Inhibition of sphingolipid synthesis improves dyslipidemia in the diet-induced hamster model of insulin resistance: evidence for the role of sphingosine and sphinganine in hepatic VLDL-apoB100 overproduction

Inhibition of ceramide de novo synthesis with myriocin affects lipid metabolism in the liver of rats with streptozotocin-induced type 1 diabetes

Nowadays diabetes is one of the most common metabolic diseases. Sphingolipids, which are vitally important constituents of intracellular signal transduction pathways, may be among the most pathogenic lipid moieties intermingled in the origin and development of diabetes. It is now well established that inhibition of de novo ceramide synthesis with myriocin exerts positive effects on lipid metabolism and glucose homeostasis in type 2 diabetes mellitus animal models. However, its influence on type I diabetes still remains unknown. Therefore, the scope of this paper is to fulfill that particular gap in our knowledge.

Effects of inhibition of serine palmitoyltransferase (SPT) and sphingosine kinase 1 (SphK1) on palmitate induced insulin resistance in L6 myotubes

Background

The objective of this study was to examine the effects of short (2 h) and prolonged (18 h) inhibition of serine palmitoyltransferase (SPT) and sphingosine kinase 1 (SphK1) on palmitate (PA) induced insulin resistance in L6 myotubes.

Methods

L6 myotubes were treated simultaneously with either PA and myriocin (SPT inhibitor) or PA and Ski II (SphK1inhibitor) for different time periods (2 h and 18 h). Insulin stimulated glucose uptake was measured using radioactive isotope. Expression of insulin signaling proteins was determined using Western blot analyses. Intracellular sphingolipids content [sphinganine (SFA), ceramide (CER), sphingosine (SFO), sphingosine-1-phosphate (S1P)] were estimated by HPLC.

Results

Our results revealed that both short and prolonged time of inhibition of SPT by myriocin was sufficient to prevent ceramide accumulation and simultaneously reverse palmitate induced inhibition of insulin-stimulated glucose transport. In contrast, prolonged inhibition of SphK1 intensified the effect of PA on insulin-stimulated glucose uptake and attenuated further the activity of insulin signaling proteins (pGSK3β/GSK3β ratio) in L6 myotubes. These effects were related to the accumulation of sphingosine in palmitate treated myotubes.

Conclusion

Myriocin is more effective in restoration of palmitate induced insulin resistance in L6 myocytes, despite of the time of SPT inhibition, comparing to SKII (a specific SphK1 inhibitor). Observed changes in insulin signaling proteins were related to the content of specific sphingolipids, namely to the reduction of ceramide. Interestingly, inactivation of SphK1 augmented the effect of PA induced insulin resistance in L6 myotubes, which was associated with further inhibition of insulin stimulated PKB and GSK3β phosphorylation, glucose uptake and the accumulation of sphingosine.

Morin reduces hepatic inflammation-associated lipid accumulation in high fructose-fed rats via inhibiting sphingosine kinase 1/sphingosine 1-phosphate signaling pathway

SphK1/S1P signaling pathway is involved in the development of hepatic inflammation and injury. But its role in high fructose-induced NAFLD has not yet been reported. The aim of this study was to elucidate the crucial role of SphK1/S1P signaling pathway in high fructose-induced hepatic inflammation and lipid accumulation in rats. Moreover, the hepatoprotective effects of morin, a flavonoid with anti-inflammatory and anti-hyperlipedimic activities, on these hepatic changes in rats were investigated. High fructose-fed rats were orally treated with morin (30 and 60mg/kg) and pioglitazone (4mg/kg) for 8 weeks, respectively. Fructose feeding induced hyperlipidemia, and activated SphK1/S1P signaling pathway characterized by the elevation of SphK1 activity, S1P production as well as SphK1, S1PR1 and S1PR3 protein levels, which in turn caused NF-κB signaling activation to produce IL-1β, IL-6 and TNF-α and inflammation in the liver of rats. Subsequently, hepatic insulin and leptin signaling impairment and lipid metabolic disorder were observed in this animal model, resulting in liver lipid accumulation. Morin restored high fructose-induced the activation of hepatic SphK1/S1P signaling pathway in rats. Subsequently, the reduced NF-κB signaling activation by morin decreased inflammatory cytokine production, recovered insulin and leptin signaling impairment to reduce lipid accumulation and injury in the rat liver. These effects of morin were confirmed in Buffalo rat liver (BRL3A) cell model stimulated with 5mM fructose. Thus, the inhibition of hepatic SphK1/S1P signaling pathway may be a novel mechanism by which morin exerts hepatoprotection in high fructose-fed rats, possibly involving liver inflammation inhibition and lipid accumulation recovery.

Sphingolipids and lifespan regulation

Diseases including cancer, type 2 diabetes, cardiovascular and immune dysfunction and neurodegeneration become more prevalent as we age, and combined with the increase in average human lifespan, place an ever increasing burden on the health care system. In this chapter we focus on finding ways of modulating sphingolipids to prevent the development of age-associated diseases or delay their onset, both of which could improve health in elderly, fragile people. Reducing the incidence of or delaying the onset of diseases of aging has blossomed in the past decade because of advances in understanding signal transduction pathways and cellular processes, especially in model organisms, that are largely conserved in most eukaryotes and that can be modulated to reduce signs of aging and increase health span. In model organisms such interventions must also increase lifespan to be considered significant, but this is not a requirement for use in humans. The most encouraging interventions in model organisms involve lowering the concentration of one or more sphingolipids so as to reduce the activity of key signaling pathways, one of the most promising being the Target of Rapamycin Complex 1 (TORC1) protein kinase pathway. Other potential ways in which modulating sphingolipids may contribute to improving the health profile of the elderly is by reducing oxidative stresses, inflammatory responses and growth factor signaling. Lastly, perhaps the most interesting way to modulate sphingolipids and promote longevity is by lowering the activity of serine palmitoyltransferase, the first enzyme in the de novo sphingolipid biosynthesis pathway. Available data in yeasts and rodents are encouraging and as we gain insights into molecular mechanisms the strategies for improving human health by modulating sphingolipids will become more apparent. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.