Introduction
Over 40 million children younger than 5 years of age were overweight worldwide in 2011 [
1], and being overweight or obese during childhood and adolescence is a predictor of adult obesity [
2,
3]. Obesity is a risk factor for type 2 diabetes, coronary heart disease, hypertension, atherosclerosis and some cancers; many of these diseases are traditionally associated with adulthood but are occurring with increasing prevalence in youth [
4]. Hyperinsulinaemia and insulin resistance are two characteristics of the obese state that have been proposed to contribute to its detrimental effects on health [
5,
6]. Circulating insulin levels are intimately related to systemic insulin responsiveness, and the most widely held paradigm posits that obesity leads to insulin resistance, causing a compensatory rise in insulin to prevent hyperglycaemia [
5,
6]. However, clinical and experimental evidence suggests that hyperinsulinaemia can precede and promote obesity [
7‐
11]. Drugs that suppress insulin secretion in hyperinsulinaemic obese rodents or humans lead to weight loss [
12‐
14]. Obese individuals with the highest insulin levels respond best to diets that reduce postprandial glycaemia and insulinaemia whereas those with less-elevated insulin show equivalent weight loss on low-fat diets [
15,
16]. Insulin is known to suppress lipolysis and stimulate lipogenesis in white adipose tissue (WAT) [
17], and mouse models with reduced adipose tissue insulin signalling are protected against obesity [
18,
19].
We exploited the existence of two rodent insulin genes (
Ins1 and
Ins2) to genetically manipulate endogenous insulin production. Recent work in our laboratory demonstrated that continuous suppression of fasting hyperinsulinaemia through reducing
Ins1 dosage (in an
Ins2 null background) prevented diet-induced obesity in male mice [
11]. However,
Ins1 is a rodent-specific gene, and there are differences in promoter elements and expression patterns between
Ins1 and
Ins2 [
11,
20‐
23]. As it is unclear whether
Ins1 and
Ins2 have distinct roles, we felt it important to examine the effects of reduced
Ins2 dosage in the development of high-fat diet (HFD)-induced obesity.
Certain life stages are important for adipocyte hyperplasia and hypertrophy but the mechanisms controlling WAT expansion, and their timing, remain to be fully elucidated [
24]. White adipocyte cell number is thought to stabilise towards the end of adolescence in non-obese humans and rodents [
24], which suggests that conditions during this programming period could influence future adiposity. Indeed, adolescence has been identified as a key life stage for the development of obesity in humans, since the presence or onset of obesity during adolescence is associated with increased incidence of its associated morbidities in adults [
25]. In our previous study, the
Ins1
+/−:
Ins2
−/− genetic manipulation resulted in lifelong prevention of hyperinsulinaemia [
11], which precluded an assessment of whether anti-obesity effects would persist without sustained repression of insulin. In the present study, we found that high-fat-fed female
Ins1
−/−:
Ins2
+/− mice had reduced insulin secretion at a young age but by 1 year they had reached a degree of hyperinsulinaemia equivalent to that of high-fat-fed
Ins1
−/−:
Ins2
+/+ littermates. This provided a unique model with which to test the hypothesis that the reduction of insulin secretion in young, growing mice can provide long-term protection against diet-induced obesity.
Discussion
Our objective was to test the hypothesis that reducing insulin secretion by partial disruption of the
Ins2 gene would prevent diet-induced obesity. A transient attenuation of insulin hypersecretion in young, growing
Ins1
−/−:
Ins2
+/− mice allowed us to test the secondary hypothesis that these anti-obesity effects would persist despite late-onset HFD-induced elevations in insulin. Our data indicate that, under these experimental conditions, reduced dosage of the ancestral
Ins2 gene can provide protection against obesity similar to that gained by reducing the dosage of rodent-specific
Ins1 [
11]. Importantly, the current study also identified the growth period of adolescence and young adulthood as a potentially critical time to suppress insulin escalation.
It is an intriguing concept that there could be key interventional periods for influencing obesity and associated health risks [
34]. In humans, the rate of BMI increase during pubescence and the maximum BMI during young adulthood (22–24 years) can both be stronger predictors of adiposity at mature adulthood (35–45 years) than adult lifestyle variables [
35]. It has previously been shown that early-life manipulations, such as short-term insulin treatment in neonatal rats, leads to increased weight gain as well as impaired glucose tolerance and insulin responsiveness into adulthood [
36]. Our experiments with short-term INSULIN2 treatment in
Ins1
−/−:
Ins2
+/− mice also pointed to possible long-term effects on weight gain, although the limited period of insulin treatment could have led to impaired insulin sensitivity and a lasting elevation of insulin secretion, as previously seen [
36]. Collectively, our investigation indicates that repression of hyperinsulinaemia in young, HFD-fed mice can attenuate obesity throughout adulthood.
Tissue analyses showed subtle dissimilarities in lipid droplet size distributions between
Ins1
−/−:
Ins2
+/− and
Ins1
−/−:
Ins2
+/+ mice in the gonadal WAT at 25 weeks, implying possible divergence in adipocyte hypertrophy or hyperplasia, although no significant transcriptional changes were detected. By the age of 50 weeks, HFD-fed
Ins1
−/−:
Ins2
+/− mice showed a tendency for increased expression of genes associated with fatty acid uptake and lipogenesis in gonadal WAT. This could indicate continued maintenance of adipocyte function and energy storage capacity, suggested to be metabolically beneficial in both humans and mice [
37‐
40], and it may have been counteracted by exogenous INSULIN2 treatment. Interestingly, we did not detect elevated adipose
Ucp1 expression in
Ins1
−/−:
Ins2
+/− female mice, unlike
Ins1
+/−:
Ins2
−/− HFD-fed male mice [
11]. Rather, interscapular depots of BAT were smaller in young
Ins1
−/−:
Ins2
+/− female mice than in
Ins1
−/−:
Ins2
+/+ littermates, similar to the effect of knocking the insulin receptor out of BAT [
41], and this may have contributed to the lower energy expenditure. Another caveat complicating the energy expenditure interpretation is that there were differences in body composition [
42], and human studies have also shown that obese individuals may not show reduced total energy expenditure even if they are less active than lean individuals [
43,
44]. Therefore, adipose-level changes distinct from those outlined in our previous study [
11] may have contributed to attenuated adiposity in the HFD-fed
Ins1
−/−:
Ins2
+/− female mice.
The HFD-fed Ins1
−/−:Ins2
+/− female mice maintained protection against obesity into adulthood, despite the fact that their suppression of fasting insulin had reverted by 1 year. At 50 weeks of age, the islet Ins2 mRNA and insulin levels of HFD-fed Ins1
−/−:Ins2
+/− mice approached those of Ins1
−/−:Ins2
+/+ littermates. Although we could not elucidate the chronology of the relationship between this late-onset hyperinsulinaemia and insulin resistance, it is clear from our results that reducing adipose tissue expansion and weight gain cannot always prevent the decline in glucose tolerance and insulin sensitivity that is associated with high-fat feeding.
In conclusion, results from this investigation support the body of literature that places hyperinsulinaemia mechanistically upstream of diet-induced obesity. The growth period of adolescence and young adulthood may be a critical time for shaping future adiposity and our study demonstrates that, in mice, repression of insulin hypersecretion during this life stage can provide long-term protection against obesity. Interestingly, pubescence in humans is associated with a transient period of reduced blood glucose responsiveness to insulin, and elevated insulin secretion [
45,
46]. It could be worthwhile to explore whether limiting insulin hypersecretion during this phase could have lasting anti-obesity effects in humans.
Acknowledgements
The authors acknowledge X. Hu, S. Karunakaran and S. O’Dwyer for assistance with animal studies, X. Hu for additional assistance with islet isolations and perifusions and T. J. Kieffer for input on study design and providing use of the Luminex multiplexing instrument. A. E. Mehran contributed to the study’s conceptualisation and a pilot study. These contributors all have the same affiliations as the authors.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.