Ceramide and Insulin Resistance: How Should the Issue Be Approached?

There is a growing amount of data indicating that ceramide is involved in the generation of insulin resistance (1–3). However, investigation into the biological function of ceramide is a complex issue. First, there are no ceramide receptors on the plasma membrane. Second, the membrane is impermeable for ceramides containing long-chain fatty acid residues and only such ceramides seem to exist in nature. There are 13 and 12 ceramides containing different long-chain fatty acid residues that have been identified in human and rat skeletal muscle, respectively (4,5). This means that none of them can be used to study a biological role of extracellular ceramide. Therefore, there remain two other options: 1) use short-chain (C2 or C6) ceramides, which penetrate the cell membrane or 2) manipulate with the level of intracellular ceramide. In the latter case, the activity of selected enzymes of its metabolism may be influenced either by different compounds or by genetic manipulations with the expression of the enzymes. However, there are several pathways of feeding into the ceramide pool in the cell. They are de novo synthesis, hydrolysis of sphingomyelin, catabolism of complex sphingolipids, acylation of sphingosine, and dephosphorylation of ceramide-1-phosphate (Fig. 1). Moreover, most reactions involved in ceramide metabolism are reversible (6,7). Ceramide is on the crossroads of sphingolipid metabolism. However, so far the contributions of different pathways to the ceramide pool have not been quantified in detail. Certainly, the most important source of ceramide for the balance of sphingolipids in the cell is its de novo synthesis. This pathway supplies, through ceramide, all other sphingolipid metabolic pathways (6,7). Studies with C2 and C6 ceramides showed that they produce insulin resistance in C2C12 and L6 myotubes (1–3). However, it should be added that it is not certain whether short-chain ceramides mimic fully the action of the long-chain ones. Other evidence of the involvement of ceramide in the generation of insulin resistance is indirect. It includes examination of a relationship between the level of muscle ceramide and insulin sensitivity. In vivo data obtained in rat and in most human studies (4,5,8) showed an existence of an inverse relationship between the level of ceramide in skeletal muscles and insulin sensitivity. Another approach was to study the role of different pathways of ceramide metabolism in the development of insulin resistance. It was shown that inhibition of the de novo synthesis pathway prevents accumulation of ceramide and reduction of insulin sensitivity in mice fed a high-fat diet and in db/db mice (Fig. 1) (9,10). Overexpression of acid ceramidase (the enzyme that deacylates ceramide) blocks saturated fatty acids–induced elevation in the level of ceramide in C2C12 myotubes with concomitant prevention in reduction of insulin sensitivity (11). On the contrary, inhibition of the enzyme markedly augmented both basaland palmitate-induced level of ceramide in C2C12 cells (12). Deficiency of ceramide kinase, the enzyme that phosphorylates ceramide to ceramide-1-phosphate, increases insulin sensitivity in mice fed a high-fat diet (13). In this case, however, the role of a reduction in the level of ceramide-1-phosphate has not been elucidated. Blockade of the conversion of ceramide to glucosphingolipids increases insulin sensitivity. The latter effect could be caused by the inhibition of the formation of GM3, an intermediate on the glucosphingolipid metabolism pathway (1,14). The existing data on modulation of acid sphingomyelinase activity (the enzyme that catalyzes the conversion of sphigomyelin to ceramide) also indicate the role of ceramide in regulation of insulin sensitivity (1). A novel approach to the problem is presented by Bruce et al. (15) in an article that appears in this issue of Diabetes. They used transgenic mice with overexpression of sphingosine kinase 1 (SphK1). The enzyme phosphorylates sphingosine, the only product of ceramide catabolism, to sphingosine-1-phosphate (S1P). The mice were fed either a standard chow or a high-fat diet for 6 weeks. Overexpression of SphK1 in chow-fed mice resulted in the reduction in the level of total ceramide in the quadriceps and soleus muscles but not in the white adipose tissue and the liver. The level of S1P increased insignificantly, and the level of sphingosine remained stable. This manipulation did not result in any changes in fasting plasma glucose, insulin, triglycerides, and glucose tolerance in mice fed a chow diet. This is very important data showing that the reduction in the level of ceramide in skeletal muscles does not affect the behavior of the examined variables in mice fed a chow diet. However, the role of SphK1 in the generation of insulin resistance evinced when the mice were fed a high-fat diet. Overexpression of the enzyme prevented ceramide—but not triglyceride—accumulation in the muscles seen in wild-type mice after the diet. The levels of other sphingolipid intermediates did not differ from the respective values in the wild-type mice fed a highfat diet. Concomitantly, glucose tolerance was improved. Also ex vivo insulin-stimulated glucose uptake and Akt phosphorylation in skeletal muscles were enhanced. Finally, the data obtained in the hyperinsulinemic euglycemic clamp experiment showed improvement of insulin sensitivity after overexpression of SphK1 in the high-fat diet group. Taken together, Bruce et al. (15) provide convincing From the Department of Physiology, Medical University of Białystok, Białystok, Poland. Corresponding author: Jan Górski, [email protected]. DOI: 10.2337/db12-1157 2012 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by -nc-nd/3.0/ for details. See accompanying original article, p. 3148.