose tissue and
the centers in the brain that control ap-
petite. Likewise, there is communication
between adipose tissue and the organs,
and the tissues that utilize fat for en-
ergy to insure that fatty acid delivered to
nonadipose tissues, such as muscle and
liver, are handled properly and do not ac-
cumulate to abnormal levels. One of the
major advances in metabolic research in
the last few years has been the discovery
that adipocytes secrete hormones that act
at speci±c sites in the body and have im-
portant effects on many aspects of energy
metabolism. This new understanding of
adipose tissue as an endocrine organ has
dramatically changed our understanding
of the signi±cance of adipocytes in the
regulation of metabolism. It is now be-
lieved that the adipose-derived hormones
(referred to as adipokines) are important
components of the integrated system of
hormonal and neural signaling pathways
that function to regulate the storage and
use of metabolic energy. The following is
a brief summary of the biology of four
of the adipokines that have relatively clear
effects on metabolism. Table 1 presents a
more complete list of hormone like pro-
teins produced by adipocytes.
The seminal contribution to the concept
that adipose tissue produces hormones
with important metabolic effects was made
by Friedman and colleagues in 1994. These
investigators identi±ed the protein prod-
uct of the
obese
(
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