Fatty acid 2-Hydroxylation in mammalian sphingolipid biology

Abstract

2-Hydroxy fatty acids (hFA) are important components of a subset of mammalian sphingolipids. The presence of hFA in sphingolipids is best described in the nervous system, epidermis, and kidney. However, the literature also indicates that various hFA-sphingolipids are present in additional tissues and cell types, as well as in tumors. Biosynthesis of hFA-sphingolipids requires fatty acid 2-hydroyxlase, and degradation of hFA-sphingolipids depends, at least in part, on lysosomal acid ceramidase and the peroxisomal fatty acid alpha-oxidation pathway. Mutations in the fatty acid 2-hydroxylase gene, FA2H, have been associated with leukodystrophy and spastic paraparesis in humans, underscoring the importance of hFA-sphingolipids in the nervous system. In the epidermis, hFA-ceramides are essential for the permeability barrier function. Physiological function of hFA-sphingolipids in other organs remains largely unknown. Recent evidence indicates that hFA-sphingolipids have specific roles in cell signaling.

Keywords: FA2H; Fatty acid 2-hydroxylase; fatty acid alpha-hydroxylase; hydroxy fatty acid; hydroxy sphingolipids.

Fig. 1. Biosynthesis of hFA-sphingolipids

The pathway is identical to biosynthesis of non-hydroxy sphingolipids, except for the fatty acid 2-hydroxylation step. Note that various fatty acids (mostly C₁₆-C₂₄) can be 2-hydroxylated and incorporated into hFA-ceramide. The 2-hydroxyl group is shown in red, and the hydroxyl group at the C-1 position of ceramide to which a head group is transferred is indicated by a red circle. DHC, dihydroceramide; CerS, dihydroceramide synthase; Cer, ceramide; SMS, sphingomyelin synthase; CGT, UDP-galactose: ceramide galactosyltransferase; GCS, UDP-glucose:ceramide glucosyltransferase; GSL, glycosphingolipids.

Fig. 2. FA2H is a membrane-bound monooxygenase

FA2H (372 aa) consists of two functional domains; the N-terminal cytochrome b₅ domain (1–92 aa) and the C-terminal catalytic domain. The topology of FA2H is postulated based on the topology of microsomal cytochrome b₅ [111] and the yeast FA2H homologue Scs7p [112, 113]. The 8 conserved histidines (H₂₃₄, H₂₃₉, H₂₅₇, H₂₆₀, H₂₆₁, H₃₃₆, H₃₃₉, H₃₄₀) are clustered in the two short segments facing the cytoplasm (shown in red). These histidines are thought to coordinate the non-heme di-iron. The 7 patients with an intronic mutation in the FA2H gene produce aberrant mRNA encoding a protein without the last three transmembrane domains that encompass the putative catalytic site.

Fig. 3. Degradation of hFA-sphingolipids

Endocytosed sphingolipids are first hydrolyzed by lysosomal hydrolases to ceramide. hFA-ceramide is hydrolyzed by acid ceramidase with the assistance of saposin D. Currently, there is no direct evidence for the recycling of hFA via a salvage pathway. The only known degradation pathway of hFA is the peroxisomal α-oxidation pathway, in which one-carbon cleavage of even-chain fatty acids (C_2n) generates odd-chain fatty acids (C_2n-1). How hFA is transported from the lysosome to peroxisome is not known. SL, sphingolipids; Sph, sphingosine; DHC, dihydroceramide; CerS, dihydroceramide synthase.