The link between the metabolic syndrome and cancer

Since the incidence of the metabolic syndrome is on the rise in the western world, its coherence to cancer is becoming more apparent. In this review we discuss the different potential factors involved in the increase of cancer in the metabolic syndrome including obesity, dyslipidemia and Type 2 Diabetes Mellitus (T2DM) as well as inflammation and hypoxia. We especially focus on the insulin and IGF systems with their intracellular signaling cascades mediated by different receptor subtypes, and suggest that they may play major roles in this process. Understanding the mechanisms involved will be helpful in developing potential therapeutics.

Decreased IGF type 1 receptor signaling in mammary epithelium during pregnancy leads to reduced proliferation, alveolar differentiation, and expression of insulin receptor substrate (IRS)-1 and IRS-2

The IGFs and the IGF type 1 receptor (IGF-1R) are essential mediators of normal mammary gland development in mice. IGF-I and the IGF-1R have demonstrated functions in formation and proliferation of terminal end buds and in ductal outgrowth and branching during puberty. To study the functions of IGF-1R during pregnancy and lactation, we established transgenic mouse lines expressing a human dominant-negative kinase dead IGF-1R (dnhIGF-1R) under the control of the whey acidic protein promoter. We provide evidence that the IGF-1R pathway is necessary for normal epithelial proliferation and alveolar formation during pregnancy. Furthermore, we demonstrate that the whey acidic protein-dnhIGF-1R transgene causes a delay in alveolar differentiation including lipid droplet formation, lumen expansion, and β-casein protein expression. Analysis of IGF-1R signaling pathways showed a decrease in P-IGF-1R and P-Akt resulting from expression of the dnhIGF-1R. We further demonstrate that disruption of the IGF-1R decreases mammary epithelial cell expression of the signaling intermediates insulin receptor substrate (IRS)-1 and IRS-2. No alterations were observed in downstream signaling targets of prolactin and progesterone, suggesting that activation of the IGF-1R may directly regulate expression of IRS-1/2 during alveolar development and differentiation. These data show that IGF-1R signaling is necessary for normal alveolar proliferation and differentiation, in part, through induction of signaling intermediates that mediate alveolar development.

Minireview: IGF, Insulin, and Cancer

In recent years, the influence of the IGF system and insulin on cancer growth has been widely studied. Observational human studies have reported increased cancer mortality in those with obesity and type 2 diabetes, which may be attributable to hyperinsulinemia, elevated IGF-I, or potentially both factors. Conversely, those with low insulin, IGF-I and IGF-II levels appear to be relatively protected from cancer development. Initial attention focused on the role of IGF-I in tumor development. The results of these investigations allowed for the development of therapies targeting the IGF-I receptor signaling pathway. However, after in vitro and in vivo studies demonstrating that insulin may also play a significant and independent role in tumorigenesis, insulin is now receiving more attention in this regard. Some studies suggest that targeting insulin receptor signaling may be an important alternative or adjunct to targeting IGF-I receptor signaling. In this minireview, we discuss some of the recent in vitro, animal, and clinical studies that have elaborated our understanding of the influence of IGF and insulin on tumorigenesis. These studies have shed more light on the interaction between insulin and IGF signaling in cancer cells. They have made possible the development of novel targeted therapies and highlighted some of the potential future directions for research and therapeutics.

Type 2 diabetes and cancer: what is the connection?

Epidemiological studies have demonstrated an association between type 2 diabetes and cancer. Type 2 diabetes is characterized by insulin resistance and hyperinsulinemia. Hyperinsulinemia may lead to cancer through insulin's effect on its cognate receptor and the insulin-like growth factor system. The effects of insulin and insulin-like growth factor I on cancer development and progression have been demonstrated in animal and human studies. Type 2 diabetes has been positively associated with cancers of the breast, colon, and pancreas. An inverse relationship has been observed between type 2 diabetes and prostate cancer, and this may be due to lower testosterone levels in men with type 2 diabetes. Medications used to treat type 2 diabetes may affect cancer cells directly or indirectly by affecting serum insulin levels. Hyperinsulinemia may be an important risk factor for cancer as well as a target for cancer therapy.

Growth hormone receptor regulates β cell hyperplasia and glucose-stimulated insulin secretion in obese mice

Insulin, growth hormone (GH), and insulin-like growth factor-1 (IGF-1) play key roles in the regulation of β cell growth and function. Although β cells express the GH receptor, the direct effects of GH on β cells remain largely unknown. Here we have employed a rat insulin II promoter-driven (RIP-driven) Cre recombinase to disrupt the GH receptor in β cells (βGHRKO). βGHRKO mice fed a standard chow diet exhibited impaired glucose-stimulated insulin secretion but had no changes in β cell mass. When challenged with a high-fat diet, βGHRKO mice showed evidence of a β cell secretory defect, with further deterioration of glucose homeostasis indicated by their altered glucose tolerance and blunted glucose-stimulated insulin secretion. Interestingly, βGHRKO mice were impaired in β cell hyperplasia in response to a high-fat diet, with decreased β cell proliferation and overall reduced β cell mass. Therefore, GH receptor plays critical roles in glucose-stimulated insulin secretion and β cell compensation in response to a high-fat diet.

Intracerebroventricular leptin infusion improves glucose homeostasis in lean type 2 diabetic MKR mice via hepatic vagal and non-vagal mechanisms

MKR mice, lacking insulin-like growth factor 1 receptor (IGF-1R) signaling in skeletal muscle, are lean yet hyperlipidemic, hyperinsulinemic, and hyperglycemic, with severe insulin resistance and elevated hepatic and skeletal muscle levels of triglycerides. We have previously shown that chronic peripheral administration of the adipokine leptin improves hepatic insulin sensitivity in these mice independently of its effects on food intake. As central leptin signaling has been implicated in the control of peripheral glucose homeostasis, here we examined the ability of central intracerebroventricular leptin administration to affect energy balance and peripheral glucose homeostasis in non-obese diabetic male MKR mice. Central leptin significantly reduced food intake, body weight gain and adiposity, as well as serum glucose, insulin, leptin, free fatty acid and triglyceride levels relative to ACSF treated controls. These reductions were accompanied by increased fat oxidation as measured by indirect calorimetry, as well as increased oxygen consumption. Central leptin also improved glucose tolerance and hepatic insulin sensitivity determined using the euglycemic-hyperinsulinemic clamps relative to pair fed vehicle treated controls, as well as increasing the rate of glucose disappearance. Hepatic vagotomy only partially reversed the ability of central leptin to improve glucose tolerance. These results demonstrate that central leptin dramatically improves insulin sensitivity independently of its effects on food intake, in a lean mouse model of type 2 diabetes. The findings also suggest that: 1) both hepatic vagal and non-vagal pathways contribute to this improvement, and 2) central leptin alters glucose disposal in skeletal muscle in this model.

The intricate role of growth hormone in metabolism

Growth hormone (GH), a master regulator of somatic growth, also regulates carbohydrate and lipid metabolism via complex interactions with insulin and insulin-like growth factor-1 (IGF-1). Data from human and rodent studies reveal the importance of GH in insulin synthesis and secretion, lipid metabolism and body fat remodeling. In this review, we will summarize the tissue-specific metabolic effects of GH, with emphasis on recent targets identified to mediate these effects. Furthermore, we will discuss what role GH plays in obesity and present possible mechanisms by which this may occur.

Mammalian target of rapamycin inhibition abrogates insulin-mediated mammary tumor progression in type 2 diabetes

Type 2 diabetes increases breast cancer risk and mortality, and hyperinsulinemia is a major mediator of this effect. The mammalian target of rapamycin (mTOR) is activated by insulin and is a key regulator of mammary tumor progression. Pharmacological mTOR inhibition suppresses tumor growth in numerous mammary tumor models in the non-diabetic setting. However, the role of the mTOR pathway in type 2 diabetes-induced tumor growth remains elusive. Herein, we investigated whether the mTOR pathway is implicated in insulin-induced mammary tumor progression in a transgenic mouse model of type 2 diabetes (MKR mice) and evaluated the impact of mTOR inhibition on the diabetic state. Mammary tumor progression was studied in the double transgenic MMTV-Polyoma Virus middle T antigen (PyVmT)/MKR mice and by orthotopic inoculation of PyVmT- and Neu/ErbB2-driven mammary tumor cells (Met-1 and MCNeuA cells respectively). mTOR inhibition by rapamycin markedly suppressed tumor growth in both wild-type and MKR mice. In diabetic animals, however, the promoting action of insulin on tumor growth was completely blunted by rapamycin, despite a worsening of the carbohydrate and lipid metabolism. Taken together, pharmacological mTOR blockade is sufficient to abrogate mammary tumor progression in the setting of hyperinsulinemia, and thus mTOR inhibitors may be an attractive therapeutic modality for breast cancer patients with type 2 diabetes. Careful monitoring of the metabolic state, however, is important as dose adaptations of glucose- and/or lipid-lowering therapy might be necessary.

Can endogenous hyperinsulinaemia explain the increased risk of cancer development and mortality in type 2 diabetes: evidence from mouse models

There is now convincing evidence of an increased risk of patients with obesity or type 2 diabetes developing cancer. To understand the possible causation, type 2 diabetic muscle lysine to arginine (MKR) mice were studied. Introduction of mammary tumour cell lines resulted in aggressive growth of the mammary tumours in the hyperinsulinaemic MKR female mice. Blocking the insulin receptor activation or reducing the hyperinsulinaemia reduced the mammary tumour burden. Thus, hyperinsulinaemia may be one of the factors in the increased breast cancer risk seen with obesity and type 2 diabetes.

The proliferating role of insulin and insulin-like growth factors in cancer

Epidemiological studies have reported an increased risk of cancer in people with type 2 diabetes (T2DM) and obesity, related in part to hyperinsulinemia, secondary to insulin resistance. Hyperinsulinemia leads to increased expression of insulin-like growth factor (IGF)-I expression. In fact, increased insulin, IGF-I and IGF-II levels are associated with tumor growth in vitro, in animal models, and in epidemiological studies in humans. In this paper, we discuss the roles of insulin, IGF-I and IGF-II, their interaction with the insulin receptor (IR) and IGF-I receptor (IGF-IR), and their signaling pathways and regulation as these pertain to tumor growth. We explain how these pathways have been deciphered by in vitro and in vivo studies, and how they are being exploited in the development of targeted cancer therapies.

MKR mice have increased dynamic glucose disposal despite metabolic inflexibility, and hepatic and peripheral insulin insensitivity

AIMS/HYPOTHESIS

Recent work has shown that there can be significant differences when glucose disposal is assessed for high-fat induced insulin resistance by static clamp methods vs dynamic assessment during a stable isotope i.p. glucose tolerance test. MKR mice, though lean, have severe insulin resistance and decreased muscle fatty acid oxidation. Our goal was to assess dynamic vs static glucose disposal in MKR mice, and to correlate glucose disposal and muscle-adipose-liver flux interactions with metabolic flexibility (indirect calorimetry) and muscle characteristics.

METHODS

Stable isotope flux phenotyping was performed using [6,6-(2)H(2)]glucose, [U-(13)C(6)]glucose and [2-(13)C]glycerol. Muscle triacylglycerol (TAG) and diacylglycerol (DAG) content was assessed by thin layer chromatography, and histological determination of fibre type and cytochrome c activity performed. Metabolic flexibility was assessed by indirect calorimetry.

RESULTS

Indirect calorimetry showed that MKR mice used more glucose than FVB/N mice during fasting (respiratory exchange ratio [RER] 0.88 vs 0.77, respectively). Compared with FVB/N mice, MKR mice had faster dynamic glucose disposal, despite increased whole-muscle DAG and TAG, and similar hepatic glucose production with higher fasting insulin and unchanged basal glucose. Fed MKR muscle had more glycogen, and increased levels of GLUT1 and GLUT4 than FVB/N muscle. Histology indicated that MKR soleus had mildly decreased cytochrome c activity overall and more type II (glycolytic) fibres compared with that in FVB/N mice.

CONCLUSIONS/INTERPRETATION

MKR muscle adapts to using glucose, with more type II fibres present in red muscle. Fasting RER is elevated and glucose disposal during an i.p. glucose tolerance test is accelerated despite increased muscle DAG and TAG. Metabolic inflexibility may result from the compensatory use of fuel that can be best utilised for energy requirements; static vs dynamic glucose disposal assessments may measure complementary aspects of metabolic flexibility and insulin sensitivity.

Overcoming challenges in Type 2 diabetes management to improve patient outcomes

As obesity and Type 2 diabetes mellitus reach epidemic proportions worldwide, glycemic control and prevention of complications have become even more critical. We searched studies published between January 2006 and January 2010 to identify and discuss the challenges facing healthcare professionals and patients in reaching glycemic targets. MEDLINE and Derwent Drug File searches were conducted with the following search terms: Type 2 diabetes mellitus, medication adherence, antihyperglycemic agents, glucose control, and clinical inertia. In this article we will outline the commonly used medications and present the advantages and disadvantages of each class of drug. Additionally, we will present the dipeptidyl peptidase-4 inhibitors and their place in the treatment of patients with Type 2 diabetes mellitus.

Insulin, insulin resistance, obesity, and cancer

Epidemiologic studies have proposed a link between obesity, type 2 diabetes, and cancer. The pathophysiologic mechanisms involved in the development of type 2 diabetes, namely hyperinsulinemia and insulin resistance, have also been implicated in cancer development. Patients with type 2 diabetes are reported to have a worse response to cancer chemotherapy, have more complications, and have a poorer prognosis than patients with cancer without diabetes. Studies also have reported that insulin, insulin secretagogues, and metformin may have effects on tumor growth. Given the escalating worldwide prevalence of obesity and type 2 diabetes, their relationship to cancer has generated great interest and research across many fields of medicine.

Insulin-sensitizing therapy attenuates type 2 diabetes-mediated mammary tumor progression

OBJECTIVE

Type 2 diabetes increases breast cancer risk and mortality, and hyperinsulinemia has been identified as a major factor linking these two diseases. Thus, we hypothesized that pharmacological reduction of elevated insulin levels would attenuate type 2 diabetes-mediated mammary tumor progression.

RESEARCH DESIGN AND METHODS

We studied mammary tumor development in MKR(+/+) mice, a nonobese, hyperinsulinemic mouse model of type 2 diabetes. MKR(+/+) mice were either crossed with mice expressing the polyoma virus middle T oncogene specifically in the mammary gland or inoculated orthotopically with the mouse mammary tumor cell lines Met-1 and MCNeuA. MKR(+/+) or control mice harboring tumors were treated with CL-316243, a specific beta3-adrenergic receptor agonist, which sensitizes insulin action but has no direct effect on the mouse mammary epithelium or Met-1 and MCNeuA cells.

RESULTS

CL-316243 treatment significantly reduced the elevated insulin levels in MKR(+/+) mice and, as a consequence, attenuated mammary tumor progression in the three tumor models tested. This effect was accompanied by reductions in phosphorylation of insulin and IGF-I receptors in transformed mammary tissue.

CONCLUSIONS

Insulin-sensitizing treatment is sufficient to abrogate type 2 diabetes-mediated mammary tumor progression. Therefore, early administration of insulin-sensitizing therapy may reduce breast cancer risk and mortality in patients with type 2 diabetes.

Biological effects of growth hormone on carbohydrate and lipid metabolism

This review will summarize the metabolic effects of growth hormone (GH) on the adipose tissue, liver, and skeletal muscle with focus on lipid and carbohydrate metabolism. The metabolic effects of GH predominantly involve the stimulation of lipolysis in the adipose tissue resulting in an increased flux of free fatty acids (FFAs) into the circulation. In the muscle and liver, GH stimulates triglyceride (TG) uptake, by enhancing lipoprotein lipase (LPL) expression, and its subsequent storage. The effects of GH on carbohydrate metabolism are more complicated and may be mediated indirectly via the antagonism of insulin action. Furthermore, GH has a net anabolic effect on protein metabolism although the molecular mechanisms of its actions are not completely understood. The major questions that still remain to be answered are (i) What are the molecular mechanisms by which GH regulates substrate metabolism? (ii) Does GH affect substrate metabolism directly or indirectly via IGF-1 or antagonism of insulin action?