The clustering of dyslipidaemia, hypertension, and glucose intolerance has been recognised
for 4 decades. However, the Metabolic Syndrome gained publicity and prominence following
Reaven's Lilly Lecture in 1988 [1]. Since that time, the number of component features has expanded to include other
lipid disturbances, elevated levels of proinsulin, and abnormalities of coagulation
and fibriolysis. More recently, several other features have been described in this
cluster, including markers of low grade inflammation (such as C-reactive protein),
microalbuminuria, and endothelial dysfunction. And it is these characteristics which,
perhaps unlike hypertension and dyslipidaemia, challenge the tenet that the central
aetiological factor in the cluster is insulin resistance.
We have shown, using a technique of Z scores, that in 107 healthy subjects, there
are close correlations between 8 metabolic syndrome variables and 4 of acute phase
activation [2]. Furthermore, the acute phase score correlated with that of 7 markers of endothelial
activation. In such a model, there was no residual relation-ship of the metabolic
syndrome score and the endothelial score. These findings suggest that low grade inflammation
might underlie both the clustering of the Metabolic Syndrome, including the presence
of insulin resistance, and its relationship with vascular damage. There are, moreover,
strong biological grounds for these epidemiological observations. In a recent paper,
mice in which constitutively active IKK-β was expressed in hepatocytes, producing
overexpression of liver NFκB, showed insulin resistance both in liver and skeletal
muscle [3], implying the existence of a signal originating from the ‘inflamed’ liver.
These observations raise the question as to the origin of the low grade inflammatory
state in these healthy subjects. We explored the possibility of chronic infection
with Helicobacter, Chlamydia, or cytomegalovirus, but relationships between antibody
titres and inflammatory markers were weak. Much stronger were the correlations between
measures of obesity, particularly central fat, and those of acute phase proteins [2]. In these subjects around 15-20% of the variance of the acute phase score was statistically
dependent upon obesity. This led to our proposing the hypothesis that the generation
of an inflammatory signal from adipose tissue underlay the clustering of metabolic
syndrome variables, including insulin resistance, with endothelial dysfunction, and
perhaps vascular disease ([Fig. 1]).
Fig. 1 The role of central obesity and adipocytokines in the clustering of ‘Metabolic Syndrome’
risk factors.LDL: low density lipoprotein; TG: triglycerides; HDL: high density lipoprotein;
NEFA: nonesterified fatty acids; PAI-1: plasminogen activator inhibitor-1; IGT: impaired
glucose tolerance; DM: diabetes mellitus; CRP: C-reactive protein; H/T: hypertension;
CHD: coronary heart disease. Based on figure in: Yudkin JS. Insulin resistance and
the Metabolic Syndrome - on the pitfalls of epidemiology. Diabetologia 2007; 50: 1576-1586:
With kind permission of Springer Science and Business Media.
This now raises a new dilemma: how do the liver, and skeletal muscle, and blood vessels
know that one is fat? The likelihood is that fat mass signals to distant tissues and
organs via the medium of a circulating signalling molecule. Nonesterified fatty acids
may well play such a role, but more recently substantial interest has focused on the
potential role of adipocytokines. Indeed the release of fatty acids may be dependent
on the degree of low grade inflammation in the adipose tissue. A number of potential
candidate adipocytokines may induce muscle and liver insulin resistance, including
interleukin-6 (IL-6), resistin, leptin, retinol binding protein-4 [4], and (inversely) adiponectin. However, the cytokine which has been best characterized
as downregulating the insulin signalling pathway, and also producing endothelial activation,
is tumour necrosis factor-α (TNF-α). Yet we have found no net release of TNF-α from
an adipose issue bed [5], suggesting that, other than in severe inflammatory illness, it is unlikely to play
a major endocrine role.
It has been recognised that in conditions of calorie excess, fat accumulation occurs
both in hepatocytes and in skeletal muscle fibres, and this may play a major role
in insulin resistance in these tissues. Obese subjects show endothelial dysfunction,
including resistance to insulin-mediated vasodilatation, so implying vascular insulin
resistance. Indeed it has been suggested that this component part of insulin resistance
may impede the ability of the hormone to augment its delivery, and that of substrate,
to skeletal muscle in the postprandial state [6]. Such action may not require an increase in total limb blood flow, but might represent
the diversion of flow from non-nutritive to nutritive circuits, something of which
insulin is capable in a short timeframe and in low physiological concentrations [7].
With colleagues in Amsterdam, we have been exploring insulin's effect on the vasculature,
using a model of an ex vivo first order arteriole isolated from a rat cremaster muscle [8]. The vessel is cannulated and maintained at 60 mmHg in an organ bath, where it is
exposed to vasoactive substances. Using an arteriole from a lean rat, insulin has
no net effect on vessel diameter, but with the use of inhibitors of insulin signalling
pathways, this can be shown to represent equivalent degrees of vasodilatation - mediated
through the PI-3 kinase pathway and with nitric oxide as the vasodilator - and vasoconstriction
- mediated by endothelin-1 via the ERK pathway. If a similar experiment is done using
a similar arteriole isolated from an obese Zucker rat, incubation with insulin produces
vasoconstriction, which is inhibited by an endothelin-1 receptor blocker. The combination
of insulin and the endothelin-1 inhibitor in the obese vessel produces no net change
in diameter, but the same combination in the vessel from the lean rat produces net
vasodilatation. These observations show that in the vessel from the obese rat, the
PI3-kinase pathway of insulin action is impaired, leaving unopposed insulin stimulated
ERK-pathway vasoconstriction, mediated by endothelin-1.
Further studies comparing the arterioles of lean with those of obese rats have shown
a reduction in the expression of endothelial nitric oxide synthase in the endothelial
cells from the obese vessels. The inhibition of PI3-kinase-mediated insulin signalling,
of nitric oxide synthase expression, and thus of insulin-mediated vasodilatation are
known consequences of the action of TNF-α[9]
[10], and we have found that incubating the cremaster arteriole of lean rats with insulin
in combination with TNF-α produces similar effects to those of obesity [8], producing vasoconstriction which can be overcome by an endothelin-1 receptor blocker.
This raises the possibility that adipocytokines such as TNF-α are secreted from fat
depots, either remote from or in close proximity, to the vessels. Yet as pointed out
above, adipose tissue does not appear to secrete this cytokine in substantial amounts,
and circulating TNF-α is bound to excess amounts of the specific binding proteins
[11], both of which factors make it improbable that TNF-α acts as a systemic circulating
signal.
We have proposed a novel mechanism to explain these observations [8]. The morphology of the rat cremaster muscle arteriole differs between lean and obese
animals, in that there is a circumscribed depot of fat only around the origin of the
vessel in the obese rat ([Fig. 2]). We have suggested that this may provide a regulatory mechanism whereby, in situations
of calorie excess or inactivity, a local fat pad develops at the vessel origin with
specialist vasoregulatory function. Adipocytokines from these pads, such as TNF-α,
inhibit the PI3-kinase signalling pathway of endothelial NO production, thus locally
inhibiting a systemic postprandial insulin-mediated vasodilatation and helping the
organism to protect its muscle from substrate over-supply.
Fig. 2 Isolated cremaster muscle from Zucker fatty rat*.*Reprinted from: Yudkin JS, Eringa
E, Stehouwer CDA. The Lancet 2005; 365: 1817-1820. Vasocrine signalling from perivascular
fat - a mechanism linking insulin resistance and vascular disease; Copyright (2005),
with permission from Elsevier.
It must be noted, however, that the localization of the fat pad in the arteriole from
the Zucker rat is proximal to the segment of vessel which shows altered insulin signalling.
For this reason, we have proposed a mechanism to explain the propagation of the signalling
function of TNF-α. It is suggested that the increased endothelial permeability, consequent
upon the cytokine’s action, allows it to access the circulation, where it locally
exceeds the binding capacity of circulating binding proteins, so producing inhibition
of insulin-mediated vasodilatation throughout the entire nutritive vascular tree.
We have used the term “vasocrine signalling” to define this proposed signalling mechanism
[8].
Our hypothesis is shown diagrammatically in [Fig. 3]. The vascular smooth muscle cell in nutritive arterioles is under dual regulation
by insulin. Under normal circumstances, postprandial insulin secretion will cause
a predominantly vasodilatory response, mediated by endothelially derived nitric oxide,
with a consequent increase in supply of substrate and hormone to skeletal muscle.
However, in circumstances of calorie excess, the accumulation of periarteriolar fat
generates production of adipocytokines, which block the vasodilatory response and
instead may even produce vasoconstriction. Such action throughout the nutritional
vascular bed could contribute to muscle insulin resistance.
Fig. 3 Vasocrine signalling from perivascular fat*.Adipocytokines secreted from perivascular
adipocytes inhibit the PI3-kinase pathway of insulin signalling, leaving unopposed
vasoconstrictor effects of endothelin-1. High concentrations of tumour necrosis factor-α
access the vascular lumen, resulting in inhibition of endothelial PI3-kinase pathway
insulin signalling in downstream vessels. Reduced insulin-mediated enhancement of
muscle nutritive blood flow will contribute to insulin resistance. EC: endothelial
cell; VSMC: vascular smooth muscle cell; eNOS: endothelial nitric oxide synthase;
NO: nitric oxide; PI3-K: phosphoinositol-3-kinase; TNF-α: tumour necrosis factor-α;
IL-6: interleukin-6; NEFA: nonesterified fatty acids; ERK: extracellular signal-related
kinase; ET-1: endothelin-1. *Reprinted from: Yudkin JS, Eringa E, Stehouwer CDA. The
Lancet 2005; 365: 1817-1820. Vasocrine signalling from perivascular fat - a mechanism
linking insulin resistance and vascular disease; Copyright (2005), with permission
from Elsevier.
Depots of adipose tissue are found around large vessels as well as small. It is proposed
that there is a homology between such perivascular depots, which secrete substantial
amounts of adipocytokines such as TNF-α, and that this is responsible for outside-to-inside
signalling, in arteries as in arterioles. We postulate that in conditions of inactivity
and calorie excess, such depots around the coronary, carotid, and femoral arteries
may contribute to inflammatory changes, and so to atherothrombosis, in the affected
arteries [12]. It is noted that there are close relationships between measures of epicardial and
those of visceral fat [13], and the predictive power of waist, and perhaps neck, circumference for coronary
heart disease may relate to more direct influences of the adipose depot on the vessels
than those mediated by circulating signals - be they nonesterified fatty acids, insulin
or adipocytokines. These speculations, however, remain as hypotheses, as a detailed
characterisation of perivascular fat, and its physiological and pathophysiological
role, remains to be elucidated
