Sphingolipid regulators of cellular dysfunction in Type 2 diabetes mellitus: a systems overview

Climbing obesity rates have contributed to worldwide increases in obesity-associated diseases, including the metabolic syndrome and Type 2 diabetes mellitus (T2DM). Sphingolipids, an important class of structural and signaling lipids, have emerged as key players in the development and pathogenesis of insulin resistance and T2DM. More specifically, sphingolipids have been demonstrated to play integral roles in lipotoxicity and other aspects of pathogenesis in T2DM, although the cellular mechanisms by which this occurs and by which sphingolipid metabolism is dysregulated in T2DM remain under investigation. This review summarizes current knowledge of sphingolipid metabolism and signaling in key organs and tissues affected by T2DM, including the pancreas, adipose tissue, skeletal muscle, cardiovascular system and liver, and highlights areas that ripe for future investigation.

SphK1 mediates hepatic inflammation in a mouse model of NASH induced by high saturated fat feeding and initiates proinflammatory signaling in hepatocytes

Steatohepatitis occurs in up to 20% of patients with fatty liver disease and leads to its primary disease outcomes, including fibrosis, cirrhosis, and increased risk of hepatocellular carcinoma. Mechanisms that mediate this inflammation are of major interest. We previously showed that overload of saturated fatty acids, such as that which occurs with metabolic syndrome, induced sphingosine kinase 1 (SphK1), an enzyme that generates sphingosine-1-phosphate (S1P). While data suggest beneficial roles for S1P in some contexts, we hypothesized that it may promote hepatic inflammation in the context of obesity. Consistent with this, we observed 2-fold elevation of this enzyme in livers from humans with nonalcoholic fatty liver disease and also in mice with high saturated fat feeding, which recapitulated the human disease. Mice exhibited activation of NFκB, elevated cytokine production, and immune cell infiltration. Importantly, SphK1-null mice were protected from these outcomes. Studies in cultured cells demonstrated saturated fatty acid induction of SphK1 message, protein, and activity, and also a requirement of the enzyme for NFκB signaling and increased mRNA encoding TNFα and MCP1. Moreover, saturated fat-induced NFκB signaling and elevation of TNFα and MCP1 mRNA in HepG2 cells was blocked by targeted knockdown of S1P receptor 1, supporting a role for this lipid signaling pathway in inflammation in nonalcoholic fatty liver disease.

Fatty acids differentially regulate insulin resistance through endoplasm reticulum stress-mediated induction of tribbles homologue 3: a potential link between dietary fat composition and the pathophysiological outcomes of obesity

AIMS/HYPOTHESIS

Previous studies have shown that saturated fatty acids cause insulin resistance (IR) that is prevented by unsaturated fatty acids. Tribbles homologue 3 (TRIB3) is a putative endogenous inhibitor of insulin signalling, but its role in insulin signalling is controversial. This study aimed to determine whether fatty acids regulate IR via TRIB3.

METHODS

We treated HepG2 cells with saturated and unsaturated fatty acids and evaluated TRIB3 expression. We then tested whether regulation of TRIB3 occurred through endoplasmic reticulum (ER) stress, and whether modulating TRIB3 and ER stress marker genes was necessary and/or sufficient for regulation of insulin signalling. To test the in vivo significance of this mechanism, we fed mice obesogenic diets with different fatty acid profiles and assessed physiological variables of diabetes, ER stress markers and Trib3 expression in the liver.

RESULTS

Our data show that fatty acids differentially regulate IR through ER stress-mediated induction of TRIB3. Intriguingly, a standard and widely used obesogenic diet high in unsaturated fats failed to induce ER stress, TRIB3 or IR. However, an alternative obesogenic diet with lower unsaturated fat recapitulated the cell studies by causing ER stress, TRIB3 induction and IR.

CONCLUSIONS/INTERPRETATION

This study revealed a novel mechanism linking dietary fat composition to IR. Given the emerging roles for ER stress in non-alcoholic liver disease, we conclude that dietary fat composition rather than total amount may mediate hepatic pathology associated with obesity.

The yeast oxysterol binding protein Kes1 maintains sphingolipid levels

The oxysterol binding protein family are amphitropic proteins that bind oxysterols, sterols, and possibly phosphoinositides, in a conserved binding pocket. The Saccharomyces cerevisiae oxysterol binding protein family member Kes1 (also known as Osh4) also binds phosphoinositides on a distinct surface of the protein from the conserved binding pocket. In this study, we determine that the oxysterol binding protein family member Kes1 is required to maintain the ratio of complex sphingolipids and levels of ceramide, sphingosine-phosphate and sphingosine. This inability to maintain normal sphingolipid homeostasis resulted in misdistribution of Pma1, a protein that requires normal sphingolipid synthesis to occur to partition into membrane rafts at the Golgi for its trafficking to the plasma membrane.

Sphingolipids in obesity, type 2 diabetes, and metabolic disease

Metabolic disease, including obesity and type 2 diabetes, constitutes a major emerging health crisis in Western nations. Although the symptoms and clinical pathology and physiology of these conditions are well understood, the molecular mechanisms underlying the disease process have largely remained obscure. Sphingolipids, a lipid class with both signaling and structural properties, have recently emerged as key players in most major tissues affected by diabetes and are required components in the molecular etiology of this disease. Indeed, sphingolipids have been shown to mediate loss of insulin sensitivity, to promote the characteristic diabetic proinflammatory state, and to induce cell death and dysfunction in important organs such as the pancreas and heart. Furthermore, plasma sphingolipid levels are emerging as potential biomarkers for the decompensation of insulin resistance to frank type 2 diabetes. Despite these discoveries, the roles of specific sphingolipid species and sphingolipid metabolic pathways remain obscure, and newly developed experimental approaches must be employed to elucidate the detailed molecular mechanisms necessary for rational drug development and other clinical applications.