DES1: A Key Driver of Lipotoxicity in Metabolic Disease

Abstract

Ceramides have emerged as important regulators of tissue metabolism that play essential roles in cardiometabolic disease. They are potent biomarkers of diabetes and heart disease and are now being measured clinically as predictors of major adverse cardiac events. Moreover, studies in rodents reveal that inhibitors of ceramide synthesis prevent or reverse the pathogenic features of type 2 diabetes, nonalcoholic fatty liver disease, atherosclerosis, and cardiomyopathy. Herein the authors discuss inhibition of dihydroceramide desaturase-1, the final enzyme in the ceramide biosynthesis pathway, as a potential therapeutic approach to lower ceramides and combat cardiometabolic disease.

The global epidemic of obesity is leading to unprecedented rates of diabetes and liver disease, and their late-stage complications, including cardiovascular and kidney disease. The sphingolipid ceramide, which is a precursor to sphingomyelins and gangliosides that has both structural and signaling functions, is an important driver of the metabolic perturbations that underlie these diseases. Investigators have identified various points in the ceramide pathway that might be the basis for pharmacological interventions to combat these pathologies. Dihydroceramide desaturase 1 (DES1), an enzyme that catalyzes the final step in the de novo synthesis of ceramide, is a particularly attractive therapeutic target.

Despite considerable progress in pharmaceutical development, type 2 diabetes (T2D) affects a staggering number of adults in the United States; patients typically fail to achieve their glycemic goals (Mittermayer et al., 2015) and experience high rates of complications, including cardiovascular disease, nephropathy, and end-stage renal disease. Moreover, nonalcoholic fatty liver disease (NAFLD) is now the most common chronic liver disease with an estimated worldwide prevalence exceeding 25%, with 25%–40% of NAFLD patients progressing to the more advanced form of disease (nonalcoholic steatohepatitis [NASH]), which carries an increased risk of cirrhosis and hepatocellular carcinoma (Noureddin and Sanyal, 2018).

A key biological phenomenon that is central to these metabolic diseases is “lipotoxicity,” a term coined in the early 1990s by Roger Unger. Unger's (1995) lipotoxicity theory posits that, as a consequence of obesity and overnutrition, excess lipids are disseminated from storage depots (e.g., adipose tissue) into tissues and cells that are ill-equipped to handle them (e.g., insulin-secreting β-cells of the pancreas). Most of these ectopic lipids conjugate with glycerol to form di- and triglycerides (e.g., in the liver), which are storage forms that are probably harmless but which serve as useful biomarkers of lipid excess. When lipid levels exceed this cellular buffering capacity, they enter the parallel biosynthetic pathway that produces ceramides. The bioactive ceramides are a principal transmission mechanism of lipotoxicity. In a seminal early study, the Unger lab demonstrated that pharmacological blockade of ceramide biosynthesis reverses lipotoxicity in pancreatic β-cells of obese ZDF rats (Shimabukuro et al., 1998).

The ensuing two decades since this foundational study produced major technological advances (highly sensitive LC/MS/MS instrumentation that spawned the field of lipidomics, genetic ablation and pharmacological inhibition of biosynthetic enzymes in the ceramide pathway, and genome-wide association studies in patients) that have revealed clear roles for ceramides in metabolic disease (Chaurasia and Summers, 2015). In geographically and ethnically diverse patient populations, various species of ceramide (particularly those consisting of C16 and C18 acyl chains) were shown to strongly associate with insulin resistance/T2D (Bergman et al., 2015; Lemaitre et al., 2018; Fretts et al., 2019; Jensen et al., 2019) and NAFLD/NASH (Luukkonen et al., 2016; Apostolopoulou et al., 2018; Yang et al., 2019), as well as resultant cardiovascular events (Havulinna et al., 2016; Anroedh et al., 2018; Mantovani et al., 2018, 2019; Poss et al., 2020). A recent study in patients demonstrated that the level of blood ceramide increases in direct proportion to the severity of NAFLD (i.e., patients with NASH had higher ceramides than those with NAFL, and those with NAFL had higher ceramides than healthy controls) (Yang et al., 2019). In fact, ceramide (and its metabolite sphingosine-1-phosphate [S1P]) has been shown to drive processes leading to hepatic fibrosis in severe NASH, including apoptosis (Osawa et al., 2005), induction of collagen gene transcription (Chen et al., 2016), and activation of profibrogenic stellate cells (Ikeda et al., 2000; Liu et al., 2011).

The correlation of ceramide levels and disease severity raises the possibility that blood ceramide could potentially be used as a biomarker to stratify the many patients with simple hepatic steatosis from those 25%–40% who are more likely to progress to advanced NASH with fibrosis. High ceramide levels have also been shown to be prognostic of cardiovascular events (independent of LDLc, HDLc, and other traditional risk factors), and the Mayo Clinic has begun using ceramide as a clinical marker to identify patients with substantial near-term risk of having major adverse cardiac events, based upon a series of landmark longitudinal studies in patients (Laaksonen et al., 2016; Anroedh et al., 2018). Moreover, blood levels of dihydroceramide (the immediate precursor to ceramide in the de novo biosynthetic pathway) are a sensitive biomarker of metabolic flux through the de novo pathway as occurs in the context of chronic overnutrition/obesity and inflammation, and have been shown to anticipate the onset of diabetes by up to 9 years (Wigger et al., 2017).

Beyond ceramide's potential utility as a biomarker, research advances have validated the ceramide pathway as a functional driver of disease and a viable source of drug targets. Genetic ablation or pharmacological inhibition of various enzymatic steps in the de novo pathway of ceramide biosynthesis yields robust disease-modification in industry-standard animal models of insulin resistance/T2D and NAFLD/NASH (Holland et al., 2007, 2011; Ussher et al., 2010; Bikman et al., 2012; Zhang et al., 2012; Raichur et al., 2014; Kasumov et al., 2015; Xia et al., 2015; Chaurasia et al., 2016; Jiang et al., 2019). The totality of animal studies conducted by disparate investigators robustly demonstrates that reduction of ceramide levels effects substantial improvement in insulin sensitivity, prevention of pancreatic β-cell dysfunction, and resolution of liver fat, hepatocyte inflammation and apoptosis, and fibrosis.

More recently, studies conducted at the University of Utah and Merck Research Laboratories utilizing inducible whole-body and tissue-specific genetic ablation or knockdown of DEGS1 (the gene encoding DES1) in genetically defined or diet-induced obese mice have further advanced our understanding of ceramide-driven metabolic dysfunction (Chaurasia et al., 2019; Kusminski and Scherer, 2019). DES1 catalyzes the final step in the de novo biosynthesis of ceramide: the insertion of a C4 double bond into the precursor dihydroceramide to form ceramide. The studies reported by Chaurasia et al., (2019) demonstrate the importance of DES1 and reveal how a relatively minor chemical modification (insertion of the C4 double bond to form the bioactive ceramide) is transformative in effecting the metabolic derangements that lead to insulin resistance and NAFLD/NASH. Inducible genetic ablation of DES1 reduces ceramide levels, restores whole-body insulin sensitivity and reverses selective insulin resistance in the liver, clears hepatic steatosis, and ultimately decreases liver transaminases indicating a substantial improvement in overall liver health.

As depicted in Figure 1, pathologically high ceramide levels damage the liver in the following ways: (1) ceramide impairs mitochondrial activity in adipocytes, thereby increasing the supply of free fatty acids (FFAs) to the liver; (2) ceramide promotes hepatocyte uptake of FFAs by altering the localization of CD36; (3) ceramide upregulates SREBP1c to drive triglyceride synthesis and hepatic steatosis; (4) ceramide inhibits Akt through PP2A and PKCζ, which increases gluconeogenesis and promotes hepatocyte apoptosis; and (5) ceramide impairs mitochondrial function leading ultimately to hepatocyte apoptosis. Several of these ceramide-mediated biological effects were assayed in vitro using cultured adipocytes and liver cells and were fully reconstituted with ceramide, but not its inactive precursor dihydroceramide. In addition to demonstrating the potential therapeutic utility of DES1 inhibition, these studies also confirm the safety of broadly inhibiting the enzyme in the adult, and are thus consistent with the established safety of chronic DES1 inhibition in humans (heterozygous inactivating single nucleotide polymorphisms or long-term chemical inhibition) (Camerini et al., 2001; Blackburn et al., 2019).

FIG. 1.

Schematic depicting the utility of DES1 inhibitors for the treatment of insulin resistance and NAFLD. Ceramides alter adipose metabolism by inhibiting insulin-stimulated glucose uptake and decreasing mitochondrial efficiency. Simultaneously, they alter liver metabolism by impairing insulin signaling, thus enhancing gluconeogenesis, and inducing triglyceride production. When ceramides get to high enough concentrations, they initiate hepatic apoptosis and fibrosis. Collectively, these actions account for the key pathogenic features of NAFLD/NASH. Inhibiting DES1, which prevents the conversion of dihydroceramides to ceramides during de novo sphingolipid synthesis, ameliorates all of these conditions. DES1, dihydroceramide desaturase-1; NAFLD, nonalcoholic fatty liver disease; NASH, nonalcoholic steatohepatitis.

The now-extensive canon of human and animal data indicates that ceramide has an essential role in disease: (1) lipidomics studies in patients demonstrate that ceramide is at “the right place at the right time”; (2) patients with genetic mutations in acid ceramidase, the enzyme that breaks down ceramide and thus have high ceramide levels, present with substantial tissue damage, including extensive liver fibrosis (Yu et al., 2018); and (3) genetic ablation or pharmacological inhibition of various enzymes in the de novo ceramide biosynthesis pathway (including DES1) ameliorate disease in animals (“loss of function” effect). In the context of NAFLD/NASH, these studies suggest that pathologically high ceramide levels likely establish an ongoing feedforward cycle that compromises the liver and initiates the fibrogenic response characteristic of advanced disease. In as much as it is a druggable disease driver, ceramide (and its metabolites, including S1P) could also be a powerful clinical biomarker to predict disease, monitor response to treatment, and potentially identify patients who are most likely to respond to ceramide lowering agents.

Notwithstanding the overwhelmingly supportive data to date, the critical test of the “ceramide hypothesis” will be the discovery of an inhibitor of ceramide biosynthesis that is effective and safe in humans. Of all the enzymes in the pathway, DES1 presents itself as an attractive therapeutic target that displays an intriguing combination of safety, efficacy, and biomarker availability. Development of inhibitors of this and other enzymes in the ceramide pathway offers exciting opportunities to reduce the cardiometabolic disease burden for a large number of suffering individuals.

Disclosure Statement

S.A.S and J.T.B are cofounders and equity holders of Centaurus Therapeutics, Inc,. For L.W., no competing financial interests exist.

Funding Information

The authors receive research support from the National Institutes of Health (DK115824, DK116888, and DK116450), the Juvenile Diabetes Research Foundation (3-SRA-2019-768-A-B), the American Diabetes Association, and the Margolis Foundation.