Circumventing leptin resistance for weight control.
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ABSTRACT: There is increasing evidence of a causal relationship between sleep-disordered breathing and metabolic dysfunction. Metabolic syndrome (MetS), a cluster of risk factors that promote atherosclerotic cardiovascular disease, comprises central obesity, insulin resistance, glucose intolerance, dyslipidemia, and hypertension, manifestations of altered total body energy regulation. Excess caloric intake is indisputably the key driver of MetS, but other environmental and genetic factors likely play a role; in particular, obstructive sleep apnea (OSA), characterized by intermittent hypoxia (IH), may induce or exacerbate various aspects of MetS. Clinical studies show that OSA can affect glucose metabolism, cholesterol, inflammatory markers, and nonalcoholic fatty liver disease. Animal models of OSA enable scientists to circumvent confounders such as obesity in clinical studies. In the most widely used model, which involves exposing rodents to IH during their sleep phase, the IH alters circadian glucose homeostasis, impairs muscle carbohydrate uptake, induces hyperlipidemia, and upregulates cholesterol synthesis enzymes. Complicating factors such as obesity or a high-fat diet lead to progressive insulin resistance and liver inflammation, respectively. Mechanisms for these effects are not yet fully understood, but are likely related to energy-conserving adaptations to hypoxia, which is a strong catabolic stressor. Finally, IH may contribute to the morbidity of MetS by inducing inflammation and oxidative stress. Identification of OSA as a potential causative factor in MetS would have immense clinical impact and could improve the management and understanding of both disorders.ILAR journal / National Research Council, Institute of Laboratory Animal Resources 02/2009; 50(3):289-306. · 1.58 Impact Factor
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ABSTRACT: We have examined the dose-dependent effects and central action of intraventricular administration of a recombinant adeno-associated virus encoding rat leptin (rAAV-leptin) in suppressing body weight (BW) gain in adult female rats. A low dose of rAAV-leptin (5x10(10) particles) suppressed weight gain (15%) without changing daily food intake (FI), but a twofold higher dose decreased BW by 30% along with a reduction in daily FI. Reduced BW was due to a loss in body adiposity because serum leptin was reduced. Serum insulin levels were decreased (96%) by only the high dose along with a slight reduction in glucose. Uncoupling protein-1 (UCP-1) mRNA expression in brown adipose tissue (BAT), reflecting energy expenditure through thermogenesis, was upregulated to the same magnitude by the two rAAV-leptin doses. We analyzed by in situ hybridization the expression in the hypothalamus of genes encoding the appetite-regulating neuropeptides. Only the high dose decreased expression of neuropeptide Y (NPY), the orexigenic peptide, and increased proopiomelanocortin (POMC), precursor of the an orexigenic peptide, alpha-MSH. Our studies show for the first time that increased availability of leptin within the hypothalamus through central leptin gene therapy dose-dependently decreases weight gain, adiposity, and serum insulin by increasing energy expenditure and decreasing FI. The decrease in FI occurs only when NPY is reduced and alpha-MSH is increased in the hypothalamus by the high dose of rAAV-leptin. Delivery of the leptin gene centrally through rAAV vectors is a viable therapeutic modality for long-term control of weight and metabolic hormones.Molecular Therapy 09/2001; 4(2):139-45. · 7.04 Impact Factor
Article: Inward currents in smooth muscle[Show abstract] [Hide abstract]
ABSTRACT: Electrophysiological studies of isolated smooth muscle cells have revealed a variety of membrane-conductance changes that contribute to depolarization and excitation. These include activation of inward Cl− and nonselective cation currents, and the suppression of several types of outward K+ currents, leading to depolarization and opening of voltage-dependent Ca2+ current. In most cases, several of these mechanisms operate together in the same cell. We now recognize that [Ca2+]u plays a critical role not only in initiating contraction but in regulating membrane-channel activity. Cl− current is Ca2+ activated, and muscarinic nonselective cation current is facilitated by Ca2+. Cl− and nonselective cation channels therefore participate in a positive feedback loop, where elevation of [Ca2+]; by entry across the membrane or by release from internal stores initiates or promotes depolarization, which in turn leads to further influx of Ca2+ through voltage-dependent Ca2+ channels. These features of excitation of smooth muscle differ from established views, where depolarization is the primary event causing Ca2+ entry and contraction.
Circumventing leptin resistance for weight control
Satya P. Kalra*
Departments of Neuroscience and Physiology, College of Medicine, University of Florida McKnight Brain Institute, Gainesville, FL 32610-0244
and the pathophysiology of obesity, there
are two outstanding insights. One is en-
hanced knowledge of the neuroanatomy
and neurochemistry of various components
of the appetite regulation and energy ex-
penditure networks in the hypothalamus,
and the other is characterization of the
neurobiology of leptin in regulating these
two hypothalamic networks (1, 2). Leptin,
an adipocyte-derived hormone normally
transported across the blood brain barrier
(BBB), was initially heralded as a major
negative feedback signal relaying informa-
tion of the body’s energy stores to these
hypothalamic networks on a moment to
moment basis to maintain body weight
around a set point. However, results of
numerous investigations in rodents and hu-
mans have been disappointing because cir-
culating leptin levels display marked varia-
tions among individuals and rise in direct
proportion to the age-related increase in
adipose tissue depots. Thus, leptin concen-
trations are greatly elevated in obese sub-
jects (2). Consequently, despite the promis-
experiments in leptin-deficient mice and
patients, endogenous hyperleptinemia
could neither curb appetite nor augment
energy expenditure in normal subjects ex-
periencing an increase in the rate of weight
gain. It soon became apparent that the
priately termed leptin resistance, develops
rapidly and that leptin therapy even at sup-
raphysiological concentrations is largely in-
effective in reducing the body weight of
clinically obese patients (3). Although these
revelations dampened the enthusiasm of
clinicians and investigators in academia and
industry alike, they presented a challenge to
devise newer therapeutic strategies that
would curtail the environmentally based
increase in the rate of weight gain and the
tries (4). The knowledge accrued over sev-
eral years that cytokines readily cross the
an action within the hypothalamus (5), and
the serendipitous finding that ciliary neuro-
trophic factor (CNTF) treatment of amyo-
tropic lateral sclerosis patients for neuro-
tropic benefits produced severe anorexia
and weight loss (6), an outcome later repli-
ent that CNTFAx15 mobilizes intracellular
n the avalanche of recent information on
the brain’s control of energy homeostasis
cated in rodents (7, 8), presented a new
avenue for therapeutic exploration. The pa-
per by Lambert et al. (9) in this issue of
PNAS has extended these findings by dem-
onstrating the efficacy of a CNTF deriva-
tive, CNTFAx15, in correcting obesity and
dependent metabolic disorders in mice. Re-
sults show that in leptin-deficient ob?ob
mice and normal mice rendered obese and
diet, CNTFAx15 treatment normalized the
obese phenotypes. Furthermore, it is appar-
signal transduction pathways in the hypo-
thalamus that are similar to those activated
by leptin, and not by interleukin-1, the pro-
totype cytokine. This explains why the
weight-reducing effects of CNTFAx15 are
free of side effects, such as fever, taste
aversion, and metabolic abnormalities that
are generally evoked by cytokines.
See companion article on page 4652.
energy expenditure. NPY, AgrP, and GABA, the appetite-stimulating signals, are coproduced in the
in the POMC?CART coexpressing perikarya in the ARC. These distinct populations of neurons project into
the two subdivisions of the PVN, the mPVN and pPVN, to activate their corresponding receptors for
regulation of appetite and energy expenditure. NPY-producing neurons also contact the POMC?CART
neurons locally in the ARC to curtail their tonic restraint on appetite. Leptin inhibits appetite through
leptin-Rb located on the NPY?AgrP?GABA- and POMC?CART-expressing cell bodies in the ARC and at
postsynaptic sites in the PVN where it regulates release of these signals and enhances energy expenditure
through the sympathetic nervous system. The results show that CNTF?CNTFAx15, through activation of the
CNTFR? located in the ARC and PVN, markedly diminish the availability of NPY for release in the PVN by
suppressing its synthesis in the ARC and concurrently attenuating postsynaptic NPYergic signaling by
decreasing NPY Y1 receptor and pCREB abundance.
Schematic representation of hypothalamic signaling pathways in the regulation of appetite and
April 10, 2001 ?
vol. 98 ?
no. 8 ?
petite and reduce body weight? The urge to
replenish the body’s depleted energy stores
is chemically coded in the appetite-regulat-
ing network resident in the arcuate and
paraventricular nuclei (ARC-PVN) of the
hypothalamus (ref. 1; Fig. 1). Basically, the
drive to restore energy stores is elicited by
the augmented release of appetite-stimulat-
ing signals, primarily the orexigenic signal,
appetite-inhibiting signals, such as the an-
orexigenic melanocortin, ?-melanocyte
stimulating hormone (?-MSH; refs. 1 and
10). Whereas the NPY- and ?-MSH-
producing neurons reside in the ARC, their
receptors (Y1 and Y5 receptors for NPY
and MC-4 receptors for ?-MSH) are local-
ized at their release sites in the magnocel-
lular PVN (mPVN) and parvocellular PVN
(pPVN), respectively (11–14). Interestingly,
stimulation of feeding by NPY is supple-
mented by two additional signals that
are coexpressed with NPY, agouti-related
feeding by com-
tion of ?-MSH
at MC-4 recep-
tors, and ?-ami-
GABAAreceptors in the mPVN (14–16).
Similarly, the peptide cocaine- and amphet-
amine-regulating transcript (CART), which
is coexpressed with ?-MSH in POMC pro-
ducing neurons, supplements the ?-MSH-
induced restraint on appetite (1, 2). Addi-
tionally, morphological and experimental
evidence suggests that NPY may directly
modulate POMC neurons (1, 17, 18). These
two connected, but functionally opposed
neuronal systems, NPY and POMC, are
leptin targets as evidenced by the presence
of the functionally active, long isoform
of the leptin receptor, leptin-Rb, on these
neurons (1, 2).
Although CNTF is not a secretable pro-
tein it can readily penetrate the BBB in a
manner similar to leptin (19). Whereas the
CNTF receptor, CNTFR?, is abundant in
feeding relevant sites, such as the ARC and
PVN in the adult hypothalamus (20, Fig. 1),
CNTF ligand expression is undetectable,
leaving open the possibility of a CNTF-
related ligand operating in these sites (21).
Administration of CNTF to wild-type lab-
oratory rats and mice rapidly induced an-
orexia and weight loss, primarily due to loss
of body fat, that lasted for a few days even
after cessation of treatment (20, 22). Impor-
tantly, CNTF?CNTFAx15 readily normal-
ized the obese phenotype in leptin-deficient
ob?ob mice, in db?db mice with impaired
signaling due to mutated leptin receptor, in
MC-4 receptor-deficient mice, and in nor-
mal wild-type mice rendered obese and lep-
tin-resistant by a high-fat diet (9, 20, 22, 23).
Thus, CNTF?CNTFAx15appear to be more
efficient than leptin because body weight
was reduced in conditions where leptin was
ineffective. That CNTF?CNTFAx15are suit-
able substitutes for leptin is also implied by
the findings that, like leptin, they diminish
hypothalamic NPYergic signaling by sup-
pressing NPY gene expression in the ARC
(ref. 9; Fig. 1). A second site of action is at
the postsynaptic level in the PVN because
CNTF?CNTFAx15 treatment suppressed
NPY-induced feeding [likely a result of di-
minished NPY Y1 receptor abundance (22,
24)] and, as shown by Lambert et al. (9),
diminished pCREB in the PVN.
CNTF belongs to the class 1 superfam-
ily of cytokines well known for their pleio-
tropic actions (25, 26). Cytokines are
causal factors for anorexia, weight loss,
and metabolic breakdown leading to ca-
chexia, the typical symp-
toms associated with
cancer, chronic infec-
tion, and prolonged im-
mune reaction (27). This
intrinsic pleiotropic na-
ture of cytokines is due
to the common signal
transduction sequalae in
target cells. However, it
is now obvious that these severe symptoms
are rarely attributed to one single cyto-
kine, but rather to the concerted action of
several cytokines. Nevertheless, it is rea-
sonable to suspect that administration of
CNTF or CNTFAx15may elicit some, if not
all, cytokine-like symptoms. Untoward
side effects, such as fever, taste aversion,
and metabolic effects, were not observed
when CNTF or CNTFAx15were adminis-
tered in doses effective in reducing weight
(9, 20). Concomitant central infusion of
NPY with CNTF completely reversed the
anorexic and weight-reducing effects of
CNTF (28), reiterating the specificity and
safety of CNTF. Despite these assurances,
careful evaluation of the proinflammatory
and cytotoxic effects of chronic systemic
CNTF?CNTFAx15 treatment is clearly
tin activates cytokine-like signal transduc-
the leptin-Rb in hypothalamic sites impli-
cated in energy homeostasis (9, 20, 29).
Additionally, leptin activates Suppressor of
Cytokine Signaling-3 (SOCS-3), which re-
duces intracellular signaling by inhibiting
JAK activity. Excessive expression of
SOCS-3 has been proposed as a potential
mechanism underlying leptin resistance by
thereby permitting overeating and reduc-
also markedly increased SOCS-3 along with
the continued severe reduction in appetite
and weight (29). Consequently, SOCS-3 ac-
tivation is unlikely to be the major signal
transduction event underlying leptin resis-
pathways activated by leptin, it is highly
likely that independent, and yet to be de-
fined, CNTFR?-induced cellular and intra-
cellular signal transduction events may un-
derlie appetite and weight reduction. That
disparate hypothalamic mechanisms oper-
ate after CNTF and leptin is also evident by
the fact that, whereas leptin reduces weight
by decreasing appetite through modulation
of the appetite-regulating network and by
of the energy expenditure network linked
with sympathetic nervous system (refs. 1
and 2; Fig. 1), so far only selective diminu-
tion of appetite via NPYergic signaling has
been identified as the mechanism responsi-
With the increase in the incidence of
obesity and abnormal weight gain and
associated metabolic disorders such as
type II diabetes and cardiovascular dis-
eases, the need for newer therapeutic
strategies to stem this epidemic continues
to grow. A rational approach may be to
exploit the anorectic and weight-reducing
properties of the naturally occurring cy-
tokines that readily gain access to hypo-
thalamic sites by saturable transport
nature of cytokines and leptin, overlap-
ping signal transduction pathways, and the
cross talk among cytokines, redesigning
the chemical structure of the cytokines to
reduce or eliminate side effects is highly
desirable. Application of CNTF and its
derivative, CNTFAx15, as weight-reducing
therapy is a step in that direction. An
alternative approach that deserves serious
consideration and has the potential for
sustained outcome is gene therapy to de-
liver the transgenes that encode desirable
cytokines. Indeed, one-time delivery of
viral vectors encoding transgenes for lep-
either systemically or directly into the
hypothalamus in a site-specific manner to
minimize the undesirable side effects of
to be effective in reducing body weight for
extended periods (30, 31).†‡Admittedly,
†Dhillon, H., Minter, R., Topping, D., Prima, V., Moldawer,
L., Muzyczka, N., Kalra, P., Kalra, S. & Zolotnkhin, S., 2nd
Annual Meeting of the American Society for Gene Ther-
apy, Washington, DC, June 9–13, 1999, abstr. 178.
‡Beretta, E., Prima, V., Dhillon, H., Moldawer, L.L., Muzyc-
CO, May 31–June 4, 2000, abstr. 186.
The paper by Lambert et al.
demonstrates the efficacy of
CNTFAx15in correcting obesity
in leptin-resistant mice.
the interventional strategies that have be-
gun to exploit the diversity in cytokine
action are in their infancy, and many
hurdles remain to be overcome before
successful application to control human
obesity is contemplated.
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