Endocrinology Vol. 140, No. 7 2935-2937
Copyright © 1999 by The Endocrine Society
Editorial: Nutrition and Fat Cell Differentiation
Jane E.-B. Reusch
Dwight J. Klemm
Denver VA Medical Center
Denver, Colorado 80220
National Jewish Hospital
Denver, Colorado 80206
Address all correspondence and requests for reprints to: Jane E.-B. Reusch, Denver VA Medical Center, 1055 Clermont Street, Denver Colorado 80220. E-mail: Reuschi{at}den-res.org
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Introduction
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Over the past decade, there has been an explosion
of knowledge about obesity and a great focus on the complex program
involved in fat cell differentiation. The temporally orchestrated dance
between transcription factors that leads to lipid loading and enhanced
insulin sensitivity is fascinating and has increased our understanding
of cell fate determination for adipocytes, as well as serving as a
model for understanding other cellular differentiation programs
(1). It is believed that elucidation of adipocyte biology will
facilitate development of strategies to combat obesity, a major problem
in the U.S. and worldwide. Additionally, because increased adiposity is
associated with insulin resistance and type 2 diabetes, the fat cell
could be the key to unlocking the pathophysiology of those diseases as
well. The article by Wang et al. in this month’s journal
demonstrates the importance of understanding normal adipocyte biology
as a backdrop for the examination of pathophysiology (2). The authors
demonstrate that metabolic signals (glucose and insulin) in excess are
capable of down-regulating CCAAT enhancer binding protein 
(C/EBP), a transcription factor essential for maintenance of
a fully differentiated adipocyte phenotype (2, 3, 4). This is a critical
observation because loss of complete differentiation correlates with
increased insulin resistance and leads to a cavalcade of metabolic
derangements. It also provides some insight into the mechanism whereby
fat cell differentiation agents, peroxisome proliferator activated
receptor 
(PPAR) agonists, lead to improvement in insulin
sensitivity in the setting of hyperglycemia.
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Adipocyte Differentiation Program
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Many reviews have been written about adipocyte differentiation (1, 5). Recently, it has been observed by two groups that a positive
feedback loop occurs between C/EBP and PPAR that is responsible
for maintenance of terminal differentiation (3, 4). If C/EBP or
PPAR are disrupted by molecular strategies before adipocyte
differentiation, cells will not reach maturity or become insulin
sensitive in terms of glucose transport. Many of the genes essential
for maintenance of a well differentiated adipocyte phenotype require
C/EBP including: insulin receptor, insulin receptor
substrate-1, fatty acid synthase, fatty acid binding
protein, and PPAR (5, 6). C/EBP also regulates glucose
transporter 4 (GLUT-4) by a more complex mechanism (7, 8). This
can be reversed by heterodimer formation with the inactive form of
C/EBPß. It has been suggested that loss of C/EBP could account for
adipocyte immaturity and insulin resistance. In most instances, the
proposed model has been incomplete differentiation of adipocytes in
culture (3, 4). The report by Wang et al. (2) suggests a
mechanism whereby an abnormal metabolic environment (high glucose in
the presence of insulin) can affect mature adipocytes leading to
partial differentiation to an insulin-resistant, immature adipocyte
(2).
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Adipocyte Differentiation and Insulin Sensitivity
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During adipocyte differentiation, immature adipocytes are exposed
to agents that induce a series of transcription factors in a specific
sequence. Initially, C/EBPß is induced, which induces PPAR and
subsequently C/EBP (1, 5, 9, 10). C/EBP and PPAR positively
regulate each other’s expression, and this feedback loop maintains fat
cell phenotype (3, 4, 5). One critical characteristic of mature adipocyte
phenotype is the induction of GLUT-4 and distribution of glucose
transporters to insulin-responsive vesicles. Only when transporters are
expressed and appropriately localized will insulin sensitivity occur.
Expression and redistribution of GLUT-4 does not occur in C/EBP
deficient cell lines (3, 4). C/EBP is necessary for GLUT-4
transcription (11, 12). It has been demonstrated in vitro
with correlative studies in vivo that cytokines such as
tumor necrosis factor (TNF) can induce insulin resistance in
adipocytes and that at least one mechanism of this resistance is a
decrease in GLUT-4 content. TNF appears to act by permitting
C/EBPß to enter the nucleus and heterodimerize with C/EBP or to
form C/EBPß homodimers (8). In this model, GLUT-4 expression is
regulated by C/EBPß translocation rather than C/EBP content.
Regardless of the mechanism, TNF is able to induce insulin
resistance and an effect that can be reversed by TNF antagonists.
Interestingly, adipocytes produce TNF. One mechanism whereby
increased adiposity leads to insulin resistance is postulated to be fat
cell production of TNF.
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Insulin Sensitizers and Fat Cell Differentiation
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Given this correlation between fat cell mass and TNF, it has
been intriguing to watch the development of the thiazolidinedione class
of antidiabetic agents and to unravel the relationship between
adipocyte differentiation and whole animal insulin sensitivity. From a
cell biology perspective, it is well established that adipocytes do not
become insulin sensitive to glucose transport until late in the
differentiation program (3, 4). So, it makes sense that agents capable
of increasing adipocyte differentiation would improve insulin
sensitivity. On the other hand, adipocyte differentiating agents will
recruit new adipocytes. This should lead to increased adiposity and
insulin resistance. The data presented by Wang et al.
suggest that derangements in nutrient availability can alter a critical
protein essential for the maintenance of adipocyte differentiation (2).
If that is the case, this could lead to insulin resistance and a
differentiating agent, such as a thiozolidinedione, could restore
differentiation and thus enhance insulin sensitivity. Perhaps we should
reframe our view of diabetes and insulin resistance as a consequence of
excess adipocity and consider instead a state of altered adipocyte
differentiation.
The induction of insulin resistance as a consequence of metabolic fuel
derangements (glucose and free fatty acids) is an established
phenomenon in animal and human models of insulin resistance and
diabetes. Wang et al. present data in vitro and
in vivo that a decline in C/EBP, a primary determinant of
adipocyte phenotype, occurs under metabolic conditions common to
insulin resistance and diabetes (2). This could be responsible for the
decrease in insulin receptor and insulin receptor substrate-1
that has been noted in animal models of insulin resistance and
hyperglycemia as both of these proteins are regulated by C/EBP.
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Transcription Factors and Adipocyte Health
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As detailed earlier, C/EBP and PPAR form a positive feedback
loop that maintains adipocyte differentiation (3, 4). Each
transcription factor positively affects the expression of the other,
either directly or indirectly. A number of laboratories have
demonstrated that phosphorylation of PPAR alters its transactivating
potential and leads to insulin resistance (13). Hyperglycemia and
intracellular reactive oxygen species can activate MAP kinase and
stimulate PPAR phosphorylation. Similarly, the authors of the paper
in this month’s journal correlate diminution of C/EBP with insulin
resistance. They demonstrate a mechanism whereby nutrient stress
down-regulates C/EBP, the other key regulator of adipocyte insulin
sensitivity (2). Down-regulation of C/EBP content or PPAR
function could be expected to impact the other transcription factor.
This expectation is fulfilled in the in vivo studies
undertaken by Wang et al. (Fig. 6) (2).
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C/EBP and Cellular Proliferation
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One of the main motivations behind the explosion of new
information regarding adipocyte biology is a desire to understand
obesity and regulation of fat stores. The hope was that understanding
adipocyte development would foster strategies for prevention of fat
cell accumulation and the related medical consequences. As the story of
adipocyte differentiation emerged and the sequential induction of
C/EBPß followed by PPAR and later C/EBP, questions arose as to
the sufficiency and necessity of each of these events for
differentiation. To test the importance of each of these transcription
factors for differentiation, molecular tools to introduce either active
or dominant negative forms of these molecules were created. In
the case of C/EBP, a large methodological problem arose. Stable cell
lines were difficult to create because C/EBP blocked mitosis by the
induction of growth arrest and DNA damage inducible protein 153
and DNA-protein kinase interacting protein 21 (14) leading to
cell cycle arrest. C/EBP arrest of cell cycle is considered a key
step in adipocyte terminal differentiation. Down-regulation of C/EBP
could present an opportunity for release into cell cycle and adipocyte
proliferation. Theoretically, insulin resistance should not be
associated with weight gain as the ability to store fuel is impaired.
Despite that logic, insulin resistance and obesity go hand in hand.
Perhaps, C/EBP down-regulation is a permissive step for adipocyte
proliferation. Thus, the response of an adipocyte to excess fuel is to
develop both insulin resistance to glucose transport and proliferative
capacity.
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C/EBP and Whole Animal Physiology
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As anticipated from observations with adipocytes in culture, Wang
et al. (2) find a tight correlation between down-regulation
of C/EBP and decreased PPAR content. Somewhat unexpected is the
fat depot specific down-regulation in epididymal fat pad with a
relative sparing of the omental fat pad. Visceral fat, represented in
this paper by omental fat, has been shown to correlate with insulin
resistance, diabetes, and mortality from atherosclerotic disease (1).
Perigonadal fat, represented in this paper by epididymal fat, is also a
hormonally active fat depot, but studies have not clearly delineated a
relationship between this fat depot and insulin resistance or
cardiovascular risk. For the purpose of this discussion, we will treat
this epididymal fat depot as representative of peripheral as opposed to
central adiposity, which may not be accurate. With the fat pads defined
in this way, there is a clear difference in the regional fat pads with
regards to the induction of an insulin resistant phenotype. This would
be expected to be associated with a more mild insulin resistance than
if the visceral fat pad were affected. It would also suggest that the
peripheral fat pad would be the target for the PPAR agonist type of
differentiating agents. Thiazolidinediones, in animal models, increase
peripheral adipose mass with a relative sparing of visceral adipose
stores (Smith, S., SmithKline Beecham-United Kingdom, personal
communication). This is not to suggest that the omental fat pad is
completely spared, indeed the combination of insulin and glucosamine
clearly decreases PPAR and appears to decrease C/EBP in Fig. 6 of
the article by Wang et al. in this issue of
Endocrinology (2). The critical point to be made by these
distinctions is that different fat depots respond differently to
metabolic stress and thus could be expected to respond differently to
hormonal or pharmaceutical manipulations.
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C/EBP and Nutrition
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There is no dispute that there is a relationship between excessive
nutrition and adiposity. What has remained more oblique is the
relationship between insulin resistance, fat cell differentiation, and
the tendency for insulin-resistant animals and humans to gain adipose
mass. The dual function of C/EBP as an inhibitor of cellular
proliferation and a key determinant of terminal fat cell
differentiation places this transcription factor in a unique position
to have important pathophysiological consequences if disregulated. The
paper by Wang et al. supports this as one mechanism whereby
hyperglycemia induces insulin resistance (2).
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Future Directions
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Much has been learned recently about glucose regulation of
transcription, and defining the specific mechanism of this
hyperglycemia-related decrease in transcription of C/EBP could
provide a target for pharmaceutical intervention. The precise mechanism
of glucose-induced C/EBP down-regulation remains to be defined. The
data demonstrating an impact of glucosamine and inhibition of the
effect in the face of GFAT inhibitors directs one’s attention to the
glucosamine pathway. Evaluation of the C/EBP promoter and defining
glucose regulatory elements is a clear area for future study. The
manuscript by Wang et al. provides an exciting lesson on how
understanding of normal adipocyte differentiation permits meaningful
interpretation of this pathological response to nutrient excess.
Received April 26, 1999.
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Introduction
Adipocyte Differentiation...
