ADMINISTRACIÓN DEL RIESGO: (1) Introducción:

associated with all the components of the MetSyn [173]. Indeed, some authors have proposed that the presence of NAFLD should be included in the diagnostic criterion for the MetSyn [179]. NAFLD is present in 50-75% of people with type 2 diabetes and 76% of obese people and has been shown to predict insulin resistance independently from presence of obesity [180]. Insulin resistance has been proposed to promote hepatic triglyceride (TAG) accumulation. The two main sources for hepatic TAG are dietary TAG, transported to the liver via chylomicrons, or as the outcome of intrahepatic synthesis using glycerol and fatty acids (Figure 1.16). The fatty acids

required for this synthesis are either sourced from plasma as non-esterified fatty acids (NEFA) or newly synthesised within the hepatocyte by de novo lipogenesis (DNL). Whilst the hepatic uptake of NEFA is not regulated and is directly related to plasma NEFA concentrations, de novo lipogenesis is a precisely regulated process. Quantification studies in hepatic TAG

accumulation in NAFLD have demonstrated that NEFA accounts for 59% of hepatic TAG, 26% from DNL and 15% from dietary sources [181].

Two molecular mediators of lipid metabolism, namely, sterol regulatory element-binding protein isoform 1c (SREBP1c) and carbohydrate response element-binding protein (ChREBP) are important in the regulation of hepatocyte lipogenesis. SREBP1-c, upon activation and transcription

upregulation secondary to insulin action [182], transcriptionally upregulates most genes involved in DNL and TAG synthesis and decreases breakdown of fatty acids (β oxidation) [183, 184]. Therefore, through activation of SREBP- 1c, normal insulin action in hepatocytes leads to fatty acid build up,

providing increased substrate for triglyceride synthesis. ChREBP is activated in hyperglycaemia [185] and, apart from upregulating production of

enzymes of DNL, upregulates pathways of glycolysis (the process of glucose metabolism to pyruvate) and, thereby through the Kreb’s cycle, provides building blocks for fatty acid synthesis [186]. Hyperglycaemia, therefore, stimulates both glycolysis and lipogenesis by facilitating conversion of glucose to fatty acids in condition of energy excess.

In the state of insulin resistance, the associated hyperglycaemia and the reactive increase in insulin concentrations lead to an unrestricted activation of ChREBP and SREBP respectively (Figure 1.17). This promotes DNL and

Figure 1.16 Schematic diagram depicting regulation of hepatocyte lipid metabolism in response to blood glucose concentrations and insulin action.

NEFA, non esterified fatty acids; ChREBP, carbohydrate response element-binding protein; SREBP, Sterol regulatory element-binding protein isoform 1c; VLDL, very low density lipoprotein.

inhibits β oxidation within hepatocytes and, hence, promotes the

accumulation of triglycerides within the cell [187].Furthermore, increased plasma concentrations of NEFA, due to absence of insulin mediated

suppression of hormone sensitive lipase in adipocytes, also leads to increase FA availability in the hepatocyte. Insulin resistance, therefore, promotes intrahepatic accumulation of triglycerides and development of hepatic steatosis.

Accumulation of lipids within the liver has also been proposed to cause insulin resistance (Table 1.7). Inherited or acquired lipodystrophy [188, 189], a condition characterised by loss or absence of adipose tissue resulting in redistribution of lipids to liver and skeletal muscles, is known to lead to insulin resistance [189]. Hepatic lipidomic analyses of human NAFLD patients have identified selective accumulation of free cholesterol, proinflammatory eicosanoids, diacylglycerides (DAG) and ceramides

increasing with NAFLD progression [190]. Hepatic accumulation of DAG, an intermediate of TAG synthesis, strongly correlates with insulin resistance

Figure 1.17 Lipid metabolism in hepatocytes in the state of insulin resistance.

Insulin resistance leads to increase in hepatocyte triglyceride synthesis through increased delivery of NEFA and activation of ChREBP and SREP1c mediators. NEFA, non esterified fatty acids; ChREBP, carbohydrate response element-binding protein; SREBP1c, Sterol regulatory element-binding protein isoform 1c; VLDL, very low density lipoprotein.

[191], and has been investigated as a mediator of insulin resistance development [192]. Increased intracellular DAG leads to activation of

hepatocyte protein kinase C (PKC) isoform ξ leading to inactivation of serine phosphorylation of IRS [193]. Furthermore, DAG upregulates inflammatory pathways via JNK and IKK (reviewed in [194]). Findings from several genetic studies with transgenic and knockout mice [193, 195-198] support these mechanisms.

Lipid induced insulin resistance- Proposed mechanisms Steatosis   hepatic DAG activated PKCξ inactivation of IRS Steatosis   hepatic DAG activation of JNK and IKK Steatosis  hepatic Ceramides inhibition of Akt/PKB Steatosis  hepatic Ceramides  activation of JNK and IKK

Steatosis  hepatic cholesterol, LPA, long chain fatty acyl-CoA

Ceramides are a family of lipid molecules composed of sphingosine and a fatty acid. As a component of sphingomyelin, ceramides are found in all cell membranes. Increased accumulation of hepatic ceramides during hepatic steatosis is considered to be secondary to its upregulated de novo synthesis and salvage processes secondary to increased saturated fats [199], oxidative stress [200] or inflammatory stress [201]. Increased availability of NEFA to hepatocytes also increases the ceramide load to the cell. Apart from being structural components of the cell membrane, ceramides are also known to take part in lipid signalling. Studies in skeletal muscles have consistently demonstrated Akt/PKB inhibition as the mechanism of promotion on insulin resistance secondary to ceramides [202]. This is supported by in vitro experiments in cultured hepatocytes [203] and by in vivo experiments of inhibition [204] or promotion [205] of hepatic ceramide synthesis.

Table 1.7 Table listing some of proposed mechanisms for development of hepatic insulin resistance in NAFLD.

Hepatic DAG, ceramides, cholesterol and long chain fatty acyl-CoA accumulation in NAFLD have all been proposed to cause insulin resistance through inactivation of IRS or activation of inflammation. DAG, diacylglycerides; PKCξ , protein kinase C isoform ξ; JNK, c-Jun N-terminal kinase; IKK, Inhibitor of κB kinase; Akt/PKB, protein kinase B; LPA, lysophosphatidic acid.

Furthermore, ceramide accumulation promotes inflammatory cascades activation via JNK and IKK pathways [206].

Other potential mediators of lipid induced hepatic insulin resistance are

cholesterol, lysophosphatidic acid (LPA) and long chain fatty acyl-CoA [207]. Experimental and epidemiological evidence, therefore, demonstrate that the presence of insulin resistance and NAFLD foster the development and

progression of each other, thereby, advancing the metabolic derangements present in the state of obesity.

1.5.3 Hepatic inflammation in obesity associated metabolic