The Journal of Clinical Endocrinology & Metabolism Vol. 82, No. 2 381-382
Copyright © 1997 by The Endocrine Society
Leptin Synthesis Is Resistant to Acute Effects of Insulin in Insulin-Dependent Diabetes Mellitus Patients1
Juha A. Tuominen,
Pertti Ebeling,
Ulf-Håkan Stenman,
Mark L. Heiman,
Thomas W. Stephens and
Veikko A. Koivisto
Departments of Medicine (J.A.T., P.E., V.A.K.) and Clinical
Chemistry (U.-H.S.), Helsinki University Central Hospital, Helsinki,
Finland; and Lilly Research Laboratory (M.L.S., T.W.S.), Indianapolis,
Indiana 46285
Address all correspondence and requests for reprints to: Juha A. Tuominen, M.D., Helsinki University Central Hospital, Department of Medicine, os 112, FIN-00290 Helsinki, Finland.
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Abstract
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Insulin stimulates ob gene expression and increases
serum leptin concentrations in mice and in noninsulin-dependent
diabetes mellitus patients. Obese women have higher ob
gene messenger ribonucleic acid levels than obese men, suggesting that
sex hormones are involved in the regulation of leptin synthesis. We
studied the relationship among leptin, insulin, and testosterone in 15
men with insulin-dependent diabetes mellitus (IDDM; age, 29 ± 2
yr; body mass index, 22.7 ± 0.5 kg/m2; body fat,
9.5 ± 1.0%; insulin dose, 44 ± 4 U/day; hemoglobin
A1c, 8.1 ± 0.3%; diabetes duration, 12.7 ± 2.0
yr) and 15 healthy control subjects (age, 27 ± 1 yr; body mass
index, 22.6 ± 0.4 kg/m2; body fat, 9.6 ± 0.5%)
in the fasting state. In addition, the effect of a 4-h euglycemic
hyperinsulinemia (
600 pmol/L) on the plasma leptin concentration was
determined. The fasting leptin concentration was negatively correlated
to plasma testosterone (r = -0.55; P < 0.05)
in IDDM patients. The fasting plasma leptin level rose 25% in healthy
subjects (from 1.0 ± 0.2 to 1.3 ± 0.3 ng/mL;
P < 0.05). The leptin levels were higher in IDDM
subjects (P < 0.01) and remained unchanged
(2.7 ± 0.2 vs. 2.7 ± 0.2 ng/mL) during
hyperinsulinemia.
We reached the following conclusions. 1) In nonobese IDDM patients,
leptin synthesis is resistant to the acute effect of insulin. 2) Serum
testosterone may contribute to the regulation of leptin synthesis in
IDDM patients.
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Introduction
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THE SATIETY factor leptin, a product of the
ob gene (1), is expressed in adipose tissue, and its
expression is stimulated by insulin (2, 3, 4). Serum leptin concentrations
in humans are positively associated with body fat percentage, body mass
index (BMI), and serum insulin and cortisol concentrations. Insulin
infusion increases serum leptin concentrations in healthy men and
noninsulin-diabetes mellitus patients (5). In overweight individuals,
serum leptin concentrations are elevated, suggesting leptin resistance,
and weight loss reduces serum leptin levels (6). In addition, sex
hormones may have a role in the regulation of leptin levels, as obese
women have higher ob messenger ribonucleic acid (mRNA)
levels than obese men (7), and female mice have higher leptin levels at
any given body fat content than males (8). Whether insulin increases
leptin concentrations in insulin-dependent diabetes mellitus (IDDM)
patients, who are characterized by insulin resistance (9), is not
known. Neither is it known whether body fat or testosterone is related
to plasma leptin levels. In the present study we examined the effects
of insulin on plasma leptin levels and possible associations between
leptin and body fat or testosterone concentrations in healthy men and
patients with IDDM.
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Subjects and Methods
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Fifteen men with IDDM (age, 29 ± 2 yr; BMI, 22.7 ±
0.5 kg/m2; body fat, 9.5 ± 1.0%; insulin dose,
44 ± 4 U/day; hemoglobulin A1c, 8.1 ± 0.3%;
diabetes duration, 12.7 ± 2.0 yr) and 15 healthy control subjects
(age, 27 ± 1 yr; BMI, 22.6 ± 0.4 kg/m2; body
fat, 9.6 ± 0.5%) were invited to participate in the study. None
of the patients had clinical signs of neuropathy, proliferative
retinopathy, or nephropathy. No subject was using any medication,
except insulin. The purpose, nature, and possible risks of the study
were explained to all subjects before informed consent was obtained.
The study protocol was approved by the ethical committee of the
Helsinki University Hospital.
After an overnight fast, a 240-min euglycemic hyperinsulinemic
clamp study with the insulin infusion rate of 9 pmol (1.5 mU)/kg·min
was performed in each subject (10, 11). Plasma glucose was determined
by a glucose oxidase method using the Beckman glucose analyzer (Beckman
Instruments, Fullerton, CA). Hemoglobin A1c (reference
range, 4.0–6.0%) was quantitated by high performance liquid
chromatography (12). Plasma leptin was determined radioimmunologically
(6, 13). The detection limit of this assay was 0.39 ng
mL-1. The intraassay coefficient of variation was 4.7%,
and the interassay variation was 6.2%. Serum testosterone, cortisol,
and insulin were determined as previously reported (11, 14). Body fat
percentage was calculated from the thickness of six skinfolds (15).
In the statistical analysis, Wilcoxon’s signed rank test was used to
calculate differences between paired observations, and the Mann-Whitney
U test was used to calculate differences between the groups.
Correlation analysis was performed with Spearman’s test.
P < 0.05 was considered significant. The results are
given as the mean ± SEM.
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Results
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Fasting values for plasma glucose, serum insulin, and
testosterone are given in Table 1
. During the insulin
infusion, plasma glucose (5.3 ± 0.1 vs. 5.1 ±
0.1 mmol/L) and insulin (625 ± 34 vs. 596 ± 27
pmol/L) were maintained at a constant level, achieving glucose disposal
rates of 47.6 ± 2.3 and 66.2 ± 2.8 µmol/kg·min
(P < 0.01) in patients and control subjects,
respectively. In healthy subjects, insulin infusion increased the
plasma leptin concentration by 25% (P < 0.05;
fasting, 1.0 ± 0.2 ng/mL; 240 min, 1.3 ± 0.3 ng/mL). In
IDDM patients, there was no change in plasma leptin concentrations
during hyperinsulinemia, and both fasting (2.7 ± 0.2 ng/mL) and
clamp end (2.7 ± 0.2 ng/mL) plasma leptin concentrations were
higher (P < 0.01) than those in the control subjects.
The plasma leptin concentration correlated with body fat percentage in
patients (r = 0.67; P < 0.01), but not in control
subjects (r = 0.32; P = NS). The correlation
between plasma leptin concentration and BMI (r = 0.19;
P = NS and r = 0.47; P = 0.06) and
fasting insulin level (r = 0.19 and r = 0.31) was not
significant in patients and control subjects, respectively, and that
between plasma leptin and the daily insulin dose (r = 0.20) was
not significant in patients. The basal leptin concentration was
inversely related to serum testosterone (P = -0.55 and
P < 0.05) in patients, but not in healthy subjects
(r = 0.39).
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Discussion
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In contrast to the acute stimulatory effect of
insulin on serum leptin concentrations, as shown in rats (3),
noninsulin-dependent diabetes mellitus patients (5), and healthy men,
hyperinsulinemia did not enhance plasma leptin concentrations in IDDM
patients, nor was there any association between plasma leptin level and
daily insulin dose. Our IDDM patients were insulin resistant, as shown
by their reduced rate of glucose uptake in the face of equal serum
insulin concentrations as those in healthy subjects. Unchanged leptin
levels indicate a resistance to the acute stimulatory effect of insulin
in leptin synthesis. Fasting plasma leptin values were higher in IDDM
patients than in control subjects. This difference can be due to
chronicly high insulin concentrations in insulin-treated patients (16, 17) regulating the leptin synthesis (18). In addition to the insulin
resistance in glucose metabolism, this is another reflection of
decreased insulin action in IDDM. As we only used one (a high
physiological) insulin concentration, we cannot exclude the possibility
that with a higher insulin dose or a longer duration of infusion, a
rise in the plasma leptin level would have occurred. Due to a diurnal
variation, there is a fall in the serum leptin concentration in the
morning in healthy men (13). Without a saline control study it is not
possible to estimate whether insulin had any effect on plasma leptin
levels in IDDM patients. If there was an effect, however, the response
was less than that in healthy subjects.
Sex hormones may have an effect on leptin synthesis, because
obese women have higher ob mRNA levels than obese men (7),
and female mice have higher leptin levels at any given body fat content
than males (8). Furthermore, leptin inhibits hypothalamic expression of
neuropeptide Y (NPY) mRNA (19), and NPY modulates gonadotropin
secretion (20). On the other hand, testosterone enhances the amount of
NPY mRNA in the arcuate nucleus of the hypothalamus (21). It also
up-regulates 
2-adrenoreceptors in male hamsters solely
in adipose tissue (22) and accordingly accentuates the antilipolytic
part of the adrenergic effect in fat tissue. We found an inverse
correlation between serum testosterone and plasma leptin levels in IDDM
patients. These data support the interaction of leptin and
testosterone. A similar correlation was not found in healthy
subjects. It is possible that the mechanisms in IDDM patients lack the
acute stimulatory effect of insulin, as shown in this study, and thus
other hormones, such as testosterone, have a greater role in the
regulation of energy homeostasis than they do in healthy men.
Taken together, our results indicate that patients with IDDM are
resistant to insulin action on leptin synthesis. Our data raise the
possibility that serum testosterone contributes to the regulation of
leptin synthesis in IDDM patients.
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Acknowledgments
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The skillful technical assistance of Ms. Elisa Kostamo and Ms.
Saija Pöyhönen is appreciated.
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Footnotes
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1 This work was supported by the Academy of Finland, the Yrjö
Jahnsson Foundation, and the Maud Kuistila Foundation. 
Received June 6, 1996.
Revised October 17, 1996.
Accepted October 28, 1996.
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Top
Abstract
Introduction
