Sphingolipids such as ceramides and glucosylceramides
Sphingolipids, such as ceramides and glucosylceramides, are an important class of bioactive lipids. The levels of these lipids change as a function of adipose tissue mass and functionality, and are partially driven by cellular availability of palmitoyl-CoA. Aberrant accumulation of ceramide, glucosylceramide, and GM3 ganglioside has been implicated in a multitude of metabolic processes, including atherosclerosis, insulin resistance, lipotoxic heart failure, β-cell apoptosis, and β-cell dysfunction (reviewed in ). In stark opposition, the phosphorylated sphingoid base Sphingosine 1-phosphate (S1P) is a potent inducer of proliferation and inhibitor of apoptosis . The conversion of ceramide to S1P consists of deacylation of ceramide by ceramidase enzymes and a subsequent phosphorylation of sphingosine by one of two sphingosine kinase isoforms . The opposing nature and simple 2-step conversion-process separating these lipids has led to speculation that the dynamic ratio of ceramide:S1P may constitute a physiological rheostat regulating in numerous cellular processes .
To gain further insights into the local physiological consequences of adiponectin and AdipoR-induced ceramidase activation we have embarked on two parallel approaches. Using a novel doxycycline-inducible model to allow for tissue-specific overexpression of 3mb ceramidase, we have determined that ceramidase activation in adipose or liver is sufficient to prevent or reverse diet-induced steatosis, insulin resistance, and glucose intolerance . Here we have generated similar models allowing for AdipoR1 or AdipoR2 to be expressed under the control of a tetracycline response element (TRE-AdipoR1 or TRE-AdipoR2). We use these in order to determine which adiponectin receptor may have the most beneficial effects on glucose tolerance and whole body insulin sensitivity, and to compare and contrast adiponectin receptor action with a lysosomal ceramidase that does not promote S1P accumulation. Furthermore, we determine whether targeting of adiponectin receptors within hepatocytes or adipocytes provides the greatest metabolic improvements and evaluate the effects of adiponectin receptor agonists for their ability to faithfully recapitulate these effects and further enhance them in transgenic mice.
Materials and methods
Results We recently reported that FGF21 rapidly promotes adiponectin secretion and lowers hepatic ceramides. Curiously, FGF21 promoted an increase in accumulation of ceramides (Supplemental Figure 1A) and glucosylceramides (Supplemental Figure 1B) within the adipose tissue. As opposed to an increase in ceramidase activity in the liver (Supplemental Figure 1C, top), ceramidase activity in the adipose tissue was decreased by FGF21 (Supplemental Figure 1C, bottom). As this coincides with a decrease in adiponectin retained within the adipocyte, these data (and others) prompted us to consider if adiponectin receptors promote ceramidase activity within adipocytes to convey adiponectin actions within adipose tissue.
Discussion These studies link adiponectin-induced ceramidase activity to metabolic improvements of non-alcoholic fatty liver disease and systemic insulin resistance in rodents. Similar to our recent report of acid ceramidase overexpression we find that adiponectin receptors functionally prevent accumulation of C16 and C18 ceramides in DIO mice. We found that C16:0 and C18:0 ceramide levels to be ∼60% of WT in livers of Alb-R1 and AlbR2 mice, which are the ceramide subspecies that most prominently contribute to the development of NAFLD and hepatic insulin resistance. Consistent with our findings, recent work has shown that mice with ceramide synthase 6 deficiency (cerS6Δ/Δ) exhibit reduced C16:0 ceramide levels and are protected from high fat diet-induced obesity and glucose intolerance . By contrast, heterozygous deletion of ceramide synthase 2 promotes a paradoxical increase in C16:0 ceramides to confer greater susceptibility to diet induced insulin resistance .