Understanding Insulin Resistance   (Part 1)

Insulin resistance - What is it and why is it relevant

Worldwide, the incidence of insulin resistance (IR) is increasing due to increasing lifespan, lifestyle habits, environmental conditions and over-nutrition.  In insulin resistant subjects, insulin secretion from the beta cell is augmented to overcome insulin resistance and maintain the blood glucose level in a physiological range. Merely twenty percent of these subjects will develop increased blood glucose concentrations, and will carry on to the manifestation of overt diabetes mellitus [1-4]. Normal glucose control will be maintained until functional abnormalities in insulin secretion from the beta cell will add on to the underlying insulin resistance and will merge in to loss of metabolic control and T2DM [5,6]. From this pathophysiological sequencing, T2DM can be seen as an end stage disease driven by progressing insulin resistance and set off by the failure of the beta cell to compensate for the increased insulin demand.
Insulin resistance not only increases the risk for T2DM, but also drives the development of arterial hypertension, dyslipidaemia, inflammation and coagulation disorders, causing atherosclerosis and the development of major vascular complications [7-11].

Insulin signalling - Understanding the pathways behind insulin resistance

The biological effects of insulin are mediated by specific insulin receptors. Great progress has been made in understanding the intracellular signal transduction pathways in both molecular and physiological contexts. Activation of the insulin receptor (IR) in insulin target cells initiate a complex downstream signalling cascade, which involves several distinct intracellular pathways. The insulin receptor (IR) phosphorylates at least nine intracellular signalling molecules including four different insulin receptor substrates (IRS), where IRS- 1 and IRS- 2 are suggested to serve as the major intracellular downstream signals for the insulin response [12]. In muscle cells, activation of IRS-1 and the phosphatidylinositol-3-kinase (PI3K) signalling pathway triggers the translocation of the glucose transporter (GLUT) 4 into the cell surface, enabling cellular uptake of glucose molecules. Within the liver, IRS-1 has been more closely linked with glucose homeostasis, whereas IRS-2 may be more closely linked with lipid metabolism [13].  In endothelial cells, the activation of IRS-1 and the PI3K signalling pathway increases the expression of endothelial nitric oxide synthase (eNOS) and triggers the release of nitric oxide (NO) [14-16]. NO stimulates the guanylate cyclase promoting vasodilatation and anti-proliferative effects on vascular smooth muscle cells. In addition, nitric oxide mediates anti-inflammatory effects, inhibits platelet aggregation [17,18], and reduces the expression of vascular adhesion molecules like VCAM-1, ICAM-1, or E-Selectin [19]. Therefore, activation of the PI3K signalling in endothelial cells will exert vasodilating and anti-atherogenic effects. Under certain conditions, insulin may activate alternate signalling pathways like the mitogen activated protein kinase cascade (MAPK) pathway in endothelial cells, or the transcription factor sterol regulatory element binding protein (SREBP)-1c in the liver, accounting for endothelial dysfunction and some of the lipid abnormalities observed in patients with IR [20,21].

Figure 1: Schematic presentation of intracellular insulin signalling in difference tissues. Presentation of PI3K and MAPK signalling cascades following binding of insulin to the insulin receptor subtype B. Mediators released from the visceral adipose tissue shift intracellular insulin signalling from the PI3K signalling to the MAPK/ERK signalling with mitogenic, atherogenic and lipogenic effects (aPKC = atypical Protein-Kinase C; eNOS = endothelial Nitric Oxide Synthase; ET1 = Endothelin 1; FOXO1 = Forkhead Box O1; IR = Insulin Receptor; IRS = Insulin Receptor Substrate; MAPK = Mitogen Activated Protein-Kinase; MEK = Mitogen Activated Protein-Kinase; NO = Nitric Oxide; PDK = Phosphoinositide-Dependent Kinase; PI3K = Phosphatidylinostol 3 Kinase; RAF = Proto-Oncogenes; SHC = Adapter Protein; SREBP-1c = sterol regulatory element binding protein 1c) Modified according to [80].

Shift of insulin signal and insulin resistance

The effectors that control the balance between insulin signalling through the distinct pathways may indicate trigger points in the pathophysiology of insulin resistance. Visceral adipose tissue plays an important role in the balance between distinct insulin signalling pathways. Visceral adipocytes release a huge number of cytokines and adipokines with endocrine, autocrine, juxtacrine, and paracrine activity. A major driver in obesity-related insulin resistance and atherogenesis is chronic inflammation with invasion of the white adipose tissue by mononuclear cells and the release of pro-inflammatory cytokines like monocyte chemotactic protein 1 (MCP-1), plasminogen activator inhibitor 1 (PAI-1), interleukin-1 (IL-1), interleukin-6 (IL-6), tumor necrosis factor alpha (TNF-α) or high sensitive C-reactive peptide (hs-CRP). In addition, changes in the release of adipokines contribute to the development of insulin resistance, hypertension, and lipid disorders [22,23]. Several of these adipokines interfere with intracellular insulin signaling, with some (e.g. resistin) contributing to the development of IR, while others (e.g. adiponectin, leptin, visfatin) evolve insulin sensitizing effects [24-30]. Obesity driven insulin resistance predominantly affects the insulin PI3K pathway with a subsequent increase in circulating insulin levels and an overdrive of the unaffected (insulin sensitive) MAPK-dependent pathway [21,28,29,31]. Fasting plasma insulin levels in normal insulin sensitive people are in the low picomolar range (50-150pm) and may rise up to the nanomolar range (1.5nm) in patients developing insulin resistance [32,33].

In endothelial cells, a shift of signalling in favour of the MAPK pathway activates the endothelial release of endothelin 1 (ET-1), which plays an important role in the development of hypertension and the progression of atherosclerosis [34-36]. Endothelin-1 induces pro-atherogenic effects by vasoconstriction [37], increased vascular permeability [38], VSMC proliferation [39] and an increased production of IL-6 by endothelial cells and monocytes [40,41]. Therefore, IR results in an imbalance in favour of vasoconstriction by reducing insulin-dependent nitric oxide production and an increase in endothelin-1 [11,42]. The use of high doses of insulin in subjects who are refractory to its glucose lowering effect will only modestly activate PI3K signalling in the endothelium but will accelerate signalling through the MAPK pathway, leading to vasoconstriction and an increase in pro-atherogenic effects [43].

Insulin resistance in the heart

The myocardium with its high energy turnover, adapts to the predominant nutrient source through complex interactions between glucose and free fatty acid (FFA) metabolism [44,45]. Under physiological conditions, there is a reciprocal relationship between FFA and glucose levels in the blood. High FFA levels during fasting conditions inhibit the uptake and oxidation of glucose within the myocardium. In the feeding state, insulin drives the uptake of glucose within the myocardium, which will decrease the utilisation of FFA’s. In patients with poorly controlled T2DM, glucose and FFA’s are simultaneously elevated, which places the heart at increased risk of nutrient overload and myocardial glucolipotoxicity [46,47]. Myocardial insulin resistance in case of poorly controlled T2DM can restrict the glucose uptake to the myocardium, thereby providing some protection against myocardial glucolipotoxicity [45].

Figure 2: Nutrient Supply to the Myocardium (A- Non-Diabetic fasting, B – Non-Diabetic- fed, C – Poorly Controlled  Type 2 Diabetes mellitus; BG – Blood Glucose; FFA – Free Fatty Acids, IR – Insulin Resistance, Glut 4 – Glucosetransporter 4)

Consequently, treating patients with poorly controlled T2DM with large amounts of exogenous insulin could override this block of glucose entry into the myocardium accelerating glucolipotoxic effects to the myocard. In patients with ischemic heart disease, exogenous insulin tripled myocardial glucose uptake without any compensatory reduction in FFA uptake, thus placing the cardiomyocyte at high risk for glucolipotoxic damage [48]. Hyperinsulinaemia and hyperglycaemia was shown to increase lipid accumulation within the heart in non-diabetic subjects and in patients with T2DM [49,50].

Therefore insulin resistance might evolve diverging consequences depending on the activation of distinct insulin signalling pathways in different cells and tissues. Overriding insulin resistance by supplementation of high insulin amounts to certain cell lines, like endothelial cells or cardiomyocytes, might exert adverse effects with regard to cardiovascular complications.   

Assessment of insulin resistance

Having understood the basics of insulin resistance and the underlying molecular pathways poses the question how all this is relevant for drug development and how assessment of insuline resistance works. This question is relevant both for animal models as well as in humans. In our next post, we will address this by discussing current technology that allows measurement of insulin resistance. Sign up for our blog updates (on the right oside of this page) to make sure you won't miss it.

Furthermore, Profil Germany is hosting a online seminar on the topic of insulin resistance in preclinical and clinical trials. In this online seminar we bring together experts from from the preclinical side and clinical trial experts to discuss how insulin resistance can be measured and how to best prepare the move from preclinical to clinical trials for a relevant drug development project. Register for this free online seminar now, as seats are limited.  

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