Insulin Resistance Syndrome in OB/OB Mouse

Review Article

Austin J Endocrinol Diabetes. 2016; 3(4): 1051.

Insulin Resistance Syndrome in OB/OB Mouse

Tatsuo Tomita*

Departments of Integrative Bioscience and Pathology, Oregon Health and Science University, USA

*Corresponding author: Tatsuo Tomita, Department of Integrative Bioscience, Oregon Health and Science University, 611 SW Campus Drive, Portland, USA

Received: August 23, 2016; Accepted: September 23, 2016; Published: October 14, 2016

Abstract

The ob/ob mouse had been extensively used as a good animal model for insulin resistance study especially obesity associated Type 2 diabetes (T2DM) in humans. Ob/ob mice develop obesity as early as 1 month of age with progressively increasing body weight and insulin levels in both plasma and pancreas due to increasing insulin resistance through leptin deficiency in the hypothalamus. Progressively hyperplastic and hypertrophic islets in ob/ob mice consist predominantly of β-cells and correspond to extreme hyperinsulinemia and hyperglycemic ob/ob mice with age responding to progressive insulin resistance with hyperglycemia. Abnormal glucose and insulin tolerance tests revealed increasing higher plasma glucose and insulin levels in ob/ob mice with age and support increasing insulin resistance with age. This study aimed to reveal increasing insulin resistance in ob/ob mice with aging, similar to humans with type 2 diabetes. Leptin treatment and leptin gene therapy dramatically improve body weight with normalizing glucose tolerance and insulin tolerance test. Insulin resistance is partly due to decreased insulin receptor sites in the target organs and ob/ob mice have been used to unfold biological interactions of Insulin-Receptor Substrate -1 (IRS-1) and IRS-2 and may elucidate a complex mechanisms of a kinase cascade, through which insulin resistance will be further analyzed at the molecular level.

Keywords: β-cells; glucose; Hyperphagia; Insulin; Insulin receptor; Insulin resistance; IRS-1; IRS-2; Islets of Langerhans; Leptin; NPY

Abbreviations

ISR: Insulin-Receptor Substrate; NPY: Neuropeptide Y; T2DM: Pi (3) kinase [PI (3) K] and type 2 diabetes

Introduction

Leptin-deficient ob/ob mouse was discovered at the Jackson Memorial Laboratory, Bar Harbor, Maine in 1949 [1]. A non-obese mouse colony was raised and produced a strain of obese offspring, which suggested that a single mutation had occurred in a hormone, leptin, regulating hunger and energy expenditure [1]. Leptin was the first fat cell-derived hormone to be discovered [2] and has the structure of a long chain helical cytokine and is expressed in adipose tissue in proportion to adipocyte size [3,4]. Leptin acts on receptors in the lateral hypothalamus to inhibit hunger by counteracting the effects of Neuropeptide Y (NPY), a potent huger promoter secreted by cells in the gut and in the hypothalamus [2] and also acts on the medial hypothalamus to stimulate satiety [1]. Absence of leptin in ob/ob mice leads to hyperphagia and resulting extreme obesity [2]. Ob/ob mice are indistinguishable from the lean littermates at birth but become heavier within 1 month after birth [5]. Insulin resistance progresses with aging through ever increasing obesity in ob/ob mice, similar to aging humans with type 2 diabetes [2]. Obese human individuals generally exhibit more leptin in the blood than lean individuals due to more leptin secreting fat cells and exogenous leptin injection does not reduce obesity in humans in resistance to leptin [6]. In a contrast, ob/ob mice will revert obesity to normal weight by exogenous leptin injection or leptin gene treatment [6-8].

Pathophysiological study

Ob/ob mice were indistinguishable from lean littermates at birth but already had a sign of chubby appearance with 1/3 heavier body weight within 1 month of age (Figure 1) [5,9]. Plasma glucose levels were already twice as much as that of lean mice (Figure 1) [9]. Body weight progressively increased up to 6 months reaching 70 g as twice as heavy as lean mice (Figure 1) [9] whereas plasma glucose peaked at 3 months and subsequently decreased in 6 months twice as much as that of lean mice (Figure 1) [9]. Glucose tolerance test, 2 mg glucose per g of body weight intraperitoneal, ip, revealed a marked difference of plasma glucose and insulin levels between ob/ob mice and lean mice: lean mice showed consistently high plasma glucose levels at 12—13 mmol/l at 30 min from 1 to 6 months of age whereas 1 month old ob/ob mice revealed the highest plasma glucose of 35 mmol at 60 min, 3.5 fold higher than that of lean mice and both 3 and 6 month old ob/ob mice revealed the highest glucose levels of 30 mmol/l at 30 or 60 min after glucose infusion (Figure 2) [10]. Plasma insulin levels of 1 month old ob/ob mice were about twice as much as that of lean mice and subsequently increased to fivefold that of lean mice at 6 months (Figure 2) [10]. Thus, glucose tolerance test showed much higher plasma glucose and insulin levels in ob/ob mice than in lean mice (Figure 2) [10]. Insulin tolerance test, porcine insulin 150 mU/ml, 1mU/g of body weight, ip revealed that lean mice responded by the lowest plasma glucose to 3 to 4 mmol/l from 30 to 120 minutes after insulin injection whereas 1 month old ob/ob mice revealed the lowest plasma glucose to 7 mmol/l in 1 month of age, 9 mmol/l in 3 months and 10 mmol/l in 6 months old, showing much higher plasma glucose levels, progressively increased from age 1 to 6 months (Figure 3) [10]. Plasma insulin levels were about the same for both ob/ob mice and lean mice of 1 month old (Figure 3) [10]. However, plasma insulin levels of ob/ob mice reached the highest levels at 3 months, peaked at 45 μmol/l of three fold higher than that of lean mice, and slightly decreased until 6 months at 40 μmol/l 30 min after insulin injection (Figure 3) [10]. Plasma insulin levels stayed 3 to 4 fold higher in ob/ob mice than in lean mice (Figure 3) [10]. Pancreatic tissue level of insulin and other pancreatic hormones: Pancreatic insulin content of ob/ob mice was about 30% higher for 1 and 2 month old mice compared to lean mice, but it progressively increased to fivefold at 6 month old while lean mouse insulin content staying at the same levels of 100 μg/g throughout 6 months of age (Figure 4) [9]. Glucagon level was 50% higher for 1 month old ob/ ob mice and also progressively increased to 2.5 fold toward 6 months compared to lean mice (Figure 4) [9]. Pancreatic polypeptide level was twice as that of lean mice at 1 month and moderately higher throughout 6 months (Figure 4) [9]. Thus, in ob/ob mice insulin level progressively increased higher with age of mice, followed by glucagon and PP compared to lean mice but somatostatin level did not increase with age of mice, staying at the same levels of that of lean mice from 1 to 6 months of age (Figure 4) [9]. These normal levels of pancreatic somatostatin correlate with normal levels of serum somatostatin, which will not reduce markedly elevated serum insulin and moderately increased serum glucagon levels [11]. The increasing islet sizes and numbers support ever increasing plasma insulin levels in response to an increasing insulin resistance with age in ob/ob mice [10]. Insulin removal in perfused livers of ob/ob mice: Karakash et al. measured insulin concentration in flow and outflow of perfusate from perfused livers in situ from 2 and 3 months old ob/ob and lean mice: the percentage of the insulin removed by the liver was considerably lower in ob/ob mice than in livers of lean mice using various insulin concentrations of the perfusate [12]. Ob/ob mice were also made normoglycemic by prior injection of streptozotocin or anti-insulin serum: insulin removal was increased by 50% by streptozotocin or anti-insulin serum. These data suggest insulin-binding and removal by the liver were much lower in hyperglycemic and hyperinsulinemic ob/ob mice and removal percentage was increased by normoglycemic ob/ob mice by streptozotocin or anti-insulin antibody injection. Hyperglycemic ob/ob mice are hyperinsulinemic partly due to lesser insulin removal in the liver, the main organ for insulin to bind and be removed. In hyperglycemic ob/ob mice more insulin circulates in the blood as a result of lesser insulin removal in the liver [11,12]. Leptin treatment and leptin gene therapy on ob/ob mice: In 5 week old ob/ob and lean mice, Harris et al. placed an Alzet mini osmotic pump (Alza Corp, Palo Alto, CA) in the peritoneal cavity of each mouse and delivered 0 to 42 μg/day human leptin for 7 days [4]. In ob/ob mice 2 to 42μg/day reduced food intake and body weight [4]. Liver weight, lipid and glycogen decreased 50%, 70% and 47%, respectively [4]. Serum glucose and insulin were reduced in all leptintreated mice and were normalized by 10μg/day leptin [4]. These doses also increased brown adipose tissue as well as increasing rectal temperature and sympathetic nerve activities [4]. Exogenous leptin treatment had shown to successfully revert hyperphagia in ob/ob mice and to reduce body weight through normalizing serum glucose, insulin, liver weight, normal lipid and glycogen content [4]. Ten to 12 months old ob/ob mice were treated with leptin (100μg/o.1 ml), ip for 7 days and their isolated islets secreted improved insulin secretion with 16.7 mmol glucose [13]. In 18 to 22 month old male mice, ob/ ob mice were treated with a recombinant adenovirus expressing the mouse leptin cDNA [8]. Treatment resulted in dramatic reductions in both food intake and body weight as well as normalization in serum insulin levels and glucose tolerance [8]. Subsequent diminishment in serum leptin levels resulted in the rapid resumption of food intake and a gradual body weight gain, correlating with the gradual return of hyperinsulinemia and insulin resistance [8] Most recently, DiSilvestro et al. produced an encapsulated fat pad containing leptin over expressing 3T3-L1 preadipocyte cell line, which was injected into visceral fat deposit in 5 week old ob/ob mice [14]. Leptin was found in the plasma of an encapsulated fat pad injected ob/ob mice but not in the ob/ob mice without fat pad injection at the end of 72 days [14]. Fat pad injected ob/ob mice had transit suppression in appetite and weight loss and had greater brown fat mass, metabolic rate and reduced resist in plasma levels compared to non-fat injected ob/ob mice [14]. By glucose tolerance test, blood glucose levels were significantly improved in fat pad injected ob/ob mice compared to non-fat pad injected ob/ob mice [12]. Thus, an encapsulated fat pad provided a long-term improvement of insulin resistance syndrome in ob/ob mice [14]. In 6 week old mice, gonadal fat pad of white adipose tissue from lean littermates was transplanted subcutaneously in ob/ ob mice, in whom 4 of 8 donors prevented obesity in ob/ob mice despite leptin levels being below 25% of lean littermates [15]. Thirteen month old ob/ob mice were also transplanted subcutaneously with white adipose tissue to test the therapeutic potential, which reduced body weight with reduced non-fasting insulin [15]. In also 6 week old ob/ob mice, brown adipose tissue transplanted from lean littermates reportedly reduced body weight with increased oxygen consumption and decreased fat mass, resulting improved insulin resistance and liver statuses without an evidence of the increased serum leptin levels [15].