Insulin resistance (IR) refers to the situation whereby insulin interaction with its receptor fails to elicit downstream signaling events such as those depicted in the Figures above. Metabolically and clinically the most detrimental effects of IR are due to disruption in insulin-mediated control of glucose and lipid homeostasis in the primary insulin-responsive tissues: liver, skeletal muscle, and adipose tissue. IR is a characteristic feature found associated with most cases of type 2 diabetes. In addition, IR is the hallmark feature of the metabolic syndrome (MetS). IR can occur for a number of reasons however, the most prevalent cause is the hyperlipidemic and pro-inflammatory states associated with obesity. How does an abnormal metabolism, as is associated with obesity, lead to the development of IR? The answer to this question can be found in the effects of excess free fatty acids (FFAs) on the insulin receptor-mediated signaling pathways in adipose tissue, liver, and skeletal muscle as well as the pro-inflammatory status induced by the toxic effects of excess FFAs principally in the liver and adipose tissues.
The precise mechanisms that underlie the promotion of a pro-inflammatory state in obese individuals in not completely established. However, both adipose tissue and liver are important mediators of systemic inflammation in obesity. One model proposes that the expansion of adipose tissue that occurs in obesity results in large adipocytes that have metabolic capacities that exceed the local oxygen supply. The resultant hypoxia leads to the activation of cellular stress response pathways induced by the hypoxia induced factor 1 (HIF-1) transcription factor. Activation of HIF-1 is associated with cell autonomous inflammation and the release of pro-inflammatory cytokines. As a part of the chronic inflammation adipocytes secrete chemokines such as IL-8 and macrophage chemotactic protein-1 (MCP-1) that attract pro-inflammatory macrophages into the adipose tissue. These activated adipose tissue macrophages secrete cytokines that further exacerbate the pro-inflammatory state. In the liver inflammatory processes are also activated due to the excess accumulation of fatty acids and triglycerides which is the consequence of activated stress response pathways. Within the liver, Kupffer cells (resident liver macrophages) become activated by the generation of reactive oxygen species (ROS) and induction of stress responses. These activated Kupffer cells release locally acting cytokines that, like in adipose tissue, exacerbates the pro-inflammatory environment. Within the vasculature, saturated FFAs can directly activate pro-inflammatory pathways in endothelial cells and myeloid-derived cells resulting in the induction and propagation of a systemic pro-inflammatory state.
Insulin resistance induced by fatty acids. Model for how excess free fatty acids (FFAs) lead to insulin resistance and enhanced inflammatory responses, predominantly in cells such as skeletal muscle and adipose tissue but also in the liver. Only the major pathways regulated by insulin relative to glucose and lipid homeostasis are shown. Black arrows represent positive actions and red T-lines represent inhibitory actions. JNK: Jun N-terminal kinase. PKC: protein kinase C. IKKβ: inhibitor of nuclear factor kappa B kinase beta. ROS: reactive oxygen species. PI3K: phosphatidylinositol-3 kinase. DAG: diacylglycerol. TAG: triacylglycerides. LCA-CoA: long-chain acyl-CoAs. NFκB: nuclear factor kappa B. PKB (protein kinase B) is a serine/threonine kinase also known as AKT. The role of ceramides in the development of insulin resistance is discussed in the section below.
Hepatic IR is induced by the excess accumulation of FFAs. Within the hepatocyte, metabolites of the FFA re-esterification process, including long-chain acyl-CoAs and diacylglycerol (DAG), accumulate. Excess FFAs also participate in the relocation of several protein kinase C (PKC) isoforms, from the cytosol to the membrane compartment. These PKC isoforms include PKC-β2, PKC-δ, and PKC-theta (PKC-θ). DAG is a potent activator of these PKC isoforms and the membrane-associated PKCs will phosphorylate the intracellular portion of the insulin receptor on serine residues which results in impairment of insulin receptor interaction with downstream signaling proteins including insulin receptor substrate 1 (IRS1) and IRS2. Loss of IRS1 and IRS2 interaction with the receptor prevents interaction with phosphatidylinositol 3-kinase (PI3K) and its' subsequent activation. In addition to serine phosphorylation of the insulin receptor, various PKCs have been shown to phosphorylate IRS1 and IRS2 further impairing the ability of these insulin receptor substrates to associate with the insulin receptor and downstream effector proteins such as PI3K.
The FFA-induced down-regulation of insulin signaling pathways results in activation of several kinases involved in stress responses. These kinases include Jun N-terminal kinase (JNK), inhibitor of nuclear factor kappa B kinase beta (IKKβ), and suppressors of cytokine signaling-3 (SOCS-3). Like PKC, JNK activity is also regulated by FFAs and is an important regulator of IR. The target of JNK action is the Ser307 of IRS-1 and this phosphorylation plays an important role in the progression to hepatic IR. Activation of IKKβ (which is required for the activation of nuclear factor kappa B, NFκB) may have the most pronounced effect on inflammatory responses in the liver and adipose tissue. NFκB is the most important transcription factor activating the expression of numerous pro-inflammatory cytokine genes such as interleukin-1 (IL-1), IL-6, and tumor necrosis factor-alpha (TNF-α) each of which have been shown to be involved in promoting IR. NFκB-dependent inflammatory mediators produced in hepatocytes act to reduce insulin sensitivity and to promote liver injury.
Analysis of the effects of FFAs on macrophages in cell culture demonstrated that they can activate inflammatory signaling through the toll-like receptors (TLRs), specifically TLR4. The TLRs are a family of cell surface receptors involved in key events triggered via the innate immune system. The TLRs are pattern recognition receptors that recognize structurally conserved molecules from microbial pathogens. TLR4 is responsive to bacterially derived lipopolysaccharide (LPS) which is an endotoxin secreted by gram-negative bacteria. LPS stimulation of TLR4 results in activation of both the JNK and IKKβ signal transduction pathways leading to secretion of pro-inflammatory cytokines such as IL-1β, IL-6, MCP-1, and tumor necrosis factor alpha (TNFα). These cell culture experiments demonstrated that FFA addition to macrophages results in activation of NFκB and that this activation was deficient in macrophages from TLR4 knock-out mice. In the livers of TLR4 knock-out mice there is reduced inflammation even in the presence of hepatic steatosis suggesting that Kupffer cell TLR4 is important in hepatic inflammatory responses to excess FFA loading.