Ceramide synthase inhibition by fumonisins: a perfect storm of perturbed sphingolipid metabolism, signaling, and disease

Abstract

Fumonisins are mycotoxins that cause diseases of plants and, when consumed by animals, can damage liver, kidney, lung, brain, and other organs, alter immune function, and cause developmental defects and cancer. They structurally resemble sphingolipids (SLs), and studies nearly 30 years ago discovered that the most prevalent fumonisin [fumonisin B₁ (FB₁)] potently inhibits ceramide synthases (CerSs), enzymes that use fatty acyl-CoAs to N-acylate sphinganine (Sa), sphingosine (So), and other sphingoid bases. CerS inhibition by FB₁ triggers a "perfect storm" of perturbations in structural and signaling SLs that include: reduced formation of dihydroceramides, ceramides, and complex SLs; elevated Sa and So and their 1-phosphates, novel 1-deoxy-sphingoid bases; and alteration of additional lipid metabolites from interrelated pathways. Moreover, because the initial enzyme of sphingoid base biosynthesis remains active (sometimes with increased activity), the impact is multiplied by the continued production of damaging metabolites. Evidence from many studies, including characterization of knockout mice for specific CerSs and analyses of human blood (which found that FB₁ intake is associated with elevated Sa 1-phosphate), has consistently pointed to CerS as the proximate target of FB₁ It is also apparent that the changes in multiple bioactive lipids and related biologic processes account for the ensuing spectrum of animal and plant disease. Thus, the diseases caused by fumonisins can be categorized as "sphingolipidoses" (in these cases, due to defective SL biosynthesis), and the lessons learned about the consequences of CerS inhibition should be borne in mind when contemplating other naturally occurring and synthetic compounds (and genetic manipulations) that interfere with SL metabolism.

Keywords: cancer; glycolipids; lipid signaling; liver; nutrition/lipids; sphingosine 1-phosphate; toxicology.

Graphical abstract

Fig. 1.

Structures of B family fumonisins and representative SLs. The major fumonisin, FB₁, is shown with highlighting of the sphingoid base-like portion in blue, additional functional groups in red, and the tricarballylic acids (TCA) in green. Hydrolyzed fumonisins (such as HFB₁) lack the TCA groups, and are sometimes referred to as “AP”. Components that differ for B₂ (HFB₂) and B₃ (HFB₃) are shown in black. Four representative mammalian sphingoid bases are displayed by the Sa backbone (in blue), So (with a 4,5-trans-double bond), and phytosphingosine (with a 4-hydroxyl-group) indicated in black, and 1dSa. The lower structures represent N-acyl derivatives of sphingoid bases and the 1-phosphates.

Fig. 2.

An overview of SL biosynthesis and turnover in mammalian cells with highlighted metabolites that are affected by inhibition of CerSs by fumonisins (FB₁). The metabolites in green are generally elevated when CerSs are inhibited by FB₁ and metabolites in red eventually decrease. The perinuclear dashed box represents the ER, where de novo sphingoid base biosynthesis occurs, with CerSs in the ER, mitochondria, and associated membranes (MAMs). In these reactions, Ser + a fatty acyl-CoA (usually palmitoyl-CoA) are incorporated into 3-ketosphinganine (not shown) and then Sa followed by N-acylation to DHCers by CerS (likewise 1dSa is made from Ala then acylated to 1dDHCer). DHCer is mostly desaturated to Cer and (DH)Cers are incorporated into more complex SLs (mainly SM and GSLs) beginning in the ER lumen (for galactosylCer), cis-Golgi (for glucosylCer, with more complex GSLs made throughout the Golgi apparatus) and trans-Golgi, and plasma membrane (for SM). The SMs and GSLs arrive at their destinations (plasma membrane, exosomes, and other locations) via vesicular transport, transport proteins, and sorting mechanisms at the trans-Golgi network (TGN). SL turnover occurs in multiple locations in the cell (hydrolases in lysosomes, autophagosomes, and other organelles) to release the sphingoid base (mainly So) that is recycled (salvaged) via CerSs or phosphorylated by So kinases to 1-phosphates in multiple locations in the cell. Sphingoid bases (and the 1-phosphates) can function in cell signaling, undergo degradation (to hexadecenal, hexadecanal, and ethanolamine phosphate), or efflux from the cell. Metabolism of Cer from SM in plasma membrane signaling might not be affected by fumonisins until SM is depleted. For more information about these processes, see the references cited in the text and Supplement A.

Fig. 3.

A scheme summarizing how inhibition of CerS(s) alters multiple SLs to affect many biochemical processes and the toxicologically relevant perturbations found in mammalian cells in culture and/or animals exposed to fumonisins. All of these relationships have been experimentally linked (for example, that sphingoid bases can induce ROS and that fumonisins elevate sphingoid bases and ROS), but most are sufficiently complex (i.e., have multiple causes) that they have not been definitively connected across all four columns of this scheme. Each of the components of this figure is described in greater detail with relevant bibliographic citations (247 references) in Supplement B.

Fig. 4.

Additional fungal secondary metabolites known to inhibit SL metabolism and/or with close structural similarity to known substrates/inhibitors. The sphingoid base-like structural features of AAL toxin TA1 and AOD are colored blue with additional functional groups in red and green, as in Fig. 1. Australifungin shares some of these features (as an alkyl polyol), but lacks the characteristic amino-group of sphingoid bases. There are also several fungal metabolites, such as myriocin (not shown), that are potent inhibitors of SPT. References for these compounds are given in the text.